Variability of surface sediments in the Peru basin: dependence on water depth, productivity, bottom water flow, and seafloor topography

We investigated surficial sediments for physico-chemical composition from n umerous sites of seven study areas in the manganese nodule field of the nor thern Peru Basin as part of a deep-sea environmental study. Major results f rom this study are strong variability with respect to water depth, producti vity in surface waters, locality, bottom water flow, and seafloor topograph y. Sediment sites are located mostly in 3900 to 4300 m water depth between the lysocline and the carbonate compensation depth (CCD). Large fluctuation s in carbonate content (0% to 80%) determine sediment density and compressi onal-wave velocity, and, by dilution, contents of opal and non-biogenic mat erial. Mass accumulation rates of biogenic components as well as geochemica l proxies (barium and phosphorus) distinguish areas of higher productivity in the northwest near equatorial upwelling and in the northeast close to co astal upwelling, from areas of lower productivity in the west and south. Co mparisons between the central Peru Basin area (Discol) and western Peru Bas in area (Sediperu) reveals, for the Sediperu area, a shallower CCD, more ca rbonate but less opal, organic carbon, and non-biogenic material in sedimen ts at the same water depth as well as larger down-core fluctuations of orga nic carbon and MnO2. Bottom water flow in the abyssal hill topography cause s winnowing of material from summits of seamounts and ridges, where organic carbon preservation is poor, to basins where organic carbon preservation i s better. Down-core measurements in box cores indicate a three-fold divisio n in the upper 50 cm of the sediment column. An uppermost semi-liquid top l ayer is dark brown, 5-15 cm thick and contains most of the ferro-manganese nodules. A 5-15 cm thick transition zone of light sediment color has increa sing shear strength, lowest opal contents and compressional-wave velocities , but highest carbonate contents and sediment densities. The lowermost laye r contains stiffer light gray sediments. (C) 2000 Elsevier Science B.V. All rights reserved.