Shallow Marine Carbonate Dissolution and Early Diagenesis-Implications from an Incubation Study

Surface carbonate sediments from sites on the Bahamas Bank with different seagrass densities were incubated across a range of O2 delivery rates, to study the controls on metabolic carbonate dissolution in these sediments. The results confirmed the 1:1 ratio between the rates of O2 consumption and carbonate dissolution, demonstrating that microbial respiration was the ratelimiting step in metabolic carbonate dissolution. Furthermore, the dissolution we observed was actually net dissolution resulting from coupled dissolution and reprecipitation. This carbonate reprecipitation occurs on the time scale of days, and significantly alters the pore water dissolved inorganic carbon (DIC) stable isotopic composition. The carbonate reprecipitation/dissolution ratios observed here were similar to those reported in the literature for other sediments. Dissolution/ reprecipitation appeared to involve preferential dissolution of high magnesium calcite and reprecipitation of a carbonate phase with a Mg content that was only slightly lower than that of the dissolving phase. This result agrees with conclusions in the literature that “Ostwald ripening” may be responsible for this reprecipitation.