Geochemistry of basal dolomitic and Fe-Mn sediments from the Tyrrhenian Sea

In the Tyrrhenian Sea (Western Mediterranean), unusual reddish, soft to lithified, dolomitic sediments up to 45 m thick overlie igneous crust at the base of thick Pliocene-Quaternary deep-sea sediment successions in the Marsili (Site 650) and Vavilov (Site 651) basins. These sediments also overlie the Gortani Ridge, a basaltic Seamount near the base of the Sardinian continental margin (Site 655). At both basinal sites (650, 651), the lowest sediments are dolomitic, with manganese oxide (MnO) segregations. Whole-rock X-ray diffraction indicates abundant dolomite and quartz, with subordinate calcite, illite (authigenic), feldspar and minor kaolinite, chlorite, and anhydrite. Chemical analyses show strong enrichment in magnesium oxide (MgO) and MnO relative to shale or deep-sea clay. Mg and Mn correlate positively and exhibit decreasing concentrations up the succession in the Marsili Basin (Site 650). The following scenario is proposed: peridotites were exposed on the seafloor in the Vavilov Basin (Site 651) and then eroded, depositing talc in local fine-grained dolomitic sediments within the igneous basement. After local magmatism ended, the igneous basement at each site subsided rapidly (about 800 m/m.y.) and was blanketed with calcareous and clay-rich oozes. During early diagenesis (from isotopic evidence; McKenzie et al., this volume) tepid fluids, of modified seawater composition, reacted with and dolomitized the overlying deep-sea sediments. At Site 651 additional Mg may have been extracted from asthenosphere peridotite cored at shallow depths (about 100 m). One can hypothesize that fluids rich in Mg and Mn were flushed from the igneous basement, triggered by extensional faulting and local tilting during subsidence of the basement, and that these fluids then dolomitized the base of the overlying sediment succession. Late tectonic movements in the Vavilov Basin (Site 651) fractured already lithified dolomitic sediments and more reducing (? hydrothermal) fluids locally remobilized Fe and Mn and corroded dolomite crystals.

Pliocene to Pleistocene calcium carbonate and organic carbon record of ODP Hole 117-722B

Site 722 provides high resolution records of percent CaCO3, magnetic susceptibility, d18O, organic carbon, and coarse fraction for the past 3.4 m.y. from the crest of the Owen Ridge, northwestern Arabian Sea. Within this time interval, most of the carbonate percent variations can be attributed to terrigenous dilution and do not reflect changes in the carbonate system. From the late Pliocene to Present, the average rate of calcium carbonate accumulation increases from 1 to 3 g/cm**2/k.y. and the average accumulation of organic carbon decreases from 75 to 30 mg/cm**2/k.y. The carbonate component is more dissolved in the older interval. The long-term variations in carbonate accumulation may reflect a greater input of organic matter in the late Pliocene, which decomposes to produce CO2 and dissolve carbonate. Magnetic susceptibility and % noncarbonate (100 - CaCO3%) reflect changes in the amount of the lithogenic component in the sediments. The period of variation of lithogenic material is the same period as the original forcing of the regional summer monsoon, however, the timing matches global aridity patterns and global ice volume (sea level) changes. This preliminary analysis suggests that the high frequency variation of lithogenic material persists for at least the last 3.4 m.y. Within the last million years, calcium carbonate accumulation has a large amplitude signal that covaries with major changes in ice volume. Both calcium carbonate and noncarbonate (mostly terrigenous) accumulation are greatest during glacial stages. Interglacial intervals are characterized by low mass accumulation rates, increased foraminifer fragmentation, and increased opal concentration. The accumulation of organic carbon matches the high frequency changes in sedimentation rates. We attribute this high correlation to enhanced preservation of organic carbon by increased sedimentation rate. Of the three major biological components studied, only opal exhibits the variations expected for a biological productivity system forced by monsoonal upwelling driven by changes in northern hemisphere summer radiation.