Stable oxygen isotope record and relative abundances of planktonic foraminifera of ODP Hole 117-728A

High resolution stratigraphy based on oxygen isotope ratios of the planktonic foraminifers Neogloboquadrina dutertrei (d'Orbigny), Globigeriniodes ruber (d'Orbigny), and Globigerina bulloides (d'Orbigny), magnetic susceptibility, and calcium carbonate content covers the sedimentary record of ODP Hole 728A drilled on the Oman Margin from approximately 10 k.y. to 525 k.y., comprising isotopic stages 1-13. Below stage 13 isotopic stage boundaries cannot be defined with certainty in our data. Sediment accumulation rates were calculated from the isotopic record of N. dutertrei by matching it with the age model SPECMAP curve. During the glacial periods sediment accumulation rates were higher than during the interglacial periods, reflecting increased input from the shelf during low-stands of sea level and increased eolian input. Periodograms for the past 524 k.y. on oxygen isotope records of N. dutertrei, G. ruber, and G. bulloides, on calcium carbonate content, magnetic susceptibility, and on a foraminiferal fragmentation record show powers matching the Milankovitch periodicities. High powers are concentrated around 103 k.y. In the spectra of oxygen isotope ratios of N. dutertrei, magnetic susceptibility, and foraminiferal fragmentation these are significant at the 80% confidence level with respect to a first order autoregressive model. Power concentrations near 43 k.y., matching obliquity, are present but subdued in all spectra. Power concentrations near 23 k.y., matching precession, are significant in the spectra of the oxygen isotope record of N. dutertrei, magnetic susceptibility, and calcium carbonate content record. Fragmentation of planktonic foraminifers increased during the interglacial periods. This is attributed to dissolution of the tests in an expanded oxygen minimum zone (OMZ), where undersaturation of calcium carbonate is caused by enhanced production in the euphotic zone, which would suggest stronger monsoonal induced upwelling during interglacial periods. Extension of the OMZ could also be increased by outflow of low oxygen marginal basin bottom water.

Pelagic, turbiditic, and contouritic sequential deposits on the Cape Verde Plateau

On the Cape Verde Plateau, Neogene deposits are composed of major pelagic and hemipelagic sediments. These sediments show climatic sequences composed of two lithologic terms that differ in their siliciclastic and carbonate contents. Several turbiditic and contouritic sequences are interbedded in these deposits. Turbidite sequences are fine grained and thin bedded with a very low frequency (about 12 sequences during the Neogene). They are composed of quartz-rich siliciclastic or volcaniclastic sediments. Quartz-rich turbidites originated from the Senegalese margin. Their slightly higher frequency during the early Pliocene indicates that the stronger turbidity currents, and probably the most abundant continental inputs, occur at that period. Volcaniclastic turbidites are only present in the early Miocene (about 17 Ma) and the early Pleistocene (1 Ma). They have flown from adjacent Cape Verde Islands and reflect two episodes of high volcanic activity in this area. Contourite sequences, composed of biogenic sandy silts, represent less than 5% of the sediment pile and seem to have been mainly deposited during the late Pleistocene. These different sequences show clay mineral variations throughout Neogene time. Kaolinite is predominant in the Miocene and lower Pliocene deposits; this mineral decreases thereafter, with an increased trend of illite in the uppermost Pliocene and Pleistocene sediments, suggesting a change in sediment sources on the Saharan continent at about 2.6 Ma.

Stable isotope record of benthic foraminifera from DSDP Site 68-502

Benthic foraminiferal delta13C data from site 502 in the Caribbean Sea (sill depth ?1800 m) indicate that throughout the past 2.6 m.y., glacial delta13C values in the middepth Atlantic were higher during glaciations than interglaciations. This is interpreted as indicating a greater proportion of Upper North Atlantic Deep Water (UNADW) relative to southern source waters during glaciations. The contribution of UNADW during interglaciations to the middepth Atlantic remained approximately constant, and the contribution during glaciations may have been as much as 10 % higher in the late Pleistocene than in the late Pliocene. This small increase is in striking contrast to the much larger decrease in glacial Lower North Atlantic Deep Water (LNADW) contribution relative to southern sources, from about 80% to about 20%, that occurred over the past 2.6 m.y. Glacial intensification over the past 2.6 m.y. was probably coupled with a decrease in northward heat transport by the upper limb of the North Atlantic circulation cell, as was previously suggested on the basis of a LNADW record alone. Late Pleistocene (1 Ma-present) delta13C values in the Caribbean Sea were approximately 0.2? higher than they were from 2.6 to 2.0 Ma. The delta13C rise is not due to an increase in the mean ocean delta13C value, nor can it be entirely attributed to an increase in the proportion of high-delta13C source waters. An increase in the delta13C value of the surface source waters must have contributed to the delta13C rise.