PP51G-01 08:00h
Separating Palaeoproductivity and Preservation
Understanding climate related changes in concentration and isotopic composition of the atmospheric carbon pool, is a prerequisite for understanding anthropogenic or natural climate forcing. The deep ocean includes the largest pool of carbon and has major influence on the atmosphere on decadal and longer time scales. Its carbon budget depends on export production rate and ventilation steered organic matter (OM) degradation. However, its influence over time is poorly quantified mainly since current proxies for productivity often suffer from modification e.g., dissolution, aerobic decay, or migration. This is often neglected, resulting in misleading interpretations and forcing-mechanism propositions. Separating diagenesis from productivity forms a key question in world-wide operating research programs such as IMAGES (International Marine Global Change Study), JGOFS (Joint Global Ocean Flux Studies) and WCRP (World Climate Research Program). Here we circumvent such modifications by using recognizable OM particles that remain at a stable position within the sediments and for which the degradation rates for various environments are known. Compilation of data on postdepositional aerobic OM degradation at natural oxidation fronts in sediments of the late Quaternary Madeira Abyssal Plain f-turbidite and the last eastern Mediterranean sapropel (S1) reveals a stable ranking of the OM components with respect to their degree of degradation. In contrast to previous work, which mainly emphasises the effects of decay, we use this to separate degradation from bioproductivity. Application of our method will be documented by a reconstruction of past export productivity and deep ocean ventilation of the eastern South Atlantic Ocean and the Southern Ocean (Atlantic sector) since 145,000 BP. We will show that export productivity is regionally defined whereas ventilation and oxygen content variations of the deep ocean occur basin-wide. We indicate that a more active working "biological pump" as a cause for lower glacial atmospheric [CO$_{2}$], might have been overestimated and address possible effects of ventilation changes and post depositional aerobic OM decay on the atmospheric \Delta$^{14}$C
PP51G-02 INVITED 08:15h
Diagenetic Controls on the Age Distribution of Marine Sedimentary Components
Molecular-level radiocarbon studies of source-specific biomarkers have revealed that different components isolated from marine surface sediments can exhibit a wide spectrum of apparent ages. Biogenic materials (e.g., phytoplankton detritus) raining down from the overlying surface ocean deliver modern sources of carbon to the sea floor. "Old" organic carbon (OC) can reflect terrigenous inputs, such as kerogen from sedimentary rock weathering or products of terrestrial photosynthesis stored in intermediate reservoirs (e.g., soils) on the continents. The presence of "pre-aged" marine OC in surface sediments indicates the occurrence of advective processes that result in redistribution of organic matter associated with low-density aggregates or fine-grained particles. These latter inputs contribute to the typically greater radiocarbon age of bulk OC relative to that of co-deposited planktonic foraminifera in surficial sediments. Once deposited, further changes in age distribution may occur by virtue of the differing reactivities and modes of delivery of organic matter inputs. In particular, recently produced biogenic materials may be preferentially degraded relative to allochthonous terrigenous and marine organic components as a result of the intrinsic chemical stability of the latter or their physical association with mineral matrices. Diagenetic changes in the proportions of these different inputs shape the sedimentary record and influence interpretation of corresponding proxies. Moreover, climate-driven changes in depositional conditions under which sedimentary components are buried likely modulate their degree of preservation. In order to investigate the influence of diagenetic processes on molecular and microfossil records, we have examined radiocarbon age relationships between different components (algal and terrestrial biomarkers, bulk OC, planktonic foraminifera) of surficial sediments recovered from a range of depositional regimes. We compare age distributions in surface sediments from settings that receive similar inputs but differ in bottom oxygen content, as well as between core samples taken within and below the sediment mixed layer. We discuss the importance of pre- and post-depositional processes on the formation of the sedimentary biomarker record and their implications for interpretation of molecular and microfossil proxies.
PP51G-03 08:30h
Diagenesis, redistribution, and dilution of biomarkers in surface sediments of the Ross Sea
Concentrations and carbon-isotopic compositions of sterols and of total organic carbon (TOC) have been measured at 18 stations in the Ross Sea. The samples were collected in December, 1997 and January, 1998 as part of the ROAVERRS program (Research on Ocean-Atmosphere Variability and Ecosystem Response in the Ross Sea). Numerous correlative observations of surficial and benthic conditions and processes are available from that program. Moreover, radiocarbon analyses are available for 39 sites within the study area. Independent of productivity, concentrations of TOC are highest at basin and slope sites and lowest on crests (sites with high measured bottom currents) and banks. Concentrations of sterols vary from 3 to 1120 (umol sterol C)/(mol TOC). Radiocarbon levels indicate that low concentrations of sterols are due mainly to dilution of autochthonous inputs by older materials. The effect is stronger at bank and crest sites and the diluent is mainly reworked sedimentary debris rather than recycling continental debris. Calculations comparing the decay of carbon-14 and the degradation of TOC and sterols indicate that the first-order rate constant for degradation of sterols exceeds that for TOC by 0.002 inverse years. Polytopic vector analysis resolves four end-member suites of sterols. The two most abundant account for 33 and 28 percent of all sterols and derive respectively from diatoms and from heterotrophs. The others (each 20 percent) derive from the Phaeocystis-dominated community associated with blooms in the Ross Sea Polynya and from a combination of dinoflagellates and microbial heterotrophs. The isotopic compositions of TOC and of algal sterols are weakly correlated (correlation coefficient = 0.51). Weighted-average delta values for algal sterols cover the range from -33.2 to -27.6 permil. In contrast, delta values for TOC cluster in two groups at -27 and -23 permil. The latter sites are concentrated at the western margin of the Ross Sea. This distribution duplicates that observed in particulate organic carbon in surface waters. At least for TOC, therefore, redistribution of organic carbon has not obscured the strongest isotopic feature in the Ross Sea.
PP51G-04 08:45h
Nitrogen isotopic composition of ammonium released during diagenesis of organic matter under steady-state and non-steady state conditions in marine anoxic sediments
The $\delta$15N of organic matter in marine sediments has been successfully used as a paleoceanographic proxy. However, diagenesis may alter the original isotopic ratio. One approach to evaluate the sign and magnitude of the diagenetic impact is to measure the isotopic composition of ammonium that is produced during the degradation of nitrogenous organic matter (N-org). If diagenesis of organic matter is the only process affecting ammonium concentration and its isotopic composition, the $\delta$15N of ammonium should reflect the isotopic ratio of the N-org plus fractionation, if any. We evaluated the impact of diagenesis on $\delta$15N of N-org on a time scale of few thousand years in two California Borderland Basins, Santa Barbara (SBB) and San Nicolas (SNB). We found ammonium to be about 2.5 \permil heavier than the bulk N-org. One interpretation is that N-org consists of multiple, isotopically distinct fractions of organic matter with variable lability. The most labile fraction must be 2.5 \permil heavier than the bulk N-org. The relatively large loss of N-org in SNB (50 %) should have resulted in a 2 \permil shift in $\delta$15N of N-org downcore, while in SBB, higher accumulation rates led to smaller loss of N-org (15%) and little change in its isotopic composition. Sediment from the Peru-Chile margin, collected during ODP Legs 201 and 202, was used to evaluate the diagenetic impact on a time scale of millennia to millions of years. In rapidly accumulating anoxic coastal sediments with constant $\delta$15N in the N-org , we found diagenetic fractionation is less than 1 \permil. Several other ODP sites are also strongly influenced by non-steady state conditions, and the isotopic composition of pore water ammonium primarily reflects changes in $\delta$15N of the source organic matter through time. Preliminary model results show that isotopically heavy ammonium released from organic matter deposited within the last 12-15 Kyr has diffused about 30 m into the sediments. This result underscores the importance of recognition and proper evaluation of non-steady state conditions.
PP51G-05 09:00h
Preservation of Plant Biomolecules and the Relevance to the Interpretation of Paleoenvironmental Signals: Tertiary {\it Metasequoia } Fossils as Examples
The degradation and preservation of biomolecules in plant tissues not only affects the inference on paleoecology of ancient plants but also bears significance in the interpretation of paleoenvironmental signals. Using a combined SEM and geochemical approach, we are able to show the source, liability, and preservation of structural biopolymers from morphologically well-preserved {\it Metasequoia } tissues from three Tertiary deposits. We detected a continuum of biomolecular preservation in this evolutionarily-conserved conifer. Pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) was applied to solvent-extracted residues from both fossil leaf and wood remains in comparison with tissues from their living counterparts. The late Paleocene-early Eocene leaves from Ellesmere Island, Canadian Arctic Archipelago, exhibit the best quality of biochemical preservation and show pyrolysis products derived from labile biomolecules characterized by large amounts of polysaccharides. These labile biomolecules are the oldest record of these kinds so far characterized by the pyrolysis technology. The middle Eocene leaf tissues from Axel Heiberg Island, Canadian Arctic Archipelago, yielded slightly lesser amounts of polysaccharide moieties, but the lignin products are similar to those identified from the Ellesmere Island fossils. Compared with these Arctic materials, the {\it Metasequoia } leaves from Miocene Clarkia, Idaho, USA, show the lowest quality of molecular preservation, characterized by a dramatic reduction of polysaccharides. This continuum of relative quality of biomolecular preservation is further confirmed by SEM observations of transverse sections of these fossil leaves. The investigation revealed tissue-specific degradation, and our data support the in-situ polymerization hypothesis for the origin of long-chain homologous pairs of aliphatic n-alk-1-enes/n-alkanes as leaf alteration products. The preferential degradation and selective removal of polysaccharides may be significant in estimating plant paleo-productivity whereas the addition of aliphatic components to the leaf wax lipid pool may potentially contribute to the accuracy of compound specific isotope analysis using these lipid markers.
PP51G-06 09:15h
Diagenesis and Paleotemperature Reconstruction: A New Look at Old Coral
The strontium to calcium ratio (Sr/Ca) of the aragonite skeleton of massive reef corals decreases with increasing seawater temperature and is widely used as a paleothermometer. However, Sr/Ca measurements of fossil coral from both submerged and uplifted reefs have yielded paleoSSTs several degrees colder than those based on other marine proxies and their accuracy has been questioned. Here we show that abiogenic (secondary) aragonite crystals begin to grow within vacant skeletal pore spaces soon after tissue uplift (evacuation) has occurred. While the presence of secondary aragonite in living and fossil corals cannot be detected by X-Ray Diffraction, these crystals are distinguished from those of the primary skeleton by their large size. (>10 microns width) and irregular orientation. Discrete SIMS ion microprobe analyses of secondary crystals in two chunks of fossil Porites coral retrieved from 79-82m depth in a Tahiti barrier reef drill core (149W, 17S), yielded Sr/Ca ratios greater than 1mmol/mol higher than that of the primary coral skeleton, equivalent to 3 degrees cooler SST. Our results suggest that bulk sample analyses of these fossil corals would yield SSTs several degrees cooler than were actually experienced. While radiocarbon analyses indicate that the secondary crystals in these samples are up to 200 years younger than the primary coral skeleton, their impact on the Tahiti radiocarbon chronology is small relative to the impact on paleoSST reconstructions. Nevertheless, using SIMS ion microprobe, we have extracted Sr/Ca-derived SSTs from these fossil corals by targeting the centers of pristine skeletal septa with a 10-m diameter ion beam spot, avoiding adjacent pore spaces occupied by the secondary needles. We obtained western Pacific SSTs ~0.5-1.5 degrees C cooler during the Bolling-Allerod relative to the present day, in good agreement with SSTs derived from Mg/Ca ratios of calcitic foraminifera. Our results indicate that employment of microanalytical techniques capable of selective analysis of pristine skeleton is warranted for accurate paleoSST reconstructions from ancient corals.
PP51G-07 INVITED 09:30h
Combined Rock Magnetic/Geochemical Proxies of Redoxomorphic Iron Mineral Diagenesis Based on Fast Core Logging Techniques - Examples from the Equatorial Atlantic
Many sediments show cyclic variations in organic carbon content representing climate controlled changes in paleoproductivity and bottom water oxygenation. In organically enriched layers Fe(3+) bearing magnetic minerals like magnetite and hematite are successively reduced to Fe(2+) and dissolved. The iron precipitates in situ as non-magnetic Fe(2+) sulfides/oxyhydroxides or diffuses upwards forming new magnetic and non-magnetic Fe(3+) minerals above the active Fe(2+)/Fe(3+) redox boundary. These processes leave characteristic imprints on all environmental magnetic parameters, in particular those related to absolute and relative magnetic mineral contents and magnetic grain size estimates. We propose new selective proxies to identify and quantify reductive iron oxide dissolution (Fe/kappa) and precipitation (kappa/Ti) based on logging data detected by X-ray fluorescence (Fe, Ti) and magnetic susceptibility (kappa) core scanning. Combining these rapid and non-destructive methods, diagenetically altered sediment sections can be detected at unprecedented speed and precision - provided that the primary composition of the terrigenous sediment fraction has been fairly constant through time. These integrated proxies help to distinguish climatic and diagenetic signal patterns, to quantify secondary magnetic mineral phases and to reconstruct the pristine magnetic susceptibility signal indicative of terrigenous content. XRF data are largely unbiased by mineralogy and do not follow susceptibility lows caused by conversion of ferri- into paramagnetic iron species. The (Fe/kappa) ratio therefore highlights intervals, where such alterations occur. An alternative hysteresis-based proxy is the ratio of high-field to low-field susceptibility. Sedimentary Ti is an inert and insoluble, hence conservative ion of terrigenous origin. Its accumulation is generally highly correlated with that of primary Fe. XRF-based Ti logs can therefore be used as normalizers for kappa and Fe logs. This yields two additional proxy ratios for ferrimagnetic iron precipitation (kappa/Ti) and total iron enrichment (Fe/Ti). All mentioned proxy parameters can be regionally calibrated by detailed rock magnetic and geochemical analyses on single samples. A collection of Late Quaternary sediment sequences from the Central Equatorial Atlantic Ocean serves as case study to demonstrate applications and interpretations.
PP51G-08 09:45h
Formation and Preservation of Authigenic Metal Sedimentary Signatures: Carbon Cycle Proxies and Insights from Transition Element Stable Isotopes
The solubility of U, Cd, Re, and Mo decreases under the reducing conditions commonly encountered within the upper few centimeters of ocean margin sediments. Because of this decrease in solubility, the presence or absence of these elements in marine sedimentary deposits has been interpreted as a signature for reducing conditions in the past. However, our understanding of these elements has yet to mature to the point where their quantitative utility is possible. One difficulty in exploiting these elements derives from the fact that two processes may foster sedimentary reducing conditions: low bottom water oxygen concentrations and high organic carbon decomposition rates. These two factors create shallow oxygen penetration depths where diagenetic reaction zones (e.g., oxygen consumption, nitrate, Mn, and Fe reduction) can be spatially compressed to the point where they functionally overlap. It is suggested here that because of this diagenetic compression, the relationship between oxygen penetration into the sediments and U and Cd accumulation is non-linear. In addition, it appears that U, Cd, and Mo are each sensitive to the organic carbon flux arriving at the seafloor, perhaps indicating that under certain circumstances these elements may serve as quantitative proxies for carbon accumulation. Our emphasis for this study is on those processes that regulate the deposition of these elements in continental margin sediments. It is within these sediments where our target elements offer the brightest potential to serve as proxies. It is also these sediments where variations in biogenic material dilution and preservation wreak the most havoc on more traditional biogenic proxies thereby necessitating a reliance on alternative or complementary proxies to interpret the sedimentary record. Because it is often difficult to separate authigenic signatures from background signatures during the earliest stages of authigenesis (particularly for Mo) we present data on Mo isotope compositions that further allow us to refine our interpretations of the sedimentary record. It is clear from this work that Mo is initially taken up in sedimentary sequences during the earliest stages of diagenesis. This initial stage of authigenesis is associated with isotope fractionation resulting in solid-phase Mo being isotopically light relative to the initial material, and this authigenic phase continues to be enriched in the heavier isotope as diagenesis proceeds.