Interstitial water chemistry of the Santa Barbara Basin sediments

Abstract

The interstitial water composition (SO 4 2−, alkalinity, sulfide, Ca 2+, Mg 2+, K +, Si(OH) 4, phosphate, NO 2 −, NO 3 −, NH 4 +, and Cl%.) of both the ‘basin’ sediment in the center of the Santa Barbara Basin and the ‘slope’ sediment shallower than the basin sill is compared. The ‘basin’ sediment is deposited in seawater which is almost anoxic (0.05–0.1 ml/l. of O 2) whereas the ‘slope’ sediment is overlain with more oxygenated seawater (0.3–0.4 ml/l. O 2). The sediments in both regions have similar clay mineralogy and are rapidly deposited (~0.4 cm/yr). The relationship between the redox state of deposition, early diagenesis, and pore water chemistry is investigated. Relative to seawater, the interstitial water of the ‘basin’ sediment shows large depletions in SO 4 2− and Ca 2+, accompanied by large enrichments in alkalinity, NH 4 +, and phosphate. In contrast, the ‘slope’ sediment pore waters show slight depletions in SO 4 2− and Ca 2+ and small increases in alkalinity, NH 4 +, and phosphate. In both sedimentary environments 1. (1) Mg 2+ depletions occur to the same extent; 2. (2) dissolved silica concentrations are maintained at high, constant values beginning within 3–6 cm of the sediment-water interface; 3. (3) NO 2 − and NO 3 − are absent. A diagenetic model, which utilizes organic C:N:P ratios of southern California basin sediments, is able to predict the measured interstitial water concentrations of alkalinity, ammonia and phosphate from reactions involving the decomposition of organic material by the reduction of pore water SO 4 2− and the authigenic precipitation of CaCO 3. The uptake of Mg 2+ by clay minerals is proposed to explain Mg 2+ depletions in marine interstitial waters. Pore water silica values appear to be maintained at equilibrium concentrations, probably resulting from the interaction of clay minerals with silica produced by the dissolution of siliceous tests. From this comparison of the slope and basin regions of the Santa Barbara Basin it appears that the interstitial water chemistry (e.g. extent of pore water SO 4 2− reduction; degree of carbonate saturation) and the preservation of the calcareous microfossil record are extremely sensitive to small changes (i.e. a decrease from 0.4 to 0.1 ml/l.) in the oxygen content of the sea-water under which deposition occurs. Implications of this oxygen control on sedimentary diagenesis may be important in both recent and past marine environments.