TIME AVERAGING ON OYSTER REEFS: IMPLICATIONS FOR ENVIRONMENTAL RECONSTRUCTION AND HISTORICAL CHANGE

As brackish water ecotone indicators, oyster reefs are useful for construction of sea level curves and for determining anthropogenic effects on water quality. Unfortunately, the magnitude of time averaging in most coastal, intertidal depositional settings is too great to yield an appropriate temporal resolution. We therefore examined the relationship among taphonomic grade, absolute age, and the magnitude of time averaging on modern oyster reefs, previously a taphonomically unstudied ecosystem, along the Southwest Florida coast. Shells from the species Crassostrea virginica were collected in 10-cm increments along transects across a high energy reef, located in an exposed coastal setting, and across a low energy reef, located in a protected bay. Four taphonomic grades were established using a set of preservational criteria. Specimens were graded on a scale of 1 to 4 (1: best preservation, 4: worst) by three people. Grades were averaged for each oyster and 16 samples (8 from each reef; 2 of each grade) were chosen for amino acid racemization dating. The degree of time averaging was less for these reefs than that found in studies of other intertidal communities. Both reefs exhibited better than millennial-scale resolution, with a time averaging minimum of 280 (protected reef) and 520 (exposed reef) years. The mean taphonomic grade for the high energy reef (3.2) was significantly worse than the low energy reef (2.6). Average taphonomic grade also worsened from the low- to high-energy ends on both reefs. Our regression analyses of amino acid racemization age versus taphonomic grade show a positive correlation for the high energy reef (R=.91) and low energy reef (R=.88), suggesting taphonomic grade is a reasonable proxy for absolute age. These results suggest that reefs experience a degree of time averaging that is reasonable for studying problems of environmental and sea level reconstruction.