Molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the Early Jurassic

Pearce, Christopher R.; Cohen, A. S.; Coe, Angela L.; Burton, Kevin W.

doi:10.1130/g24446a.1

Cited by 231 publications

(233 citation statements)

References 27 publications

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“…During the Toarcian OAE, for example, the Re and Mo enrichments of ORM are muted, indicating drawdown of the oceanic Re and Mo reservoirs in response to expanded anoxia (Pearce et al, 2008;Owens et al, 2016). Conversely, during the Cretaceous OAEs 1a and 2, which were associated with LIP emplacement, Re concentrations of ORM are often not muted and are mildly correlated with excursions to higher Os concentrations and unradiogenic 187 Os/ 188 Os, suggesting delivery of magmatic Re (and Os) to seawater in sufficient quantities to offset the increased burial of Re (and Os) into anoxic sediments (Turgeon and Creaser, 2008;Bottini et al, 2012;du Vivier et al, 2014;Kendall, 2014).…”

Section: Temporal Trends In Re Concentrationsmentioning

confidence: 99%

A model for the oceanic mass balance of rhenium and implications for the extent of Proterozoic ocean anoxia

Sheen

Kendall

Reinhard

et al. 2018

Geochimica et Cosmochimica Acta

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Emerging geochemical evidence suggests that the atmosphere-ocean system underwent a significant decrease in O 2 content following the Great Oxidation Event (GOE), leading to a mid-Proterozoic ocean (ca. 2.0-0.8 Ga) with oxygenated surface waters and predominantly anoxic deep waters. The extent of mid-Proterozoic seafloor anoxia has been recently estimated using mass-balance models based on molybdenum (Mo), uranium (U), and chromium (Cr) enrichments in organic-rich mudrocks (ORM). Here, we use a temporal compilation of concentrations for the redox-sensitive trace metal rhenium (Re) in ORM to provide an independent constraint on the global extent of mid-Proterozoic ocean anoxia and as a tool for more generally exploring how the marine geochemical cycle of Re has changed through time. The compilation reveals that mid-Proterozoic ORM are dominated by low Re concentrations that overall are only mildly higher than those of Archean ORM and significantly lower than many ORM deposited during the ca. 2.22-2.06 Ga Lomagundi Event and during the Phanerozoic Eon. These temporal trends are consistent with a decrease in the oceanic Re inventory in response to an expansion of anoxia after an interval of increased oxygenation during the Lomagundi Event. Mass-balance modeling of the marine Re geochemical cycle indicates that the mid-Proterozoic ORM with low Re enrichments are consistent with extensive seafloor anoxia. Beyond this agreement, these new data bring added value because Re, like the other metals, responds generally to lowoxygen conditions but has its own distinct sensitivity to the varying environmental controls. Thus, we can broaden our capacity to infer nuanced spatiotemporal patterns in ancient redox landscapes. For example, despite the still small number of data, some mid-Proterozoic ORM units have higher Re enrichments that may reflect a larger oceanic Re inventory during transient episodes of ocean oxygenation. An improved understanding of the modern oceanic Re cycle and a higher temporal resolution for the Re compilation will enable further tests of these hypotheses regarding changes in the surficial Re geochemical cycle in response to variations in atmosphere-ocean oxygenation. Nevertheless, the existing Re compilation and model results are in agreement with previous Cr, Mo, and U evidence for pervasively anoxic and ferruginous conditions in mid-Proterozoic oceans.

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Section: Temporal Trends In Re Concentrationsmentioning

confidence: 99%

A model for the oceanic mass balance of rhenium and implications for the extent of Proterozoic ocean anoxia

Sheen

Kendall

Reinhard

et al. 2018

Geochimica et Cosmochimica Acta

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“…[11] Our work here on Toarcian events also reinterprets the d 98 Mo from Pearce et al [2008] Reitz et al [2007].…”

Section: Working Modelmentioning

confidence: 99%

“…The uncertainty of the analyses lie within the size of the symbols used to plot concentrations on our figures. We calculated degree of pyritization based on total Fe (DOP-T) as 0.95TS/(Total Fe) to allow for organic S. For the exaratum Subzone, we supplemented our data with the Mo, TOC, d 98 Mo, Re, and TOC data of Pearce et al [2008] who sampled at a higher density than we did in that interval. The methods of analysis used by Pearce et al [2008] are documented in their paper and its Supplementary Information.…”

Section: Chemical Analysismentioning

confidence: 99%

Basinal restriction, black shales, Re‐Os dating, and the Early Toarcian (Jurassic) oceanic anoxic event

et al. 2008

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[1] Profiles of Mo/total organic carbon (TOC) through the Lower Toarcian black shales of the Cleveland Basin, Yorkshire, United Kingdom, and the Posidonia shale of Germany and Switzerland reveal water mass restriction during the interval from late tenuicostatum Zone times to early bifrons Zone times, times which include that of the putative Early Toarcian oceanic anoxic event. The degree of restriction is revealed by crossplots of Mo and TOC concentrations for the Cleveland Basin, which define two linear arrays with regression slopes (ppm/%) of 0.5 and 17. The slope of 0.5 applies to sediment from the upper semicelatum and exaratum Subzones. This value, which is one tenth of that for modern sediments from the Black Sea (Mo/TOC regression slope 4.5), reveals that water mass restriction during this interval was around 10 times more severe than in the modern Black Sea; the renewal frequency of the water mass was between 4 and 40 ka. The Mo/TOC regression slope of 17 applies to the overlying falciferum and commune subzones: the value shows that restriction in this interval was less severe and that the renewal frequency of the water mass was between 10 and 130 years. The more restricted of the two intervals has been termed the Early Toarcian oceanic anoxic event but is shown to be an event caused by basin restriction local to NW Europe. Crossplots of Re, Os, and Mo against TOC show similar trends of increasing element concentration with increase in TOC but with differing slopes. Together with modeling of 187 Os/ 188 Os and d 98 Mo, the element/TOC trends show that drawdown of Re, Os, and Mo was essentially complete during upper semicelatum and exaratum Subzone times (Mo/TOC regression slope of 0.5). Drawdown sensitized the restricted water mass to isotopic change forced by freshwater mixing so that continental inputs of Re, Os, and Mo, via a low-salinity surface layer, created isotopic excursions of up to 1.3% in d 98 Mo and up to 0.6% for 187 Os/ 188 Os. Restriction thereby compromises attempts to date Toarcian black shales, and possibly all black shales, using Re-Os chronology and introduces a confounding influence in the attempts to use d 98 Mo and initial 187 Os/ 188 Os for palaeo-oceanographic interpretation.

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“…They Lias metals anomaly paper Version: Final proposed a lipped sill linked to the Neo-Tethys Ocean so that the sill restricted full oceanic overturn in the shallow epeiric sea except during oxygenation events during periods of higher sea-level. An alternative hypothesis (Jenkyns, 1988;Jenkyns and Clayton, 1997;Cohen et al, 2004;Pearce et al, 2008) (Weedon, 1986;Smith, 1989;Paul et al, 2008;Ruhl et al, 2010;Bonis et al, 2010). Similar limestone-shale cycles described form the Lower Jurassic Redcar Mudstone Formation (Cleveland Basin) have also been interpreted as a result of orbitally induced cyclicity (Van Buchem et al, 1994), although in the Cleveland Basin the siliciclastic beds are less rich in organic material (Powell, 1980;2010).…”

Section: Palaeogeography and Sedimentationmentioning

confidence: 99%

“…There has been much debate concerning the origin and distribution in space and time of Lower Jurassic euxinic black shales and their metal content. For instance, in the stratigraphically higher Early Toarcian oceanic/regional anoxic event (Jenkyns, 1988, Cohen et al, 2004McArthur et al, 2008;Pearce et al, 2008;Caruthers et al, 2013) of the Cleveland Basin (Powell, 2010), it is uncertain whether the causal mechanism for variation in Mo/TOC and associated metals in black shales is due to local/regional or global anoxia on the seafloor from the semicelatum through exaratum Subzones (Jet Rock) estimated at about 930 kyr duration (McArthur et al, 2000;Cohen et al, 2004). One hypothesis (McArthur et al, 2008;Wignall et al, 2005) favours regional sea-floor anoxia in restricted silled basins (Küspert, 1982) that developed below a pycnocline of lowsalinity, fresh water resulting from enhanced fluvial run-off from the hinterland.…”

Section: Palaeogeography and Sedimentationmentioning

confidence: 99%

Anomalous enrichment of molybdenum and associated metals in Lower Jurassic (Lias Group) black shales of central England, as revealed by systematic geochemical surveys

Breward¹,

Kemp

Ambrose

et al. 2015

Proceedings of the Geologists' Association

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Molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the Early Jurassic

Cited by 231 publications

References 27 publications

A model for the oceanic mass balance of rhenium and implications for the extent of Proterozoic ocean anoxia

A model for the oceanic mass balance of rhenium and implications for the extent of Proterozoic ocean anoxia

Basinal restriction, black shales, Re‐Os dating, and the Early Toarcian (Jurassic) oceanic anoxic event

Anomalous enrichment of molybdenum and associated metals in Lower Jurassic (Lias Group) black shales of central England, as revealed by systematic geochemical surveys

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