SULFUR ISOTOPE AND POREWATER GEOCHEMISTRY OF FLORIDA ESCARPMENT SEEP SEDIMENTS

Distributions of porewater constituents, SO4=, NH4+, Cl-, SIGMACO2, and H2S, solid phase iron, and sulfur concentrations, and the sulfur isotopic CoMposition of dissolved and solid phases were investigated in sediments from abyssal seeps at the base of the Florida escarpment. Despite the apparent similarity of seep sediment porewater chemistry to that of typical marine sediments undergoing early diagenesis, relationships between chemical distributions and isotopic measurements revealed that the distribution of pore fluid constituents was dominated by processes occurring within the platform rather than by in situ microbial processes. Ammonium and sulfate concentrations were linearly correlated with chloride concentrations, indicating that variations in porewater chemistry were controlled by the admixture of seawater and a sulfate depleted brine with a chlorinity of 27.5 +/- 1.9 parts per thousand and 2.2 +/- 1.3 mM ammonium concentration. At sites dominated by seepage, dissolved sulfate isotopic composition remained near seawater values despite depletion in porewater concentrations. Porewater SIGMACO2 concentrations were found to be elevated relative to seawater, but not to the extent predicted from the observed sulfate depletion. Sediment solid phase sulfur was predominantly pyrite, at concentrations as high as 20% S by weight. In contrast to typical marine deposits, pyrite concentrations were not related to the quantity of sedimentary organic matter. Pyrite deltaS-34 values ranged from -29 parts per thousand to +21 parts per thousand (CDT). However, only positive deltaS-34 values were observed at sites associated with high-pyrite concentrations. Isotopically heavy pyrite was observed at sites with porewater sulfate of seawater-like isotopic composition. Isotopically light pyrite was associated with sites where porewater sulfate exhibited deltaS-34 values greater than those in seawater, indicating the activity of in situ microbial sulfate reduction. Thus, dual sulfide sources are suggested to explain the range in sediment pyrite isotopic composition: a deltaS-34 enriched (+ 10 to +20 parts per thousand) source advected from within the Florida platform, and a lighter S-34 depleted component generated in situ from microbial reduction of seawater sulfate. The degree of pyritization of seep sediments was as high as 0.9 and was controlled by pyrite concentrations, which varied over a wider range than did the non-pyrite solid phase iron concentrations. The highest non-sulfide solid phase iron concentrations were observed in sediments that are believed to be at the ''front'' of the advancing seep fluids (i.e., hemipelagic sediments newly exposed to the seep fluids), indicating that dissolution of hemipelagic background sediment may be the source of at least half of the iron to the highly pyritized seep sediments. Porewater sulfide concentrations were variable, reaching a maximum of 5.7 mM, and were not correlated with the degree of pyritization of the sediments, suggesting that iron was not particularly limiting to pyrite formation.