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.

Relative abundance of pteropod species of the plankton pump samples in the surface water of the eastern North Atlantic Ocean (Table 4)

Plankton pump samples and plankton tows (size fractions between 0.04 mm and 1.01 mm) from the eastern North Atlantic Ocean contain the following shell- and skeleton-producing planktonic and nektonic organisms, which can be fossilized in the sediments: diatoms, radiolarians, foraminifers, pteropods, heteropods, larvae of benthic gastropods and bivalves, ostracods, and fish. The abundance of these components has been mapped quantitatively in the eastern North Atlantic surface waters in October - December 1971. More ash (after ignition of the organic matter, consisting mostly of these components) per cubic meter of water is found close to land masses (continents and islands) and above shallow submarine elevations than in the open ocean. Preferred biotops of planktonic diatoms in the region described are temperate shallow water and tropical coastal upwelling areas. Radiolarians rarely occur close to the continent, but are abundant in pelagic warm water masses, even near islands. Foraminifers are similar to the radiolarians, rarer in the coastal water mass of the continent than in the open ocean or off oceanic islands. Their abundance is highest outside the upwelling area off NW Africa. Molluscs generally outnumber planktonic foraminifers, implying that the carbonate cycle of the ocean might be influenced considerably by these animals. The molluscs include heteropods, pteropods, and larvae of benthic bivalves and gastropods. Larvae of benthic molluscs occur more frequently close to continental and island margins and above submarine shoals (in this case mostly guyots) than in the open ocean. Their size increases, but they decrease in number with increasing distance from their area of origin. Ostracods and fish have only been found in small numbers concentrated off NW Africa. All of the above-mentioned components occur in higher abundances in the surface water than in subsurface waters. They are closely related to the hydrography of the sampled water masses (here defined through temperature measurements). Relatively warm water masses of the southeastern branches of the Gulf Stream system transport subtropical and southern temperate species to the Bay of Biscay, relatively cool water masses of the Portugal and Canary Currents carry transitional faunal elements along the NW African coast southwards to tropical regions. These mix in the northwest African upwelling area with tropical faunal elements which are generally assumed to live in the subsurface water masses and which probably have been transported northwards to this area by a subsurface counter current. The faunas typical for tropical surface water masses are not only reduced due to the tongue of cool water extending southwards along the coast, but they are also removed from the coastal zone by the upwelling subsurface water masses carrying their own shell and skeleton assemblages. Tropical water masses contain much more shelland skeleton-producing plankters than subtropical and temperate ones. The climatic conditions found at different latitudes control the development and intensity of a separate continental coastal water mass with its own plankton assemblages. Extent of this water mass and steepness of gradients between the pelagic and coastal environment limit the occurrence of pelagic plankton close to the continental coast. A similar water mass in only weakly developed off oceanic islands.