Carbonate concentrations of ODP Leg 115 samples

We measured carbonate concentrations in Pleistocene and Pliocene sediments deposited at Sites 709, 710, and 711. Carbonate concentrations exhibit low-amplitude, long-wave length (300-400 k.y. period) variations at the shallowest sites (709 and 710). Before 2.47 Ma, all three sites exhibit higher frequency (100 k.y. period) variations. The deepest site (711) exhibited low-amplitude variations and very low concentrations up to the Gauss/Matuyama magnetic reversal (2.47 Ma), then concentrations abruptly increased. After 2.47 Ma, carbonate concentrations at Site 711 exhibited the same periodic changes as at Site 709. Although a long wave-length periodicity (260-280 k.y.) occurs at these sites after 2.47 Ma, the 100 k.y. period is absent. The dominant periods observed in these data are those found in the eccentricity component of the earth's orbital geometry. Estimates of carbonate accumulation at Sites 709 and 710 document that surface-water productivity decreased near the Gauss/Matuyama magnetic reversal whereas accumulation at Site 711 increased. These results indicate that the rate of carbonate preservation in the deep Indian Ocean increased at that time. This increase in preservation may have re- sulted from a decrease in the production rate of carbonate in tropical oceans of the world. Carbonate accumulation esti- mated from sediments in shallow locations (~3000-3800 m) of the Atlantic and Pacific oceans also indicates that carbonate production decreased at this time. A consequence of lowered surface-water productivity is increased carbonate ion concentration of the deep ocean and better preservation of carbonate on the seafloor.

Carbonat-free sediment components of ODP Leg 115 holes

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.