INTERPRETING STABLE ISOTOPES IN TROPICAL STALAGMITES: MASKING OF INTER-ANNUAL CLIMATE PROXY SIGNALS BY RESIDUAL TROPICAL CYCLONE WATER FROM HURRICANE MITCH

Paleotempestology aims to reconstruct the pre-historic activity of tropical cyclones (hurricanes, typhoons, and tropical storms). Tropical cyclone precipitation has stable oxygen isotope ratios (d18O values) about 6 per mil lower than other summer rains. Calcite speleothems (cave formations) from tropical cyclone-prone regions can record past tropical cyclone activity. Hurricane rainfall results in infiltration of a slug of isotopically anomalous water through the epikarst conduit network. A very high-resolution stable isotope record from a modern Belize stalagmite showed a clear link between local historical tropical cyclone rainfall events and brief excursions in stalagmite d18O values.

I have suggested that the extent to which tropical stalagmite stable isotope records can resolve individual storm events depends upon both sampling intensity and the degree of homogenization of storm water during transport. I infer that most infiltrating tropical cyclone water reached the Belize cave as a coherent slug; however, diffusion and dispersion during transport require that some storm water passes more slowly through the system. If significant, this residual tropical cyclone water's anomalous stable isotopic composition may affect the isotopic composition of cave drip waters for some time after deposition of the initial storm water “spike.” This paper presents mass-balance arguments and weather correlations to show that residual storm waters affected speleothem d18O values on both decadal and inter-annual timescales. Of particular concern is the observation that inter-annual climatic correlations between El Nino events and d18O values were briefly masked by rainfall from Hurricane Mitch in 1998. Interestingly, stable carbon isotope ratios (d13C values) were a better recorder of El Nino events without any interference from tropical cyclone activity. For stalagmites collected from tropical cyclone regions, the interpretation of stable isotope climate proxy records during intervals for which the tropical cyclone activity is unknown may be limited by this masking effect, which could lead to paleo-precipitation over-estimates during intervals of high tropical cyclone activity. The modulation of the stable isotope climate proxy accuracy by tropical cyclone activity is potentially problematic and requires further investigation.