Sedimentation in the vicinity of Leg 92 Drill Sites: Studies of site survey cores

Surveys of the areas surrounding the sites drilled on the Leg 92 19°S transect showed that sedimentation at all except the oldest site is dominated by calcium carbonate deposition. The sediments in the area of the oldest site, west of the Austral Fracture Zone, are being deposited beneath the calcium carbonate compensation depth and are dominated by terrigenous and metal-rich hydrogenous and hydrothermal sediments. The noncarbonate sediments in all of the areas east of the Austral Fracture Zone are dominated by hydrothermal sediment similar in composition to that presently being deposited at the East Pacific Rise. Although no biogenic microfossils were present in smear slides of the sediment, geochemical partitioning suggests that a remnant signal of siliceous biogenic deposition may be preserved, especially in gravity core (GC) 8, which was collected from a high heat flow zone near Site 600. The siliceous sediment may also result from the deposition of amorphous hydrothermal silica from the higher concentrations of pore water SiO2 characteristic of the upwelling waters. Sedimentation on the broad plateaus that characterize each area is quite uniform and suggests that sites on these plateaus will be broadly representative of pelagic sedimentation in the area.