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

本研究将调查乔治亚州南部、南卡罗来纳州和德克萨斯州沿海松树年轮中α纤维素（木材组织的基本成分）氧同位素比值的潜在用途，作为飓风活动的代理记录。树轮α -纤维素有机氧同位素主要反映了源水的同位素组成。在浅根针叶树中，源水与降水直接相关。飓风产生的大量降水的氧同位素比率明显低于正常的地表水，而这种低同位素比率可能会在地表水和土壤水中持续数周。这些异常轻的氧同位素组成很可能与树轮纤维素结合在一起，捕捉到飓风活动的同位素记录。飓风通常在8月到10月期间影响美国东南部，这几个月对应于单个树木年轮的晚木部分。飓风活动可能导致晚木d18O值为负（氧-18与氧-16相对于国际公认标准的比值），并且在年轮中早木和晚木的同位素组成差异较大。在没有明显飓风活动的年份，早木-晚木的差异预计可以忽略不计。研究人员首先将单个树木年轮分成早木和晚木的次年轮，然后去除树脂。α纤维素的氧同位素组成将使用高温碳还原元素分析仪连接到同位素比质谱仪进行测量。由于干旱可能潜在地掩盖飓风引起的同位素信号，测量的环宽度将与实际的氧同位素值进行比较。发生干旱的年份将导致树木年轮宽度变窄，树木年轮早期部分的氧同位素值异常重。通过结合详细的树木年代学和同位素分析，该项目有可能开发出比现代仪器或历史文献早几个世纪的飓风活动代用记录。这种较长的替代记录将有助于评估飓风活动是否以几十年尺度的周期发生，以及在独立于这些周期的情况下，较长的记录是否表明20世纪后期飓风活动增加。这些结果将有助于完善飓风活动的长期记录，这可能会增强对飓风的长期统计预测，并将有助于确定我们是否可以采取具体行动来减轻飓风频率的潜在增加。

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.