2003 Seattle Annual Meeting (November 2–5, 2003)

Paper No. 5
Presentation Time: 1:30 PM-5:30 PM


MURGULET, Valeriu1, AHARON, Paul1, WHEELER, Christopher W.2 and GRAHAM, Elizabeth Y.3, (1)Department of Geological Sciences, Univ of Alabama, Box 870338, Tuscaloosa, AL 35487, (2)Deparment of Environmental Science, California State University-Channel Islands, Camarillo, CA 93012, (3)Department of Geological Sciences, Univ of Alabama, Box 870338, 202 Bevill Building, Tuscaloosa, AL 35487, murgu001@bama.ua.edu

Niue Island is a large uplifted former atoll in the tropical South Pacific Ocean where karst is pervasive in the carbonate cap. The elemental chemistry and stable isotopes of dripwaters from which speleothems are actively forming in two adjacent coastal caves were determined in order to identify their sources. This study also examined the differences and similarities between vadose (cave dripwaters) and phreatic groundwaters on the island. Water chemistry data indicate that cave dripwaters are a Na-Ca-HCO3 type with Na, Ca and Mg as the dominant cations and HCO3 as the dominant anion. Relatively elevated concentrations of Na (up to 327 ppm) suggest mixing of sea-spray with rainwater at the points of recharge. Differences in Mg/Ca molar ratios of dripwaters from adjacent caves suggest variability intrinsic to the specific infiltration pathways. Cave dripwaters exhibit elevated DIC values (6 ±1 mM, n=7) and are 13C-depleted (d13C=-11 ±2 per mil PDB, n=7) attesting to contribution of soil CO2. On the basis of material balance it is estimated that about 40% of the DIC is derived from soil CO2 and the remainder 60% from dissolution of the carbonate cap. d18O values of dripwaters (-4.6 ±0.5 per mil SMOW, n=7) are statistically indistinguishable from those of the phreatic groundwaters (-4.2 ±0.8 per mil SMOW, n=19) and are compatible with the d18O values of winter monsoon rainfall. Phreatic groundwaters sampled from 19 wells around the island are clustered into two distinct chemical groups: (1) Cl-HCO3 water with Na and Ca as the dominant cations within 1 km of the coast and (2) Ca-Mg-HCO3 water over most of the island interior. Carbonate speciation data indicate that phreatic groundwaters are slightly undersaturated with respect to calcium carbonate whereas cave dripwaters are all supersaturated thus explaining the active formation of speleothems in the Niuean caves.