Phosphorus partition and acumulation rates in sediments of ODP Legs 130 and 138

Understanding phosphorus (P) geochemistry and burial in oceanic sediments is important because of the role of P for modulating oceanic productivity on long timescales. We investigated P geochemistry in seven equatorial Pacific sites over the last 53 Ma, using a sequential extraction technique to elucidate sedimentary P composition and P diagenesis within the sediments. The dominant P-bearing component in these sediments is authigenic P (61-86% of total P), followed in order of relative dominance by iron-bound P (7-17%), organic P (3-12%), adsorbed P (2-9%), and detrital P (0-1%). Clear temporal trends in P component composition exist. Organic P decreases rapidly in younger sediments in the eastern Pacific (the only sites with high sample resolution in the younger intervals), from a mean concentration of 2.3 µmol P/g sediment in the 0-1 Ma interval to 0.4 µmol/g in the 5- 6 Ma interval. Over this same time interval, decreases are also observed for iron-bound P (from 2.1 to 1.1 µmol P/g) and adsorbed P (from 1.5 to 0.7 µmol P/g). These decreases are in contrast to increases in authigenic P (from 6.0-9.6 µmol P/g) and no significant changes in detrital P (0.1 µmol P/g) and total P (12 µmol P/g). These temporal trends in P geochemistry suggest that (1) organic matter, the principal shuttle of P to the seafloor, is regenerated in sediments and releases associated P to interstitial waters, (2) P associated with iron-rich oxyhydroxides is released to interstitial waters upon microbial iron reduction, (3) the decrease in adsorbed P with age and depth probably indicates a similar decrease in interstitial water P concentrations, and (4) carbonate fluorapatite (CFA), or another authigenic P-bearing phase, precipitates due to the release of P from organic matter and iron oxyhydroxides and becomes an increasingly significant P sink with age and depth. The reorganization of P between various sedimentary pools, and its eventual incorporation in CFA, has been recognized in a variety of continental margin environments, but this is the first time these processes have been revealed in deep-sea sediments. Phosphorus accumulation rate data from this study and others indicates that the global pre-anthropogenic input rate of P to the ocean (20x10**10 mol P/yr) is about a factor of four times higher than previously thought, supporting recent suggestions that the residence time of P in the oceans may be as short as 10000-20000 years.

Composition of sediment pore water and isotopes in carbonate and benthic foraminifera at DSDP Holes 68-501, 69-505 and Site 69-504

Two sites on the southern flank of the Costa Rica Rift were drilled on DSDP Legs 68 and 69, one on crust 3.9 m.y. old and the other on crust 5.9 m.y. old. The basement of the younger site is effectively cooled by the circulation of seawater. The basement of the older site has been sealed by sediment, and an interval in the uppermost 560 meters of basement recently reheated to temperatures of 60 to 120°C. Although the thickness of the sediments at the two sites is similar (150-240 m versus 270 m), the much rougher basement topography at the younger Site 505 produces occasional basement outcrops, through which 80 to 90% of the total heat loss apparently occurs by advection of warm seawater. This seawater has been heated only slightly, however; the temperature at the base of the sediments is only 9°C. Changes in its composition due to reaction with the basement basalts are negligible, as indicated by profiles of sediment pore water chemistry. Bacterial sulfate reduction in the sediments produces a decrease in SO4 (and Ca) and an increase in alkalinity (and Sr and NH3) as depth increases to an intermediate level, but at deeper levels these trends reverse, and all of these species plus Mg, K, Na, and chlorinity approach seawater values near basement. Si, however, is higher, and Li may be lower. At the older site, Site 501/504, where heat loss is entirely by conduction, the temperature at the sediment/basement contact is 59°C. Sediment pore water chemistry is heavily affected by reaction with the basaltic basement, as indicated by large decreases in d18O, Mg, alkalinity, Na, and K and an increase in Ca with increasing depth. The size of the changes in d18O, Mg, alkalinity, Ca, Sr, and SO4 varies laterally over 500 meters, indicating lateral gradients in pore water chemistry that are nearly as large as the vertical gradients. The lateral gradients are believed to result from similar lateral gradients in the composition of the basement formation water, which propagate upward through the sediments by diffusion. A model of the d18O profile suggests that the basement at Site 501/504 was sealed off from advection about 1 m.y. ago, so that reaction rates began to dominate the basement pore water chemistry. A limestone-chert diagenetic front began to move upward through the lower sediments less than 200,000 yr. ago.

Geochemical and biogeochemical parameters of bottom sediments from the area of the Spitsbergen (Svalbard) Archipelago

Quantitative characteristics for rates of diagenetic processes in the upper (0-30 cm) layer of sedimentary deposits in the area of the Spitsbergen (Svalbard) Archipelago (78°-80°N) were obtained by lithologo-geochemical, radioisotope (35S, 14C), and stable isotope (d34S, d13C) studies. It was proved that rates of diagenetic processes in polar deposits at 123-395 m depth affected by the East Spitsbergen ''warm'' current are mostly determined by bioproductivity and are commensurate with rates of processes in shelf deposits of temperate latitudes. High contents of migratory methane (up to 263 ml/dm**3) and isotopically-light organic carbon (Corg, d13C = -30 per mil PDB) were found in the 1 m layer of shelf deposits (at 123 m sea depth) with low bacterial in situ production of methane. It was shown that methane is not utilized in the deposits by the methanotrophic bacterial community and it may be supplied to the water mass and, probably, to the atmosphere.

Geochemistry and stable carbon isotope composition of kerogen of DSDP Site 124-767

Celebes Basin sediments from Ocean Drilling Program Site 767 (Leg 124) containing both marine and terrestrial organic matter have been investigated through palynofacies and geochemical analyses. The main degradation processes affecting or having affected organic matter are recorded in the sedimentary column as shown by ammonium, phosphate and sulfate pore-water profiles, and by petrographic and geochemical analyses of sediments. In the upper part of the sedimentary section (down to 200 mbsf), the decrease of the ratio of total organic carbon to sulfur (TOC/S) with depth, generally related to the sulfate reduction process, is accompanied by an increase of framboidal pyrite content in the marine organic matter, and by an increasing amount of amorphous marine organic matter relative to the total organic matter. However, as the terrestrial organic input also varies with depth, dilution effects are superimposed on diagenesis. This continental supply affects the TOC/S ratio by increasing total organic carbon and decreasing the ability of the bulk organic matter to be metabolized through sulfate reduction. A positive relationship between the TOC/P ratio and the amount of degraded organic matter of marine origin clearly displays the effect of an organic source on the composition of the sediment. Each lithostratigraphic unit possesses its own characteristics in terms of composition and preservation of organic matter. The effects of diagenesis can only be appreciated within a single lithostratigraphic unit and mainly affect the less-resistant marine organic matter.

Authigenic carbonate of ODP SIte 104-643

Different generations of complex authigenic carbonates formed in siliceous muds (lithologic Unit IV) and hemipelagic clays (lithologic Unit V) of ODP Site 643, Leg 104 Norwegian Sea. The dominant phase in Unit IV is an early diagenetic Mn, Fe-calcite with a strong negative d13C ( -14 to -16 per mil) signature, and slightly negative d180 values. The strong negative d13C results from extensive incorporation of 12C-enriched CO2 derived from bacterial degradation of marine organic matter into early Mn, Fe - calcite cements. Concomitant framboidal pyrite precipitation and abundant SEM microtextures showing excellent preservation of delicate structures of fragile diatom valves by outpourings with early Mn-calcites strongly support their shallow burial formation before the onset of compaction. Later generations of authigenic mineralizations in lithologic Unit IV include minor amounts of a second generation of calcite with platy crystals, possibly precipitated along with opal-A dissolution, and finally opal-CT crystallization in deeper seated environments overgrowing earlier precipitates with films and lepispheres. The last mineralization is collophane (fluor apatite) forming amorphous aggregates and tiny hexagonal crystals. Authigenic mineral assemblages in lithologic Unit V consist of rhodochrosites, transitional rhodochrosite/manganosiderites, and apatite. A negative d13C ( -7.1 to -15.6 per mil) and a fluctuating d18O signal indicates that the micritic to sparitic rhodochrosites, transitional rhodochrosites/manganosiderites were formed at various burial depths. CO2 resulted from organic matter degradation in the lowermost sulfate reduction zone and from biogenic methane generation in the lowermost sediments, resulting in variable and negative d13C signals. The change in carbonate mineralogy reflects major compositional differences compared to sediments in Unit IV. Most prominent is an increase in altered ash as a primary sediment component and a sudden decrease of siliceous microfossils. Upward diffusion of cations, lowered salinities in pore waters, and elevated temperatures provide diagenetic environments favoring increased remobilization processes.

Contencentrations of reduced inorganic sulfur forms in Mediterranean Sea waters

The first data on content of inorganic reduced sulfur compounds [H2S, S°, S2O3(2-), SO3(2-)] were obtained at two stations in the northeastern Levant Sea (Mediterranean Basin). With lower detection limit for the mentioned sulfur forms of 30 nM, sulfide forms were not found, while thiosulfate concentration varied from 178 to 890 nM (from 24 to 78 % of total reduced S), and S° varied from 156 to 1090 nM. Vertical distribution of these compounds showed irregular character; correlation between total reduced S maxima, fluorescence, and increase of nutrient element content near the lower pycnocline boundary was observed. The maximum total sulfur concentration in the surface layer was likely due an anthropogenic influence. The ''starting'' mechanism that controls appearance and distribution of sulfur compounds in oxygen-containing water is the process of bacterial sulfate reduction in micropatches of fresh organic detritus. Reduced sulfur forms participate further in a series of chemical and biochemical processes. Contribution of hydrolysis of organic sulfur-containing compounds is insignificant for the region in study.

Stable carbon isotope ratios and the formation of dolomite in ODP Leg 175 sites

We examine the link between organic matter degradation, anaerobic methane oxidation (AMO), and sulfate depletion and explore how these processes potentially influence dolomitization. We determined rates and depths of AMO and dolomite formation for a variety of organic-rich sites along the west African Margin using data from Ocean Drilling Program (ODP) Leg 175. Rates of AMO are calculated from the diffusive fluxes of CH4 and SO4, and rates of dolomite formation are calculated from the diffusive flux of Mg. We find that the rates of dolomite formation are relatively constant regardless of the depth at which it is forming, indicating that the diffusive fluxes of Mg and Ca are not limiting. Based upon the calculated log IAP values, log K(sp) values for dolomite were found to narrowly range between -16.1 and -16.4. Dolomite formation is controlled in part by competition between AMO and methanogenesis, which controls the speciation of dissolved CO2. AMO increases the concentration of CO3[2-] through sulfate reduction, favoring dolomite formation, while methanogenesis increases the pCO2 of the pore waters, inhibiting dolomite formation. By regulating the pCO2 and alkalinity, methanogenesis and AMO can regulate the formation of dolomite in organic-rich marine sediments. In addition to providing a mechanistic link between AMO and dolomite formation, our findings provide a method by which the stability constant of dolomite can be calculated in modern sediments and allow prediction of regions and depth domains in which dolomite may be forming.

Stable carbon and oxygen isotope record of diagenetic carbonates from the New Jersey shelf

Carbon cycling is an important but poorly understood process on passive continental margins. In this study, we use the ionic and stable isotopic composition of interstitial waters and the petrology, mineralogy, and stable isotopic composition of authigenic carbonates collected from Ocean Drilling Program (ODP) Leg 174A (Sites 1071 and 1072) to constrain the origin of the carbonates and the evolution of methane on the outer New Jersey shelf. The pore fluids of the New Jersey continental shelf are characterized by (1) a fresh-brackish water plume, and (2) organic matter degradation reactions, which proceed through sulfate reduction. However, only minor methanogenesis occurs. The oxygen isotopic composition of the pore fluids supports a meteoric origin of the low salinity fluids. Authigenic carbonates are found in nodules, thin (~1-cm) layers, and carbonate cemented pavements. Siderite is the most common authigenic carbonate, followed by dolomite and calcite. The oxygen isotopic composition of the authigenic carbonates, i.e. 1.3-6.5 per mil PeeDee Belemnite (PDB), indicates an origin in marine pore fluids. The carbon isotopic composition of dolomite cements range from -16.4 to -8.8 per mil PDB, consistent with formation within the zone of sulfate reduction. Siderite d13C values show a greater range (-17.67-16.4 per mil), but are largely positive (mean=2.8 per mil) and are interpreted to have formed throughout the zone of methanogenesis. In contrast, calcite d13C values are highly negative (as low as -41.7 per mil)and must have formed from waters with a large component of dissolved inorganic carbon derived from methane oxidation. Pore water data show that despite complete sulfate reduction, methanogenesis appears not to be an important process presently occurring in the upper 400 m of the outer New Jersey shelf. In contrast, the carbon isotopic composition of the siderites and calcites document an active methanogenic zone during their formation. The methane may have been either oxidized or vented from shelf sediments, perhaps during sea-level fluctuations. If this unaccounted and variable methane flux is an areally important process during Neogene sea-level fluctuations, then it likely plays an important role in long-term carbon cycling on passive continental margins

Plio-Pleistocene magnetic polarity stratigraphies of ODP Leg 177 Sites from the South Atlantic

Magnetic polarity stratigraphies from ODP Leg 177 'high resolution' sites indicate Brunhes sedimentation rates in the 12-25 cm/kyr range, with a trend of decreasing sedimentation rates with increasing age. Magnetite is the principal remanence-carrying mineral. Downcore alteration of magnetite and authigenic growth of iron sulfides introduces a high coercivity diagenetic remanence carrier (pyrrhotite). The change in pore water sulfate with depth in the sediment tends to be in step with the decrease in magnetization intensity, indicating the link between sulfate reduction and magnetite dissolution. Shipboard pass-through magnetometer data are generally very noisy due to a combination of weak magnetization intensities, drilling-related core deformation, and the influence of authigenic iron sulfides. Post-cruise progressive demagnetization of discrete samples aids the magnetostratigraphic interpretation, as these measurements are less influenced by low magnetization intensities and drilling-related deformation. The magnetostratigraphic interpretations provide much-needed calibration for biostratigraphic events in the high latitude southern oceans. Apart from the ODP Hole 745B (Kerguelen Plateau), published Plio-Pleistocene magnetostratigraphies from ODP sites in the Southern Ocean are poorly constrained. For this reason, we compare interpolated ages of 11 radiolarian events and one diatom event that occur at Hole 745B and Leg 177 sites.

Methane concentration profiles and isotopic composition of sediment cores from the Black Sea

We present high resolution profiles for the methane concentration and the carbon isotope composition of methane from surface sediments and from the sediment-water transition in the Black Sea. At shallow water sites methane migrates from the sediment into the water column, and the magnitude of this upward migrating flux depends on the depth of the sulfate-methane transition (SMT) in the sediment. The isotope data reveal that the sediments at shallow water sites are a source for methane depleted in 13C relative to the isotope composition of methane in the water column. At deep water sites the methane concentration first decreases with depth in the sediment to reach lowest values at the Unit I to Unit II transition. Below this transition the concentration increases again. Numerical modeling of methane concentration and isotope data shows that high methane oxidation rates occur in the surface sediment layer, indicating that the removal of methane in the surface sediments is not related to the anaerobic oxidation of methane coupled to sulfate reduction that occurs a few meters deep in the sediment, at the SMT. Instead, near-surface methane consumption in the euxinic Black Sea sediments appears to be related to lithological stratification. Furthermore, a map of the diffusive methane fluxes in the Black Sea surface sediments indicates that approximately half of the Black Sea seafloor acts as a sink for methane and thus limits the flux of methane to the atmosphere.

Sulfur speciation and isotopic composition in waters and sediments from river and seawater mixing zones

Studies of sulfur behavior in the water column and in sediments in river and seawater mixing zone were conducted in three areas of the Black and Azov Seas. These investigations showed constancy of sulfate concentrations versus chlorinity. Sulfur isotope composition in sulfates of surface, bottom, and pore waters depended on sulfate contents and salinity. The dependence was complicated by partial sulfate depletion in pore water due to bacterial sulfate reduction and also by alteration of isotope composition. Surface sediments in mixing zones are characterized by intensive sulfate reduction, great variability of content and isotopic composition of reduced sulfur, and a low mean isotopic fractionation factor of sulfur.

Interstitial water chemistry, Deep Sea Drilling Project, Leg 96

On Leg 96 of the Deep Sea Drilling Project (DSDP), holes were drilled in Orca and Pigmy basins on the northern Gulf of Mexico continental slope and on the Mississippi Fan. The holes on the fan encountered interbedded sand, silt, and mud deposited extremely rapidly, most during late Wisconsin glacial time. Pore-water chemistry in these holes is variable, but does not follow lithologic changes in any simple way. Both Ca and SO4 are enriched in the pore water of many samples from the fan. Two sites drilled in the prominent central channel of the middle fan show rapid SO4 reduction with depth, whereas two nearby sites in overbank deposits show no sulfate reduction for 300 m. Calcium concentration decreases as SO4 is depleted and Li follows the same pattern. Strontium, which like Li, is enriched in samples enriched in Ca, does not decrease with SO4 and Ca. Potassium in the pore water decreases with depth at almost all sites. Sulfate reduction was active at the two basin sites and, as on the fan, this resulted in calcium carbonate precipitation and a lowering of pore water Ca, Mg, and Li. The Orca Basin site was drilled through a brine pool of 258? salinity. Pore-water salinity decreases smoothly with depth to 50 m and remains well above normal seawater values to the bottom of the hole at about 90 m. This suggests constant sedimentation under anoxic hypersaline conditions for at least the last 50,000 yr.

Sulfur isotope composition of pore fluids at ODP Sites 128-798 and 128-799

Micro-crystalline barites recovered by deep-sea drilling from Site 684 on the Peru margin and Site 799 in the Japan Sea are highly enriched in the heavy sulfur isotope relative to seawater ( d34S up to +84?). This isotopic composition is consistent with remobilization of biogenic barite triggered by sulfate reduction, and subsequent reprecipitation as a diagenetic barite front. The high levels of barium sulfate in these deposits (10-50%) cannot be explained by a diffusive transport model in sediments experiencing a constant rate of sedimentation. When sedimentation rates change radically, the barite front will remain at a given depth interval leading to large accumulations of barium sulfate. Such conditions may have generated the barite deposits at Site 799. At Site 684, on the other hand, there is evidence that the barite deposits are a result of the tectonically-driven advection of sulfate-bearing fluids through the sediment column.

Biogeochemical measurements associated to deep-sea wood falls

Large organic food falls to the deep sea - such as whale carcasses and wood logs - support the development of reduced, sulfidic niches in an otherwise oxygenated, oligotrophic deep-sea environment. These transient hot spot ecosystems may serve the dispersal of highly adapted chemosynthetic organisms such as thiotrophic bivalves and siboglinid worms. Here we investigated the biogeochemical and microbiological processes leading to the development of sulfidic niches. Wood colonization experiments were carried out for the duration of one year in the vicinity of a cold seep area in the Nile deep-sea fan (Eastern Mediterranean) at depths of 1690 m. Wood logs were deployed in 2006 during the BIONIL cruise (RV Meteor M70/2 with ROV Quest, Marum, Germany) and sampled in 2007 during the Medeco-2 cruise (RV Pourquoi Pas? with ROV Victor 6000, Ifremer, France). Wood-boring bivalves played a key role in the initial degradation of the wood, the dispersal of wood chips and fecal matter around the wood log, and the provision of colonization surfaces to other organisms. Total oxygen uptake measured with a ROV-operated benthic chamber module was higher at the wood (0.5 m away) in contrast to 10 m away at a reference site (25 mmol m-2 d-1 and 1 mmol m-2 d-1, respectively), indicating an increased activity of sedimentary communities around the wood falls. Bacterial cell numbers associated with wood increased substantially from freshly submerged wood to the wood chip/fecal matter layer next to the wood experiments, as determined with Acridine Orange Direct Counts (AODC) and DAPI-stained counts. Microsensor measurements of sulfide, oxygen and pH were conducted ex situ. Sulfide fluxes were higher at the wood experiments when compared to reference measurements (19 and 32 mmol m-2 d-1 vs. 0 and 16 mmol -2 d-1, respectively). Sulfate reduction (SR) rates at the wood experiments were determined in ex situ incubations (1.3 and 2.0 mmol m-2 d-1) and fell into the lower range of SR rates previously observed from other chemosynthetic habitats at cold seeps. There was no influence of wood deposition on phosphate, silicate and nitrate concentrations, but ammonium concentrations were elevated at the wood chip-sediment boundary layer. Concentrations of dissolved organic carbon were much higher at the wood experiments (wood chip-sediment boundary layer) in comparison to measurements at the reference sites, which may indicate that cellulose degradation was highest under anoxic conditions and hence enabled by anaerobic benthic bacteria, e.g. fermenters and sulfate reducers. Our observations demonstrate that, after one year, the presence of wood at the seafloor had led to the creation of sulfidic niches, comparable to what has been observed at whale falls, albeit at lower rates.

Stable and radiogenic isotope ratios in pore waters from ODP Leg 127 sites

Interstitial waters from four sites of the Japan Sea (794 to 797) have been analyzed for stable isotopes (delta D, delta11B, delta18O, and delta34S) and 87Sr/86Sr, besides major and minor ions. The isotopic composition is dominated by organic matter degradation, alteration of ash layers and volcaniclastic sands, silica transformation (opal A/CT), and basement alteration. Organic matter degradation and corresponding sulfate reduction leads to 32S depletion and is dependent upon sedimentation rate. The remaining sulfate reservoir is characterized by very "heavy" delta34S ratios, up to +93 ? (rel. CDT = Canyon Diabolo Troilite). "Barite fronts," which may develop in such sediments, should also be characterized by very "heavy" sulfur isotopes. The alteration of volcaniclastic material in the Quaternary sections influences the delta18O (-1.5 ? shift) and delta11B (desorption and later adsorption of "labile"11B). A pronounced positive delta11B anomaly at Site 795 represents the depth range of preferential 10B uptake by alteration products of the ash layers. At Site 796 delta D, delta11B, and 87Sr/86Sr are severely affected by alteration processes of volcaniclastic sands. The opal A/CT transformation may influence the oxygen isotopes and serves as a potential source for B, which is liberated at this interval at Site 795. This positive B anomaly is not reflected in the delta11B profile. Basement alteration processes dominate the sedimentary sequence below the opal A/CT transition, which serves as a chemical and physical boundary. The decreases in delta D and delta18O are probably related to a "paleo ocean water reservoir" situated in the permeable Layer II of the oceanic crust, as is indicated by the positive correlation between these two parameters. Besides Mg, alkalies and delta18O basement rocks also serve as a sink for 11 B (Site 795) and are the source for the Ca and Sr increases, as is documented by the less radiogenic 87Sr/86Sr ratio. 87Sr/86Sr ratios for the lowermost pore waters from Site 795 (0.70529) are comparable to those from volcaniclastic rocks from the "Green Tuff' region (0.704 to 0.706) and oil field brines from the Niigata Oil Field.