Introduction

Diatoms have great potential for studies of marine paleoecology and paleoceanography, especially in high latitudes and coastal regions. In these settings they are diverse and abundant, usually being the dominant group in the fossil assemblage. Elsewhere, their use may be limited by the relatively poor preservation of biogenic silica. The red clays of the central oceanic gyres contain no diatoms at all; most calcareous sediments contain only fragments of a few robust forms.

The poor preservation results from two interrelated conditions. (i) Diatoms as a group are at a competitive disadvantage in conditions of low nutrient supply. In addition to the universal requirement for nitrogen and phosphorus, they will be limited by availability of silicon, and trace metals such as iron may be limiting also (Martin & Gordon, 1988; Coale et al., 1996). Therefore, over large areas of the world ocean, diatoms are a minor component of the phytoplankton, and those taxa which are present are frequently very weaklysilicified. (ii) Seawater and sediment porewaters are usually undersaturated with respect to biogenic silica (Tréguer et al., 1995), so that dissolution of the frustules occurs rapidly, especially when pH is relatively high, as it often is in calcareous sediments. Preservation is good only in sediments with a high component of rock particles (ice-rafted detritus, coastal sediments, volcanic ash beds) or in siliceous (diatom) oozes, which result from a combination of high rates of diatom production and exclusion of other sediment components.