The Formation of Dolomite in Sediments from the Continental Margin of Northeastern Queensland

Abstract

The occurrence of dolomite in young Pleistocene sediments off the northeastern coast of Australia offers a special opportunity to examine dolomite formation relative to the original mineralogy and the remineralization of organic material through the processes of sulfate reduction and methanogenesis. The four cores examined in this study form a transect from shallow to deep water (Sites 821, 820, 819, and 822). At all sites, dolomite occurs in sediments of Pleistocene age and is more abundant closer to the continental margin (Site 821) than in sediments found in deeper water (Sites 819 and 822). In spite of the fact that the classic diagenetic zones of sulfate reduction and methanogenesis were all well developed at Sites 819, 820, 821, and 822, the δ13C of the interstitial dissolved inorganic carbon (DIC) showed no evidence of these diagenetic processes. The dolomites retrieved from sediments showed δ13C values similar to that of the DIC. The isotopic composition of the dolomite revealed no evidence of having been formed in either the sulfate reduction zone or the zone of methanogenesi s. The Leg 133 sites differ from other anoxic hemipelagic areas in which extensive sulfate reduction and methanogenesis have been documented in that the sediments are dominated by CaCO3. Recrystallization of calcium carbonate apparently buffers the isotopic composition of the system, masking large isotopic changes that might be induced by additions of isotopically light or heavy CO2 derived from the processes of sulfate reduction and methanogenesis. The dolomite does not appear to be preferentially located in the portion of the cores in which sulfate is absent. Based on the association between the abundance of high-Mg calcite (HMC) and dolomite, it is thought that dolomites formed in response to the dissolution of HMC.