VARIATION IN TAPHONOMIC SIGNATURES AMONG MODERN OYSTER REEFS ACROSS AN ESTUARINE ENVIRONMENTAL GRADIENT: IMPLICATIONS FOR INTERPRETING SEA-LEVEL RISE HISTORY

The interplay of sedimentation rate on intertidal oyster reefs and rate of sea-level rise has controlled the geomorphology of the Southwest Florida coast through the late Holocene. Cores from this coast exhibit complex reef histories that require better paleoenvironmental interpretive methods. This study investigates the differences in fossilization potential and sedimentology among modern oyster reefs along a salinity and energy gradient to refine those interpretations. We hypothesize that taphonomic, sedimentologic, and architectural characteristics are significantly different between marine, high-energy and brackish, low-energy settings. Three transects on 2 modern reefs, one located within a protected bay and the second from the exposed gulf in the Ten Thousand Islands, were compared. Ten randomly selected dead left valves of Crassostrea virginica were collected from 1-m² quadrats spaced at 3-m lengths along each transect. For each shell, height and width was measured; valve thickness, crenulation depth, and their normalized equivalents (dividing by height) were measured to assess predation effects; and percent shell surface experiencing encrustation, bioerosion, luster loss, and margin loss were measured as indicators of time averaging. Morphology of each clump was described, and the sedimentary matrix was characterized.

The reefs exhibit significantly different architecture and sedimentology. The marine reef has few small clumps, oysters develop recumbent pavements, and the matrix is dominated by valves, shell gravel, and quartz sand. The brackish reef contains numerous large clumps, and carbonate mud and valves dominate the sediment. The marine reef contains oyster valves that exhibit more pronounced anti-predatory morphologies: greater shell thickness and deeper valve crenulations. These statistically significant differences disappear when those measures are normalized for ontogeny, suggesting that greater predation pressure on the marine reef is skewing the population and no difference in growth rate exists. The brackish water reef exhibits taphonomic characteristics indicative of shorter time averaging; median and mean values for encrustation, bioerosion, and margin loss are smaller for the brackish reef, though differences are not always statistically significant.