Abstract

Neritic dinoflagellates from periplatform (slope) carbonates of the Clino borehole, located on the western, leeward margin of the Great Bahama Bank, record environmental fluctuations on the platform top. A lower Pliocene interval (3.6-4.2 Ma) contains platform-top sediments shed onto the lower slope when the platform was open and ramplike. Despite this open topography, abundant Polysphaeridium zoharyi indicate the presence of restricted marine environments on the platform top. Terrestrial palynomorphs are rare throughout this interval and imply a mostly or fully submergent platform top. By late Pliocene times (about 2.1-2.3 Ma) the platform had become flat-topped and steep-sided, with the Clino site located on its upper slope. Samples characteristic of sea-level highstands and lowstands were selected for analysis. Polysphaeridium zoharyi is abundant only in lowstand samples and may have thrived in proximity to terrestrial vegetation. In highstand samples Lingulodinium machaerophorum replaces P. zoharyi. perhaps in response to less restricted marine environments on the platform top. This change in assemblages, along with apparent variations in cyst influx, reflects a fluctuating history of currents and salinities over the platform top in the late Pliocene. Upper Pliocene lowstand samples contain anomalously high proportions of terrestrial palynomorphs, allowing the identification of two phases of emergence and vegetation of the platform top. Palynology therefore appears to be a sensitive indicator of short-term (4th-order) sea-level change on carbonate platforms. Dinoflagellate concentrations correlate positively with carbonate compaction, and infer that compacted layers have undergone dissolution of their metastable constituents. Dinoflagellate concentrations therefore can be useful in the often difficult task of assessing compaction and dissolution in fine-grained limestones where other indicators are absent. The following dinoflagellate species are formally proposed: Operculodinium bahamense Head new species. Operculodinium? megagranum Head new species, and Spiniferites rhizophorus Head new species.

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