Oxygen Isotope Compositions of Tree-Ring Cellulose as a High-Resolution Proxy Record of Hurricane Activity

University Of Tennessee Knoxville, Knoxville TN

Investigators

Abstract

This research will investigate the potential use of the oxygen isotope ratios of alpha cellulose (the basic component of wood tissue) in tree rings of pines from southern Georgia, South Carolina, and coastal Texas as a proxy record of hurricane activity. Organic oxygen isotopes in tree-ring alpha cellulose mainly reflect the isotopic composition of the source water. In shallowly rooted conifers, the source water is directly related to precipitation. Hurricanes produce large amounts of precipitation with distinctly lower oxygen isotope ratios than normal surface water, and these low isotope ratios may persist in surface and soil water for several weeks. These anomalously light oxygen isotopic compositions are likely incorporated into the tree-ring cellulose, capturing an isotopic record of hurricane activity. Hurricanes most typically affect the southeastern United States during August through October, months that correspond to the latewood portion of the individual tree ring. Hurricane activity may result in negative latewood d18O values (the ratio of oxygen-18 to oxygen-16 relative to an internationally accepted standard) and relatively large differences in the isotopic compositions of earlywood and latewood in an annual ring. In years without significant hurricane activity, earlywood-latewood differences are expected to be negligible. The investigators will first separate individual tree rings into sub-annual segments of earlywood and latewood, then resins will be removed. Oxygen isotope compositions of alpha cellulose will be measured using a high temperature carbon reduction elemental analyzer interfaced to an isotope ratio mass spectrometer. Because drought can potentially mask the isotope signal caused by hurricanes, measured ring widths will be compared with the actual oxygen isotope values. Years in which drought occurred will result in both narrow tree-ring widths and anomalously heavy oxygen isotope values for the earlywood portion of the tree ring. This project has the potential to develop a proxy record of hurricane activity that extends several centuries earlier than modern instrumental or historical documents by combining detailed dendrochronological and isotopic analysis. This longer proxy record will be useful to evaluate whether hurricane activity occurs in multi-decadal-scale cycles and, independently of these cycles, whether the longer record indicates an increase in hurricane activity during the latter 20th century. These results should contribute to a refined, long-term record of hurricane activity that may enhance statistical long-range future forecasts of hurricanes, and will help to determine if there are specific actions we can take to mitigate potential increases in hurricane frequency.