Environmental Quality and Preservation:Reefs, Corals, and Carbonate Sands; Guides to Reef-Ecosystem Health and Environments

United States Geological Survey

SUMMARY: The geologic record of the Florida Keys reef tract is a rich archive of evidence for numerous significant past environmental changes. (1) Thick cumulative Pleistocene and continuous Holocene coral-growth records verify previous existences of healthy reef ecosystems and thus the optimal conditions under which they grew. (2) Dated mangrove peat found beneath offshore reefs documents the times and locations of earlier, seaward, shorelines. (3) Landward reef migration is evidence of rising sea level. (4) Shifts from head-coral zones to branching-coral zones within a reef are signs of changing conditions from quiet water to presence of surf and are thus evidence for flooding of once protective offshore reef islands. (5) Annual stress bands in corals indicate periods of severe water-temperature changes, and fluorescent bands verify weather variations that produced high freshwater influx to the reef. (6) Sands record geologic processes of transport direction, scouring of backreef troughs, in-place formation, strength and direction of currents, infilling of bedrock depressions, and accretion behind bedrock highs, as well as biologic processes of grazing for algae and resultant bioerosion of coral skeletons. (7) Sands are bioindicators of the types of reef organisms and the general health of corals through space and time. These geologic signatures are guides to reef-ecosystem health and environment and prove that conditions have been both good and bad for reef ecosystems in the past. Is the fragile modem ecosystem undergoing yet another natural decline, or are human activities responsible or, at the very least, an added component? We cannot know without first understanding the evolutionary history and characteristics of ancient reefs, the processes that produced or destroyed them, and the responses they had to environmental change. The reefs, corals, and carbonate sands are far more important than simply being major components in a reef ecosystem. Their history and what they can tell us about past environmental conditions comprise one of the baselines from which understanding the decline of the modem reef system must be derived.

Environmental Quality and Preservation:Reefs, Corals, and Carbonate Sands; Guides to Reef-Ecosystem Health and Environments

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