Stable Isotopic and Geochemical Variability within Shallow Groundwater beneath a Hardwood Hammock and Surface Water in an Adjoining Slough (Everglades National Park, FL)

Data from a ten-month monitoring study during 2007 in south Florida provide insight into the variation of δ18O, δD, and δ13C of DOC in surface water and shallow groundwater of the Everglades ecosystem. Bi-monthly samples were taken from surface water and time-averaged precipitation at Taylor Slough, and shallow groundwater from a well and a small cave within Palma Vista Hammock, an exposure of the Upper Pleistocene Miami Limestone. δ18O and δD values in shallow groundwater from the well and cave remain near the mean of -2.4 ‰ and -12 ‰, respectively (VSMOW scale). 18O and D are enriched in surface water compared to shallow groundwater. δ18O and δD values in surface water fluctuate in sync with, but to a lesser amplitude than, those measured in rainfall. The local meteoric water line (LMWL) for precipitation is in close agreement to the global meteoric water line (GMWL); however, the local evaporation line (LEL) for surface water and shallow groundwater is δD = 5.6 δ18O + 1.5 (R2=0.97), a sign that these waters have experienced evaporation. The intercept of the LMWL and LEL indicates that the primary recharge to the Everglades occurs primarily from tropical or frontal sources. Local convection merely recycles available water. Time-series of deuterium excess (Dex), clearly reveals two moisture sources for precipitation; an evaporation-dominated source with Dex\u3e10 and a source significantly influenced by transpiration with Dex\u3c10. Samples with higher Dex cluster in the fall and winter, and appear to be associated with maritime moisture carried along the Trade Winds. Samples with lower Dex cluster in the late spring and summer, and could reflect continental moisture carried by the Westerlies or local convection. Values of δ13CDOC between -22.6 and -28.0‰ suggest C-3 vegetation as the primary source of DOC at all sample sites. C:N ratios of DOC averaging 20:1 at the cave indicate that organic matter originates from woody material, while an average of 15:1 at the well along with δ13CDOC similar to the cave indicate further decomposition of the organic matter entering the cave. C:N ratios of DOC the slough averaged 15:1, and with δ13CDOC values, suggest sources of organic matter not present at the cave and well

Hydrologic and microclimate characterizations of Thornton’s Cave, West-Central Florida (USA)

A cave’s environment is controlled by a suite of factors unique to the environments in which they formed, including, but not limited to, regional geologic and climate settings. These factors collectively owe to wide variations in cave biology, geomorphology and overall speleogenesis. This report combines local climate, hydrologic, and CO2 data collected over the course of a two-year study at Thornton’s Cave, a partially-flooded cave in the West-Central Florida karst belt, to characterize its current environment and yield insight regarding how changes in regional climate and hydrology impact its past and future speleogenesis. Data loggers continuously monitoring cave and surface air temperatures, water levels and surface rainfall documented immediate responses in the cave to long- and short-term fluctuations in these parameters at the surface. Atmospheric CO2 in the cave and at the surface demonstrate seasonal trends, though the cave maintains higher concentrations and lower δ13C of CO2 than the surface, suggesting decomposition of organic matter, and to a lesser degree macroorganisms are contributing proportionally more CO2 to the cave. Collectively, these interpretations provide insight on the impact of surface processes on cave formation here, and suggest that a biotic model of speleogenesis using bacterially-sourced CO2 as a component of dissolution may be possible. Further, they can be used to further the understanding of the karstification in West-Central Florida and analogous karst regions

Tracing Groundwater Geochemistry Using δ\u3csup\u3e13\u3c/sup\u3eC on San Salvador Island (Southeastern Bahamas): Impliations for Carbonate Island Hydrogeology and Dissolution

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 δ13CDIC 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