BELIZEAN CORAL SCLEROCHRONOLOGY AND GEOCHEMISTRY PROVIDE INSIGHT INTO ENVIRONMENTAL CONDITIONS DURING RECENT CORAL GROWTH

Temporal changes in the growth rate or the stable isotopic composition of Scleractinian coral carbonate may reflect variations in sea-surface temperature and/or the isotopic composition of surrounding waters. Three cores of Montastraea faveolata coral (one shallow-water patch reef coral and two deeper water corals) were collected in June 2002 from south-central Belize. Geochemical and growth rate data extracted from these modern coral cores appear to reflect changes in the local hydrology, and perhaps even fluctuations in recent El Niño Southern Oscillation (ENSO) activity.

Analyses of annual growth patterns reveal distinct seasonal banding punctuated by anomalous high-density stress bands that often formed contemporaneously between corals. These stress bands often correlated with unusual but well documented environmental phenomena, such as El Niño events, associated coastal flooding, and hurricanes. Stable isotopic data were collected from the most recent 30 years of shallow water coral growth to examine recent climate correlations in detail. Mean annual d13C of the coral skeleton decreased while d18O increased over the 30-year time period, with accelerated rates over the last decade. These long-term trends in isotopic composition may reflect potentially significant effects of coastal and agricultural development (influx of light terrestrial organic carbon) and water consumption (decreased freshwater input) on local waters.

Perturbations in growth rate and geochemical patterns recorded by these corals may also reflect ENSO activity. All three coral cores displayed intense stress banding, accompanied by a significant decrease in growth, correlating with the extreme 1997-1998 El Niño event. Short-term (decadal scale) decreases in d18O during El Niño years likely reflect temporary increases in sea surface temperature recorded by the coral skeleton, and the seasonal cycle of d13C and d18O are poorly represented during the summers of 1999 and 2001. This may represent a cessation of coral colony growth, probably accompanied by coral bleaching. The occurrence of Hurricane Mitch in the autumn of 1998 likely exacerbated stress on the coral colony. This is recorded by the depletion of d13C during that time, as a potential proxy for increased storm-related terrestrial matter in the reef environment.