Composition of pore water and diagenetic carbonates of ODP Site 123-765

Degradation of organic matter in slightly organic-rich (1 wt% organic carbon) Neogene calcareous turbidites of the Argo Basin at Site 765 by sulfate reduction results in pore-water phosphate, ammonium, manganese, and carbonate alkalinity maxima. Pore-water calcium and magnesium decrease in the uppermost 100 meters below seafloor (mbsf) in response to the precipitation of calcian dolomite with an average composition of Ca1.15Mg0.83Fe0.02(CO3)2. Clear, euhedral dolomite rhombs range from <1 to 40 µm in diameter and occur in trace to minor amounts (<1-2 wt%) in Pleistocene to Pliocene sediment (62-210 mbsf) The abundance of dolomite increases markedly (2-10 wt%) in Miocene sediment (210-440 mbsf). The dolomite is associated with diagenetic sepiolite and palygorskite, as well as redeposited biogenic low-Mg calcite and aragonitic benthic foraminifers. Currently, dolomite is precipitating at depth within the pore spaces of the sediment, largely as a result of aragonite dissolution. The rate of aragonite dissolution, calculated from the pore-water strontium profile, is sufficient to explain the amount of dolomite observed at Site 765. A foraminiferal aragonite precursor is further supported by the carbon and oxygen isotopic compositions of the dolomite, which are fairly close to the range of isotopic compositions observed for Miocene benthic foraminifers. Dolomite precipitation is promoted by the degradation of organic matter by sulfate-reducing bacteria because the lower pore-water sulfate concentration reduces the effect of sulfate inhibition on the dolomite reaction and because the higher carbonate alkalinity increases the degree of saturation of the pore waters with dolomite. Organic matter degradation also results in the precipitation of pyrite and trace amounts of apatite (francolite), and the release of iron and manganese to the pore water by reduction of Fe and Mn oxides. Spherical, silt-sized aggregates of microcrystalline calcian rhodochrosite occur in trace to minor amounts in Lower Cretaceous sediment from 740 to 900 mbsf at Site 765. A negative carbon isotopic composition suggests that the rhodochrosite formed early in the sulfate reduction zone, but a depleted oxygen isotopic composition suggests that the rhodochrosite may have recrystallized at deeper burial depths.

Physical properties of ODP Sites 123-765 and 123-766

During Ocean Drilling Program Leg 123, two sites were drilled in the deep Indian Ocean. Physical properties were measured in soft Quaternary and Lower Cretaceous sediments to relatively fresh, glass-bearing pillow lavas and massive basalts. Porosities ranged from 89% near the seafloor to 1.6% for the dense basalts. This self-consistent set of measurements permitted some descriptive models of physical properties to be more rigorously tested than before. Predictive relationships between porosity and compressional-wave velocity have generally been based upon the Wyllie time average equation. However, this equation does not adequately describe the actual relationship between these two parameters, and many have attempted to improve it. In most cases, models were derived by testing them against a set of data representing a relatively narrow range of porosity values. Similarly, the use of the Wyllie equation has often been justified by a pseudolinear fit to the data over a narrow range of porosity values. The limitations of the Wyllie relationship have been re-emphasized here. A semi-empirical acoustic impedance equation is developed that provides a more accurate porosity-velocity transform, using realistic material parameters, than has hitherto been possible. A closer correlation can be achieved with this semi-empirical relationship than with more theoretically based equations. In addition, a satisfactory empirical equation can be used to describe the relationship between thermal conductivity and porosity. If enough is known about core sample lithologies to provide estimates of the matrix and pore water parameters, then these predictive equations enable one to describe completely the behavior of a saturated rock core in terms of compressional-wave velocity, thermal conductivity, porosity, and bulk density.