Late Quaternary sea-level position: Evidence from Bahamian carbonate deposition and dissolution cycles

Abstract

The Bahama Islands offer a unique opportunity to examine Late Quaternary sea-level position, as the island group places significant constraints on the time and space boundary conditions for climatic events recorded there. The Bahamian Archipelago extends as a series of large and small islands on shallow carbonate banks of various sizes for 1400 km, across a climatic gradient wet in the northwest to dry in the southeast. The entire island group is tectonically stable, and consists solely of Quaternary carbonate sediments with interspersed paleosols. The islands reflect glacioeustatic sea-level highstand positions in their eolian and subtidal suite of limestones, a depositional record of platform flooding and carbonate sediment production. A coeval record of phreatic cave development in the island's fresh-water lenses also documents sea-level highstand positions. Absolute dating of late Pleistocene fossil corals and cave stalagmites by U/Th methods has produced an excellent record of the last interglacial (marine isotope substage (MIS) 5e). Holocene eolianites have been dated by 14C to reveal platform flooding at ∼5000 ybp, and sea-level stabilization at ∼3000 ybp. Pleistocene eolianites have been dated by amino acid racemization analysis, but subdivisons of the eolianites into separate MIS 5a, 5c and 5e deposits are controversial. Paleosols, representing integrated aerosol dust deposition of ∼100 ka-long platform exposure events during glaciations, have been differentiated by paleomagnetic secular variation measurements. Previous interpretations of a beach unit to indicate a +20 m MIS stage 11 sea-level highstand, and of large boulders to represent a mega-tsunami, appear to be incorrect. Phreatic caves in today's subaerial environment have been ascribed to MIS 5e, but some may have formed earlier. Carbonate deposition and dissolution cycles create a mass flux of CO 2 sufficient to explain the atmospheric CO 2 excursions of the Quaternary.