DECADE-SCALE TREND IN SEA-WATER SALINITY REVEALED THROUGH DELTA-O-18 ANALYSIS OF MONTASTRAEA-ANNULARIS ANNUAL GROWTH BANDS

Stable oxygen isotope ratios (delta18O) of coral skeletons are influen ced by ambient water temperature and by the oxygen isotope ratio in th e surrounding sea water, which, in turn, is linked to evaporation (sal inity) and precipitation. To investigate this relationship more thorou ghly, we collected hourly temperature data from the Hen and Chickens R eef in the Florida Keys between 1975 and 1988 and compared them to the deltaO-18 of Montastraea annularis skeleton that grew during the same interval. To ensure that we obtained the correct oxygen isotopic rang e in the skeleton we typically sampled the coral at a resolution of 20 -30 samples in 1 year; in 1 year we sampled the coral at a resolution of 70 samples.year-1. Despite our high-resolution sampling, we were un able to obtain the full temperature-induced deltaO-18 range in the ske leton. Our data suggest that, during the summer, evaporation causes is otopic enrichment in the water, partially masking the temperature-indu ced signal. Our data also show that oxygen isotopic composition of sea water at the reef has increased since 1981. This increase indicates th at salinity has increased slightly during the past decade, perhaps as a result of increased evaporation in waters of Florida Bay and the Key s. This phenomenon is probably not caused by a decrease in the outflow of freshwater into Florida Bay from the Everglades but may be related to the measured deficit in precipitation that has occurred over the p ast decade.