Interstitial Water Studies, Leg 25

Abstract

Interstitial water analyses from sediments collected during Leg 25 of the Deep Sea Drilling Project have revealed that in the southwest Indian Ocean, great chemical activity exists in sediments in various depositional environments. Variable sedimentation rates allow us to set some interesting boundary conditions on chemical and transport processes in these interstitial waters, particularly with regard to the distribution of dissolved sulfate. In terrigenous rapidly deposited sediments, large depletions are observed in magnesium and potassium, whereas relatively small decreases in dissolved calcium occur. In slowly deposited detrital sediments, also, large decreases in potassium and magnesium coincide with very large calcium increases. In truly pelagic sediments, a one to one replacement of magnesium by calcium is observed in the interstitial waters, presumably due to reactions in the basal sediment layers. Biogenous deposits have great influence on dissolved silica (sponge spicules and radiolarians) and on dissolved strontium (carbonate recrystallization). Otherwise, dissolved silica reflects the clay mineralogy and shows variations which seem particularly dependent on the presence or absence of kaoUnite. Variable dissolved manganese values reflect reducing conditions and/or availability of manganese in the solid phases for mobilization in reducing sediments.

Interstitial Water Studies, Leg 24

“…We suggest the formation of high magnesium silicates from the alteration of volcanic ash is responsible for such changes, similar to those reported at Site 239 (ΔCa/ΔMg = 1.8; Gieskes, 1974).…”

“…The data reported are subject to uncertainties because of possible seawater contamination (Gieskes, 1974) and/or temperature of squeezing effects (cf Mangelsdorf et al, 1969;Bischoff et al, 1970;Fanning and Pilson, 1971;Sayles et al, 1973;Gieskes, 1973;Manheim, 1974). Seawater contamination effects are most easily seen in concentration gradients of dissolved Cl, Ca, Mg, and sulphate, where they would appear as anomalous points (closer to seawater values) on the otherwise smooth curves.…”

“…High values (up to 22 ppm) were measured in Units 3 and 5. Other studies (Gieskes, 1974) have noted the increase of dissolved Mn related to a decrease in sulphate and have postulated the increased mobility of Mn under reducing conditions. However, the dissolved Mn values for this site do not show such correlation with sulphate and probably reflect local concentration gradients in the sediments.…”

“…Also described are the analytical methods for alkalinity, pH, and salinity. The analytical methods for laboratory analysis reported in this chapter are as described in Leg 25 Initial Reports (Gieskes, 1974) with the modifications noted below.…”

Introduction and Explanatory Notes

“…Interstitial waters were analyzed on board according to the techniques described by Gieskes (1974). All water samples were squeezed from sediments recovered by coring.…”