Bivalve taphonomy in tropical mixed siliciclastic-carbonate settings. I. Environmental variation in shell condition

Contrary to the geological stereotype of pure-carbonate reef platforms, app roximately 50% of shallow shelf area in the Tropics is accumulating silicic lastic and mixed siliciclastic-carbonate sediments. Taphonomic characteriza tion of these settings is thus essential for assessing variation among majo r facies types within the Tropics, as well as for eventual comparison with higher-latitude settings. Our grab samples and dredge samples of bivalve de ath assemblages from nine stations in five subtidal habitats in a large mar ine embayment of Caribbean Panama (Bocas del Toro) provide the first actual istic information on the taphonomic condition of shells in Recent tropical siliciclastic sediments. Focusing on unambiguous damage to bivalve shell in teriors, we found that the quality of shell preservation in fine-grained si liciclastics is superb: commonly much less than 10% of specimens are affect ed by encrustation, boring, edge-rounding, and fine-scale surface alteratio n via dissolution, microbioerosion, and maceration. Pure-carbonate and mixe d siliciclastic-carbonate environments containing hard substrata (patch ree fs, Halimeda gravelly sand, mud among patch reefs) contain higher numbers o f more severely damaged shells (generally >25%) and also higher diversities of fossilizable encrusters and borers. Disarticulation and fragmentation a re pervasive across all environments and are probably related to predation rather than to postmortem processes. As in other shallow subtidal study are as, the taxonomic compositions of death assemblages have not been homogeniz ed by postmortem transport but show high spatial fidelity to the distributi on of living species. Assemblages from the five sedimentary environments ha ve distinct taphonomic signatures, but the strongest differences are betwee n the two fine-grained, exclusively soft-sediment siliciclastic environment s on the one hand and the three environments containing hard substrata on t he other. Experimental tests for rates and agents of damage, still in progr ess, indicate that the most critical environmental variables are exhumation cycles and burial rate. Bivalve death assemblages from Bocas del Toro demo nstrate that damage levels in tropical fine-grained silicielastic environme nts are much lower than in closely associated reefs and algal sands, and su ggest a less filtered record of biological information.