40Ar/39Ar analysis for incremental heating experiments at DSDP Holes 77-537 and 77-538A

Drilling penetrated pre-Mesozoic crystalline basement beneath abbreviated sedimentary sequences overlying fault blocks in the southeastern Gulf of Mexico. At Hole 538A, located on Catoche Knoll, a foliated, regional metamorphic association of variably mylonitic felsic gneisses and interlayered amphibolite is intruded by post-tectonic diabase dikes. Hornblende from the amphibolite displays internally discordant 40Ar/39Ar age spectra, suggesting initial post-metamorphic cooling at about 500 Ma followed by a mild thermal disturbance at about 200 Ma. Biotite from the gneiss yields a plateau age of 348 Ma, which is interpreted to result from incorporation of extraneous argon components when the biotite system was opened during the about 200 Ma thermal overprint. A whole-rich diabase sample from Hole 538A records a crystallization age of 190.4 ± 3.4 Ma. A lower grade phyllitic metasedimentary sequence was penetrated at Hole 537, drilled about 30 km northwest of Catoche Knoll. Whole-rock phyllite samples display internally discordant 40Ar/39Ar age spectra, but plateau segments clearly document an early Paleozoic metamorphism at about 500 Ma. The age and lithologic character of the basement terrane penetrated at Holes 537 and 538A suggest that the drilled fault blocks are underlain by attenuated fragments of continental crust of "Pan-African" affinity. This supports pre-Mesozoic tectonic reconstructions that locate Yucatan in the present Gulf recess during the amalgamation of Pangea.

Relative abundance and isotopic composition of calcite, dolomite and siderite from ODP Leg 164 sites

Authigenic carbonate mineral distributions are compared to pore-water geochemical profiles and used to evaluate diagenesis within sedimentary sections containing gas hydrates on the Blake Ridge (Ocean Drilling Program Sites 994, 995, and 997). Carbonate mineral distributions reveal three distinct diagenetic zones. (1) Carbonate minerals in the upper 20 m are primarily biogenic and show no evidence of diagenesis. The d13C and d18O values of calcite within this zone reflects marine carbonate (~0 per mil Peedee belemnite [PDB]) formed in equilibrium with seawater. (2) Between 20 and 100 mbsf, calcite d13C values are distinctly negative (as low as -7.0 per mil), and authigenic dolomite is common (~2-40 wt%) with d13C values between -3.6 per mil and 13.7 per mil. (3) Below 100 mbsf, dolomite abundance decreases to trace amounts, and disseminated siderite becomes the pervasive (~2-30 wt%) authigenic carbonate. Both siderite textures and stable isotope values indicate direct precipitation from pore fluids rather than dolomite replacement. The d13C and d18O values of siderite vary from 5.0 per mil to 10.9 per mil and 2.9 per mil to 7.6 per mil, respectively. Comparisons between the d13C profiles of dissolved inorganic carbon (DIC) and pore-water concentration gradients, with the d13C and d18O values of authigenic carbonates, delineate a distinct depth zonation for authigenic carbonate mineral formation. Coincidence of the most negative d13CDIC values (<=-38 per mil) and negative d13C values of both calcite and dolomite, with pore-water alkalinity increases, sulfate depletion, and decreases in interstitial Ca2+ and Mg2+ concentrations at and below 20 mbsf, suggests that authigenic calcite and dolomite formation is initiated at the base of the sulfate reduction zone (~21 mbsf) and occurs down to ~100 mbsf. Siderite formation apparently occurs between 120 and 450 mbsf; within, and above, the gas hydrate-bearing section of the sediment column (~200-450 mbsf). Siderite d13C and d18O values are nearly uniform from their shallowest occurrence to the bottom of the sedimentary section. However, present-day pore-water d13CDIC values are only similar to siderite d13C values between ~100 and 450 mbsf. Furthermore, calculated equilibrium d18O values of siderite match the measured 18O values of siderite between 120 and 450 mbsf. This interval is characterized by high alkalinity (40-120 mM) and low Ca2+ and Mg2+ concentrations, conditions that are consistent with siderite formation.

Early Cenozoic Atlantic and Pacific benthic foraminiferal isotopes from 12 sites

Oxygen and carbon isotope records are important tools used to reconstruct past ocean and climate conditions, with those of benthic foraminifera providing information on the deep oceans. Reconstructions are complicated by interspecies isotopic offsets that result from microhabitat preferences (carbonate precipitation in isotopically distinct environments) and vital effects (species-specific metabolic variation in isotopic fractionation). We provide correction factors for early Cenozoic benthic foraminifera commonly used for isotopic measurements (Cibicidoides spp., Nuttallides truempyi, Oridorsalis spp., Stensioina beccariiformis, Hanzawaia ammophila, and Bulimina spp.), showing that most yield reliable isotopic proxies of environmental change. The statistical methods and larger data sets used in this study provide more robust correction factors than do previous studies. Interspecies isotopic offsets appear to have changed through the Cenozoic, either (1) as a result of evolutionary changes or (2) as an artifact of different statistical methods and data set sizes used to determine the offsets in different studies. Regardless of the reason, the assumption that isotopic offsets have remained constant through the Cenozoic has introduced an 1-2°C uncertainty into deep sea paleotemperature calculations. In addition, we compare multiple species isotopic data from a western North Atlantic section that includes the Paleocene-Eocene thermal maximum to determine the most reliable isotopic indicator for this event. We propose that Oridorsalis spp. was the most reliable deepwater isotopic recorder at this location because it was best able to withstand the harsh water conditions that existed at this time; it may be the best recorder at other locations and for other extreme events also.