CHARACTERIZING AND INTERPRETING THE MICROSTRATIGRAPHY OF A STROMATOLITE/THROMBOLITE FROM A HYPERSALINE INLAND LAKE, SAN SALVADOR, BAHAMAS

Extremophiles are organisms that thrive in extreme environmental conditions. Several Bahamian inland lakes, characterized by extreme, hyper- or hypo saline conditions, are perfect for such organisms and may provide models for extremophiles in the geologic record. Storrs Lake on the Bahamian island of San Salvador is one such inland lake that harbors easily accessible bacterial, archaean, and eukaryotic extremophile assemblages in the form of thrombolites and stromatolites. Storrs Lake is a turbid, hypersaline, alkaline lake with widely varying pH and salinity due to inflow from a marine karstic system, rain, and evaporation; it is close to the ocean and easily influenced by storms. The stromatolites in Storrs Lake are carbonate structures precipitated in micritic, laminated layers to thrombolitic clots with voids. Indeed, in a single, sectioned, stromatolite head collected from Storrs Lake, a recognizable macroscopic pattern in the form of four distinct ‘sequences' appears to be present; each sequence shows a recognizable and repetitive pattern of microstratigraphy. Each sequence is generally composed of layers of loose sand, followed by dense, micritic laminae, and succeeded by a thicker layer of thrombolitic clots. The loose sand apparently reflects storm accumulation of reworked debris from the lake and is followed by a laminar layer of micrite that represents re-establishment of the micro-extremophile community and precipitation of micrite to stabilize underlying sands. With subsequent lake deepening, it becomes necessary for the extremophile community to secrete ever higher accumulations of micrite in order to obtain sufficient sunlight in the deepening, very turbid waters, so that the change to clots may reflect increasing phototropism in deepening, turbid waters. A new storm restarts the sequence with a fresh layer of sand. Hence, the microstratigraphy of any individual stromatolite head may be more indicative of local environmental conditions in a given locale than of any lake-wide events. Although the microstratigraphy in individual heads may not easily relate, the record of isotope variations may be more telling and truly correlative throughout the lake.