Reconstructing estuarine conditions: oyster shells as recorders of environmental change, Southwest Florida

Live-collected shells of the oyster, Crassostrea virginica, contain geochemical records of modern temperature and salinity, so records of prehistoric conditions may be obtained from subfossil shells. Restoration of channelized watersheds in Florida is receiving much attention, and plans for targeted watersheds require information about estuarine conditions before channelization. Lack of historical records necessitates alternative methods to understand pre-disturbance conditions. A C-14-calibrated, amino-acid geochronology based on racemization of glutamic acid yielded ages ranging from 190-1220 AD and from 1270-1860 AD for subfossil oysters from Blackwater River (near-natural watershed) and for Faka-Union Bay (channelized watershed), respectively. delta(18)O and delta(13)C values of subfossil shells from Blackwater River indicate salinity and summer temperatures similar to present. Winter temperatures recorded in shells from 190, 590, 720, and 1050 AD appear 1-5 degreesC colder than present winter temperatures, whereas the shell from 1220 AD records winter temperatures similar to modern winter temperatures. These temperature shifts may indicate change in climate or natural seasonal variation of winter temperature from year to year. Subfossils from Faka-Union Bay may reflect a complicated hydrology, which cannot be evaluated by isotopic compositions alone and demonstrates the need for development of independent elemental proxies for temperature and salinity. Decreases in delta(13)C from subfossil to modern shells may in part result from CO2 added to the atmosphere from fossil fuel burning (the Suess effect). Subfossil delta(13)C that is > 1parts per thousand more positive than modern shells suggest a change in the dominant carbon sources from terrestrial C-4 or aquatic plants to C-3 plants (mangroves). (C) 2003 Elsevier Ltd. All rights reserved.