Composition of Interstitial Waters of Modern Sediments

Interstitial waters were squeezed from several hundred samples of modern oceanic sediments in twenty-two cores from six general areas: the central equatorial Atlantic Ocean, the southwest Atlantic, the northwest Atlantic, the continental shelf off Cape Cod, Massachusetts, the Gulf of California, and the Experimental Mohole (Guadalupe site). Samples were analyzed for Ca and Mg (versene titration), Na and K (cationic glass electrodes), Cl (AgNO₃ titration), and SiO₂ (molybdate colorimetry). Direct determinations of pH and saturometry were made on board ship. Chlorinity varies about a mean close to that of average sea water but shows significant variations both within and between cores from different locations. Chlorinity differences are probably due to semipermeable behavior of compacting clays rather than to "paleosalinities." Increase in CO₂ pressure that results from decay of organic matter is the cause of the lowering of pH in almost all cores. Ca and Mg are similar to that in sea water but Mg is slightly depleted as a result of takeup by chlorites and possibly incipient dolomitization. Na varies sympathetically with chlorinity, but in most samples K is enriched as a consequence of continued hydrolysis of detrital K-feldspars that are ubiquitous in the samples. SiO₂ consistently is enriched in pore waters and is traceable to dissolution of diatoms. The general conclusion is that, in the average marine clayey sediment, early diagenesis is minimal but that the composition of pore waters is a guide to incipient mineral transformations.