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Abstract:
In The Bahamas, caves and blue holes provide clues to the geologic and climatic history of archipelago but are now emerging as windows into the ecological and cultural past of islands. Cave environments in The Bahamas alternate cyclically between vadose and phreatic conditions with sea-level change, thereby providing unique but ephemeral fossil capture and preservation conditions.

A diverse assemblage of fossil plants and animals from Sawmill Sink, an inland blue hole on Abaco Island in the northern Bahamas, has revealed a prehistoric terrestrial ecosystem with exquisitely preserved fossil assemblages that result from an unusual depositional setting. The entrance is situated in the pine forest and opens into a flooded collapse chamber that intersects horizontal conduits at depths to 54 meters. The deepest passages are filled with sea water up to an anoxic mixing zone at depths of 14 to 9 meters and into the upper surface fresh-water layer. The collapse chamber is partially filled with a large talus pile that coincides with an anoxic halocline and direct sunlight for much of the day.

During glacioeustatic sea-level lowstands in the late Pleistocene, Sawmill Sink was a dry cave, providing roosting sites for bats and owls. Accumulations of bones deposited in depths of 25 to 30 meters were subsequently preserved by sea-level rise in the Holocene. The owl roost deposits are dominated by birds but also include numerous small vertebrate species that were actively transported by owls to the roost sites.

As sea levels rose in the Holocene, Sawmill Sink became a traditional passive pitfall trap. Significant quantities of surface derived organic material collected on the upper regions of the talus at the halocline where decaying plant material produced a dense layer of peat within an anoxic mixing zone enriched with hydrogen sulfide. Vertebrate species that drowned were entombed in the peat, where conditions inhibited large scavengers, microbial decomposition, and mechanical disarticulation, contributing to the superb preserva­tion of the fossil assemblage in the upper regions of the talus.