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The Geological Record of Upwelling Evolution |
Department of Geological Sciences, Brown University, Providence, RI 02912, USA
Pelagic sediments recovered during Leg 117 of the Ocean Drilling Program (ODP) from the Owen Ridge, northwestern Arabian Sea, record regional upwelling variations and global climatic oscillations since the Late Neogene. Miocene to Recent deposits exhibit characteristic light and dark cycles (0.5–1.0 m thick) that are well correlated to calcium carbonate concentration; light layers are carbonate-rich and dark layers carbonate-poor. The deposits are largely composed of biogenic calcite and eolian derived terrigenous materials. Opaline silica and organic carbon comprise less than 10% of the sediment composition. Within the last 370 000 years, the major compositional oscillations are synchronous with global ice volume changes. These cycles result from terrigenous dilution rather than changes in carbonate production or dissolution. Noncarbonate concentration characterizes these cycles and is a good approximation of temporal changes in terrigenous input for the past 3.4 My. Prior to 2.5 Ma, the major compositional oscillations are dominated by high-frequency cycles possibly related to earth’s precessional changes. Within the past 2.5 My there is a shift in variance to much longer periods characteristic of high-latitude climatic variations. Based on late Pleistocene associations between lithogenic grain size and accumulation, the sedimentary cycles produced by changes in the lithogenic flux are attributed to changes in source area aridity rather than monsoon wind strength.
Calcite, opal and organic carbon accumulation rates, commonly used to infer palaeoproductivity, are compared to records of monsoon wind strength in the Arabian Sea. Changes in calcite accumulation by as much as 3 mg/cm2/a between glacial and interglacial intervals are attributed to dissolution rather than calcite production. The accumulation of organic carbon matches the changes in sedimentation rate. We attribute this high correlation to enhanced preservation of organic carbon by increased sedimentation rate, not to palaeoproductivity variations. Opal accumulation provides a good match to other indices of monsoon wind strength and upwelling intensity. These monsoon indices exhibit a strong concentration of variance in the precessional band, but lag the expected forcing by
Major variations in the mean accumulation rates of sediment components, thought to reflect large changes in monsoon-driven upwelling, are quantified for the late Pliocene to Present. The average rate of calcium carbonate accumulation increases from 1 to 3 mg/cm2/a and organic carbon accumulation decreases from 75 to 30 µg/cm2/a. The long-term increase in carbonate accumulation is the result of enhanced preservation of Recent deposits compared to the upper Pliocene. The high rate of organic carbon accumulation between 1.5 and 3.0 Ma, also observed at Oman Margin sites, implies that oceanographic conditions were more favourable for high production and preservation of organic carbon in the Arabian Sea during the Late Pliocene compared to the Present.
9 ky.
