Title: Cenozoic Planktonic Biostratigraphy and Paleoceanography of the Tropical Western Indian Ocean
Abstract: Eight sites drilled during DSDP Leg 24 in the tropical western Indian Ocean provide a nearly complete record of major Cenozoic and paleoceanographic events.Sites 231, 232, and 233 in the Gulf of Aden penetrated Neogene hemipelagic calcareous oozes; these expanded sections are of special interest for investigation of planktonic events.Sites 234 and 235 in the Somali Basin lay close to or below the CCD throughout the time of deposition and are not well suited for biostratigraphic investigation.Sites 236, 237, and 238 in the central western Indian Ocean recovered Pleistocene through Oligocene calcareous oozes and Eocene and Paleocene chert-bearing chalk.The occurrence of well-preserved calcareous and siliceous microfossils in the late Neogene and Eocene of Sites 238 and 237, respectively, permits the direct comparison and correlation of zonation schemes based on the two microfossil types for these time intervals.Sediment-based contacts were penetrated, and basal sediment ages determined, at four sites.The opening of the Gulf of Aden predates 13 m.y.The sundering of the Chagos and Saya de Malha regions is at least as old as 32-34 m.y.B.P. The basement age at Site 235 (65-67 m.y.B.P.) implies the presence of a discontinuity between this site and the magnetic lineations identified to the east, and of an offset of the magnetic anomalies to the north.A discrepancy between the biostratigraphic age of the basal sediments (57-58 m.y.B.P.) at Site 236 and the magnetic age of basement (10 m.y.older according to the prevailing paleomagnetic time scale) is consistent with similar discrepancies noted at other DSDP sites.This finding supports the adjustments proposed for the Heirtzler et al. (1968) paleomagnetic scale, including a reduction of the ages for the early Cenozoic.Similar tropical planktonic assemblages recovered at the various sites reflect a common Oceanographic history throughout the Cenozoic.Prior to the late Paleocene, a thick sequence accumulated rapidly (68 m/m.y.) at Site 237 in upper bathyal depths, with an added influx of displaced neritic sediments from nearby shallow banks.After the deepening of this site in the late Paleocene, normal pelagic sedimentation proceeded at a rate about one-sixth as fast, similar to contemporaneous rates at Site 236.An interval of much-reduced accumulation or nondeposition spans the Paleocene/Eocene boundary at both sites.In the late Eocene-early Oligocene, when Antarctic Bottom Water moved northward into the Indian Ocean, the accumulation rate and preservation of calcareous ooze decreased.Preservation of planktonic foraminifera remained poor during the Oligocene and early Miocene, and hiatuses or condensed series occurred during this interval.A change in the circulation system occurred by middle Miocene time.When India moved north of the equator, the development of the modern south equatorial circulation pattern resulted in higher surface productivity, reflected in the late Neogene sequences by a higher content of siüceous microfossils and by sedimentation rates about three times as high as those 1111 E. VINCENT prevailing during the preceding Oligocene and early Miocene.An episode of reworking in the early late Miocene, following a 2-m.y.interval of reduced sedimentation spanning the middle/late Miocene boundary, suggests the advent of large-scale bottom current activity in the central western Indian Ocean, Gulf of Aden, and Arabian Sea.Water Site Latitude Longitude Depth (m)