Middle Valley

Bent Hill Area

Water collected at 73 mbsf from Hole 856H prior to drilling has 511 ppm methane indicating that methane is moving advectively through the sulfide deposit. Gas removed from lithified sediments (191 mbsf) consisted of equal amount of methane (18,000 ppm) and CO₂ with traces of propane and ethane. The occurrence of soot carbon in the same interval indicates that the gas may be adsorbed onto the carbon. Organic and carbonate carbon contents are low. Significant CH₄ and CO₂ concentrations were only encountered at depth in Holes 1035A and 1035G. Methane concentration increases dramatically at the bottom of Hole 1035A. Extractable bitumen shows accelerated maturation with depth for Holes 1035A, 1035D, and 1035E, but the low concentration indicates in situ alteration without migration. In the shallow sections of all holes a terrigenous source was identified as the dominant form of organic carbon.

Inorganic geochemistry

Pore fluid chemistry was investigated in the sediment cover of the sulfide mound in Holes 1035A, 1035D, 1035E, and 1035G. In Holes 1035D and 1035E, most constituents of the pore fluids display only minor changes with depth. Increase of Ca, Sr, and SO₄ above seawater values can be explained by the retrograde dissolution of anhydrite due to decreasing temperature. Conductive heating of pore water cannot produce the observed amount of anhydrite, but substantial downward circulation of seawater through the upper sediment is a likely cause. A small chloride maximum at about 25 mbsf is interpreted as a remnant of higher chloride content of seawater during the last glaciation. In Hole 1035A, Cl concentration shows a gradual increase with depth and a steep decrease below 150 mbsf. This decrease can be explained by an increasing contribution of the BHMS endmember hydrothermal fluid due to lateral advection. It is likely that at this depth an impermeable barrier prevents the influence of hydrothermal water in the upper sediments. Similar sharp enrichment is also seen for lithium and boron. The upper sediments are not significantly affected by hydrothermal water. One pore fluid sample collected in the feeder zone just under the sulfide mound shows strong similarities with the hydrothermal fluid previously sampled with Alvin from the active vent on the ODPMS mound. This hydrothermal fluid is easily distinguishable by its low chloride content from the fluid venting in the Dead Dog hydrothermal field 4 km to the northwest.

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