A primary objective of Ocean Drilling Program (ODP) Leg 199 is to determine the position and strength of upwelling zones and the extent of primary production in the equatorial zones of the Pacific Ocean during the Paleogene. Calculating mass accumulation rates of the biogenic components in the sediments (specifically opal, calcium carbonate, and organic carbon) is essential for determining the locations and migrations of paleoupwelling zones. Our laboratory planned to generate the opal data for Leg 199 cores by employing a commonly used wet-digestion method, but we were concerned about the feasibility of obtaining good opal measurements using a sodium carbonate (Na2CO3) leach in light of previous studies that suggested the robust tests of Eocene radiolarians would be solution resistant. For example, Moore (1969) recognized that opaline assemblages of Eocene and Miocene surface sediments are more resistant to dissolution in seawater relative to Quaternary assemblages. The reasons that explain this observation have not been fully elucidated but may reflect possibilities such as bond changes in the opaline skeletons upon aging (Heath, 1974) and/or morphologically dissimilar shell remains between the two age groups. That is, Eocene fossil assemblages contain proportionately more opaline skeletons, which are larger and thicker compared to late Neogene opal assemblages, whose individuals exhibit thinner more delicate structures. Hence, the relatively low surface area per unit mass ratio of Eocene fossils might explain their slower dissolution rate under natural conditions. Therefore, we anticipated that the Leg 199 sediments will present special problems with regard to accurately measuring the biogenic silica fraction and, consequently, challenge our ability to achieve a primary objective of the cruise.
A commonly used method for rapidly measuring biogenic silica is the wet-digestion technique described by Mortlock and Froelich (1989). They successfully employed a 2-M Na2CO3 solution to efficiently digest sediments with a wide range of opal content (3%-100%). However, the samples used in their study span a relatively short geologic time interval (Pliocene-Holocene), ~5 m.y. The relatively young age of those sediments is contrasted to Leg 199 samples, where drilling recovered a sedimentary record spanning the Miocene-Paleocene, a span of geologic time nearly an order of magnitude older than the samples used in the Mortlock and Froelich (1989) study. Also, the opaline fossil assemblages differ between the two studies. Preliminary work during Leg 199 site survey samples (EW9709) indicates that the sediments are dominated by radiolarian oozes, which contrasts to Mortlock and Froelich's (1989) diatom-rich samples. The problem of dissolution-resistant radiolarians when using a Na2CO3 treatment is convincingly illustrated in Figure F1A and F1B, which show photographs of the smear slides with solid sediment residue that remained after 9- and 14-hr 2-M Na2CO3 digestions of two samples of Eocene radiolarian ooze (collected on the site survey cruise). Abundant radiolarian fossils remained undissolved, even after nearly doubling and tripling the recommended digestion time of 5 hr by Mortlock and Froelich (1989). Hence, their method's efficiency for rapidly determining biogenic opal in marine sediments may be limited to "young" diatom-rich, radiolarian-poor sediments. In this study, we employed a harsher alkaline treatment to dissolve the opaline fraction of site survey samples from Core EW9709 to determine this methods robustness for providing reasonable measurements of biogenic silica for radiolarian-rich sediments. Site survey cores were recovered during the September 1997 cruise of the Maurice Ewing. Samples from three EW9709 cores and other sites used in this study (see "Results for Site Survey Samples") were digested in a 2-M potassium hydroxide (KOH) solution, and aliquots of the supernatant were analyzed by spectrophotometry for SiO2. The sediment residue remaining after the digestion was recovered and mounted on a smear slide for microscopic analysis. Both the analytical and smear slide results, after the KOH digestion, were compared to an independent leach, supernatant analysis, and smear slide inspection of the residue after a 2-M Na2CO3 extraction. (A second aliquot of undigested sediment sample was used in the Na2CO3 leach). This analytical regimen (two independent leaches of the sampled interval: one sediment sample digested in Na2CO3, a second sediment sample in KOH, followed by recovery of the undigested residues for microscopic inspection) was applied to Pacific Ocean sediments comprising different opal assemblages (diatom-rich samples vs. radiolarian-rich samples) and spanning the Eocene-Holocene.