PP31C-1495
Chromium Isotope Stratigraphy of the Toarcian OAE From Yorkshire, NE England
Recent advances in mass spectrometry now permit the routine measurement of a very wide range of stable isotope ratios and provide a new approach for the investigation of paleoclimatic and paleoenvironmental changes from sedimentary records. Transition-metal trace elements are sensitive indicators of redox conditions, controlled by chemical processes that can be dramatically altered in response to environmental change. Chromium has particular advantages as a tracer of past ocean-water oxygenation because its two principal stable oxidation states show very different chemical behaviour in the natural environment. Chromium is expected to act in a similar way to molybdenum, for which published data shows a 2 permille excursion in the 98Mo/95Mo ratio across the Early Jurassic (Toarcian) Oceanin Anoxic Event (OAE) interpreted as a drawdown of marine Mo into a sediment sink as euxinic conditions envelop the water column (Pearce et al., 2008). The two elements have different oceanic residence times; approximately 800,000 years for Mo and about 10,000 years for chromium, raising the possibility of investigating the onset of anoxia in unprecedented detail. However, to obtain accurate and precise measurements a method for extracting chromium from rock samples that delivers a high yield is required. A high yield is important to ensure that the natural signal is not masked by isotope fractionation induced by the separation process. We have recently developed a method which consistently delivers chromium yields of 95% and greater from sedimentary samples. Here we present preliminary results for chromium abundance and 53Cr/52Cr isotope ratios across the Jurassic, Toarcian OAE, from the Jet Rock section at Hawsker Bottoms, Yorkshire, UK. Samples were collected at a 10 cm spacing giving an equivalent temporal resolution of approximately 6,000 years.
PP31C-1496
Basinal Restriction, Black Shales, and the Early Toarcian (Jurassic) Oceanic Anoxic Event.
Profiles of Mo/TOC through the Lower Toarcian black shales of the Cleveland Basin, Yorkshire, UK, 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. In this interval, faunal diversity was reduced because of the long period of restriction. The degree of restriction is revealed best by cross-plots of Mo and TOC concentrations for the Cleveland Basin, which define two linear arrays with regression slopes (ppm per %) of 0.5 and 17. The slope of 0.5 is one-tenth that for modern sediments from the Black Sea (Mo/TOC 4.5) and reveals severe water-mass restriction during upper-semicelatum and exaratum Sz. times, a period often known as the Early Toarcian oceanic anoxic event. The frequency of water-mass renewal was between 4 and 40 kyrs, a frequency ten time lower than in the current Black Sea. The Mo/TOC regression slope of 17 for the overlying falciferum and commune Szs. shows lesser restriction and renewal times of 10 – 130 years i.e. substantially more frequent than before. The putative Early Toarcian oceanic anoxic event (OAE) is thus shown not to be global but to reflect an event local to NW Europe caused by basin restriction. Cross-plots of Re/TOC, Os/TOC, and Mo/TOC are similar and, together with modelling of 187Os:188Os and del98Mo, show that drawdown of Re, Os, and Mo, was essentially complete during the more restricted interval (Mo/TOC slope 0.5). Drawdown sensitized the restricted water-mass to isotopic change so that continental inputs of Re, Os, and Mo, via a low-salinity surface layer, created isotopic excursions of up to 1.3 ‰ in del98Mo and up to 0.6 for 187Os:188Os. 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 del98Mo and initial-187Os/188Os for palaeo-oceanographic interpretation of organic- rich strata.
PP31C-1497
Timing and evolution of ocean anoxic event during Early Cambrian in south China
The Precambrian/Cambrian (PC-C) interval is one of the most interesting intervals in the evolution of life because of the sudden diversification of animals with mineralized skeletons, known as "Cambrian Explosion". The Yangtze Platform in south China is one of the best occurrences that can provide excellent insights into the palaeo-environmental and biological changes across the PC-C boundary. Our study show that the ocean anoxia were widespread during the Early Cambrian period, however, the start of this anoxic event was not from the PC-C boundary (i.e., 542 Ma), but some 7 Ma later (~535 Ma) when the Niutitang Formation black rock series (black phosphorite, chert, and black shale) deposited along a thousand kilometer long NEE zone in the transitional facies in the Yangtze Platform, while the major Cambrian radiation (Changjiang fauna) took place during 521-511 Ma. During the Niutitang period, the depositional environment of the Early Cambrian sedimentary sequence in south China have evolved from an initial oxic/dysoxic to a major anoxic/euxinic environment, and then back to dysoxic/oxic environment. A Ni-Mo sulfide layer occurred in the lower part of the Niutitang black shales which contains extremely enrichments of many metals, and can serve as a marker layer in south China when the depositional environment turned into euxinic condition. Re-Os isotope study of the sulfide ores and host black shales show an age of 535 Ma. Initial Os isotopic compositions, Mo isotopic compositions, and rare earth elements and Pt group element geochemistry suggest involvement of submarine hydrothermal fluids during the metal enrichments in black shale.
PP31C-1498
Oceanic hypoxia and mass extinction at the Cenomanian-Turonian boundary triggered by a flare-up of arc volcanism
The Cenomanian-Turonian Boundary (CTB) at 93.5 Ma was characterized by global depletion of oxygen in the deep oceans as evidenced by global deposition of black shales and the mass extinction of benthic organisms. These conditions have been attributed to a combination of greenhouse-induced oceanic stagnation and high biological productivity. Because the CTB overlapped in time with the emplacement of the submarine Caribbean flood basalt province (95-88 Ma), the flood basalt-mass extinction coincidence is generally thought to be causal. The hypothesis is that oxygen was immediately consumed in the deep ocean by direct oxidation of reduced Fe in the basalt or, alternatively, such Fe was leached out and transported to surface waters, enhancing biological productivity. However, oxygen consumption by direct or indirect oxidation of Fe in submarine flood basalts is shown here to be difficult, complicating this causal link. We present here an alternative hypothesis based on the under-appreciated coincidence in time between the CTB and a flare-up of continental arc volcanism along the North America margin, which extended from southern Mexico up through Canada. Unlike submarine flood basalt volcanism, arc volcanoes generate ash, which is erupted into the stratosphere and deposited around the world. We show that the magnitude of the ash flux was sufficient enough to fertilize surface waters with reduced Fe, thereby generating a global phytoplanktonic bloom, which exceed the threshold of biological productivity above which bottom waters would be depleted of oxygen and benthic organisms suffocated. We conclude that although the CTB was coincident with both submarine flood basalt volcanism and subaerial arc volcanism, the latter process may have been more effective at triggering oxygen depletion and mass extinction. Mass extinctions are often thought to be triggered by catastrophic and episodic processes, such as flood basalts and meteor impacts, hence our hypothesis, if correct, suggests that more mundane plate tectonic processes, such as subduction and arc volcanism, may occasionally cause mass extinctions.
PP31C-1499
Fractionation of Sulfur Isotopes in the Cariaco Basin
The detailed distribution of sulfur isotopes was studied in the water column of the Cariaco Basin, with high depth resolution across the redox transition zone, in Nov. 2007 and May 2008 as part of the on-going CARIACO (CArbon Retention In A Colored Ocean) time series project. Long term monitoring (1995 to present) of the vertical position of the chemocline (200-350 m) and thickness of the suboxic zone (0-50 m) reveals highly dynamic spatial and temporal variability tied to the influx of oxygenated water into the restricted basin. The link between the basin hydrography and chemoautotrophic activity is potentially expressed in chemical speciation and composition of sulfur compounds cycled within the chemocline. For Nov. 2007 cruise, we measured the δ34S of total dissolved sulfide (δ34SH2S) and sulfate (δ34SSO4), and the δ18O of sulfate (δ18OSO4) from water samples. In close agreement with results from the Black Sea, δ34SH2S, δ34SSO4 and δ18OSO4 were relatively constant in the deep anoxic water, with the δ34SH2S depleted in 34S relative to δ34SSO4 by roughly 50 permil. However, near the oxic-anoxic interface, δ34SH2S was 3 permil higher than that in the deep water due to sulfide oxidation and/or in situ sulfide production. In May 2008, we collected water samples to measure three sulfur isotopes (33S, 34S and 36S) in sulfate, sulfide and elemental sulfur to explore the contribution of biological processes (sulfate reduction, sulfur intermediates disproportionation, and chemoautotrophic sulfur oxidation) to the isotopic composition of sulfur compounds in the basin. Also during the same cruise, an oxygen intrusion event was observed and we will present sulfur isotope data associated with that event.
PP31C-1500
Hypoxic Conditions off the Oregon coast - A Modern Occurrence or Part of Larger Oceanographic Cycles?
Coastal shelf hypoxic events are toxic environments for the benthic community resulting in die-offs of fish and other organisms. Since 2000, hypoxic (<0.5 ml l-1 oxygen) to anoxic conditions have been observed off the Oregon coast, though historical records from 1950 to 1999 show no previous occurrences of these conditions. While other hypoxic events (i.e. the Gulf of Mexico) have been attributed to anthropogenic nutrient pollution, it has been postulated that the shelf hypoxia off the Oregon coast is driven by basin-scale fluctuations in atmosphere-ocean processes. These fluctuations have altered the oxygen content and intensity of upwelling in the region, corresponding to increases in productivity and slow oxygen replenishment. This study seeks to construct past records to determine if longer records reveal water column anoxia prior to 1950 and to determine if human influence, changes in ocean circulation, or long term climatic cycles are at play. To identify past anoxic events, trace metal concentrations, specifically redox-sensitive metals such as uranium, vanadium, and molybdenum, along with export production proxies have been analyzed. In addition, carbon and nitrogen isotopic ratios have been measured throughout the core. These preliminary results will provide a baseline for identifying the cyclicity and extent of anoxic cycles throughout the Holocene. The ability to predict the recurrence and evolution of these events will be critical for the formulation of appropriate mitigation measures.
PP31C-1501
Rapid Oxic/Anoxic Changes in the Sediment-Water Interface During the Eocene- Oligocene Transition
The Eocene-Oligocene Transition (EOT) refers to a 500 kyr period of time, which includes the Eocene- Olicgocene Boundary, stretching from 33.5-34.0 Ma (Pearson et al. 2008). It includes all of the changes associated with the expansion of Antarctic glaciation and the concurrent changes in marine fauna and productivity, as well as any physical oceanographic changes that occurred. Shallow-level drilling recovered cores from southern Tanzania and Java, Indonesia. The cores sampled outer-shelf to slope, organic-rich marine clays which show excellent preservation of microfossils and a very low thermal maturity. As the drill sites are from either side of the Indian Ocean, at approximately 10° south of the equator, the comparison of results from these sites will allow insights into the global or regional nature of oceanic signals. Results from the Tanzanian cores have highlighted rapid oscillations in the oxic/anoxic nature of the sediment-water interface throughout the EOT. This is currently interpreted as a direct result of changes occurring in the ocean currents surrounding Antarctica. In the present, Sub-Antarctic Mode Waters (SAMW) deliver high-latitude climatic signals to the tropical Indian Ocean (Dunkley Jones et al., 2008). We suggest that during the EOT these Antarctic currents were in an infantile state, switching on and off during the transitional period, until cooling reached a stable state and the currents were able to fully establish. This could have caused the oxygenation state of the Indian Ocean bottom waters to oscillate during the EOT, only being oxygenated in a stable way after 33.5 Ma. However, the cores from Java will help us verify whether these signals are global or regional in nature. Recent studies have indicated that the heightened productivity of the time is more global than regional, and does not occur just in the high latitudes around Antarctica. Preliminary results from Indonesia indicate that the same increase in productivity was occurring in the Indian Ocean, suggesting that a similar oscillation in oxic/anoxic conditions may have occurred there too.
PP31C-1502
Observations of the Effect of Non-steady State Injections of Oxygen Into Anoxic Waters of the Cariaco Basin, Venezuela
Traditionally, the Cariaco Basin has been considered to be a classic example of an anoxic basin where transport of organic rich material from the surface layers to depth, restricted vertical mixing, and anaerobic diagenesis dominate. Many studies explicitly or implicitly assume that distributions of chemical species and microbial activity change relatively gradually and that sediment properties solely reflect processes in the overlying water. However the CARIACO time series has repeatedly obtained evidence that intruding oxygenated water must be extremely important in controlling both water chemistry and microbial activity. In May 2008 repeated cruises took place over a period of weeks which clearly demonstrated that relatively large volumes of oxygenated water had recently intruded to depths of at least 300 m over a period of days to weeks. We saw clear evidence of deep oxygen maxima, minima in methane and sulfide concentrations below the oxygen/sulfide interface and apparent perturbations in other measured parameters including nutrients, sulfur species and microbial activity. Data suggest, among other things, that microbial activity lags production of sulfur intermediates like elemental sulfur, implying that chemoautotrophic bacteria in the system may use elemental S. We will present data on changes in hydrography, nutrient distributions, microbial rates and other variables resulting from the intrusions and which allow us to estimate the zone of influence of this intrusion. Ultimately, properties (such as N and S chemistry) of sediments in such systems could be affected by changes in chemistry of the system caused by lateral injections of oxidants and other material from outside the basin.
PP31C-1503
High-Resolution pCO2 Record for the Cenomanian-Turonian OAE2 based on Fossil Plant Cuticle
The Cenomanian-Turonian oceanic anoxic event 2 (OAE2) represents a major perturbation to the ocean- atmosphere-terrestrial system that lasted at least 600 ka. Marine extinction associated with OAE2 is thought to have occurred due to global marine anoxia, and increased primary production is the likely driver. However, the causal factors leading to increased production have remained poorly resolved. Recent research indicates that a pulse of submarine volcanism occurred in association with OAE2, and it has been suggested that this event led to overturn of the stratified ocean, possibly bringing P-rich deep waters to the surface. 208/204Pbinitial ratios taken from the Bonarelli horizon suggest that CO2 expelled by the volcanic event may have come from the Caribbean LIP. This is supported by a rapid negative shift in both 186/187Os and 87/86Sr isotopes just prior to OAE2, suggesting a marine source for the CO2 pulse. Some prior studies suggested that OAE2 resulted in a major drawdown of high ambient pCO2 levels. Only recently has a volcanic-sourced CO2 pulse been suggested for the event and its implications have yet to be fully explored. Two previous studies attempted to reconstruct pCO2 levels through OAE2 using geochemical proxy methods. Both studies show a decline in pCO2 at the onset of the δ13C excursion, but with decreases ranging from 140 to at least 400ppmV. Since neither study extended analyses to the interval preceding onset of the δ13C excursion it is difficult to assess the longer term trend. In addition, marine-based proxy methods contain large uncertainties that may be avoided in a non-marine proxy of pCO2, such as the stomatal frequency record of fossil plant cuticles. The stomatal based paleo-CO2 proxy relies on a genetically controlled inverse relationship between pCO2 and stomatal frequency in terrestrial plant leaves. Abundant plant cuticle is preserved in the paralic sections of the Dakota SS in SW Utah, which has been correlated to the Cenomanian-Turonian GSSP in Colorado via bio-, chemo-, and tephro-stratigraphy. Stomatal index and density data collected from Dakota SS cuticle material suggest a steady, long term increase in pCO2 through the Late Cenomanian, with a rapid pCO2 pulse just prior to the onset of OAE2. A less rapid pCO2 decline is observed within the OAE2 interval. The CO2 pulse seen in fossil plants prior to the event coincides with changes in Pb, Os, and Sr isotopes, and is consistent with evidence for elevated ocean temperatures derived from δ18O and Tex86 data in the tropical Atlantic.
PP31C-1504
Response of marine biota to a period of oceanic anoxia during the Toarcian (Early Jurassic)
The early Toarcian Oceanic Anoxic Event (OAE; 183 Ma) was associated with a species level extinction of marine fauna and a crisis in the marine phytoplankton. The event lasted c. 250 ka and was characterised by a large, negative C-isotope excursion (CIE) of ~-7 per mil in marine organic matter, marine carbonates and fossilized wood. Geochemical evidence suggests that there was a contemporaneous increase in seawater temperature of 6-13° C that was accompanied by a large increase in the rate of global weathering. The present study documents changes in marine macrofauna in the early Toarcian at a high resolution and explores how species composition and biometric measurements are linked to geochemical changes. Reanalysis of the published palaeontological data for the Toarcian OAE suggests three apparent extinction horizons on a global and regional scale. The youngest of these horizons coincides exactly with the initial decrease in δ13C, and with the initial increases in sea surface temperature, continental weathering rates and seawater anoxia. New species range data were collected during this study from Toarcian sections in N Yorkshire, England. The results show distinct relationships with high resolution geochemical datasets (Cohen et al. 2007; Pearce et al. 2008). For example, there was an almost complete absence of fauna for 1750-12500 years immediately after each of the four abrupt shifts that make up the overall CIE. Only one bivalve species, Pseudomytiloides dubius, occurs in high abundance throughout the event, except within these discrete horizons. Increased epifaunal bivalve diversity and the reappearance of infauna indicate a brief return to relatively oxygenated conditions towards the end of the CIE. Biometric data were obtained for the two dominant bivalve species P. dubius and Bositra radiata from over 226 stratigraphic levels across the event. The data show that shell size is related to fluctuating seawater anoxia as recorded from Mo abundance and Mo-isotope data. These relationships are apparent both over the duration of the entire event and on 20 ka time-scales. Cohen, A. S. et al. 2007. J. Geol. Soc. 164 Pearce, C. R. et al. 2008. Geology 36
PP31C-1505
Distinctive stable carbon isotope ratios indicative of carbon-cycle perturbations at the Permian-Triassic boundary
Though there have been many proposed causes for the severe biotic crisis at the Permian-Triassic boundary (PTB), significant evidence exists for the presence of anoxic oceans coincident with the event. This evidence, in the form of distinct sediment lithologies , C and S isotopic anomalies and biomarker evidence of both anoxic sedimentary deposition and lipids produced by organisms that thrive in anoxic environments is prevalent in PTB sections from a range of palaeolocations. The organic input to the sedimentary record that is deposited beneath a stratified ocean with anoxic deep waters is distinctly different relative to that of a fully ventilated ocean because of fundamental differences in carbon and nutrient cycling that occur under each of these conditions. Arguably one of the most useful tools for understanding the cycling of elements in ancient ecosystems is the stable isotope ratio of compounds in the rock record. The C-isotopic ratios of two groups of compounds, n-alkane hydrocarbons and isoprenoid hydrocarbons, have provided insight into the carbon cycling at the Proterozoic-Phanerozoic boundary, since these compounds are produced by heterotrophs and autotrophs with different isotopic values. In numerous Proterozoic marine sections, the n-alkanes are enriched relative to the isoprenoids, a reversal of the pattern predicted by biosynthetic relationships. Previously, this reversal has been interpreted to signify significant heterotrophic reworking of the organic matter high in the water column, consistent with a stratified ocean with deep-water anoxia. A similar reversal in the isotopic relationship was found at a PTB section in the Perth Basin, Western Australia in samples from the Hovea-3 well (Grice et al., 2005). Here we report on the C-isotopic analysis of n-alkanes and isoprenoids from a locations where biomarker evidence has been well documented supporting the presence of hydrogen sulfide in the water column at P-T sections in the Peace River Basin, Canada. Preliminary results confirm an isotopic reversal similar that observed in Perth Basin samples. These data provide further support for ocean anoxia at this time period, and significantly, show a correlation between these conditions and the carbon cycle perturbations associated with the extinction event. Grice, K., et al. (2005). Science 307: 706-709.
PP31C-1506
Climatic Control of Oceanic Suboxic Zones on Centennial to Million Year Timescales
Suboxic intermediate waters of the world's oceans, though constituting a small fraction of total volume, dominate important redox-sensitive biogeochemical processes. They host a large proportion of marine denitrification (heterotrophic as well as anammox) and thus are a significant control on overall oceanic combined nitrogen balance. Climatic forcing on a variety of time scales can be studied through paleo- records of denitrification intensity. Denitrification increases the 15N/14N ratio (δ15N) of the residual nitrate and this signal is transferred to and preserved in underlying sediments. In the Northern ETP (ETNP), δ15N varies strongly with glacial-interglacial cycles for the past 4 Ma but shows dramatic decrease in average and amplitude of variation prior to establishment of a strong ETP cold tongue. In the Southern ETP off Peru (ETSP), past denitrification intensity has showed a very sharp and early rise at the onset of the last deglaciation. These events in the ETNP and ETSP are not correlated with productivity proxies. These regions also have a high degree of similarity in their δ15N records which is also supportive of common forcing through SAMW ventilation. Together these results indicate important interactions and feedback between global climate and suboxic zone extent and intensity. The Peru system is nevertheless unique as compared to the other large, oceanic suboxic zones in having clear, centennial to millennial-scale oscillation in denitrification intensity especially during the late Holocene. This variability is likely due to the particular ENSO sensitivity of this region and these records are evidence for longer period versions of the known interannual and decadal-scale phenomena. In this case, strong linkage with productivity is evident and suggestive of the mechanism for the fastest responses to future climate change.
PP31C-1507
Condition for Global Ocean Anoxia Obtained From a One Dimensional Ocean Biogeochemical Cycle Model
Ocean anoxic events (OAEs) have occurred sporadically during the Phanerozoic. Previous studies proposed several oxygen-depleted mechanisms, such as stagnant ocean circulation, high primary productivity in surface ocean, sea level change, and low atmospheric oxygen level. There is a possibility that OAEs may have been caused by combination of these factors. There are however very few quantitative studies which examine the effects of these mechanisms. In this study, we constructed a vertical one-dimensional ocean biogeochemical cycle model to investigate conditions for the occurrence of anoxia in the ocean. In addition to the biogeochemical processes in oxic environments, this model includes decomposition processes of particulate organic matter due to nitrate and sulfate and oxidation processes of ammonium and hydrogen sulfide in aerobic water column. We divided the ocean anoxic conditions into "intermediate water anoxia" and "deep water anoxia" based on the vertical profile of dissolved oxygen in the water column. We investigate these two conditions systematically with respect to the two critical parameters, that is, ocean ventilation intensity and riverine nutrient (phosphorus) supply rate, which have been proposed as promising candidates for the mechanisms of ocean anoxic events. The effect of the redox condition on reactive phosphorus burial in seafloor sediments and the reduced solubility of oxygen due to increased sea surface temperature are also examined. Because of the characteristic behaviors of marine phosphorus cycle under the anoxic ocean condition, we found that primary production can be enhanced in spite of stagnant ocean circulation. Our results also suggest that the positive feedback between oxygen concentration, phosphorus regeneration, and primary productivity plays an important role in controlling the oceanic redox conditions.
PP31C-1508
Molybdenum Cycling in Upwelling Sediments: An Example from Namibian Margin Sediments
The paleo-redox application of molybdenum (Mo) isotopes is strongly tied to our knowledge of the modern marine Mo cycle. Elemental mass balance indicates that ~47% of the Mo supplied to the oceans is removed to deep sea sediments, leaving the remaining Mo to "near-shore" reducing sediments (1). The Black Sea is likely the best studied reducing environment with regards to Mo isotopes, yet accounts for only a small fraction of the Mo mass balance. The accumulation of Mo in continental margin sediments has been recently re-assessed and may account for a larger fraction of the marine Mo reservoir than previously thought (2). In the presence of sulfide, the molybdate anion is transformed, by the replacement of oxygen with sulfur, to particle reactive oxy-thiomolybdates (3). This is often cited as the mechanism by which Mo removal proceeds in the Black Sea where sulfide concentrations in the water are high. In contrast, in continental margin settings, the removal mechanism is poorly understood, and the extent to which sulfur cycling plays a role remains un-quantified. To better understand removal/cycling processes in a continental margin setting, where sulfide may only be present in the pore waters and not in the water column, Mo was studied in an array of marine settings off the Namibian coast. Surface sediments were collected across a transect from near-shore/high productivity to deep water/low productivity sediments. These sediments were incubated in bag experiments to study the relationship between sulfur and Mo cycling. Molybdenum concentrations in the Namibian sediments range from detrital values at the lowest productivity site to 25 ppm in surface sediments with high productivity. Preliminary results allude to a correlation between sulfate reduction rates and Mo accumulation in these sediments. Detailed studies of Mo, Mo isotopes, other trace metals, and sulfur investigations from both sediment cores and bag experiments will be presented. (1)Bertine and Turekian (1973), Geochim. Cosmochim. Acta 87, 1415. (2)McManus et al. (2006), Geochim. Cosmochim. Acta 70, 4643. (3)Erickson and Helz (2000) Geochim. Cosmochim. Acta 64, 1149.
PP31C-1509
High-resolution Reconstruction of the Bottom-water Oxygenation Condition of Panthalassa During the Early Triassic Based on Sediment Fabric Analysis of Pelagic Sequence in Central Japan
After the most severe mass extinction event in the Phanerozoic at the Permian/Triassic (P/T) boundary, biotic recovery from the extinction required about 5 Myr that corresponds to the entire Early Triassic. The absence of radiolarian bedded chert across the P/T boundary is widely recognized in the pelagic sequence of Japan and considered as resulted from prevalence of anoxic deep water. However, previous studies examined the bottom water oxygenation condition based on compilation of discontinuous sequence of fragmental lithologic intervals from several different regions. To examine the extent, duration and stability of the anoxic condition in superocean Panthalassa during the Early Triassic, high-resolution reconstruction of the continuous Lower Triassic pelagic sequence within the Jurassic accretionary complex in central Japan was conducted for the first time based on the detailed geologic mapping and lithostratigraphic correlation of the sequence in Inuyama area. The reconstructed Early Triassic pelagic sequence is approximately 11 m thick, and is divided into 7 lithologic units. Carbon isotopic analysis of total organic carbon was conducted to reconstruct high- resolution carbon isotopic record covering the entire Early Triassic, which is compared with inorganic carbon isotopic records from Tethys. The comparison suggests overall similar pattern of the two curves that allows correlation of the shallow marine sequence of Tethys with pelagic sequence of Panthalassa. We reconstructed bottom-water oxygenation condition in the deep-sea environment of Panthalassa during the Early Triassic based on the degree of lamina preservation observed on soft-X-ray radiographs. The observation revealed relatively oxygen-poor but unstable bottom-water oxygenation condition oscillating between oxic and anoxic with high frequency from Griesbachian to Smithian, followed by the gradual recovery to the oxic environment during Spathian. The most severe period of anoxic condition occurs in middle to late Griesbachian, when organic carbon isotopic ratio from Panthalassa shifts from heavier to lighter values, while oxic to suboxic conditions are dominant in late Dienerian and late Smithian, when organic carbon isotopic ratio shifts from lighter to heavier values. Organic carbon isotopic ratio does not exhibit distinct trend in middle Smithian when relatively oxygen-poor condition occurs, although some of carbonate carbon isotopic ratios reported from Tethys shift from heavier to lighter values during this period. In general, these temporal changes in deep-sea bottom-water oxygenation condition between oxic and anoxic may correspond to carbon isotopic changes between heavier and lighter values, respectively. We will discuss the relationship between occurrence of oceanic anoxic condition and carbon isotopic oscillation in shallow and deep-water environment during the Early Triassic.
PP31C-1510
Iron Isotopes in Lake Pavin (French Massif Central): A Window to the Precambrian Ocean
Consequences and timing of ocean oxygenation are still debated. It is generally considered that early ocean was anoxic and passed through a stratified state (oxic at the surface and anoxic in the deeper part) before being completely oxidized. Stratified ocean may have persisted more than 1Ga, thus representing an important period of the Earth history. The Fe isotope record in Precambrian sediments may provide constraints on the ocean evolution but a major question is what do the rocks tell us about the water column? This is an indirect way to get information and the geochemical signal may be modified later by diagenesis and metamorphism. The study of present stratified water body associated with sediments may help to decipher the isotopic signal from old sediments. Unlike marine complex environments, smaller and well-defined ecosystems such as lake can be useful to understand element transfer and processes. Lake Pavin is characterized by the presence of two stratified layers. The upper layer (mixolimnion) extends from the surface to 60m depth, and is oxidized. The deeper layer (monimolimnion), which extends from 70 to 92m depth, is permanently anoxic and separated from the mixolimnion by the mesolimnion where a drastic dissolved Fe gradient exists. In lake Pavin, Fe is oxidized and precipitated as Fe(III) in the mixolimnion. It is then dissolved in the deep layers (60 to 92m) and within the sediments and accumulates as aqueous Fe(II). Fe(II) either diffuses towards the mixolimnion or reacts with sulfides or phosphates to form FeS colloids and secondary minerals such as pyrite, vivianite or siderite. In order to characterize Fe isotope fractionations in this system, we measured Fe content and isotopic composition of aqueous Fe(II) along a vertical profile at different depths from the oxic-anoxic interface to the bottom of the lake. Fe content increases with depth, from less than 2 μmol/L above 60m depth to more than 1200 μmol/L at the bottom of the lake. δ56Fe values also increases with depth, from -1.33 to +0.31‰. The large concentration gradient associated with negative Fe isotope composition observed below the oxic-anoxic interface is interpreted as the signature of intense bacterial dissimilatory Fe reduction, which seems to be the main process controlling the variation of Fe isotope composition in the water column. In a further study, we plan to analyze sediments and associated pore water to get a full picture of the system.
PP31C-1511
Milankovitch cycles detected in the middle Triassic bedded chert (Inuyama, Japan) and its relation with bottom water oxygenation changes
The Permo-Triassic mass extinction is known as the largest mass extinction event in the Phanerozoic, and the Early to Middle Triassic period is characterized by the interval of delayed biotic and environmental recovery. One of the main cause of the delayed recovery is considered to have been glong-lasted anoxiah of deep ocean (Superanoxia; Isozaki, 1997). However, the reality of long-lasting ganoxiah, its maintaining mechanism, and details of the recovery process during the middle Triassic are largely unknown. In this study, we established high-resolution and continuous reconstruction of the middle Triassic bedded chert sequence that deposited in the middle of Panthalassa, and explored the possibility of the Milankovitch cycle origin of the sedimentary rhythms observed in the bedded chert sequence so as to establish the cyclostratigraphic framework for the sequence. We also examined the temporal change in the redox condition of the bottom water based on the lamina preservation index (LPI) and abundance of redox sensitive trace elements in order to reconstruct the detailed recovery process from glong-lasted anoxiah. The middle Triassic pelagic sequence in Inuyama area consists of bedded chert with periodical change in color. The sequence is divided into lower gray chert (lower Anisian), lower red chert (lower-middle Anisian), upper grey chert (middle-upper Anisian), and upper red chert (upper Anisian-Ladinian) units, in ascending order. The radiolarian biostratigraphy has been already established for the sequence by Sugiyama (1997). We examined cyclicities of chert and shale bed thicknesses using wavelet and spectral analyses. Previous studies suggested the possibility of Milankovitch cycle origin for the sedimentary rhythms of bedded chert. This idea was based on the observation that one chert-shale couplet represent about 20 ky duration, corresponding to precession cycle (e.g. Hori et al., 1993). However, agreement with just one periodicity of the Milankovitch cycle is insufficient as a proof for Milankovitch origin. So, wavelet analysis was conducted to demonstrate the dominant periodicities (with respect to bed numbers) for variations of the chert and shale thicknesses based on an assumption that one chert-shale couplet correspond to ca. 20 ky precession cycle. The results of spectrum analyses of chert and shale bed thickness revealed ca. 20 beds and 100 beds cyclicities that corresponds to 0.4 Myr and 2.4 Myr eccentricity cycles, respectively. In addition, ca. 20 beds cycle changes its length between 17 and 25 beds with the periodicity of about 100 beds. The amplitude of ca. 20 bed cycle is also modulated with the periodicity of ca. 100 beds. Similar types of modulations are observed in insolation curve (La04; Laskar et al., 2004). These results support the idea that the cyclicities observed in middle Triassic bedded chert sequence are controlled by Milankovitch cycle. We also reconstructed the bottom-water redox condition by the LPI based on the observation of the soft-X ray radiographs and by the trace metal abundance measured by the X-ray fluorescence. We will discuss the relationship between longer Milankovitch cycles and the bottom-water oxygenation changes in middle Triassic super-ocean Panthalassa.
PP31C-1512
The Early Aptian Oceanic Anoxic Event 1a at ODP Site 641C Galicia Margin, Tropical Atlantic
The most prominent example for rapid climate change related to the release of CO2 from gas hydrates in the Cretaceous is thought to be represented by an abrupt and stepped negative carbon isotope excursion at the base of the Early Aptian Oceanic Anoxic Event 1a (OAE 1a). To reconstruct the chain of processes affecting the Early Aptian land-ocean-atmosphere system in tropical regions of the Atlantic we investigated ODP Site 641C from the Galicia Margin for sea surface temperatures, organic carbon composition and burial, bulk carbon and oxygen isotopes, and inorganic markers for productivity, ocean redox and continental supply. Bulk carbon isotope data from carbonates and organic matter clearly identify the onset of OAE 1a by a marked negative δ13C shift of 1.25 permil associated with a pronounced increase in TOC from 0.5 to more than 2%. The diagnostic initial negative isotope excursion is followed by a sudden drop in carbonate content from more than 40% to close to zero and a 3 permil increase in δ13Ccarb. First TEX86-SST reconstructions support persistent warm surface waters clustering around 37°C. Elevated Ba/Al ratios suggest an overall increase in bioproductivity that commenced before the onset of OAE 1a. The relationships between TOC-Fe-S support normal marine conditions for most of the studied interval except for the termination of the event where stronger oxygen depletion is indicated. Redox-sensitive element ratios fluctuate throughout the record and are only moderately enriched compared to average shale values supporting variable and at times anoxic, but probably never sulfidic bottom water oxygenation levels. Linked climate effects are inferred from inorganic proxy data. A minimum in Mg/Al and Na/Al and high CIA values (chemical index of alteration) during the event suggest a contribution of more aluminum-rich clay minerals (e.g. kaolinite) and hence more intense continental weathering. Decreasing amplitudes of the Mg/Al, K/Al and Na/Al records starting at the onset of OAE 1a further seem to indicate a reorganization towards less variable climate conditions.
PP31C-1513
Dynamics of sulfidic and anoxic, non-sulfidic waters along the Cenomanian-Turonian NW- African shelf at Tarfaya (SW-Morocco)
Understanding the timing, mechanisms and feedbacks capable of driving parts of the ocean from oxic to either anoxic or even sulfidic (euxinic) conditions is critical to identify possible scenarios for the future ocean on a warmer Earth. The mid Cretaceous is well-known as a time of extreme greenhouse conditions, and linked to this were a series of major black shale deposition events associated with global perturbations of the carbon cycle (ocean anoxic events; OAEs). Previous high-resolution geochemical and biofacies records from the Cenomanian-Turonian NW-African shelf have shown that the redox state of the water column and associated carbon burial varied considerably on Milankovitch, and maybe even shorter, time scales. Despite these stimulating implications, the degree of oxygenation over different depth intervals of the water column, in particular the dynamics of contrasting conditions at the lower end of the redox scale, and associated effects on redox-sensitive element and nutrient cycles are still not well-constrained. We report new millennial-scale geochemical records of Fe speciation, molecular markers for photic zone euxinia (PZE) and other nutrient and redox sensitive elements from site S57, drilled close to the centre of the Tarfaya shelf basin in SW-Morocco, to address the nature and pacing of shallow marine redox cycles associated with the initial onset of OAE2. We observe previously unrecognized pronounced fluctuations between anoxic non-sulfidic and sulfidic conditions, identified as distinct repetitive enrichments in unsulfidized highly reactive Fe (anoxic non-sulfidic conditions), in a sediment column otherwise characterised by high degrees of pyritization (sulfidic conditions). These changes appear to have occurred at orbital frequencies (possibly at obliquity time scales), supporting a direct link between ocean redox and fluctuations in climate and upwelling intensity. The PZE biomarker record is highly variable over the study interval, with lowest concentrations typically in non-sulfidic sections. We speculate that periods of strong upwelling of nutrients, shoaling of the chemocline into the photic zone and enhanced carbon burial, alternated with intervals of nutrient depletion that finally led to relatively short intervals (on the order of 10,000 years) of non-sulfidic but still anoxic conditions.
PP31C-1514
Assessing Mass-dependent Isotope Fractionation of Cerium
Earth surface and ocean oxygenation is governed by a variety of processes and is recorded in the concentration-, speciation- and isotope-distribution of several redox sensitive elements in sediments throughout the history of earth. To quantify the redox state from a known distribution is complicated though, since the distribution often also is influenced by non-redox processes and other poorly constrained inputs. Hence a multi-proxy approach is required to constrain the oxidation history of earth. Here the REE (Rare Earth Elements) and yttrium patterns provide us with a valuable tool, as they both can be used as chemical fingerprint for seawater and also contain the redox sensitive element Cerium. In present days seawater Cerium is less abundant than its neighboring REE (negative cerium anomaly), since trivalent cerium is readily oxidized to the less soluble tetravalent cerium and subsequently scavenged by settling particles. The onset of a negative cerium anomaly can thus help to pinpoint, when the redox conditions of the oceans changed from reducing to oxidizing. In the present study we have developed a double spike technique to determine Cerium mass dependent isotope fractionation using a TIMS. The spike composition was chosen to minimize the propagated error in the data deconvolution and to reduce the sensitivity to barium interferences. For samples the barium interference was minimized by a oxidation-reduction of cerium on ln-resin. The Double-spike technique was compared to ICP-MS standard sample bracketing technique for pure cerium standard solutions, where the analytical precision of the δ142/140Ce was better than 0.01% (2SD). Also some oxidation, precipitation experiments at different pH have been performed in order to determine whether there is an isotopic fractionation associated with the oxidation. Preliminary results will be presented.
PP31C-1515
Biogeochemical evidence for environmental constraints on cyanobacterial populations during an Oceanic Anoxic Event
2-Methylhopanoids provide a signature for cyanobacterial contributions to organic matter in sediments as old as the Paleoproterozoic. All cyanobacteria biosynthesize hopanoids, but controls regulating production of 2- methyl analogs remain unknown. 2-MHI values (index of 2-methylhopanes to hopanes) are often high during intervals of oceanic dysoxia known as Oceanic Anoxic Events (OAE), and vary throughout the sedimentary record reflecting proportions of cyanobacteria within microbial communities. A new perspective emerges from assessment of hopanoid distributions in sediments from Shatsky Rise (ODP Site 1207) deposited in the tropical Pacific during the Early Aptian (OAE1a; 120 Myr). Here, δ15N values indicative of N2 fixation by cyanobacteria occur throughout the section, whereas 2-MHI values increase during episodes of cooler temperatures and more oxygenated waters, based on the TEX86 proxy and biomarkers, respectively. Thus, environmental changes during OAE1a affected cyanobacterial populations. In modern marine systems higher temperatures favor filamentous N2-fixing non-heterocystous cyanobacteria, and exclude heterocystous species, whereas unicellular cyanobacteria are favored by low pO2. 2-MHI variations during OAE1a suggest that N2-fixing cyanobacterial populations changed from heterocystous species during cooler, more oxygenated intervals to unicellular and non-heterocystous species during warmer, dysoxic intervals. This coupled response of hopanoids, cyanobacteria, and O2 levels during OAE bears on their relationships in earlier times, especially during evolutionary diversification of cyanobacteria on the early Earth. An origin for 2-MH in N2-fixing heterocystous cyanobacteria would explain their presence in the Late Archean during a global-scale expansion of oxygenated habitats. It would also require biosynthesis of 2-MH by these cyanobacteria prior to the initial accumulation of atmospheric O2, consistent with the fact that these these cyanobacteria are phylogenetically more evolved members of the clade. Moreover, intermittent oceanic anoxia over geological time may have served to sustain those cyanobacteria whose evolutionary inheritance best equips them for low O2 environments.
PP31C-1516
Extreme organic carbon and pyrite burial during the Toarcian OAE and possible consequences for the marine trace metal inventory
The Toarcian Ocean Anoxic Event (OAE) was a time of extreme perturbation of the global carbon cycle caused by large-scale burial of organic matter under anoxic to euxinic conditions and possibly large-scale methane release. However, many questions remain. Foremost among the remaining questions is the global versus local nature of the event. Unlike Cretaceous OAEs, the Toarcian lacks an available deep ocean record and, consequently, most studies have focused on geochemical data from stratigraphic sections in the north European epicontinental seaway (NEES). Underlying this concern is the observation that black shales within the NEES, assumed to have been deposited under euxinic conditions, show little or no enrichment in some redox-sensitive elements (e.g., Mo) beyond crustal concentrations. Since the nature of the connection between the NEES and open ocean is highly contested, muted metal enrichments can be interpreted as due to a drawdown of either global or local reservoirs. We present geochemical data from within and outside the NEES to test the global nature of the records preserved there. Carbon and sulfur isotope data (from both carbonate-associated sulfate and pyrite) from the NEES (Cleveland and Southern Germany sub-basins) and a carbonate platform on the margin of the Tethyan Ocean (Southern Italy) show parallel, positive excursions. This relationship suggests that burial of organic carbon and pyrite occurred on a scale that perturbed global budgets. Additionally, we will present stratigraphic geochemical data (DOP, Fe/Al, FeR/Fetotal, δ34Spyrite) to constrain the local redox conditions within the sub-basins of the NEES. These data will be compared with trace metal concentrations and isotope ratios to test whether enrichment patterns represent changes in local redox within the NEES or a drawdown of the global marine reservoir.
PP31C-1517
Phosphorus burial in Cariaco Basin sediments through the last glacial – interglacial transition
On geologic timescales, phosphate availability in the oceans limits global organic carbon burial rates in marine sediments. The phosphate inventory of the oceans is strongly influenced by early diagenetic regeneration of phosphate in marine sediments. Oxic conditions are generally thought to favor phosphate retention in marine sediments, whereas anoxic conditions permit a greater fraction of reactive particulate phosphate to be released to pore waters and return to the overlying water column. Feedbacks with the marine phosphorus cycle can act to stabilize ocean anoxic events and control atmospheric oxygen concentrations. We analyzed phosphorus phase associations in Cariaco Basin sediments through the transition from oxidized sediments that characterized deposition during the last glacial period through laminated anoxic sediments that characterize Holocene deposition. We have used a sequential extraction technique (SEDEX) on samples from ODP core 1002 in order to target adsorbed phosphate, ferric iron associated P, authigenic and biogenic P phases, detrital apatite, and organic P. Preliminary results show a correlation between total organic carbon and reactive phosphate in the sediments. The anoxic sediments are dominated by the authigenic and biogenic P pool, which typically represents 60-80% of the total P. These values, coupled with high C-org/P-org (> 600) and C-org/P- reactive (> 300) ratios, suggest post-depositional redistribution of P from organic matter to authigenic carbonate fluorapatite and substantial regeneration of phosphate to the overlying water column. C/P ratios are much lower in the oxic sediments, though the authigenic and biogenic P pool remains quantitatively most significant. Throughout the core, changes in C:P ratios and the concentration of authigenic P appear to correlate with changes in the Mo/Al ratio, further linking P burial and diagenesis to water column anoxia.
PP31C-1518
Suboxic conditions at the Permian-Triassic boundary in the NE Panthalassic Ocean suggest a different extinction mechanism compared to Paleotethys anoxia
The Permian-Triassic boundary marks the most important mass extinction event recorded in Earth history. Based on numerous studies of Permian-Triassic sites, most of them located around the Paleotethys, an anoxic event has been assumed to be the most likely killing mechanism. Here we present a high-resolution study of a Permian-Triassic section on the north- eastern shore of the Panthalassic Ocean. The Opal Creek shale section in SW Alberta was sampled over 40 m with a 50 cm resolution and at a 10 cm-scale around the extinction event; paleontological and geochemical data were collected. The extinction event is correlated by conodont biostratigraphy and a ~5‰ carbon isotope negative trend. The onset of suboxic/euxinic conditions is suggested by trace elements (V, Mo, U) and organic carbon data and a negative trend of non-acid volatile sulfur isotope data to a minimum of -31.2‰ just above the extinction horizon. However, this episode appears to be very short-lived as all geochemical tracers return to background values over a ~50 cm interval. Our results from the Opal Creek section seem to argue against the model of a prolonged euxinic ocean as seen in Paleotethys sections. Such discrepancy may be explained by contrasting geography and climate. The semi-closed, equatorial Paleotethys would have been much more prone to reaching euxinic conditions because of high continental nutrient delivery. On the open shore of the Panthalassic global ocean, with a much lower terrigenous input, lower temperatures and hence presumably lower bioproductivity, sustaining euxinia would have been difficult. In spite of the lack of evidence for strong prolonged anoxia, extinction does occur at Opal Creek, albeit at a lesser scale than in the Paleotethys, which may imply a different mechanism for the prolonged delay in biotic recovery.
PP31C-1519
Re-Os Isotopic Disturbances at Unconformities: Challenges and Opportunities
Re-Os geochronology of organic material from black shales is a proven technology, based on multiple studies of sections ranging from Neoarchean to Jurassic. We now have the opportunity to use our experience to predict which sections best preserve Re-Os isotope systematics and provide robust geochronology, and to extract information from problematic sections. In two cases, we have analyzed two suites of samples from the same section, acquiring statistically solid isochrons from one of the suites, but scattered data from the other suite. In each case, the scatter can be explained by oxidation processes that leave little visible trace in the rocks. An 80-m-thick section of black shale from the Neoproterozoic Biri Formation is exposed in a steep bedrock stream channel in south Norway. A suite of samples collected in a 4 m stratigraphic interval mid-way through the section yields a statistically robust Model 1 isochron age of 561 ± 4 Ma (initial 187Os/188Os = 1.10 ± 0.03, MSWD = 0.5, n = 6). A suite of samples taken within 3 m of the upper contact scatter about a 600 Ma reference line. The top of the Biri Formation is here marked by an unconformity and overlain by a fluvial conglomerate (Ring Formation). Clearly there was a period of exposure and potential oxidation of the uppermost Biri shale. The disturbed section is not visibly altered and shares features with the undisturbed section: the shales are very black, have total organic carbon (TOC) concentrations of about 1%, and show fine pyrite framboids. A 90-m-thick section of black shale from the Middle Triassic Botneheia Formation is exposed in sharp cliffs and incised gullies in western Spitsbergen (Svalbard). A suite of samples collected over a 70 cm stratigraphic interval about 10 m above the base of the section yields a well-constrained Model 3 isochron age of 241 Ma (initial 187Os/188Os = 0.83, MSWD = 16, n = 6, see Xu et al., this meeting). Samples taken in a 1 m stratigraphic interval 4 m below the top of the shale section scatter above and below the 241 Ma isochron. The Botneheia Formation is overlain at this location by reddish siltstones of Tschermakfjellet Formation, representing a pro-delta environment at the base of the largely deltaic Storfjorden Group. As with the Biri sections, the disturbed samples are not visibly altered. TOC contents of 2-8% are representative of the shale section. The Biri shales were clearly exposed, partially eroded, and subsequently covered by fluvial sediments. During exposure, the upper part of the shale may have been above the water table and subject to oxidative weathering. The extent of exposure of the Botneheia shales prior to deposition of deltaic sediments is uncertain, but the water column at the time of Tschermakfjellet deposition was clearly oxidizing. Because the shales contain high concentrations of strong reducing agents (organic carbon and sulfides), and because downward flow of infiltrating fluids could transport reaction products away from the zone of oxidation, evidence for limited oxidation events may not be readily visible. Yet this process would mobilize and redistribute redox-sensitive elements such as Re and Os. The isotopic disturbance may therefore provide a marker for the vertical extent of oxidation of shale bedrock underlying fluvial-deltaic systems.
PP31C-1520
Carbon, Sulfur, and Iron Cycling During the Cenomanian-Turonian Oceanic Anoxic Event (OAE2)
The Cenomanian-Turonian oceanic anoxic event (OAE2) is characterized by widespread deposition of organic rich sediments and a positive C isotope excursion in both carbonate and organic carbon. The biogeochemical cycles of C and S are linked via bacterial sulfate reduction (BSR) such that enhanced production and delivery of organic carbon to marine bottom waters and sediments should manifest as an increase in BSR and associated pyrite formation, assuming reactive iron is available. Within this context, anoxic conditions should promote an increase in both pyrite burial and a measure called Degree of Pyritization (DOP), which provides an estimate of the extent to which the iron available for pyrite formation (highly reactive iron) was actually transformed to pyrite. Additionally, water column anoxia is purported to facilitate elevated ratios of highly reactive iron to total iron (FeHR/FeT). We present pyrite concentration, S isotope, and Fe speciation measurements from the Western Interior Seaway (WIS) of North America and Ocean Drilling Program sites 1258 and 1261 (Demerara Rise, southern proto-North Atlantic) that span OAE2. Results from the WIS yield: 1) elevated DOP before and after OAE2 with depressed values during the event; 2) elevated FeHR/FeT before and after OAE2 with decreased ratios during the event; 3) elevated pyrite concentrations before OAE2, a sharp decrease at the onset of the event with variable but increasing values thereafter; and 4) an increase in Δ34S (defined as the S isotope difference between seawater sulfate and simultaneously deposited sedimentary pyrite) at the onset of OAE2 and elevated values thereafter. By contrast, Demerara Rise measurements show: 1) consistently high DOP before, during, and after OAE2; 2) consistently low FeHR/FeT before and after OAE2 with a slight decrease during the event; 3) consistently lower pyrite concentrations before OAE2, a rapid and sharp increase at the onset with variable and slightly elevated values thereafter; and 4) a general (although variable) increase in Δ34S at the onset of OAE2 and elevated values thereafter. In combination, these data indicate that increased OC burial did not lead to substantial increases in pyrite burial at these two localities and that reactive Fe was the limiting factor in pyrite formation. However, δ34Spyrite results indicate that shifting redox levels did influence the location of (water column vs. sediment) and/or microbial reactions involved with pyrite formation. To conclude, the talk will explore hypotheses for the uncharacteristically low FeHR/FeT at Demerara Rise.
PP31C-1521
Uranium Isotope Systematic in Saanich Inlet
As a redox-sensitive element Uranium has become the focus of stable isotope studies. Based on the nuclear field shift effect [1], U isotope fractionation was predicted as a function of U(IV)-U(VI) exchange reactions with the insoluble reduced U(IV) species being heavier than the soluble oxidized U(VI) species. Recently, variations in 238U/235U were reported in low temperature aqueous and sedimentary environments [2,3] indicating that U deposited in well-oxygenated environments is characterized by light isotopic composition, whereas suboxic and anoxic deposits tend towards a heavy isotopic signature. U isotope fractionation has been hence proposed as a promising new paleo-redox proxy. In order to test the efficacy of U isotope fractionation to record oxidation states in marine systems, we are investigating sediment samples deposited over a range of redox conditions in the seasonally anoxic Saanich Inlet, on the east coast of Vancouver Island. We have also made δ238U measurements for water samples from above and below the redoxcline. The measurements were carried out by MC-ICPMS using 233U/236U-double spike technique. The data are reported as δ238U relative to NBL 112a with a 238U/235U ratio of 137.88 (2sd). External precision is better than 0.10 permil (2sd). Eleven analyses of seawater performed over the course of this work yielded δ238U of -0.41±0.07 permil (2sd). No clear difference in δ238U values has been found, thus far, in water samples collected at 10m (O2~380μM) and 200m (O2~1μM) depths from a single location in the middle of the inlet. The mean of two measurements of the deepwater sample yielded -0.43±0.01 permil (2sd). Two measurements of the shallow water sample yielded a mean value of -0.38±0.03 permil (2sd). The δ238U values for HF-HNO3 digestions of the organic rich sediments, one taken in the middle of the basin (3.11% organic carbon) below seasonally anoxic bottom waters (-0.22±0.01 permil, n=2), and the other taken from the sill (1.29% organic carbon) below well-oxygenated bottom waters (-0.22 permil, n=1) are identical. The δ238U value matches previously reported values for suboxic sediments from the Peru margin [3], but is lighter than organic rich sediments from the Black Sea [3], where the bottom waters are strongly euxinic. The consistency in δ238U vaues between previously investigated suboxic sediment samples [3] and our two sediment samples indicates that the magnitude of the U isotopic fractionation is identical between seawater and sediments deposited under a range of bottom water oxygen conditions from oxygenated to anoxic. However, differences between the U isotope compositions in Saanich Inlet and those from the Black Sea remain to be explained, if U isotope fractionation is be used as a quantitative proxy for paleoredox in ancient oceans. [1] Schauble (2007) GCA 71, 2170- 2189. [2] Stirling et al. (2007) EPSL 264, 208-225. [3] Weyer et al. (2007) GCA 72, 345-399.