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    Coupling of oceanic carbon and nitrogen facilitates spatially resolved quantitative reconstruction of nitrate inventories (2018)
    Nature Research
    In:  Nature Communications, 9 (Article number 1217).
    Details
    Publication Date: 2021-03-19
    Description: Anthropogenic impacts are perturbing the global nitrogen cycle via warming effects and pollutant sources such as chemical fertilizers and burning of fossil fuels. Understanding controls on past nitrogen inventories might improve predictions for future global biogeochemical cycling. Here we show the quantitative reconstruction of deglacial bottom water nitrate concentrations from intermediate depths of the Peruvian upwelling region, using foraminiferal pore density. Deglacial nitrate concentrations correlate strongly with downcore δ13C, consistent with modern water column observations in the intermediate Pacific, facilitating the use of δ13C records as a paleo-nitrate-proxy at intermediate depths and suggesting that the carbon and nitrogen cycles were closely coupled throughout the last deglaciation in the Peruvian upwelling region. Combining the pore density and intermediate Pacific δ13C records shows an elevated nitrate inventory of 〉10% during the Last Glacial Maximum relative to the Holocene, consistent with a δ13C-based and δ15N-based 3D ocean biogeochemical model and previous box modeling studies.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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    DOI: 10.1038/s41467-018-03647-5
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    Gas hydrate dissociation off Svalbard induced by isostatic rebound rather than global warming (2018)
    Nature Research
    In:  Nature Communications, 9 (Article number: 83 ).
    Details
    Publication Date: 2021-03-19
    Description: Methane seepage from the upper continental slopes of Western Svalbard has previously been attributed to gas hydrate dissociation induced by anthropogenic warming of ambient bottom waters. Here we show that sediment cores drilled off Prins Karls Foreland contain freshwater from dissociating hydrates. However, our modeling indicates that the observed pore water freshening began around 8 ka BP when the rate of isostatic uplift outpaced eustatic sea-level rise. The resultant local shallowing and lowering of hydrostatic pressure forced gas hydrate dissociation and dissolved chloride depletions consistent with our geochemical analysis. Hence, we propose that hydrate dissociation was triggered by postglacial isostatic rebound rather than anthropogenic warming. Furthermore, we show that methane fluxes from dissociating hydrates were considerably smaller than present methane seepage rates implying that gas hydrates were not a major source of methane to the oceans, but rather acted as a dynamic seal, regulating methane release from deep geological reservoirs.
    Type: Article , PeerReviewed
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    DOI: 10.1038/s41467-017-02550-9
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Periodic changes in the Cretaceous ocean and climate caused by marine redox see-saw (2019)
    Nature Research
    In:  Nature Geoscience, 12 (6). pp. 456-461.
    Details
    Publication Date: 2022-01-31
    Description: Periodic changes in sediment composition are usually ascribed to insolation forcing controlled by Earth’s orbital parameters. During the Cretaceous Thermal Maximum at 97–91 Myr ago (Ma), a 37–50-kyr-long cycle that is generally believed to reflect obliquity forcing dominates the sediment record. Here, we use a numerical ocean model to show that a cycle of this length can be generated by marine biogeochemical processes without applying orbital forcing. According to our model, the restricted proto-North Atlantic and Tethys basins were poorly ventilated and oscillated between iron-rich and sulfidic (euxinic) states. The Panthalassa Basin was fertilized by dissolved iron originating from the proto-North Atlantic. Hence, it was less oxygenated while the proto-North Atlantic was in an iron-rich state and better oxygenated during euxinic periods in the proto-North Atlantic. This redox see-saw was strong enough to create significant changes in atmospheric pCO2. We conclude that most of the variability in the mid-Cretaceous ocean–atmosphere system can be ascribed to the internal redox see-saw and its response to external orbital forcing.
    Type: Article , PeerReviewed
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    DOI: 10.1038/s41561-019-0359-x
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    Permian–Triassic mass extinction pulses driven by major marine carbon cycle perturbations (2020)
    Nature Research
    Details
    Publication Date: 2023-02-08
    Description: The Permian/Triassic boundary approximately 251.9 million years ago marked the most severe environmental crisis identified in the geological record, which dictated the onwards course for the evolution of life. Magmatism from Siberian Traps is thought to have played an important role, but the causational trigger and its feedbacks are yet to be fully understood. Here we present a new boron-isotope-derived seawater pH record from fossil brachiopod shells deposited on the Tethys shelf that demonstrates a substantial decline in seawater pH coeval with the onset of the mass extinction in the latest Permian. Combined with carbon isotope data, our results are integrated in a geochemical model that resolves the carbon cycle dynamics as well as the ocean redox conditions and nitrogen isotope turnover. We find that the initial ocean acidification was intimately linked to a large pulse of carbon degassing from the Siberian sill intrusions. We unravel the consequences of the greenhouse effect on the marine environment, and show how elevated sea surface temperatures, export production and nutrient input driven by increased rates of chemical weathering gave rise to widespread deoxygenation and sporadic sulfide poisoning of the oceans in the earliest Triassic. Our findings enable us to assemble a consistent biogeochemical reconstruction of the mechanisms that resulted in the largest Phanerozoic mass extinction.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    Serpentine alteration as source of high dissolved silicon and elevated δ30Si values to the marine Si cycle (2020)
    Nature Research
    Details
    Publication Date: 2023-02-08
    Description: Serpentine alteration is recognized as an important process for element cycling, however, related silicon fluxes are unknown. Pore fluids from serpentinite seamounts sampled in the Mariana forearc region during IODP Expedition 366 were investigated for their Si, B, and Sr isotope signatures (δ 30 Si, δ 11 B, and 87 Sr/ 86 Sr, respectively) to study serpentinization in the mantle wedge and shallow serpentine alteration to authigenic clays by seawater. While serpentinization in the mantle wedge caused no significant Si isotope fractionation, implying closed system conditions, serpentine alteration by seawater led to the formation of authigenic phyllosilicates, causing the highest natural fluid δ 30 Si values measured to date (up to +5.2 ± 0.2‰). Here we show that seafloor alteration of serpentinites is a source of Si to the ocean with extremely high fluid δ 30 Si values, which can explain anomalies in the marine Si budget like in the Cascadia Basin and which has to be considered in future investigations of the global marine Si cycle.
    Type: Article , PeerReviewed
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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