Interstitial Water Studies on Small Core Samples, Leg 22

Abstract

Interstitial waters from Leg 22 in the Indian Ocean revealed two unique results: Site 214, on the Ninetyeast Ridge, penetrated through a 30-meter sequence of fine-grained basalt and reentered hard, silty clay containing carbonate skeletal debris. Such a basalt layer may well have been impervious and extensive enough to seal off underlying (fossil) seawater of Paleocene age. However, except for a marked increase in calcium and a slight increase in chloride, no appreciable changes in pore fluid chemistry could be confirmed. Site 217, at the northernmost end of the Ninetyeast Ridge, demonstrated record concentrations of interstitial calcium in clayey nannofossil oozes and a relatively small but significant increment in chloride with depth. Presumably, these increments signal the existence of evaporitic sediments or evaporiteinfluenced brines at considerably greater depth than penetrated. INTRODUCTION A total of 125 sediment samples collected from eight Leg 22 sites was squeezed for interstitial water. Sediment samples were collected immediately following the recovery of the core by the removal of discrete segments of sediment-filled plastic liner (mini-cores). This sampling method is essentially the same as was used for the special geochemical studies carried out on Leg 15 (Horowitz et al., in press). In most instances a 10-cm mini-core was cut from the top end of a 150-cm core section. This sample was first used for the resistivity measurements reported elsewhere in this volume (Chapter 32). Mini-cores were split lengthwise and one-half the available sediment was squeezed. All samples were squeezed at ambient room temperature. When soft sediments were encountered, two contiguous minicores were removed in order to make punch-in electrode pH measurements. pH measurements were also made on the squeezed, unfiltered pore water immediately following recovery. The fluid was injected directly from the receiver syringe into an Orion miniature flow-through electrode. The water content of all samples was determined from weight loss upon oven drying of approximately 1 -g portions of sediment taken from the mini-cores prior to processing. Analytical methods were identical to those outlined in earlier volumes of this series. Most of the analyses and data reduction were performed by John Mahoney. The analytical data are summarized in Tables 1 and 2. 1 Contribution No. 3100 of the Woods Hole Oceanographic Institution. RESULTS AND DISCUSSION Rather than cover sites comprehensively, as in previous interstitial water studies, effort was concentrated on a few sites, from which a number of samples were analyzed to provide continuity of information. A total of 41 samples as selected from Site 212 in the Wharton Basin, about 1500 km northwest of Australia; Sites 214 and 217 on the Ninetyeast Ridge; and Site 218 in the southern portion of the Central Bengal Fan. A greater effort was made on this leg to squeeze pore fluid from consolidated sediment than previously. Sufficient water for analysis was obtained from moderately lithified sediments (water content as low as 18%). This may help account for the more marked evidence of diagenetic reactions, as discussed below. Site 212 At this site, in the Wharton Basin, clayey nannofossil oozes showed moderate depletion in K, Mg, and SO4 and a 2.5-fold increase in Ca. Alkalinity remained roughly constant at values little higher than in seawater when corrected for a systematic analytical error. The mean rate of accumulation for this site was 0.65 cm/1000 years. Site 214 Located on the crest of the Ninetyeast Ridge, in water depth of 1655 meters, this site revealed an upper sequence of foraminiferal and nannofossil ooze, becoming glauconitic toward the base of the Eocene at about 330 meters depth below the sea floor. A remarkable series of increasingly shallow water to possibly lagoonal sediments culminated in lignitic and volcanoclastic sediments interbedded with igneous extrusive rock. Interestingly, one sample was