Geologic development of Buck Island Reef National Monument

Hubbard, Dennis

Nine cores were recovered from the reefs of Buck Island using the SCARID drilling system developed by West Indies Laboratory. Four cores taken during a companion NSF study and several cores taken from Tague Reef by the Smithsonian Institution provide comparative information on the development of nearby reefs on St. Croix. Thirtyone radiocarbon dates constrained the timing of reef development as revealed by the cores. The oldest reefs on eastern St. Croix formed on southern Lang Bank 9000 years ago. As sea level rose, reefs developed on progressively shallower substrates, first on Buck Island Bar, to the north of Buck Island (ca. 6950 ybp - yrs before present) and then around eastern Buck Island and NE St. Croix (ca.6100-6600 ybp). The reefs that rim Buck Island formed in 2-4 m of water on a broad platform 12-16 m below present sea level. The reefs were broad carpets of mixed corals rather than the distinct ridges that exist today. By 5000 ybp, a well-zoned reef rimmed Buck Island in an arc 50 m landward of its present position. The rapid rise in sea level over the past 2000 years had all but outpaced shelf-edge reefs that rimmed Lang Bank, and a slower-growing community of head corals replaced the Acropora palmata reefs that had flourished previously. The reef near Buck Island was actively accreting, but zonation would remain indistinct for another 1500 years. Between 5000 and 1000 ybp, the main reef accreted upward and seaward. Throughout this interval, the southern reef crest was shallower than its northern counterpart, a condition that persists today. Observations after Hurricane Hugo in 1989 imply that this difference in elevation is related to storm waves piling debris ripped from the southern forereef onto the reef crest. The mix of coral assemblages within the interior of the south-facing reef is consistent with post-storm observations of corals being moved from the zones in which they lived to other sites of deposition either shallower or deeper on the reef. By 1000 ybp, the reefs of Buck Island had assumed their present character. Along the main reef, the crest continued to track slowly rising sea level. The upward accretion of Buck Island Bar ceased at this time. The reasons for this are unclear, but may include heavy wave action, sudden sediment or nutrient stress, or the onset of something similar to the "white band disease" that is devastating the Acropora populations in the park today. The shallow-water reefs of Buck Island share somewhat similar morphologies, coral types and accretionary histories with reefs of the Florida Keys. This is due to common wave regimes and parallels in the sea-level histories of Florida and much of the eastern Caribbean. Differences are related primarily to variations in the depths and morphologies of the antecedent platforms on which the Florida and St. Croix reefs formed. The shallowest reefs in the Keys retreated landward, while their shallow-water counterparts on St. Croix have prograded seaward. In deeper water (10-20m), the shelf-edge reefs rimming eastern St. Croix have been left behind by rising sea level. The deeper depth of the underlying Pleistocene shelf beneath these reefs resulted in earlier an earlier start-up, when sea level was rising at a rate too great for them to keep pace. In addition to information on the geologic history of the reefs in the park, this study provides insight into the controls of reef development on a larger scale, especially antecedent topography and sea-level change. Comparisons between the reefs of Buck Island and the Florida Keys demonstrate the importance of physical oceanographic variables in determining the character of Caribbean reefs throughout the Holocene. Also, observations after Hurricane Hugo led to the recognition of signatures left in the reef interior by past storms. The understanding of such relationships are of paramount importance if we are to ever separate natural perturbations from man-induced catastro