Tracing groundwater geochemistry using δ13C on San Salvador Island (southeastern Bahamas): implications for carbonate island hydrogeology and dissolution

摘要

Mixing dissolution is a widely accepted process of karstification on carbonate platforms, but regional differences in climate and geology indicate that a universal application of this model is insufficient to assess water–limestone interactions in more specific island settings. A two-phase study investigating δ13C, carbon concentration, and other geochemical parameters took place on San Salvador Island, The Bahamas, to better understand its hydrologic characteristics and identify local controls on dissolution. In the initial phase, Crescent Pond and adjacent Crescent Top Cave, both with conduit connections to one another and to open marine water, were monitored over 1.5 normal tidal cycles and found to have little geochemical variation. Contrasting geochemical compositions between these two sites and the ocean illustrates the complexity of subsurface hydrology, while lower pH and δ13C_DIC values in the cave suggest the potential for bacterially mediated dissolution. The second phase included a more comprehensive geochemical survey of 12 of the island’s surface/subsurface water bodies, and found that water geochemistry was governed primarily by connectivity to the ocean and secondarily by topographic and vegetative settings. Geochemical relationships illustrated by regression analyses showed that biologic activity exerted additional controls over water geochemistry, with photosynthesis removing biotically respired CO₂ and elevating organic carbon in surface waters, while biotically respired CO₂ accumulates and supports dissolution in the subsurface. These data underscore the importance of including the role of biotic processes with climate and geologic settings when identifying dissolution mechanisms and using them to estimate modern and historical dissolution processes.