(Table 1) Sulfur isotopes, total sulfur, and degree of oxidation of basalt samples from DSDP Hole 69-504B and 70-504B

About 150 basalt samples from Hole 504B, near the Costa Rica Rift were analyzed for sulfur content and sulfur-isotope composition. The basement in Hole 504B can be divided into an upper part, which has oxidative alteration (274.5-550 m below sea floor), and a lower part, which has nonoxidative alteration (550-835 m below sea floor) (the interval from 540 to 585 meters actually is transitional). This division is reflected in both the sulfur content and the sulfurisotope composition. Oxidative alteration of basalts by sea water at low temperatures has resulted in a depletion in sulfur in the upper part of the hole (mostly less than 600 ppm S) as compared to fresh sulfur-saturated oceanic tholeiites (900-1200 ppm S). High amounts of sulfur in the lower part of the hole are a result of precipitation of secondary pyrite under non-oxidative or weakly oxidative conditions from solutions which dissolved igneous sulfides. The average sulfur-isotope composition of the primary igneous sulfides is d34S = -0.01 per mil, which is close to the assumed mantle sulfur composition (d34S = 0 per mil. Pyrite and sulfate sulfur extracted together in a separate preparation step (as "pyrite-sulfate" sulfur) indicate addition of sea-water sulfate to the upper part of the basalts. The d34S of secondary pyrite isolated by hand-picking varies between -8.0 and +5.8 per mil; the "pyrite-sulfate" sulfur (d34S = -4.8 to +10.5 per mil), as well as that of the isolated pyrite, may have originated in the precipitation of pyrite from solutions containing sulfur from the dissolution of igneous sulfides, but addition of sulfur transported by hydrothermal solutions cannot be excluded.

Late Neogene sediment composition and accumulation rates of ODP Hole 120-751A

Through scanning electron microscope analysis of sediment microfabric, we have evaluated variations in high-resolution shipboard physical properties (index properties and shear strength), sediment components (smear slide determinations), and shore-based calcium carbonate and biogenic silica data from Site 751 (Kerguelen Plateau). The stratigraphic section at this site records a change in biogenic ooze composition from predominantly calcareous (nannofossil) to siliceous (diatom) ooze from ~23 Ma to the present, reflecting expansion of Antarctic water masses during the late Neogene. The profound change in physical properties and sediment character at 40.1 mbsf (~5-6 Ma) evidently records the northward movement of the Polar Front and a change in absolute accumulation rates of sediment at this site. Trends in geotechnical properties with depth at Site 751 allowed us to subdivide the sedimentary column into a number of geotechnical units that reflect changes in depositional and postdepositional processes with time. Geotechnical properties are sensitive to changing sedimentary inputs of primarily siliceous and calcareous microfossils. This allows us to study the physical nature of biostratigraphically-identified hiatuses and variations in environmental conditions linked to the migration of the Polar Front across this region. The analysis of geotechnical properties permits a more detailed division of the sedimentary column than is possible from shipboard lithologic descriptions alone. Our study of the sedimentary microfabric indicates that randomly oriented, elongate pennate diatom valves compose the sediments with highest porosity and water content values, and the lowest density values (wet bulk, dry bulk, and grain density). Conversely, sediments composed of nannofossils and disassociated nannofossil crystallites and little or no siliceous remains have the lowest porosity and water content values, and the highest density values. Samples of mixed siliceous/calcareous composition have intermediate physical property values, but these vary according to the nature of the sedimentary matrix and the state of preservation of individual skeletal elements.

(Table 1) Presence of sublittoral macroalgae in the investigated profiles in Potter Cove, King George Island, Antarctica

The vegetation of a small fjord and its adjacent open shore was documented by subaquatic video. The distribution of individual species of macroalgae and the composition of assemblages were compared with gradients of light availability, hydrography, slope inclination, substratum, and exposition to turbulence and ice. The sublittoral fringe is usually abraded by winterly ice floes and devoid of large, perennial algae. Below this zone, the upper sublittoral is dominated by Desmarestia menziesii on steep rock faces, where water movements become irregular, or by Ascoseira mirabilis and Palmaria decipiens on weakly inclined slopes with steady rolling water movements. In the central sublittoral above 15 m, where turbulence is still active, Desmarestia anceps is outcompeting all other species on solid substratum, However, the species is not able to persist on loose material under these conditions. Instead, Himantothallus grandifolius may occur. Deeper, where turbulence usually is negligible, Desmarestia anceps also covers loose material. The change of dominance to Himantothallus grandifolius in the deep sublittoral cannot completely be explained at present. Himantothallus grandifolius also prevails in a mixed assemblage under the influence of grounding icebergs. Most of the smaller algae are opportunists with different degrees of tolerance for turbulence, but some apparently need more stable microhabitats and thus are dependent from continuing suppression of competitive large phaeophytes.