Mineralogy and interstitial-water chemistry of ODP Leg 101 sites

Concentrations of dissolved Ca2+, Sr2+, Mg2+, SO4[2-], and alkalinity were measured in pore waters squeezed from sediments taken from ODP Holes 626C and 626D in the Florida Straits; Holes 627A and 627B, 628A, and 630A and 630C north of Little Bahama Bank; Holes 631 A, 632A and 632B, and 633A in Exuma Sound; and Holes 634A and 635A and 635B in Northeast Providence Channel. These data are compared with the mineralogy and strontium content of the sediments from which the waters were squeezed. Contrasts in the geochemical profiles suggest that significantly different processes govern pore-water signatures at each group of sites. In Little Bahama Bank, strong positive Ca2+ gradients are correlated with weak negative Mg2+ profiles. These trends are analogous to those seen at DSDP sites where carbonate deposits immediately overlie mafic basement, but as the depth to basement may be in excess of 5000 m, we suggest that diffusion gradients are initiated by an underlying sedimentary unit. In contrast, Ca2+ and Mg2+ gradients in Exuma Sound are not developed to any appreciable extent over similar thicknesses of sediment. We suggest that the pore-water chemistry in these deposits is principally controlled by diagenetic reactions occurring within each sequence. The location and extent of carbonate diagenesis can be estimated from dissolved Sr2+ profiles. In Little Bahama Bank and Exuma Sound, Sr2+ concentrations reach a maximum value of between 700 and 1000 µmol/L. Although the depths at which these concentrations are achieved are different for the two areas, the corresponding age of the sediment at the dissolved Sr2+ maximum is similar. Consequently, the diffusive flux of Sr2+ and the calculated rates of recrystallization in the two areas are likewise of a similar magnitude. The rates of recrystallization we calculate are lower than those found in some DSDP pelagic sites. As the waters throughout most of the holes are saturated with respect to SrSO4, celestite precipitation may cause erroneously low Sr2+ production rates and, consequently, low calculated rates of recrystallization. We therefore encourage only the discriminate use of Sr2+ profiles in the quantification of diagenetic processes.

Interstitial water chemistry of ODP Leg 110 holes

Major-element compositions (Cl-, SO4[2-], Ca2+, Mg2+ , Li+ , K+, Na+ , Sr2+) of interstitial waters obtained from sediment cores along the ODP Leg 110 transect across the Northern Barbados accretionary prism have shown that a complex set of geochemical processes are of importance in this area. In the volcanic ash-rich Pleistocene-Pliocene sediments, alteration reactions involving volcanic ash lead to depletions of Mg2+ and K+. This process is confirmed by the much lower than contemporaneous seawater values of the 87Sr/86Sr ratios of dissolved strontium. In the deeper sediments recovered below the zone of decollement (Sites 671 and 672) large increases in Ca2+ and gradual decreases in Mg2+ , Na+, and d18O (H2O) indicate a potential contribution to the interstitial water chemistry by exchange with underlying basement rocks. This process has been hard to confirm because the drill holes were terminated well short of reaching basement. However, the concentration gradient pattern is consistent with observations in a large number of DSDP drill holes. Finally, but most importantly, low Cl- concentrations in the decollement zone and underlying sand layers, as well as in fault zones at Sites 673 and 674, indicate dilution of interstitial waters. The potential origins of the low Cl- concentrations are discussed, though we are not able to distinguish any mechanism in particular. Our evidence supports the concept of water migration along the decollement and through the underlying sandstones as well as along recent fault zones in the accretionary complex. Interstitial water concentration depth profiles are affected by faulting, thrusting, and overturn processes in the accretionary prism. These processes have caused a diminished diffusive exchange with the overlying ocean, thus explaining increased depletions in Mg2+ and SO4[2-] in sites farther onto the accretionary prism.

Pore-water chemistry of ODP sites 111-677 and 111-678

Sites 677 and 678 were drilled on ODP Leg 111 to test hypotheses about the nature and pattern of hydrothermal circulation on a mid-ocean ridge flank. Together with earlier results from DSDP Site 501/504 and several heatflow and piston coring surveys covering a 100-km**2 area surrounding the three drill sites, they confirm that hydrothermal circulation persists in this 5.9-m.y.-old crust, both in basement and through the overlying sediments (Langseth et al., 1988, doi:10.2973/odp.proc.ir.111.102.1988). Profiles of sediment pore-water composition with depth at the three drill sites show both vertical and horizontal gradients. The shapes of the profiles and their variation from one site to another result from a combination of vertical and horizontal diffusion, convection, and reaction in the sediments and basement. Chemical species that are highly reactive in the siliceous-calcareous biogenic sediments include bicarbonate (alkalinity), ammonium, sulfate, manganese, calcium, strontium, lithium, silica, and possibly potassium. Reactions include bacterial sulfate reduction, mobilization of Mn2+, precipitation of CaCO3, and recrystallization of calcareous and siliceous oozes to chalk, limestone, and chert. Species with profiles more affected by reaction in basaltic basement than in the sediments include Mg, Ca, Na, K, and oxygen isotopes. Reaction in basement at 60?C and at higher temperatures has produced a highly altered basement formation water that is uniform in composition over distances of several kilometers. As inferred from the composition of the basal sediment pore water at the three sites, this uniformity extends from up flow zone to downflow zone in basement and the sediments. It exists in spite of large variations in heat flow and depth to basement, apparently as a result of homogenization by hydrothermal circulation in basement. Profiles for chlorinity, Na, Mg, and other species in the sediment pore waters confirm that Site 678, drilled on a localized heatflow high identified by Langseth et al. (1988), is a site of long-lived upwelling of warm water from basement through the sediments at velocities of 1 to 2 mm/yr. The upflow through the anomalously thin sediments is apparently localized above an uplifted fault block in basement. This site and other similar sites in the survey area give rise to lateral diffusion and possibly flow through the sediments, which produces lateral gradients in sediment pore-water composition at sites such as 501/504. The complementary pore-water profiles at the low-heatflow Site 677 2 km to the south indicate that downflow is occurring through the sediments there, at comparable rates of 1 to 2 mm/yr.