Dissolution of high-Mg calcite fossils and the formation of biomolds during mineralogical stabilization

Abstract

Biomolds formed by the dissolution of high-Mg calcite (HMC) porcellaneous foraminifera occur in Holocene limestones from the Schooner Cays, Bahamas, and in Pleistocene limestones from the Belmont Formation of Bermuda and the Lucayan Formation of the Bahamas. In all of these cases, the HMC biomolds are associated with either a water table or a subaerial exposure surface. A single foraminiferal test may exhibit both a moldic fabric and a low-Mg calcite (LMC) recrystallization fabric that preserves original texture. Other types of HMC bioclasts, such as red algae and echinoids, do not exhibit biomolds but are mineralogically stabilized. The fact that foraminifera are more susceptible to dissolution and biomold formation than other types of HMC bioclasts indicates that an intrinsic factor, such as skeletal microstructure, is partially responsible. However, because both mold formation and texture-preserving recrystallization may occur in the same grain, intrinsic properties of the bioclasts alone do not determine the alteration pathway. An extrinsic factor is also implicated. It is proposed that this factor is CO2 flux across diagenetic interfaces during mineralogical stabilization. Abrupt and large influxes of CO2 across a subaerial exposure surface or water table will reduce saturation states in bulk pore waters and within skeletal grains. Recrystallization by spatially and temporally synchronous dissolution and precipitation, which is the normal HMC alteration process, will cease and only dissolution of HMC will occur. Bioclasts with high reactive surface areas, such as foraminifera, will be most susceptible to this dissolution. Once the CO2 influx has been buffered, HMC-to-LMC recrystallization will resume. In this manner, an originally HMC bioclast can exhibit the products of both dissolution and recrystallization pathways. This would be particularly true if recharge, and thus CO2 flux, is a seasonal phenomenon, as is the case in most carbonate regimes.