OXYGEN ISOTOPE COMPOSITIONS OF TREE RING a-CELLULOSE AS A PROXY FOR PALEO-HURRICANE ACTIVITY

The effect of global warming on hurricane activity in the Atlantic Ocean is a topic of considerable research and speculation. Key to understanding the relationship between hurricane activity and climate is development of geological proxies that extend the record of hurricane activity beyond historical documents. Tree rings may preserve excellent, high-resolution records of climate and climate change. We investigated the potential use of O-isotope ratios of a-cellulose in slash pine (Pinus elliottii) from southern Georgia (30.84°N; -83.25°E) to detect hurricane signals recorded within tree rings. Organic O-isotopes in a-cellulose mainly reflect source water, which is directly related to precipitation and relative humidity; shallow root systems of slash pines result in negligible groundwater influence on the isotopic signal. Upon landfall, rainout of hurricanes and tropical storms lead to an isotopic “amount effect,” depleting ¹⁸O in storm precipitation relative to average rainfall. Slash pine tree rings preserve distinct earlywood (EW; May-June) and latewood (LW; July-October) components that can be separately analyzed. Hurricanes impact southern Georgia mainly in the LW season, therefore we hypothesize that hurricane landfall results in relatively large EW-LW ¹⁸O fractionations (D _EW-LW). In years without a hurricane event, D_EW-LW will be negligible. We analyzed d¹⁸O of a-cellulose in tree rings grown during decades for which we have meteorological records of hurricane impact at the study site. The results support our hypothesis: years of known hurricane activity exhibit large D_EW-LW (e.g., 1958/Hurr. Helen in Sept.-Oct./D_EW-LW=3.17 and 1994/Hurr. Gordon in Nov/D_EW-LW=2.4) whereas non-eventful years record small D_EW-LW (1997/no major storm/D_EW-LW=0.02). Lower intensity extratropical storms (ETS) do not result in a significant D_EW-LW (e.g., 1956/Hurr. Flossy is ETS when it hit southern Georgia in Sept./D_EW-LW=0.75). Other special events that may affect a-cellulose D_EW-LW include hurricanes/TS that occur in June (between EW and LW growth), drought, and ENSO-driven changes in precipitation patterns.