Chemical Changes in Interstitial Waters from Continental Shelf Sediments

Résumé du livre

The chemical characteristics of interstitial waters from cores of shelf sediments form the inner and outer shelf off Long Island, New York, have been compared with those of the overlying sea waters. At each coring station one sample of surface sea water and of bottom sea water, and one or more samples of interstitial waters within the core were collected. The pH and Eh values were measured aboard ship. Salinity and the concentrations of calcium, magnesium, potassium, rubidium, strontium and lithium were determined at the home laboratory. Cores from inner shelf consist of clean sands, whereas those from the outer shelf contain a mud fraction. In both core types, chlorinity (Cl) and the Ca/Cl, K/Cl and Rb/Cl ratios are higher in the interstitial waters than in the overlying waters. The Mg/Cl and Li/Cl ratios are about the same, but the Li/Cl ratios are higher on the inner shelf than on the outer shelf. The Sr/Cl ratio is highest for the surface waters, lowest for the interstitial waters, and of intermediate values in bottom waters. Values of pH and Eh are lower in the core waters than in overlying waters. The decrease in pH and Eh below the water/sediment interface are attributed to the activity of anaerobic bacteria. The reasons for the other chemical changes are not known at present, but are thought to result from diagenetic changes within the sediment. One explination previously proposed for the increased salinity beneath the water/sediment interface is that clays acts as semipermeable membranes during compaction and retain salt while espressing less saline water. While this explanation may apply where clays are abundant, it does not seem applicable to the clean sands of the inner shelf in which clays are not abundant. The increase of the Ca/Cl ratio might be explained as a result of dissolution of small amounts of aragonite from shell material in the enclosing noncarbonate sediments. If true, it is surprising that the Sr/Cl ratio decreases as the Ca/Cl ratio increases. The increase in the K/Cl and Rb/Cl ratios may result from dissolution of K-feldspar.