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Analyses of water samples taken by means of an in-hole sampler generally show good agreement with analyses of samples collected by routine shipboard squeezing techniques. At Sites 438 and 439, a decrease in salinity with depth is related to former freshwater flow from an aquifer that crops out at an anticline on a deep sea terrace between Japan and the top of the trench slope of the Japan Trench. This former subaerial recharge suggests significant late Cenozoic subsidence of the terrace, because it now lies at a water depth of 1500 meters. Samples from the trench slope at Site 440 have extremely high values of alkalinity and ammonia, presumably because of a favorable combination of high sedimentation rate and organic carbon content. Diagenetic conditions on the trench slope favor formation of the Fe-Mg carbonate mineral, ankerite; at Site 440 it first occurs at a depth below the sea floor of only 29 meters in late Pleistocene strata. Undissolved diatoms persist to relatively great depth at the sites of Leg 57 because of a low geothermal gradient caused by subduction. Secondary silica lepispheres first appear at 851 meters at the most landward and warmest site, Site 438, in strata 16 million years old with an ambient temperature of 31 °C. SAMPLING AND ANALYTICAL PROCEDURES During Leg 57 (Figure 1), samples of interstitial water were collected at all of the sites by standard shipboard squeezing techniques and at Sites 438 and 440 by means of the IPOD in-hole sampler, which collects both water for chemical analysis in a stainless steel tubular coil and water for gas analysis in a connected copper coil (Barnes et al., 1979). The in-hole sampler was used to a depth below the sea floor of 449 meters at Site 438 and 235 meters at Site 440. In this chapter we present results of shipboard chemical analyses and subsequent analyses carried out in the laboratory of J. M. Gieskes at Scripps Institution of Oceanography. Analytical methods follow those described by Gieskes (1974), with minor modifications. Shipboard in-hole data on pH, alkalinity, chlorinity, calcium, and magnesium (except Mg from Site 438) generally show good agreement with the samples obtained by routine shipboard squeezing techniques (Tables 1 and 2; Figures 2 and 3). The shipboard results for the inhole samples of Site 438 show large differences for magnesium between these in situ samples and those obtained from routine shipboard squeezing methods. We reanalyzed in-hole sample splits stored in copper tubes provided by Dr. R. Barnes by atomic absorption spectroscopy to investigate this discrepancy. The new results indicate good agreement with shipboard squeezings. Thus it is possible that the shipboard results for magnesium from the in-hole sampler are in error, although no reason is apparent to explain the discrepancy. The results for ammonia and silica were obtained from the overflow water of the in-hole sampler and had to be corrected for dilution by distilled water. Agreement for Site 438 with routinely collected samples is good (Figure 2), but for Site 440 the agreement is poor. This can be explained by larger amounts of copper in the overflow water due to the corrosive nature of the high-alkalinity pore water at Site 440. Copper ions interfere with the colorimetric determination of ammonia and silica. In general, however, we conclude that the in-hole samples essentially confirm the validity of the interstitial water sample data collected by routine squeezing, even for the minor constituents, silica and ammonia. This is in agreement with the conclusions of Barnes and others (1979). SALINITY DECREASE WITH DEPTH An interesting phenomenon at Sites 438 and 439 (water depth about 1600 m) is a downhole decrease in salinity and chlorinity. The chlorinity at the deepest sample at Site 438 is only about 50 per cent of that of seawater. These data imply that fresh water has been advected to the sites from an outcrop area. A possible source of fresh water is through sandstone and conglomerate layers in the deeper strata of the sites. Because boulders in the conglomerate are very large,

Interaction between Sediment and Interstitial Water near the Japan Trench, Leg 57, Deep Sea Drilling Project