Summary

Petrographic information for sands lining the Florida reef tract is neither plentiful nor areally extensive. To rectify the paucity, surface sediments collected shelf-wide off the Florida Keys in 1989 were recently examined in thin section. Depending upon location off land or tidal passes, the sands were dominated by one of three principal skeletal components: coral, Halimeda, or mollusc. The sedimentary signature correlates with observed reef vitality, indicating that in Florida a natural link exists among vitality, geomorphology, and the coral fraction of the sand. This link results in differences in biologic and geologic processes that produce broad seaward and southwesterly trends in sand composition. Comparison with previous analogous data shows a progressive 37-year increase in coral grains in those directions. The new data show for the first time that the coralgal and molluscan components are reliable proxy indicators for reef vitality. The new data contribute to and enhance a model that predicts effects of nutrification on benthic biota. The sediment record thereby also contains new evidence of reef decline.

In Florida, sand composition is influenced by location and geomorphology. (a) Sands are formed in place and essentially remain there. Intense winds and waves of storms and hurricanes redistribute but do not remove them from their place of origin. (b) The Keys form a discontinuous chain of islands broken by numerous tidal passes of varied widths and depths. The passes link turbid, nutrient-enriched lagoonal waters from Biscayne Bay, Florida Bay, and the Gulf of Mexico with clear, nutrient-poor oceanic waters on the reefs. Field observations show that vitality is enhanced in reefs off protective land barriers. In 1989, upper Keys sands were dominated by Halimeda or mollusc grains in sinuous, random bands paralleling the margin. Composition in the middle and lower Keys clearly demarcated the inner and outer shelf, where Halimeda was dominant nearshore and coral debris offshore. The primary natural mechanism for coral breakdown is bioerosion of weakened, damaged, or dead skeletons. Until 1983, the principal coral-sand producer was the herbivorous black spiny sea urchin, Diadema antillarum.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah