| 1. |
- Drake, Henrik, 1979-, et al.
(författare)
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Ancient microbial activity in deep hydraulically conductive fracture zones within the Forsmark target area for deep geological nuclear waste disposal, Sweden
- 2018
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Ingår i: Geosciences. - : MDPI AG. - 2076-3263. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- Recent studies reveal that organisms from all three domains of life—Archaea, Bacteria, and even Eukarya—can thrive under energy-poor, dark, and anoxic conditions at large depths in the fractured crystalline continental crust. There is a need for an increased understanding of the processes and lifeforms in this vast realm, for example, regarding the spatiotemporal extent and variability of the different processes in the crust. Here, we present a study that set out to detect signs of ancient microbial life in the Forsmark area—the target area for deep geological nuclear waste disposal in Sweden. Stable isotope compositions were determined with high spatial resolution analyses within mineral coatings, and mineralized remains of putative microorganisms were studied in several deep water-conducting fracture zones (down to 663 m depth), from which hydrochemical and gas data exist. Large isotopic variabilities of 13Ccalcite (?36.2 to +20.2‰V-PDB) and 34Spyrite (?11.7 to +37.8‰V-CDT) disclose discrete periods of methanogenesis, and potentially, anaerobic oxidation of methane and related microbial sulfate reduction at several depth intervals. Dominant calcite–water disequilibrium of 18O and 87Sr/86Sr precludes abundant recent precipitation. Instead, the mineral coatings largely reflect an ancient archive of episodic microbial processes in the fracture system, which, according to our microscale Rb–Sr dating of co-genetic adularia and calcite, date back to the mid-Paleozoic. Potential Quaternary precipitation exists mainly at ~400 m depth in one of the boreholes, where mineral–water compositions corresponded.
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| 2. |
- Katrantsiotis, Christos, et al.
(författare)
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Seasonal variability in temperature trends and atmospheric circulation systems during the Eemian (Last Interglacial) based on n-alkanes hydrogen isotopes from Northern Finland
- 2021
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Ingår i: Quaternary Science Reviews. - Amsterdam : Elsevier. - 0277-3791 .- 1873-457X. ; 273, s. 107250-107250
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Tidskriftsartikel (refereegranskat)abstract
- The Last Interglacial warm period, the Eemian (ca. 130-116 thousand years ago), serves as a reference for projected future climate in a warmer world. However, there is a limited understanding of the seasonal characteristics of interglacial climate dynamics, especially in high latitude regions. In this study, we aimto provide new insights into seasonal trends in temperature and moisture source location, linked to shifts in atmospheric circulation patterns, for northern Fennoscandia during the Eemian. Our study is based on the distribution and stable hydrogen isotope composition (dD) of n-alkanes in a lake sediment sequence from the Sokli paleolake in NE Finland, placed in a multi-proxy framework. The dD values of predominantly macrophyte-derived mid-chain n-alkanes are interpreted to reflect lake water dD variability influenced by winter precipitation dD (dDprec), ice cover duration and deuterium (D)-depleted meltwater. The dD values of terrestrial plant-derived long-chain n-alkanes primarily reflect soil water dD variability modulated by summer dDprec and by the evaporative enrichment of soil and leaf water. The dDprec variability in our study area is mostly attributed to the temperature effect and the moisture source location linked to the relative dominance between D-depleted continental and polar air masses and Denriched North Atlantic air masses. The biomarker signal further corroborates earlier diatom-based studies and pollen-inferred January and July temperature reconstructions from the same sediment sequence. Three phases of climatic changes can be identified that generally follow the secular variationsin seasonal insolation: (i) an early warming trend succeeded by a period of strong seasonality (ii) a midoptimum phase with gradually decreased seasonality and cooler summers, and (iii) a late climatic instability with a cooling trend. Superimposed on this trend, two abrupt cooling events occur in the early and late Eemian. The Sokli dD variability is generally in good agreement with other North Atlantic and Siberian records, reflecting major changes in the atmospheric circulation patterns during the Eemian as a response to orbital and oceanic forcings.
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| 3. |
- Weis, Franz A., et al.
(författare)
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Magmatic water contents determined through clinopyroxene : Examples from the Western Canary Islands, Spain
- 2015
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Ingår i: Geochemistry Geophysics Geosystems. - 1525-2027. ; 16:7, s. 2127-2146
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Tidskriftsartikel (refereegranskat)abstract
- Water is a key parameter in magma genesis, magma evolution, and resulting eruption styles, because it controls the density, the viscosity, as well as the melting and crystallization behavior of a melt. The parental water content of a magma is usually measured through melt inclusions in minerals such as olivine, a method which may be hampered, however, by the lack of melt inclusions suitable for analysis, or postentrapment changes in their water content. An alternative way to reconstruct the water content of a magma is to use nominally anhydrous minerals (NAMs), such as pyroxene, which take up low concentrations of hydrogen as a function of the magma's water content. During magma degassing and eruption, however, NAMs may dehydrate. We therefore tested a method to reconstruct the water contents of dehydrated clinopyroxene phenocrysts from the Western Canary islands (n=28) through rehydration experiments followed by infrared and Mossbauer spectroscopy. Employing currently available crystal/melt partitioning data, the results of the experiments were used to calculate parental water contents of 0.710.07 to 1.490.15 wt % H2O for Western Canary magmas during clinopyroxene crystallization at upper mantle conditions. This H2O range is in agreement with calculated water contents using plagioclase-liquid-hygrometry, and with previously published data for mafic lavas from the Canary Islands and comparable ocean island systems elsewhere. Utilizing NAMs in combination with hydrogen treatment can therefore serve as a proxy for pre-eruptive H2O contents, which we anticipate becoming a useful method applicable to mafic rocks where pyroxene is the main phenocryst phase.
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| 4. |
- Alvarez, Belinda, et al.
(författare)
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Assessing the potential of sponges (Porifera) as indicators of ocean dissolved Si concentrations
- 2017
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Ingår i: Frontiers in Marine Science. - : Frontiers Media SA. - 2296-7745. ; 4
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Tidskriftsartikel (refereegranskat)abstract
- We explore the distribution of sponges along dissolved silica (dSi) concentration gradients to test whether sponge assemblages are related to dSi and to assess the validity of fossil sponges as a palaeoecological tool for inferring dSi concentrations of the past oceans. We extracted sponge records from the publically available Global Biodiversity Information Facility (GBIF) database and linked these records with ocean physiochemical data to evaluate if there is any correspondence between dSi concentrations of the waters sponges inhabit and their distribution. Over 320,000 records of Porifera were available, of which 62,360 met strict quality control criteria. Our analyses was limited to the taxonomic levels of family, order and class. Because dSi concentration is correlated with depth in the modern ocean, we also explored sponge taxa distributions as a function of depth. We observe that while some sponge taxa appear to have dSi preferences (e.g., class Hexactinellida occurs mostly at high dSi), the overall distribution of sponge orders and families along dSi gradients is not sufficiently differentiated to unambiguously relate dSi concentrations to sponge taxa assemblages. We also observe that sponge taxa tend to be similarly distributed along a depth gradient. In other words, both dSi and/or another variable that depth is a surrogate for, may play a role in controlling sponge spatial distribution and the challenge is to distinguish between the two. We conclude that inferences about palaeo-dSi concentrations drawn from the abundance of sponges in the stratigraphic records must be treated cautiously as these animals are adapted to a great range of dSi conditions and likely other underlying variables that are related to depth. Our analysis provides a quantification of the dSi ranges of common sponge taxa, expands on previous knowledge related to their bathymetry preferences and suggest that sponge taxa assemblages are not related to particular dSi conditions.
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| 5. |
- Steinthorsdottir, Margret
(författare)
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Global trends of pCO2 across the Cretaceous-Paleogene boundary supported by the first Southern Hemisphere stomatal proxy-based pCO2 reconstruction
- 2016
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Ingår i: Palaeogeography, Palaeoclimatology, Palaeoecology. - Amsterdam : Elsevier. - 0031-0182 .- 1872-616X. ; 464, s. 143-152
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Tidskriftsartikel (refereegranskat)abstract
- Reliable reconstructions of atmospheric carbon dioxide concentrations (pCO2) are required at higher resolution than currently available to help resolve the relationship between mass extinctions and changes in palaeo-pCO2 levels. Such reconstructions are needed: 1, at a high temporal resolution for constraining the pre- and post-extinction atmospheres; and 2, at a sufficient spatial resolution to constrain potential inter-hemispheric differences. Here we estimate pCO2 based on fossil Lauraceae leaf cuticle specimens derived from three localities with strata spanning the latest Cretaceous to the mid-Paleocene, including a new Cretaceous–Paleogene boundary (K–Pg) locality, in New Zealand. We use two independent methods of stomatal density-based pCO2 reconstructions; a transfer function calibrated using herbarium material and the stomatal ratio method, producing three calibration sets. Our results based on the mean values of each of the three calibration methods indicate pCO2 ranging between ca. 460 and 650 ppm during the latest Cretaceous, falling precipitously to average values between ca. 360 and 430 ppm across the K–Pg boundary, and further to ca. 305–320 ppm in the mid-Paleocene. A ‘spike’ of extremely high pCO2 at the K–Pg could not be confirmed, but our results are, nonetheless, consistent with previously published pCO2 records from the Northern Hemisphere, and show that stomatal density worldwide was responding to significant changes in pCO2 across the K–Pg.
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| 6. |
- Capo, Eric, et al.
(författare)
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Oxygen-deficient water zones in the Baltic Sea promote uncharacterized Hg methylating microorganisms in underlying sediments
- 2022
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Ingår i: Limnology and Oceanography. - : Wiley. - 1939-5590 .- 0024-3590. ; 67:1, s. 135-146
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Tidskriftsartikel (refereegranskat)abstract
- Human-induced expansion of oxygen-deficient zones can have dramatic impacts on marine systems and its resident biota. One example is the formation of the potent neurotoxic methylmercury (MeHg) that is mediated by microbial methylation of inorganic divalent Hg (HgII) under oxygen-deficient conditions. A negative consequence of the expansion of oxygen-deficient zones could be an increase in MeHg production due to shifts in microbial communities in favor of microorganisms methylating Hg. There is, however, limited knowledge about Hg-methylating microbes, i.e., those carrying hgc genes critical for mediating the process, from marine sediments. Here, we aim to study the presence of hgc genes and transcripts in metagenomes and metatranscriptomes from four surface sediments with contrasting concentrations of oxygen and sulfide in the Baltic Sea. We show that potential Hg methylators differed among sediments depending on redox conditions. Sediments with an oxygenated surface featured hgc-like genes and transcripts predominantly associated with uncultured Desulfobacterota (OalgD group) and Desulfobacterales (including Desulfobacula sp.) while sediments with a hypoxic-anoxic surface included hgc-carrying Verrucomicrobia, unclassified Desulfobacterales, Desulfatiglandales, and uncharacterized microbes. Our data suggest that the expansion of oxygen-deficient zones in marine systems may lead to a compositional change of Hg-methylating microbial groups in the sediments, where Hg methylators whose metabolism and biology have not yet been characterized will be promoted and expand.
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| 7. |
- Smit, Matthijs, et al.
(författare)
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Early evolution of the Pamir deep crust from Lu-Hf and U-Pb geochronology and garnet thermometry
- 2014
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 42:12, s. 1047-1050
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Tidskriftsartikel (refereegranskat)abstract
- Determining early orogenic processes within the Pamir-Tibet orogen represents a critical step toward constructing a comprehensive model on the tectonic evolution of the region. Here we investigate the timing and cause of prograde metamorphism of Cenozoic metamorphic rocks from the Pamir plateau through Lu-Hf geochronology, U-Pb rutile thermochronology, and garnet thermometry. Regional prograde metamorphism and heating to 750–830 °C, as constrained by thermometry, occurred between 37 and 27 Ma. Prograde growth of garnet first occurred in the South Pamir and spread to the Central Pamir during the following 10 m.y. The early metamorphism is attributed to high mantle heat flow following the ca. 45 Ma break-off of the Indian slab south of the Pamir. Our investigation confirms a long-lived thermal history of the Pamir deep crust before the Miocene, and provides a causal link between break-off, enhanced mantle heat flow, and prograde heating of the subduction hanging wall.
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| 8. |
- Hutchinson, David K., et al.
(författare)
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The Eocene-Oligocene transition: a review of marine and terrestrial proxy data, models and model-data comparisons
- 2021
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Ingår i: Climate of the Past. - : European Geosciences Union (EGU). - 1814-9324 .- 1814-9332. ; 17:1, s. 269-315
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Tidskriftsartikel (refereegranskat)abstract
- The Eocene–Oligocene transition (EOT) was a climate shift from a largely ice-free greenhouse world to an icehouse climate, involving the first major glaciation of Antarctica and global cooling occurring ∼ 34 million years ago (Ma) and lasting ∼ 790 kyr. The change is marked by a global shift in deep-sea δ18O representing a combination of deep-ocean cooling and growth in land ice volume. At the same time, multiple independent proxies for ocean tempera- ture indicate sea surface cooling, and major changes in global fauna and flora record a shift toward more cold-climate- adapted species. The two principal suggested explanations of this transition are a decline in atmospheric CO2 and changes to ocean gateways, while orbital forcing likely influenced the precise timing of the glaciation. Here we review and synthesise proxy evidence of palaeogeography, temperature, ice sheets, ocean circulation and CO2 change from the marine and terrestrial realms. Furthermore, we quantitatively com- pare proxy records of change to an ensemble of climate model simulations of temperature change across the EOT. The simulations compare three forcing mechanisms across the EOT: CO2 decrease, palaeogeographic changes and ice sheet growth. Our model ensemble results demonstrate the need for a global cooling mechanism beyond the imposition of an ice sheet or palaeogeographic changes. We find that CO2 forcing involving a large decrease in CO2 of ca. 40 % (∼ 325 ppm drop) provides the best fit to the available proxy evidence, with ice sheet and palaeogeographic changes play- ing a secondary role. While this large decrease is consistent with some CO2 proxy records (the extreme endmember of decrease), the positive feedback mechanisms on ice growth are so strong that a modest CO2 decrease beyond a critical threshold for ice sheet initiation is well capable of triggering rapid ice sheet growth. Thus, the amplitude of CO2 decrease signalled by our data–model comparison should be consid- ered an upper estimate and perhaps artificially large, not least because the current generation of climate models do not in- clude dynamic ice sheets and in some cases may be under- sensitive to CO2 forcing. The model ensemble also cannot exclude the possibility that palaeogeographic changes could have triggered a reduction in CO2.
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| 9. |
- Sahlström, Fredrik, et al.
(författare)
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Iron isotopes constrain sub-seafloor hydrothermal processes at the Trans-Atlantic Geotraverse (TAG) active sulfide mound
- 2022
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Ingår i: Communications Earth & Environment. - : Springer Science and Business Media LLC. - 2662-4435. ; 3:1
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Tidskriftsartikel (refereegranskat)abstract
- Sub-seafloor hydrothermal processes along volcanically active plate boundaries are integral to the formation of seafloor massive sulfide deposits and to oceanic iron cycling, yet the nature of their relationship is poorly understood. Here we apply iron isotope analysis to sulfide minerals from the Trans-Atlantic Geotraverse (TAG) mound and underlying stockwork, 26°N Mid-Atlantic Ridge, to trace hydrothermal processes inside an actively-forming sulfide deposit in a sediment-free mid-ocean ridge setting. We show that data for recently formed chalcopyrite imply hydrothermal fluid–mound interactions cause small negative shifts (<−0.1‰) to the δ56Fe signature of dissolved iron released from TAG into the North Atlantic Ocean. Texturally distinct types of pyrite, in turn, preserve a δ56Fe range from −1.27 to +0.56‰ that reflects contrasting precipitation mechanisms (hydrothermal fluid–seawater mixing vs. conductive cooling) and variable degrees of progressive hydrothermal maturation during the >20 kyr evolution of the TAG complex. The identified processes may explain iron isotope variations found in fossil onshore sulfide deposits.
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| 10. |
- Osterwalder, S., et al.
(författare)
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Critical Observations of Gaseous Elemental Mercury Air-Sea Exchange
- 2021
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Ingår i: Global Biogeochemical Cycles. - : American Geophysical Union (AGU). - 0886-6236 .- 1944-9224. ; 35:8
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Tidskriftsartikel (refereegranskat)abstract
- Air-sea exchange of gaseous elemental mercury (Hg-0) is not well constrained, even though it is a major component of the global Hg cycle. Lack of Hg-0 flux measurements to validate parameterizations of the Hg-0 transfer velocity contributes to this uncertainty. We measured the Hg-0 flux on the Baltic Sea coast using micrometeorological methods (gradient-based and relaxed eddy accumulation [REA]) and also simulated the flux with a gas exchange model. The coastal waters were typically supersaturated with Hg-0 (mean +/- 1 sigma = 13.5 +/- 3.5 ng m(-3); ca. 10% of total Hg) compared to the atmosphere (1.3 +/- 0.2 ng m(-3)). The Hg-0 flux calculated using the gas exchange model ranged from 0.1-1.3 ng m(-2) h(-1) (10th and 90th percentile) over the course of the campaign (May 10-June 20, 2017) and showed a distinct diel fluctuation. The mean coastal Hg-0 fluxes determined with the two gradient-based approaches and REA were 0.3, 0.5, and 0.6 ng m(-2) h(-1), respectively. In contrast, the mean open sea Hg-0 flux measured with REA was larger (6.3 ng m(-2) h(-1)). The open sea Hg-0 flux indicated a stronger wind speed dependence for the Hg-0 transfer velocity compared to commonly used parameterizations. Although based on a limited data set, we suggest that the wind speed dependence of the Hg-0 transfer velocity is more consistent with gases that have less water solubility than CO2 (e.g., O-2). These pioneering flux measurements using micrometeorological techniques show that more such measurements would improve our understanding of air-sea Hg exchange.
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