PP33A-1522
Late Pliensbachian (Early Jurassic) Cold Seep Carbonates: Methane Release Prior to the Toarcian Oceanic Anoxic Event
We present evidence for methane seepage during the Early Jurassic (~ 185 Ma) in the form of newly discovered extensive occurrences of carbonate concretions that resemble the subsurface plumbing system of better known Cenozoic to Recent examples of cold seep carbonates. Columnar carbonate concretions of up to 1 m in length that are perpendicular to bedding, occur abundantly in the Upper Pliensbachian (upper Amaltheus margaritatus Zone, gibbosus Subzone) in outcrops in the vicinity of Riviere-sur-Tarn, southern France. Stable isotope analyses of these nodules show depleted δ13C values that decrease from the rim to the center from -18.8 to -25.7‰ (V-PDB), but normal marine δ18O values (-1.8‰). Computer tomographic (CT) scanning of the columnar concretions show one or more central canals that are lined or filled entirely with pyrite and late diagenetic minerals. Septarian cracks are also filled with secondary calcite and/or siderite. Based on our preliminary geochemical and sedimentological observations we suggest that these concretions formed as a combination of the anaerobic oxidation of methane (AOM) and sulfate reduction within the sediment. Previously, these concretions with one, two or more central tubes have been ascribed to the activity of an enigmatic organism, possibly with annelid or arthropod affinities, known as Tisoa siphonalis. Our results suggest tisoan structures are abiogenic. Interestingly, Tisoa siphonalis has been described from many locations in the Grands Causses Basin in southern France, and from northern France and Luxemburg, always occurring at the same stratigraphic level. Upper Pliensbachian cold seep carbonates thus possibly cover an area of several thousand square kilometers, largely distributed across the basin centres of the NW European epicontinental seaway. Our findings may have far reaching implications for understanding the Toarcian Oceanic Anoxic Event, which is interpreted to bear the hallmarks of catastrophic methane release from gas hydrates in the form of a pronounced negative C-isotope excursion. Carbon isotope analyses of Late Pliensbachian bulk carbonate (matrix) samples show clearly decreasing C-isotope values across the margaritatus Zone and reach -3‰ within the uppermost Pliensbachian spinatum Zone. We attribute this decrease to seeping fluids that led to induration and diagenesis. Isotope analyses of coeval belemnite rostra do not document such a negative C-isotope trend with values remaining stable around +2‰. Hence, if methane was seeping prior to the Toarcian OAE, it appears not to have imprinted global carbon reservoirs.
PP33A-1523
Local Perturbations to the Carbon and Oxygen Pools in the Lower Mississippian Madison Limestone, Montana and Wyoming
The δ13Ccarb values of samples from 22 localities across the Madison ramp and into the Antler foreland basin, exhibit an overall increase and subsequent decrease throughout the Kinderhookian and lower Osagean succession reaching maximum values of up to +7.6 ‰ PDB. The δ13Ccarb values show no significant isotopic variability between bulk carbonate and microsampled micrite, calcitic brachiopods and marine cements and the secular changes in the δ13Ccarb values are generally independent of facies including dolomitized inner ramp and calcareous outer ramp deposits. Several shorter-term fluctuations in δ13Ccarb values are superimposed on the long term trend (Kinderhookian-Osagesn) both in outcrop and in subsurface samples, both of which have been successfully used as chronostratigraphic correlation tools in the subsurface and outcrop. The shorter-term trends are intimately associated with sea-level fluctuations. Trends towards more positive values correlate with the transgressive hemicycle with maximum δ13Ccarb values occurring at the maximum flooding surface. Trends towards minimum values correlate with the regressive hemicycle or highstand systems tract with the minimum values occurring at sequence boundaries. These data indicate that transgressions are dominated by ocean basins which are capable of preserving light organic 12C while regressions are dominated by relatively well-mixed ocean basins with relatively more oxygenated waters. The increase in the global carbon isotope composition during the Early Mississippian in North America are 2 ‰ more positive than European successions and is attributed to enhanced primary productivity and storage of detrital organic matter in stratified ocean basins of North America. Superimposed on this global isotope variability is a geographic variation that is observed in both the δ13Ccarb and δ18Ocarb values at and around 3rd-order maximum flooding surfaces. The δ13Ccarb values show a progressive trend towards more depleted values (¡Ö 4 ‰) in a landward direction and the δ18Ocarb values culminate in a trend towards more enriched values (¡Ö 1.5 ‰) in a landward direction and with shallowing sedimentary facies. The geographic trends in δ13Ccarb and δ18Ocarb values have been interpreted as increased alkalinity and salinity, respectively, in a water mass characterized by increasing restriction towards land. Results from this study suggest that caution should be taken when interpreting a global seawater δ13Ccarb signal from shallow water carbonates.
PP33A-1524
Environmental Implications of Ediacaran C-isotopic Shifts
Compound-specific carbon isotope analyses of biomarkers show a widespread reversal in isotopic patterns in the Ediacaran. We analyzed oils and/or rocks from Eastern Siberia, Oman and Australia and confirmed that, in sediments and oils older than ~550 Ma, n-alkanes are enriched in 13C relative to the acyclic isoprenoids pristane and phytane. In younger sediments, the n-alkanes are depleted compared to these isoprenoids with the possible exception of those deposited during Phanerozoic oceanic anoxic events.1 Pristane and phytane are considered to be derived from photosynthetic primary inputs and, based on established biosynthetic relationships of organisms that dominate the modern ocean, should be 13C- enriched relative to n-alkanes from the same source. Therefore, the presence of n-alkanes with anomalously enriched isotopic compositions before 550 Ma may signify a high relative abundance of bacterial heterotrophs that extensively recycled organic matter (Corg) in the water column.2 The switch from anomalous isotopic ordering to isoprenoid: n-alkyl biosynthetic relationships characteristic of the Phanerozoic is observed to take place in the Ediacaran. In Oman, this coincides with the termination of the Shuram Excursion when marine carbonates show very negative δ13C values with no corresponding shift in the isotopic composition of co-occurring Corg.3 This has been attributed to the oxidation of a large pool of Corg in the deep ocean3 with a corresponding fundamental change in C-cycle dynamics.4 Several hypotheses, many ultimately linked to release of molecular oxygen via enhanced Corg burial, have been proposed to explain these phenomena. They include the evolution of: bilaterian animals with guts that rapidly export organic matter to the ocean floor as fecal pellets, reducing the amount of heterotrophy in the water column,2 biomineralization, providing ballast for organic export,4 and algae with decay-resistant biopolymers.4 Alternatively, tectonism and the rifting of Rodinia may have indirectly increased the burial flux of Corg. Additionally, the success of sponges and other filter-feeding Ediacaran fauna at the time may have been an important factor. References 1. e.g. Grice et al., 2005. Science 307, 706-709. 2. Logan et al., 1995. Nature 376, 53-56. 3. Fike et al., 2006. Nature 444, 744-747. 4. Rothman et al., 2003. PNAS 100, 8124-8129.
PP33A-1525
Mid-Miocene Carbon Isotope Maximum: Marine Versus Terrestrial Carbon Burial
A prominent, middle Miocene (17.5 to 13.5 Ma) carbon-isotope excursion ubiquitously recorded in carbonate sediments has been attributed to enhanced marine productivity and sequestration of 13C depleted organic carbon in marine sediments or enhanced carbon burial in peat/lignite deposits on land. We use a multi-proxy approach including estimates of paleoproductivity based on benthic foraminiferal accumulation rates, elemental ratios (Ba/Al, P/Al), the δ13C of bulk sedimentary organic matter, dissolution indices, and a geochemical model to test whether or not an increase in marine productivity can explain the observed positive shift in middle Miocene δ13C records. Our results indicate that marine paleoproductivity in the Atlantic Ocean is not related to the benthic foraminiferal δ13C excursion. The numerical box model confirms that marine productivity cannot account for the δ13C maximum. The model shows that sequestration of 1.5×1018 mol C in the terrestrial realm over a period of 3 myr leads to a 0.9 permil δ13C increase in the deep ocean, which is near to the observed records. Therefore, an increase in continental organic carbon sequestration is the most plausible way to enrich the ocean's carbon pool with 13C, which is consistent with coeval lignite deposits worldwide. The δ13C values of bulk sedimentary organic matter parallel the δ13C of dissolved inorganic carbon as reflected by benthic foraminiferal δ13C values suggesting no significant change in atmospheric pCO2 levels over the investigated period. These results are consistent with other geochemical studies highlighting the importance of factors other than, or in addition to, pCO2 in mid- Miocene climatic warmth.
PP33A-1526
Deciphering Carbon Isotope Excursions in Separated Biogenic and Diagenetic Carbonates
The long-term evolution of the carbon-isotope ratio in the sedimentary archive is classically linked with
changes in primary productivity and organic matter burial. There have been sudden and pronounced shifts,
so-called Carbon Isotope Excursions (CIEs) in the long-term trends as evidenced by synchronous shifts from
various basins. These geochemical perturbations may have various explanations such as changes of the
efficiency of the carbon sink; sudden infusion of isotopically-light carbon into the Ocean-Atmosphere system;
or advection of 12C-rich source from bottom water in a stratified water column. Beside the record of
primary changes in seawater chemistry, a possible diagenetic overprint may also mime such CIEs in the
sedimentary record.
The aim of this contribution is to illustrate through three critical intervals (the Early Toarcian, the K-P
boundary and the Mid-Miocene Montery Event) how the various micron-sized sedimentary particles
specifically record these CIEs, which are respectively associated with major paleoceanographical events. New
techniques for getting monotaxic calcareous nannofossil assemblages from the sediment (Minoletti et al.,
accepted) enable the isotopic measurement at various depths within the surface water and from bottom water
by analyzing early diagenetic precipitations (rhombs and micarbs). The integration of these high-resolution
isotopic signals in terms of amplitudes affords to recognize diagenetic artifacts in some sections displaying
coeval decrease in the carbonate content. For both Early Toarcian and K-P events, corroborative records of
CIE records in both primary calcite and bottom water carbonate indicate a global C-isotope perturbation of
the water column. For the Monterey event, the evolution of calcareous nannoplankton and the foraminifera
isotopic records are in overall agreement, but in detail, the coccolith-discoaster and foraminifer ratio in the
sediment, related to environmental changes, is likely to produce isotopic shift in the bulk carbonate record.
Contrasts in the amplitude of the carbon-isotope excursion at the single-species level compared to inorganic
calcite and organic subtrate, should improve our understanding of the evolution of the water column
composition through these major C-cycling perturbation events, and how marine calcifiers have fedback
during such events, and eventually contribute for better understanding ocean-climate dynamics through time
and into the future.
Minoletti, F., Hermoso, M. and Gressier, V. (accepted). Separation of sedimentary micron-sized particles for
palaeoceanography and calcareous nannoplankton biogeochemistry. Nature protocols.
PP33A-1527
Transient carbon isotope changes in complex systems: Finding the global signal, embracing the local signal
Global, transient carbon isotope excursions (CIEs) in the geological record are increasingly invoked as evidence of short-lived changes in carbon fluxes to/from the ocean-atmosphere-biosphere (exogenic) system. Reconstructing the dynamics of carbon cycle perturbation and response during such events requires that the global extent, magnitude, and temporal pattern of carbon isotope change are well understood. Unfortunately, no simple, globally integrated measure of exogenic δ13C change exists in the geological record: during major global perturbations even the best-case candidates such as deep-ocean carbonate δ13C values likely respond to a complex of factors including ocean carbonate chemistry and circulation. Here we consider the utility of organic carbon isotope records from two complex depositional systems common in the geological record, fossil soils and continental margin sediments, which are of interest in terms of their relationship to organic carbon cycling and records of past ecological change. Within both systems changes in ecology, climate, carbon source, residence time, and molecular composition have clear potential to modulate the preserved record of global exogenic δ13C change, compromising 1st-order interpretations of bulk or compound-specific isotopic records. Process-explicit eco- geochemical models, ideally combined with multi-substrate data, provide one approach to the isolation of global δ13C change and identification of local or regional processes reflected in such records. Examples from both systems drawn from ongoing work on the Paleocene-Eocene thermal maximum illustrate the potential pitfalls, as well as opportunities, afforded by coupled data/model assessment of transient δ13C changes in complex systems.
PP33A-1528
Further evidence of a global carbon cycle disturbance at the Silurian-Devonian boundary: A study of the δ13C record in the eastern-most Central Appalachian Basin (Ulster County, New York)
The Silurian-Devonian (Pridoli-Lochkovian) boundary has been scrutinized and researched more than any other boundary worldwide, and within the last fifteen years, a large carbon isotope disturbance over the boundary has been noted in Europe, Australia, and parts of North America. In this study, a carbon isotope analysis across the Silurian-Devonian boundary in the upper Rondout/Lower Helderberg carbonate sequences of the eastern-most Appalachian Basin has revealed a +2.6 per mil excursion over the transition (approximately 5 m of material), suggesting that the S-D carbon cycle disturbances measured elsewhere are indeed related. δ13C values for the studied section range from +0.6 per mil to +4.8 per mil. Sampling has revealed some variability limited to and directly below the excursion, with values ranging from +2.4 per mil to +4.8 per mil. Following the excursion, δ13C values level off around +1.3 per mil, before gradually increasing to around +2.8 per mil. Samples were also analyzed for δ18O, but early diagenesis appears to have altered these values. Given the positive δ13C values, and correlation with sections from around the world, the carbon isotopes in these samples are thought to have remained unaffected by diagensis. Additionally, the discovery of the positive excursion (noted at two sites approximately 20 km apart) suggests that the S-D boundary exists within the Upper Thatcher/Lower Ravena members of the Manlius/Coeymans Formations of the Helderberg Group. Regional unconformities and limited biostratigraphic records have allowed the precise location of the boundary to remain debatable; however, the addition of chemostratigraphic evidence supports an Upper Thatcher/Lower Ravena location for the transition. Many studies hypothesize that the excursion is related to heightened surface water productivity following a rapid sea-level regression and increased input of phosphate into ocean waters. Understanding of the S-D event is critical in understanding the future of carbon cycling under recent anthropogenic forcing.
PP33A-1529
Post-depositional Diagenetic Carbonate Precipitation, Methane Production and Climate- Driven Sedimentary Processes in the Northeastern Pacific Nitinat Fan
Ocean Drilling Program (ODP) Core 888B (48°10'N, 126°39'W), from the Nitinat Fan, Cascadia Margin is dominated by sediment deposited during glacial conditions and contains unconformities due to both non- deposition and turbidity current erosion. However, this core also displays a unique chemical signature indicative of post-depositional diagenetic CaCO3 precipitation due to CH4 oxidation. Climate history has been reconstructed based on core lithology, δ13C and δ18O of Globigerina bulloides, magnetic susceptibility, coiling ratios of Neogloboquadrina pachyderma, and 14C dates. The δ13C of marine carbonate, usually related to nutrient utilization, cannot account for the extremely negative G. bulloides δ13C at depths 110 mbsf (-6.5‰), 115 mbsf (-3.0‰), and 225 mbsf (-3.5‰). Instead, we posit that these spikes are a post-depositional diagenetic result of CaCO3 precipitation occurring where porewater alkalinity is rapidly changing due to CH4 oxidation. This secondary CaCO3 is strongly depleted in 12C due to the anaerobic oxidation of CH4 mediated by bacteria, which both favor the 12C isotope and consume CH4 with very negative δ13C. Finally, a telling correlation appears to exist between core lithology and CH4 peaks, leading us to conclude that the CH4 peaks and resulting diagenesis are thus a secondary consequence of climate- driven sedimentary processes. The first CH4 peak (93 ppmv; 78-113 mbsf) occurs within a sandy sediment facies containing wood fragments, possibly deposited during an early glacial period (Marine Isotope Stage 4), in which advancing ice carried terrigenous organic matter to the shelf edge. This wood matter then slowly decayed, consuming oxidants to the extent that methanogenesis occurred. The second CH4 peak (6863 ppmv; 185-240 mbsf), also correlated with a coarse sand facies, lacks evidence of terrigenous organic matter and thus may be related to lateral CH4 gas flow through the porous facies. Therefore, by providing coarse-grained and organic material to ODP Hole 888B, climate-driven deposition is in part responsible for the present microbial activity and the resulting post-depositional diagenetic carbonate precipitation within the sediments.
PP33A-1530
Scrutiny of a Diamictite to Cap-Carbonate Contact: Neoproterozoic Scout Mountain Member, Pocatello Formation, Idaho
It is common in Neoproterozoic strata worldwide to find 'cap carbonates' overlying inferred glaciogenic diamictites. Despite the fact that many of the contacts are sharp, the relationship between these carbonate units and the underlying diamictites is commonly interpreted to be conformable and to indicate post-glacial transgressive alkalinity events immediately following the aftermath of low-latitude glaciation. If, however, these cap-carbonate units are not conformable with the underlying diamictites, it might imply that they are recording local or global alkalinity events independent of the glacial conditions recorded in the diamictites. The cap carbonate in the Scout Mountain Member of the Pocatello Formation in southeastern Idaho is an excellent site to study the nature of the diamictite to cap-carbonate relationship because the contact is bracketed by two absolute ages: the contact lies ~100 m above a tuff that is 709 Ma and a reworked tuff ~50 m above the contact has been dated at 667 Ma (Fanning and Link, 2004). There is thus ca. 42 million years of time represented in a ~150-m-thick stratigraphic interval implying that there should be at least one unconformity present in this relatively thin stratal package. Most workers have placed a single major sequence boundary/unconformity in this 150-m-thick interval at the top of the cap carbonate, associated with a dolomite-chip breccia unit. Our facies analysis suggests that the most likely place for an unconformity is at the contact between the diamictite and the cap carbonate. The cap carbonate displays a genetic relationship with adjacent and overlying facies, and sits sharply on the diamictite unit (~50 m thick). The cap carbonate comprises pink laminated peloidal dolomicrite (1 m thick) and exhibits rare symmetric and interference ripples, possible hummocky-cross stratification, and negative δ13Ccarb values. Approximately 2 kms along strike, this cap dolomicrite facies is absent and the dolomite-chip breccia facies (< 2 m thick) sits directly on the diamictite instead. The breccia facies is clast-supported with an arkosic arenite matrix. The clasts are typically platy, up to 52 cm in length, and are composed of the cap dolomicrite facies. These clasts exhibit bedding-parallel and edgewise (chaotic and fan-shaped) fabrics, and rarely define scour surfaces by changes in clast fabric. Some clasts show soft-sediment deformation indicating that the cap dolomicrite parent rock was partially soft just prior to erosion and that these two facies were deposited synchronously. Both of the dolomite facies are sharply overlain by an arkosic arenite interval that is medium- to thickly-bedded, massive to trough-crossbedded, and contains load and groove casts. This arenite facies (> 14 m thick) also contains rip-up clasts of the cap dolomicrite facies at the base of some beds (ave. clast size of 3 cm dia.) and is interbedded with cm-thick intervals of the cap dolomicrite facies, suggesting contemporaneous deposition of all three facies. The three facies collectively suggest a wave- dominated shelf that was affected by large storms. This interpretation does not require an unconformity above the cap carbonate as previously suggested (represented by the breccia facies), but, instead, an unconformity at the base of the cap unit needs to be considered. This hypothesis has implications for understanding the Earth system's recovery from low-latitude glaciation.
PP33A-1531
Changes in terrestrial organic matter input to the Mendeleev Ridge, western Arctic Ocean, during the Late Quaternary
Hydrocarbons and glycerol dialkyl glycerol tetraethers (GDGTs) were analyzed in Late Pleistocene sediments of Core HLY0503-08JPC collected at the Mendeleev Ridge during the Healy-Oden Trans Arctic Expedition 2005 (HOTRAX"05) to investigate environmental changes in the western Arctic Ocean during the last full glacial cycle, ca. 130 kyr. Variations in long-chain n-alkane and GDGT concentrations correspond to alternated color banding, brown (interglacial/interstadial) and grayish (glacial/stadial) layers. Grayish layers are characterized by abundant higher plant n-alkanes and branched GDGTs, implying larger contribution of terrestrial plant and soil organic matter (OM) in glacial environments, possibly due to the deposition of fine-grained products of glacial erosion in the Amerasian basin. Concentration of lithic n- alkanes derived from mature OM, reflected as carbon preference index (CPI) reversely, shows sixteen major maximal peaks mainly at the boundary between grayish and brown layers. Some peaks are correlated to events of iceberg discharge and freshwater outbursts from proglacial lakes of the Eurasian and, possibly, Laurentide ice sheets, suggesting that other peaks may correspond to similar events.
PP33A-1532
Hydrodynamic Controls on Archaeal Tetraether Lipid Compositions in Washington Margin Sediments: Insights From Compound-Specific Radiocarbon Measurements
Continental margin sediments represent a large sink of organic carbon derived from marine and terrestrial sources. Archaeal glycerol dibiphytanyl glycerol tetraether lipids (GDGTs) are derived from both marine and terrestrial sources and have been used both for reconstruction of paleo sea surface temperatures and as an index of terrestrial carbon input to the marine sediments. However, the sources and modes of supply as well as the preservation of GDGTs in marginal sediments are poorly understood. The distribution and deposition of GDGTs is further complicated by hydrodynamic processes. We have analyzed a suite of surface sediment samples collected along a transect from the mouth of the Columbia River, across the Washington Margin, to the Cascadia Basin in the northeast Pacific Ocean. Sediments were separated according to their grain size and hydrodynamic properties, and the organic matter characterized in terms of its bulk elemental, isotopic, and molecular properties. Here we present radiocarbon measurements on individual GDGTs, alkenones, and fatty acids from size-fractionated sediments from shelf and slope sediments, and discuss the results in the context of previous studies of the molecular abundances and isotopic compositions of sedimentary organic matter for in this region. Systematic variations in elemental, isotopic and molecular-level composition are observed across the different particle classes. Moreover, these variations are manifested in the isotopic composition of different molecular markers of both marine and terrestrial sources organic matter. Both marine-derived lipids, including alkenones and marine archaeal tetraethers, and soil microbe-derived tetraether lipids show strong distributional and isotopic variations among the size-fractionated sediments. These variations in terrestrial and marine biomarker properties inform on the sources, particle dynamics, and transport history of organic matter buried on river-influenced continental margins. The implications of these findings for the application of molecular markers as proxies of organic matter input, and on the interpretation of past marine and continental environmental conditions from sedimentary records will also be discussed.
PP33A-1533 [WITHDRAWN]
The Late Cambrian SPICE Event: A Global Carbon Cycle Perturbation
The Late Cambrian SPICE event is an inorganic carbon isotope excursion that is documented in carbonate rocks around the world. At most localities, the excursion begins near 0 per mil and rises to between + 4 and +5 per mil. These localities include China, Siberia, Kazakhstan, Australia, and North America (Great Basin, US midcontinent, and Applachian regions). There is only one locality, in western Newfoundland, in which absolute values are significantly offset from the global pattern, perhaps reflecting regional influences that offset, but did not obliterate, the global signal. Parallel studies of carbonate associated sulfate, pyrite, and organic carbon at several of these sections also reveal positive excursions that closely follow the positive shift in inorganic carbon. Taken together, these parallel isotopic proxy records suggest that global organic carbon and pyrite burial increased; numerical modeling is used to explore the magnitude of the burial event and its implications for atmospheric oxygen. The event is characteristically marine, as expected; later Paleozoic events display decoupled C and S isotope shifts, which we interpret as heralding the introduction of significant quantities of terrestrial organic matter into the sedimentary record.