Isotopic composition and chemistry of pore waters from ODP Site 195-1201 sediments, West Philippine Basin

This report summarizes chemical and isotopic data from Ocean Drilling Program Leg 195 Site 1201. Pore water is divided into three intervals based on the rate of chemical change with depth. The shallowest interval is the red clay unit between 1.26 and 56.40 meters below seafloor (mbsf). In this section, there are overall decreases in the concentrations of alkalinity, boron, lithium, magnesium, potassium, sodium, and sulfate, whereas concentrations of calcium and chloride increase. Values of d18O and dD plot near standard mean ocean water to the right of the global meteoric water line (GMWL). Five samples from 72.60 and 83.33 mbsf yielded pore water for analyses. These samples help define a trend in the second interval, which is between 56.4 and 238.98 mbsf. Here, concentrations of magnesium, potassium, sodium, and sulfate decease, whereas concentrations of boron, calcium, and chloride increase. Concentrations of alkalinity and lithium remain roughly constant. The deepest interval, between 238.04 and 504.8 mbsf, has comparatively slower decreases of sodium and sulfate, increases of calcium and chloride, slow increases of alkalinity and lithium, and roughly constant concentrations of magnesium, potassium, and boron. Values of d18O and dD in pore water between 146.98 and 504.80 mbsf plot in a linear trend to the right of the GMWL.

(Table 1) Porewater analysis with respect to gypsum from different ODP holes

The stability of gypsum in marine sediments has been investigated through the calculation of its saturation index at the sediment in situ temperature and pressure, using the entire ODP/IODP porewater composition database (14416 samples recovered from sediments collected during 95 ODP and IODP Legs). Saturation is reached in sediment porewaters of 26 boreholes drilled at 23 different sites, during 12 ODP/IODP Legs. As ocean bottom seawater is largely undersaturated with respect to gypsum, the porewater Ca content or its SO4 concentration, or both, must increase in order to reach equilibrium. At several sites equilibrium is reached either through the presence of evaporitic gypsum layers found in the sedimentary sequence, and/or through a salinity increase due to the presence of evaporitic brines with high concentrations of Ca and SO4. Saturation can also be reached in porewaters of seawater-like salinity (~ 35 per mil), provided sulfate reduction is limited. In this case, saturation is due to the alteration of volcanogenic material which releases large amounts of Ca to the porewaters, where the Ca concentration can reach 55 times its seawater value as for example at ODP Leg 134 site 833. At a few sites, saturation is reached in hydrothermal environments, or as a consequence of the alteration of the basaltic basement. In addition to the well known influence of brines on the formation of gypsum, these results indicate that the alteration of sediments rich in volcanogenic material is a major process leading to gypsum saturation in marine sediment porewaters. Therefore, the presence of gypsum in ancient and recent marine sediments should not be systematically interpreted as due to hypersaline waters, especially if volcanogenic material is present.

Geochemical data and mineralogical properties of bulk sediment composition of ODP Hole 195-1202B

During Leg 195 of the Ocean Drilling Program, Site 1202 was drilled in the subtropical northwestern Pacific Ocean beneath the Kuroshio (Black Current) between northern Taiwan and the Ryukyu Island Arc on the northern flank of the I-Lan Ridge at 1274 m water depth. The upper 110 m of the Site 1202 section, composed of dark grey calcareous silty clay, provide an expanded record of environmental changes during the last 28 kyr. The sediments were deposited at high sedimentation rates between 3.0 and 5.0 m/kyr and peak values of 9.0 m/kyr between 15.1 and 11.2 ka BP. Variations in the modes and sources of detrital sediment input, as inferred from sediment granulometry, mineralogy, and elemental XRF-scanner data, reflect changes in environmental boundary conditions related to sea-level changes, Kuroshio variability, and the climate-driven modes of fluvial runoff. The provenance data point to increased sediment supply from northwestern Taiwan between 28 and 19.5 ka BP and from East China sources between 19.5 and 11.2 ka BP. The change in provenance at 19.5 ka BP reflects increased fluvial runoff from the Yangtze River and strong sediment reworking from the East China Sea shelf in the course of increased humidity and postglacial sea-level rise, particularly after 15.1 ka BP. The Holocene was dominated by sediments that originated from rivers in northeastern Taiwan. For the pre-Holocene period prior to 11.2 ka BP, low portions of sortable silt (63-10 ?m) show that the Kuroshio did not enter the Okinawa Trough, because of low sea-level. In turn, high proportions of sortable silt and sediment provenance from northeastern Taiwan point to strong ocean circulation under the direct and persistent influence of the Kuroshio during the Holocene. The reentrance of the Kuroshio to the Okinawa Trough was heralded by two pulses in relative current strengthening at 11.2 and 9.5 ka BP, as documented by stepwise increases in sortable silt in the lower Holocene section. From a global perspective, environmental changes in the southern Okinawa Trough show affinities to climate change in the western Pacific warm pool with little influence of climate teleconnections from the North Atlantic realm, otherwise seen in many other marine and terrestrial palaeoclimate records from southeastern Asia.