ALBERT

Description: Calcites from the basaltic basement at Deep Sea Drilling Project Site 597, containing 3.4 to 4.4 mol % MgCO3, were analyzed for d13C, d18O, 87Sr/86Sr, K, Rb, and Sr concentrations. The d13C values range from 1.9 to 2.4 per mil (relative to PDB) and are typical for calcites from the extrusive layer of the oceanic crust. The d18O values of calcites are very uniform and unusually high (2.33 to 2.77 per mil, PDB), which suggest temperatures of formation of 1.5 to 2.9°C and 0.03 to 1.7°C using the calibrations of O'Neil et al. (1969) and Epstein et al. (1953), respectively (after correction for MgCO3), and assuming - l per mil for the late Oligocene/early Miocene bottom water. Paleogene bottom waters are thought to have been warmer than those of present day, and hence the calibration of Epstein et al. (1953) yields more reasonable temperatures for late Oligocene/early Miocene bottom waters. K and Rb concentrations are very low, which is consistent with their incompatible character. Sr/Ca ratios of calcites from sub-basement depths less than 35 m are typical for calcites precipitated from pure seawater, whereas the lowermost sample, from a sub-basement depth of 78 m, appears to contain substantial amounts of basaltic Ca. The 87Sr/86Sr ratios of seawater calcites range from 0.708192 to 0.708349, suggesting that they precipitated from 20 to 28 Ma seawater, or within 8 m.y. after formation of the oceanic crust at Site 597. Our data for calcites from Site 597, drilled on fast-spreading oceanic crust, are similar to data for carbonates from slowspreading crust, except that the high d18O values of the former suggest low temperatures of formation. The low temperatures of calcite formation apparently indicate that the oceanic crust at Site 597 was unusually permeable, allowing rapid circulation of seawater.

Description: Offset of prograde hydrothermal alterations by retrograde reactions is evident in sediments from a piston core (PC6, 8 m long) from the Guaymas Basin, Gulf of California. Geochemical and mineralogical analyses of sediments and pore fluids show superimposed geochemical signals that indicate currently active reactions are modifying previously formed secondary solids. Hydrothermal barite was formed in a previously active flow channel between 150 and 250 cm depth. Hydrothermal gypsum is presently dissolving at depths 〉780 cm, below which hydrothermal fluid flow is channeled by a secondary dolomite layer. Thermal stress of organic matter generates hydrothermal gas and petroleum having wide ranges of compositions and maturities. A significant amount of hydrothermally generated oil has been transported laterally through a porous debris flow (approx. 240 cm subbottom), overwhelming the indigenous bituminous matter. Water soluble petroleum constituents have disseminated throughout the cored sediments. From 400 to 700 cm depth the immature organic matter continues to experience thermally-enhanced diagenesis, and an even higher thermal stress has occurred at the bottom of the core. Enhanced bacterial degradation of organic matter has led to depletion of pore water sulfate and enrichment of ammonium (NH4+) in both pore fluid and sediment, resulting in formation of NH4+ bearing secondary clay minerals at depths below 780 cm.

Description: The basaltic rocks of Hole 794D drilled during Leg 128 are strongly altered. Microprobe analyses and XRD spectra on small quantities of matter extracted from thin sections show that primary minerals and glassy zones of the groundmass are totally or partially replaced by clay minerals with chlorite/saponite mixed-layer composition whatever the rock sample considered. This mixed-layer was also identified in veins and vesicles where it crystallizes in spheroidal aggregates. The largest veins and vesicles are filled by a zoned deposit: the chlorite/saponite mixed-layer always occupies the central part and is rimmed by pure saponite. Calcite crystallizes in secondary fractures which crosscut the clayey veins and vesicles. Chemographic analysis based on the M+-4Si-3R2+ projection shows that the chemical composition of the saponite component in the mixed-layer is identical to that of the free saponite. This indicates that the clay mineral crystallization was controlled by the chemical composition of the alteration fluids. From petrographic evidence, it is suggested that both chlorite/saponite mixed-layer and free saponite belong to the same hydrothermal event and are produced by a temperature decrease. This is supported by the stable isotopic data. The isotopic data show very little variation: d18O saponite ranges from 13.1 per mil to 13.5 per mil, and dD saponite from -73.6 per mil to -70.0 per mil. d18O calcite varies from +19.7 per mil to +21.9 per mil vs SMOW and d13C from -3.2 per mil to +0.4 per mil vs. PDB. These values are consistent with seawater alteration of the basalt. The formation of saponite took place at 150°-180°C and the formation of calcite at about 65°C.

Description: This paper summarizes the biostratigraphic and magnetostratigraphic results from the recovery of holes drilled during Legs 127 and 128 in the Sea of Japan. Siliceous microfossils (i.e., diatoms, radiolarians, silicoflagellates, and ebridians) were most useful as biostratigraphic datum markers in the upper part of the section (Pliocene-Pleistocene), as was the magnetostratigraphy. Time series of other biostratigraphic, climatostratigraphic, and lithostratigraphic markers indicate that a high-resolution picture of the late Pliocene-Pleistocene oceanographic and climatic history of the Sea of Japan is possible. These data include coiling direction changes in Neogloboquadrina pachyderma, oxygen isotopes, percent opal, and alternating dark/light rhythms. Although the magnetostratigraphy was somewhat ambiguous in corroborating age determinations in the older part of the sections, calcareous microfossils and foraminifers indicate that some of the holes penetrated as far back in time as the early Miocene. These data provide a new time frame for theories on the age of formation and early history of the Sea of Japan.