Recovered sediments from ten sites (Sites 1023-1032) consist of turbidites (silt to sand), hemipelagic mud (clayey silt to silty clay), and nannofossil-rich mud to mudstone of variable thicknesses (Shipboard Scientific Party, 1997). In total, 562 samples were taken aboard ship with particular emphasis on obtaining clayey sediments in close proximity to the pore-water sampling points and to provide an even distribution of sampling points through the sediment column. The mineral composition of the clayey sediments is relatively homogeneous, involving plagioclase, K-feldspar, quartz, amphibole, biotite, illite, chlorite, smectite, calcite, and pyrite, based on XRD and smear-slide examinations in the shipboard lab. Smectite weight percentage of the sediments quantified by XRD ranged from 0 to 48 wt%, and it was relatively lower in sandy sediments and higher in clayey sediments. Total organic carbon (TOC) content of the sediments is usually less than 1 wt%, but locally exceeds more than 1 wt%. Details of the lithological and sedimentological characteristics, in addition to geophysical and geochemical details, are contained in the companion Leg 168 Initial Reports volume (Shipboard Scientific Party, 1997).
The samples collected were first freeze dried, dispersed, and, to avoid a mixed signal reflecting pore waters and cations exchanged onto minerals in the bulk sediment, dialyzed with distilled water until chloride could not be detected in supernatant solution. Chloride-free samples were then cation exchanged. About 3 g of the samples were put in a polyethylene bottle with 50 mL of 0.1-N strontium chloride solution to extract exchangeable cations from the sediments. After keeping the bottles for 4-6 days at room temperature, the suspension was separated into solid and solution phases by centrifugation. Then Na, K, Mg, and Ca in the supernatant solutions were analyzed as exchangeable cations of the bulk sediment by atomic absorption spectroscopy. The accuracy of exchangeable cation determination in the present study was about 0.05 meq/100 g sample. Other exchangeable cations were not analyzed, but it was estimated that the contributions to the total CEC are generally negligible in the sediments studied. Equivalent fractions (Xi) of exchangeable sediment cations were calculated from: XNa + XK + XMg + XCa = 1 using values of Na, K, Mg, and Ca derived from the exchange experiments.