| Abstrakt: |
The tropical-subtropical western Atlantic realm harbours abundant fringing reefs, two major barrier reefs (Belize, Florida), several atolls and large reef-fringed carbonate platforms (Bahamas). Data from Belize and south Florida indicate that major barrier reefs were established during the high sea-levels of the long and warm marine isotope stage 11. Reefs formed during Pleistocene sea-level highstands, whereas erosion, non-deposition and the formation of subaerial exposure surfaces prevailed during lowstands of sea-level (Florida, Bahamas); exposed reefs and platforms with siliciclastic hinterlands were characterized by deposition of clays and sands on fluvial plains (Belize). Elevated terraces and outcrops of reefs of the last interglacial of marine isotope stage 5e are evidence of higher than present sea-level and temperature that sustained large reef-fringed carbonate platforms and ramps in the Florida-Bahamas region and Central America (Belize, Mexico), including ooid deposition in warm and shallow waters. During the highstands of marine isotope stages 5c and 5a, at sea-levels several metres lower than today, fringing reefs developed in parts of the Caribbean, for example, in south Florida ('outlier reefs'). Data from the island of Barbados support the contention that the postglacial sea-level rise from about −120 m was rapid and punctuated by two meltwater pulses Ia and Ib, which bracket the beginning and end of the Younger Dryas, respectively. After reflooding of shelves in the early Holocene, reefs developed preferentially on highs of the underlying karstified Pleistocene limestone surface and on former river channel bars. Many reefs in the western Atlantic region kept pace with Holocene sea-level rise whereas some were drowned. Reasons for reef drowning are debated and include sea-level rise, elevation of antecedent topography, and deterioration of environmental conditions such as high nutrient input. Apart from sea-level and antecedent topography, factors such as exposure to waves and currents, storms, and sedimentation have controlled reef development. The historical decline of acroporid corals in the Caribbean realm has been attributed to cooling following the Holocene thermal optimum; however, historical die-offs of reef-building corals and other reef organisms suggest that some if not all recent declines were unprecedented during the Holocene. Rising temperatures, decreasing carbonate saturation, increased frequency of major storms, high nutrient concentrations and sedimentation, overfishing, and disease certainly played a role in decline, but systematic studies relating these environmental factors to Holocene reef-building are only in the early stages. [ABSTRACT FROM AUTHOR] |
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