Pore-water chemistry of ODP Leg 115 samples

Analyses of the Sr2+ concentrations of interstitial fluids obtained from sediments squeezed during Leg 115 were used to estimate the rates and total amount of recrystallization of biogenic carbonates. The total amount of recrystallization calculated using this method varies from less than 1 % in sediments at Site 706 to more than 40% at Site 709 in sediments of 47 Ma. Five of the sites drilled during Leg 115 (Sites 707 through 711) were drilled in a depth transect within a restricted geographic area so that theoretically they received similar amounts of sediment input. Of these, the maximum rate of recrystallization occurred in the upper 50 m of Site 710 (3812 m). The amount of recrystallization decreased with increasing water depth at Sites 708 (4096 m) and 711 (4428 m), presumably as a result of the fact that most of the reactive calcium carbonate was dissolved before burial. We also observed significant alkalinity deficits at many of these sites, a condition which most likely resulted from the precipitation of calcium carbonate either in the sedimentary column, or during retrieval of the core. Precipitation of CaCO3 as a result of pressure changes during core retrieval was confirmed by the comparison of Ca2+ and alkalinity from water samples obtained using the in-situ sampler and squeezed from the sediments. At Sites 707 and 716, the shallowest sites, no calcium or alkalinity deficits were present. In spite of our estimations of as much as 45% recrystallization at Site 709, all the carbonate sites exhibited what would be previously considered conservative Ca2+/Mg2+ profiles, which varied from -1 to -0.5. By virtue of the position of these sites relative to known basaltic basement or through the actual penetration of basalt (i.e., Sites 706, 707 and 712), these sites are all known to be underlain by basalt. Our results suggest, therefore, that more positive Ca2 + /Mg2+ gradients cannot necessarily be used as indicators of the nature of basement material.

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.