A SPELEOTHEM ISOTOPE PROXY FOR PALEOTEMPESTOLOGY

Despite a great deal of study, the likely future impact of climate change on tropical cyclone activity (hurricane and tropical storms) remains unclear. Historical hurricane records are too brief to settle the issue, and modeling studies have been equivocal. It has been suggested that enthalpy transport by tropical cyclones may constitute a key climate control in the Earth System. Geologic and biogeochemical proxy records of hurricane events can provide information about past tropical cyclone activity. The isotopic signature of tropical cyclone rainwater is anomalously low compared to other summer precipitation, providing a potential isotopic spike which may be recorded by various organic and mineral archives such as coral skeletal material, fish otoliths, bivalve shells, and speleothems (cave deposits). Calcite speleothems have several advantages: they can record variations in groundwater composition at very high temporal resolution, can be radiometrically dated, may contain annual layers, and record environmental conditions between storm events. Cave depositional settings are also very common around tropical cyclone basins.

We demonstrate a proxy for individual tropical cyclone rainfall events using high-resolution stable oxygen isotope ratio analyses of speleothem calcite. A record of calcite oxygen-18 values in a stalagmite from Belize measured at ~weekly temporal resolution over the last few decades contains brief, low isotopic “spikes” that correspond in time to known tropical cyclone rain events near the cave site. Decadal variations in recurrence intervals calculated from the speleothem record and Belize Weather Service data for the period of interest agree well. The speleothem correctly shows increased landfall frequency during the 1990’s compared to the quiescent decade of the 1980’s. This speleothem proxy is capable of resolving multiple storm events within a single year, separated by months to weeks. We also investigate the sensitivity of this new proxy to storm intensity and/or storm track proximity to the cave site. With further calibration of this approach, speleothems can provide a useful new tool for paleotempestology.