| 1. |
- Bradshaw, Clare, et al.
(författare)
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Physical Disturbance by Bottom Trawling Suspends Particulate Matter and Alters Biogeochemical Processes on and Near the Seafloor
- 2021
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Ingår i: Frontiers in Marine Science. - : Frontiers Media SA. - 2296-7745. ; 8
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Tidskriftsartikel (refereegranskat)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.
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| 2. |
- Gäng, Frederik, et al.
(författare)
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Critical assessment of U, Ba and Ni as redox and productivity proxies in organic-rich sediments underneath dynamic, highly productive waters
- 2023
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Ingår i: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037 .- 1872-9533. ; 348, s. 206-220
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Tidskriftsartikel (refereegranskat)abstract
- Sediments of the continental shelf anoxic zones of the Benguela upwelling system (BUS) and the Peruvian upwelling system are present-day hotspots of trace element accumulation. However, contribution of the lithogenic trace element fraction and early diagenetic transformation processes are poorly constrained. The identification of source and accumulation mechanisms is necessary for the validation of trace elements as proxies for productivity and redox cycling, notably in highly dynamic upwelling systems such as the BUS. Here, we analyzed redox- and biosensitive elements (U, Ba, Ni), lithogenic tracers (Al, Ti, Zr), total organic carbon (TOC) and P in eleven short sediment cores from the Namibian shelf (ca. 22–25°S) as well as grain size fractionated hinterland samples from the Tsauchab Valley at Sossusvlei (∼25°S). These samples help to constrain the lithogenic trace element contributions, a prerequisite for a realistic interpretation of marine authigenic trace element data. Our findings corroborate previous findings that Zr is a sensitive tracer of the coarse fraction in marine sediment for dust input and/or sediment reworking. Hence, the combined analysis of the coarse fraction (Zr), organic matter, and P enrichments (TOC/P) differentiates between a calm, organic matter-rich central shelf environment and a more energetic phosphorite-rich southern shelf environment. A significant correlation of U with P in cores from the southern shelf corroborates previous findings of U incorporation into apatite, the initial formation of which requires oscillating oxic to sulfidic redox conditions. Non-apatite associated U dominates in the cores from the central shelf, where it is only enriched in the buried, anoxic parts of the sediment. This also supports oscillating oxic to sulfidic redox conditions in surface sediment, where U may be recycled. Hence, U accumulates either (i) in apatite under oscillating oxic to sulfidic conditions or (ii) in buried anoxic sediment sections. This strongly questions the use of U as indicator of suboxic conditions but rather anoxic conditions. While lithogenic fractions of U were negligible, the lithogenic fractions of Ba and Ni were found to be elevated. The calculation of authigenic Ba when accounting for lithogenic Ba from the Sossusvlei fine fractions gives more realistic authigenic Ba values. The high Ba enrichment in the BUS is interpreted as uptake of Ba by diatoms and/or nucleation of barite in P-rich diatom remains, as suggested previously. Given the shallow water depth of the cores, barite formation must already take place in subsurface waters (<50 m water depth) as opposed to previous suggesting greater water depths. The lithogenic background of Ni strongly varies from 23°S to 25°S based on Sossusvlei data and the literature. When authigenic Ni contents are calculated accordingly and plotted versus TOC, more systematic trends are seen (compared to total Ni versus TOC). Essentially all cores show an increase in the authigenic Ni/TOC ratio with sediment depth suggesting better preservation and retention of Ni as compared to TOC consistent with previous observations in sediments from the Peruvian upwelling system. This trend corroborates the use of Ni as a productivity indicator for upwelling sediments.
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| 3. |
- Jakobsson, Martin, et al.
(författare)
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Ryder Glacier in northwest Greenland is shielded from warm Atlantic water by a bathymetric sill
- 2020
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Ingår i: Communications Earth & Environment. - : Springer Science and Business Media LLC. - 2662-4435. ; 1
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Tidskriftsartikel (refereegranskat)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.
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| 4. |
- Lundevall-Zara, Maysoon, et al.
(författare)
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Sea-Air Exchange of Methane in Shallow Inshore Areas of the Baltic Sea
- 2021
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Ingår i: Frontiers in Marine Science. - : Frontiers Media SA. - 2296-7745. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- We report sea-air fluxes of methane in physically and biologically distinct inshore habitats of the Baltic Sea with the goal to establish empirical relationships that allow upscaling of local site-specific flux measurements. Flux measurements were conducted using floating chambers with and without bubble shields, and by using a boundary layer gas transfer model before, during, and after an annually occurring algal bloom from June to October 2019. Water and air temperature, salinity, wind, sediment organic content, and organic content of floating algal biomass were found to successfully discriminate the different habitats in terms of methane flux, both over periods of days and over a season. Multivariate statistical analysis was used to establish the relative environmental forcing of methane emissions over one growth season for each flux method. Floating algal biomass carbon and sediment organic content were identified as the most important controlling factors for methane emissions based on flux chamber measurements over a period of days to weeks, whereas water and air temperature and wind velocity were the most important factors based on the gas transfer model on these time scales. Over the season, water and air temperature were the most important controlling factors with both methods. We present a first attempt how our observations can be extrapolated to determine the coastal methane emission along the coastline.
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| 5. |
- Morys, Claudia, et al.
(författare)
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Impacts of bottom trawling on benthic biogeochemistry in muddy sediments : Removal of surface sediment using an experimental field study
- 2021
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Ingår i: Marine Environmental Research. - : Elsevier BV. - 0141-1136 .- 1879-0291. ; 169
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Tidskriftsartikel (refereegranskat)abstract
- Experimental benthic dredging was conducted in an unfished, muddy area in the Baltic Proper to mimic the impact of trawling by removing surface sediment, with a focus on benthic biogeochemical processes. Sediment cores were taken on the track and compared to undisturbed controls. Benthic fluxes were immediately affected and an upward shift in pore water DIC profiles was detected. The time needed for the sediment to readjust to a new biogeochemical state seemed to be nutrient-specific. Sediment properties (profiles of chlorophyll, organic carbon and water content) were found to change significantly. Macrofauna was removed completely by the dredge pointing out the potential loss of highly valuable functions that are associated with them. In the Baltic Sea, in areas which were previously the most heavily fished, the frequency of trawling may have left little time for readjustment and potentially kept the seabed in a permanent state of transient biogeochemical cycling.
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| 6. |
- Prytherch, John, 1980-, et al.
(författare)
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Central Arctic Ocean surface-atmosphere exchange of CO2 and CH4 constrained by direct measurements
- 2024
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Ingår i: Biogeosciences. - : Copernicus Publications. - 1726-4170 .- 1726-4189. ; 21:2, s. 671-688
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Tidskriftsartikel (refereegranskat)abstract
- The central Arctic Ocean (CAO) plays an important role in the global carbon cycle, but the current and future exchange of the climate-forcing trace gases methane (CH4) and carbon dioxide (CO2) between the CAO and the atmosphere is highly uncertain. In particular, there are very few observations of near-surface gas concentrations or direct air-sea CO2 flux estimates and no previously reported direct air-sea CH4 flux estimates from the CAO. Furthermore, the effect of sea ice on the exchange is not well understood. We present direct measurements of the air-sea flux of CH4 and CO2, as well as air-snow fluxes of CO2 in the summertime CAO north of 82.5 N from the Synoptic Arctic Survey (SAS) expedition carried out on the Swedish icebreaker Oden in 2021. Measurements of air-sea CH4 and CO2 flux were made using floating chambers deployed in leads accessed from sea ice and from the side of Oden, and air-snow fluxes were determined from chambers deployed on sea ice. Gas transfer velocities determined from fluxes and surface-water-dissolved gas concentrations exhibited a weaker wind speed dependence than existing parameterisations, with a median sea-ice lead gas transfer rate of 2.5cmh-1 applicable over the observed 10m wind speed range (1-11ms-1). The average observed air-sea CO2 flux was -7.6mmolm-2d-1, and the average air-snow CO2 flux was -1.1mmolm-2d-1. Extrapolating these fluxes and the corresponding sea-ice concentrations gives an August and September flux for the CAO of -1.75mmolm-2d-1, within the range of previous indirect estimates. The average observed air-sea CH4 flux of 3.5μmolm-2d-1, accounting for sea-ice concentration, equates to an August and September CAO flux of 0.35μmolm-2d-1, lower than previous estimates and implying that the CAO is a very small (‰ 1%) contributor to the Arctic flux of CH4 to the atmosphere.
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| 7. |
- Roth, Florian, et al.
(författare)
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High spatiotemporal variability of methane concentrations challenges estimates of emissions across vegetated coastal ecosystems.
- 2022
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Ingår i: Global change biology. - : Wiley. - 1365-2486 .- 1354-1013. ; 28:14, s. 4308-4322
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Tidskriftsartikel (refereegranskat)abstract
- Coastal methane (CH4 ) emissions dominate the global ocean CH4 budget and can offset the "blue carbon" storage capacity of vegetated coastal ecosystems. However, current estimates lack systematic, high-resolution, and long-term data from these intrinsically heterogeneous environments, making coastal budgets sensitive to statistical assumptions and uncertainties. Using continuous CH4 concentrations, δ13 C-CH4 values, and CH4 sea-air fluxes across four seasons in three globally pervasive coastal habitats, we show that the CH4 distribution is spatially patchy over meter-scales and highly variable in time. Areas with mixed vegetation, macroalgae, and their surrounding sediments exhibited a spatiotemporal variability of surface water CH4 concentrations ranging two orders of magnitude (i.e., 6-460nM CH4 ) with habitat-specific seasonal and diurnal patterns. We observed (1) δ13 C-CH4 signatures that revealed habitat-specific CH4 production and consumption pathways, (2) daily peak concentration events that could change >100% within hours across all habitats, and (3) a high thermal sensitivity of the CH4 distribution signified by apparent activation energies of ~1eV that drove seasonal changes. Bootstrapping simulations show that scaling the CH4 distribution from few samples involves large errors, and that ~50 concentration samples per day are needed to resolve the scale and drivers of the natural variability and improve the certainty of flux calculations by up to 70%. Finally, we identify northern temperate coastal habitats with mixed vegetation and macroalgae as understudied but seasonally relevant atmospheric CH4 sources (i.e., releasing≥100μmol CH4 m-2 day-1 in summer). Due to the large spatial and temporal heterogeneity of coastal environments, high-resolution measurements will improve the reliability of CH4 estimates and confine the habitat-specific contribution to regional and global CH4 budgets.
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| 8. |
- Roth, Florian, et al.
(författare)
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Methane emissions offset atmospheric carbon dioxide uptake in coastal macroalgae, mixed vegetation and sediment ecosystems
- 2023
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 14
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Tidskriftsartikel (refereegranskat)abstract
- Coastal ecosystems can efficiently remove carbon dioxide (CO2) from the atmosphere and are thus promoted for nature-based climate change mitigation. Natural methane (CH4) emissions from these ecosystems may counterbalance atmospheric CO2 uptake. Still, knowledge of mechanisms sustaining suchCH4 emissions and their contribution to net radiative forcing remains scarce for globally prevalent macroalgae, mixed vegetation, and surrounding depositional sediment habitats. Here we show that these habitats emit CH4 in the range of 0.1 – 2.9 mg CH4 m−2 d−1 to the atmosphere, revealing in situ CH4 emissions from macroalgae that weresustained by divergent methanogenic archaea in anoxic microsites. Over an annual cycle, CO2-equivalent CH4 emissions offset 28 and 35% of the carbon sink capacity attributed to atmospheric CO2 uptake in the macroalgae and mixed vegetation habitats, respectively, and augment net CO2 release of unvegetated sediments by 57%. Accounting for CH4 alongside CO2 sea-air fluxes and identifying the mechanisms controlling these emissions is crucial to constrain the potential of coastal ecosystems as net atmospheric carbon sinks and develop informed climate mitigation strategies.
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| 9. |
- Steinbach, Julia, et al.
(författare)
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Source apportionment of methane escaping the subsea permafrost system in the outer Eurasian Arctic Shelf
- 2021
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 118:10
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Tidskriftsartikel (refereegranskat)abstract
- The East Siberian Arctic Shelf holds large amounts of inundated carbon and methane (CH4). Holocene warming by overlying seawater, recently fortified by anthropogenic warming, has caused thawing of the underlying subsea permafrost. Despite extensive observations of elevated seawater CH4 in the past decades, relative contributions from different subsea compartments such as early diagenesis, subsea permafrost, methane hydrates, and underlying thermogenic/ free gas to these methane releases remain elusive. Dissolved methane concentrations observed in the Laptev Sea ranged from 3 to 1,500 nM (median 151 nM; oversaturation by similar to 3,800%). Methane stable isotopic composition showed strong vertical and horizontal gradients with source signatures for two seepage areas of delta C-13-CH4 = (-42.6 +/- 0.5)/(-55.0 +/- 0.5) % and delta D-CH4 = (-136.8 +/- 8.0)/(-158.1 +/- 5.5) %, suggesting a thermogenic/ natural gas source. Increasingly enriched delta C-13-CH4 and delta D-CH4 at distance from the seeps indicated methane oxidation. The Delta C-14-CH4 signal was strongly depleted (i.e., old) near the seeps (-993 +/- 19/-1050 +/- 89%). Hence, all three isotope systems are consistent with methane release from an old, deep, and likely thermogenic pool to the outer Laptev Sea. This knowledge of what subsea sources are contributing to the observed methane release is a prerequisite to predictions on how these emissions will increase over coming decades and centuries.
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| 10. |
- Stranne, Christian, 1979-, et al.
(författare)
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Anaerobic oxidation has a minor effect on mitigating seafloor methane emissions from gas hydrate dissociation
- 2022
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Ingår i: Communications Earth & Environment. - : Springer Nature. - 2662-4435. ; 3:1
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Tidskriftsartikel (refereegranskat)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.
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