Stable isotope study of precipitation and cave drip water in Florida (USA): implications for speleothem-based paleoclimate studies†

Revised version of a paper presented at the 9th, Symposium of the European Society for Isotope Research (ESIR), 23 to 28 June 2007, Cluj-Napoca, Romania.

Abstract

Stable isotopes of hydrogen and oxygen were used to examine how the isotopic signal of meteoric water is modified as it travels through soil and epikarst into two caves in Florida. Surface and cave water samples were collected every week from February 2006 until March 2007. The isotopic composition of precipitation at the investigated sites is highly variable and shows little seasonal control. The δ¹⁸O vs. δ²H plot shows a mixing line having a slope of 5.63, suggesting evaporation effects dominate the isotopic composition of most rainfall events of less than 8 cm/day, as indicated by their low d-excess values. The δ¹⁸O values of the drip water show little variability (<0.6‰), which is loosely tied to local variations in the seasonal amount of precipitation. This is only seen during wintertime at the Florida Caverns site.

The lag time of over two months and the lack of any relationship between rainfall amount and the increase in drip rate indicate a dominance of matrix flow relative to fracture/conduit flow at each site. The long residence time of the vadose seepage waters allows for an effective isotopic homogenisation of individual and seasonal rainfall events. We find no correlation between rainfall and drip water δ¹⁸O at any site. The isotopic composition of drip water in both caves consistently tends to resemble the amount-weighted monthly mean rainfall input. This implies that the δ¹⁸O of speleothems from these two caves in Florida cannot record seasonal cycle in rainfall δ¹⁸O, but are suitable for paleoclimate reconstructions at inter-annual time scales.†

Revised version of a paper presented at the 9th, Symposium of the European Society for Isotope Research (ESIR), 23 to 28 June 2007, Cluj-Napoca, Romania.

Keywords:

• cave water
• hydrogen-2
• Florida
• natural variations
• oxygen-18
• paleoclimate
• precipitation
• rainfall

Acknowledgements

This study was supported by grants from the University of South Florida (BPO), Geological Society of America Graduate Research Grant, National Speleological Society Research Grant, University of South Florida, and Florida Studies Center Research Grant (KPG). We wish to thank Southwest Florida Water Management District (Steve Saxton) for providing the equipment for meteorological logging, Colleen Werner, Mike Gordon, and Florida Caverns State Park for providing permits to enter and collect samples from Legend, Jennings, and Florida Caverns, respectively. The authors are indebted to T. Turner, R. Brooks, J. Hagan, M. Gordon, L. Florea, L. Soto, B. Fratesi, A. Persoiu, and G. Hunt for field assistance. The isotope database for each research site is available at: http://www.nrel.colostate.edu/projects/usnip/.

Notes

Revised version of a paper presented at the 9th, Symposium of the European Society for Isotope Research (ESIR), 23 to 28 June 2007, Cluj-Napoca, Romania.