Abstract

Geochemical studies of metalliferous sediments from the Atlantis II Deep reveal a variable history of hot brine activity. Radiocarbon dating indicates that sedimentation began in the deep about 28,000 yrs ago. The lowermost unit (DOP) consists mainly of detrital biogenic carbonates with occasional thin beds of iron oxide or sulfide minerals. Sulfur isotope composition and carbon-sulfur relations indicate that some of these sulfide layers have a hydrothermal source whereas others formed by bacterial sulfate reduction in the sediments. DOP zone sedimentation rates were about 25 cm/1,000 yr.Continuous brine activity began in the west basin of the Atlantis II Deep about 15,000 yrs ago with deposition of the lower sulfide zone (SU 1 ). Sulfur isotope values average 5.4 (range 1 to 14) in this unit as in the upper sulfide (SU 2 ) and amorphous silicate (AM) zone. Overall hydrothermal sedimentation rates average 100 cm/1,000 yr. However, low sulfur values and negative sulfur isotope values in the central oxide zone (CO) indicate a period of less vigorous brine influx. Facies variations are related to the rate of brine influx and the size of the brine pool rather than changes in the bulk composition or temperature of the inflowing brine.Organic carbon and sulfide sulfur values in the SU 2 , SU 1 , and AM zones indicate direct hydrothermal addition of sulfide. Sulfur isotope values can best be explained by inorganic sulfate reduction in the subsurface brine and nonequilibrium sulfur isotope fractionation.

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