Geochemistry, clay mineralogy and grain size distribution in various sediment cores drilled during ODP Leg 119 on the Antarctic continental shelf off Prydz Bay

Drilling was undertaken at five sites (739-743) on ODP Leg 119 on a transect across the continental shelf of Prydz Bay, East Antarctica, to elucidate the long-term glacial history of the area and to examine the importance of the area with respect to the development of the East Antarctic ice sheet as a whole. In addition to providing a record of glaciation spanning 36 m.y. or more, Leg 119 has provided information concerning the development of a continental margin under the prolonged influence of a major ice sheet. This has allowed the development of a sedimentary model that may be applicable not only to other parts of the Antarctic continental margin, but also to northern high-latitude continental shelves. The cored glacial sedimentary record in Prydz Bay consists of three major sequences, dominated by diamictite: 1. An upper flat-lying sequence that ranges in thickness from a few meters in inner and western Prydz Bay to nearly 250 m in the outer or eastern parts of the bay. The uppermost few meters consist of Holocene diatom ooze and diatomaceous mud with a minor ice-rafted component overlying diamicton and diamictite of late Miocene to Quaternary age. The diamictite is mainly massive, but stratified varieties and minor mudstone and diatomite also occur. 2. An upper prograding sequence cored at Sites 739 and 743, unconformly below the flat-lying sequence. This consists of a relatively steep (4° inclination) prograding wedge with a number of discrete sedimentary packages. At Sites 739 and 743 the sequence is dominated by massive and stratified diamictite, some of which shows evidence of slumping and minor debris flowage. 3. A lower, more gently inclined, prograding sequence lies unconformably below the flat-lying sequence at Site 742 and the upper prograding sequence at Site 739. This extends to the base of both sites, to 316 and 487 mbsf, respectively. It is dominated by massive, relatively clast-poor diamictite which is kaolinite-rich, light in color, and contains sporadic carbonate-cemented layers. The lower part of Site 742 includes well-stratified diamictites and very poorly sorted mudstones. The base of this site has indications of large-scale soft-sediment deformation and probably represents proximity to the base of the glacial sequence. Facies analysis of the Prydz Bay glacial sequence indicates a range of depositional environments. Massive diamictite is interpreted largely as waterlain till, deposited close to the grounding line of a floating glacier margin, although basal till and debris flow facies are also present. Weakly stratified diamictite is interpreted as having formed close to or under the floating ice margin and influenced by the input of marine diatomaceous sediment (proximal glaciomarine setting). Well-stratified diamictite has a stronger marine input, being more diatom-rich, and probably represents a proximal-distal glaciomarine sediment with the glaciogenic component being supplied by icebergs. Other facies include a variety of mudstones and diatom-rich sediments of marine origin, in which an ice-rafted component is still significant. None of the recovered sediments are devoid of a glacial influence. The overall depositional setting of the prograding sequence is one in which the grounded ice margin is situated close to the shelf edge. Progradation was achieved primarily by deposition of waterlain till. The flat-lying sequence illustrates a complex sequence of advances and retreats across the outer part of the shelf, with intermittent phases of ice loading and erosion. The glacial chronology is based largely on diatom stratigraphy, which has limited resolution. It appears that ice reached the paleoshelf break by earliest Oligocene, suggesting full-scale development of the East Antarctic ice sheet by that time. The ice sheet probably dominated the continental margin for much of Oligocene to middle Miocene time. Retreat, but not total withdrawal of the ice sheet, took place in late Miocene to mid-Pliocene time. The late Pliocene to Pleistocene was characterized by further advances across, and progradation of, the continental shelf. Holocene time has been characterized by reduced glacial conditions and a limited influence of glacial processes on sedimentation.

On the fine structure of oxygen distribution in surface water layers in the area between Iceland and Faroe Islands

During the international "Overflow-Expedition'' 1973 on R.V. "Meteor" oxygen concentrations in surface layers were measured in order to determine the oxygen gradients within the first two meters and to add some informations to the mechanisms of oxygen exchange at the air-sea interface. These investigations may be interesting also with regard to longterm- observations of the oxygen distribution in the Atlantic, especially the problem of the A.O.U. (apparent oxygen utilization) determination. To measure oxygen gradients a special sampler was built which is able to take water samples each 20 cm of the first 2 meters. These data were supplemented by further samples down to 150 m, taken by conventional water samplers, from which samples were also taken to measure N2/O2-relations. By comparing these relations with theoretical relations in air-saturated water the influence of biological production and consumption on the oxygen contents in water could be estimated. A simple glass apparatus was built to extract gas from the water samples, and hereafter the N2/O2-relations were determined by mass spectrometry. Most distributions of the oxygen anomaly show a negative oxygen balance which varies largely, probably due to strong mixing processes in the Iceland-Faroe ridge area. The distribution of surface oxygen saturation values are of two different types. The values of the stations 260, 262 and 270 stem from mixed water and show homogeneous supersaturations, as can be found instantly when whitecaps appear. The values of 9 other stations are from water, sampled during calm periods which has been mixed and supersaturated before. They show a decreasing oxygen saturation towards the sea surface and often undersaturation in the upper decimeters up to 98 % and even 91 %. So at the air-sea interface even less initial oxygen saturation than 100 % can be found after supersaturation during heavy weather periods.

Grains size and components distribution in turbidite layesr of ODP Leg 101

Textural and compositional differences were found between gravity-flow sheets in an open-ocean environment on the northern slope of Little Bahama Bank (Site 628, Pliocene turbidite sequence) and in a closed-basin depositional setting (Site 632, Quaternary turbidite sequence). Mud-supported debris-flow sheets were cored at Site 628. Average mean grain size of the turbidite samples was lower, mud content was higher, and sorting was poorer than in comparable samples from Site 632. This reflects the deposition of proximal, low-energy turbidity currents and debris flows on a base-ofslope carbonate apron. No mud-supported debris-flow sheets were deposited in the investigated sediment sequence of Hole 632A. Many larger turbidity currents from around the margins of Exuma Sound may have reached this central basin setting, depositing sediments that had been transported over longer distances. Planktonic components dominate in the grain-sized fraction (500-1000 µm) of turbidite samples from Hole 628A, while platform detritus is rare. We interpreted this as resulting from the erosion and reworking of a large area of open-ocean slope sediments by gravity flows. In contrast, large amounts of benthic and platform components were found in the turbidite samples of Hole 632A. This may be explained by the fact that the slopes of the enclosed Exuma Sound are steep, and turbidity currents bypassed much of these slopes through pronounced channels, delivering more shallow-water detritus to the deep basin. Erosion of slope sediments, a possible source area of planktonic detritus, is assumed to be low. The small slope area in relation to the larger surrounding platform areas and lower production of planktonic components in the enclosed waters of Exuma Sound may also explain the observed low number of planktonic components at Hole 632A. Turbidite material from both open-ocean and enclosed-basin environments was deposited at Site 635.

Diatom distribution in sediments of the Peruvian coastal upwelling

Two shelf and upper-slope sites (681 and 686) near present cores of active coastal upwelling on the Peruvian margin were sampled continuously at 7.5 cm intervals down to approximately 25 meters below the seafloor (mbsf). Sedimentary structures were subdivided into homogeneous/bioturbated, laminated, and laminated-varved intervals. Diatom analysis and statistical treatment of data revealed almost persistent upwelling over the last 400,000 yr, with slightly increased upwelling during oxygen-isotope Stages 3, 5, 7, and 8 that resulted in anoxic bottom-water conditions and deposition of laminated/undisturbed primary sedimentary structures.

CaCO3 size distribution in surface sediments

Lysocline reconstructions play an important role in scenarios purporting to explain the lowered atmospheric CO2 content of glacial time. These reconstructions are based on indicators such as the CaCO3 content, the percent of coarse fraction, the ratio of fragments to whole foraminifera shells, the ratio of solution-susceptible to solution-resistant species, and the ratio of coarse to fine CaCO3. All assume that changes with time in the composition of the input material do not bias the result. However, as the composition of the input material does depend on climate, none of these indicators provides an absolute measure of the extent of dissolution. In this paper we evaluate the reliability of the ratio of >63 µm CaCO3 to total CaCO3 as a dissolution indicator. We present here results that suggest that in today's tropics this ratio appears to be determined solely by CO3= ion concentration and water depth (i.e., the saturation state of bottom waters). This finding offers the possibility that the size fraction index can be used to reconstruct CO3= ion concentrations for the late Quaternary ocean to an accuracy of ±5 µmol/kg.