Marine redox conditions during deposition of Late Ordovician and Early Silurian organic-rich mudrocks in the Siljan ring district, central Sweden

Lu, Xuanyu; Kendall, Brian; Stein, Holly J.; Li, Chao; Hannah, Judith L.; Gordon, Gwyneth W.; Ebbestad, Jan Ove R.

doi:10.1016/j.chemgeo.2017.03.015

Cited by 43 publications

(11 citation statements)

References 173 publications

(345 reference statements)

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“…Over the last decade, the uranium (U) isotope system ( 238 U/ 235 U) has emerged as a valuable marine redox proxy, which, when coupled to an isotope mass balance approach, has been used to estimate the global extent of anoxic seafloor area in past oceans (e.g., Bartlett et al, 2018; Brennecka et al, 2011; Clarkson et al, 2018; Elrick et al, 2017; Kendall et al, 2015; Lau et al, 2016, 2017; Lu et al, 2017; Montoya‐Pino et al, 2010; Wei et al, 2018; White et al, 2018; Yang et al, 2017; Zhang, Xiao, et al, 2018; Zhang, Algeo, et al, 2018; Zhang, Romaniello, et al, 2018). Because U has a modern marine residence time on the order of ~400,000 kyr (Ku et al, 1977)—orders of magnitude longer than ocean mixing times—the concentration and isotopic composition of U in the open marine system should be relatively homogenous, potentially providing insight about global‐scale processes.…”

Section: Introductionmentioning

confidence: 99%

Uranium Isotope Fractionation in Non‐sulfidic Anoxic Settings and the Global Uranium Isotope Mass Balance

Cole

Planavsky

Longley

et al. 2020

Global Biogeochemical Cycles

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Uranium isotopes (238U/235U) have been used widely over the last decade as a global proxy for marine redox conditions. The largest isotopic fractionations in the system occur during U reduction, removal, and burial. Applying this basic framework, global U isotope mass balance models have been used to predict the extent of ocean floor anoxia during key intervals throughout Earth's history. However, there are currently minimal constraints on the isotopic fractionation that occurs during reduction and burial in anoxic and iron‐rich (ferruginous) aquatic systems, despite the consensus that ferruginous conditions are thought to have been widespread through the majority of our planet's history. Here we provide the first exploration of δ238U values in natural ferruginous settings. We measured δ238U in sediments from two modern ferruginous lakes (Brownie Lake and Lake Pavin), the water column of Brownie Lake, and sedimentary rocks from the Silurian‐Devonian boundary that were deposited under ferruginous conditions. Additionally, we provide new δ238U data from core top sediments from anoxic but nonsulfidic settings in the Peru Margin oxygen minimum zone. We find that δ238U values from sediments deposited in all of these localities are highly variable but on average are indistinguishable from adjacent oxic sediments. This forces a reevaluation of the global U isotope mass balance and how U isotope values are used to reconstruct the evolution of the marine redox landscape.

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Section: Introductionmentioning

confidence: 99%

Uranium Isotope Fractionation in Non‐sulfidic Anoxic Settings and the Global Uranium Isotope Mass Balance

Cole

Planavsky

Longley

et al. 2020

Global Biogeochemical Cycles

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“…All ORM units in Figure 3 lack evidence of strong Fe-Mn particulate influence on δ 98 Mo (Mo is not greatly enriched relative to U; after [6]), thus lighter δ 98 Mo is likely a function of weakly euxinic conditions [46,48,[50][51][52][53] and in one case may also reflect a possible local hydrothermal source [47]. Additionally, if particulate shuttling was active during the deposition of these ORM, it is expected that the V-δ 98 Mo trend would resemble that observed in Figure 2 (strong negative correlation), however, a weak (but not insignificant; p-value = 0.0001) positive trend is noted instead.…”

Section: Hyper-enrichments In Euxinic Semi-restricted Basinsmentioning

confidence: 99%

“…A comparison of coeval geological units suggests this scenario is probably not correct. The terminal Ediacaran Blovice Complex and other Ara Group locations [47][48][49], Late Ordovician Fjäcka Shale [50], Late Devonian Bakken Formation (not in compilation because no Mo isotope data is available; [9]), Early Jurassic Gordondale Member (not in compilation; [69]), and Pleistocene Sapropels at Site 969D [53] are approximately coeval with the units depicted in Figure 4 and feature V hyper-enrichments.…”

Section: Ancient Localities Without V Hyper-enrichmentsmentioning

confidence: 99%

Molybdenum Isotope Constraints on the Origin of Vanadium Hyper-Enrichments in Ediacaran–Phanerozoic Marine Mudrocks

2020

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Vanadium is an important redox-sensitive trace metal for paleoenvironmental reconstructions. Modern organic-rich sediments persistently contain sediment V enrichments <500 μg/g, but many ancient marine organic-rich mudrocks record enrichments >500 μg/g. Previous studies propose that ancient V enrichments of these magnitudes (“V hyper-enrichments”) were deposited from hyper-sulfidic bottom-waters with higher H2S levels (≥10 mM) than observed in modern euxinic basins. To test the importance of hyper-sulfidic conditions for generating V hyper-enrichments, we compare V concentrations with Mo isotope (δ98Mo) compositions from mudrock samples ranging in age from Ediacaran to Pleistocene. In the modern ocean, sediments deposited from strongly euxinic bottom waters ([H2S]aq > 11 μM) closely record global seawater δ98Mo because conversion of molybdate to tri- and tetra-thiomolybdate is quantitative. By contrast, large Mo isotope fractionations occur during Mo adsorption to Fe-Mn particulates or because of incomplete formation of the most sulfidic thiomolybdates in weakly euxinic settings ([H2S]aq < 11 μM), which both favor removal of lighter-mass Mo isotopes to sediments. We find multiple examples when mudrocks with V hyper-enrichments are associated with a wide range of δ98Mo for a single time interval, including values at or below oceanic input δ98Mo (0.3–0.7‰). This observation suggests significant isotopic offset from reasonable seawater values (typically ≥1.0‰). Thus, we conclude that hyper-sulfidic conditions were not responsible for many V hyper-enrichments in Ediacaran–Phanerozoic mudrocks. Instead, sediment V hyper-enrichments can be explained by high Fe-Mn particulate fluxes to weakly euxinic sediments or by moderately restricted euxinic settings with strongly euxinic ([H2S]aq > 11 μM but not necessarily > 10 mM) or weakly euxinic (with slow clastic sedimentation rates and high organic carbon fluxes) bottom waters where vigorous water exchange provides a continuous V supply from the open ocean.

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“…The ratio of the two long-lived isotopes of U ( 238 U/ 235 U; expressed as  238 U, which is the per mil 238 U/ 235 U deviation of a sample relative to the CRM-112a standard) has been extensively used to track the global extent of oceanic anoxia through time (Stirling et al 2007;Weyer et al 2008;Montoya-Pino et al 2010;Brennecka et al 2011;Asael et al 2013;Kendall et al 2013Kendall et al , 2015Kendall et al , 2020Andersen et al 2014Andersen et al , 2016Andersen et al , 2018Dahl et al 2014Dahl et al , 2017Dahl et al , 2019Azmy et al 2015;Tissot and Dauphas 2015;Hood et al 2016;Lau et al 2016Lau et al , 2017Wang et al 2016Wang et al , 2018Wang et al , 2020Elrick et al 2017;Jost et al 2017;Lu et al 2017;Song et al 2017;Yang et al 2017;Bartlett et al 2018;Bura-Nakić et al 2018Chen et al 2018aChen et al , 2018bClarkson et al 2018;Herrmann et al 2018;Phan et al 2018;Wei et al 2018Wei et al , 2020White et al 2018;Zhang et al 2018aZhang et al , 2018bZhang et al , 2018cZhang et al , 2019a…”

Section: Introductionmentioning

confidence: 99%

The uranium isotopic record of shales and carbonates through geologic time

Chen

Tissot

Jansen

et al. 2021

Geochimica et Cosmochimica Acta

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This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Marine redox conditions during deposition of Late Ordovician and Early Silurian organic-rich mudrocks in the Siljan ring district, central Sweden

Abstract: Marine redox conditions during deposition of Late Ordovician and Early Silurian organic-rich mudrocks in the Siljan ring district, central Sweden. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Chemge(2017),

Cited by 43 publications

References 173 publications

Uranium Isotope Fractionation in Non‐sulfidic Anoxic Settings and the Global Uranium Isotope Mass Balance

Uranium Isotope Fractionation in Non‐sulfidic Anoxic Settings and the Global Uranium Isotope Mass Balance

Molybdenum Isotope Constraints on the Origin of Vanadium Hyper-Enrichments in Ediacaran–Phanerozoic Marine Mudrocks

The uranium isotopic record of shales and carbonates through geologic time

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