Tracing groundwater geochemistry using δ~(13)C on San Salvador Island (southeastern Bahamas): implications for carbonate island hydrogeology and dissolution

摘要

Mixing dissolution is a widely accepted processof karstification on carbonate platforms, but regional dif-ferences in climate and geology indicate that a universalapplication of this model is insufficient to assess water–limestone interactions in more specific island settings. Atwo-phase study investigating δ~(13)C, carbon concentration,and other geochemical parameters took, place on San Sal-vador Island, The Bahamas, to better understand itshydrologic characteristics and identify local controls ondissolution. In the initial phase, Crescent Pond and adjacentCrescent Top Cave, both with conduit connections to oneanother and to open marine water, were monitored over 1.5normal tidal cycles and found to have little geochemicalvariation. Contrasting geochemical compositions betweenthese two sites and the ocean illustrates the complexity ofsubsurface hydrology, while lower pH and δ~(13)C_(DIC)valuesin the cave suggest the potential for bacterially mediateddissolution. The second phase included a more compre-hensive geochemical survey of 12 of the islands surface/subsurface water bodies, and found that water geochemis-try was governed primarily by connectivity to the oceanand secondarily by topographic and vegetative settings.Geochemical relationships illustrated by regression analy-ses showed that biologic activity exerted additional controls over water geochemistry, with photosynthesisremoving biotically respired CO_2 and elevating organiccarbon in surface waters, while biotically respired CO_2 accumulates and supports dissolution in the subsurface.These data underscore the importance of including the roleof biotic processes with climate and geologic settings whenidentifying dissolution mechanisms and using them toestimate modern and historical dissolution processes.