Ocean Drilling Program (ODP) Leg 175 shipboard and shore-based diatom studies, based on smear-slide analysis, showed evidence of a pronounced and prolonged maximum in diatom export production during the late Pliocene and early Pleistocene at Walvis Ridge (Site 1081) and in Walvis (Sites 1082 and 1083) and Cape Basins (Sites 1084 and 1085) (Wefer, Berger, Richter, et al., 1998). This prolonged maximum is reached within the early Matuyama Chron; it is centered at ~2.2 Ma and follows a rapid increase in diatom deposition near 3.1 Ma (Lange et al., 1999).

In order to precisely define the Matuyama diatom maximum (MDM) for future comparison with other records in the region and elsewhere (e.g., equatorial Atlantic: Ruddiman and Janecek, 1989; subantarctic region: Froelich et al., 1991), we selected Sites 1082 and 1084 for the measurements of biogenic opal content (Fig. F1).

The goals of the present work are threefold: (1) to find ways to merge opal data from different laboratories for time series analysis, (2) to compare the Pliocene-Pleistocene opal records at Sites 1082 and 1084, and (3) to establish whether overall diatom abundance constitutes a reliable proxy for opal content.

Several techniques have been used to measure the biogenic opal content in marine sediments. The wet alkaline methods are the most widely used. Advantages and limitations of the most common extraction procedures were discussed by DeMaster (1991), illustrating the variations in the methodology and showing that different methods do not necessarily give the same results. More recently, Conley (1998) made an interlaboratory comparison that confirmed a wide range of variability in the measurement of biogenic opal, though no significant differences were found between the two most widely used techniques: (1) DeMaster (1981) and (2) Mortlock and Froelich (1989). In this study, we demonstrate that these two methods delivered results that can be merged.