PP33D-01 INVITED
Early Neoproterozoic Ocean Chemistry: Fe-S Systematics from the Chuar Group
Recent work suggests that Fe-rich, Archean-like ocean conditions returned during the Neoproterozoic. This provocative finding raises a number of questions about the triggers for this transition and the geochemical and biological consequences. To better address these questions, we present a high-resolution geochemical record through the early Neoproterozoic Chuar Group. The Chuar Group is composed of more than 1500 meters of gently folded, unmetamorphosed inter-bedded fossiliferous shales and meter scale carbonate- sandstones. Focusing primarily on the shales, Fe-speciation and sulfur isotope data provide the backbone of the study. Fe-speciation data, in particular, allows for the story of Neoproterozoic ocean anoxia to be updated and extended. Interestingly, these data illustrate transitions between Fe-rich and S-rich anoxic conditions, and shed light on how these reversions behave and are initiated. Just as important as the local information gained from the Chuar sediments, these data further elucidate our understanding of the early Neoproterozoic world. Specifically, we gain information about one of the most influential and complex factors in Neoproterozoic geochemistry - the size and speciation of the marine sulfur reservoir. For instance, necessarily low levels of seawater sulfate 1) limit the prominence of sulfate reducing bacteria in remineralization reactions, 2) increase the relative iron efflux from hydrothermal systems, and 3) may have consequences for the marine methane hydrate budget. When paleontological and biomarker records are interpreted in light of these geochemical findings, questions ranging from the abundance and distribution of microfossils, to transitions in dominant primary producers, and potential inhibitors to eukaryotic evolution, are more fully understood. Together, these data contribute to an evolving picture of the Neoproterozoic world.
PP33D-02
Chromium stable isotopes and their use as redox tracers through time
Stable chromium isotopes have the potential to trace redox processes in seawater and bear the potential to reflect the magnitude of surface water oxygenation.. In order to use these isotopes as a valuable tracer, one first needs to understand the behavior of Cr, particularly with respect to its fractionation during precipitation and adsorption, two processes which potentially can remove dissolved Cr from a water column. Our preliminary experiments show that isotopic fractionation of Cr during adsorption onto graphite is minimal, a result which translates positively to the use of stable Cr isotopes for characterizing strongly organic-rich (black) shales. The capacity of Fe(II) to reduce Cr(VI), and the insolubility of Cr(III) which preferentially is stripped from a water column by precipitation as a mixed Fe(III)-Cr(III) (oxy-) hydroxide, also enables the application of Cr stable isotopes to Fe-rich chemical precipitates, best represented by banded iron formations and Fe-rich cherts. We postulate that the Cr isotopic composition of BIFs indirectly reflect the former presence of positively fractionated Cr(VI) in the surface waters, and that the magnitude of the observed fractionation bears information regarding the level to which the ambient surface waters were oxygenated. Our NIST 3112a Cr standard (relative to which we report Cr isotope ratios of our samples) reproducibility over the last 8 months amounts to δ(53Cr/52Cr) = +/- 0.12 (2σ) permil at intensity levels comparable to the samples. Eoarchean (~3.7 Ga) oxide-facies BIFs from Isua (W Greenland) show minimal enrichment of the light Cr isotopes [δ(53Cr/52Cr) = ~ -0.15 permil]. Similarly, Neoarchean (~2.9 Ga?) oxide-facies BIFs from the Black Hills (South Dakota) have δ (53Cr/52Cr) values of ~ -0.12 permil and ~2.7 and ~2.5 Ga oxide-facies BIFs from within the Botswana-Zimbabwe craton (Matsiloje; Pitshane) yielded δ(53Cr/52Cr) = -0.11 to -0.30 permil. These values are compatible with the ~ -0.12 +/- 0.10 permil average of Cr in magmatic rocks and Cr-bearing (high-T) minerals and implies that Cr(VI) was essentially absent in Archean seawater. First results of positively fractionated Cr isotopes (δ(53Cr/52Cr) = ~ +0.15 permil in 1.8-2.05 Ga iron formations from the Black Hills (South Dakota, USA) and strongly fractionated Cr isotopes (δ(53Cr/52Cr)) up to +4.5 permil) from Late Ediacaran Fe-cherts (Yerbal Fm., Arroyo del Soldato Group, Uruguay) might indicate the presence of (hyper)oxic surface waters that formed as a result of the major atmospheric oxygenation events at ~2.3-2.4 Ga and during the Late Neoproterozoic.
PP33D-03
Transition metal isotope proxies for ancient anoxia: ground-truthing in a modern anoxic basin
Insights into biogeochemical cycling in ancient anoxic environments can be gleaned from studies of their modern day analogues. One such setting is the Golfo Dulce, a ~200 m deep semi-restricted basin on the Pacific coast of Costa Rica. As a result of limited exchange with the open ocean, the deeper parts of the basin experiences sub-oxic to anoxic conditions, but little or no sulphate-reduction, for the majority of the year. The Golfo Dulce thus provides a natural laboratory to study biogeochemical cycling of trace metals under conditions that are perhaps analogous to those of Archaean oceans. Redox sensitive metals, such as molybdenum (Mo), have recently emerged as potentially powerful proxies for reconstruction of past oceanic and atmospheric redox statee.g. 1,2. The palaeoproxy power of Mo rests on its redox-bimodal aqueous geochemistry and associated isotopic fractionation during removal to oxic or euxinic sedimentary environments. Here we focus on Fe and Mo systematics on pore waters extracted from approximately 50 cm sediment cores across the basin, allowing investigation of cycling of these elements across pronounced redox gradients. Cores taken from shallow sediments beneath oxic bottom waters record diagenetic processes that are dominated by Mn and Fe reduction, as suggested by large releases first of dissolved Mn followed by dissolved Fe into the pore waters. Cores taken from depths beneath the water-column oxycline have a narrower zone of Mn and Fe reduction. Complete depletion of dissolved Fe from 10 cm downwards strongly suggests sulphate reduction being the dominant diagenetic process at the deeper, anoxic site. At the deeper anoxic site, δ98Mo increases by approximately 2‰ from a low starting value for seawater of 1.3‰, perhaps suggesting that reduction of Mn-oxides has already begun in the water column. The same pattern is seen in the deeper, more reducing section of the core retrieved from beneath oxic bottom water, where δ98Mo increases by approximately 1‰ from seawater values to a δ98Mo value of 3.5‰. These increases in δ98Mo are accompanied by a corresponding decrease in Mo concentration. In the presence of low dissolved sulphide concentrations Mo is expected to be removed as a thiomolybdate species, thought to preferentially remove the lighter isotopes3. Such a process would drive residual pore water to heavier δ98Mo values, offering a potential explanation for the fractionation observed here. These data will be interpreted in light of their accompanying solid phase systematics along with emerging data on experimental controls on Mo isotope fractionation during early diagenesis. 1 C. Scott et al, Nature, 2008, 452, 456-459 2 A.D. Anbar et al, Science, 2007, 317, 1903-1906 3 R.L. Poulson et al, Geology, 2006, 34, 617-620
PP33D-04
New insights on the Frasnian/Famennian mass extinction: a role for soil erosion?
The Frasnian/Famennian (F/F) mass extinction, which killed off a previously thriving tabulate coral- stromatoporoid reef community, was the most severe biotic crisis of the middle Paleozoic. The present study examines the geochemistry of a 28-m stratigraphic interval straddling the F/F boundary in the West Valley drillcore from the northern Appalachian Basin (western New York State), comprising bioturbated shales of the Hanover Formation and mostly laminated shales of the overlying Dunkirk Formation. Paleoredox proxies (DOP, FeT/Al, δ98Mo) indicate an increase in the frequency and intensity of anoxia at the F/F boundary. Proxies for hydrographic conditions (Mo/TOC, Re/TOC, U/TOC) suggest that the depositional basin experienced an interval of deepwater restriction around the boundary, possibly as a consequence of eustatic fall. The boundary is characterized by a large decrease in Zr/Al, indicating lower silt:clay ratios, and by a large decrease in excess Ba (i.e., total Ba-detrital Ba), implying reduced levels of primary productivity. Organic C- and N-isotopic data provide evidence of a major change in organic matter fluxes commencing ~7 meters below the boundary and persisting ~10 m above it. This change is characterized by ca. +5‰ and +15‰ excursions in kerogen δ13C and total organic δ13C, respectively, and by short- term excursions in organic δ15N to as low as -1‰ CDT (from background values of +1 to +2‰) that may provide evidence of cyanobacterial N fixation. Biomarker analysis, still in progress, may provide additional clues concerning changes in organic matter sources. The existing data are consistent with a model of enhanced terrigenous siliciclastic flux to the northern Appalachian Basin at the F/F boundary linked to climatic cooling, eustatic regression, and soil erosion. The rapid development of soils as a consequence of the spread of vascular land plants during the Middle and Late Devonian (Algeo et al., 1995, GSA Today, v. 5(5)) may have created the potential for precipitating marine ecological crises through soil erosion events.
PP33D-05 INVITED
Investigating The Cause And Consequences Of Oceanic Anoxia
Episodes of ancient oceanic anoxia were often associated with intervals of global warmth. Fundamental questions remain about the relationship between climate and anoxia: is warming alone sufficient to generate anoxia, or are other factors, including "ocean stagnation," paleogeographic configuration, or nutrient inventory equally important? What feedbacks amplify or damp the oceanic response to warming? Few studies have addressed these questions quantitatively, and most of those have used box models for which the physics of the ocean must be specified. We instead explore these questions using Genie, an Earth-system model of intermediate complexity. We use Genie to hindcast the onset and spread of anoxia for our example application, the Late Permian. We find that global warming (perhaps created by Siberian Traps greenhouse gas emissions) reduces the oceanic oxygen concentration, but does not create widespread anoxia. Phosphate accumulation resulting from enhanced weathering-derived delivery and reduced sinks under anoxic conditions, and enhanced nitrogen fixation are needed to carry the ocean into the anoxic and then euxinic (hydrogen-sulfide rich) state indicated by biomarkers in Late Permian sediments. Sensitivity analyses are used to evaluate the relative importance of changes in ocean circulation rate, atmospheric oxygen levels, nutrient levels, and continental configuration to the development of anoxic conditions.
PP33D-06
Biogeochemical indicators of euxinic oceans presaging the end-Permian mass extinction at Meishan, China
The Permian-Triassic Boundary (PTB) event at 252.25 Ma is the largest extinction of the Phanerozoic and one that coincided with a protracted and extreme oceanic anoxic event (OAE). The body-stratotype of the Changhsingian Stage is bracketed by the PTB and Changhsingian-base GSSPs at Meishan in southern China and is well-constrained with respect to geochronology and the pattern of extinction. Secular trends in C- and N-isotopic parameters and lipid biomarkers in a core spanning 200m of stratigraphic section across the PTB and the entire Changhsingian interval at Meishan reveal dramatic shifts in paleoenvironmental conditions and in plankton communities well before the extinction event. Specifically, patterns of steroids and triterpenoids indicate a marine plankton community that was heavily dominated by bacteria during the late Wuchaipingian, middle Changhsingian and early Griesbachian stages. Hydrocarbons diagnostic for green sulphur bacteria show these intervals were characterized by euxinic conditions in the photic zone. Reducing and sulfidic conditions prevailed during the entire Changhsingian stage. These results, together with a chronology established from intercalated ash beds, indicate that conditions unfavourable for aerobiosis existed in the marine photic zone at Meishan for more than one million years prior to the main phase of the biological extinction. The widespread induction of marine euxinic conditions at the end of the Permian was likely a consequence of the aggregation of Pangea and the weathering and transport of terrestrial organic matter to the ocean for at least 1.5 million years prior to the PTB. The protracted nature of the ensuing OAE suggests a causal association with the extinction.
PP33D-07
Continental Feedback Between Terrestrial Climate Change, Carbon Burial and Associated Ocean Redox Change in the Tropical Cretaceous Atlantic
Continental margin sediments constitute the dominant sink of organic carbon resulting from the interface of terrestrial and marine processes and facilitated by accommodation space. Here we show the importance of terrestrial processes sensitive to rapid shifts in continental climate strongly influencing burial of organic carbon and the associated development of ocean anoxia in a Late Cretaceous (Coniacian "oceanic anoxic event 3") black shale from a deep water setting in the tropical Atlantic. Sediments recovered from ODP 959 from West Africa document a systematic variation of mineralogical feedback between continental climate change, redox change across the entire water column and organic carbon preservation and burial in deep water sediments in the middle Cretaceous tropical Atlantic. TOC of up to 16 percent is developed in repetitive cycles over meter scales of the core coinciding with movement of the intertropical convergence zone (ITCZ) on precessional time scales (22 kys) (Beckmann et al., 2005) and associated freshwater and nutrient runoff from tropical Africa. We analyzed clay mineral composition in four of these cycles at cm scale and observe a strong relation (r=0.81; n =105) between TOC and the concentration of 2:1 (smectite) clay minerals in the sediment. This relation suggests a control of organic enrichment by clay minerals over these abrupt changes in TOC as occurs in modern continental margin sediments through organic carbon preservation by mineral surfaces. Since the detrital clay minerals in these sediments are a product of terrestrial weathering conditions, the mineral surface preservation provides a direct link between organic carbon preservation in marine sediments and changes in hydrology and soil formation on land that track changing climate. Changes in terrestrial soil forming conditions in tropical west Africa during the late Cretaceous thus affected a strong influence on organic carbon burial patterns in deep water Atlantic sediments and provide evidence of a strong continental feedback between terrestrial climate change and carbon burial.
PP33D-08
Mid-Miocene Global Cooling: Insights from New Integrated Data From the Organic-Rich Monterey Formation, California
The mid-Miocene represents one of the four major cooling steps of the last c. 50 Myr of Earth history but the driving mechanism for this climate transition is contentious. Mechanisms proposed for the cooling include: the large-scale burial of organic rich-mudrocks (The Monterey Hypothesis); the uplift and weathering of the Himalaya leading to an increase in the rate of chemical weathering and enhanced drawdown of carbon dioxide; changes in ocean circulation; and the congruence of orbital parameters. This interval of time is represented in the Circum-Pacific area by the deposition of organic-rich mudrocks and in some sections the amount of organic matter increases during the pronounced cooling. In order to quantify the stages of this climatic transition and to understand the underlying driving mechanism, we have accurately documented the sedimentary and geochemical characteristics of the Monterey Formation near Santa Barbara, California through high-resolution graphic logging and geochemical analysis. Eleven organic-rich mudrock facies have been defined. These facies are distributed cyclically on length-scales ranging from c. 0.5 to 20 m and Fourier analysis of geochemical and physical parameters reveals regular cycles. Using new diatom ages (this study) and nannofossil ages (Follmi et al. 2005), the regular cycles have been correlated to the Laskar orbital solution (Laskar et al. 2004) for about 16 to 13 Myr to produce an accurate astronomical timescale. The distribution of organic-rich facies changes on timescales that range from the sub-Milankovitch to the million- year and these changes are clearly linked to climate. Our new high-resolution C-isotope data can be tied to the distinctive CM events from deep sea sections. The record from the Monterey Formation shows a number of very rapid large C-isotope shifts which may relate to either methane hydrate release or to changes in the organic-C source. Our combined high resolution observations will allow us to discriminate between the different mechanisms proposed for mid-Miocene cooling.