The Relationship Between Modern Mollusk Assemblages and Their Expression in Subsurface Sediment in a Carbonate Lagoon, St. Croix, US Virgin Islands

Much work has been done on the fidelity of a death assemblage to its present-day living community. Few studies, however, have extended this into the deeper subsurface in modern environments. This study examines the molluskan faunas accumulating at the surface in a reef/lagoon system on St. Croix, US Virgin Islands and compares those to the prefossilized assemblages in the subsurface to examine how faithfully surface assemblages are reflected in the sediment below. Data from 12 vibrocores through the Holocene section indicate that there is a very strong taphonomic filter in effect in the lagoon. The faunal constituents, their taphonomic signature, and the size classes represented in the surface assemblage are quite different from molluskan accumulations in deeper subsurface sediments. In fact, intense bioturbation by callianassid shrimp as deep as 3 m into the sediment has affected the entire sedimentary package resulting in a largely homogeneous section devoid of subsurface shell beds. The one exception to that pattern is a single shelly lag found at the bottom of the sediment package resting on the hard pre-Holocene surface. Most importantly, we find that the lag fauna does not resemble the modern life/death assemblage accumulating at the sediment surface. Dominant taxa in the modern life or death assemblage include small epifaunal gastropods and large bivalves. In contrast, small, thin-shelled infaunal bivalves dominate the lag along with agglutinated polychaete tubes, decapod remnants, and shell fragments. Our data suggest that the shell-rich accumulation deep within the sediment package is a time-averaged concentration of predominantly small infaunal mollusks deposited over time by deep burrowing callianassid crustaceans. Moreover, the extent of bioturbation within the lagoon suggests that some of the widely accepted live–dead paradigms may not be as robust as generally assumed.