Acid-soluble sulfides in upper layer bottom sediments from the Tsemess and Gelendzhik bays and adjacent shelf area of the Black Sea

Contents of labile (acid-soluble) sulfides were determined in the upper layer of bottom sediments at 80 stations on the Caucasian shelf of the Black Sea. Maximum values of this parameter occurred in black mud accumulated in zones of intense pollution in the Gelendzhik and Tsemess bays and in shelf areas adjacent to large health resort objects and to seaports. Contents of acid-soluble sulfides in sediments varied from 400 to 900 mg S/dm**3 of wet mud. In zones of moderate pollution they varied from 200 to 400 mg S/dm**3. Rate of sulfate reduction was 10-40 mg S/dm**3 of wet sediment per day. Obtained data show that accumulation of labile sulfides in the upper layer of shelf bottom sediments is directly related to anthropogenic pollution and is one of the most hazardous environmental aftereffects.

Isotopic composition of oxygen, carbon, and sulfur in interstitial water and cores at DSDP Leg 59 Holes

Comprehensive isotopic studies based on data from the Deep Sea Drilling Project have elucidated numerous details of the low- and high-temperature mechanisms of interaction between water and rocks of ocean crustal seismic Layers 1 and 2. These isotopic studies have also identified climatic changes during the Meso-Cenozoic history of oceans. Data on the abundance and isotopic composition of sulfur in the sedimentary layer as well as in rocks of the volcanic basement are more fragmentary than are oxygen and carbon data. In this chapter we specifically concentrate upon isotopic data related to specific features of the mechanisms of low-temperature interaction of water with sedimentary and volcanogenic rocks. The Leg 59 data provide a good opportunity for such lithologic and isotopic studies, because almost 600 meters of basalt flows and sills interbedded with tuffs and volcaniclastic breccias were cored during the drilling of Hole 448A. Moreover, rocks supposedly exposed to hydrothermal alteration play an important role at the deepest horizons of that mass. Sulfur isotopic studies of the character of possible biogenic processes of sulfate reduction in sediments are another focus, as well as the nature and origin of sulfide mineralization in Layer-2 rocks of remnant island arcs. Finally, oxygen and carbon istopic analyses of biogenic carbonates in the cores also enabled us to investigate the effects of changing climatic conditions during the Cenozoic. These results are compared with previous data from adjacent regions of the Pacific Ocean. Thus this chapter describes results of isotopic analyses of: oxygen and sulfur of interstitial water; oxygen and carbon of sedimentary carbonates and of calcite intercalations and inclusions in tuffs and volcaniclastic breccias interbedded with basalt flows; and sulfur of sulfides in these rocks.

Paleomagnetic of DSDP Leg 90

Sediments recovered during Leg 90 (Sites 587-594, plus Site 586 cored during Leg 89) are, in general, extremely weakly magnetized carbonate oozes and chalks with NRM intensities seldom greater than 0.05 µG. The quality of the paleomagnetic records deteriorates with increasing depth caused by the combined effects of removal of primary magnetic oxides by sulfate reduction processes and the dispersal of magnetic grains during compaction. Magnetic reversal sequences are generally recognizable back to the Gilbert, 3.4 to 5.35 m.y., except at equatorial Site 586 where only the Brunhes/Matuyama boundary could be identified. Longer reversal records were obtained at Site 588 (to Chron 13, about 13 m.y.) and Site 594 (base of Chron 5, about 5.9 m.y.). Sediments are characterized by extremely high calcium carbonate contents (90-100%) with almost no biosiliceous components. Blebs and streaks of pyrite are common, and the presence of iron sulfides with poor magnetic stabilities is suspected, although not yet positively identified. Viscous components of magnetization are common, sometimes to the extent of dominating the primary remanence, and there is evidence to suggest that a magnetic remanence is imparted during core recovery. Siliceous carbonate oozes provide better paleomagnetic records than pure carbonate oozes.

Carbon isotopic compositions and concentrations of major fatty acids in the Beggiatoa covered sediment core at the crest of southern Hydrate Ridge

Membrane fatty acids were extracted from a sediment core above marine gas hydrates at Hydrate Ridge, NE Pacific. Anaerobic sediments from this environment are characterized by high sulfate reduction rates driven by the anaerobic oxidation of methane (AOM). The assimilation of methane carbon into bacterial biomass is indicated by carbon isotope values of specific fatty acids as low as -103 per mill. Specific fatty acids released from bacterial membranes include C 16:1 omega 5c , C 17:1 omega 6c , and cyC 17:0 omega 5,6 , all of which have been fully characterized by mass spectrometry. These unusual fatty acids continuously display the lowest d13 C values in all sediment horizons and two of them are detected in high abundance (i.e., C 16:1 omega 5c and cyC 17:0 omega 5,6 ). Combined with microscopic examination by fluorescence in situ hybridization specifically targeting sulfate-reducing bacteria (SRB) of the Desulfosarcina/Desulfococcus group, which are present in the aggregates of AOM consortia in extremely high numbers, these specific fatty acids appear to provide a phenotypic fingerprint indicative for SRB of this group. Correlating depth profiles of specific fatty acid content and aggregate number in combination with pore water sulfate data provide further evidence of this finding. Using mass balance calculations we present a cell-specific fatty acid pattern most likely displaying a very close resemblance to the still uncultured Desulfosarcina/Desulfococcus species involved in AOM.

(Table T2) Sulfur isotopes of pore water from ODP Leg 201 sites

Fifty-seven interstitial water samples from six sites (Ocean Drilling Program Sites 1225-1229 and 1231) in the eastern equatorial Pacific Ocean and the Peru margin were analyzed for the stable sulfur isotopic composition (34S/32S) of dissolved sulfate along with major and minor ions. With the exception of Site 1231, sulfate from the interstitial fluids (d34S values as much as 89 per mil vs. the SF6-based Vienna-Canyon Diablo troilite standard) is found at depth to be enriched in 34S with respect to modern seawater sulfate (d34S = ~21 per mil), indicating that microbial sulfate reduction (MSR) took place to different extents at all investigated sites. Deeper sediments at Sites 1228 and 1229 are additionally influenced by diffusion of a sulfate-rich brine that has already undergone sulfate reduction. The intensity of MSR depends on the availability of substrate (organic matter), sedimentation conditions, and the active bacterial community structure. Formation of isotopically heavy diagenetic barite at the sulfate-methane transition zone is expected at Sites 1227 (one front), 1229 (two fronts), and probably Site 1228. At Site 1231, the constant sulfur isotopic composition of sulfate and concentrations of minor pore water ions indicate that suboxic (essentially iron and manganese oxide based) diagenesis dominates and no net MSR occurs.

Relative abundance and isotopic composition of calcite, dolomite and siderite from ODP Leg 164 sites

Authigenic carbonate mineral distributions are compared to pore-water geochemical profiles and used to evaluate diagenesis within sedimentary sections containing gas hydrates on the Blake Ridge (Ocean Drilling Program Sites 994, 995, and 997). Carbonate mineral distributions reveal three distinct diagenetic zones. (1) Carbonate minerals in the upper 20 m are primarily biogenic and show no evidence of diagenesis. The d13C and d18O values of calcite within this zone reflects marine carbonate (~0 per mil Peedee belemnite [PDB]) formed in equilibrium with seawater. (2) Between 20 and 100 mbsf, calcite d13C values are distinctly negative (as low as -7.0 per mil), and authigenic dolomite is common (~2-40 wt%) with d13C values between -3.6 per mil and 13.7 per mil. (3) Below 100 mbsf, dolomite abundance decreases to trace amounts, and disseminated siderite becomes the pervasive (~2-30 wt%) authigenic carbonate. Both siderite textures and stable isotope values indicate direct precipitation from pore fluids rather than dolomite replacement. The d13C and d18O values of siderite vary from 5.0 per mil to 10.9 per mil and 2.9 per mil to 7.6 per mil, respectively. Comparisons between the d13C profiles of dissolved inorganic carbon (DIC) and pore-water concentration gradients, with the d13C and d18O values of authigenic carbonates, delineate a distinct depth zonation for authigenic carbonate mineral formation. Coincidence of the most negative d13CDIC values (<=-38 per mil) and negative d13C values of both calcite and dolomite, with pore-water alkalinity increases, sulfate depletion, and decreases in interstitial Ca2+ and Mg2+ concentrations at and below 20 mbsf, suggests that authigenic calcite and dolomite formation is initiated at the base of the sulfate reduction zone (~21 mbsf) and occurs down to ~100 mbsf. Siderite formation apparently occurs between 120 and 450 mbsf; within, and above, the gas hydrate-bearing section of the sediment column (~200-450 mbsf). Siderite d13C and d18O values are nearly uniform from their shallowest occurrence to the bottom of the sedimentary section. However, present-day pore-water d13CDIC values are only similar to siderite d13C values between ~100 and 450 mbsf. Furthermore, calculated equilibrium d18O values of siderite match the measured 18O values of siderite between 120 and 450 mbsf. This interval is characterized by high alkalinity (40-120 mM) and low Ca2+ and Mg2+ concentrations, conditions that are consistent with siderite formation.

Sulfur and iron speciations in bottom sediments from the southwest Caspian Sea

Speciations of sulfur (sulfide S, pyrite S, sulfate S) and of reactive iron (Fe3+, Fe2+, sulfide Fe) in bottom sediments have been studied in gravity cores and drill cores collected on the shelf of the southwest Caspian Sea. It has been shown that intensity of reduction processes, in particular sulfate reduction, as well as speciations of S and reactive Fe reflect the change of transgressive and regressive stages of the Caspian basin. Characteristic features for the investigated area are high sedimentation rate and high reactivity of organic matter entering bottom sediments.

Paleomagnetic analysis of ODP Site 133-820

Paleomagnetic analysis of sediment samples from Ocean Drilling Program (ODP) Leg 133, Site 820, 10 km from the outer edge of the Great Barrier Reef, is undertaken to investigate the mineral magnetic response to environmental (sea level) changes. Viscous remanent magnetization (VRM) of both multidomain and near-superparamagnetic origin is prevalent and largely obscures the primary remanence, except in isolated high-magnetization zones. The Brunhes/Matuyama boundary cannot be identified, but is expected to be below 120 mbsf. The only evidence that exists for a geomagnetic excursion occurs at about 33 mbsf (-135 k.y.). Only one-half the cores were oriented, and many suffered from internal rotation about the core axis, caused by coring and/or slicing. The decay of magnetic remanence below the surface layer (0-2 mbsf) is attributed to sulfate reduction processes. The magnetic susceptibility (K) record is central for describing and understanding the magnetic properties of the sediments, and their relationship to glacio-eustatic fluctuations in sea level. Three prominent magnetic susceptibility peaks, at about 7, 32, and 64 mbsf, are superimposed on a background of smaller susceptibility oscillations. Fluctuations in susceptibility and remanence in the ôbackgroundö zone are controlled predominantly by variations in the concentration, rather than the composition of ferrimagnetics, with carbonate dilution playing an important role (type-A properties). The sharp susceptibility maxima occur at the start of the marine transgressions following low stands in sea level (high d18O, glacial maxima), and are characterized by a stable single-domain remanence, with a significant contribution from ultra-fine, superparamagnetic grains (type-C properties). During the later marine transgression, the susceptibility gradually returns to low values and the remanence is carried by stable single-domain magnetite (type-B properties). The A, B, and C types of sediment have distinctive ARM/K ratios. Throughout most of the sequence a strong inverse correlation exists between magnetic susceptibility and both CaCO3 and d18O variations. However, in the sharp susceptibility peaks (early transgression), more complex phase relationships are apparent among these parameters. In particular, the K-d18O correlation switches to positive, then reverts to negative during the course of the late transgression, indicating that two distinct mechanisms are responsible for the K-d18O correlation. Lower in the sequence, where sea-level-controlled cycles of upward-coarsening sediments, we find that the initial, mud phase of each cycle has been enriched in high-coercivity magnetic material, which is indicative of more oxic conditions. The main magnetic characteristics of the sediments are thought to reflect sea-level-controlled variations in the sediment source regions and related run-off conditions. Some preliminary evidence is seen that biogenic magnetite may play a significant role in the magnetization of these sediments.

Paleomagnetic of ODP Hole 133-823A

Hole 823A covers the upper 120 m (Subunits IA and IB) of Site 823 at the bottom of the Queensland Trough. This hole contains an abundance of gravity-flow deposits, but is thought to have a monotonic age sequence. Above 32 mbsf, a strong, stable (normal) magnetic remanence having a relatively small viscous remanent magnetization (VRM) is seen. Below 32 mbsf, the sediments are subject to widespread VRM, which appears to obliterate the primary magnetization and precludes identification of the Brunhes/Matuyama boundary. Progressive alternating field (AF) demagnetization is limited to low fields (typically <400 Oe) by the weak magnetization in these sediments. As a consequence, the possibility of a high-coercivity component of primary magnetization cannot be ruled out. Lowrie-Fuller tests indicate that this VRM overprinting does not have a multidomain origin. An approximately linear relationship exists between median destructive field (MDF) and the logarithm of the natural remanent magnetization (NRM). Carbonate dilution does not appear to be a dominant factor in controlling variations in concentration-dependent magnetic parameters, such as magnetic susceptibility. The sedimentological distinction between Subunits IA and IB does not show up in the magnetic record. However, a sharp change in magnetic properties does occur at 32 mbsf, with low background magnetizations below this level and high background magnetizations above it. The boundary coincides with a change from thick (>10 cm thick) to thin (<10 cm thick) turbidite deposition, and is also near the boundary separating the sulfate-reduction zone in the upper part of the sequence from the sulfate-free zone beneath. The abrupt nature of the magnetic boundary is evidence that nannofossil subzone CN14b is not condensed, but is missing in a hiatus at 32 mbsf. Nine peaks have been identified in the susceptibility (K) record that are superimposed on ôbackgroundö signals. ARM/K ratios are uniformly low for the background sediments below 32 mbsf, intermediate for strong susceptibility peaks, and high for background sediments above 32 mbsf and weak susceptibility peaks. Comparisons with results from Site 820 suggest that (1) the background sediments above 32 mbsf and the weak susceptibility peaks carry a stable single-domain magnetization, and (2) the high susceptibility peaks are caused by the addition of a superparamagnetic contribution. Expectations are that the distinctive features of the Hole 823A magnetic record are linked to major environmental changes.

Geochemical investigation of ODP Leg 113 sites

Dissolved organic carbon (DOC) was determined in pore water extracted from pelagic and hemipelagic sediments recovered during Leg 113. DOC concentration varied between 1.82 and 13.6 mg C/L which is one to two orders of magnitude less than previously reported for hemipelagic sediments. It is argued that this difference is related to differences in the intensity of degradation of organic matter. As a first approximation it is found that in reducing sediments, the level of DOC is proportional to the intensity of sulfate reduction. It is suggested that DOC is formed by different mechanisms in oxic and reducing environments.

Texture, geochemistry, isotope composition, petrography and epoch at DSDP Site 93-603

Abundant Fe-Mn carbonate concretions (mainly siderite, manganosiderite, and rhodochrosite) were found in the hemipelagic claystones of Site 603 on the eastern North American continental rise. They occur as nodules, micronodules, or carbonate-replaced burrow fills and layers at a subbottom depth of between ~ 120 (Pliocene) and 1160 m (Albian-Cenomanian). In general, the Fe-Mn carbonate concretions form from CO3- produced by the microbiological degradation of organic matter in the presence of abundant Fe + or Mn + and very low S- concentrations. However, there is also some evidence for diagenetic replacement of preexisting calcite by siderite. The carbon isotope composition of diagenetic Fe-Mn carbonate nodules is determined by CO2 reduction during methanogenesis. Carbonate nodules in Cretaceous sediments at sub-bottom depths of 1085 and 1160 m have distinctly lower d13C values (- 12.2 and - 12.9 per mil) than Neogene siderites, associated with abundant biogenic methane in the pore space (-8.9 to 1.7 per mil between 330 and 780 m depth). Since no isotopic zonation could be detected within individual nodules, we assume that the isotopic composition reflects more or less geochemical conditions at the present burial depth of the carbonate nodules. Carbonates did not precipitate within the zone of sulfate reduction (approximately 0.01 to 10 m), where all of the pyrite was formed. The oxygen isotope composition indicates precipitation from seawater-derived interstitial waters. The d18O values decrease with increasing burial depth from + 5.1 to - 1.2 per mil, suggesting successively higher temperatures during carbonate formation.

High-resolution in situ oxygen microprofiles, porewater and solid phase geochemistry from the Crimean shelf (Black Sea) from Maria S. Merian cruise MSM15/1

The outer western Crimean shelf of the Black Sea is a natural laboratory to investigate effects of stable oxic versus varying hypoxic conditions on seafloor biogeochemical processes and benthic community structure. Bottom-water oxygen concentrations ranged from normoxic (175 µmol O2/L) and hypoxic (< 63 µmol O2/L) or even anoxic/sulfidic conditions within a few kilometers' distance. Variations in oxygen concentrations between 160 and 10 µmol/L even occurred within hours close to the chemocline at 134 m water depth. Total oxygen uptake, including diffusive as well as fauna-mediated oxygen consumption, decreased from 15 mmol/m**2/d on average in the oxic zone, to 7 mmol/m**2/d on average in the hypoxic zone, correlating with changes in macrobenthos composition. Benthic diffusive oxygen uptake rates, comprising respiration of microorganisms and small meiofauna, were similar in oxic and hypoxic zones (on average 4.5 mmol/m**2/d), but declined to 1.3 mmol/m**2/d in bottom waters with oxygen concentrations below 20 µmol/L. Measurements and modeling of porewater profiles indicated that reoxidation of reduced compounds played only a minor role in diffusive oxygen uptake under the different oxygen conditions, leaving the major fraction to aerobic degradation of organic carbon. Remineralization efficiency decreased from nearly 100 % in the oxic zone, to 50 % in the oxic-hypoxic zone, to 10 % in the hypoxic-anoxic zone. Overall, the faunal remineralization rate was more important, but also more influenced by fluctuating oxygen concentrations, than microbial and geochemical oxidation processes.

Methane and hydrogen sulfide in porewater of sediment cores from the upwelling area off Namibia

Porewater concentrations of sulfate, methane, and other relevant constituents were determined on four sediment cores from the high productivity upwelling area off Namibia which were recovered from the continental slope at water depths of 1300 and 2000 m. At all four stations a distinct sulfate-methane transition zone was observed several meters below the seafloor in which both sulfate and methane are consumed. Nutrient porewater concentration profiles do not show gradient slope changes at the depths of the transition zones. Flux calculations carried out on the basis of the determined porewater profiles revealed that anaerobic methane oxidation accounts for 100% of deep sulfate reduction within the sulfate-methane transition zone and consumes the total net diffusive sulfate flux. A significant contribution of organic carbon oxidation to the reduction of sulfate at these depths could, therefore, be excluded. We state that porewater profiles of sulfate with constant gradients above the transition zones are indicative for anaerobic methane oxidation controlling sulfate reduction.