Pore-Water Chemistry and Early Diagenesis of Nearshore Marine Sediments: ABSTRACT

Abstract

End_Page 801------------------------------Pore-water chemistry and mineralogy of carbonate and terrigenous sediments from Kaneohe Bay, Oahu, Hawaii, were analyzed to determine differences in pore-water compositions, in nature and extent of early diagenetic reactions, and in fluxes of constituents between these sediment types. Pore waters, extracted at in-situ temperatures and analyzed for pH and concentrations of Ca++, Mg++, K+, Na+, Cl-, SO=4, reduced sulfides, SiO2, NH+4, PO 4, NO-3,and Sr++, of 23 1-3 m gravity cores show chemical gradients of dissolved species with sediment-burial depth. The sediment pore waters are anaerobic, exhibiting an increase in reduced sulfides, H+, and alkalinity and a decrease of SO=4 with depth. Owing to reactions resulting in the formation of diagenetic ferrous sulfide, the pH's of terrigenous sediment pore waters are higher at an equivalent depth than those of carbonates, whereas reduced sulfides are lower and sulfate reduction is more rapid. Calcium and Mg++ are removed from pore waters with increasing depth as a result of (1) formation of protodolomite, or (2) precipitation of calcite and substitution of Mg++ for Fe++ in clay minerals resulting in formation of ferrous sulfide (Drever reaction). Dissolved SiO2 increases with depth by solution of siliceous plankton or amorphous aluminosilicates, whereas NH+4, PO^identity4, and NO-3 increase with depth because of bacterial oxidation of organic matter. Sodium, K+, and Cl- vary sympathetically, reflecting the original salinity of the pore waters. Lateral gradients of dissolved species in bay pore waters reflect the fact that the southern end of the bay is a more efficient trap for organic matter. These gradients imply a lateral component of flux of dissolved constituents. This study shows that (1) calculations of fluxes in and out of marine sediments must take into account variability of pore-water compositions and fluxes among sediment types, and (2) nutrient regeneration in pore waters can be a significant source of nutrients to overlying waters, whereas reduced sulfide fluxes may be significant enough to inhibit infaunal and epifaunal growth. End_of_Article - Last_Page 802------------