Browsing by Author "Lepland, Aivo"

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    Effects of early marine diagenesis and site-specific depositional controls on carbonate-associated sulfate: Insights from paired S and O isotopic analyses
    (Elsevier, 2021-12) Richardson, Jocelyn A.; Lepland, Aivo; Hints, Olle; Prave, Anthony R.; Gilhooly, William P., III; Bradley, Alexander S.; Fike, David A.; Earth Science, School of Science
    Carbon, sulfur and oxygen isotope profiles in Silurian strata of the Baltoscandian Basin (Estonia), coincident with the Ireviken Bioevent, provide insights into basin-scale and platform-specific depositional processes. Paired carbon isotope records preserve a positive isotope excursion during the early Wenlock, coincident with faunal turnover, yet δ13C variability of this excursion compared to other locations within the paleobasin reflects local depositional influences superimposed on a global signal. In comparison, sulfur isotope records do not preserve a systematic isotopic excursion over the same interval. Instead, sulfur isotope records have high sample-to-sample stratigraphic variability, particularly in shallow-water carbonate rocks (scatter up to ~10‰ for δ34SCAS and ~ 25‰ for δ34Spyr). This pattern of isotopic variability is also found between sites from the same carbonate platform, where the magnitude and isotopic variability in δ34SCAS and δ34Spyr differ depending on relative local sea level (and therefore facies). Such facies-dependent variability reflects more closed- versus more open-system diagenetic conditions where pulses of increased sedimentation rate in the shallow water environments generates greater isotopic variability in both δ34SCAS and δ34Spyr. Increased reworking and proximity to the shoreline results in local sulfide oxidation, seen as a decrease in δ34SCAS in the most proximal settings. Platform-scale evolution of isotopically distilled pore-fluids associated with dolomitization results in increased δ34SCAS in deep water settings. Correlations in paired δ34SCAS-δ18OCAS data support these conclusions, demonstrating the local alteration of CAS during deposition and early marine diagenesis. We present a framework to assess the sequence of diagenetic and depositional environmental processes that have altered δ34SCAS and find that δ34S of ~27–28‰ approximates Silurian seawater sulfate. Our findings provide a mechanism to understand the elevated variability in many deep-time δ34SCAS records that cannot otherwise be reconciled with behavior of the marine sulfate reservoir.
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    Effects of early marine diagenesis and site-specific depositional controls on carbonate-associated sulfate: Insights from paired S and O isotopic analyses
    (Elsevier, 2021) Richardson, Jocelyn A.; Lepland, Aivo; Hints, Olle; Prave, Anthony R.; Gilhooly, William P., III; Bradley, Alexander S.; Fike, David A.; Earth and Environmental Sciences, School of Science
    Carbon, sulfur and oxygen isotope profiles in Silurian strata of the Baltoscandian Basin (Estonia), coincident with the Ireviken Bioevent, provide insights into basin-scale and platform-specific depositional processes. Paired carbon isotope records preserve a positive isotope excursion during the early Wenlock, coincident with faunal turnover, yet δ13C variability of this excursion compared to other locations within the paleobasin reflects local depositional influences superimposed on a global signal. In comparison, sulfur isotope records do not preserve a systematic isotopic excursion over the same interval. Instead, sulfur isotope records have high sample-to-sample stratigraphic variability, particularly in shallow-water carbonate rocks (scatter up to ~10‰ for δ34SCAS and ~ 25‰ for δ34Spyr). This pattern of isotopic variability is also found between sites from the same carbonate platform, where the magnitude and isotopic variability in δ34SCAS and δ34Spyr differ depending on relative local sea level (and therefore facies). Such facies-dependent variability reflects more closed- versus more open-system diagenetic conditions where pulses of increased sedimentation rate in the shallow water environments generates greater isotopic variability in both δ34SCAS and δ34Spyr. Increased reworking and proximity to the shoreline results in local sulfide oxidation, seen as a decrease in δ34SCAS in the most proximal settings. Platform-scale evolution of isotopically distilled pore-fluids associated with dolomitization results in increased δ34SCAS in deep water settings. Correlations in paired δ34SCAS-δ18OCAS data support these conclusions, demonstrating the local alteration of CAS during deposition and early marine diagenesis. We present a framework to assess the sequence of diagenetic and depositional environmental processes that have altered δ34SCAS and find that δ34S of ~27–28‰ approximates Silurian seawater sulfate. Our findings provide a mechanism to understand the elevated variability in many deep-time δ34SCAS records that cannot otherwise be reconciled with behavior of the marine sulfate reservoir.
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    Iron cycling in Arctic methane seeps
    (Springer, 2020) Hong, Wei-Li; Latour, Pauline; Sauer, Simone; Sen, Arunima; Gilhooly, William P., III; Lepland, Aivo; Fouskas, Fotios; Earth Sciences, School of Science
    Anoxic marine sediments contribute a significant amount of dissolved iron (Fe2+) to the ocean which is crucial for the global carbon cycle. Here, we investigate iron cycling in four Arctic cold seeps where sediments are anoxic and sulfidic due to the high rates of methane-fueled sulfate reduction. We estimated Fe2+ diffusive fluxes towards the oxic sediment layer to be in the range of 0.8 to 138.7 μmole/m2/day and Fe2+ fluxes across the sediment-water interface to be in the range of 0.3 to 102.2 μmole/m2/day. Such variable fluxes cannot be explained by Fe2+ production from organic matter–coupled dissimilatory reduction alone. We propose that the reduction of dissolved and complexed Fe3+ as well as the rapid formation of iron sulfide minerals are the most important reactions regulating the fluxes of Fe2+ in these cold seeps. By comparing seafloor visual observations with subsurface pore fluid composition, we demonstrate how the joint cycling of iron and sulfur determines the distribution of chemosynthesis-based biota.