OS21A-1194 0800h
Marl-Chalk Rhymicity in Plio-Pleistocene Black Sea Sediments
During Leg 42B of the Deep Sea Drilling Project in the Black Sea, two calcium carbonate-rich Plio-Pleistocene sequences totaling about 250 m in thickness were retrieved at Site 380 near the Bosporus Straits. These "Chalk units" are characterized by hundreds of rhythmically deposited, cm-to dm-thick marl-chalk couplets marked by varying amounts of carbonate and organic matter contents. Whereas the couplets in the Upper Chalk unit are largely structureless, in the Lower Chalk unit the chalk half-couplets occur as (1) finely laminated beds, (2) moderately bioturbated beds exhibiting a lensoidal pattern, or (3) intensely bioturbated homogeneous structureless chalk beds. In contrast, the marl half-couplets are by and large, structureless. Textural and mineralogical studies show that the carbonates in the marls and chalks in both the Chalk units are microcrystalline low-magnesian calcite. The paucity of biogenic and detrital carbonate in the sediments, the presence of bioturbation features along the contacts between the basal marl and the overlying chalk, and covariant increases in carbonate content and the $\delta$$^{13}$C values of the carbonate fraction across the couplets indicate that the carbonate phases in these rhythmicities are a direct result of authigenic precipitation from the surface waters. Because (i) the CaCO$_{3}$-C$_{org}$ systematics indicate that primary biological productivity variations during the times of deposition of these lithologies were not were not significant enough to promote the development of the rhythmites, and (ii) the Black Sea remained a gigantic, deep, hydrologically-closed lacustrine water body through most of the Plio-Pleistocene, we contend that the carbonate production in the basin was largely sustained by abiotic precipitation from the surface waters, with bio-induced carbonate precipitates constituting only a minor portion of the total carbonate fraction. Bedding thickness considerations indicate that systematic fluctuations in terrigenous flux rates and carbonate production could have been of significance in the formation of the couplets in the Lower and Upper Chalk sequences respectively. Published pollen records preserved in the sediments indicate that steppe pollen (reflecting cooler and more arid climatic conditions) is dominant in chalk half-couplets, whereas forest pollen (indicative of relatively warmer and more humid climate) dominates basal marl half-couplets. This, together with the observed systematic variations in the carbon and oxygen isotope composition across the couplets indicate the strong influence of climate in promoting the deposition of these rhythmicities. Our studies indicate that the individual marl-chalk couplets of both Chalk Units represent alternating warm-humid and cool-arid climatic conditions which in turn controlled rhythmic flucuations between evaporation and inflow rates, with average periods occurring approximately every 4200 years. However, whereas the short-term climatic fluctuations during the times of deposition of the Upper Chalk sequences were within an overall cold phase of early Pleistocene, similar perturbations which characterized the Upper Chalk times occurred during a relatively warm preglacial epoch in the mid-to late Pliocene.
OS21A-1195 0800h
Barium Geochemistry of a Marginal Cold-seep Basin: Abundance and Provenance of Total Barium and Barite in the San Clemente Basin.
Barium in marine sediments is found in various solid phases including barite, aluminosilicates, carbonates, organic material and oxyhydroxides. Barite, the dominant Ba-bearing solid phase in pelagic settings, is of particular interest as it has the potential for use as a tracer for paleoproductivity and past isotopic and rare earth element composition of seawater. Barite crystals form by abiotic precipitation in supersaturated microenvironments of decaying organic material in the water column and then settle as aggregates to the sediments. In areas of cold seep activity, discharge of barium-rich seep fluid also results in large in situ barite deposits at the seafloor. Transport of particles from these deposits leads to significantly increased barium concentrations in surrounding sediments. The San Clemente basin located about 100km off the coast of San Diego, is one such location where massive barite deposits have been observed in association with cold seep activity along the San Clemente fault that transects the basin. Analysis of surface sediments from the San Clemente basin has shown total barium enrichment of up to 140% higher than surface sediments from the neighboring Santa Catalina basin. Cold seeps are not present in the latter location, where the major source of barite is likely to be that associated with biogenic material. The significant input of barium from authigenic barite deposits to surrounding sediments affects global barium geochemistry and could possibly be used to provide a paleorecord for cold seep activity.
OS21A-1196 0800h
Formation of carbonate concretions in deep-sea sediment below the CCD and above an active gas hydrate system
Site 1230 of the Ocean Drilling Program targeted the chemistry and microbiology of an active deep-water gas hydrate system in the Peru Trench. The site is noteworthy because, at nearly 6000 m water depth, it lies well below the carbonate compensation depth and the sediments comprise mostly terrigenous clays and biogenic silica. Shipboard work at this site delineated a prominent sulfate-methane transition (SMT) at 8-10 m below seafloor (mbsf) as well as some carbonate horizons. In this study, we present calcium and strontium data for pore waters and sediments at this site, including across the SMT. Concentration profiles show that dissolved Ca$^{2+}$ diffuses downward from the seafloor toward the SMT, where a sharp inflection indicates consumption of Ca$^{2+}$ into an authigenic phase. Dissolved Sr$^{2+}$, on the other hand, diffuses upward from depth toward the SMT. Again, however, a prominent inflection suggests removal of Sr$^{2+}$ to sediment. The inferences from pore water profiles are borne out by sediment chemistry. Large peaks in the calcium and strontium content of sediment mark the SMT. The calcium and strontium fronts reach $\sim$2700 and $\sim$5 mmol/kg, respectively, at 9 mbsf, which are much greater than average background values of $\sim$10 and $\sim$1 mmol/kg. These authigenic fronts are primarily composed of carbonate minerals, as determined by acetic acid extractions and x-ray diffraction. Because the calcium and strontium fronts coincide with both the SMT and changes in dissolved chemistry, it is proposed that the carbonates are currently forming as follows: methane rising from the underlying gas hydrate system reacts with dissolved sulfate through anaerobic oxidation of methane which releases HCO$^{3-}$ and alkalinity and causes carbonate precipitation. The overall process has been observed elsewhere; the Peru Trench is interesting, however, because the process leads to carbonate in sediments otherwise devoid of carbonate.
OS21A-1197 0800h
Dissolved barium and authigenic barite above gas hydrates: Microbially mediated reactions at the Sea of Japan UT04 Ridge and Seep Area.
Only recently described, the UT04 Ridge and Seep Area is one of Japan's most significant gas hydrate systems. Bathymetric profiles reveal numerous pockmarks and more than 30 major seeps, each approximately 100m in diameter. We present both shipboard and land-based geochemical data from 15 piston cores recovered on a recent cruise to the Ridge and discuss the influence of microbial oxidation of methane on pore-fluid and sediment chemistry. Although evidence for bioturbation is apparent in many of the cores, the relatively smooth profiles of pore-fluid chemistry throughout the Ridge suggest that regional-scale diffusion plays a greater role in fluid transport within the upper 5 meters than bioturbation or bioirrigation. The sulfate-methane transition (SMT) in the coring sites ranges in depth from 2 meters to 4 meters below the sea floor, and is characterized by high H$_{2}$S concentrations and an inflection in alkalinity profiles. Barium concentrations above the SMT are generally less than 1 $\mu$M. Pore-fluid profiles indicate active barite dissolution just below the SMT, as a result of sulfate depletion in the surrounding pore fluids, and concentrations rapidly approach maximum values of 20-50 $\mu$M. Dissolved barium produced in this region presumably diffuses upward and is reprecipitated as barite. In the one core where massive hydrate was recovered, authigenic concretions were interdispersed among sediment and hydrates throughout the entire sample interval (2.5 m). In this particular core, dissolved barium concentrations were much lower than can be accounted for by pore fluid dilution caused by hydrate dissociation. Ongoing analyses of authigenic barite accumulation will indicate the duration of active gas venting along the Ridge.
OS21A-1198 0800h
Distribution Patterns of Glaucony From Lower Cenomanian Deposits of Northern France
Vertical and lateral distribution of autochthonous glaucony from a stratigraphically condensed interval of Early Cenomanian age in Northern France is shown through detailed sedimentological investigation of five sections (from south to north: Cap de La Heve, Cauville, Octeville, St. Jouin, and Fecamp), extending along spectacularly exposed cliffs. The condensed interval is up to 3m thick and includes 70-96% glaucony in the southern outcrops. To the north (St. Jouin and Fecamp), the thickness of the condensed interval decreases to about 1m, and glaucony content to 40-60%. Vertical distribution of glaucony invariably shows a decreasing upward tendency, generally with maxima in the basal 50 cm. Geochemical and mineralogical analyses of 53 samples from the condensed horizon enable precise characterization of glaucony maturity, through determination of K2O content and the distance "d" between (001) and (020) peaks. X-ray diffraction analyses reveal patterns characteristic of well-ordered, highly-evolved glauconite, with sharp and narrow (001) reflections. The distance "d" is equal to 10,8 cm in 50 samples. The K2O content varies in a consistent manner, showing values narrowly constrained between 7% and 8%. These data allow classification of the green grains as evolved glaucony, suggesting a depositional hiatus of several hundreds of ka. Upsection, thinner Lower to Middle Cenomanian glaucony-bearing horizons are recorded. These differ from the major condensed horizon by the lower glaucony content (less than 15%) and lower maturity of the green grains (K2O = 6-7%; "d" = 10.9-11.2 cm). These major differences are laterally persistent, suggesting a lower degree of stratigraphic condensation (a few tens of ka), and providing the basis for basinwide correlations. The glauconitic minerals do not show significant changes in terms of maturity throughout the entire condensed interval, even when they represent less than 10% of the sand fraction. This suggests a geochemically open system in early diagenetic phases, resulting in homogeneous glaucony composition over thicknesses much higher than previously thought.
OS21A-1199 0800h
Origin of the diagenetic carbonate crusts and concretions from the mud volcanoes of the Nile deep-sea fan
During the NAUTINIL cruise (September -October 2003), 22 submersible dives have been realized in the Nile deep-sea fan area to investigate by a multidisciplinary approach, selected mud volcanoes which are very abundant and of various morphologies in the whole area (Loncke et al., 2004). The deepest site (3019 m) located in the western part of the deep-sea fan, corresponds to a large caldera (about 8 km of diameter) where brines are seeping along the flanks of the structure and are sometimes collected in pools and lakes. The other sites in the central and eastern parts of the Nile deep-sea fan correspond respectively to pock-marks located at 2120 m and to a mud volcano located at 1130 m where active fluid ventings were identified by the presence of living benthic organisms (mainly vestimentiferan worms; rarely bivalves). At these three sites, hard carbonate crusts cover irregularly the sea floor and are sometimes present as dispersed fragments within the topmost sediments. The sediments from the venting areas are organic-rich and have a strong H$_{2}$S smell which is indicative of active sulfate reduction. Petrographic observations and XRD analyses of the carbonate crusts indicate that aragonite, calcite, Mg-calcite are the dominant authigenic carbonate phases with a minor contribution of dolomite ; small concretions of ankerite occur occasionally in the sediments of the eastern delta. Millimeter sized barite concretions have also been discovered in the pock-marks sediments. The oxygen and carbon isotopic compositions of the bulk carbonate from crusts and concretions exibit large variations : -0.67 $<$ \delta$^{18}$O\permil PDB $<$ 4.66 -44.17$<$ \delta$^{13}$C \permil PDB $<$ 3.10 The distribution of the isotopic values is explained by the mixing of the authigenic carbonates with the sedimentary matrix which corresponds itself to a mixture of pelagic sediments and mud breccia issued from the mud volcano activity. The rather large range of \delta$^{18}$O values might reflect variable sources of diagenetic fluids. Typically, the very low \delta$^{13}$C values of the authigenic carbonates indicate that CH$_4$ was the major source of carbon which was oxidized as CO$_{2}$, either through bacterial sulfate reduction within the sediment, or via bacterial aerobic oxidation at the sea floor. Similar isotopic values were previously measured in the diagenetic carbonate crusts from the mud volcanoes of the Mediterranean Ridge area (Aloisi et al., 2000) as well as in other areas of cold seeps outside the Mediterranean sea (for instance Gulf of Mexico, Cascadia margin, Barbados prism). References: Aloisi G., Pierre C., Rouchy J.M., Foucher J.P., Woodside J. and the Medinaut Scientific Party, 2000. E.P.S.L., 184, 321-338. Loncke L., Gaullier V., Bellaiche G., and Mascle J., 2004. A.A.P.G. Bull
OS21A-1200 0800h
Reaction Vessel Design for Sampling Nanoparticles During Growth Under Controlled T, P, pH and Oxidation State
Simulation of authigenic mineral formation and mineral catalysis of aqueous reactions requires experimental systems that simulate natural environments by buffering the pH and fO$_{2}$ to relevant environmental conditions. The fO$_{2}$ is commonly buffered in experiments using minerals (e.g., Ni-NiO; Fe$_{2}$O$_{3}$-Fe$_{3}$O$_{4}$), however at low temperatures, the kinetics of the mineral buffer reactions are too slow for accurate control of oxidation state, and the minerals themselves may interfere with the reactions under investigation. To study authigenic clay mineral reactions ($<$200nm particles), we built a new titanium reaction vessel that utilizes a Pd/Ag alloy (75%/25%) semi-permeable hydrogen membrane. At temperatures above 200\deg C, a known pressure of hydrogen on the backside of the membrane controls the concentration of dissolved hydrogen within the vessel, and therefore controls the fO$_{2}$. The pH can be controlled either by the mineral reaction or by addition of acid/base. The reaction system can run at conditions up to 400\deg C and 30 MPa. The fO$_{2}$ dictated by the hydrogen membrane is monitored by the electrochemical potential between a Pt wire in Hg/HgO within an Yytrium doped zirconium (YSZ) reference electrode and a gold source electrode. The reference electrode can also be used to monitor pH with the addition of a titanium source electrode. A Pt-Rd thermocouple continuously monitors the temperature within the vessel, and a liquid chromatography pump and adjustable pressure relief valve control pressure. A sampling valve system allows capture of a 2 ml aliquot of mineral/fluid suspension at T and P for analysis of suspended minerals or of aqueous species. The use of the hydrogen membrane in concert with the hydrogen electrode allows for in situ control and monitoring of the oxidation state within the vessel, without the addition of mineral phases that may influence the reaction progress during crystal growth.
OS21A-1201 0800h
Petrologic and Geochemical Evidence for Biologic Contributions to Carbonate Formation in the Panoche-Tumey Hills Paleoseep, California
The Panoche-Tumey Hills (PTH) paleoseep is a Paleocene seep system that extends greater than 20 km nearly continuously along strike along the western edge of the Great Valley. The seep fauna is dominated by tubeworms but includes bivalves, gastropods, and solitary corals. The entire system, including feeder sandstone intrusions, is 800 m thick. Carbonate deposits occur in the upper 150 m, and are most concentrated in the upper 50 m. Authigenic carbonates include several generations of cements and veins. The carbonates are mainly calcite, but there is evidence for calcite replacing aragonite in pipe structures. Textures indicate the seep system evolved from an open, relatively permeable sedimentary matrix to one of increasingly focused flow. Late stages of cementation included formation of pipe structures with flow velocities vigorous enough to allow precipitation of carbonate relatively free of silt and sand. \delta$^{13}$C$_{PDB}$ values as low as -45 permil are consistent with a component of biogenic methane during carbonate formation. Pronounced laminated structures in the carbonates suggest microorganisms played an important role in carbonate formation, by binding cements and/or varying the P$_{CO2}$ in distinct microenvironments. Biomarker analysis will help to constrain the importance of bacterially mediated carbonate precipitation. During the fluid evolution of the seep organic and inorganic structures served as conduits, and isolated fluid reservoirs at small scales. Carbon isotopes differ outside and inside tubeworms and pipe structures by 18 permil and 45 permil, respectively, but in opposite directions. Clearly fluid sources, as characterized by carbon isotope signatures, can vary substantially over short distances and presumably over short time scales. The geochemical data along with the cement textures indicate that individual seep sites within the system were doomed by their own carbonate- precipitating success. As one seep site became clogged and less permeable, seep activity migrated to a new location within the system. The fickle nature of fluid seepage in modern systems is well documented at convergent plate boundaries. The PTH system provides a particularly large and well-exposed ancient analog for understanding the evolution of these types of seeps in four dimensions.
OS21A-1202 0800h
Hydrogeochemistry Of A Modern Dolomite-Forming Lagoon System, Cabo Frio-RJ, Brazil
Two hypersaline, dolomite-forming lagoons near Cabo Frio, Brazil, and associated ground- and surface waters were sampled in a comparative sediment and fluid geochemical (solutes, stable isotopes) investigation. Although microbial mediation via sulfate reducers has been invoked to explain dolomite formation in these lagoons, we showed that dolomites are associated with sulfide oxidation (Moreira et al., 2004). Sulfide oxidation is thought to promote dolomite formation by causing undersaturation for competing carbonate phases such as Mg-calcite. Herein, we consider the larger hydrogeologic and temporal setting to further elucidate hydrogeochemical and geochemical constraints on rates and mechanisms of dolomite formation in the two lagoons. The lagoons, Brejo do Espinho (BE) and Lagoa Vermelha (LV), are shallow marginal marine systems flanked by quartz sand dunes separating them from Atlantic open seawater to the south and from Araruama lagoon, a large, hypersaline water body, to the north. In both lagoons, about 1 m of high Mg-calcite and dolomite mud has accumulated over the last 5,000 years on an underlying aquifer composed of highly permeable, quartz-rich coquinas. BE has a proximal relation to recharge from Araruama lagoon, while LV is more closely associated with meteoric recharge from lacustrine and riverine systems. BE is shallower, at 0.5 m water depth, than LV (2 m), permitting BE waters to remain oxic. Oxygen isotope values and chloride mass balances of pore waters and of fluids sampled from shallow ground water wells identify the different water and solute sources the lagoons. BE overlying brines and pore waters appear to be produced by evapoconcentration of Araruama source brines and meteoric precipitation. In contrast, LV derives from evapoconcentrated seawater mixed with regional lake and ground water sources. We envision a scenario in which dense, Mg-SO$_{4}$-rich brines from Araruama migrate along a permeable flowpath in limited contact with the atmosphere. These fluids undergo progressive sulfate reduction, then are drawn up by capillary forces to the shallow BE lagoonal system, where sulfide is oxidized. Thus, the locus of dolomite formation at BE appears to occupy the interface between discharging sulfide-rich fluids and the thin layer of oxygen-rich overlying brine. In contrast, sulfide oxidation is not an important process in the LV lagoonal-sediment system. XRD profiles and sediment chemistry indicate that BE sediments indeed contain a significantly higher percentage dolomite than those of LV. Sr systematics support this conclusion and place the lagoonal dolomites in the context of carbonates throughout the rock record. The Sr contents of BE carbonates decrease markedly in the most dolomite-rich intervals, approaching a value of 800 ppm, the high end of expected values for dolomite (Land, 1980). LV carbonates fluctuate in Sr value from 2000-3000 ppm, values more typical of high Mg-calcite. Some precipitation of dolomite has occurred in both lagoons, however, reflected by increasing pore water Sr with depth as Mg-calcite is replaced by dolomite. Recent work documenting changes in the elemental compositions of the oceans, including Ca, Mg, CO$_{3}$, and SO$_{4}$, raises the potential for significant variation in marine saturation states for carbonate minerals and in carbon-sulfur cycling through time. Our model may provide clues to the relationship between marine elemental composition and varying rates of dolomitization over the geologic record.
OS21A-1203 0800h
Authigenic Mineral Precipitation at Cold Seeps in Continental Margin Sediments - A Comparative Study.
Chemosynthetic biological communities and authigenic mineral formation are common along continental margins worldwide where they indicate the availability of reduced compounds on or immediately below the seafloor. Some of these sites, historically termed "cold seeps", are associated with obvious hydrocarbon venting (e.g., Gulf of Mexico, Santa Barbara Basin, and Eel River Basin), whereas fluid flow at other sites has been inferred by the presence of chemosynthetic communities and authigenic mineral deposits on the seafloor (e.g., Monterey Bay). Authigenic mineral deposits include carbonate-group minerals, most commonly aragonite, Mg-calcite, and dolomite, which form via microbially mediated anaerobic oxidation of methane. However, depending on the chemistry of the expelled fluids, minerals other than carbonate (i.e., barite) may form deposits on the seafloor. To develop a more comprehensive understanding of the complex geochemical, physical, and biological interactions at seep sites, we present a comparative study that illustrates the diverse processes associated with the flux of fluids and gases to the seafloor and the precipitation of authigenic minerals at cold seeps. In this study, we will discuss the formation and early diagenesis of authigenic mineral deposits in a variety of geologic and geochemical environments including the southern Gulf of Mexico, Monterey Bay, Santa Barbara Basin, Eel River Basin, North Sea, and the Sea of Okhotsk. Authigenic carbonates from these study areas exhibit a wide range of mineralogical and stable isotopic compositions. The mineralogy of the precipitates ranges from dolomite and high-Mg-calcite to aragonite. The carbon isotopic composition of carbonates shows a wide variation, indicating a complex carbon source from both $^{13}$C-depleted and residual, $^{13}$C-enriched, fluids. The large variability of $\delta$$^{18}$O values also demonstrates the geochemical complexity of these sites with some samples pointing toward a heavy, $^{18}$O-enriched oxygen source, possibly related to the decomposition of gas hydrate, whereas other samples indicate the local presence of meteoric water during carbonate precipitation. In one instance, fluid flow in response to slumping and mass wasting resulted in mixing of barium-rich pore fluids and sulfate-rich bottom water, causing barite, not carbonate, precipitation on the seafloor.
OS21A-1204 0800h
Authigenic Dolomite in Marine Marginal Sediments: An Indicator of Fossil Microbial Activity
Dolomite formation under Earth surface conditions remains a long-standing enigma in Earth science. Although it is abundant in the rock record, dolomite does not commonly form in modern marine sedimentary environments and attempts to precipitate dolomite in the laboratory at Earth surface conditions have been largely unsuccessful. In recent years, however, microbial mediation of dolomite formation has been tested in cultural experiments and in natural environments. A new microbial factor has been added to the list of factors, which are thought to promote dolomite precipitation. This microbial model for dolomite formation has found wide acceptance and has been applied to explain modern and ancient dolomite formation. In particular, deep-sea margin sediments provide an ideal setting to apply the microbial dolomite model. With the discovery of dolomite in Quaternary deep-sea sediments in the late 1970's, e.g. during Deep Sea Drilling Program (DSDP) Legs 63 and 64 on the California Margin and in the Gulf of California, a new environment for dolomite formation was recognized. We have applied the microbial dolomite model in two different marine marginal settings and will present the results of SEM, isotopic and mineralogical studies of the recovered dolomite. The discovery of a field of dolomite chimneys strewn on the Gulf of Cadiz seafloor provided an opportunity to study the fossil microbial activity, which had mediated dolomite formation in the flow path of upward migrating methane on this tectonically active margin. In contrast, modern microbial activity was investigated in organic carbon-rich hemipelagic sediments recovered in drill cores obtained from the Peru Margin during ODP Leg 201, which was dedicated to the study of the deep biosphere. The microbial impact on the pore-water geochemistry could be linked directly to the diagenetic processes occurring in the sediments, i.e. dolomite formation. Combining the results obtained from our two study areas, we propose that dolomite formation in marine marginal settings is probably directly linked with microbial activity associated with the geochemical transition between the zones of sulfate reduction and methanogenesis.
OS21A-1205 0800h
The Meaning of 87Sr/86Sr Ages of Authigenic Seamount Phosphorite
Phosphorite is a common deposit on many seamounts of world oceans. The phosphate mineral of the deposit is apparently carbonate-fluorapatite (CFA) and contains significant amount of Sr. Evidence shows the mineral to be a marine authigenic phase. The Sr-isotope (i.e. 87Sr/86Sr) ages of the mineral samples from many seamounts of Pacific Ocean show that they were formed episodically. Their presents thus represent periods of unique ocean chemistry in world-ocean history. The implicit assumption of the application of and the episodic conclusion based on the 87Sr/86Sr is rapid formation and insignificant post-formation isotopic alteration of the mineral. This study is positioned to investigate the validity of the assumption to clarify the significance of the episodic conclusion. 87Sr/86Sr of Sr samples extracted sequentially from an essentially pure marine authigenic CFA was measured to determine the isotopic homogeneity in the mineral. The results show that the isotopic composition of first few percents of Sr-extract is significantly different from the rest. This several percents of isotopically different Sr appears to be a mixture of modern seawater and the mineral\-bulk Sr. The contribution of Sr from modern seawater, however, appears to be minor. Thus, the rate of post-formation Sr exchange between the CFA and seawater appears to be slow and shall not affect the strontium-isotope derived age of formation of the CFA significantly. The fact that 87Sr/86Sr of the rest Sr-extracts which constitutes dominant portion of the CFA is homogeneous indicates the rapidity of its formation. The results of this study are consistent with the notion that the authigenic seamount CFA of Pacific basin studied so far were formed rapidly and suffered little post-formation Sr-isotope alteration. This thus renders further support for the proposition that there were episodic genesis of phosphate deposits in Cenozoic world Oceans.
OS21A-1206 0800h
Lithium Contents and Isotopic Compositions of Ferromaganese Deposits from the Global Oceans
Trace elements and isotopes in marine ferromanganese crusts have been used to reconstruct the history of water-mass movement and composition in global oceans. Here we evaluate the feasibility of using Li concentration and isotopic composition as proxies of past seawater composition in response to varying mantle and continental contributions to the ocean. This is possible because hydrothermal and fluvial inputs have distinct Li isotope signatures. We analyzed a suite of ferromanganese deposits from the Indian, Atlantic and Pacific Oceans. There are three main types of deposits: hydrogenetic crust precipitated from seawater, stratabound deposits from hydrothermal fluids, and mixed hydrogenetic and hydrothermal Fe/Mn oxides. The hydrothermal deposits were collected from midplate hot spots, back-arc basins, volcanic arcs and mid-ocean ridges. Two Li fractions were examined: the more mobile fraction that is leachable by acetic acid and the relatively immobile fraction that remains in the oxide phase. Hydrogenetic and diagenetic deposits contains $<$1 to 10 ppm Li of which $25$ to $73%$ are mobile. Hydrothermal deposits are strongly enriched in Li (150 to 1400 ppm) that is predominantly in the leachable fraction, suggesting that almost all hydrothermal Li is loosely sorbed on the oxides. Mixed hydrogenetic/hydrothermal crusts show intermediate concentration levels. $\delta^{7}$Li ranges of acetic acid extractable Li on hydrogenetic $(23.7$ to $31.6%_o)$, mixed crust $(16.7$ to $32.8%_o)$, and hydrothermal $(30.3$ to $33.5%_o)$ deposits are mostly seawater-like $(32%_o)$. The residual Li in hydrothermal crusts $(28%_o)$ also resembles seawater. Mid-ocean ridge hydrothermal fluids worldwide have an average $\delta^{7}$Li of $7\pm 1 %_o$. Our results suggest that hydrothermal deposits have exchanged Li with seawater and lost the mantle signature with time. We conclude that Li concentration on ferromanganese deposits may serve as an index of hydrothermal flux to the ocean but its mantle isotopic signature may be erased by exchange with seawater.
OS21A-1207 0800h
Thermodynamic control of microbial sulfate reduction rates and consequences for the form and distribution of metal sulfide nanoparticles
Sulfate reducing bacteria are widespread in natural environment. They derive energy for growth by reducing sulfate to sulfide. In the presence of metal ions, the sulfide byproduct precipitates as metal sulfide nanoparticles. Microorganisms can reduce sulfate through two metabolic pathways, i.e., an incomplete and a complete pathway. In the former case, microorganisms, such as {\it Desulfovibrio} sp., oxidize lactate, butyrate, ethanol, etc. to acetate, whereas in the latter, microorganisms, such as {\it Desulfobacter} sp., oxidize acetate to bicarbonate. It is important to study the kinetics of microbial sulfate reduction because microbial metabolic rates may influence the form, mobility, and reactivity of biogenic minerals. The rate of sulfate reduction is controlled not only by substrate concentrations in environments, but also by the thermodynamic driving force. The driving force is the difference between the energy available from environment and the energy conserved. The amount of energy conserved is about 80 kJ/mol lactate and 90 kJ/mol acetate for incomplete and complete pathway, respectively. Where the energy available from environments is much higher than the energy conserved, the driving force is large and sulfate reduction proceeds at high rate. The particle size of metal sulfide minerals precipitated under such conditions is extremely small. However, where the energy available is close to the energy conserved, the driving force is small and the rate of sulfate reduction is small. These conditions could favor nanoparticle transport because particle advection and dispersion may be strong enough to remove nanoparticles before large immobile aggregates can form.
OS21A-1208 0800h
Precambrian Limestones: Products of Microbial Diagenesis?
The anomalous abundance and texture of Precambrian sedimentary dolomites have not precluded their C-isotopic compositions being interpreted in terms of paleoclimatology and chemostratigraphy (e.g. Snowball Earth Hypothesis; Hoffman et al. 1998, Science, v. 281, p. 1342-1346). Neoproterozoic dolomitic and calcitic limestones from the Australian Adelaidean, are, as elsewhere, comprised of interlocking, 5-50 $\mu$m dolomite and calcite microspar, as opposed to particulate, $<$5 $\mu$m, modern micrite. The microspar crystals are commonly zoned, with a 3-4 $\mu$m core having $<$100 ppm of Mn$^{2+}$ plus Fe$^{2+}$, surrounded by outwardly increasing levels of these reduced cations (to $>$6 wt%). Timing criteria demonstrate that these crystals represent an initial micrite precipitated from oxidized fluids (i.e. Neoproterozoic seawater), which was subsequently overgrown by a microspar cement precipitated from early diagenetic pore fluids with low Eh due to ongoing microbial decomposition of organic matter. Dolomitisation is thermodynamically advantaged over calcification by sulphate reduction and high carbonate alkalinity. Lacking both a substantive foodweb and bioturbative substrate ventilation, the Precambrian was a time when the relative flux of labile organic matter entering the substrate was maximized. Such an increased flux will have driven, through microbial consumption, more rapid anoxia and higher carbonate alkalinity. Thus the abundance of Precambrian sedimentary dolomites, and their microspar texture, reflects the importance of substrate-based microbial processes in the pre-metazoan carbon cycle. "Reduced" microspar overgrowths contribute $>$75% of virtually all analyzed Adelaidean calcitic and dolomitic limestones. Since their petrography is representative of Neoproterozoic limestones in general, we suggest that the existing Neoproterozoic C-isotope database most likely records early diagenetic microbial processes, not seawater carbon biogeochemistry as currently interpreted. We further suggest that the wide range in global Neoproterozoic carbonate C-isotope data (+10 to -8 \permil) is entirely consistent with the compositions of modern early diagenetic carbonates, rather than extreme secular oscillations in the C-isotopic signature of seawater.