Chemical Geology (v.324-325, #C)

Reprint of: New Applications of Trace Metals as Proxies in Marine Paleoenvironments by Thomas J. Algeo; Jennifer Morford; Anna Cruse (1-5).

Recent studies have identified a range of new applications of trace-metal concentration data in the analysis of paleoceanographic systems. In restricted anoxic marine systems, trace-metal/TOC ratios can provide insight into the degree of watermass restriction and estimates of deepwater renewal times. In such systems, secular changes in sediment trace-metal ratios may provide evidence of the chemical evolution of basinal deepwaters in response to differential rates of trace-metal removal to the sediment. The degree of deepwater restriction in silled basins is generally controlled by eustatic elevations, with higher (lower) sea levels resulting in lesser (greater) watermass restriction. Short-term (ca. million-year) drawdown of the trace-metal inventory of seawater has occurred repeatedly in conjunction with oceanic anoxic events, which result in elevated rates of trace-metal removal to widespread anoxic facies. Long-term (eon-scale) changes of the trace-metal composition of seawater have occurred in response to secular changes in atmospheric–oceanic redox conditions and their effects on trace-metal cycling. Caution must be exercised in evaluating trace-metal patterns in paleomarine systems, however: (1) hydrographic analyses based on trace-metal/TOC relationships can be undertaken only on systems that had anoxic deepwaters, and (2) the influence of redox variation on trace-metal accumulation patterns outweighs that of hydrographic factors in some paleomarine systems.► Sediment trace-metal data allow reconstruction of paleohydrographic properties. ► Properties include deepwater renewal time and aqueous trace-metal concentrations. ► Secular variation in trace metals provides clues to long-term Earth-system changes.
Keywords: Molybdenum; Uranium; Vanadium; Zinc; Anoxia; Watermass restriction;

The redox-sensitive transition element molybdenum is present in the Earth's crust at trace concentrations but is abundant in seawater, marine sediments, and ancient sedimentary rocks. These occurrences have led to the development of a suite of paleoredox proxies based on the bulk concentration of Mo, its isotopic composition, and the covariation of Mo with total organic carbon (TOC) in ancient black shales. However, these proxies have almost exclusively targeted the identification and interpretation of euxinic environments, where bottom waters are both anoxic and sulfidic. Here we present a discussion of the Mo geochemistry of non-euxinic sediments, where hydrogen sulfide is sometimes present yet always restricted to pore waters. We propose a new paleoredox application that uses Mo concentrations to help distinguish between environments where sulfide was present in the bottom waters and environments where sulfide was restricted to pore waters. Under ideal conditions, it is also possible to infer the presence of Mn-oxides recycling in non-euxinic paleoenvironments based on the Mo isotope composition of ancient black shales, providing a critical constraint on bottom water oxygenation.► In non-euxinic environments sulfide is restricted to the sediments. ► In euxinic environments sulfide is present in the bottom waters. ► In non-euxinic environments Mo enrichments remain below 25 ppm. ► In euxinic environments Mo enrichments exceed 25 ppm. ► Mo isotopes can be used to identify presence of Mn-oxide cycling.
Keywords: Molybdenum; Euxinic; Non-euxinic; Black shales;

This study investigates the controls on organic carbon and molybdenum (Mo) accumulation in sediments deposited within the Western Interior Seaway across the Cenomanian–Turonian boundary interval (94.34–93.04  Ma) including Oceanic Anoxic Event 2 (OAE2). Carbon fluxes to the sediment–water interface (reflecting changes in primary productivity) and bottom-water oxygen concentrations (reflecting preservation effects) are reconstructed from field data and used to constrain a benthic model that simulates the geochemistry of unconsolidated sediments as they were deposited. The results show that increased availability of reactive iron prevents Mo sequestration as thiomolybdate (MoS4 2 −) during OAE2 (O2  ~ 105 μM) by (i) inhibiting sulfate reduction, and (ii) buffering any free sulfide that becomes available. In the post-OAE2 period (O2  ~ 50 μM), Mo accumulation is favored by a large reduction in iron flux. Importantly, this occurs in parallel with oxygenated bottom waters and high rates of aerobic carbon degradation in the surface sediments, implying that elevated Mo burial fluxes in ancient marine facies do not necessarily reflect euxinic or even anoxic conditions within the water column. Our findings suggest that both an increase in production and preservation lead to enrichment in organic carbon in the Western Interior Seaway. More generally, the results demonstrate that a careful consideration of the coupling between iron, carbon and oxygen cycles during the early stages of diagenesis is critical for interpreting geochemical proxies in modern and ancient settings.►Reaction-transport modeling is used to reconstruct Mo and C diagenesis in Cretaceous sediments. ► Molybdenum accumulation is not a robust proxy for bottom water anoxia. ► Molybdenum accumulation is determined by reactive Fe and C availability. ► Models provide a mechanistic interpretation of paleoproxy signals.
Keywords: Cretaceous; Ocean Anoxic Event 2; Model; Molybdenum; Iron; Sediment;

Patterns of uranium–molybdenum covariation in marine sediments have the potential to provide insights regarding depositional conditions and processes in paleoceanographic systems. Specifically, such patterns can be used to assess bottom water redox conditions, the operation of metal-oxyhydroxide particulate shuttles in the water column, and the degree of water mass restriction. The utility of this paleoenvironmental proxy is due to the differential geochemical behavior of U and Mo: (1) uptake of authigenic U by marine sediments begins at the Fe(II)–Fe(III) redox boundary (i.e., suboxic conditions), whereas authigenic Mo enrichment requires the presence of H2S (i.e., euxinic conditions), and (2) transfer of aqueous Mo to the sediment may be enhanced through particulate shuttles, whereas aqueous U is unaffected by this process. In the present study, we examine U–Mo covariation in organic-rich sediments deposited mostly in the western Tethyan region during oceanic anoxic events (OAEs) of Early Jurassic to Late Cretaceous age. Our analysis generally confirms existing interpretations of redox conditions in these formations but provides significant new insights regarding water mass restriction and the operation of particulate shuttles in depositional systems. These insights will help to address contentious issues pertaining to the character and origin of Mesozoic OAEs, such as the degree to which regional paleoceanographic factors controlled the development of the OAEs.► We examine U–Mo covariation in Tethyan, organic-rich sediments of Mesozoic OAEs. ► We provide new insights regarding water mass restriction in some depositional systems. ► We provide new insights regarding the operation of particulate shuttles. ► These insights regard issues pertaining to the character and origin of Mesozoic OAEs.
Keywords: Sedimentary geochemistry; Redox conditions; Anoxia; Oceanic anoxic event; Jurassic; Cretaceous;

The geochemistry of black shales can faithfully record paleoenvironmental conditions. However, the original geochemical signatures are obscured by post-depositional events such as diagenesis and weathering. Resolving the relative influence of these processes on the geochemistry of shales requires a combination of geochemical and chemometric (chemical statistics) approaches to data analysis that identify the sources of variance in major and trace element geochemistry. The Permian Gungri Shale (Kuling Group) at Attargoo (Spiti Valley, Himachal Pradesh, India) is a gray to black shale capped, unconformably by an iron-rich pebble-sand layer (“ferruginous layer”) marking, ostensibly, the Permian–Triassic boundary. High resolution analysis of the Gungri at Attargoo (~ 3 cm intervals) reveals a geochemical record complicated by low-grade metamorphism/diagenesis and modern weathering. Using multivariate chemometric techniques (e.g., Principal Component Analysis) we identified variance in the geochemical record associated with the original detrital and depositional conditions. We also identified 4 beds within the Gungri that mark “depositional events” which may correlate to those identified in other PT sections (e.g., Guryul Ravine, Kashmir). Identification of sources of variance allowed us to identify geochemical signatures that can be used for paleooceanographic reconstruction. The Attargoo section records a sequence of discrete events including transient iron-enrichments attributed to euxinic depositional conditions and late diagenetic formation of siderite and regression condensation surfaces associated with pulsed transgression–regression towards the uppermost Permian. Our results show that chemometric analysis of geochemical data enables the resolution of paleoenvironmental conditions in shales impacted by post-depositional alteration.► The Gungri Shale at Attargoo has not been impacted by K2O metasomatism ► Iron-enrichments occur in several beds up to 30 cm below the Permian–Triassic boundary ► Paleoenvironmental indicators such as Ce-anomaly do not appear affected by post-depositional alteration ► Event strata in the Gungri Shale at Attargoo can be resolved using a combination of geochemical and statistical approaches
Keywords: Black Shale; Geochemistry; Multivariate statistics; Permian Gungri Shale;

Rhenium geochemical cycling: Insights from continental margins by Jennifer L. Morford; William R. Martin; Caitlin M. Carney (73-86).
We measured rhenium (Re) in sediments (both pore waters and solid phase) from three locations on the Mid-Atlantic Bight (MAB) from the eastern margin of the United States: a northern location on the continental shelf off Massachusetts (OC426, 75 m water depth), and two southern locations off North Carolina (EN433-1, 647 m water depth and EN433-2, 2648 m water depth). These sediments underlie high oxygen bottom waters (250–270 μM), but become reducing below the sediment–water interface due to the relatively high organic carbon oxidation rates in sediments. Greater ‘reduction intensity’ is reflected by shallower oxygen penetration, greater ammonium concentrations at depth, and shallower first appearances of Mn and Fe in pore waters. This comparison suggests that for the three sites along the MAB the ‘reduction intensity’ from greatest to least is: EN433-1 > OC426 > EN433-2.All of the pore water profiles show removal of Re from pore waters. Modeled pore water Re fluxes follow the trend in reducing conditions (EN433-1: 2.0 ± 0.2 pmol/cm2/y, OC426: 0.9 ± 0.1 pmol/cm2/y, and EN433-2: 0.60 ± 0.03 pmol/cm2/y, respectively). Solid phase profiles show authigenic Re enrichment in sediments from all three locations, and the calculated Re accumulation rates also mirror the trend in reducing conditions, with the values decreasing from EN433-1 > OC426 > EN433-2 (1.5 ± 0.2 pmol/cm2/y; 0.29 ± 0.09 pmol/cm2/y; 0.19 ± 0.09 pmol/cm2/y, respectively).Controls on Re accumulation are determined from the MAB data and from the literature, and include: 1) the extent of reducing conditions, with greater removal of Re from pore waters with greater anoxic organic carbon oxidation as determined by the net dissolved ammonium production, consistent with the expected removal of pore water Re coincident with Fe(III) and sulfate reduction; 2) irrigation, which results in net removal of Re to sediments, augments the diffusive flux of Re across the sediment–water interface, and is more important in shallow, coastal sediments rather than locations from deeper water depths (MAB sites); and 3) seasonal remobilization of Re from surface sediments due to deepening oxygen penetration depth (from 0.2 cm to 1 cm), although smaller seasonal changes in oxygen penetration depth (e.g., 0.2 cm to 0.6 cm) are less conducive for remobilizing Re from sediments.There is an empirical relationship with larger Re accumulation rates present when the oxygen penetration depth is shallow. However, there is a stronger correlation between organic carbon oxidation and Re accumulation rates. This latter relationship is more appropriate, since the oxidation of labile organic matter in sediments dictates the sedimentary reducing conditions that are then conducive for Re accumulation in the solid phase.► Greater Re uptake from pore water as anoxic organic carbon oxidation rate increases. ► Irrigation yields net Re removal to sediments; more important in coastal sediments. ► Seasonal Re remobilization from surface sediments due to deepening O2 penetration. ► Re accumulation rate increases with increasing organic carbon (Corg) oxidation rate.
Keywords: Rhenium; Redox-sensitive trace metals; Continental margin sediments; Mid-Atlantic Bight; Oxygen penetration depth; Organic carbon oxidation;

Anomalous molybdenum isotope trends in Upper Pennsylvanian euxinic facies: Significance for use of δ98Mo as a global marine redox proxy by Achim D. Herrmann; Brian Kendall; Thomas J. Algeo; Gwyneth W. Gordon; Laura E. Wasylenki; Ariel D. Anbar (87-98).
The use of molybdenum isotope data (δ98Mo) from organic-rich shales to draw inferences concerning marine paleoredox conditions at a global scale is predicated upon the assumptions of (1) a residence time of Mo in seawater much greater than the ocean mixing time, and (2) quantitative removal of Mo from a strongly euxinic ([H2S]aq  > 11 μM) water column to the sediment, thus preserving the seawater δ98Mo signature. In this study we analyze Mo isotopic variation in the Hushpuckney Shale, a 73-cm-thick unit representing the late transgressive to early regressive stages of a glacio-eustatic cyclothem (Swope Formation) deposited in the Late Pennsylvanian Midcontinent Sea (LPMS) of North America. The Hushpuckney can be subdivided into four stratigraphic zones of distinctive geochemical character. Zones I and III, which accumulated under weakly euxinic conditions, acquired relatively high δ98Mo values (+0.9 to +1.1‰), whereas Zone II, which accumulated under intensely euxinic conditions, acquired lower δ98Mo values (~+0.6‰). Zone IV, which accumulated under suboxic conditions in the water column, acquired the heaviest δ98Mo values (+1.1 to +1.8‰). These results contrast with the pattern of redox — δ98Mo covariation in modern marine environments, in which the heaviest δ98Mo values are found in the most intensely euxinic facies.We evaluate three different hypotheses to account for the Mo isotopic patterns of the Hushpuckney Shale. One hypothesis, seawater–freshwater mixing, is rejected owing to isotopic mass balance considerations. A second hypothesis is a local control on δ98Mo by water-column redox cycling of Mn, with particulate Mn-oxyhydroxides adsorbing isotopically light Mo and transferring it to the sediment, a process that was most active during deposition of Zone II. The significance of this scenario is that euxinic black shales may not reliably record global seawater δ98Mo in areas where a Mn-particulate shuttle is operative. A third hypothesis is based on rapid secular variation of the Mo isotope composition of Late Pennsylvanian global seawater. In order to account for δ98Mo trends within the Hushpuckney Shale, seawater δ98Mo must have varied by ~ 1.2‰ at a ~ 100-kyr timescale, which would have been possible only if the residence time of Mo in Late Pennsylvanian seawater was < 100 kyr. Although both the second and third hypotheses are viable based on the present limited δ98Mo dataset, we discuss how each model might be tested through additional Mo isotope data.► The lowest δ98Mo values are observed in the euxinic facies of cyclothems ► This is contrary to modern day euxinic sediments (which have high δ98Mo values). ► Results are inconsistent with a simple redox control on Mo isotopes in epeiric seas. ► Different paleoceanographic hypotheses can explain the Mo geochemistry in epeiric seas.
Keywords: Cyclothem; Black shale; Swope Formation; Superestuarine circulation; Epeiric sea; Hushpuckney Shale;

U/Mo ratios and δ98/95Mo as local and global redox proxies during mass extinction events by Lian Zhou; Paul B. Wignall; Jie Su; Qinglai Feng; Shucheng Xie; Laishi Zhao; Junhua Huang (99-107).
When used in conjunction with trace metal redox proxies, the use of U/Mo ratios and δ98/95Mo values can considerably enhance the interpretation of ocean oxygenation trends both locally and globally. This is illustrated with studies from two, biogenic-chert dominated records of mass extinction: the Ordovician–Silurian boundary and the Late Permian in sections from the Yangtze Block in South China. The latest Ordovician mass extinction is well known to coincide with oxygenation episode in the Hirnantian Stage here revealed by trace metal proxies (V/V + Ni, V/Cr, Ni/Co) that show it sandwiched between anoxic/euxinic black shales. The predominantly anoxic/euxinic Ordovician–Silurian strata provide a record of seawater δ98/95Mo values that are seen to gradually rise during the interval from 0.5 to 1.0‰. The exception to the δ98/95 Mo stability is the datum of the slight negatively δ98/95Mo, associated with decreasing of V/Cr and V/(V + Ni) ratio suggesting the Hirnantian oxygenation event. The Upper Permian chert succession at Shangsi reveals an overall better ventilated record in which the best oxygenated strata show the highest U/Mo values that is interpreted as relative enrichment of U in suboxic conditions. At a few levels anoxic/euxinic conditions show the δ98/95Mo values to be ~ 2‰, close to modern values, implying that latest Permian oceans were as well ventilated as those today. The well-known end-Permian oceanic anoxic event is therefore suggested to only develop at the end of this period.► Investigated the variation of palaeo-oxygenation indexes and δ98/95Mo during two extinction events ► Calibrated the U/Mo ratio using established redox trace metal proxies ► The used in conjunction with trace metal redox proxies and δ98/95Mo values ► Enhanced the interpretation of ocean oxygenation trends both locally and globally
Keywords: U/Mo ratio; Redox index; Ordovician; Silurian; Permian; South China;

Chemical signals for oxidative weathering predict Re–Os isochroneity in black shales, East Greenland by Svetoslav Georgiev; Holly J. Stein; Judith L. Hannah; Hermann M. Weiss; Bernard Bingen; Guangping Xu; Elin Rein; Vibeke Hatlø; Helge Løseth; Micaela Nali; Stefan Piasecki (108-121).
Re and Os isotopes in shales are increasingly used for precise and accurate geochronology and for tracking rates of continental weathering in the geologic past. Moreover, trace element compositions of organic-rich shales reveal paleoredox conditions and properties of shale organic matter (OM) useful for lithostratigraphic correlations. Chemical weathering alters the original shale composition and hence may compromise geologic interpretations based on geochemical data. Here we compare Re–Os isotopes, trace elements and OM properties in correlative shales from outcrops and nearby drillcore to quantify the effects of weathering. We define chemical parameters that distinguish weathered from pristine shale in samples that are macroscopically identical.Black shale from the Ravnefjeld Formation in East Greenland yields disturbed Re–Os isotope systematics for outcrop samples, and highly precise Re–Os isochron ages for drillcore. Geochemical data show that Re, Os and common Os are hosted in the organic matter rather than sulfides. Re and Os both show mobility during weathering. Combined use of Rock-Eval pyrolysis indices and sulfur contents documents oxidation of both OM and pyrite, thereby providing a valuable chemical criterion to assess invisible shale weathering. Further studies on chemically diverse shales will test the widespread application of these criteria as weathering indicators.Systematic differences in major and trace element content and kerogen quality between weathered and fresh shale samples are used to characterize and quantify weathering effects. The character of the OM in shales is most telling. Ravnefjeld data are compared with data from time correlative shales from the mid-Norwegian shelf [Georgiev et al., 2011. Hot acidic Late Permian seas stifle life in record time. Earth Planet. Sci. Lett., 310, 389–400] to present a strategy for identification of Re–Os isochroneity in advance of Re–Os analytical work.Our recent compilation and comparison of Upper Permian shales from widely separated sequences on Pangea's margins reveal exceptionally high Re/Os ratios on a global scale. We suggest Upper Permian shales may have significantly influenced the Cenozoic (e.g. Eocene) 187Os/188Os ratio of seawater on their exposure and weathering.► Re–Os isochroneity is used to distinguish fresh from weathered Late Permian shales. ► Weathering results in pyrite loss and oxidation of organic matter. ► Kerogen character signals subtle weathering better than major or trace element data. ► Both Re and Os reside in organic matter and are mobile during shale weathering. ► High Re/Os shales may release highly radiogenic Os to seawater during weathering.
Keywords: Shale weathering; Re–Os; Rock-Eval; Trace elements; Late Permian; East Greenland;

Traditionally, analytical techniques such as wavelength-dispersive x-ray fluorescence (WD-XRF), inductively-coupled plasma (ICP) and mass spectrometry (ICP-MS), instrumental neutron activation analysis (INAA), or stationary (bench-top) energy-dispersive x-ray fluorescence (ED-XRF) have been used to generate quantitative geochemical results. However, a more efficient means of data collection using portable ED-XRF instrumentation now allows the investigator to acquire rapid, non-destructive, quantitative measurements on drill core and clean, flat rock surfaces, in addition to pressed powder pellets typically used in WD-XRF analysis. Similar to traditional XRF methods, quantification using the handheld ED-XRF requires a matrix-specific calibration. Unfortunately, very few internationally-accepted mudrock or shale reference materials exist, and their elemental ranges provide inadequate coverage for the geochemical diversity of mudrocks. In order to return reliable, calibrated results, a unique set of reference materials has been developed that incorporates a wide range of mudrock elemental compositions. The current method provides elemental calibrations for major elements heavier than sodium, and the following trace elements: Ba, V, Cr, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Mo, Th, and U.A comparison of handheld energy-dispersive and wavelength-dispersive x-ray fluorescence (ED-XRF and WD-XRF) results from pressed powder pellets of Mississippian-age Barnett Shale of North-Central Texas, USA, is presented in order to evaluate the reliability of the reference calibration and the quantification of unknown samples using two different instrument platforms. It will be demonstrated that calibrated results from the handheld ED-XRF effectively define chemostratigraphic changes in real-time. As a consequence, quantified results can be used immediately to assess changes in bulk mineralogy, paleo-redox conditions, and to link down-core geochemical changes to stratigraphic, sedimentological, and paleoenvironmental observations.► Development of a diverse suite of mudrock geochemistries. ► Development of a 24-element handheld ED-XRF calibration for analyzing drill cores and flat mudrock surfaces. ► Application of calibrated results to answering paleoceanographic questions.
Keywords: Major and trace elements; Calibration; Shale; Mudstone;

Digestion methods for trace element measurements in shales: Paleoredox proxies examined by Guangping Xu; Judith L. Hannah; Bernard Bingen; Svetoslav Georgiev; Holly J. Stein (132-147).
Trace elements such as U, Mo, Re, V and Ni in marine shales are commonly used to probe paleoenvironmental conditions. Trace elements may have either a detrital or seawater source, or both. The seawater contribution is often modelled from whole-rock sediment analyses. Using whole-rock trace element ratios as paleoredox proxies, however, presents limitations. In this study, we compare three different methods of digestion using Middle Triassic black shales from Svalbard and Svalis Dome, and Late Permian black shales from the mid-Norwegian shelf and East Greenland. Multi-acid total digestion provides whole-rock compositions, including both detrital and seawater contributions. Aqua regia and inverse aqua regia digestions dissolve organic matter, sulfides and carbonates, leaving behind major detrital silicate components. Thus, trace element concentrations by aqua regia digestion provide a direct, though still imperfect approximation of the seawater component.For most trace elements, the concentrations and their ratios derived from aqua regia and inverse aqua regia digestions agree within 10%, roughly the uncertainty of the analytical procedures. However, inverse aqua regia may not dissolve apatite completely, which leads to slightly lower Ca, P, Sc, U and Pb contents than aqua regia digestion. Multi-acid whole-rock total digestion and aqua regia partial digestion yield comparable abundances of P, S, Co, Ni, Cu, Zn, Mn, Mo and Pb, which are dominantly of seawater origin in our samples. However, concentrations of U, V, Sc and Cr derived from aqua regia digestion are up to 80% lower than those from whole-rock digestion, suggesting significant contributions from both detrital and seawater sources.We conclude that: (1) trace metals released by aqua regia or inverse aqua regia predominantly reflect seawater components, and thus have value as paleoredox proxies, (2) if the absolute trace metal concentrations are used to infer the paleoredox conditions, the total amount of organic carbon and sulphur must be taken into account, and (3) threshold values for trace metal ratios based on whole-rock compositions (e.g., Th/U, V/Sc, V/V + Ni) are not globally applicable to infer redox conditions, as these ratios reflect a combination of both detrital and seawater components. ► Aqua regia effectively releases trace metals of seawater origin from black shale. ► Aqua regia may also dissolve or leach trace metals from certain silicates. ► Aqua regia and inverse aqua regia digestions yield similar concentrations. ► U, V, Sc and Cr may derive significant contributions from detrital sources. ► Ratios involving U, V, Sc, Cr from bulk composition may not reflect paleoredox conditions.
Keywords: Black shale; Trace element; Paleoredox; Organic matter; Aqua regia; Digestion;