Pliocene-Plistocene deposition of carbonate and organic carbon in ODP Hole 159-959C

During Ocean Drilling Program (ODP) Leg 159, four sites (Sites 959-962) were drilled along a depth transect on the Côte d'Ivoire/Ghana Transform Margin. In this study, the Pliocene-Pleistocene history of carbonate and organic carbon accumulation at Hole 959C is reconstructed for the eastern equatorial Atlantic off the Ivory Coast/Ghana based on bulk carbonate, sand fraction, organic carbon, and other organic geochemical records (d13Corg, marine organic matter percentages derived from organic petrology, hydrogen index, C/N). Pliocene-Pleistocene sedimentation off the Ivory Coast/Ghana was strongly affected by low mean sedimentation rates, which are attributed to persistently enhanced bottom-water velocities related to the steep topography of the transform margin. Sand fraction and bulk carbonate records reveal typical glacial/interglacial cycles, preserved, however, with low time resolution. Intermediate carbonate accumulation rates observed throughout the Pliocene-Pleistocene suggest intense winnowing and sediment redistribution superimposed by terrigenous dilution. 'Atlantic-type' sand and carbonate cycles, consistent with records from pelagic areas of the eastern equatorial Atlantic, are encountered at Hole 959C prior to about 0.9 Ma. Total organic carbon (TOC) records are frequently inversely correlated to carbonate contents, indicating mainly productivity-driven carbonate dissolution related to changes in paleoproductivity. During Stages 22-24, 20, 16, 12, 8, and 4, sand and carbonate records reveal a 'Pacific-type' pattern, showing elevated contents during glacials commonly in conjunction with enhanced TOC records. Formation of 'Pacific-type' patterns off the Ivory Coast/Ghana is attributed to drastically increased bottom-water intensities along the transform margin in accordance with results reported from the Walvis Ridge area. Short-term glacial/interglacial changes in paleoproductivity off the Ivory Coast/Ghana are to some extend recognizable during glacials prior to 1.7 Ma and interglacial Stages 21, 19, 13, 9, and 1. Enhanced coastal upwelling during interglacials is attributed to local paleoclimatic and oceanographic conditions off the Ivory Coast/Ghana. Quantitative estimates of marine organic carbon based on organic petrologic and d13Corg records reveal an offset in concentration ranging from 15% to 60%. Highest variabilities of both records are recorded since ~0.9 Ma. Discrepancies between the isotopic and microscopic records are attributed to an admixture of C4 plant debris approaching the eastern equatorial Atlantic via atmospheric dust. Terrestrial organic material likely originated from the grass-savannah-covered Sahel zone in central Africa. Estimated C4 plant concentrations and accumulation rates range from 10% to 37% and from almost zero to 0.006 g/cm**2/k.y., respectively. The strongest eolian supply to the northern Gulf of Guinea is indicated between 1.9 and 1.68 Ma and during glacial isotopic Stages 22-24, 20, 14, and 12. The presence of grass-type plant debris is further supported by organic petrologic studies, which reveal well-preserved cell tissues of vascular plants or tube-shaped, elongated terrestrial macerals showing different levels of oxidation.

Distribution of foraminifera and other components in ODP Leg 188 sites

During Leg 188 of the Ocean Drilling Program (ODP), employing JOIDES Resolution, we drilled holes at three sites in the southern Indian Ocean in and near Prydz Bay, East Antarctica, between 28 January and 29 February 2000. The objectives of the voyage were to: - Core through sediments deposited when Antarctica underwent the transition from "greenhouse" to the modern "icehouse" state late in the Eocene or early in the Oligocene, at sites obtaining their sediment from the currently subglacial Gamburtsev Mountains that probably were the site of nucleation of the ice sheet (principally Site 1166); - Obtain a sediment record from times at which major changes in the ice sheet volume and characteristics took place as judged from oxygen isotope records, especially at ~23.7 Ma (Oligocene/Miocene boundary), 12-16 Ma (middle Miocene), and 2.7 Ma (late Pliocene) (mainly Site 1165); and - Sample through the upper Pliocene and Quaternary in an attempt to document fluctuations in the extent of the ice sheet over the continental shelf during the Quaternary (especially Site 1167). Paleogene foraminifer-bearing marine sections were not intersected, and thus discussion of marine sections is restricted to the Neogene. Foraminifers are not major contributors to Leg 188 chronostratigraphy but contribute to paleoenvironmental interpretation, to issues such as carbonate compensation depth (CCD) effects and source and history of sediment, and provide a basis for Sr and d18O studies. Chronostratigraphy for the various sections was compiled from diatoms, radiolarians, and paleomagnetism (Shipboard Scientific Party, 2001, doi:10.2973/odp.proc.ir.188.101.2001). Foraminifers were sporadic rather than continuous except in short intervals; however, the Neogene foraminifers from the region are very poorly known and the new records proved to be of significant value in paleoenvironmental interpretation. Only at Site 1167 did drilling intersect a section that yielded foraminifers virtually throughout. Other than for the very young section at each site, there is virtually no continuity of assemblages between sites and thus each section is treated here as separate and unrelated.

Sedimentary and subsidence history of ODP Sites 135-840 and 135-841

Sedimentary sections recovered from the Tonga platform and forearc during Ocean Drilling Program Leg 135 provide a record of the sedimentary evolution of the active margin of the Indo-Australian Plate from late Eocene time to the Present. Facies analyses of the sediments, coupled with interpretations of downhole Formation MicroScanner logs, allow the complete sedimentary and subsidence history of each site to be reconstructed. After taking into account the water depths in which the sediments were deposited and their subsequent compaction, the forearc region of the Tofua Arc (Site 841) can be seen to have experienced an initial period of tectonic subsidence dating from 35.5 Ma. Subsidence has probably been gradual since that time, with possible phases of accelerated subsidence, starting at 16.2 and 10.0 Ma. The Tonga Platform (Site 840) records only the last 7.0 Ma of arc evolution. However, the increased accuracy of paleowater depth determinations possible with shallow-water platform sediments allows the resolution of a distinct increase in subsidence rates at 5.30 Ma. Thus, sedimentology and subsidence analyses show the existence of at least two, and possibly four, separate subsidence events in the forearc region. Subsidence dating from 35.5 Ma is linked to rifting of the South Fiji Basin. Any subsidence dating from 16.2 Ma at Site 841 does not correlate with another known tectonic event and is perhaps linked to localized extensional faulting related to slab roll back during steady-state subduction. Subsidence from 10.0 Ma coincides with the breakup of the early Tertiary Vitiaz Arc because of the subduction polarity reversal in the New Hebrides and the subsequent readjustment of the plate boundary geometry. More recently, rapid subsidence and deposition of a upward-fining cycle from 5.30 Ma to the Present at Site 840 is thought to relate to rifting of the Lau Basin. Sedimentation is principally controlled by tectonic activity, with variations in eustatic sea level playing a significant, but subordinate role. Subduction of the Louisville Seamount Chain seems to have disrupted the forearc region locally, although it had only a modest effect on the subsidence history and sedimentation of the Tonga Platform as a whole.