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Salinity Reflux and Dolomitization of Southern Australian Slope Sediments

Rivers, John M.*1; Kyser, T. Kurtis 2; James, Noel P.2
(1) ExxonMobil Upstream Reseach Center, Houston, TX (2) Department of Geological Sciences & Geological Engineering, Queen's University, Kingston, ON, Canada.

Sampling during Leg 182 of the Ocean Drilling Program shows two lenses of pore waters in southern Australian slope sediments, a shallow lens (<50 meters below the seafloor) flushed by seawater with normal pH, alkalinity, and trace element contents, and a deeper lens with decreased pH, and increased salinity and alkalinity. Presence of the anomalously saline pore waters, and isotopic analyses of shelf sediment, indicate that Pleistocene-Holocene salinity reflux is occurring along the southern Australian margin. Whereas no dolomite has been found in the shallow lens, it has been identified in association with the deeper saline lens. Changes in pore water Mg content indicate dolomite formation is ongoing.

Isotopic and trace-element analysis of the dolomite indicates two stages of formation. The first stage is a nucleation phase that occurs in waters of relatively low pH (~7) due to oxidation of H2S diffusing from below. Such dolomite, which precipitates in sediments less than 80 meters below the seafloor, has δ13C values consistent with having formed from normal seawater (-1 - +2 ‰ VPDB). Sr content of the dolomite indicates stage 1 precipitation can occur prior to substantial metastable carbonate dissolution. 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. Stage 2 is an overgrowth stage, and is associated with metastable carbonate dissolution in the acidified seawater at > 80 meters below the seafloor. Analysis of large dolomite rhombs demonstrates that at these depths dolomite grows on dolomite cores precipitated during stage 1. Stage 2 dolomite has high Sr values relative to stage 1 dolomite, and δ13C values similar to those of the surrounding bulk carbonate, consistent with having formed in waters of affected by aragonite and calcite dissolution. These observations challenge the more commonly accepted paradigm that inhibition of dolomitization by seawater is overcome by effectively increasing the saturation state of dolomite.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California