Carbonate Preservation in Shallow Marine Environments: Unexpected Role of Tropical Siliciclastics

Coordinated taphonomic, geochronologic, and geochemical studies of bivalve death assemblages and their sedimentary environments of San Blas, Caribbean Panama, permit us to identify the major factors controlling skeletal degradation in mixed carbonate-siliciclastic tropical shelf sediments. Ten sites were studied along environmental gradients including water nutrients, grain size, and sediment chemistry (carbonate, organic carbon, and reactive iron contents). Taphonomic data were derived from naturally occurring bivalve death assemblages and experimentally deployed specimens of Mytilus edulis and Mercenaria mercenaria to determine environmental controls on types and intensities of postmortem damage to skeletal hardparts and to quantify short-term rates of damage accrual. Death assemblage shells were dated using C and amino acid racemization techniques to examine shell persistence, scales of time averaging, and long-term rates of damage accrual, including correlations between shell damage and shell age. Pore water and sediment geochemical analyses were used to determine the pathways and extent of early diagenetic change in the different sediment–pore water environments. We found that carbonate shell preservation is enhanced in dominantly siliciclastic sediments compared to dominantly carbonate sediments. The most important factors limiting the postmortem persistence of shell material are (1) exposure above the sediment-water interface, which is enhanced in coarser-grained carbonate sediments and permits attack by bioeroders and encrusters; (2) the availability of abundant reactive iron mineral phases in the sediments, which promotes supersaturated pore waters and limits acid production; and (3) shell microstructure (rather than mineralogy), particularly organic content that is the focus of intense microbial attack. Thus, there is significant potential for enhanced carbonate shell preservation in areas receiving ferric-rich tropical weathering products, which are common in much of the tropics today and are associated with subduction systems in the geologic past. This suggests that paleodiversity estimates from carbonate tropical settings are minima and that siliciclastic settings are probably underestimated regions for carbon burial, given the large proportion of tropical shelf area characterized by such conditions and the relatively high proportional capture there of local carbonate production.