Pliocene stable isotope and carbonate stratigraphy at DSDP Holes 85-572C and 85-573A

Holes 572C and 573A provide high resolution (about 5000-yr. sampling interval) records of oxygen and carbon isotope stratigraphy (Globigerinoides sacculifera) and carbonate stratigraphy for the Pliocene of the equatorial Pacific. These data enable detailed correlation of carbonate events between sites and provide additional resolution to the previous carbonate stratigraphy. Comparison of calcium carbonate and d18O data reveal a "Pacific-type" carbonate stratigraphy throughout the Pliocene. The d18O data have two modes of variability with a boundary at 2.9 Ma. The planktonic d18O record does not have a steplike enrichment at 3.2 Ma, which is observed in benthic records elsewhere, suggesting that this event does not represent the proposed initiation of northern hemispheric glaciation. Hole 572C does record a distinct d18O enrichment event at about 2.4 Ma, which has been previously associated with the onset of major ice rafting in the North Atlantic.

Carbon chemistry of ODP Leg 105 sites

The quantity and quality of organic carbon of Eocene to Holocene sediments from ODP Sites 645, 646, and 647 were investigated to reconstruct depositional environments. Results were based on organic-carbon and nitrogen determinations, Rock-Eval pyrolysis, and kerogen microscopy. The sediments at Site 645 in Baffin Bay are characterized by relatively high organic-carbon values, most of which range from 0.5% to almost 3%, with maximum values in the middle Miocene. Distinct maxima of organic-carbon accumulation rates occur between 18 and 12.5 Ma and between 3.4 and 0 Ma. At Sites 646 and 647 in the Labrador Sea, organic-carbon contents vary between 0.1% and 0.75%. Cyclic 'Milankovitch-type' changes in organic-carbon deposition imply climate-controlled mechanisms that cause these fluctuations. The composition of organic matter at Site 645 is dominated by terrigenous components throughout the entire sediment sequence. An increased content of marine organic carbon was recorded only in the late-middle Miocene. At Sites 646 and 647, the origin of the organic matter most probably is marine. Oceanic paleoproductivity values were estimated, based on the amount of marine organic carbon. During most of the Neogene time interval at Site 645, productivity was low, i.e., similar or less than that measured in Baffin Bay today. Higher values of up to 150 (200) gC/m**2/y may have occurred only in the Miocene. At Sites 646 and 647, mean paleoproductivity values vary between 90 and 170 gC/m**2/y; i.e., these are also similar to those measured in the Labrador Sea today. Lower values of 40 to 70 gC/m**2/y were estimated for the early Eocene and (middle) Miocene.

Physical properties of firn cores from the 1995/96 and 1996/97 field seasons

The paper focuses on studies of snow-pit samples and shallow firn cores taken during the 1995-96 and 1996-97 field seasons at Amundsenisen, Dronning Maud Land, Antarctica. The dating of the firn is based on the artificial tritium distribution in the snow cover and on several reference horizons identified by electrical measurements. The early 1964 through 1965 horizon is marked by the deposition of sulfate released to the atmosphere during the eruption of the Agung volcano in March 1963; this horizon was detected by dielectric profiling and electrical conductivity measurements; the proof by chemical analysis has still to be seen. At the ten investigated sites on Amundsenisen the 1964-65 horizon was identified 4.1-5.7 m below the surface. The accumulation rates on Amundsenisen are 41-91 kg/m**2/a. The cores are up to 100 years old. A relationship between isotope content and the mean air temperature on a regional scale can be based on measurements of firn temperature at 10 m depth at the drilling sites. Between Neumayer station at the coast and Heimefrontfjella, the temperature gradient of the deuterium content is 9.6 per mil/K. South of Heimefrontfjella, on the Amundsenisen plateau, it is only 5.5 per mil/K. Time series of yearly accumulation rates show no significant trend. For the isotope records a significant trend to higher values with gradients of 0.1-0.2 d2H per mil/a can be seen in five of the ten time series.