Syndepositional dissolution of calcium carbonate in neritic carbonate environments: geological recognition, processes, potential significance

Within carbonate sediments below tropical-subtropical oceanic surface waters, syndepositional "chemical" dissolution of CaCO3 driven by organic matter oxidation can modify substantially the textural, compositional and early diagenetic characteristics of the resulting rock. Main actuogeological evidence for "chemical" dissolution includes pore-water chemistry of carbonate sediments, and corrosion of bioclasts. Geological evidence includes taphonomic bias towards bioclasts of primary low-magnesium calcite, ghosts of aragonitic or high-magnesium calcitic bioclasts or fossils, lateral variations in early lithification, corroded early cements, pores overprinted by dissolution, and aragonite relicts in microspar. To date, evidence for syndepositional dissolution has been identified, with gaps in documentation, in Silurian to Cretaceous limestones. During organic matter oxidation in the sediment, aerobic respiration, sulfate reduction and oxidation of reaction by-products (e.g. H2S) may result in local undersaturation for CaCO3. Depending on the degree of openness of the diagenetic system, microbial sulfate reduction and, in open systems, reactions involving reaction by-products may in one case lead to precipitation, in another to dissolution of calcium carbonate. Both organic matter oxidation and fluctuations in pore water carbonate saturation are amplified by bioturbation. In burrowed carbonate sediments, carbonate dissolution is coupled to sulfate reduction and oxidation of hydrogen sulfide [Geochim. Cosmochim. Acta 63 (1999) 2529]. Part of the dissolved CaCO3 is recycled to the sea, but the amount of dissolution recycling is difficult to estimate. Below the bioturbated layer, perhaps much of the dissolved calcium carbonate reprecipitates. In Phanerozoic neritic carbonate environments, syndepositional dissolution proceeded at least largely independent from aragonite seas or calcite seas, and appears mainly controlled by site-related factors. Over Phanerozoic time, both bioerosion and factors favourable for "chemical" dissolution within the sediment increased. (C) 2003 Elsevier Ltd. All rights reserved.