Carbonate aggregate mineralogy and stable isotopes at DSDP Site 87-583

Crystalline aggregates composed of calcium carbonate were recovered in the uppermost 50 m of Nankai Trough sediments during DSDP Leg 87A. These aggregates decomposed with time to masses of sandy calcite as determined by X-ray diffraction analysis. Petrographic and scanning electron microscopy revealed textures suggestive of a precursor phrase prior to calcite, and this precursor has been tentatively identified as the mineral ikaite, CaCO3*6H2O. Stable isotope data suggest a large component of terrigenous organic matter as the carbon source, consistent with the appearance of these aggregates in highly reducing pyritic sediments containing abundant plant remains. We propose that these nodules formed in euxinic basins on the upper part of the Trough slope under normal seafloor conditions of pressure and temperature. Calculated temperatures of formation of this phase are not unusually low. The specimens from Site 583 are the first reported occurrences of ikaite in active margin sediments.

Stable oxygen isotope composition of the sea water in the Mediterranean Sea

During the cruises No 17 and 22 of the German research vessel "Meteor", 45 water samples were taken at 4 stations in the central part of the Mediterranean Sea. Mass spectrometrical analyses showed that systematic, but time variable changes of the oxygen isotope ratios occur. Deep water samples (T> 500 m) have a ± constant isotopic composition of d18O = +1.79? (SMOW) and a Chlorinity of 21.399?. These data are discussed with respect to paleotemperature determinations.

Planktonic foraminifera in Core ML2_66, North Atlantic

Evolution of approaches and methods for reconstruction of paleoenvironmental conditions from microfossils contained in bottom sediments is assessed. Authors elaborated a new actualistic basis for such reconstructions, consisting of a database on contents of tests of planktonic foraminifers in the surface layer of Atlantic sediments and a package of mathematical tools for computer data processing. Structure of the database is described. It contains data on test contents for 29 species and varieties of planktonic foraminifers in 381 samples. A mathematical model designed for reconstructions is based on factor analysis and multidimensional spline interpolation. The model allows one to deduce Quaternary hydrological parameters (paleotemperature, paleosalinity) for standard hydrological levels down to depth of 250 m for the four seasons of the year. Reconstructions are illustrated by an example of a sedimentary core from the North Atlantic representing a period of 300 ky. During the next to last and the last maxima of continental glaciation (oxygen isotope stages 8, 6, 4, and 2), the subarctic water mass was spread here. Winter and summer surface water temperatures comprised 1-5° and 5-7°C, respectively. During interglacials and in Holocene the conditions were close to present ones: winter and summer surface water temperatures comprised 10-12 and 15-17°C, respectively. Vertical paleohydrological profiles compiled for peaks of climatostratigraphic intervals suggest that during cold intervals water stratification was stronger than during the warm ones. At depth 50 m seasonal salinity oscillations did not exceed 0.4 per mil and commonly salinity was minimum in winter and maximum in summer.

Alkenone UK37 measured in the upper water column at pump stations CariacoTrench, Peru and Vertex

A series of long-chain (C37, C38, C39), primarily di and tri-unsaturated methyl and ethyl ketones, first identified in sediments from Walvis Ridge off West Africa and from Black Sea (de Leeuw et al., 1979), has been found in marine sediments throughout the world (Brassell et al., 1986 doi:10.1038/320129a0). The marine coccolithophorid Emiliania huxleyi and members of the class Prymnesiophyceae are now the recognized sources of these compounds (Volkman et al., 1979; Marlowe, et al., 1984). Experiments with laboratory cultures of algae showed the degree of unsaturation in the ketone seris biosynthesized depends on growth temperature (Brassell et al., 1986; Marlowe, 1984), a physiological respons observed for classical membrane lipids (vanDeenen et al., 1972). Brassell and co-workers (Brassell et al., 198; Brassell et al., 1986b) thus proposed that systematic fluctuations in the unsaturation of these alkenones noted down-core in sediments from the Kane Gap region of the north-east tropical Atlantic Ocean and correlated with glacial-interglacial cycles provide an organic geochemical measure of past sea-surface water temperatures. Using laboratory cultures of E. huxleyi, we have calibrated changes in the unsaturation pattern of the long-chain ketone series versus growth temperature. The calibration curve is linear and accurtely predicts unsuturation patterns observed in natural particulate materials collected from oceanic waters of known temperature. We present evidence supporting the proposed paleotemperature hypothesis (Brassell et al., 1986, Brassel et al., 1986b) and suggesting absolute 'sea-surface temperatures' for a given oceanic location can be estimated from an analysis of long-chain ketone compositions preserved in glacial and interglacial horizons of deep-sea sediment cores.

Silicoflagellates in ODP Leg 108 holes

The occurrence of Cenozoic silicoflagellates at three Ocean Drilling Program (ODP) Holes (660A, 662A, and 667A) was investigated to determine biostratigraphic and relative paleotemperature relations in the tropical Atlantic Ocean. This report presents the data obtained from a study of 37 samples and some preliminary comments on the data. The age of the single sparse assemblage at Hole 660A is late middle Eocene or late Eocene (Dictyocha hexacantha Zone); the sparse to common assemblages of Hole 667A are Oligocene and early Miocene and the common to abundant assemblages of Hole 662A are early Pliocene to Quaternary. Dissolution thinning of silicoflagellates is noted in most samples, even in Hole 662A, which is under the present productive Benguela Current.

Skeletal density of Porites

Paleotemperature estimates based on coral Sr/Ca have not been widely accepted because the reconstructed glacial-Holocene shift in tropical sea-surface temperature (~4-6°C) is larger than that indicated by foraminiferal Mg/Ca (~2-4°C). We show that corals over-estimate changes in sea-surface temperature (SST) because their records are attenuated during skeletogenesis within the living tissue layer. To quantify this process, we microprofiled skeletal mass accumulation within the tissue layer of Porites from Australasian coral reefs and laboratory culturing experiments. The results show that the sensitivity of the Sr/Ca and d18O thermometers in Porites will be suppressed, variable, and dependent on the relationship between skeletal growth rate and mass accumulation within the tissue layer. Our findings help explain why d18O-SST sensitivities for Porites range from -0.08 per mil/°C to -0.22 per mil/°C and are always less than the value of -0.23 per mil/°C established for biogenic aragonite. Based on this observation, we recalibrated the coral Sr/Ca thermometer to determine a revised sensitivity of -0.084 mmol/mol/°C. After rescaling, most of the published Sr/Ca-SST estimates for the Indo-Pacific region for the last ~14,000 years (-7°C to +2°C relative to modern) fall within the 95% confidence envelope of the foraminiferal Mg/Ca-SST records. We conclude that two types of calibration scales are required for coral paleothermometry; an attenuated Porites-specific thermometer sensitivity for studies of seasonal to interannual change in SST and, importantly, the rescaled -0.084 mmol/mol/°C Sr/Ca sensitivity for studies of 20th-century trends and millennial-scale changes in mean SST. The calibration-scaling concept will apply to the development of transfer functions for all geochemical tracers in corals.