PP21B-1411
An Episode of Late Archean Euxinia and Enhanced Continental Weathering Revealed by Iron Speciation in the Mt. McRae Shale
Recent high-resolution chemostratigraphy for sediments of the late Archean Hamersley Basin has revealed an episode of pronounced enrichment of the redox-sensitive elements molybdenum and rhenium, a primary sedimentary feature that has been accurately dated to 2501.1 ± 8.2 Ma. These enrichments are not easily explained through postdepositional addition or syndepositional hydrothermal input to the ocean and have thus been interpreted to reflect mild oxidative weathering 50-100 million years prior to the significant increase in Earth's atmospheric oxygen level referred to as the "Great Oxidation Event." To further explore this feature of the late Archean record, we have generated complementary high-resolution iron speciation data for the Mt. McRae Shale and the underlying Mt. Sylvia Formation. Using a calibrated sequential extraction, biogeochemically reactive iron phases were separated into Fecarb (siderite or dolomite-ankerite), FeOx (reducible iron oxides such as goethite or hematite), Femag (magnetite), and FePY (pyrite). Values for FeOx are uniformly low for the entire Mt. McRae Shale, indicating water column and pore fluid conditions that were reducing with respect to iron. The observation of FePY concentrations of 0.4-1.5 wt% in the lower Mt. McRae Shale suggests significant sulfide production by microbial sulfate reduction, but values of FePY/FeHR averaging ~0.3 for this unit indicate reactive iron in excess of dissolved sulfide. This reactive iron may have been repartitioned in situ by dissimilatory iron reduction, as reducible iron oxide concentrations are low ([FeOx] ~ 0.1- 0.2 wt%), but may also have been externally sourced by hydrothermal fluids. As a result of this reactive iron excess, variations in FeHR within the lower Mt. McRae Shale are governed primarily by differences in Fecarb, suggesting conditions that were anoxic but non-sulfidic. Values for FePY/FeHR in the upper Mt. McRae Shale generally exceed 0.8, and for many samples are ~1.0, indicating that authigenic enrichment of molybdenum in this interval was facilitated by a transient episode of euxinia. In modern marine environments authigenic molybdenum enrichments of this magnitude require equilibration of dissolved MoO42- with [H2S] >10μM on hundred-day timescales, which corresponds to total dissolved sulfide concentrations ([ΣS2-]) on the order of 200-300μM. Evidence for iron-limitation of pyrite formation, despite high concentrations of both iron and organic carbon, suggests that water column sulfate concentrations were at least transiently elevated. This assertion is supported by sulfur isotope data for pyrites immediately above and below the euxinic interval, which show an attenuation of non-mass-dependent sulfur isotope anomalies most parsimoniously interpreted to reflect dilution of atmospherically produced sulfur by oxidatively weathered continental sulfur.
PP21B-1412
Clay Mineralogy and Organic Carbon Burial in Proterozoic Basins
Pedogenic, or soil-derived, clay minerals have long been implicated in the efficiency of organic matter (OM) burial and coincident accumulation of atmospheric oxygen. As diagenesis and metamorphism obscure pedogenic clays in many Precambrian rocks, clay mineralogy and its role in OM burial through much of geologic time remains incompletely understood. In this study we analyzed the mineralogy and total organic carbon (TOC) of a number of organic rich shales deposited in Late Archean to Early Cambrian sedimentary basins. Across all samples, diagenetic transformation of pre-existing smectite minerals has led to the predominance of glauconite and the diagenetic 1M and 1Md illite polytypes, which, collectively, can be thought of as "proto-smectite". The correlations between TOC and illite crystallinity suggest that OM burial and preservation in the Proterozoic proceeded by the physical aggregation of OM and pedogenic clays upon deposition. This association, in turn, led to the interference of OM with the illitization process, resulting in the ubiquitous relationship between high surface area (or, finely crystalline) material and high TOC. This interpretation is consistent with suggestions that the preservation of OM after burial proceeds by physical exclusion, with mineral surfaces effectively isolating OM from enzymatic breakdown. Together, it appears that the deposition of pedogenic clays has remained broadly constant over Proterozoic time and into the Early Cambrian, which is incompatible with the hypothesis that late Neoproterozoic oxygenation was influenced by increases in pedogenic clay production. As no clear temporal relationship exists between clays and OM, Precambrian oxygenation was likely controlled by other mechanisms.
PP21B-1413
The Isotopic Evolution of the Cryogenian Ocean: a Record from Southwest Mongolia.
Geochemical analyses of Neoproterozoic sedimentary rocks inform our understanding of the large-scale climatic and biological events that mark this era. Much remains to be learned, however, about the drivers and feedbacks associated with these events. Critical information is also lacking during one of the most intriguing periods, the Cryogenian, and specifically between the Sturtian and Marinoan glaciations. To address these and related questions, we look to the Dzabkhan Basin of southwest Mongolia. Here, two Neoproterozoic glacial diamictites are separated by hundreds of meters of carbonate strata. The upper diamictite is capped by a micropeloidal dolomite with sedimentological structures (i.e. tubestone stromatolites, giant wave ripples, and former aragonite crystal fans) characteristic of the ca. 635 Ma basal Ediacaran cap carbonate. The underlying Cryogenian lower Tsagaan Oloom Formation consists of ~575 m of dark, organic-rich limestone. Thus, these carbonates represent an ideal record for testing changes in ocean chemistry and the behavior of prominent elemental cycles. Herein we report chemical and isotopic analyses, specifically C-isotopes of carbonate and organic carbon, in order to assay the behavior of the biosphere and pinpoint the mechanisms driving carbon isotope excursions. These data further extend the use of C-isotopes as a correlation tool and focus not only carbonate carbon, but also utilize the co-variation of carbonate carbon with organic carbon.
PP21B-1414
Covariation in the carbon isotopes of carbonate and organic carbon across the Neoproterozoic Bitter Springs Stage
Through most of the Phanerozoic, δ13Ccarb and δ13Corg chemostratigraphic records covary, demonstrating that the organic carbon was derived from contemporaneous dissolved inorganic carbon. In contrast, there is a reported absence of covariation in δ13Ccarb and δ13Corg records from late Neoproterozoic sedimentary successions. These data have been used to develop a model for the Neoproterozoic carbon cycle that decouples the reservoirs of inorganic and organic carbon and explains the relatively invariant δ13Corg values as a result of a large reservoir of oceanic dissolved organic carbon. This model has played an important role in recent thinking about Neoproterozoic Earth history as it has been argued that: (1) the onset of δ13Ccarb and δ13Corg covariation in the late Ediacaran is evidence for increased oxygenation that lead to the demise of this large oceanic reservoir of organic carbon, and (2) this mode of the carbon cycle would inhibit the onset of a Snowball Earth climate state due to the remineralization of the organic carbon pool as the solubility of O2 increased in cooling seawater. New δ13Ccarb and δ13Corg data sets with high stratigraphic resolution, developed from both outcrop and drill core section (120 km apart from one another within a single basin) of the ~ 800 Ma Bitter Springs Formation, central Australia, demonstrate covariation across the >4‰ negative δ13Ccarb excursion that marks the onset of the Bitter Springs Stage and throughout the stage itself. At the termination of the Bitter Springs Stage, δ13Ccarb values shift abruptly from - 2.7‰ to +5.3‰. There is a corresponding δ13Corg shift from -29.9‰ to - 26.7‰. These sympathetic shifts indicate that prior to the Sturtian glaciation there was not an oceanic reservoir of organic carbon that was large enough to inhibit variation in δ13Corg. While the shifts in δ13Ccarb and δ13Corg are sympathetic in direction across the end of the Bitter Springs Stage, their differing amplitudes result in a step-wise, and sustained, shift in Δδ13C from 27.4‰ to 31.5‰. This change in Δδ13C is likely a result of a change in the fractionation between CO2(aq) and the primary biomass resulting from paleoenvironmental and biological changes across the end of the Bitter Springs stage.
PP21B-1415
Geochemical Insights Into Paleobiological Patterns of the Middle-Upper Cambrian Port au Port Group, Western Newfoundland
The Cambro-Ordovician interval is marked by significant changes in the marine realm, both biological and geochemical. The first mineralized skeletons appear near the Proterozoic-Cambrian boundary, but the major radiation of carbonate skeletonized crown group taxa occurred ~40 million years later in the Ordovician. Recent work on the carbonate-clastic Middle and Upper Cambrian Port au Port Group exposed in western Newfoundland has focused on quantifying skeletal abundances and analyzing δ13Ccarbonate and δ34S(sulfate and pyrite) to determine how environmental fluctuations may be tied to the abundance of skeletal organisms during the Cambro-Ordovician interval. The geochemical data show systematic shifts in δ34Ssulfate of >15‰ over relatively short stratigraphic distances (10 m, likely <1 Myr), low average Î"34Ssulfate-pyrite (ca. 23‰) and a general positive coupling between δ13Ccarbonate and δ34Ssulfate. In combination, these results indicate that Middle to Late Cambrian sulfate concentrations were low and that the sulfate reservoir was more sensitive to isotopic variability than it was in either terminal Neoproterozoic or Cenozoic oceans. However, a simple C and S isotope box model of the Late Cambrian ocean illustrates that low sulfate concentrations alone fail to account for the >15‰ δ34Ssulfate shifts recognized in Port au Port strata. These large δ34S shifts can be generated only if fluctuating oceanic redox is invoked, whereby marine anoxia forces reduced C/S and elevated Î"34S, driving larger δ34S changes per mole of OC buried. These geochemical data have been generated in concert with quantification of fossil skeletal abundances from the same Cambro-Ordovician sections of western Newfoundland. In general, skeletal abundance is low during the Middle and Late Cambrian and shows a small increase in the first half of the Early Ordovician. The major radiation of carbonate skeletal organisms occurs in the second half of the Early Ordovician, indicating that conditions were favorable for diversification of skeleton-secreters. Future work will determine if the radiation of skeletal organisms occurs concurrently with stabilization of the carbon and sulfur cycles and/or the disappearance of anoxia.
PP21B-1416
SIMS-based Approaches to Understanding Sulfur Cycling Over Earth History
Secondary Ion Mass Spectrometry (SIMS) can be used to generate relatively non-destructive sulfur isotope ratio data at high spatial resolution using a Cs+ primary ion beam between 50nm and 10um in size. Data can be collected either in spot mode (averaging over a larger region of interest (e.g., 5-100um) to obtain more precise results (d34S precision of 0.2 permil)) or in ion image mode (where the spatial record of geochemical change at scales approaching the beam size is uncompromised, at the cost of the increased precision of spot analyses). Here, we highlight a variety of SIMS-based analyses that help improve our understanding of sulfur cycling – both in modern sedimentary environments and throughout the geologic record. The ability to resolve numerous (50 - 500) analyses over a length scale of just a few mm allows for the identification of significant isotopic trends that are not possible to observe using conventional techniques. Specifically, by analyzing samples in a regular grid framework, we can meaningfully link the data from successive measurements, allowing for the identification of 'long-range' (e.g., mm) trends. Organizing measurements into a regular grid also provides an additional control on instrument drift (beyond sample- standard bracketing). We demonstrate the utility of our approach by examining coherent sulfide isotope variability in both modern microbial mats and sulfur-rich Archean black shales over several mm to 1 cm with data collected as spot measurements using a Cameca 7F/GEO SIMS instrument. We also highlight the potential to resolve even finer scale trends using ion imaging on the Cameca NanoSIMS 50L. Though pyrites and other sulfide-bearing minerals are ideal for SIMS analysis (because of their ready ionization), we show that these techniques can also be meaningfully applied to sulfate minerals preserved in the geologic record.
PP21B-1417
Multiple Sulfur Isotope Signatures Across Environmental Gradients in the Modern Black Sea
We present a multiple sulfur isotope study of sediment cores collected from the southern basin during Leg 4 of the 1988 R/V Knorr Black Sea Expedition, which are representative of sedimentary deposition below oxic and euxinic bottom waters. Comparison of oxic (shelf) with euxinic (basin) sediments provides a diverse array of depositional conditions for investigating mass conservative isotope effects in natural systems. Paleo- redox conditions independently recognized using well-tested geochemical proxies (iron chemistry, trace metal enrichments, inorganic sulfur, δ34S, and TOC) provide a back-drop with which to compare rare sulfur isotopic (33S/32S and 36S/32S) compositions of coexisting sulfates and sulfides (AVS and pyrite). Consistent with previous proxy data, δ33S and δ36S pyrite results from the presently oxic shelf appear to track localized enrichments in solid phase Mn, pyrite, and reactive iron, which are diagnostic of a shoaling of the impinging chemocline approximately 250 ybp. Sulfate isotope enrichments within anoxic sediments along the rapidly accumulating euxinic margin appear to track Rayleigh-type fractionation during diagenetic pyrite formation. Δ33S relationships within muddy turbidite layers in the deep basin are suggestive of an admixture of diagenetic sulfides slumped from margin sediments with syngenetic pyrite formed within the chemocline of the deep basin. This exploration of rare sulfur isotope systematics combined with multiple paleoreodox proxies within the environmental context of an oxygen deficient basin is the first study of its kind and offers insights into the utility and preservation of multiple sulfur isotope signatures.
PP21B-1418
S-Isotope Fractionation Behavior of Desulfobacter Latus Under Sulfide Stress Conditions
We investigate whether the fractionation of sulfur isotopes during microbial sulfate reduction is affected by cell external sulfide concentrations (Brunner and Bernasconi~2005). We report time series data of δ34S ratios of dissolved sulfate measured in two sets of batch culture experiments with the marine sulfate reducer Desulfobacter latus. The initial sulfide conditions for the 1st set were 0.3~mM reaching up to ~~21~mM at the end whereas the 2nd set grew from 22.2~mM to ~~40~mM sulfide. We obtain enrichment factors (ε) of 12.9±0.1~‰ and 13.5±0.3~‰ for the first and second experiment respectively, assuming a Rayleigh-type fractionation process. However, the residuals of the regression analysis for the 2nd experiment reveal a curved trend which suggests that the data are not linearly correlated whereas the 1st set of data has a linear trend. To evaluate our experimental results, we develop a single box model that allows forward and backward fluxes between the microbial cell and respective cell external sulfate and sulfide pools (Brunner and Bernasconi~2005). By introducing a backward flux for sulfide, we are now able to minimize the residuals between modelled curve and experimental data significantly, which results in a better fit of the measured values in the 2nd experiment. Assuming that the backward flux increases with a fixed ratio based on the total sulfide concentration outside the cell, we obtain enrichment factors of ~~12.7~‰ for the 1st and ~~16.3~‰ for the 2nd experiment. Our data suggest that Rayleigh-derived enrichment factors might be underestimated in cases where sulfide re-flux is present but not identified. Brunner, B. and S. M. Bernasconi (2005). A revised isotope fractionation model for dissimilatory sulfate reduction in sulfate reducing bacteria. Geochim. Cosmochim. Acta 69(20): 4759-4771
PP21B-1419
The Oxygen - Sulfur Isotope Relationships of Dissimilatory Sulfate Reduction: Deciphering the Isotopic Composition of Sulfates from the Geologic Record
Dissimilatory sulfate reduction (DSR) leads to an overprint of the oxygen isotope composition of water on residual sulfate. The exchange of oxygen isotopes between water and sulfate is assumed to occur between sulfur intermediates formed in the sulfate reduction pathway, and are subsequently reoxidized by reversible enzymatic reactions. An additional oxygen source is the oxygen used in the reoxidation of the sulfur intermediates. Consequently, the apparent rate of oxygen isotope exchange between sulfate and water should depend on cell-internal reduction and reoxidation processes, the very same processes that control the extent of sulfur isotope fractionation expressed by DSR. We show that the ratio of the apparent oxygen isotope exchange rate to the sulfate reduction rate can be derived from the S-O isotope relationship of residual sulfate. Our model predicts rapid apparent oxygen isotope exchange for cases where the sulfur isotope fractionation factors are large and slow apparent exchange for cases where the sulfur isotope fractionation factor is small. We tested our model with data from DSR experiments with water enriched in 18O by +700‰ and found that the measurements agree well with the model predictions, but cover only a small part of possible S- O isotope relationships. This indicates that the reversibility of individual DSR steps is not fully independent from the reversibility of the other steps (e.g. sulfate uptake, conversion to APS, etc) in the sulfate reduction pathway. Our approach is a first step towards a new tool for the interpretation of sulfates that have been affected by microbial sulfur cycling.
PP21B-1420
Temperature and Oxygen Isotope Composition of The Ediacaran Ocean: Constraints From Clumped Isotope Carbonate Thermometry
The temperature and chemical variations of the early oceans on Earth are highly debated, particularly for periods associated with significant evolutionary change and/or extinction. The temperature of past oceans has been estimated based on conventional carbonate-water and/or silicate-water stable oxygen isotope thermometry. Precambrian carbonates and silicates both exhibit a long-term secular trend of increasing δ18O values with decreasing age. This trend has been used to support two opposite - though related - interpretations: the Earth's oceans gradually cooled over the course of the Proterozoic eon, from a maximum of ~ 60-90°C at ~ 2.5Ga (and were, on average, relatively warm during much of the Paleozoic era) [1]. This interpretation has been supported by Si-isotope proxies and the thermal tolerances of proteins in various classes of microbial organisms [2-3]. Alternatively, the δ18O value of the oceans has gradually increased through time [4-5], and mean Earth surface temperatures varied over a narrow range similar to modern conditions. In other terms, one either assumes an ocean of constant δ18O and infers that climate varied dramatically, or vise versa. Finally, it is possible that post- depositional processes (e.g., diagenesis, burial metamorphism, weathering) has modified the δ18O values of all or most Precambrian sedimentary carbonates and silicates, overprinting any paleoclimatic variations. Carbonate 'clumped isotope' thermometry provides a new way to independently test these hypotheses because it allows one to determine the apparent growth temperatures of carbonate minerals based on their abundances of 13C-18O bonds, as reflected by the 'Δ47' value of CO2 extracted by phosphoric acid digestion [6]. This method is thermodynamically based and independent of the δ18O of water from which the carbonate grew. We will report the initial results of measurements of 'Δ47 for a suite of carbonates from the Sultanate of Oman. This Ediacaran age (~ 635 to ~ 548 Myr) suite was buried to a range of depths that generally decrease with distance from the basin center. These Omanese dolomites vary in δ18OPDB from ~ -7 to +1 ‰, inversely correlated with their apparent crystallization temperatures from 28 to 76°C (using the unpublished calibration for temperature dependence of Δ47 of dolomite). The lowest δ18O values and highest temperatures are measured for samples with petrographic evidence for post-depositional re-crystallization (e.g. coarse crystals and low porosity) and likely to have experienced the most intense burial metamorphism. The correlation we observe between δ18Ocarb. and temperature suggests that the maximum temperature of the Ediacaran oceans was 28°C and that burial metamorphism has systematically modified the δ18O values of most of the analyzed carbonates. The δ18OSMOW for seawater for this period was between -1‰ (if depositional temperatures were 28°C — the maximum permitted by our data) and -4‰ (in the unlikely case that the ocean was at its minimum possible temperature permitting the existence of liquid water). [1] Knauth and Lowe, EPSL. (1978), 41, 209-222. [2] Robert and Chaussidon (2006), Nature, 443, 969-972. [3] Gaucher et al. (2008), Nature, 451, 704-707. [4] Shields and Veizer (2002), G3, 3. [5] Jaffres et al., (2006), ESR, 83-122. [6] Ghosh et al., (2006), GCA, 70, 1439-1456.
PP21B-1421
Uptake of inorganic carbon in conical structures formed by cyanobacteria
Strongly influenced by environmental processes such as waves, currents and sedimentation, macroscopic shapes of stromatolites are at best ambiguous biomarkers. Conical stromatolites are an exception to this, because their shape is thought to indicate the former presence of photosynthetic microbes that moved toward light. We investigate the biological basis of the formation of macroscopic cones in the cultures of modern cone-forming cyanobacteria from Yellowstone National Park. We demonstrate that these cyanobacteria can form cones oriented in the opposite direction from light, implying that phototaxis is not the only mechanism that generates conical shapes. We hypothesize that, under the conditions of low flow and inorganic carbon limitation, conical shapes may be advantageous for the uptake and cycling of carbon within the microbial aggregate. Our models of the diffusion of inorganic carbon predict that more vigorous uptake of inorganic carbon by microbes in the tips of cones can be expected simply due to the geometry of the microbial aggregate. We test this model by investigating the uptake and incorporation of carbon in cones and the adjacent flat mats by using 13C labeling measured by nanoscale secondary ionization mass spectrometer (nano-SIMS). Steady state rate of carbon uptake through the cone shows that 13C is taken up throughout the structure. Both carbon uptake and the cell density are high in the tips, supporting the idea that cone tip is an area that favors microbial growth. These measurements, combined with the ability of cyanobacteria to form cones against a light gradient suggest that the faster growth of tips can be influenced by the internal distribution of nutrients, in addition to phototaxis. Conical shapes in microbial mats and fossil stromatolites can thus be viewed as a macroscopic consequence of the biological response to environmental gradients at a microscale.
PP21B-1422
Geobiological Association of Cold (Methane) Seeps and Ancient to Modern Glacial Marine Sequences
Recent discovery of a chemoautotrophic ecosystem beneath the disintegrated Larsen Ice Shelf in Antarctica has been confirmed as a methane based cold seep associated with bacterial mats and large vesicomid clams. The source of the methanogenesis remains unresolved, although the most likely candidates include methanogenic microbial activity within the organic rich till (which lies below a thin ice shelf mud) and deeper seated thermogenic hydrocarbon seepage. Recent documentation of ancient cold seep sequences now include examples from both Neoproterozoic (Marinoan glacial event 635 mya) and Late Paleozoic glacigenic and succeeding marine shale successions. The recent example from the Lasen Ice Shelf is clearly associated with unusual conditions of organic matter abundance and availability (during ice shelf cover) followed by a large influx of phytodetritus accompanying the catastrophic ice shelf collapse. Interestingly, the chemotrophic system is observed to be in a state of decline, at least locally, and has possibly been impacted in some way by the rapid sediment flux to the seafloor induced by glacial surge and ice berg rafting. We compare the above examples of methane seep formation and glacial to deglacial stratigraphy in order to examine any common elements to the association. We postulate that glacial (ice shelf) conditions are conducive to chemoautotrophy on the seafloor and that environmental catastrophe (rapid deglaciation) may be a trigger for rapid changes in benthic ecosystems and the pace of evolution in permissive ecosystems.
PP21B-1423
Possible Record of Neoproterozoic Ice Sheet Collapse: The Kapp Lyell Diamictite Sequence of southwest Spitsbergen, Svalbard
The Late Proterozoic Kapp Lyell diamictites of northern Wedel Jarlsberg Land, southwest Spitsbergen, have long been recognized as ancient glacial deposits, but their place within the global stratigraphic framework of 'Snowball Earth' has remained unclear owing to the complexity of superimposed Caledonian deformation and to the inaccessible terrain in which they occur. Newly deglaciated exposures of the rocks now provide a more complete picture of the changing environment in which they were deposited and their place in the global chronostratigraphy. The Kapp Lyell Sequence (KLS) is the higher of two diamictic to conglomeratic Neoproterozoic units in SW Spitsbergen. The lower of these, the Konglomeratfjellet Formation, is not unambiguously glaciogenic, but it is topped by a carbonate sequence with textures similar to 'cap' carbonate units in other Neoproterozoic sequences. These carbonates are succeeded by ca. 2000 m of black phyllite with rare lone stones. The phyllite is overlain with apparent conformity by the 2000-3000 m thick KLS, which actually consists of three distinct types of diamictite, all apparently glaciomarine. The KLS begins with 500- 1000 m of finely laminated diamictite punctuated by pebble- to boulder-sized lone stones, presumably ice- rafted. The cm-scale laminae are defined by layers of sand-to-silt sized particles of detrital dolomite alternating with thin films of graphitic phyllite. Preliminary delta 13C values from this graphitic material are very negative, pointing to a biogenic origin. The extraordinary uniformity of the laminae indicates repetitive, possibly seasonal, depositional cycles, but the density of lone stones increases upsection, recording accelerating delivery of ice-rafted debris to the basin. Carbonate clasts predominate, and many have distinctive textures (cm-sized ooids, digitate stromatolites) that link them with the 'cap' carbonate unit lower in the section. The laminated diamictite interval ends abruptly with the appearance of ca. 500 m of unlayered, unsorted diamicite in beds 1-5 m thick, and some intervening graded layers of conglomerate to sandstone. The massive, unsorted "Heinrich"-type layers contain sand- to boulder-sized material and tend to be laterally discontinuous, while the graded beds are traceable for hundreds of meters. In the uppermost 1000 -1500 m of the sequence, such coarse graded beds become predominant, with only rare unsorted intervals. These beds are the highest preserved Neoproterozoic strata in the region. Overall, the transition from laminated, to unsorted, to graded diamictites may represent change from 1) a stable ice margin that released rare icebergs into a deep, quiet basin to 2) a collapsing ice sheet that unleashed flotillas of icebergs and large volumes of sediment to 3) submarine landslides that triggered turbidity flows from the rapidly deposited, gravitationally unstable sediments. No absolute ages are available for the Kapp Lyell sequence itself, but indirect evidence suggests that it records the ca. 635 Ma Marinoan stage of the "Snowball Earth" glaciations. In southernmost Spitsbergen, a unit correlative with the underlying Konglomeratfjellet Fm has yielded a metamorphic monazite age of 653 +/- 39 Ma (Majka et al 2007). Because there is no known stratigraphic discontinuity or metamorphic contrast between this older unit and the Kapp Lyell sequence, it seems most likely that the Kapp Lyell sequence and Konglomeratfjellet Fm. represent the Marinoan and Sturtian glaciations, respectively. If so, the sedimentary characteristics of the Kapp Lyell sequence suggest a catastrophic end to a Marinoan ice sheet, an ironic conclusion to be drawn from rocks named for the father of Uniformitarianism.
PP21B-1424
Orbital forcing of Devonian Lacustrine Ecosystems
Devonian lacustrine strata of the Caithness Flagstone Group of the Orcadian Basin in North Scotland are famous for fossil fishes, lithological rhythms, and a stunning record of terrestrialization. While the marked sedimentary cyclicity has recently been attributed to climatic variation with Milankovitch cyclical control, there has been no quantitative analysis of the cyclicity in outcrop or attempts to examine the stable isotope geochemistry of the sequence in the context of the Milankovitch climate hierarchy. We measured bulk carbon and nitrogen stable isotopes, molecule-specific carbon isotopes of plant biomarkers, and proxies of water depth sensitive sedimentary facies (depth ranks) from eight sections of the Caithness to explore the quantitative aspects of this cyclicity and the way it affected lacustrine ecosystems. We sampled long stratigraphic sections as well as multiple shorter sections that cross geography (along the Achanarras fish bed horizon) to reveal both long-term environmental changes and higher frequency variations. Sedimentary facies and bulk isotopes show distinctive hierarchical Milankovitch frequency relationships (i.e., a strong signal of precession and the eccentricity-driven modulators of precession and obliquity.) Orbitally paced changes in lake depth modulate ecosystem dynamics with deep-water intervals tending to preserve relatively more 13C-depleted labile organic matter derived mostly from phytoplankton while shallower lake episodes preserve the emergent vascular plant material. Given the great thickness of the Orcadian basin sequence (>3 km) and the apparent well-developed Milankovitch periodicities, the Caithness Flagstone and contiguous units could, with the acquisition of long core records, provide a basis for an astronomically calibrated timescale for the Devonian, calibrating chaotic drift in solar system behavior, and a natural laboratory for understanding ecosystem dynamics through a major part of the terrestrialization process.
PP21B-1425
Paired 238U-230Th and 14C measurements on two Enallopsammia rostrata specimens collected from the Northwest Hawai'ian Islands
Deep-sea corals have received scientific attention because of interest in conserving the ecosystems they help form, and also because of their still largely untapped potential for recording past environmental change. Much is still unknown concerning the life history, growth characteristics and the ecosystem role of deep-sea corals. The paleoclimatic utility of deep-sea corals depends on identification of geochemical tracers of water mass properties as well as the development of a robust and precise chronologic framework. In this study, the vertical and radial growth rates of a cosmopolitan deep-sea coral species, Enallopsammia rostrata, are investigated by precise U-Th dating using MC-ICPMS methods specifically developed for younger, low 230Th samples. A living and fully tissue-covered specimen of E.rostrata from the Northwest Hawaiian Islands collected at 1108 m has been dated at 900 +- 12 years. The vertical extension rate is calculated to be .15 mm/yr, with over 650 yrs of time represented in 100 mm of growth. Another specimen, collected on the same dive, but at 534 m, is also precisely dated to be at least 1,380 years old, but may have portions of discontinuous growth. This is the oldest living specimen of E.rostrata that has been reported to date. This specimen differs in that its living tissue was confined to two of its branches and did not extend down to its base, unlike the 1108 m specimen. Dates from the center portion of one of the tissue-covered branches, however, give a vertical extension rate of .8 mm/yr with 120 years represented in 95 mm. Differences in the two specimens' growth rates likely reflect the differing environmental conditions of 1108 m versus 534 m, as well as the availability of exported particulate organic carbon. We will present radiocarbon dates from the same samples analyzed for U-Th age. This will give a history of 14C variability, which can be interpreted in some sense as a paleoventilation, from the North Pacific gyre spanning about 900 years at 1108 m, and possibly even a longer history at 534 m. The North Pacific is climatically dynamic, and these results could reveal important information concerning climate variability in this region.
PP21B-1426
Paleofires in South Brazilian Amazonia
Fire is a critical Earth-system process that has broad consequences for vegetation dynamics, biogeochemical cycling, and atmospheric chemistry. Variations in fire activity were an important trigger of past biotic reorganizations, and they are implicated as a primary agent of ecosystem change in the future. On century and millennial time scales, changes in fire activity are linked to changes in atmospheric and ocean circulation that affect regional vegetation patterns and fuel conditions. On longer time scales (>103 yr), fire occurrence is often related to variations in effective moisture arising from changes in the seasonal cycle of insolation, atmospheric composition, and land–ocean interactions. Past, present, and future human activities also affect fire–climate–vegetation linkages at all spatial scales. This study was carried out between Alta Floresta city, in the Northern portion of the Mato Grosso state and São Benedito region, located in Pará state in an area of the Central Crystalline Plateau covered by Dense Tropical Forest. This region has a climate type Am according to Köppen's classification, humid with a very intense dry season, between June to August with 1700mm of annual precipitation. The results for water content, density, granulometry and Hg concentrations allow some observations and suppositions about the sedimentation environment and climate conditions of the lake where the samples were collected. For the core sampled at the margin of the lake (9.118214°S, 56.265373°W), distinct sections could be identified. They consist of an upper organic-rich layer almost homogenous from the top of the core to 200cm, with the water content about 75% and the density between 0.2 and 0.3g/cm3. The granulometry is basically silt and clay, with an input of coarse sediments between 180 and 200cm. The Hg concentrations varied between 130 and 170ppb, with a peak of 260ppb between 50 and 65cm. Other phase is showed between 200 and 250cm, with highest values of density (near 1g/cm3) and lowest values of water content (below 50%). This phase showed a great input of sand and coarse sediments and the highest values of highest concentrations of Hg (285ppb) and charcoal particles in the entire core. These results may suggest that it was a dry phase, with biomass burning and episodes of intense rains (that caused erosion). From 250cm to the base of the core, the density values became highest (from 0.5 to 1.5 g/cm3) and the water content (60 to 25%) and Hg concentrations (280 to 80ppb) decreased.
PP21B-1427
Application of Clumped-Isotope Thermometry to Paleotemperature Reconstruction from Lacustrine Carbonates
Clumped-isotope thermometry (Eiler, 2007) is a novel method for paleotemperature determination based on the measurement of the abundance of 13C-18O bonds in carbonates. Gosh et al. (2006) have shown that the formation of 13C-18O bonds in carbonates is temperature dependent and the difference from the measured abundance compared to a stochastic isotope distribution can be related to the temperature of formation of the carbonate. With this method it is possible to obtain a temperature estimate that can be used in combination with the δ18O of the carbonate to reconstruct the oxygen isotope composition of the water. We will present first data obtained from the application of clumped-isotope thermometry to lacustrine carbonates from Lake Zurich. Lacustrine carbonates, because of their wide distribution, are important archives of climate change on the continents. However, the extraction of climatic information from carbonate oxygen isotope records is hampered by the dependency of δ18O of carbonates from both the temperature and the isotopic composition of the lake water. Clumped-isotope thermometry has the capability to solve this long standing problem. We are evaluating the potential of the method by analyzing Lake Zurich sediments and comparing it with historical water temperature measurements for the last 60 years. We have analyzed samples from a short core collected at the deepest point of the lake and annual varves were sampled from the years 1901 to 2000. We will show a comparison from the annual mean temperature of the Lake Zurich surface water derived by clumped-isotope thermometry from measured varves compared to recorded temperatures as well as first data from the authigenic carbonates, precipitated during 2008 in the surface water of Lake Zurich. Eiler, J. M., 2007. Clumped-isotope geochemistry - The study of naturally-occurring, multiply-substituted isotopologues. EPSL 262, 309-327. Ghosh, P., Adkins, J., Affek, H., Balta, B., Guo, W., Schauble, E. A., Schrag, D., and Eiler, J. M., 2006. 13C-18O bonds in carbonate minerals: A new kind of paleothermometer. GCA 70, 1439-1456.
PP21B-1428
Application of Molybdenum isotopes as proxy for weathering conditions
Molybdenum isotopes have become an important tracer for paleo-redox conditions in the oceans and paleo- oceans. Mo is also an essential enzyme co-factor in organisms and therefore important in studies of nitrogen fixation and reduction. With an increasing amount of data on the biogeochemical behavior of Mo in the oceans, it is now possible to better constrain the general behavior of Mo in the marine environment. However, the terrestrial geochemistry of Mo remains very poorly understood. For example, in contrast to earlier assumptions, recent measurements of river-waters (Archer & Vance(2008) Nature Geoscience, Vol. 1, P. 597; Pearce et al. (2008) Geology, Vol. 36, 3, P. 231) suggest that the Mo isotopic input into the oceans is more variable and more enriched in heavy isotopes then previously assumed. So far these variations do not easily correlate with the isotope composition of the source rocks in the catchment area of rivers. Therefore, fractionation during weathering and transport does indeed occur and it is essential to understand the processes controlling the Mo isotope composition of rivers, if one wants to utilize Mo isotopes as quantitative (paleo-)proxy for marine redox-conditions. The aim of this study is to investigate the biogeochemistry of molybdenum in weathering profiles. Results are promising and suggest that molybdenum isotopes may also become useful tracers for redox-conditions during weathering. Results show a clear correlation of Mo isotope compositions with rainfall gradients (i.e. redox conditions) in saprolites from Hawaii. Weathering profiles from Puerto Rico also imply a strong dependence of the Mo isotope composition on pH shifts within a weathering profile. More data are necessary to judge the influence of these processes on the overall Mo isotope composition of rivers and therefore the Mo input to the oceans. However, it becomes clear that the processes affecting Mo isotopes in the terrestrial environment might be complex and need to be investigated over geological time scales.
PP21B-1429
Using the Difference in 18O-enrichment in Pedogenic Carbonates and Freshwater Mollusk Shells as a Paleoaridity Proxy
Aridity is an important climatic attribute, yet few proxies exist to reconstruct this parameter in the past. Here we present initial results from a study using the difference in oxygen isotope value between pedogenic carbonate and freshwater mollusk shells as a proxy for aridity. These carbonates record the oxygen isotope value of the soil water and surface water from which they precipitate, respectively, as well as temperature- dependent isotopic fractionation. Evaporation causes 18O-enrichment of water that may influence the isotopic composition of both reservoirs. Soil water is more susceptible to evaporative enrichment, however, whereas surface waters more closely track the oxygen isotope value of precipitation. If both carbonates are collected from the same region, and if we assume they form at essentially the same temperature, the 18O-enrichment of soil carbonate (soil water) relative to bivalve carbonate (surface water) may reflect aridity. Alternatively, it is possible to determine the temperature of formation of each carbonate independently using the carbonate clumped isotope thermometer (Ghosh, et al., 2006), and then to solve for the oxygen isotope value of soil and surface water. To test the premise that the extent of 18O-enrichment in pedogenic vs. bivalve carbonate will reflect aridity, we collected pedogenic carbonate, freshwater mollusk shells, and stream water samples across an aridity gradient in the midwestern United States (MN, IA, NE, SD). We discovered that while pedogenic carbonates apparently formed from soil waters that are 18O-enriched relative to meteoric water, samples from drier regions are not more strongly 18O-enriched than those from wetter regions. We will extend the study to include samples from even drier regions, such as those in the southwestern US, as 18O enrichment may only become highly pronounced under very arid conditions. While our results have not yet established this approach as a viable tool for reconstructing aridity, they do offer insights into the influence of evaporation on the oxygen isotope composition of soil water. Evaporation may only have a significant impact under very arid conditions, and therefore it may be of less concern for paleoclimate reconstruction, which often relies on pedogenic carbonate from more humid environments, than previously thought. Reference: (1) Ghosh et al., 2006, GCA, 70: 1439-1456.
PP21B-1430
Ice at the Late Cambrian Equator: Climate Extremes During a Greenhouse Earth and the Last Gasp of Proterozoic-like Conditions
Upper Cambrian siliciclastics deposited along the equatorial shoreline of Laurentia preserve a record of freezing terrestrial conditions at a time commonly thought to have been a significant Greenhouse Earth. Evidence for freezing occurs within a discrete stratigraphic interval at several outcrops of the Furongian (~501-488 Ma) Jordan Formation in southeastern Minnesota, USA, and consists of large-scale (up to 1.5 meter) sandstone intraclasts preserved in swash-zone lithofacies. These clasts are interpreted to represent brecciated frozen beach and dune sand on the Late Cambrian shoreline. Intraclasts are presently uncemented; nowhere is the original cement preserved. However, intraclast features allow for interpretation of syndepositional mechanical behavior of clasts and characterization of early cement properties. Many allochthonous intraclasts preserve sharp, angular corners and delicate irregular edges, which indicate that such clasts were hard in the depositional environment. Conversely, rare clasts are folded, and relationships with host sediment dictate that folding occurred prior to burial. Such observations require that some clasts behaved ductilely in the depositional environment. Intraclasts also show apparent in situ disaggregation, suggesting that the cement was ephemeral in the sedimentary environment. Ice-cemented sand on modern frozen beaches provides a precise analogue for these sandstone intraclasts, exhibiting the critical inferred behaviors, physical characteristics and recurring morphologies observed in the Cambrian examples. Late Cambrian freezing conditions coincide with an abrupt faunal turnover recorded in these siliciclastics as well as in coeval circum-Laurentian carbonate rock. Thus, frozen shoreline deposits provide physical evidence for the long-postulated cold-water event that initiated this turnover. Brief episodes of equatorial cooling and an offshore carbonate platform dominated by microbialite, oolite, and ribbon rock indicate that the Late Cambrian was more akin to the Proterozoic than to the Phanerozoic. Given documented carbon isotope excursions and repeated mass extinctions of shelf fauna during this time interval, we propose that the Late Cambrian represents the last gasp of fluctuating, Proterozoic-like climatic extremes before the more buffered system of the Paleozoic.
PP21B-1431
Late Miocene to early Pliocene planktonic foraminiferal sea surface temperature estimates from DSDP Site 103 (northern Blake-Bahama Outer Ridge) based upon the Modern Analog Technique
The final closure of the Central American Seaway had significant and well-documented implications for global climate and oceanographic conditions, particularly in the northern hemisphere. In the North Atlantic, primarily at high latitudes, the most significant change was an overall increase in sea surface temperature (SST) via the intensification of the Gulf Stream leading to the onset of northern hemisphere glaciation. This study addresses the question of exactly how the closure of the Central American Seaway impacted the Gulf Stream prior to the complete isolation of the Caribbean from the Pacific and how this affected tropical and subtropical northwestern Atlantic SST. In order to answer these questions, planktonic foraminifers from the late Miocene through early Pliocene sediments of DSDP Site 103 (Blake-Bahama Outer Ridge) were analyzed using multivariate analyses and the Modern Analog Technique. These analyses, combined with qualitative faunal interpretation, suggest a trend of increasing SST between 9.7 and 3.5 Ma. Modern analog results provide mean annual SST estimates of 24.03°C at 9.7 Ma, 24.53°C at 8.2 Ma, 26.03°C at 5.2 Ma, and 27.98°C at 3.5 Ma. The increase in SST during the late Miocene and early Pliocene suggests considerable deflection of the Circumtropical Current and resulting intensification of the Gulf Stream in the period prior to complete closure of the Central American Seaway. These SST estimates indicate that significant oceanographic changes occurred in waters not only proximal to the Pacific- Caribbean gateway, but also in waters outside the Central American region, and provides evidence for the timing and magnitude of gradual oceanographic reorganization prior to complete closure of the Central American Seaway.
PP21B-1432
Simulating the Late Ordovician (445Ma) with the fully coupled Community Climate System Model (CCSM3).
The first of earth's five major extinctions occurred during the late Ordovician (445Ma) and was the only extinction to occur during a glaciated climate. Current theory suggests a rigorous ocean circulation, present during the onset of glaciation, increased ocean ventilation and promoted eutrophic conditions and a massive marine die-off. Despite glacial conditions, C02 levels for this period were thought to be elevated. Estimates range from as high as 15X to, more recently, 4X pre-industrial levels. We apply the fully coupled Climate System Model, Version 3 (CCSM3), to a series of experiments simulating the earth's climate for the late Ordovician and show results from both our 15X CO2 and 4X CO2 integrations. Of technical note, we are the first modelling group to accurately represent, in a fully coupled (or ocean only) model, the geographical land distribution for the Orodivican, a period with virtually no land in the northern mid- or polar latitudes. Previously, ocean models have required a small land mass at the north pole for numerical purposes. We apply a grid rotation technique to eliminate this problem. Preliminary results from simulations show a very well ventilated Panthalassic Ocean and an atmospheric general circulation which are both supported by proxy records.
PP21B-1433
An objective statistical test for eccentricity forcing of Oligo-Miocene climate
We seek a maximally objective test for the presence of orbital features in Oligocene and Miocene δ18O records from marine sediments. Changes in Earth's orbital eccentricity are thought to be an important control on the long term variability of climate during the Oligocene and Miocene Epochs. However, such an important control from eccentricity is surprising because eccentricity has relatively little influence on Earth's annual average insolation budget. Nevertheless, if significant eccentricity variability is present, it would provide important insight into the operation of the climate system at long timescales. Here we use previously published data, but using a chronology which is initially independent of orbital assumptions, to test for the presence of eccentricity period variability in the Oligocene/Miocene sediment records. In contrast to the sawtooth climate record of the Pleistocene, the Oligocene and Miocene climate record appears smooth and symmetric and does not reset itself every hundred thousand years. This smooth variation, as well as the time interval spanning many eccentricity periods makes Oligocene and Miocene paleorecords very suitable for evaluating the importance of eccentricity forcing. First, we construct time scales depending only upon the ages of geomagnetic reversals with intervening ages linearly interpolated with depth. Such a single age-depth relationship is, however, too uncertain to assess whether orbital features are present. Thus, we construct a second depth-derived age-model by averaging ages across multiple sediment cores which have, at least partly, independent accumulation rate histories. But ages are still too uncertain to permit unambiguous detection of orbital variability. Thus we employ limited tuning assumptions and measure the degree by orbital period variability increases using spectral power estimates. By tuning we know that we are biasing the record toward showing orbital variations, but we account for this bias in our statistical assessment of the significance of these orbital features. We use synthetic data with a priori known spectrum to model the effect our tuning algorithm has on the data, estimating how much power can be induced at eccentricity periods by tuning. In addition, we explore the implications of tuning only to obliquity or precession for the power concentrated at eccentricity periods.