GEOGRAPHIC VARIATION IN SHELL PRESERVATION ACROSS THE NORTHERN GULF OF MEXICO

Historical samples of mollusk shells (death assemblages) are increasingly used to infer the state of coastal ecosystems before the onset of certain human activities. However, a variety of taphonomic processes affect these assemblages, which could result in biased baseline data. Environmental differences could lead to differences in shell preservation, thereby complicating comparisons of death assemblages along environmental gradients. Environmental variation across the northern Gulf of Mexico is strongly shaped by fluvial influence. Regions of the continental shelf that are adjacent to large watersheds, such as the Mississippi River, experience elevated sediment, nutrient, and freshwater loads in comparison with regions, such as the Florida Panhandle, that are adjacent to much smaller watersheds. Consequently, variation in species abundance and composition across the northern Gulf could reflect environmental differences in post-mortem processes. To assess geographic and environmental variation in shell preservation, we collected live and dead bivalve mollusks from surficial sediment samples along the -20m isobath at 15 sites that were distributed across three regions (Louisiana, Alabama, and Florida). For each site, we gathered a random subsample of at least 100 dead bivalve shells and scored each fragment according to eight taphonomic variables (articulation, fragmentation, edge modification, encrustation, internal and external luster, bioerosion, and thickness). We performed a principal components analysis on the resulting dataset, and the first two axes explained 77% and 11% of the variance, respectively. In general, most shells were disarticulated, fragmentary, and eroded and/or pitted. While the condition of fragments at individual sites can vary greatly, we observed no clear geographic separation among sites. Edge modification, luster, and fragmentation contributed the most to taphonomic variation among sites. Because regional differences in environment across the northern Gulf do not have consistent taphonomic outcomes, geographic variation in the bivalve death assemblages likely reflects genuine biological differences, not taphonomic biases. This information will be critical when using death assemblages to generate biological baselines for the northern Gulf.