Salinity reflux and dolomitization of southern Australian slope sediments: the importance of low carbonate saturation levels

Anomalously saline waters in Ocean Drilling Program Holes 1127, 1129, 1130, 1131 and 1132, which penetrate southern Australian slope sediments, and isotopic analyses of large benthic foraminifera from southern Australian continental shelf sediments, indicate that Pleistocene–Holocene meso-haline salinity reflux is occurring along the southern Australian margin. Ongoing dolomite formation is observed in slope sediments associated with marine waters commonly exceeding 50& salinity. A well-flushed zone at the top of all holes contains pore waters with normal marine trace element contents, alkalinities and pH values. Dolomite precipitation occurs directly below the well-flushed zone in two phases. Phase 1 is a nucleation stage associated with waters of relatively low pH (ca 7) caused by oxidation of H2S diffusing upward from below. This dolomite precipitates in sediments <80 m below the sea floor and has dC values consistent with having formed from normal sea water ()1& to +1& Vienna Pee Dee Belemnite). The Sr content of Phase 1 dolomite indicates that precipitation can occur prior to substantial metastable carbonate dissolution (<300 ppm in Holes 1129 and 1127). Dolomite nucleation is interpreted to occur because the system is undersaturated with respect to the less stable minerals aragonite and Mg-calcite, which form more readily in normal ocean water. Phase 2 is a growth stage associated with the dissolution of metastable carbonate in the acidified sea water. Analysis of large dolomite rhombs demonstrates that at depths >80 m below the sea floor, Phase 2 dolomite grows on dolomite cores precipitated during Phase 1. Phase 2 dolomite has dC values similar to those of the surrounding bulk carbonate and high Sr values relative to Phase 1 dolomite, consistent with having formed in waters affected by aragonite and calcite dissolution. The nucleation stage in this model (Phase 1) challenges the more commonly accepted paradigm that inhibition of dolomitization by sea water is overcome by effectively increasing the saturation state of dolomite in sea water.