Stable Hydrogen and Oxygen Isotopic Variations in Natural Waters in North Florida

There are no IAEA-GNIP stations in Florida. In order to improve our understanding of the proxy climate records preserved in speleothems, tree rings and lake sediments and to explore the utility of stable oxygen and hydrogen isotopes in the investigation of the water cycle in North Florida, there was a need to establish rainwater sampling stations to fill a gap in the oxygen and hydrogen records of rainfall in North Florida. Rain samples collected from Tallahassee and Pensacola in North Florida were analyzed for stable oxygen and hydrogen isotopic ratios. The Tallahassee samples span a time period from May 2006 to January 2011 and the Pensacola samples were collected from November 2004 to January 2010. Water samples were also collected in late 2010 and early 2011 from two coastal lakes (Eastern Lake and Western Lake in Walton County) and two ponds (Pond 1 and Bluesink in Apalachicola National Forest) in North Florida for stable isotope analyses. The δ18O and δD values of rain samples all plot either on or close to the Global Meteoric Water Line (GMWL) on a δ18O vs δD plot while most of the lake/pond water samples plot below the GMWL indicating that the lake waters in those area have been significantly affected by evaporation. Stable isotopic ratios of the Tallahassee samples ranged from -13.6 to 0.4‰ for δ18O and -99.4 to -2.2‰ for δD values (n=43) while those of the Pensacola samples ranged from -16.0 to 1.5‰ for δ18O and -115.8 to 118.9‰ for δD (n=260). Lake waters yielded δ18O values ranging from -4.7 to 1.9‰ and δD from -31.8 to 14.2‰. Pensacola samples have weighted monthly mean δ18O and δD values that ranged from -7.3 to -1.5‰ and from -4.3 to -18.7‰, respectively. The annual weighted mean δ18O and δD values for Tallahassee for study period are -4.5 and -23.3‰ respectively and -4.3 and -18.7‰ for Pensacola. The average d-excess value is 14‰ for Tallahassee and 9‰ for Pensacola. There is a negative correlation between the precipitation amount and the stable isotopic ratios of preciptation during the study period in North Florida. However, the "amount effect" can only explain less than 20% of the variance in the δ18O values of precipitation likely due to variations in the source and history of atmosphric moisture. There is a weak (but not significant) positive correlation between the average air temperature and the stable isotopic compositions of precipitation, suggesting that temperature is not a major factor in controlling the stable isotopic composition of precipitaion in this area. In Talahassee, rain samples for winter months had δ18O values ranging from -5.3 to -2.6‰, δD values from -30 to -6.2‰, and d-excess from 9.7 to 18.8. During the summer season, rain in Tallahassee displayed a larger range of isotopic variation, with δ18O and δD values ranging from -13.6 to 0.4 and from -99.4 to -7.7‰, respectively, and d-excess ranging from -11.2 to 14.7. The larger isotopic variability observed in the summer is primarily due to tropical storms. Rain samples from Pensacola had slightly more enriched monthly mean δ18O values than Tallahassee samples probably because Pensacola is located closer to the main moisture source ( the Gulf of Mexico). Tropical storms (such as Alberto and Fay) not only produced large amounts of precipitation but also are characterized by very negative δ18O and δD values. The much lower than normal δ18O and δD values of tropical storms are likely recorded in tree rings and speleothems, and thus identification of storm signals in tree rings or speleothems would allow reconstruction of paleostorm history in the area. The unique isotopic signatures of tropical storms can also serve as a tracer to study the source and flow path of groundwater.