Cenozoic ostracoda of ODP Leg 104 holes

Ostracodes are less common than might be normally expected at Sites 642, 643, and 644, perhaps pointing to the fact that the marine habitat below the overlying Pleistocene ice covers was a severe environment. This explanation, however, would not apply to the Pliocene and Miocene deposits from which ostracodes are just as poorly represented. In the latter case the Iceland-Faeroe Ridge might still have acted as a submerged barrier that did not allow an open ocean circulation of bottom waters. Thus the barrier presumably prevented an exchange of cold subarctic bottom water with that of the open Atlantic and therefore benthic deep-sea migration from the south was impeded. Some Quaternary species are, for the first time, recorded to extend to the Pliocene and/or Miocene.

Cenozoic plaktonic foraminfers from the Celebes and Sulu Seas

This paper presents the planktonic foraminifer biostratigraphy of the sites drilled during Ocean Drilling Program Leg 124 in the Celebes and Sulu Seas. It discusses preservation of foraminifers in pelagic sediments and in calcareous turbidites. In the Celebes Sea, pelagic carbonates are only found in the Eocene and Oligocene at Site 770. The faunas are poorly preserved due to severe dissolution and offer little biostratigraphic detail. In the Sulu Sea, pelagic carbonates are found in the upper Pliocene and Pleistocene at Sites 768 and 769 and throughout the recovered sequence at the shallower Site 771. The foraminifer faunas from these sediments allow for recognition of most standard zones. Variations in preservation of pelagic foraminifer faunas with time are due to changes in the depth of the lysocline. Shifts to improved preservation at Sites 768 and 769 are synchronous in the upper Pliocene/Pleistocene and may be related to global sea-level cycles. Planktonic foraminifers are also abundant in calcareous turbidites, which were deposited in both basins from the late Miocene onward. However, the turbidites are fine-grained, and biostratigraphic marker species are absent as a result of size-sorting during transport. In the Celebes Sea, shelf-derived material was a major component of early-late Miocene and middle Pliocene to early Pleistocene turbidites. Changes in the composition of the turbidites may correspond to global sea-level changes. In the Sulu Sea, a shift from shelf-derived material in Pliocene calcareous turbidites to a pelagic source in the Pleistocene may be related to subsidence of the Cagayan Ridge.

Oxygen and carbon isotope composition of Globoquadrina venezuelana and Oridorsalis umbonatus at DSDP Leg 85 Holes

Oxygen and carbon isotope stratigraphies are given for the planktonic foraminifer Globoquadrina venezuelana (a deep-dwelling species) at three DSDP sites located along a north-south transect at approximately 133°W across the Pacific equatorial high-productivity zone. The records obtained at Sites 573 and 574 encompass the lower Miocene. At Site 575 the record includes the middle Miocene and extends into the lowermost lower Miocene. The time resolution of the planktonic foraminifer isotope record varies from 50,000 to 500,000 yr. The benthic foraminifer Oridorsalis umbonatus was analyzed for isotope composition at a few levels of Site 575. Isotope stratigraphies for all three sites are compared with carbonate, foraminifer preservation, and grain size records. We identified a number of chemostratigraphic signals that appear to be synchronous with previously recognized signals in the western equatorial Pacific and the tropical Indian Ocean, and thus provide useful tools for chronostratigraphic correlations. The sedimentary sequence at Site 573 is incomplete and condensed, whereas the sequences from Sites 574 and 575 together provide a complete lower Miocene record. The expanded nature of this record, which was recovered with minimum disturbance and provides excellent calcareous and siliceous biostratigraphic control, offers a unique opportunity to determine the precise timing of early Miocene events. Paleomagnetic data from the hydraulic piston cores at Site 575 for the first time allow late early Miocene paleoceanographic events to be tied directly to the paleomagnetic time scale. The multiple-signal stratigraphies provide clues for paleoceanographic reconstruction during the period of preconditioning before the major middle Miocene cooling. In the lowermost lower Miocene there is a pronounced shift toward greater d13C values (by -1%) within magnetic Chron 16 (between approximately 17.5 and 16.5 Ma). The "Chron 16 Carbon Shift" coincides with the cessation of an early Miocene warming trend visible in the d18O signals. Values of d13C remain high until approximately 15 Ma, then decrease toward initial (early Miocene) values near 13.5 Ma. The broad lower to middle Miocene d13C maximum appears to correlate with the deposition of organic-carbon-rich sediments around the margin of the northern Pacific in the Monterey Formation of California and its lateral equivalents. The sediments rimming the Pacific were probably deposited under coastal upwelling conditions that may have resulted from the development of a strong permanent thermocline. Deposition in the upwelling areas occurred partly under anaerobic conditions, which led to the excess extraction of organic carbon from the ocean. The timing of the middle Miocene cooling, which began after the Chron 16 Carbon Shift, suggests that the extraction of organic carbon preconditioned the ocean-atmosphere system for subsequent cooling. A major carbonate dissolution event in the late early Miocene, starting at approximately 18.7 Ma, is associated with the enrichment in 13C. The maximum dissolution is coeval with the Chron 16 Carbon Shift. It corresponds to a prominent acoustic horizon that can be traced throughout the equatorial Pacific.

Geochemistry of Neogene volcanic ashs in ODP Leg 124 holes

Mineralogical and geochemical analyses were performed on 40 ash layers of Pleistocene to late Miocene age, recovered during Leg 124 in the Celebes and Sulu Seas (Sites 767, 768, and 769). They provide information on alteration processes related to burial diagenesis. The zonal distribution of secondary volcanic products emphasizes a major diagenetic change, characterized by the complete replacement of volcanic glass by an authigenic smectite-phillipsite assemblage, in tephra layers dated at 3.5-4 Ma. This diagenetic "event" occurs simultaneously in the two basins, and, on the basis of isotopic data, under low-temperature conditions. It is independent of distinct sedimentation rates and related to a relative quiescence of on-land volcanic activity. This period suggests a more uniform paleooceanographic situation having tectonic significance, and probably reflects a kinetic and environmental control of diagenetic reactions.

Chemistry of volcanic ash from Celebes and Sulu Sea Basins

During Leg 124, off the Philippines, volcanic material was recovered in deep-sea sediments dating from the late Oligocene in the Celebes Sea Basin, and from the early Miocene in the Sulu Sea Basin. Chemical and petrological studies of fallout ash deposits are used to characterize volcanic pulses and to determine their possible origin. All of the glass and mineral compositions belong to medium-K and high-K calc-alkaline arc-related magmatic suites including high-Al basalts, pyroxene-hornblende andesites, dacites, and rhyolites. Late Oligocene and early Miocene products may have originated from the Sunda arc or from the Sabah-Zamboanga old Sulu arc. Late early Miocene Sulu Sea tuffs originated from the Cagayan arc, whereas early late Miocene fallout ashes are attributed to the Sulu arc. A complex magmatic production is distinguished in the Plio-Quaternary with three sequences of basic to acidic lava suites. Early Pliocene strata registered an important activity in both Celebes Sea and Sulu Sea areas, from the newly born Sangihe arc (low-alumina andesite series) and from the Sulu, Zamboanga, and Negros arcs (high-alumina basalt series and high-K andesite series). In the late Pliocene and the early Pleistocene, renewal of activity affects the Sangihe-Cotobato arc as well as the Sulu and Negros arcs (same magmatic distinctions). The last volcanic pulse took place in the late Pleistocene with revival of all the present arc systems.