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.

Major ion concentrations and chlorine isotope composition of water-soluble chloride and structurally bound chloride in DSDP/ODP serpentinized ultramafic rocks

Eight DSDP/ODP cores were analyzed for major ion concentrations and d37Cl values of water-soluble chloride (d37Clwsc) and structurally bound chloride (d37Clsbc) in serpentinized ultramafic rocks. This diverse set of cores spans a wide range in age, temperature of serpentinization, tectonic setting, and geographic location of drilled serpentinized oceanic crust. Three of the cores were sampled at closely spaced intervals to investigate downhole variation in Cl concentration and chlorine isotope composition. The average total Cl content of all 86 samples is 0.26±0.16 wt.% (0.19±0.10 wt.% as water-soluble Cl (Xwsc) and 0.09±0.09 wt.% as structurally bound Cl (Xsbc)). Structurally bound Cl concentration nearly doubles with depth in all cores; there is no consistent trend in water-soluble Cl content among the cores. Chlorine isotope fractionation between the structurally bound Cl**- site and the water-soluble Cl**- site varies from -1.08? to +1.16?, averaging to +0.21?. Samples with negative fractionations may be related to reequilibration of the water-soluble chloride with seawater post-serpentinite formation. Six of the cores have positive bulk d37Cl values (+0.05? to +0.36?); the other two cores (173-1068A (Leg-Hole) and 84-570) have negative bulk d37Cl values (-1.26? and -0.54?). The cores with negative d37Cl values also have variable Cl**-/SO4**2- ratios, in contrast to all other cores. The isotopically positive cores (153-920D and 147-895E) show no isotopic variation with depth; the isotopically negative core (173-1068A) decreases by ~1? with depth for both the water-soluble and structurally bound Cl fractions. Non-zero bulk d37Cl values indicate Cl in serpentinites was incorporated during original hydration and is not an artifact of seawater infiltration during drilling. Cores with positive d37Cl values are most likely explained by open system fractionation during hydrothermal alteration, with preferential incorporation of 37Cl from seawater into the serpentinite and loss of residual light Cl back to the ocean. Fluid / rock ratios were probably low as evidenced by the presence of water-soluble salts. The two isotopically negative cores are characterized by a thick overlying sedimentary package that was in place prior to serpentinization. We believe the low d37Cl values of these cores are a result of hydration of ultramafic rock by infiltrating aqueous pore fluids from the overlying sediments. The resulting serpentinites inherit the characteristic negative d37Cl values of the pore waters. Chlorine stable isotopes can be used to identify the source of the serpentinizing fluid and ultimately discern chemical and tectonic processes involved in serpentinization.

Lead 210 and silicate profiles from sediments of the equatorial Pacific and South Atlantic

Particle mixing rates have been determined for 5 South Atlantic/Antarctic and 3 equatorial Pacific deep-sea cores using excess 210Pb and 32Si measurements. Radionuclide profiles from these siliceous, calcareous, and clay-rich sediments have been evaluated using a steady state vertical advection diffusion model. In Antarctic siliceous sediments210Pb mixing coefficients (0.04-0.16 cm**2/y) are in reasonable agreement with the 32Si mixing coefficient (0.2 or 0.4 cm**2/y, depending on 32Si half-life). In an equatorial Pacific sediment core, however, the 210Pb mixing coefficient (0.22 cm**2/y) is 3-7 times greater than the 32Si mixing coefficient (0.03 or 0.07 cm**2/y). The difference in 210Pb and 32Si mixing rates in the Pacific sediments results from: (1) non-steady state mixing and differences in characteristic time and depth scales of the two radionuclides, (2) preferential mixing of fine-grained clay particles containing most of the 210Pb activity relative to coarser particles (large radiolaria) containing the 32Si activity, or (3) the supply of 222Rn from the bottom of manganese nodules which increases the measured excess 210Pb activity (relative to 226Ra) at depth and artificially increases the 210Pb mixing coefficient. Based on 32Si data and pore water silica profiles, dissolution of biogenic silica in the sediment column appears to have a minor effect on the 32Si profile in the mixed layer. Deep-sea particle mixing rates reported in this study and the literature do not correlate with sediment type, sediment accumulation rate, or surface productivity. Based on differences in mixing rate among three Antarctic cores collected within 50 km of each other, local variability in the intensity of deep-sea mixing appears to be as important as regional differences in sediment properties.

Silicate content in sediment ofthe Santa Monica Basin, California

Seafloor recycling of organic materials in Santa Monica Basin, California was examined through in situ benthic chamber experiments, shipboard whole-core incubations and pore water studies. Mass balance calculations indicate that the data are internally consistent and that the estimated benthic exchange rates compare well with those derived from deep, moored conical sediment traps and hydrographic modeling. Pore water and benthic flux observations indicate that the metabolizable organic matter at the seafloor must be composed of at least two fractions of very different reactivities. While the majority of reactive organic compounds degrade quickly, with a half-life of <=6.5 years, 1/4 of the total metabolizable organic matter appears to react more slowly, with a half-life on the order of 1700 years. Down-core changes in pore water sulfate and titration alkalinity are not explained by stoichiometric models of organic matter diagenesis and suggest that reactions not considered previously must be influencing the pore water concentrations. Measured recycling and burial rates indicate that 43% of the organic carbon reaching the basin seafloor is permanently buried. The results for Santa Monica Basin are compared to those reported for other California Borderland Basins that differ in sedimentation rate and bottom water oxygen content. Organic carbon burial rates for the Borderland Basins are strongly correlated with total organic carbon deposition rate and bulk sedimentation rate. No significant correlation is observed between carbon burial and bottom water oxygen, extent of oxic mineralization and sediment mixing. Thus, for the California Borderlands, it appears that carbon burial rates are primarily controlled by input rates and not by variations in preservation.

Silicate in marine sediments from the Emerland Basin, North Atlantic

A method was developed to measure porosity and dissolved interstitial silicate at millimeter intervals or less in a sediment core. In cores from Emerald Basin (Scotian Shelf), interstitial concentrations near the sediment surface did not drop rapidly to bottom-water concentrations as measured in bottle casts (28 µM) but remained as high as 166 µM in the upper 0.5 mm of sediment High rates of benthic silicate release were measured which could not be accounted for by interstitial concentration gradients or by ventilation of macro-invertebrate burrows. The silicate discontinuity observed between the sediments and water column suggests that a diffusive sublayer exists in a zone of viscous flow above the sediment surface. This is possible only if a surface reaction is primarily responsible for silicate release. By assuming a linear concentration gradient across this diffusive sublayer, the silicate release rates were used to estimate the thickness of the sublayer to be about 2 mm.