SwePub
Sök i SwePub databas

  Extended search

Träfflista för sökning "WFRF:(Stranne Christian) "

Search: WFRF:(Stranne Christian)

  • Result 1-50 of 59
Sort/group result
   
EnumerationReferenceCoverFind
1.
  • Löwemark, Ludvig, et al. (author)
  • Practical guidelines and recent advances in the Itrax XRF core-scanning procedure
  • 2019
  • In: Quaternary International. - : Elsevier BV. - 1040-6182 .- 1873-4553. ; 514, s. 16-29
  • Journal article (peer-reviewed)abstract
    • XRF core scanning has evolved to become a standard analytical technique for the rapid assessment of elemental, density and textural variations in a wide range of sediments and other materials, with applications ranging from palaeoceanography, paleoclimatology, geology, and environmental forensics to environmental protection. In general, scanning provides rapid, non-destructive acquisition of elemental and textural variations at sub-millimetre resolution for a wide range of materials. Numerous procedural adaptations have been developed for the growing number of applications, such as analyses of unconsolidated, water-rich sediments, powdered soil samples, or resin bags. Here, practical expertise and guidance from the Itrax community, gained over 15 years, is presented that should provide insights for new and experienced users.
  •  
2.
  • Akhoudas, Camille Hayatte, 1992-, et al. (author)
  • Isotopic evidence for an intensified hydrological cycle in the Indian sector of the Southern Ocean
  • 2023
  • In: Nature Communications. - 2041-1723. ; 14
  • Journal article (peer-reviewed)abstract
    • The hydrological cycle is expected to intensify in a warming climate. However, observational evidence of such changes in the Southern Ocean is difficult to obtain due to sparse measurements and a complex superposition of changes in precipitation, sea ice, and glacial meltwater. Here we disentangle these signals using a dataset of salinity and seawater oxygen isotope observations collected in the Indian sector of the Southern Ocean. Our results show that the atmospheric water cycle has intensified in this region between 1993 and 2021, increasing the salinity in subtropical surface waters by 0.06 ± 0.07 g kg−1 per decade, and decreasing the salinity in subpolar surface waters by -0.02 ± 0.01 g kg−1 per decade. The oxygen isotope data allow to discriminate the different freshwater processes showing that in the subpolar region, the freshening is largely driven by the increase in net precipitation (by a factor two) while the decrease in sea ice melt is largely balanced by the contribution of glacial meltwater at these latitudes. These changes extend the growing evidence for an acceleration of the hydrological cycle and a melting cryosphere that can be expected from global warming.
  •  
3.
  • Barrientos, Natalia, et al. (author)
  • Arctic Ocean benthic foraminifera Mg/Ca ratios and global Mg/Ca-temperature calibrations: New constraints at low temperatures
  • 2018
  • In: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037 .- 1872-9533. ; 236, s. 240-259
  • Journal article (peer-reviewed)abstract
    • We explore the use of Mg/Ca ratios in six Arctic Ocean benthic foraminifera species as bottom water palaeothermometers and expand published Mg/Ca-temperature calibrations to the coldest bottom temperatures (<1 °C). Foraminifera were analyzed in surface sediments at 27 sites in the Chukchi Sea, East Siberian Sea, Laptev Sea, Lomonosov Ridge and Petermann Fjord. The sites span water depths of 52–1157 m and bottom water temperatures (BWT) of −1.8 to +0.9 °C. Benthic foraminifera were alive at time of collection, determined from Rose Bengal (RB) staining. Three infaunal and three epifaunal species were abundant enough for Mg/Ca analysis. As predicted by theory and empirical evidence, cold water Arctic Ocean benthic species produce low Mg/Ca ratios, the exception being the porcelaneous species Quinqueloculina arctica. Our new data provide important constraints at the cold end (<1 °C) when added to existing global datasets. The refined calibrations based on the new and published global data appear best supported for the infaunal species Nonionella labradorica (Mg/Ca = 1.325 ± 0.01 × e^(0.065 ± 0.01 × BWT), r2 = 0.9), Cassidulina neoteretis (Mg/Ca = 1.009 ± 0.02 × e^(0.042 ± 0.01 × BWT), r2 = 0.6) and Elphidium clavatum (Mg/Ca = 0.816 ± 0.06 + 0.125 ± 0.05 × BWT, r2 = 0.4). The latter is based on the new Arctic data only. This suggests that Arctic Ocean infaunal taxa are suitable for capturing at least relative and probably semi-quantitative past changes in BWT. Arctic Oridorsalis tener Mg/Ca data are combined with existing O. umbonatus Mg/Ca data from well saturated core-tops from other regions to produce a temperature calibration with minimal influence of bottom water carbonate saturation state (Mg/Ca = 1.317 ± 0.03 × e^(0.102 ± 0.01 BWT), r2 = 0.7). The same approach for Cibicidoides wuellerstorfi yields Mg/Ca = 1.043 ± 0.03 × e^(0.118 ± 0.1 BWT), r2 = 0.4. Mg/Ca ratios of the porcelaneous epifaunal species Q. arctica show a clear positive relationship between Mg/Ca and Δ[CO32−] indicating that this species is not suitable for Mg/Ca-palaeothermometry at low temperatures, but may be useful in reconstructing carbonate system parameters through time.
  •  
4.
  •  
5.
  • Björk, Göran, 1956, et al. (author)
  • Bathymetry and oceanic flow structure at two deep passages crossing the Lomonosov Ridge
  • 2018
  • In: Ocean Science. - : Copernicus GmbH. - 1812-0784 .- 1812-0792. ; 14:1, s. 1-13
  • Journal article (peer-reviewed)abstract
    • The Lomonosov Ridge represents a major topographical feature in the Arctic Ocean which has a large effect on the water circulation and the distribution of water properties. This study presents detailed bathymetric survey data along with hydrographic data at two deep passages across the ridge: a southern passage (80-81°ĝN), where the ridge crest meets the Siberian continental slope, and a northern passage around 84.5°ĝN. The southern channel is characterized by smooth and flat bathymetry around 1600–1700ĝm with a sill depth slightly shallower than 1700ĝm. A hydrographic section across the channel reveals an eastward flow with Amundsen Basin properties in the southern part and a westward flow of Makarov Basin properties in the northern part. The northern passage includes an approximately 72ĝkm long and 33ĝkm wide trough which forms an intra-basin in the Lomonosov Ridge morphology (the Oden Trough). The eastern side of the Oden Trough is enclosed by a narrow and steep ridge rising 500–600ĝm above a generally 1600ĝm deep trough bottom. The deepest passage (the sill) is 1470ĝm deep and located on this ridge. Hydrographic data show irregular temperature and salinity profiles indicating that water exchange occurs as midwater intrusions bringing water properties from each side of the ridge in well-defined but irregular layers. There is also morphological evidence that some rather energetic flows may occur in the vicinity of the sill. A well expressed deepening near the sill may be the result of seabed erosion by bottom currents.
  •  
6.
  • Björk, Göran, 1956, et al. (author)
  • On the Arctic Ocean ice thickness response to changes in the external forcing
  • 2012
  • In: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 39:12, s. 3007-3018
  • Journal article (peer-reviewed)abstract
    • Submarine and satellite observations show that the Arctic Ocean ice cover has undergone a large thickness reduction and a decrease in the areal extent during the last decades. Here the response of the Arctic Ocean ice cover to changes in the poleward atmospheric energy transport, F wall, is investigated using coupled atmosphere-ice-ocean column models. Two models with highly different complexity are used in order to illustrate the importance of different internal processes and the results highlight the dramatic effects of the negative ice thickness—ice volume export feedback and the positive surface albedo feedback. The steady state ice thickness as a function of F wall is determined for various model setups and defines what we call ice thickness response curves. When a variable surface albedo and snow precipitation is included, a complex response curve appears with two distinct regimes: a perennial ice cover regime with a fairly linear response and a less responsive seasonal ice cover regime. The two regimes are separated by a steep transition associated with surface albedo feedback. The associated hysteresis is however small, indicating that the Arctic climate system does not have an irreversible tipping point behaviour related to the surface albedo feedback. The results are discussed in the context of the recent reduction of the Arctic sea ice cover. A new mechanism related to regional and temporal variations of the ice divergence within the Arctic Ocean is presented as an explanation for the observed regional variation of the ice thickness reduction. Our results further suggest that the recent reduction in areal ice extent and loss of multiyear ice is related to the albedo dependent transition between seasonal and perennial ice i.e. large areas of the Arctic Ocean that has previously been dominated by multiyear ice might have been pushed below a critical mean ice thickness, corresponding to the above mentioned transition, and into a state dominated by seasonal ice.
  •  
7.
  • Björk, Göran, 1956, et al. (author)
  • The Sensitivity of the Arctic Ocean Sea Ice Thickness and Its Dependence on the Surface Albedo Parameterization
  • 2013
  • In: Journal of Climate. - : American Meteorological Society. - 0894-8755 .- 1520-0442. ; 26:4, s. 1355-1370
  • Journal article (peer-reviewed)abstract
    • In this study, the response of sea ice thickness to changes in the external forcing is investigated and particularly how this response depends on the surface albedo formulation by means of a one-dimensional coupled ocean-ice-atmosphere model. The main focus is on the thickness response to the atmospheric heat advection F-wall, solar radiation F-SW, and amount of snow precipitation S-prec. Different albedo parameterization schemes [ECHAM5, CSIRO, and Community Climate System Model, version 3 (CCSM3)] representing albedos commonly used in global climate models are compared together with more simplified schemes. Using different albedo schemes with the same external forcing produces large differences in ice thickness. The ice thickness response is similar for all realistic albedo schemes with a nearly linear decrease with increasing F-wall in the perennial ice regime and with a steplike transition into seasonal ice when F-wall exceeds a certain threshold. This transition occurs at an annual-mean ice thickness of 1.7-2.0 m. Latitudinal differences in solar insolation generally leads to increasing ice thickness toward the North Pole. The snow response varies significantly depending on which albedo scheme is used. The ECHAM5 scheme yields thinner ice with S-prec, the CSIRO scheme gives ice thickness nearly independent of S-prec, and with the CCSM3 scheme the ice thickness decreases with S-prec. A general result is that the modeled ice cover is rather sensitive to positive perturbations of the external heat supply when it is close to the transition such that just a small increase of, for example, F-wall can force the ice cover into the seasonal regime.
  •  
8.
  • Bradshaw, Clare, et al. (author)
  • Physical Disturbance by Bottom Trawling Suspends Particulate Matter and Alters Biogeochemical Processes on and Near the Seafloor
  • 2021
  • In: Frontiers in Marine Science. - : Frontiers Media SA. - 2296-7745. ; 8
  • Journal article (peer-reviewed)abstract
    • Bottom trawling is known to affect benthic faunal communities but its effects on sediment suspension and seabed biogeochemistry are less well described. In addition, few studies have been carried out in the Baltic Sea, despite decades of trawling in this unique brackish environment and the frequent occurrence of trawling in areas where hypoxia and low and variable salinity already act as ecosystem stressors. We measured the physical and biogeochemical impacts of an otter trawl on a muddy Baltic seabed. Multibeam bathymetry revealed a 36 m-wide trawl track, comprising parallel furrows and sediment piles caused by the trawl doors and shallower grooves from the groundgear, that displaced 1,000 m3 (500 t) sediment and suspended 9.5 t sediment per km of track. The trawl doors had less effect than the rest of the gear in terms of total sediment mass but per m2 the doors had 5× the displacement and 2× the suspension effect, due to their greater penetration and hydrodynamic drag. The suspended sediment spread >1 km away over the following 3–4 days, creating a 5–10 m thick layer of turbid bottom water. Turbidity reached 4.3 NTU (7 mgDW L–1), 550 m from the track, 20 h post-trawling. Particulate Al, Ti, Fe, P, and Mn were correlated with the spatio-temporal pattern of suspension. There was a pulse of dissolved N, P, and Mn to a height of 10 m above the seabed within a few hundred meters of the track, 2 h post-trawling. Dissolved methane concentrations were elevated in the water for at least 20 h. Sediment biogeochemistry in the door track was still perturbed after 48 h, with a decreased oxygen penetration depth and nutrient and oxygen fluxes across the sediment-water interface. These results clearly show the physical effects of bottom trawling, both on seabed topography (on the scale of km and years) and on sediment and particle suspension (on the scale of km and days-weeks). Alterations to biogeochemical processes suggest that, where bottom trawling is frequent, sediment biogeochemistry may not have time to recover between disturbance events and elevated turbidity may persist, even outside the trawled area.
  •  
9.
  • Broman, Elias, et al. (author)
  • Low Abundance of Methanotrophs in Sediments of Shallow Boreal Coastal Zones With High Water Methane Concentrations
  • 2020
  • In: Frontiers in Microbiology. - : Frontiers Media SA. - 1664-302X. ; 11
  • Journal article (peer-reviewed)abstract
    • Coastal zones are transitional areas between land and sea where large amounts of organic and inorganic carbon compounds are recycled by microbes. Especially shallow zones near land have been shown to be the main source for oceanic methane (CH4) emissions. Water depth has been predicted as the best explanatory variable, which is related to CH4 ebullition, but exactly how sediment methanotrophs mediates these emissions along water depth is unknown. Here, we investigated the relative abundance and RNA transcripts attributed to methane oxidation proteins of aerobic methanotrophs in the sediment of shallow coastal zones with high CH4 concentrations within a depth gradient from 10–45 m. Field sampling consisted of collecting sediment (top 0–2 cm layer) from eight stations along this depth gradient in the coastal Baltic Sea. The relative abundance and RNA transcripts attributed to the CH4 oxidizing protein (pMMO; particulate methane monooxygenase) of the dominant methanotroph Methylococcales was significantly higher in deeper costal offshore areas (36–45 m water depth) compared to adjacent shallow zones (10–28 m). This was in accordance with the shallow zones having higher CH4 concentrations in the surface water, as well as more CH4 seeps from the sediment. Furthermore, our findings indicate that the low prevalence of Methylococcales and RNA transcripts attributed to pMMO was restrained to the euphotic zone (indicated by Photosynthetically active radiation (PAR) data, photosynthesis proteins, and 18S rRNA data of benthic diatoms). This was also indicated by a positive relationship between water depth and the relative abundance of Methylococcales and pMMO. How these processes are affected by light availability requires further studies. CH4 ebullition potentially bypasses aerobic methanotrophs in shallow coastal areas, reducing CH4 availability and limiting their growth. Such mechanism could help explain their reduced relative abundance and related RNA transcripts for pMMO. These findings can partly explain the difference in CH4 concentrations between shallow and deep coastal areas, and the relationship between CH4 concentrations and water depth.
  •  
10.
  • Detlef, Henrieka, et al. (author)
  • Seasonal sea-ice in the Arctic's last ice area during the Early Holocene
  • 2023
  • In: Communications Earth & Environment. - : Springer Science and Business Media LLC. - 2662-4435. ; 4:1
  • Journal article (peer-reviewed)abstract
    • According to climate models, the Lincoln Sea, bordering northern Greenland and Canada, will be the final stronghold of perennial Arctic sea-ice in a warming climate. However, recent observations of prolonged periods of open water raise concerns regarding its long-term stability. Modelling studies suggest a transition from perennial to seasonal sea-ice during the Early Holocene, a period of elevated global temperatures around 10,000 years ago. Here we show marine proxy evidence for the disappearance of perennial sea-ice in the southern Lincoln Sea during the Early Holocene, which suggests a widespread transition to seasonal sea-ice in the Arctic Ocean. Seasonal sea-ice conditions were tightly coupled to regional atmospheric temperatures. In light of anthropogenic warming and Arctic amplification our results suggest an imminent transition to seasonal sea-ice in the southern Lincoln Sea, even if the global temperature rise is kept below a threshold of 2 °C compared to pre-industrial (1850–1900).
  •  
11.
  • Geoffroy, Gaspard, 1991- (author)
  • Spiritum sidus : The star of our life – On internal tides in the ocean
  • 2024
  • Doctoral thesis (other academic/artistic)abstract
    • Tidal currents flowing over rough bathymetry generate internal tides. These internal waves with tidal frequency can be decomposed into vertical modes. Low modes generally travel thousands of kilometers, until they break due to shear flow instabilities, while high modes are believed to break close to the generation site. The power released by breaking internal tides is thought not only to shape the overturning circulation, but also to mix the upper ocean. Both processes have a large influence on the climate system, most notably for their key role in regulating the heat and carbon uptake by the ocean.The generation of internal tides, or tidal conversion, can be calculated from the bottom topography, the ocean stratification and the tidal currents. Global computations of the tidal conversion have been based on linear wave theory. However, such linear calculations are only valid if the seafloor slope is subcritical, and it is not known how to treat supercritical slopes. This is especially true for the conversion decomposed into vertical modes, which, taken individually, behaves very differently from the total conversion (the sum of the contributions from all modes).In the first paper of this thesis, we looked into the validity of linear theory in the supercritical limit. Specifically, we translated the critical slope condition, a notion defined for the superposition of all modes, into a mode-wise condition on the topographic height. The findings were applied to estimates of the global M2-tide conversion into the first 10 vertical modes (in the open ocean, excluding the continental shelves and slopes). The results unveil the rapid increase with mode number of the oceanic area where linear theory fails. In terms of conversion, this shows that linear theory is unadapted to quantify the role played by high modes in closing the internal wave energy budget.Typically, continental slopes are supercritical, and hence locations where the linear theory fails. Because of their characteristic shape, they are also an important source of low-mode internal tides. In the second paper of this thesis, we constructed a computationally inexpensive method to compute the tidal conversion by continental slopes and applied it at the global scale. It uses the usual observational data as inputs but relies on a reduced-physics numerical model rather than on linear theory to estimate the tidal conversion.Unveiling the global pattern of the dissipation of internal tides (i.e. where they break) has been a challenging objective for a few decades. This can be explained by the lack of suitable observations to compare theory with. Until recently, the only observational data of internal tides with global coverage were based on satellite altimetry. However, only the part of the wave field that is exactly phase-locked to the astronomical forcing can be identified from altimetry data.In the third paper of this thesis we created a new observational data set of internal tides, with global coverage, based on Argo park-phase data. These data are recorded while the floats are adrift at 1000 m depth, between two vertical profiling sequences. Thanks to the high sampling rate of Argo floats, the records capture the full amplitude of the waves, including the non-phase-locked part. This component turned out to be several times larger than previously thought. In the fourth paper of this thesis, we validated the internal tides in a realistic global ocean simulation with Argo data. Incidentally, this also worked to validate the Argo observations.
  •  
12.
  • Gustafsson, Erik, et al. (author)
  • Carbon cycling on the East Siberian Arctic Shelf – a change in air-sea CO2 flux induced by mineralization of terrestrial organic carbon
  • 2017
  • In: Biogeosciences. - : Copernicus GmbH. - 1726-4170 .- 1726-4189.
  • Journal article (peer-reviewed)abstract
    • Measurements from the SWERUS-C3 and ISSS-08 Arctic expeditions were used to calibrate and validate a new physical-biogeochemical model developed to quantify key carbon cycling processes on the East Siberian Arctic Shelf (ESAS). The model was used in a series of experimental simulations with the specific aim to investigate the pathways of terrestrial dissolved and particulate organic carbon (DOCter and POCter) supplied to the shelf. Rivers supply on average 8.5 Tg C yr−1 dissolved inorganic carbon (DIC), and further 8.5 and 1.1 Tg C yr−1 DOCter and POCter respectively. Based on observed and simulated DOC concentrations and stable isotope values (δ13CDOC) in shelf waters, we estimate that only some 20 % of the riverine DOCter is labile. According to our model results, an additional supply of approximately 14 Tg C yr−1 eroded labile POCter is however required to describe the observed stable isotope values of DIC (δ13CDIC). Degradation of riverine DOCter and POCter results in a 1.8 Tg C yr−1 reduction in the uptake of atmospheric CO2, while degradation of eroded POCter results in an additional 10 Tg C yr−1 reduction. Our calculations indicate nevertheless that the ESAS is an overall small net sink for atmospheric CO2 (1.7 Tg C yr−1). The external carbon sources are largely compensated by a net export from the shelf to the Arctic Ocean (31 Tg C yr−1), and to a smaller degree by a permanent burial in the sediments (2.7 Tg C yr−1).
  •  
13.
  • Hogan, Kelly A., et al. (author)
  • Glacial sedimentation, fluxes and erosion rates associated with ice retreat in Petermann Fjord and Nares Strait, north-west Greenland
  • 2020
  • In: The Cryosphere. - : Copernicus GmbH. - 1994-0416 .- 1994-0424. ; 14:1, s. 261-286
  • Journal article (peer-reviewed)abstract
    • Petermann Fjord is a deep ( > 1000 m) fjord that incises the coastline of north-west Greenland and was carved by an expanded Petermann Glacier, one of the six largest outlet glaciers draining the modern Greenland Ice Sheet (GrIS). Between 5 and 70 m of unconsolidated glacigenic material infills in the fjord and adjacent Nares Strait, deposited as the Petermann and Nares Strait ice streams retreated through the area after the Last Glacial Maximum. We have investigated the deglacial deposits using seismic stratigraphic techniques and have correlated our results with high-resolution bathymetric data and core lithofacies. We identify six seismoacoustic facies in more than 3500 line kilometres of subbottom and seismic-reflection profiles throughout the fjord, Hall Basin and Kennedy Channel. Seismo-acoustic facies relate to bedrock or till surfaces (Facies I), subglacial deposition (Facies II), deposition from meltwater plumes and icebergs in quiescent glacimarine conditions (Facies III, IV), deposition at grounded ice margins during stillstands in retreat (grounding-zone wedges; Facies V) and the redeposition of material downslope (Facies IV). These sediment units represent the total volume of glacial sediment delivered to the mapped marine environment during retreat. We calculate a glacial sediment flux for the former Petermann ice stream as 1080-1420 m(3) a(-1) per metre of ice stream width and an average deglacial erosion rate for the basin of 0.29-0.34 mm a(-1). Our deglacial erosion rates are consistent with results from Antarctic Peninsula fjord systems but are several times lower than values for other modern GrIS catchments. This difference is attributed to fact that large volumes of surface water do not access the bed in the Petermann system, and we conclude that glacial erosion is limited to areas overridden by streaming ice in this large outlet glacier setting. Erosion rates are also presented for two phases of ice retreat and confirm that there is significant variation in rates over a glacial-deglacial transition. Our new glacial sediment fluxes and erosion rates show that the Petermann ice stream was approximately as efficient as the palaeo-Jakobshavn Isbra at eroding, transporting and delivering sediment to its margin during early deglaciation.
  •  
14.
  • Holmes, Felicity A., et al. (author)
  • Calving at Ryder Glacier, Northern Greenland
  • 2021
  • In: Journal of Geophysical Research - Earth Surface. - 2169-9003 .- 2169-9011. ; 126:4
  • Journal article (peer-reviewed)abstract
    • Recent evidence has shown increasing mass loss from the Greenland ice sheet, with a general trend of accelerated mass losses extending northwards. However, different glaciers have been shown to respond differently to similar external forcings, constituting a problem for extrapolating and upscaling data. Specifically, whilst some outlet glaciers have accelerated, thinned, and retreated in response to atmospheric and oceanic warming, the behavior of other marine terminating glaciers appears to be less sensitive to climate forcing. Ryder glacier, for which only a few studies have been conducted, is located in North Greenland and terminates with a floating ice tongue in Sherard Osborn Fjord. The persistence or disintegration of floating ice tongues has impacts on glacier dynamics and stability, with ramifications beyond, including sea level rise. This study focuses on understanding the controls on calving and frontal ablation of the Ryder glacier through the use of time-lapse imagery and satellite data. The results suggest that Ryder glacier has behaved independently of climate forcing during recent decades, with fjord geometry exerting a first order control on its calving.
  •  
15.
  • Humborg, Christoph, et al. (author)
  • High Emissions of Carbon Dioxide and Methane From the Coastal Baltic Sea at the End of a Summer Heat Wave
  • 2019
  • In: Frontiers in Marine Science. - : Frontiers Media SA. - 2296-7745. ; 6
  • Journal article (peer-reviewed)abstract
    • The summer heat wave in 2018 led to the highest recorded water temperatures since 1926 - up to 21 degrees C - in bottom coastal waters of the Baltic Sea, with implications for the respiration patterns in these shallow coastal systems. We applied cavity ring-down spectrometer measurements to continuously monitor carbon dioxide (CO2) and methane (CH4) surface-water concentrations, covering the coastal archipelagos of Sweden and Finland and the open and deeper parts of the Northern Baltic Proper. This allowed us to (i) follow an upwelling event near the Swedish coast leading to elevated CO2 and moderate CH 4 outgassing, and (ii) to estimate CH4 sources and fluxes along the coast by investigating water column inventories and air-sea fluxes during a storm and an associated downwelling event. At the end of the heat wave, before the storm event, we found elevated CO2 (1583 mu atm) and CH4 (70 nmol/L) concentrations. During the storm, a massive CO2 sea-air flux of up to 274 mmol m(-2) d(-1) was observed. While water-column CO2 concentrations were depleted during several hours of the storm, CH4 concentrations remained elevated. Overall, we found a positive relationship between CO2 and CH4 wind-driven sea-air fluxes, however, the highest CH4 fluxes were observed at low winds whereas highest CO2 fluxes were during peak winds, suggesting different sources and processes controlling their fluxes besides wind. We applied a box-model approach to estimate the CH4 supply needed to sustain these elevated CH4 concentrations and the results suggest a large source flux of CH4 to the water column of 2.5 mmol m(-2) d(-1). These results are qualitatively supported by acoustic observations of vigorous and widespread outgassing from the sediments, with flares that could be traced throughout the water column penetrating the pycnocline and reaching the sea surface. The results suggest that the heat wave triggered CO2 and CH4 fluxes in the coastal zones that are comparable with maximum emission rates found in other hot spots, such as boreal and arctic lakes and wetlands. Further, the results suggest that heat waves are as important for CO2 and CH4 sea-air fluxes as the ice break up in spring.
  •  
16.
  • Jakobsson, Martin, et al. (author)
  • Bathymetric properties of the Baltic Sea
  • 2019
  • In: Ocean Science. - : Copernicus GmbH. - 1812-0784 .- 1812-0792. ; 15:4, s. 905-924
  • Journal article (peer-reviewed)abstract
    • Baltic Sea bathymetric properties are analysed here using the newly released digital bathymetric model (DBM) by the European Marine Observation and Data Network (EMODnet). The analyses include hypsometry, volume, descriptive depth statistics, and kilometre-scale seafloor ruggedness, i.e. terrain heterogeneity, for the Baltic Sea as a whole as well as for 17 sub-basins defined by the Baltic Marine Environment Protection Commission (HELCOM). We compare the new EMODnet DBM with IOWTOPO the previously most widely used DBM of the Baltic Se aproduced by the Leibniz-Institut fur Ostseeforschung Warnemiinde (IOW), which has served as the primary gridded bathymetric resource in physical and environmental studies for nearly two decades. The area of deep water exchange between the Bothnian Sea and the Northern Baltic Proper across the Aland Sea is specifically analysed in terms of depths and locations of critical bathymetric sills. The EMODnet DBM provides a bathymetric sill depth of 88 m at the northern side of the Aland Sea and 60 m at the southern side, differing from previously identified sill depths of 100 and 70 m, respectively. High-resolution multibeam bathymetry acquired from this deep water exchange path, where vigorous bottom currents interacted with the seafloor, allows us to assess what presently available DBMs are missing in terms of physical characterization of the seafloor. Our study highlights the need for continued work towards complete high-resolution mapping of the Baltic Sea seafloor.
  •  
17.
  •  
18.
  • Jakobsson, Martin, et al. (author)
  • Evidence for an ice shelf covering the central Arctic Ocean during the penultimate glaciation
  • 2016
  • In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 7
  • Journal article (peer-reviewed)abstract
    • The hypothesis of a km-thick ice shelf covering the entire Arctic Ocean during peak glacial conditions was proposed nearly half a century ago. Floating ice shelves preserve few direct traces after their disappearance, making reconstructions difficult. Seafloor imprints of ice shelves should, however, exist where ice grounded along their flow paths. Here we present new evidence of ice-shelf groundings on bathymetric highs in the central Arctic Ocean, resurrecting the concept of an ice shelf extending over the entire central Arctic Ocean during at least one previous ice age. New and previously mapped glacial landforms together reveal flow of a spatially coherent, in some regions41-km thick, central Arctic Ocean ice shelf dated to marine isotope stage 6 (similar to 140 ka). Bathymetric highs were likely critical in the ice-shelf development by acting as pinning points where stabilizing ice rises formed, thereby providing sufficient back stress to allow ice shelf thickening.
  •  
19.
  • Jakobsson, Martin, et al. (author)
  • Post-glacial flooding of the Bering Land Bridge dated to 11 cal ka BP based on new geophysical and sediment records
  • 2017
  • In: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 13:8, s. 991-1005
  • Journal article (peer-reviewed)abstract
    • The Bering Strait connects the Arctic and Pacific oceans and separates the North American and Asian landmasses. The presently shallow (similar to 53 m) strait was exposed during the sea level lowstand of the last glacial period, which permitted human migration across a land bridge today referred to as the Bering Land Bridge. Proxy studies (stable isotope composition of foraminifera, whale migration into the Arctic Ocean, mollusc and insect fossils and paleobotanical data) have suggested a range of ages for the Bering Strait reopening, mainly falling within the Younger Dryas stadial (12.9-11.7 cal ka BP). Here we provide new information on the deglacial and post-glacial evolution of the Arctic-Pacific connection through the Bering Strait based on analyses of geological and geophysical data from Herald Canyon, located north of the Bering Strait on the Chukchi Sea shelf region in the western Arctic Ocean. Our results suggest an initial opening at about 11 cal ka BP in the earliest Holocene, which is later than in several previous studies. Our key evidence is based on a well-dated core from Herald Canyon, in which a shift from a near-shore environment to a Pacific-influenced open marine setting at around 11 cal ka BP is observed. The shift corresponds to meltwater pulse 1b (MWP1b) and is interpreted to signify relatively rapid breaching of the Bering Strait and the submergence of the large Bering Land Bridge. Although the precise rates of sea level rise cannot be quantified, our new results suggest that the late deglacial sea level rise was rapid and occurred after the end of the Younger Dryas stadial.
  •  
20.
  • Jakobsson, Martin, et al. (author)
  • Potential links between Baltic Sea submarine terraces and groundwater seeping
  • 2020
  • In: Earth Surface Dynamics. - : Copernicus GmbH. - 2196-6311 .- 2196-632X. ; 8:1, s. 1-15
  • Journal article (peer-reviewed)abstract
    • Submarine groundwater discharge (SGD) influences ocean chemistry, circulation, and the spreading of nutrients and pollutants; it also shapes sea floor morphology. In the Baltic Sea, SGD was linked to the development of terraces and semicircular depressions mapped in an area of the southern Stockholm archipelago, Sweden, in the 1990s. We mapped additional parts of the Stockholm archipelago, areas in Blekinge, southern Sweden, and southern Finland using high-resolution multibeam sonars and sub-bottom profilers to investigate if the sea floor morphological features discovered in the 1990s are widespread and to further address the hypothesis linking their formation to SGD. Sediment coring and sea floor photography conducted with a remotely operated vehicle (ROV) and divers add additional information to the geophysical mapping results. We find that terraces, with general bathymetric expressions of about 1 m and lateral extents of sometimes > 100 m, are widespread in the surveyed areas of the Baltic Sea and are consistently formed in glacial clay. Semicircular depressions, however, are only found in a limited part of a surveyed area east of the island of Asko, southern Stockholm archipelago. While submarine terraces can be produced by several processes, we interpret our results to be in support of the basic hypothesis of terrace formation initially proposed in the 1990s; i.e. groundwater flows through siltier, more permeable layers in glacial clay to discharge at the sea floor, leading to the formation of a sharp terrace when the clay layers above seepage zones are undermined enough to collapse. By linking the terraces to a specific geologic setting, our study further refines the formation hypothesis and thereby forms the foundation for a future assessment of SGD in the Baltic Sea that may use marine geological mapping as a starting point. We propose that SGD through the submarine sea floor terraces is plausible and could be intermittent and linked to periods of higher groundwater levels, implying that to quantify the contribution of freshwater to the Baltic Sea through this potential mechanism, more complex hydrogeological studies are required.
  •  
21.
  • Jakobsson, Martin, et al. (author)
  • Ryder Glacier in northwest Greenland is shielded from warm Atlantic water by a bathymetric sill
  • 2020
  • In: Communications Earth & Environment. - : Springer Science and Business Media LLC. - 2662-4435. ; 1
  • Journal article (peer-reviewed)abstract
    • The processes controlling advance and retreat of outlet glaciers in fjords draining the Greenland Ice Sheet remain poorly known, undermining assessments of their dynamics and associated sea-level rise in a warming climate. Mass loss of the Greenland Ice Sheet has increased six-fold over the last four decades, with discharge and melt from outlet glaciers comprising key components of this loss. Here we acquired oceanographic data and multibeam bathymetry in the previously uncharted Sherard Osborn Fjord in northwest Greenland where Ryder Glacier drains into the Arctic Ocean. Our data show that warmer subsurface water of Atlantic origin enters the fjord, but Ryder Glacier’s floating tongue at its present location is partly protected from the inflow by a bathymetric sill located in the innermost fjord. This reduces under-ice melting of the glacier, providing insight into Ryder Glacier’s dynamics and its vulnerability to inflow of Atlantic warmer water.
  •  
22.
  • Jakobsson, Martin, et al. (author)
  • The Holocene retreat dynamics and stability of Petermann Glacier in northwest Greenland
  • 2018
  • In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 9
  • Journal article (peer-reviewed)abstract
    • Submarine glacial landforms in fjords are imprints of the dynamic behaviour of marine-terminating glaciers and are informative about their most recent retreat phase. Here we use detailed multibeam bathymetry to map glacial landforms in Petermann Fjord and Nares Strait, northwestern Greenland. A large grounding-zone wedge (GZW) demonstrates that Petermann Glacier stabilised at the fjord mouth for a considerable time, likely buttressed by an ice shelf. This stability was followed by successive backstepping of the ice margin down the GZW's retrograde backslope forming small retreat ridges to 680 m current depth (similar to 730-800 m palaeodepth). Iceberg ploughmarks occurring somewhat deeper show that thick, grounded ice persisted to these water depths before final breakup occurred. The palaeodepth limit of the recessional moraines is consistent with final collapse driven by marine ice cliff instability (MICI) with retreat to the next stable position located underneath the present Petermann ice tongue, where the seafloor is unmapped.
  •  
23.
  • Ketzer, João Marcelo, et al. (author)
  • Discovery of a major seafloor methane release site in Europe : The Landsort deep, Baltic Sea.
  • 2024
  • In: EGU General Assembly 2024. - : European Geosciences Union (EGU).
  • Conference paper (peer-reviewed)abstract
    • A recently acquired multidisciplinary dataset comprising acoustic surveys (high-resolution sub- bottom profiles, multi-beam bathymetry, and broad band mid-water echo sounder), geochemistry (gas chemical and isotopic composition, porewater chemistry), and sedimentology (core lithology and X-ray CT) in the area of the Landsort deep (450 m of depth), south of Stockholm Archipelago, revealed the existence of an extensive (20 km2) region of the seafloor where massive gas release is occurring in the form of multiple bubble streams. This new discovery represents a major seafloor methane release site in Europe and is comparable in area to other large sites worldwide such as the ones in Svalbard and in the South Atlantic Ocean associated with gas hydrate provinces. The gas is formed mostly by methane of microbial origin. Surprisingly, bubbles rise 100’s of meters above the seafloor and reach surface waters above the halocline/oxycline at around 80 m of depth. Some bubbles appear to reach the sea-air interface and their potential methane contribution to the atmosphere is under investigation. Another surprising observation is the absence of major seafloor features like pockmarks in the gas release area. The reasons for the seafloor methane release in the Landsort deep are still not entirely clear, but our preliminary acoustic and sedimentological data suggest that bottom currents may have acted to facilitate the accumulation of organic-rich sediments in a thick drift deposit during the Holocene and the modern warm period (latest 100 years). Our data further suggest that the high sedimentation rate in the drift deposit continuously supplies fresh organic matter that is quickly buried below a thin sulphate reduction zone, fueling vigorous methanogenesis and abundant methane formation. Similar methane release sites might be discovered in other known large drift deposits in the Baltic Sea. 
  •  
24.
  • Ketzer, João Marcelo, et al. (author)
  • Near seafloor methane flux in the world's largest human-induced dead zone is regulated by sediment accumulation rate
  • 2024
  • In: Marine Geology. - : Elsevier. - 0025-3227 .- 1872-6151. ; 468
  • Journal article (peer-reviewed)abstract
    • The vast oxygen-depleted area of the central Baltic Sea is the largest human-induced dead zone in the world with 70,000 km(2) or approximately three times the second largest one in the Gulf of Mexico. Methane occurs in high concentrations in bottom waters (3200 nM) and sediments (30 mM), and its dynamics is better constrained for the water column, but still poorly understood on sediments. Here we show that sediment accumulation rate plays a major role in regulating the quantity of organic matter and its residence time in the sulphate reduction and methanogenesis zones and, therefore, affects methane generation, consumption, and diffusive flux in sediments near the seafloor (< 1 m). High fluxes found in high sediment accumulation rate areas and competition for substrate (organoclastic sulphate reduction vs. anaerobic oxidation of methane with sulphate), compromise the ability of the thin microbial filter to consume and prevent methane diffusion through the seafloor.
  •  
25.
  • Miller, Clint M., et al. (author)
  • Pore water geochemistry along continental slopes north of the East Siberian Sea : inference of low methane concentrations
  • 2017
  • In: Biogeosciences. - : Copernicus GmbH. - 1726-4170 .- 1726-4189. ; 14:12, s. 2929-2953
  • Journal article (peer-reviewed)abstract
    • Continental slopes north of the East Siberian Sea potentially hold large amounts of methane (CH4/in sediments as gas hydrate and free gas. Although release of this CH4 to the ocean and atmosphere has become a topic of discussion, the region remains sparingly explored. Here we present pore water chemistry results from 32 sediment cores taken during Leg 2 of the 2014 joint Swedish-Russian-US Arctic Ocean Investigation of Climate-Cryosphere-Carbon Interactions (SWERUS-C3) expedition. The cores come from depth transects across the slope and rise extending between the Mendeleev and the Lomonosov ridges, north of Wrangel Island and the New Siberian Islands, respectively. Upward CH4 flux towards the seafloor, as inferred from profiles of dissolved sulfate (SO42-), alkalinity, and the delta C-13 of dissolved inorganic carbon (DIC), is negligible at all stations east of 143 degrees E longitude. In the upper 8m of these cores, downward SO42- flux never exceeds 6.2 mol m(-2) kyr(-1), the upward alkalinity flux never exceeds 6.8 mol m(-2) kyr(-1), and delta C-13 composition of DIC (delta C-13-DIC) only moderately decreases with depth (3.6% m 1 on average). Moreover, upon addition of Zn acetate to pore water samples, ZnS did not precipitate, indicating a lack of dissolved H2S. Phosphate, ammonium, and metal profiles reveal that metal oxide reduction by organic carbon dominates the geochemical environment and supports very low organic carbon turnover rates. A single core on the Lomonosov Ridge differs, as diffusive fluxes for SO42- and alkalinity were 13.9 and 11.3 mol m(-2) kyr(-1), respectively, the delta C-13-DIC gradient was 5.6% m(-1), and Mn2+ reduction terminated within 1.3 m of the seafloor. These are among the first pore water results generated from this vast climatically sensitive region, and they imply that abundant CH4, including gas hydrates, do not characterize the East Siberian Sea slope or rise along the investigated depth transects. This contradicts previous modeling and discussions, which due to the lack of data are almost entirely based on assumption.
  •  
26.
  • Muchowski, Julia C., 1989- (author)
  • Uncovering Ocean Mixing near Rough Bathymetry : Using Broadband Acoustics
  • 2023
  • Doctoral thesis (other academic/artistic)abstract
    • Ocean mixing related to rough bathymetry is highly dynamic and exhibits large spatial and temporal variability. Therefore, established in-situ methods as well as numerical models often lack the resolution to capture this type of mixing. In this thesis, acoustics are used to observe, map and quantify stratified mixing at unprecedented resolution. Acoustic broadband data from a Simrad EK80 and co-located microstructure data from a Sea & Sun Technology MicroStructure profiler (MSS) were collected during two cruises on R/V Electra in Feb-March 2019 and 2020 in the Southern Quark region in the northern Åland Sea, Baltic Sea. In the first manuscript (M1), an existing acoustic model to quantify turbulent mixing from acoustic backscatter is revisited and applied to the co-located data sets. Possibilities and limitations of applying the acoustic model are investigated in detail. In a following case study (M2), the acoustic model is applied to a subset of the data where rough bathymetry reaches into stratified flow and highly increases mixing across the halocline. The dominant mixing mechanism is revealed to be wake vortices and their impact is estimated using a diffusion model. The final case study (M3) emphasizes the impact of mixing in the Southern Quark on exchange processes between the distinctly different Northern Baltic Proper and the Bothnian Sea. The acoustic observations uncover the detailed structure (M1-M3) and temporal development (M3) of turbulent diapycnal mixing in heterogeneous flow over rough bathymetry. This thesis is a step towards increased applicability and automatized analysis of acoustic broadband data for identifying and quantifying turbulent diapycnal mixing.
  •  
27.
  • Muchowski, Julia, 1989-, et al. (author)
  • Diapycnal Mixing Induced by Rough Small-Scale Bathymetry
  • 2024
  • Other publication (other academic/artistic)abstract
    • Diapycnal mixing in the oceans is crucial for local ecosystems as well as the large-scale circulation because it impacts vertical transport rates of heat, salt, oxygen, and other dissolved substances. We investigate the effect of extremely rough bathymetry on mixing and energy dissipation in a coastal region characterized by small-scale seafloor features penetrating a strongly-stratified density interface. While most studies of this type focus on tidal flow, we present shear microstructure measurements and co-located acoustic observations from the non-tidal Baltic Sea. Acoustic observations indicate temperature and salinity microstructure variance and therefore regions of diapycnal mixing. Due to their high resolution, acoustics enable us to resolve the variability and intermittency of stratified turbulence in the vicinity of the obstacles. Scale analysis and acoustic imaging suggest that the underlying mixing mechanisms are topographic wake eddies and, to a smaller extent, breaking internal waves. Depth averaged dissipation rates (1.1∙10-7W kg-1) and turbulent vertical diffusivities (7∙10-4m2s-1) in the halocline exceed those at a nearby reference station with smooth bathymetry by up to two orders of magnitude. Our study emphasizes the importance of rough small-scale (<1km) bathymetric features for energy dissipation and vertical transport of e.g. salt, heat and oxygen in coastal areas.
  •  
28.
  • Muchowski, Julia, 1989-, et al. (author)
  • Diapycnal Mixing Induced by Rough Small-Scale Bathymetry
  • 2023
  • In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 50:13
  • Journal article (peer-reviewed)abstract
    • Diapycnal mixing impacts vertical transport rates of salt, heat, and other dissolved substances, essential for the overturning circulation and ecosystem functioning in marine systems. While most studies have focused on mixing induced by individual obstacles in tidal flows, we investigate the net effect of non-tidal flow over multiple small-scale (<1 km) bathymetric features penetrating a strongly-stratified density interface in a coastal region. We combine high-resolution broadband acoustic observations of turbulence microstructure with traditional shear microstructure profiling, to resolve the variability and intermittency of stratified turbulence related to the rough bathymetry. Scale analysis and acoustic imaging suggest that underlying mixing mechanisms are related to topographic wake eddies and breaking internal waves. Depth averaged dissipation rates (1.1 × 10−7 Wkg−1) and turbulent vertical diffusivities (7 × 10−4 m2s−1) in the halocline exceed reference values by two orders of magnitude. Our study emphasizes the importance of rough small-scale bathymetric features for the vertical transport of salt in coastal areas.
  •  
29.
  • Muchowski, Julia, et al. (author)
  • Potential and Limitations of a Commercial Broadband Echo Sounder for Remote Observations of Turbulent Mixing
  • 2022
  • In: Journal of Atmospheric and Oceanic Technology. - 0739-0572 .- 1520-0426. ; 39:12, s. 1985-2003
  • Journal article (peer-reviewed)abstract
    • Stratified oceanic turbulence is strongly intermittent in time and space, and therefore generally underresolved by currently available in situ observational approaches. A promising tool to at least partly overcome this constraint are broadband acoustic observations of turbulent microstructure that have the potential to provide mixing parameters at orders of magnitude higher resolution compared to conventional approaches. Here, we discuss the applicability, limitations, and measurement uncertainties of this approach for some prototypical turbulent flows (stratified shear layers, turbulent flow across a sill), based on a comparison of broadband acoustic observations and data from a free-falling turbulence microstructure profiler. We find that broadband acoustics are able to provide a quantitative description of turbulence energy dissipation in stratified shear layers (correlation coefficient r = 0.84) if the stratification parameters required by the method are carefully preprocessed. Essential components of our suggested preprocessing algorithm are 1) a vertical low-pass filtering of temperature and salinity profiles at a scale slightly larger than the Ozmidov length scale of turbulence and 2) an automated elimination of weakly stratified layers according to a gradient threshold criterion. We also show that in weakly stratified conditions, the acoustic approach may yield acceptable results if representative averaged vertical temperature and salinity gradients rather than local gradients are used. Our findings provide a step toward routine turbulence measurements in the upper ocean from moving vessels by combining broadband acoustics with in situ CTD profiles.  
  •  
30.
  • Muchowski, Julia, 1989-, et al. (author)
  • Water Modification in the Åland Sea and Impacts on Exchange Flow between Central and Northern Baltic Sea
  • Other publication (other academic/artistic)abstract
    • We present observational oceanographic data together with openly accessible high-resolution bathymetry from a prototypical sill and an adjacent deep channel in the Southern Quark located in the Åland Sea, connecting the Northern Baltic Proper with the Bothnian Sea. Our data were acquired during two one-week cruises in February-March 2019 and 2020 and include high resolution broadband acoustic observations of turbulent mixing, in situ microstructure profiler measurements and current velocities from Acoustic Doppler Current Profilers. A temporally reversing non-tidal stratified flow is observed over the steep bathymetric sill, creating a dynamic and extremely energetic environment. Saltier, warmer, and less oxygenated deep water south of the sill is blocked, the flow is at times hydraulically controlled with hydraulic jumps occurring on both sides of the sill depending on the flow direction, and sub-mesoscale processes in the surface layer leading to high spatial variability at small scales. Mixing and vertical salt flux rates are increased by 3-4 orders of magnitude in the entire water column in the vicinity of the sill compared to reference stations not directly influenced by the overflow. We suggest that underlying mechanisms causing the highly increased mixing across the halocline are a combination of shear and topographic lee waves breaking at the halocline interface. We discuss the resulting deep-water and surface-water modification in the Southern Quark, which in turn impacts exchange processes between the Bothnian Sea and the Northern Baltic Proper. The here observed mixing is hypothesized to be important for the development of oxygen and nutrient conditions in the Bothnian Sea.
  •  
31.
  • Nilsson, Johan, 1965-, et al. (author)
  • Hydraulic suppression of basal glacier melt in sill fjords
  • 2023
  • In: The Cryosphere. - 1994-0416 .- 1994-0424. ; 17:6, s. 2455-2476
  • Journal article (peer-reviewed)abstract
    • Using a conceptual model, we examine how hydraulically controlled exchange flows in silled fjords affect the relationship between the basal glacier melt and the features of warm intermediate Atlantic Water (AW) outside the fjords. We show that an exchange flow can be forced to transit into the hydraulic regime if the AW interface height decreases, the AW temperature increases, or the production of glacially modified water is boosted by subglacial discharge. In the hydraulic regime, the heat transport across the sill becomes a rate-limiting factor for the basal melt, which is suppressed. An interplay between processes near the ice-ocean boundary and the hydraulically controlled exchange flow determines the melt dynamics, and the sensitivity of the basal melt to changes in the AW temperature is reduced. The model results are discussed in relation to observations from the Petermann, Ryder, and 79∘ N glaciers in northern Greenland.
  •  
32.
  • Nilsson, Johan, 1965-, et al. (author)
  • Ice-shelf damming in the glacial Arctic Ocean: dynamical regimes of a basin-covering kilometre-thick ice shelf
  • 2017
  • In: Cryosphere. - : Copernicus GmbH. - 1994-0416 .- 1994-0424. ; 11:4, s. 1745-1765
  • Journal article (peer-reviewed)abstract
    • Recent geological and geophysical data suggest that a 1 km thick ice shelf extended over the glacial Arctic Ocean during Marine Isotope Stage 6, about 140 000 years ago. Here, we theoretically analyse the development and equilibrium features of such an ice shelf, using scaling analyses and a one-dimensional ice-sheet-ice-shelf model. We find that the dynamically most consistent scenario is an ice shelf with a nearly uniform thickness that covers the entire Arctic Ocean. Further, the ice shelf has two regions with distinctly different dynamics: a vast interior region covering the central Arctic Ocean and an exit region towards the Fram Strait. In the interior region, which is effectively dammed by the Fram Strait constriction, there are strong back stresses and the mean ice-shelf thickness is controlled primarily by the horizontally integrated mass balance. A narrow transition zone is found near the continental grounding line, in which the ice-shelf thickness decreases offshore and approaches the mean basin thickness. If the surface accumulation and mass flow from the continental ice masses are sufficiently large, the ice-shelf thickness grows to the point where the ice shelf grounds on the Lomonosov Ridge. As this occurs, the back stress increases in the Amerasian Basin and the ice-shelf thickness becomes larger there than in the Eurasian Basin towards the Fram Strait. Using a one-dimensional ice-dynamic model, the stability of equilibrium ice-shelf configurations without and with grounding on the Lomonosov Ridge are examined. We find that the grounded ice-shelf configuration should be stable if the two Lomonosov Ridge grounding lines are located on the opposites sides of the ridge crest, implying that the downstream grounding line is located on a downward sloping bed. This result shares similarities with the classical result on marine ice-sheet stability of Weertman, but due to interactions between the Amerasian and Eurasian ice-shelf segments the mass flux at the downstream grounding line decreases rather than increases with ice thickness.
  •  
33.
  • O'Regan, Matt, et al. (author)
  • Surface heat flow measurements from the East Siberian continental slope and southern Lomonosov Ridge, Arctic Ocean
  • 2016
  • In: Geochemistry Geophysics Geosystems. - 1525-2027. ; 17:5, s. 1608-1622
  • Journal article (peer-reviewed)abstract
    • Surface heat flow data in the Arctic Ocean are needed to assess hydrocarbon and methane hydrate distributions, and provide constraints into the tectonic origins and nature of underlying crust. However, across broad areas of the Arctic, few published measurements exist. This is true for the outer continental shelf and slope of the East Siberian Sea, and the adjoining deep water ridges and basins. Here we present 21 new surface heat flow measurements from this region of the Arctic Ocean. On the Southern Lomonosov Ridge, the average measured heat flow, uncorrected for effects of sedimentation and topography, is 574 mW/m(2) (n=4). On the outer continental shelf and slope of the East Siberian Sea (ESS), the average is 5710 mW/m(2) (n=16). An anomalously high heat flow of 20328 mW/m(2) was measured at a single station in the Herald Canyon. With the exception of this high heat flow, the new data from the ESS are consistent with predictions for thermally equilibrated lithosphere of continental origin that was last affected by thermotectonic processes in the Cretaceous to early Cenozoic. Variability within the data likely arises from differences in radiogenic heat production within the continental crust and overlying sediments. This can be further explored by comparing the data with geophysical constraints on sediment and crustal thicknesses.
  •  
34.
  • O'Regan, Matt, et al. (author)
  • The De Long Trough: A newly discovered glacial trough on the East Siberian continental margin
  • 2017
  • In: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 13:9, s. 1269-1284
  • Journal article (peer-reviewed)abstract
    • Ice sheets extending over parts of the East Siberian continental shelf have been proposed for the last glacial period and during the larger Pleistocene glaciations. The sparse data available over this sector of the Arctic Ocean have left the timing, extent and even existence of these ice sheets largely unresolved. Here we present new geophysical mapping and sediment coring data from the East Siberian shelf and slope collected during the 2014 SWERUS-C3 expedition (SWERUS-C3: Swedish - Russian - US Arctic Ocean Investigation of Climate-Cryosphere-Carbon Interactions). The multibeam bathymetry and chirp sub-bottom profiles reveal a set of glacial landforms that include grounding zone formations along the outer continental shelf, seaward of which lies a > 65m thick sequence of glacio-genic debris flows. The glacial landforms are interpreted to lie at the seaward end of a glacial trough - the first to be reported on the East Siberian margin, here referred to as the De Long Trough because of its location due north of the De Long Islands. Stratigraphy and dating of sediment cores show that a drape of acoustically laminated sediments covering the glacial deposits is older than similar to 50 cal kyr BP. This provides direct evidence for extensive glacial activity on the Siberian shelf that predates the Last Glacial Maximum and most likely occurred during the Saalian (Marine Isotope Stage (MIS) 6).
  •  
35.
  • O'Regan, Matt, et al. (author)
  • The Holocene dynamics of Ryder Glacier and ice tongue in north Greenland
  • 2021
  • In: The Cryosphere. - : Copernicus GmbH. - 1994-0416 .- 1994-0424. ; 15:8, s. 4073-4097
  • Journal article (peer-reviewed)abstract
    • The northern sector of the Greenland Ice Sheet is considered to be particularly susceptible to ice mass loss arising from increased glacier discharge in the coming decades. However, the past extent and dynamics of outlet glaciers in this region, and hence their vulnerability to climate change, are poorly documented. In the summer of 2019, the Swedish icebreaker Oden entered the previously unchartered waters of Sherard Osborn Fjord, where Ryder Glacier drains approximately 2 % of Greenland's ice sheet into the Lincoln Sea. Here we reconstruct the Holocene dynamics of Ryder Glacier and its ice tongue by combining radiocarbon dating with sedimentary facies analyses along a 45 km transect of marine sediment cores collected between the modern ice tongue margin and the mouth of the fjord. The results illustrate that Ryder Glacier retreated from a grounded position at the fjord mouth during the Early Holocene (> 10.7±0.4 ka cal BP) and receded more than 120 km to the end of Sherard Osborn Fjord by the Middle Holocene (6.3±0.3 ka cal BP), likely becoming completely land-based. A re-advance of Ryder Glacier occurred in the Late Holocene, becoming marine-based around 3.9±0.4 ka cal BP. An ice tongue, similar in extent to its current position was established in the Late Holocene (between 3.6±0.4 and 2.9±0.4 ka cal BP) and extended to its maximum historical position near the fjord mouth around 0.9±0.3 ka cal BP. Laminated, clast-poor sediments were deposited during the entire retreat and regrowth phases, suggesting the persistence of an ice tongue that only collapsed when the glacier retreated behind a prominent topographic high at the landward end of the fjord. Sherard Osborn Fjord narrows inland, is constrained by steep-sided cliffs, contains a number of bathymetric pinning points that also shield the modern ice tongue and grounding zone from warm Atlantic waters, and has a shallowing inland sub-ice topography. These features are conducive to glacier stability and can explain the persistence of Ryder's ice tongue while the glacier remained marine-based. However, the physiography of the fjord did not halt the dramatic retreat of Ryder Glacier under the relatively mild changes in climate forcing during the Holocene. Presently, Ryder Glacier is grounded more than 40 km seaward of its inferred position during the Middle Holocene, highlighting the potential for substantial retreat in response to ongoing climate change.
  •  
36.
  • Preto, Pedro, et al. (author)
  • Geothermal evidence for groundwater flow through Quaternary sediments overlying bedrock aquifers below Lake V?ttern, Sweden
  • 2019
  • In: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 141:2, s. 106-120
  • Journal article (peer-reviewed)abstract
    • Groundwater discharge into lakes is an important component of the fluid and nutrient budgets, and a possible route for contaminant transport. However, groundwater flow beneath lakes is difficult to investigate due to the need for drilling deep boreholes. In 2012, a 2,000 m deep borehole was drilled in Lake Vättern, the second largest lake in Sweden. A continuous temperature profile was collected from the borehole. The geothermal gradient in the upper 180 m is highly non-linear, and not controlled by variability in the measured thermal properties of the sediments and rocks. The anomalous temperature profile is best explained by fluid flow into the borehole and subsequent vertical flow of warm waters towards the lake floor. Combining the temperature profile with stratigraphic information from drilling logs and seismic data, we find that fluid flow into the borehole occurs in glacial and glaciofluvial sediments deposited on top of a large sandstone aquifer (the Visingsö Group). The warm waters flowing through the glacial and glaciofluvial sediments are likely sourced from the underlying Visingsö Group sandstones. There is no evidence for substantial vertical migration of these waters through the overlying glacial and postglacial sediments. We speculate that they escape either along lake margins where overlying sediments become thinner, or along faults that are known to exist in the deeper basin.These results highlight an important hydraulic transport pathway between recognised regional aquifers and Lake Vättern. Further work is needed to evaluate the significance of groundwater discharge on the water and nutrient budget of the lake.
  •  
37.
  • Sayedi, Sayedeh Sara, et al. (author)
  • Subsea permafrost carbon stocks and climate change sensitivity estimated by expert assessment
  • 2020
  • In: Environmental Research Letters. - : IOP Publishing. - 1748-9326. ; 15:12
  • Journal article (peer-reviewed)abstract
    • The continental shelves of the Arctic Ocean and surrounding seas contain large stocks of organic matter (OM) and methane (CH4), representing a potential ecosystem feedback to climate change not included in international climate agreements. We performed a structured expert assessment with 25 permafrost researchers to combine quantitative estimates of the stocks and sensitivity of organic carbon in the subsea permafrost domain (i.e. unglaciated portions of the continental shelves exposed during the last glacial period). Experts estimated that the subsea permafrost domain contains similar to 560 gigatons carbon (GtC; 170-740, 90% confidence interval) in OM and 45 GtC (10-110) in CH4. Current fluxes of CH4 and carbon dioxide (CO2) to the water column were estimated at 18 (2-34) and 38 (13-110) megatons C yr(-1), respectively. Under Representative Concentration Pathway (RCP) RCP8.5, the subsea permafrost domain could release 43 Gt CO2-equivalent (CO(2)e) by 2100 (14-110) and 190 Gt CO(2)e by 2300 (45-590), with similar to 30% fewer emissions under RCP2.6. The range of uncertainty demonstrates a serious knowledge gap but provides initial estimates of the magnitude and timing of the subsea permafrost climate feedback.
  •  
38.
  • Shibley, Nicole C., et al. (author)
  • Analysis of Acoustic Observations of Double-Diffusive Finestructure in the Arctic Ocean
  • 2020
  • In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 47:18
  • Journal article (peer-reviewed)abstract
    • Double-diffusive convection may occur if both temperature and salinity increase with depth, as in the Arctic Ocean. The process is identifiable by a staircase structure, with mixed layers separated by high-gradient interfaces in temperature and salinity. These staircases, which persist if turbulence levels are weak, are widely present in the Arctic Ocean and responsible for transporting heat toward the overlying sea ice. Acoustic observations (reflection coefficients) from a broadband echo sounder are analyzed here to track the detailed evolution of interfaces in the Arctic's double-diffusive staircase. We infer interface thicknesses from reflection coefficient profiles and find that thicknesses appear to be related to water column displacements. Further, we relate reflection coefficients to interface stratification and interpret stratification changes in the context of turbulence acting to thicken interfaces. The high-resolution capabilities of the echo sounder allow for insights into how double-diffusive heat fluxes and inferred mixing levels may vary in space/time.
  •  
39.
  • Snoeijs-Leijonmalm, Pauline, et al. (author)
  • A deep scattering layer under the North Pole pack ice
  • 2021
  • In: Progress in Oceanography. - : Elsevier BV. - 0079-6611 .- 1873-4472. ; 194
  • Journal article (peer-reviewed)abstract
    • The 3.3 million km marine ecosystem around the North Pole, defined as the Central Arctic Ocean (CAO), is a blind spot on the map of the world's fish stocks. The CAO essentially comprises the permanently ice-covered deep basins and ridges outside the continental shelves, and is only accessible by ice-breakers. Traditional trawling for assessing fish stocks is impossible under the thick pack ice, and coherent hydroacoustic surveys are unachievable due to ice-breaking noise. Consequently, nothing is known about the existence of any pelagic fish stocks in the CAO, although juveniles of Boreogadus saida richly occur at the surface associated with the sea ice and ice-associated Arctogadus glacialis has been reported as well. We here present a first indication of a possible mesopelagic fish stock in the CAO. We had the opportunity to analyse a geophysical hydroacoustic data set with 13 time windows of usable acoustic data over a transect from 84.4 °N in the Nansen Basin, across the North Pole (90.0 °N), to 82.4 °N in the Canada Basin. We discovered a deep scattering layer (DSL), suggesting the presence of zooplankton and fish, at 300–600 m of depth in the Atlantic water layer of the CAO. Maximum possible fish abundance and biomass was very low; values of ca. 2,000 individuals km and ca. 50 kg km were calculated for the DSL in the North-Pole area according to a model assuming that all acoustic backscatter represents 15-cm long B. saida and/or A. glacialis. The true abundance and biomass of fish is even lower than this, but cannot be quantified from this dataset due to possible backscatter originating from pneumatophores of physonect siphonophores that are known to occur in the area. Further studies on the DSL of the CAO should include sampling and identification of the backscattering organisms. From our study we can conclude that if the central Arctic DSL contains fish, their biomass is currently too low for any sustainable fishery.
  •  
40.
  • Sohn, Robert A., et al. (author)
  • Explosive volcanism on the ultraslow-spreading Gakkel ridge, Arctic Ocean
  • 2008
  • In: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 453:7199, s. 1236-1238
  • Journal article (peer-reviewed)abstract
    • Roughly 60% of the Earth's outer surface is composed of oceanic crust formed by volcanic processes at mid-ocean ridges. Although only a small fraction of this vast volcanic terrain has been visually surveyed or sampled, the available evidence suggests that explosive eruptions are rare on mid-ocean ridges, particularly at depths below the critical point for seawater (3,000 m)1. A pyroclastic deposit has never been observed on the sea floor below 3,000 m, presumably because the volatile content of mid-ocean-ridge basalts is generally too low to produce the gas fractions required for fragmenting a magma at such high hydrostatic pressure. We employed new deep submergence technologies during an International Polar Year expedition to the Gakkel ridge in the Arctic Basin at 85° E, to acquire photographic and video images of 'zero-age' volcanic terrain on this remote, ice-covered ridge. Here we present images revealing that the axial valley at 4,000 m water depth is blanketed with unconsolidated pyroclastic deposits, including bubble wall fragments (limu o Pele)2, covering a large (>10 km2) area. At least 13.5 wt% CO2 is necessary to fragment magma at these depths3, which is about tenfold the highest values previously measured in a mid-ocean-ridge basalt4. These observations raise important questions about the accumulation and discharge of magmatic volatiles at ultraslow spreading rates on the Gakkel ridge5 and demonstrate that large-scale pyroclastic activity is possible along even the deepest portions of the global mid-ocean ridge volcanic system.
  •  
41.
  • Sohn, Robert A., et al. (author)
  • Observations and Modeling of a Hydrothermal Plume in Yellowstone Lake
  • 2019
  • In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 46:12, s. 6435-6442
  • Journal article (peer-reviewed)abstract
    • Acoustic Doppler current profiler and conductivity-temperature-depth data acquired in Yellowstone Lake reveal the presence of a buoyant plume above the Deep Hole hydrothermal system, located southeast of Stevenson Island. Distributed venting in the similar to 200 x 200-m hydrothermal field creates a plume with vertical velocities of similar to 10 cm/s in the mid-water column. Salinity profiles indicate that during the period of strong summer stratification the plume rises to a neutral buoyancy horizon at similar to 45-m depth, corresponding to a similar to 70-m rise height, where it generates an anomaly of similar to 5% (-0.0014 psu) relative to background lake water. We simulate the plume with a numerical model and find that a heat flux of 28 MW reproduces the salinity and vertical velocity observations, corresponding to a mass flux of 1.4 x 10(3) kg/s. When observational uncertainties are considered, the heat flux could range between 20 to 50 MW.
  •  
42.
  • Stranne, Christian, et al. (author)
  • Acoustic mapping of mixed layer depth
  • 2018
  • In: Ocean Science. - : Copernicus GmbH. - 1812-0784 .- 1812-0792. ; 14:3, s. 503-514
  • Journal article (peer-reviewed)abstract
    • The ocean surface mixed layer is a nearly universal feature of the world oceans. Variations in the depth of the mixed layer (MLD) influences the exchange of heat, fresh water (through evaporation), and gases between the atmosphere and the ocean and constitutes one of the major factors controlling ocean primary production as it affects the vertical distribution of biological and chemical components in near-surface waters. Direct observations of the MLD are traditionally made by means of conductivity, temperature, and depth (CTD) casts. However, CTD instrument deployment limits the observation of temporal and spatial variability in the MLD. Here, we present an alternative method in which acoustic mapping of the MLD is done remotely by means of commercially available ship-mounted echo sounders. The method is shown to be highly accurate when the MLD is well defined and biological scattering does not dominate the acoustic returns. These prerequisites are often met in the open ocean and it is shown that the method is successful in 95% of data collected in the central Arctic Ocean. The primary advantages of acoustically mapping the MLD over CTD measurements are (1) considerably higher temporal and horizontal resolutions and (2) potentially larger spatial coverage.
  •  
43.
  • Stranne, Christian, et al. (author)
  • Acoustic Mapping of Thermohaline Staircases in the Arctic Ocean
  • 2017
  • In: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 7
  • Journal article (peer-reviewed)abstract
    • Although there is enough heat contained in inflowing warm Atlantic Ocean water to melt all Arctic sea ice within a few years, a cold halocline limits upward heat transport from the Atlantic water. The amount of heat that penetrates the halocline to reach the sea ice is not well known, but vertical heat transport through the halocline layer can significantly increase in the presence of double diffusive convection. Such convection can occur when salinity and temperature gradients share the same sign, often resulting in the formation of thermohaline staircases. Staircase structures in the Arctic Ocean have been previously identified and the associated double diffusive convection has been suggested to influence the Arctic Ocean in general and the fate of the Arctic sea ice cover in particular. A central challenge to understanding the role of double diffusive convection in vertical heat transport is one of observation. Here, we use broadband echo sounders to characterize Arctic thermohaline staircases at their full vertical and horizontal resolution over large spatial areas (100 s of kms). In doing so, we offer new insight into the mechanism of thermohaline staircase evolution and scale, and hence fluxes, with implications for understanding ocean mixing processes and ocean-sea ice interactions.
  •  
44.
  • Stranne, Christian, 1979-, et al. (author)
  • Anaerobic oxidation has a minor effect on mitigating seafloor methane emissions from gas hydrate dissociation
  • 2022
  • In: Communications Earth & Environment. - : Springer Nature. - 2662-4435. ; 3:1
  • Journal article (peer-reviewed)abstract
    • Continental margin sediments contain large reservoirs of methane stored as gas hydrate. Ocean warming will partly destabilize these reservoirs which may lead to the release of substantial, yet unconstrained, amounts of methane. Anaerobic oxidation of methane is the dominant biogeochemical process to reduce methane flux, estimated to consume 90% of the methane produced in marine sediments today. This process is however neglected in the current projections of seafloor methane release from gas hydrate dissociation. Here, we introduce a fully coupled oxidation module to a hydraulic-thermodynamic-geomechanical hydrate model. Our results show that for seafloor warming rates > 1 degrees C century(-1), the efficiency of anaerobic oxidation of methane in low permeability sediments is poor, reducing the seafloor methane emissions by <5%. The results imply an extremely low mitigating effect of anaerobic oxidation of methane on climate warming-induced seafloor methane emissions. Microbial anaerobic oxidation of methane may not substantially mitigate projected warming-induced emissions of methane from marine hydrate-bearing sediments, according to a coupled hydraulic-thermodynamic-geomechanical hydrate model.
  •  
45.
  • Stranne, Christian, 1979, et al. (author)
  • Analysis and modeling of hydrothermal plume data acquired from the 85°E segment of the Gakkel Ridge
  • 2010
  • In: Journal of Geophysical Research. - 0148-0227. ; 115:C06028
  • Journal article (peer-reviewed)abstract
    • We use data from a CTD plume-mapping campaign conducted during the Arctic Gakkel Vents (AGAVE) expedition in 2007 to constrain the nature of hydrothermal processes on the Gakkel Ridge at 85 degrees E. Thermal and redox potential (Eh) anomalies were detected in two discrete depth intervals: 2400-2800 m (Interval 1) and 3000-3800 m (Interval 2). The spatial and temporal patterns of the signals indicate that the Interval 1 anomalies were most likely generated by a single large, high-temperature (T > 100 degrees C) vent field located on the fault terraces that form the NE axial valley wall. In contrast, the Interval 2 anomalies appear to have been generated by up to 7 spatially distinct vent fields associated with constructional volcanic features on the floor of the axial valley, many of which may be sites of diffuse, low-temperature (T < 10 degrees C) discharge. Numerical simulations of turbulent plumes rising in a weakly stratified Arctic Ocean water column indicate that the high-temperature field on the axial valley wall has a thermal power of 1.8 GW, similar to the Trans-Atlantic Geotraverse and Rainbow fields in the Atlantic Ocean, whereas the sites on the axial valley floor have values ranging from 5 to 110 MW.
  •  
46.
  • Stranne, Christian, 1979, et al. (author)
  • Arctic Ocean perennial sea ice breakdown during the Early Holocene Insolation Maximum
  • 2014
  • In: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791 .- 1873-457X. ; 92, s. 123-132
  • Journal article (peer-reviewed)abstract
    • Arctic Ocean sea ice proxies generally suggest a reduction in sea ice during parts of the early and middle Holocene (similar to 6000-10,000 years BP) compared to present day conditions. This sea ice minimum has been attributed to the northern hemisphere Early Holocene Insolation Maximum (EHIM) associated with Earth's orbital cycles. Here we investigate the transient effect of insolation variations during the final part of the last glaciation and the Holocene by means of continuous climate simulations with the coupled atmosphere sea ice ocean column model CCAM. We show that the increased insolation during EHIM has the potential to push the Arctic Ocean sea ice cover into a regime dominated by seasonal ice, i.e. ice free summers. The strong sea ice thickness response is caused by the positive sea ice albedo feedback. Studies of the GRIP ice cores and high latitude North Atlantic sediment cores show that the Bolling Allerod period (c. 12,700-14,700 years BP) was a climatically unstable period in the northern high latitudes and we speculate that this instability may be linked to dual stability modes of the Arctic sea ice cover characterized by e.g. transitions between periods with and without perennial sea ice cover.
  •  
47.
  • Stranne, Christian, et al. (author)
  • Can anaerobic oxidation of methane prevent seafloor gas escape in a warming climate?
  • 2019
  • In: Solid Earth. - : Copernicus GmbH. - 1869-9510 .- 1869-9529. ; 10:5, s. 1541-1554
  • Journal article (peer-reviewed)abstract
    • Assessments of future climate-warming-induced seafloor methane (CH4) release rarely include anaerobic ox- idation of methane (AOM) within the sediments. Consider- ing that more than 90 % of the CH4 produced in ocean sed- iments today is consumed by AOM, this may result in sub- stantial overestimations of future seafloor CH4 release. Here, we integrate a fully coupled AOM module with a numerical hydrate model to investigate under what conditions rapid re- lease of CH4 can bypass AOM and result in significant fluxes to the ocean and atmosphere. We run a number of different model simulations for different permeabilities and maximum AOM rates. In all simulations, a future climate warming sce- nario is simulated by imposing a linear seafloor temperature increase of 3 ◦C over the first 100 years. The results presented in this study should be seen as a first step towards under- standing AOM dynamics in relation to climate change and hydrate dissociation. Although the model is somewhat poorly constrained, our results indicate that vertical CH4 migration through hydraulic fractures can result in low AOM efficien- cies. Fracture flow is the predicted mode of methane trans- port under warming-induced dissociation of hydrates on up- per continental slopes. Therefore, in a future climate warm- ing scenario, AOM might not significantly reduce methane release from marine sediments. 
  •  
48.
  • Stranne, Christian, 1979-, et al. (author)
  • Conductive heat flow and nonlinear geothermal gradients in marine sediment—observations from Ocean Drilling Program boreholes
  • 2016
  • In: Geo-Marine Letters. - : Springer Science and Business Media LLC. - 0276-0460 .- 1432-1157. ; 36:1, s. 25-33
  • Journal article (peer-reviewed)abstract
    • A basic premise in marine heat flow studies is that the temperature gradient varies with depth as a function of the bulk thermal conductivity of the sediments. As sediments become more deeply buried, compaction reduces the porosity and causes an increase in the bulk thermal conductivity. Therefore, while the heat flow may remain constant with depth, the thermal gradient is not necessarily linear. However, it has been argued that measurements showing increased sediment thermal conductivity with burial depth may be caused by a horizontal measurement bias generated by increasing anisotropy in sediments during consolidation. This study reanalyses a synthesis of Ocean Drilling Program data from 186 boreholes, and investigates the occurrence of nonlinear geothermal gradients in marine sediments. The aim is to identify whether observed downhole changes in thermal conductivity influence the measured temperature gradient, and to investigate potential errors in the prediction of in-situ temperatures derived from the extrapolation of near-surface thermal gradients. The results indicate that the measured thermal conductivity does influence the geothermal gradient. Furthermore, comparisons between shallow measurements (<10 m) from surface heat flow surveys and the deeply constrained temperature data from 98 ODP boreholes indicate that the shallow gradients are consistently higher by on average 19 °C km–1. This is consistent with higher porosity and generally lower thermal conductivity in near-seafloor sediments, and highlights the need to develop robust porosity–thermal conductivity models to accurately predict temperatures at depth from shallow heat flow surveys.
  •  
49.
  • Stranne, Christian, et al. (author)
  • Dynamic simulations of potential methane release from East Siberian continental slope sediments
  • 2016
  • In: Geochemistry Geophysics Geosystems. - 1525-2027. ; 17:3, s. 872-886
  • Journal article (peer-reviewed)abstract
    • Sediments deposited along continental margins of the Arctic Ocean presumably host large amounts of methane (CH4) in gas hydrates. Here we apply numerical simulations to assess the potential of gas hydrate dissociation and methane release from the East Siberian slope over the next 100 years. Simulations are based on a hypothesized bottom water warming of 3 degrees C, and an assumed starting distribution of gas hydrate. The simulation results show that gas hydrate dissociation in these sediments is relatively slow, and that CH4 fluxes toward the seafloor are limited by low sediment permeability. The latter is true even when sediment fractures are permitted to form in response to overpressure in pore space. With an initial gas hydrate distribution dictated by present-day pressure and temperature conditions, nominally 0.35 Gt of CH4 are released from the East Siberian slope during the first 100 years of the simulation. However, this CH4 discharge becomes significantly smaller (approximate to 0.05 Gt) if glacial sea level changes in the Arctic Ocean are considered. This is because a lower sea level during the last glacial maximum (LGM) must result in depleted gas hydrate abundance within the most sensitive region of the modern gas hydrate stability zone. Even if all released CH4 reached the atmosphere, the amount coming from East Siberian slopes would be trivial compared to present-day atmospheric CH4 inputs from other sources.
  •  
50.
  •  
Skapa referenser, mejla, bekava och länka
  • Result 1-50 of 59
Type of publication
journal article (48)
other publication (4)
doctoral thesis (3)
reports (2)
conference paper (2)
Type of content
peer-reviewed (50)
other academic/artistic (9)
Author/Editor
Stranne, Christian (39)
Jakobsson, Martin (36)
O'Regan, Matt (19)
Stranne, Christian, ... (18)
Björk, Göran, 1956 (9)
Nilsson, Johan (9)
show more...
Pearce, Christof (8)
O'Regan, Matthew (7)
Humborg, Christoph (7)
Cronin, Thomas M. (6)
Jakobsson, Martin, 1 ... (6)
Mörth, Carl-Magnus (6)
Kirchner, Nina (5)
Thornton, Brett F. (4)
Brüchert, Volker (4)
Anderson, Leif G, 19 ... (4)
Semiletov, Igor (4)
Barrientos, Natalia (4)
Muchowski, Julia (4)
Ketzer, João Marcelo (4)
Reilly, Brendan (4)
Mayer, L. (3)
Backman, Jan (3)
Norkko, Alf (3)
Gårdfeldt, Katarina, ... (3)
Eriksson, Björn (3)
Liljebladh, Bengt, 1 ... (3)
Nilsson, Johan, 1965 ... (3)
Coxall, Helen (3)
Arneborg, Lars (3)
Mayer, Larry (3)
Gemery, Laura (3)
Ulfsbo, Adam, 1985 (2)
Bonaglia, Stefano (2)
Dickens, Gerald R. (2)
Muschitiello, France ... (2)
Morlighem, Mathieu (2)
Coxall, Helen K. (2)
Johansson, Carina (2)
Norkko, Joanna (2)
Holmes, Felicity A. (2)
de Boer, Agatha (2)
Sun, Xiaole (2)
Gustafsson, Bo (2)
Detlef, Henrieka (2)
Hogan, Kelly A. (2)
Åkesson, Henning (2)
Koshurnikov, Andrey (2)
Swärd, Henrik (2)
Geibel, Marc C. (2)
show less...
University
Stockholm University (51)
University of Gothenburg (16)
Chalmers University of Technology (4)
Linnaeus University (4)
Swedish University of Agricultural Sciences (2)
Lund University (1)
Language
English (59)
Research subject (UKÄ/SCB)
Natural sciences (59)
Social Sciences (1)

Year

Kungliga biblioteket hanterar dina personuppgifter i enlighet med EU:s dataskyddsförordning (2018), GDPR. Läs mer om hur det funkar här.
Så här hanterar KB dina uppgifter vid användning av denna tjänst.

 
pil uppåt Close

Copy and save the link in order to return to this view