Carbonate-Free Sediment Components and Aspects of Silica Diagenesis at Sites 707, 709, and 711(Leg 115, Western Indian Ocean)

Abstract

From the equatorial Indian Ocean, carbonate-free portions of sediment samples of Paleocene to Miocene calcareous oozes and chalks from Sites 707, 709, and 711 were studied using X-ray diffraction measurements and the scanning electron microscope. Downhole variations in biogenic opal, quartz, barite, and clinoptilolite were investigated. The abundance patterns of these major mineral phases show several similarities and may be used for additional lithologic correlations.Variations in biogenic opal contents reflect biogenic silica productivity. Beside the general pattern, a succession in biogenic silica decrease through time is generally recorded since the Oligocene. This succession started earliest at northernmost Site 711 and latest at southernmost Site 707, including Site 709 within these two. Opal-A variations as well as the barite distribution may be influenced by the paleoposition of the sites in relation to the high-productivity zone, which today lies south of the equator. Authigenic clinoptilolite apparently formed in two different modes. In deeper sediment intervals, clinoptilolite was the last mineral phase formed associated with enhanced silica diagenesis. In late Oligocene to middle Miocene sediments, clinoptilolite was the only authigenic silica phase encountered where otherwise strong opal dissolution was observed. The sponge spicules showed special dissolution features probably related to microbiological activity. Silica concretions mainly composed of opal-CT and authigenic quartz occur in carbonate-rich environments and are formed during later diagenesis when burial depth causes the sediments to reach higher temperatures. Opal-CT concretions in carbonate-free siliceous oozes were found at Site 711 and are probably formed during an early stage of silica diagenesis.

Last glacial sea surface temperatures and sea‐ice extent in the Southern Ocean (Atlantic‐Indian sector): A multiproxy approach

“…For the determination of BSi (calculated for bound water content of 10%), an automated leaching method was applied [Müller and Schneider, 1993]. In addition, BSi contents were estimated from the maximal peak height of the broad opal hump in x-ray diffractograms, calibrated with standard samples [Eisma and van der Gaast, 1971;Hempel and Bohrmann, 1990;Leg 177 Shipboard Scientific Party, 1999].…”