EVALUATING THE ENVIRONMENTAL HISTORY OF A CARBONATE LAGOON USING TAPHOFACIES ANALYSIS (ST. CROIX, US VIRGIN ISLANDS)

A reef lagoon accumulates a package of sediment over time. Mollusk shells accumulating within the sedimentary record can record changes in lagoon environments as the reef builds and the sediment package thickens. The lagoon behind Tague Bay reef in St. Croix, USVI has been accumulating for at least 5,000 years based on radiocarbon ages from reef cores. In this study we cored the lagoon to study mollusk assemblages over time and discovered that the sediment package is highly bioturbated by callianassid shrimp that move shell material in the subsurface and concentrate it in shelly lags. Preliminary results that compared surface assemblages to the fauna in the lag showed that the assemblages were different. Two hypotheses are proposed: 1) The shrimp are sorting shell and concentrate only certain sizes/shapes/densities of shell, or only concentrate infaunal shells; or 2) the lag represents a long term, time averaged accumulation of shell that was typical over much of the 5,000 year history and the surface fauna has recently changed.

For this analysis we compared taphofacies because taphonomic condition of shells is a reliable indicator of the environment from which shells originate We also compared guild membership and size of mollusks between modern and subsurface assemblages. We found that the surface beds were more similar to each other than to lags regardless of surface habitat (seagrass, open sand), and subsurface beds were also more like one another. Surface mollusks had larger shell sizes and more epifaunal species, while subsurface beds were dominated by smaller shells and infaunal species. Surface mollusks were more taphonomically altered than subsurface ones.

The dominance of infaunal guilds and the scarcity of epifaunal guilds in the subsurface suggests that it is much more difficult for the shrimp to bring down surface shells. The decrease in taphonomic alteration in the lower beds suggests that shrimp are not pulling shells down by size alone but rather by life guild, favoring infaunal over epifaunal organisms. Since infaunal organisms are less subject to taphonomic alteration than epifaunal ones, guild membership is driving the overall taphonomic signal and influences the results for species, and size as well. Therefore, these data support a hypothesis for preferential shrimp sorting over environmental change.