Orthophosphate: Effect on the Relative Stability of Aragonite and Magnesian Calcite During Early Diagenesis

Abstract

ABSTRACT Recent, shallow-marine carbonate sands are composed primarily of aragonite and magnesian calcite. During subsequent subaerial exposure of such sediments, these mineral phases are progressively transformed to calcite and/or dolomite. Consistent with their relative thermodynamic stability, magnesian calcites are usually altered first, followed by aragonite. Adsorbed orthophosphate, in micromolar levels, has been found to inhibit strongly the dissolution of biogenic magnesian calcites, but does not affect the dissolution of aragonite. The presence of sufficient levels of orthophosphate could thus change the direction of early carbonate diagenesis and permit the preferential dissolution of aragonite. An example of one area where this process appears to be taking place is at Boiler Bay, St Croix, U.S.V.I. Here, variation in composition of beach sediments and pore waters in the backbeach area indicates early, preferential dissolution of aragonite in a sand initially containing roughly equal amounts of aragonite and magnesian calcite. Levels of dissolved and adsorbed phosphate at this site appear sufficient to inhibit magnesian calcite dissolution. Additional work needs to be done, however, to evaluate the role of other potential inhibitory agents at this beach and in other coastal carbonate systems. The potential effect of orthophosphate as an inhibitor should be greatest in systems where there is terrestrial production and input of phosphate into the zone of diagenesis and where waters are only slightly undersaturated with respect to magnesian calcite and aragonite.