USGS Home
Contact USGS
Search USGS

Systematic Mapping of Bedrock and Habitats along the Florida Reef Tract—Central Key Largo to Halfmoon Shoal (Gulf of Mexico)

USGS Professional Paper 1751

by Barbara H. Lidz, Christopher D. Reich, and Eugene A. Shinn

Introduction: Table of Contents Project Overview Project Objective Geologic Setting Primary Datasets Primary Products - Overview Maps & Evolution Overview: Introduction Depth to Pleistocene Bedrock Surface Reef & Sediment Thickness Benthic Ecosystems & Environments Sedimentary Grains in 1989 Summary Illustration Index Map Evolution Overview Tile-by-Tile Analysis Organization of Report Tiles: 1, 2, 3, 4, 5, 6, 7/8, 9/10, 11 Summary Acknowledg- ments References Disclaimer Related Publications

Tile 2 Geography Sector-Specific Studies Sclerochronology Geologic Highlights Sclerochronology: Sclerochronology, the study of coral growth bands, is the marine counterpart to dendrochronology, the study of tree rings. That corals produce annual skeletal bands has been known for some time (e.g., Ma, 1934). Banding results from variations in skeletal density. Ma (1934) considered the bands to be a measure of coral growth rates and correlated growth-rate changes to changes in seawater temperatures. Knutson et al. (1972) pioneered use of x-radiography to illustrate the banding and conclusively showed their dependence on seasonally varying environmental conditions. Subsequent studies to learn how the bands form have focused on coral skeletogenesis (e.g., Macintyre and Smith, 1974; Stolarski, 2003), biomineralization (e.g., Gautret et al., 1997), and scanning electron microscopy of skeletal structure and microarchitecture (e.g., Sorauf and Buster, 2007). Hudson et al. (1976) and Hudson (1981) were the first to use sclerochronology in the reconstruction of ecologic history in specific areas of the Florida Keys. Their test cases used Montastrea annularis from Hen and Chickens patch reef, located on the inner shelf off Plantation Key (Fig. 42B; see Benthic Ecosystems for Tile 2). Hen and Chickens patch reef consists primarily of large (2- to 4-m-diameter) head corals, about 80% of which are M. annularis. This patch reef was chosen because it suffered 80 to 90% mortality in the winter of 1969-1970. Those corals sampled that did not die contained wider-than-normal bands that correlated with the 1969-1970 cold-water winter temperatures (Hudson et al., 1976). Benthic Ecosystems for Tile 2 [larger version] Because of its massive hemispherical shape, M. annularis is a species likely to yield a complete chronology when cored along its central growth axis (Fig. 43). Annual bands are visible to some degree in a slab from the center of the coral core (Fig. 44) and are readily visible when the slab is x-rayed (Fig. 45A, 45B). The age of the coral can be determined by counting the bands downcore (i.e., backward in time) along the x-radiograph beginning with the band for the year in which the core was taken. Any irregularity in banding can be compared to known meteorologic events for the band year. The x-radiographs show that each band consists of a high- and low-density couplet. Figure 43. Using a hydraulic drill adapted for underwater operation, a scuba diver drills a core from a head of Montastrea annularis coral at Hen and Chickens patch reef off Plantation Key (upper Keys, Fig. 42B). Coral cores are 2.54 or 7.62 cm (1 or 3 in.) in diameter, depending on the size of core barrel used. After the core is removed, the top of the hole in the coral head is plugged with a numbered cork-shaped plug made of underwater cement. A healthy coral will overgrow the plug in a few years, so, for future reference, care must be taken to note precisely which corals in a reef or patch reef have been cored. [larger version] Figure 44. To study annual skeletal bands in a coral, the core is sliced slightly off center using an electric rock saw with a diamond-tipped blade. A slab ~6 mm thick is then cut from the thicker piece of core for x-radiography. The densest annual bands in the slab are visible to the naked eye. All bands become visible in an x-radiograph. [larger version] Figure 45. X-radiographs of coral core slabs show annual growth bands in Montastrea annularis from Hen and Chickens patch reef. (A) Note abnormally wide stress band deposited by the coral in response to an unknown cause during the winter of 1941-42. (B) Correlation of banding in seven different M. annularis. The four at left are from heads killed by cold-water temperatures in the winter of 1969-70. The three at right are from corals that were alive and healthy when sampled in September 1974. The topmost stress bands joined by a dark line correlate with demise of the corals in the left x-radiographs. The stress band for 1941-42 indicates that conditions must have been as severe as in 1969-70 although the corals survived the earlier stress. Other minor stress bands are very faint or do not correlate from coral to coral. Normal annual bands are correlative from coral to coral (modified from Hudson et al., 1976). [larger version] Thin, high-density bands develop sometime during July, August, and September, the warmest parts of the year (Hudson et al., 1976). These are the coral's "normal" bands. Wide bands are often interspersed at irregular intervals among the thin bands. Comparison of annual bands in Hen and Chickens corals with air temperatures recorded in Key West shows a direct correlation between years with the coldest winter-average air temperatures and bands with greatest widths and highest densities (Hudson et al., 1976). However, the Key West record shows that air temperatures for the 1941-1942 year were not only below normal winter lows but also above normal summer highs (E.A. Shinn, personal observation). Thus, there may be a correlation between wide coral bands and years with the warmest summer air temperatures as well. In any case, the wide bands have become known as "stress" bands (Hudson et al., 1976). Proximity of Hen and Chickens patch reef to tidal passes makes the reef vulnerable to massive discharges of bay water chilled by severe and prolonged cold fronts. Although the Hen and Chickens study indicates that temperature is a major factor inducing stress bands in the Florida corals, other factors might cause stress bands in Florida corals, in corals elsewhere, or in coral species other than Montastrea. Therefore, stress bands in the fossil coral record may be indicative of a type of stress other than extreme temperatures. Some coral bands also fluoresce, luminesce, or stand out as bright bands under ultraviolet light. Fluorescence in coral bands is believed to result from sudden environmental change. Bright bands in corals from the Florida Keys correlate with years of severe hurricanes, indicating the bands represent times of high runoff that delivers high concentrations of terrestrial organic acids to the reefs (Halley, 1993). Luminescent bands in corals elsewhere have been found to correlate well with coastal rainfall and river runoff (e.g., Smith et al., 1989), but the conclusion regarding cause is different. The freshwater influx also reduces salinity, which correlates with more holes or cavities in the luminescent bands and thus with less-dense skeletal material (Barnes and Taylor, 2001). Coral bands, like the composition and in-situ production of skeletal sands (see Thin Sections of Surface Sediments section), are thus good biologic (source) and geologic (skeletal record) indicators of past environments. continue to: Nearshore Rock Ledge

Coastal & Marine Geology Program > Center for Coastal & Watershed Studies > Professional Paper 1751