Mineralogy and isotopic composition of sulfur in ODP Hole 118-735B gabbros

Sulfide mineralogy, sulfur contents, and sulfur isotopic compositions were determined for samples from the 500-m gabbroic section of Ocean Drilling Program Hole 735B in the southwest Indian Ocean. Igneous sulfides (pyrrhotite, chalcopyrite, pentlandite, and troilite) formed by accumulation of immiscible sulfide droplets and crystallization from intercumulus liquids. Primary sulfur contents average around 600 ppm, with a mean sulfide d34S value near 0 per mil, similar to the isotopic composition of sulfur in mid-ocean ridge basalt glass. Rocks from a 48-m interval of oxide gabbros have much higher sulfur contents (1090-2530 ppm S) due to the increased solubility of sulfur in Fe-rich melts. Rocks that were locally affected by early dynamothermal metamorphism (e.g., the upper 40 m of the core) have lost sulfur, averaging only 90 ppm S. Samples from the upper 200 m of the core, which underwent subsequent hydrothermal alteration, also lost sulfur and contain an average of 300 ppm S. Monosulfide minerals in some of the latter have elevated d34S values (up to +6.9 per mil), suggesting local incorporation of seawater-derived sulfur. Secondary sulfides (pyrrhotite, chalcopyrite, pentlandite, troilite, and pyrite) are ubiquitous in trace amounts throughout the core, particularly in altered olivine and in green amphibole. Pyrite also locally replaces igneous pyrrhotite. Rocks containing secondary pyrite associated with late low-temperature smectitic alteration have low d34S values for pyrite sulfur (to - 16.6 per mil). These low values are attributed to isotopic fractionation produced during partial oxidation of igneous sulfides by cold seawater. The rocks contain small amounts of soluble sulfate (6% of total S), which is composed of variable proportions of seawater sulfate and oxidized igneous sulfur. The ultimate effect of secondary processes on layer 3 gabbros is a loss of sulfur to hydrothermal fluids, with little or no net change in d34S.

Clay mineraology and grain size composition of surface sediments from South Pacific Ocean

The reconstruction of low-latitude ocean-atmosphere interactions is one of the major issues of (paleo-)environmental studies. The trade winds, extending over 20° to 30° of latitude in both hemispheres, between the subtropical highs and the intertropical convergence zone, are major components of the atmospheric circulation and little is known about their long-term variability on geological time-scales, in particular in the Pacific sector. We present the modern spatial pattern of eolian-derived marine sediments in the eastern equatorial and subtropical Pacific (10°N to 25°S) as a reference data set for the interpretation of SE Pacific paleo-dust records. The terrigenous silt and clay fractions of 75 surface sediment samples have been investigated for their grain-size distribution and clay-mineral compositions, respectively, to identify their provenances and transport agents. Dust delivered to the southeast Pacific from the semi- to hyper-arid areas of Peru and Chile is rather fine-grained (4-8 µm) due to low-level transport within the southeast trade winds. Nevertheless, wind is the dominant transport agent and eolian material is the dominant terrigenous component west of the Peru-Chile Trench south of ~ 5°S. Grain-size distributions alone are insufficient to identify the eolian signal in marine sediments due to authigenic particle formation on the sub-oceanic ridges and abundant volcanic glass around the Galapagos Islands. Together with the clay-mineral compositions of the clay fraction, we have identified the dust lobe extending from the coasts of Peru and Chile onto Galapagos Rise as well as across the equator into the doldrums. Illite is a very useful parameter to identify source areas of dust in this smectite-dominated study area.

Geothermal measurements in the Tyrrhenian Sea

Geothermal data obtained during Cruise 12 of the R/V Vityaz-II and published data on heat flux of the Tyrrhenian Sea are analyzed. The thermal field is related to principal tectonic structures of the basin. Distribution of heat flux indicates that the initial stage of rifting occurs in the central basin of the Tyrrhenian Sea.

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.