Biogenic opal preservation in pelagic sediments of a small area in the eastern tropical Pacific

Abstract

A 630 km2 area of the sea floor near 14.degree. N, 117.degree. W in the E Pacific was surveyed and sampled extensively to determine the pattern of siliceous microfossil preservation, silica concentrations in the sediments and interstitial waters and clay mineralogy. Siliceous microfossil preservation is uniformly excellent in the surface sediments throughout the area, but deteriorates markedly with depth, to the point where biogenic opal is totally absent within 1-2 m below the sea floor. Sediment redistribution by bottom currents concentrates siliceous microfossils in topographic depressions, causing good preservation to extend to greater depths beneath the sea floor in these areas than on surrounding topographic highs. The down-core change in preservation [surface sediments containing siliceous microfossils, radiolarian and diatom bearing brown clays, brown and tan clays containing fish skeletal debris, a Miocene marl containing Sphenolithus heteromorphus, Cyclicargolithus floridanus, Catapsydrax dissimilis, Globorotalia peripheronda, Globoquadrina altispira] may be due to an ever-increasing input rate of biogenic opal to the sea floor over the past one-half million years and postburial dissolution. Chemical and mineralogical analyses of the sediments indicate that some silica released by postburial dissolution may be used in the formation of authigenic smectite.