PP53B-1388 1340h
Trace and Rare Earth Elements in Microbial Deposits of Upper Jurassic Coral and Sponge reefs: Palaeoenvironmental Implications
Carbonate microbial deposits (microbialites) are abundantly represented throughout the Precambrian and Phanerozic, and developed in a wide range of palaeogeographic settings. These biologically induced carbonates can be used as paleoceanographic proxies, because they are characterized by a rapid lithification and their development well-above the sea floor reduced the contamination by porewaters. Moreover, rare earth elements (REE) are incorporated with uniform partition coefficients from seawater suggesting no biological fractionation occurred (Webb and Kamber, 2000). Major and trace elements (REE and Y, Zr, U, Th) were analyzed on 17 microbialites of Upper Jurassic coral and sponge reefs located on the northwestern Tethys and eastern Atlantic Ocean margins. According to the different palaeogeographic position of bioconstructions, microbialites were collected in lagoon, external-ramp, and deep-shelf to epicontinental basin settings (France). The very low content of siliciclastic material (SiO2 between 0.1 to 7%) recorded in microbialites of thrombolitic fabric does not significantly disturb the REE patterns. In contrast, lateral and intra-reef sediments, and to a lesser degree stromatolitic microbial deposits, are more subject to terrigenous contamination. Major discrepancies of both Ce and heavy REE concentrations recorded in microbialites from different sites hence reflect changes in seawater chemistry. The cerium anomalies in the range of 0 to 0.15 (negative) suggest the formation of microbial deposits in these coral and sponge reefs occurred both in oxic and suboxic seawaters. The microbialitic development, usually linked to a change in environmental conditions, would be induced by an increase in nutrient content rather than a high alkalinity. Microbialites collected in lagoon and deep-shelf settings have REE patterns similar to those of the actual seawater. These REE results confirm the use of microbialites in palaeoenvironmental reconstruction and Nd isotopes analyses in progress will place additional constraints on the marine chemistry. Webb and Kamber, GCA 64, 1557-1565 (2000)
PP53B-1389 1340h
Consistent, but Regional, Patterns of Organic Carbon and Thorium Normalized Calcite Paleo-fluxes for the late Pleistocene in the Eastern Equatorial Pacific
Biogenic production, and fluxes of labile organic carbon and calcite from the surface ocean to the deep sea, respond to both equatorial and extra-tropical forcing in the eastern Equatorial Pacific (EEP). This is an issue important to global climate change since the EEP is a major center for the transfer of carbon (as CO2) from the ocean to the atmosphere, and from the shallow ocean to the deep sea. The production and the fluxes depend on rates of coastal and equatorial divergence upwelling as well as the sources and chemistries of upwelled water. Additionally, the micronutrients Fe and Si currently limit production and the supply of these depends on both tropical and extra-tropical source areas. It has been proposed that glacial-interglacial changes in Si supply to the equatorial ocean, caused by Fe fertilization of the Southern Ocean, could have enhanced cold phase diatom production in the tropics and played a role in the Ice Age reductions of atmospheric CO2 content. If so, this effect should have been most important in the EEP region fed by upwelling off the coast of Peru (in the area of the South Equatorial Current). The Peru upwelling is fed by deep Equatorial Undercurrent water having the subantarctic of the SW Pacific as a source. We use a benthic foraminiferal transfer function for labile org. C. flux, calcite fluxes reconstructed using Thorium-normalization and correction for seabed dissolution, and Th-normalized opal accumulation rates to examine flux and diatom community response in the EEP over glacial-interglacial time. Using a suite of cores in the EEP we find that flux response is regional, but consistent for org C and calcite. Both of these show lower values along the equator and in the region affected by Peru upwelling during glacials. Both show somewhat higher values for glacials in off equatorial regions. While this might be caused by glacial shifts of the ITCZ towards the equator (away from its current position in the northern hemisphere), this will not account for opal accumulation rate data which show increases during glacials in the EEP influenced by Peru upwelling. A pattern not seen in other regions to the west and south. Our data are most consistent with biogeochemical shifts in the EEP that favor diatom production during glacials with increased HNLP conditions during interglacials. Regional production patterns and the timing of shifts indicate that the forcing came from the southern hemisphere.
PP53B-1390 1340h
Does the Rain Ratio Reign in the Eastern Equatorial Pacific? In Search of the Ideal Carbonate Dissolution Index and Changes in Bottom Water Carbonate Ion Saturation over the Last 25,000 Years
Testing mechanisms for changing atmospheric pCO2 on millennial timescales depends on accurate reconstructions of deep sea carbonate ion concentration and the organic carbon to calcite flux ratio [rain ratio] for both core-top and down-core samples. Both of these reconstructions require a reliable carbonate dissolution proxy. Accurate percent calcite dissolved values combined with pH data can be used to calculate dissolved inorganic carbon. To this end, we tested the sensitivity of two methods in estimating percent calcite dissolved at the sea floor: the menardii fragmentation index [MFI] and the foram weight loss method. Although MFI's response to dissolution is consistent with regional oceanographic parameters combining bottom water [CO3=] and respiration related dissolution associated with variable org C flux, our foram weight loss data show that only P. obliquiloculata lose weight in response to dissolution. Obliquiloculata weight loss, however, also shows a linear response to [CO3=] at 100 m water depth, the rain ratio and apparent oxygen utilization at thermocline depths. G. menardii and N. dutertrei shell weights appear to have multiple populations in the Eastern Equatorial Pacific [EEP] with different original shell weights. Furthermore, we present 230-Thorium-normalized calcite flux data for closely spaced core-top samples from the EEP. This new data is crucial for ground-truthing the rain ratio map for the EEP we had previously generated using biogeochemical modeling and MFI alone. Application of the MFI-reverse modeling approach to estimating paleo-delta calcite downcore in the EEP reveals a consistent LGM to Present pattern in four cores. Using MFI and reverse modeling, we estimate a 20 umol/kg increase in carbonate ion saturation during the LGM. Foram weight loss data for these cores, however, are unreliable in reconstructing paleo- delta carbonate. This is possibly related to the influence of strong gradients in productivity and pCO2 in surface waters as well as differing biological niches.
PP53B-1391 1340h
Influence of Sedimentation Rate on Diagenetic Alteration of Geophysical Signals by Anaerobic Oxidation of Methane
Geochemical and rock magnetic investigations of sediments from three sites on the continental margin off Argentina and Uruguay were carried out to study diagenetic processes driven by anaerobic oxidation of methane (AOM) focusing on iron minerals. The western Argentine Basin represents a suitable sedimentary environment to study non-steady state processes because it is characterized by highly dynamic sedimentary conditions such as gravity driven mass flow deposits. Mineralogical and bulk solid phase data document that the sediment mainly consists of terrigenous material with high contents of iron minerals. As a typical feature of these deposits distinct minima in magnetic susceptibilty are observed. Pore water data reveal that these 'susceptibility gaps' coincide with the current depth of the sulfate/methane transition (SMT) where hydrogen sulfide is generated by the process of AOM. The released hydrogen sulfide reacts with the abundant iron (hydr)oxides resulting in the precipitation of iron sulfides accompanied by a nearly complete loss of magnetic susceptibility. Modelling of geochemical data showed that the magnetic record in this area is highly influenced by drastic changes in sedimentation rates (SR) which occurred during the transition from the last glacial to the Holocene. We assume that the strong decrease in SR encountered during this glacial/interglacial transition induced a fixation of the SMT at a specific depth. The stagnation is likely to have caused an enhanced diagenetic overprint of iron (hydr)oxides within a distinct sediment interval. This assumption was further substantiated by the numerical modelling in which the SR was decreased from 100 cm/kyr during glavial times to 5 cm/kyr in the Holocene and the methane flux from below was fixed to a constant value. To obtain the observed geochemical and geophysical patterns a time period of approximately 8,000 years is needed - closely correlating to the timing of the glacial/Holocene transition.
PP53B-1392 1340h
Influence of Environmental Factors over the Incorporation of U, Cd, and Minor Metals into Foraminiferal Calcite: Results of Summer 2004 Culture Experiments
During the summer of 2004, we conducted laboratory culture experiments with planktonic foraminifera ({\it O. universa} and {\it G. bulloides}) to constrain the relationships between U, Cd, Mg, and Sr and environmental factors (temperature, salinity, carbonate ion concentration, and light levels). In order to test the hypothesis that photosynthesis by algal symbionts influences U incorporation through its effect on local pH, we cultured {\it O. universa} (a symbiont-bearing species) under low light conditions in seawater modified to produce a range of carbonate ion concentrations. We will compare this curve to one we generated in previous experiments using {\it O. universa} cultured under high light conditions. We also cultured {\it G. bulloides} (a symbiont-free species) in seawater of variable carbonate. In addition, we conducted experiments to evaluate the influence of salinity and temperature on U in {\it G. bulloides}, and the influence of salinity and temperature in {\it O. universa}. Finally, we evaluated the influence of temperature on Cd in {\it G. bulloides}. We will report the results of these experiments at the Fall AGU Meeting.
PP53B-1393 1340h
Implications of Mn-Mg-rich Contaminant Phases for Mg/Ca Past Temperature Reconstructions
In recent years, many paleoceanographic studies have employed the foraminiferal elemental composition as a proxy of sea water paleoenvironmental conditions. In particular, the foraminiferal Mg/Ca ratio has been widely used as a sea water paleotemperature estimator. Nevertheless, this technique is still in development and special attention needs to be focussed on those factors which can introduce potential biases in the Mg/Ca ratio and therefore leading to inaccurate reconstructions of past-oceanographic scenarios. This study aims to solve or reduce overestimations of past sea water temperatures derived from a contaminant Mg-rich phase associated to Mn-enrichments. We address this problem using samples from ODP 1240 (0°01.311'N, 86°27.758'W, 2921m water depth) from the Panama basin which is a Mn-enriched basin. By means of the LA-ICP-MS capabilities at the Research School of Earth Sciences (Australia) we have documented and identified the presence of Mn-Mg-rich contaminant phases at the inner part of the foraminifera walls. In order to asses the efficiency of the classical Mg/Ca cleaning protocols when dealing with the presence of these contaminants, we have selected a set of samples with different contents in Mn and cleaned them with both the oxidative and the full reductive methods. LA-ICP-MS and bulk ICP-MS results show that the reductive cleaning successfully removes the Mn-Mg rich layers from the samples whereas the oxidative cleaning does not effectively eliminate the contaminant phases from the selected samples. In order to test the efficiency of the different cleaning steps we have analyzed (ICP-MS) the residual fractions from each cleaning step using two sample sub-sets. The order in the reductive and oxidative step was reversed for each sub-set. Results suggest that the Mn-enrichment occurs in two different phases, one which is eliminated by either the oxidative or the reductive step but a second one which only is removed by the reductive step. This second one is also associated to Mg-enrichments. As a consequence of this contamination phase, temperature reconstructions can provide overestimations in the range of 0.5-4°C. In addition, our results clearly demonstrate that there are no significant differences in the Mg/Ca ratio when the order between reductive and oxidative steps is altered. A further insight on the nature of these Mn-Mg-rich contaminant phases has been obtained by means of complementary analytical techniques (XRD, SEM, EDX). The results obtained demonstrate that the contaminant phase is mainly a Mn-Mg carbonate known as Kutnahorite. This mineral has been previously identified in Mn-rich crusts of marine sediments but this study has been able to identify it for the first time within foraminiferal tests.
PP53B-1394 1340h
Evaluating the Mn/Ca Ratio of Foraminiferal Calcite Determined by Flow-Through ICP-MS as a Proxy for Terrigenous Input, Upwelling, and Carbon Rain Rate
The Mn/Ca ratio of the biogenic calcite preserved in deep-sea sediments has potential as a tracer of terrestrial input, upwelling, and carbon rain rate over geologic time scales. The basis for this potential lies in features of the Mn cycle in the oceans, which are well known. Manganese is a biogeochemically reactive element, but has a lower affinity for dissolved oxygen and organic matter than iron, making it more stable over short time scales, and less affected by speciation. Depth profiles of Mn in oligotrophic ocean waters show a sharp contrast between low concentrations in deep water (0.20 nM) and relatively high concentrations in the mixed layer (2-5 nM). Mn oxides are stable in high oxygen environments but reduced in the suboxic conditions found in the oxygen minimum zone (OMZ). This behavior makes the intermediate water to surface water concentration ratio of Mn sensitive to the intensity of the OMZ, an artifact of the carbon rain rate, and dust/river input. In sediments, suboxic dissolution is balanced by the formation of carbonate making Mn highly reactive during early diagenesis. These features of the Mn cycle in seawater make the Mn/Ca ratio of foraminifera an attractive paleoproxy, but only if the primary signature can be recovered after diagenetic alteration. Recently our laboratory developed a flow-through extraction system that gives us fresh insight into this problem by making it possible to separate mineral phases associated with the foraminiferal fraction by differences in their solubilities. This paper examines foraminiferal Mn/Ca ratios in core tops and down core records from the eastern equatorial Pacific determined with this new technique. We access the potential of flow-through Mn/Ca by comparing its record to those of Mg/Ca and stable isotopes.
http://www.wmkeck-icpms.coas.oregonstate.edu
PP53B-1395 1340h
$^{15}$N depleted nitrogen isotope values in Cretaceous black shales: paleoceanographic event or diagenesis.
Nitrogen isotopic values of bulk sediment samples in black shales are almost exclusively near 0 $\permil$ and C/N ratios are high (20-35). Sequential extraction of exchangeable and non-exchangeable N fractions demonstrates that the inorganic N fraction is negligible and bulk sediment data reflect the organic N fraction. The trend in \delta$$^{15}$N and C/N has been observed in numerous localities and depositional environments in the mid-Cretaceous (Demarara Rise, DSDP Sites 367, 603B and 530, the Cretaceous Western Interior Seaway, Wunstorf, Germany, and Bahloul, Tunisia) as well as the Toarcian of England and in Quaternary Mediterranean Sapropels. Three explanations are considered: 1) That primary production during black shale deposition was dominated by a unique community composed of nitrogen-fixing cyanobacteria; or 2) utilization of a $^{15}$N depleted ammonium source by another set of biota; or 3) that the values are the result of diagenetic loss of N prevailing in C$_{org}$-rich strata with a low capacity for N adsorption. The depleted nitrogen isotopic values suggest that nitrogen fixation or utilization of a depleted nitrogen source (e.g. ammonium) may have been important which is plausible in consideration of nitrogen deficiencies that might characterize widespread deep-water anoxia. Secular variations in nitrogen isotope values across the Cenomanian-Turonian Boundary of ODP Site 1261, Demarara Rise, show a shift from -1 to -3$\permil$. One interpretation of the origin of the excursion could be that a fraction of the organic matter was produced utilizing a $^{15}$N depleted ammonium source, assuming that the nitrogen isotopic composition of fixed oceanic nitrogen does not change. However, it should be noted that large (greater than 1$\permil$) variations in nitrogen isotope values are observed above and below the Cenomanian-Turonian Boundary Event and have no known paleoceanographic forcing. The variations in nitrogen isotope values are matched by antithetic variations in C/N indicating diagenetic N-loss that could be interpreted as the primary control on the nitrogen isotope variations. The high C/N ratios probably result from selective removal of N-enriched compounds in the water column and during burial diagenesis. Clay-poor, C$_{org}$-rich sediments have a low sorptive capacity, allowing diffusion of dissolved N out of the C$_{org}$-rich sediments resulting in higher C/N ratios. Pore water ammonium concentrations are mM, indicating low sorption and high rates of diffusion into overlying strata. Coupled C/N and N-isotope variations occur by the removal of a $^{15}N$ enriched fraction. Protein degradation has been suggested to result in negative isotopic shifts but would not result in large increases in C/N and the isotopic shifts are limited by the protein richness of the primary organic matter and the internal isotopic heterogeneity of the organic matter pools. Although we favor an explanation that involves a primary signature, diagenesis cannot be excluded in low $\delta$$^{15}$N values.
PP53B-1396 1340h
Evaluation of Geochemical Proxies Preserved in the Sapropel Record from the Eastern Mediterranean Within the Pliocene-Holocene Time Interval
Considerable research has been devoted during the last decade to productivity and oxygenation proxies of sapropel deposition in the Mediterranean. Pliocene-Holocene sapropel layers from Mediterranean basins are considered to be an important key to assess the role of productivity vs. anoxia in organic matter (OM) accumulation. Evaluation of proxy preservation within these sediments is therefore crucial to further investigate forcing mechanism for OM deposition. With this aim, diverse layers from Pliocene, Pleistocene and Holocene sediments recovered at ODP Site 964 in the eastern Mediterranean have been analyzed. Ba excess is recognized as a reliable proxy for enhanced productivity because it derives from barite crystals originated in the water column. Additionally, their S isotope composition revealed that barite is an authigenic phase (Paytan et al., 2004). This proxy also revealed of exceptional importance in Mediterranean basins because some sapropels have been partially or totally oxidized. Assessment of productivity variations cannot be based therefore on the OM record which is strongly subjected to diagenetic preservation. Certain redox-sensitive elements as Mn were also subjected to remobilization during oxidation precipitating upon encountering the oxidation front. However some well preserved trace-element ratios, used as oxygenation proxies, are still recognized. These consistently point to a relatively oxic environment during the deposition of Quaternary sapropels while Pliocene sapropels tend to be characterized by lower oxygen conditions. Ba excess also indicates that barite accumulation rates within sapropels decreased through time, being considerable higher during the Pliocene. Thus, elevated primary export production could have enhanced oxygen consumption leading to lower oxygen levels at this time. Paytan, A., F. Martinez-Ruiz, M. Eagle, A. Ivy, and S.D. Wankel (2004) Using sulfur isotopes in barite to elucidate the origin of high organic matter accumulation events in marine sediments. Sulfur Biogeochemistry, GSA Special Paper, 379, 151-160.