(Table 1) Chemical composition of oceanic phosphates and their selenium content

Selenium content of phosphate material from the ocean bottom ranges from 0.2 to 4.7 mg/kg. Phosphorites of various ages from the Atlantic and Pacific Oceans contain 1.0-2.4 mg/kg of selenium, phosphatized coproliths 0.7-1.2 mg/kg, fish bones 0.2-1,4 mg/kg, and bones of marine mammals 0.5-4.7 mg/kg. Recent diatom muds on the shelf of Namibia are considerably enriched in selenium (12.2-13.8 mg/kg) than phosphorites that form within them. Accumulation of selenium in phosphate material on the ocean bottom results from diagenetic reduction, causing it to be precipitated from liquid phase and to concentrate in organic components and sulfides.

Sulfur isotope rations in oceanic basement samples

Low temperature alteration of oceanic basement rocks is characterized by net gain of sulfur, which commonly yields low d34S values, suggesting involvement of microbial sulfate reduction. In order to test whether secondary sulfide minerals are consistent with a biogenic source, we apply high precision multiple sulfur isotope analysis to bulk rock sulfide and pyrite isolates from two contrasting types of altered oceanic basement rocks, namely serpentinized peridotites and altered basalts. Samples from two peridotite sites (Iberian Margin and Hess Deep) and from a basalt site on the eastern flank of the Juan de Fuca Ridge yield overlapping d34S values ranging from 0 per mil to -44 per mil. In contrast, sulfides in the basalt site are characterized by relatively low D33S values ranging from -0.06 per mil to 0.04 per mil, compared to those from peridotite sites (0.00 per mil to 0.16 per mil). The observed D33S signal is significant considering the analytical precision of 0.014 per mil (2 sigma). We present a batch reaction model that uses observed d34S and D33S relationships to quantify the effect of closed system processes and constrain the isotope enrichment factor intrinsic to sulfate reduction. The estimated enrichment factors as large as 61 per mil and 53 per mil, for peridotite and basalt sites respectively, suggest the involvement of microbial sulfate reduction. The relatively high D33S values in the peridotite sites are due to sulfate reduction in a closed system environment, whereas negative D33S values in the basalt site reflect open system sulfate reduction. A larger extent of sulfate reduction during alteration of peridotite to serpentinite is consistent with its higher H2 production capacity compared to basalt alteration, and further supports in-situ microbial sulfate reduction coupled with H2 production during serpentinization reactions.