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

摘要

We investigated surficial sediments for physico-chemical composition from numerous sites of seven study areas inthe manganese nodule field of the northern Peru Basin as part of adeep-sea environmental study. Major results from this study arestrong variability with respect to water depth, productivity insurface waters, locality, bottom water flow, and seafloortopography. Sediment sites are located mostly in 3900 to 4300mwater depth between the lysocline and the carbonate compensationdepth (CCD). Large fluctuations in carbonate content (0% to 80%)determine sediment density and compressional-wave velocity, and,by dilution, contents of opal and non-biogenic material. Massaccumulation rates of biogenic components as well asgeochemical proxies (barium and phosphorus) distinguish areas ofhigher productivity in the northwest near equatorial upwelling andin the northeast close to coastal upwelling, from areas of lowerproductivity in the west and south. Comparisons between thecentral Peru Basin area (Discol) and western Peru Basin area(Sediperu) reveals, for the Sediperu area, a shallower CCD, morecarbonate but less opal, organic carbon, and non-biogenic materialin sediments at the same water depth as well as larger down-corefluctuations of organic carbon and MnO_2. Bottom water flow inthe abyssal hill topography causes winnowing of material fromsummits of seamounts and ridges, where organic carbonpreservation is poor, to basins whee organic carbon preservation isbetter. Down-core measurements in box cores indicate a thick andcotnains most of the ferro-manganese nodules. A 5-15 cm thicktransition zone of light sediment color has incrasing shear strength,lowest opal contents and compressional-wave velocities, buthighest carbonate contents and sediment densities. The lowermostlayer contains stiffer light gray sediments.