• D K Sinha

      Articles written in Journal of Earth System Science

    • Ocean circulation in the tropical Indo-Pacific during early Pliocene (5.6 - 4.2 Ma): Paleobiogeographic and isotopic evidence

      M S Srinivasan D K Sinha

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      A Comparison of late Neogene planktic foraminferal biogeography and stable isotopic records of shallow dwelling and deep dwelling planktic foraminifera from DSDP sites 214 (Ninetyeast Ridge, northeast Indian Ocean) and 586B (ontong-Java Plateau, western Equatorial Pacific) provides a clue to the nature of the ocean circulation in the tropical Indo-Pacific during early Pliocene.

      The Present study reveals that the late Neogene planktic foraminiferal data from the eastern and western sides of the Indonesian Seaway are very similar. The only distinct inter-ocean difference however is the absence of Pulleniatina spectablis from the Indian Ocean. This species makes its first evolutionary appearance in the Equatorial Pacific at about 5.6 Ma (Early Gilbert reversed) and ranges up to 4.2 Ma (Top Conhiti subchron). The complete absence of Pulleniatina spectablis from the Indian Ocean is attributed to blocking of westward flow of tropical waters of the Pacific to the Indian Ocean resulting in a major change in the tropical Pacific and Indian oceans during 5.6 to 4.2 ma.

      In order to understand the nature of this blockage, isotopic depth ranking of selected planktic foraminifera and thus may be interpreted that the shallow sills that mark the Seaway in modern times were present as early as 5.6 Ma.

      The distribution of Pulleniatina spectablis throughout the Equatorial Pacific reveals that Modern Equatorial Pacific Under Current (Cromwell Current) flowing towards east at a depth of 200-300 m (which is also the depth habitat of Pulleniatina spectablis) was present at the beginning of the Pliocene (5.6 Ma).

      As a dequel to the blocking of the Indonesian Seaway and the resultant interruption in the flow of central Equatorial Current System of the Pacific to the west there was an increase in the western Pacific Warm Pool Waters and strengthening of the gyral circulation in the Pacific and Indian Oceans. This eventually triggered the intensification of the Asian Monsoon System.

    • Coccolithophores from the central Arabian Sea: Sediment trap results

      Lina P Mergulhao Rahul Mohan V S N Murty M V S Guptha D K Sinha

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      Sediment trap samples collected from a depth of 1018 m in the Central Arabian Sea Trap (CAST) at 14°28.2′N, 64°35.8′E were analyzed for temporal variation of coccolithophore fluxes from October 1993 to August 1994. Out of the twenty species of coccolithophores encountered,Gephyrocapsa oceanica, Emiliania huxleyi, Umbilicosphaera sibogae andUmbellosphaera irregularis were the most abundant. The total coccolithophore fluxes ranged from 28.5 × 106m-2d-1 to 50.3 × 106m-2d-1 showing seasonality with higher fluxes during the northeast (NE) monsoon and lower fluxes during the spring intermonsoon. The higher fluxes were attributed to the enhancement of primary production in the central Arabian Sea due to southward extent of nutrients from the northeast Arabian Sea by the prevailing surface currents. Similarly, the occurrences of relatively lower coccolithophore fluxes during the spring intermonsoon and southwest (SW) monsoon were attributed to the low nutrients in the warm, shallow surface mixed layer and downwelling to the south of Findlater Jet respectively in the central Arabian Sea. Some of the coccolithophore species such asE. huxleyi, G. oceanica, Calcidiscus leptoporus andUmbellosphaera tenuis showed signs of dissolution.

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