Reconstructing Precipitation Rates from C & H Isotopes in Mangrove Lipids

在过去的35年里，全球降雨模式发生了变化，特别是在热带地区，高降水地区变得更加湿润，而低降水地区变得更加干燥。此外，亚热带干旱区已向极地扩展了约125-250英里。由于气候系统的复杂性和对热带降水机制的不完全了解，这些变化的根本原因尚不清楚。然而，启发式论证和最先进的气候模型都表明，如果温室气体在大气中积累的速度继续超过海洋和陆地生物圈吸收的速度，这种趋势将继续下去。如果有更长的降水记录，尤其是低纬度海域的降水记录，这些预测的可信度就会提高。不幸的是，海洋上的降水记录开始于35年前，与卫星气象时代相吻合。因此，将热带降雨记录追溯到过去的唯一方法是通过间接手段。本项目的目的是开发一种新的方法，从整个热带和亚热带潮间带红树林产生的脂质中氢和碳同位素比值来重建热带和亚热带降雨率。研究小组将继续使地方政府机构和非政府组织参与对原始和受人为影响的系统的快速生态评估，并将为密克罗尼西亚学校教师举办关于红树林和气候的讲习班。资金还支持博士生的教育和培训，并为华盛顿大学的几名本科生提供研究和实地经验。这种确定降雨量的新方法是基于红树林对盐度的敏感生化和生物物理响应，以及由此产生的对红树林叶片中脂质氢和碳同位素组成的影响。最近的研究表明，当盐度增加时，红树林脂质中2H/1H分馏有系统地增加，13C/12C分馏有系统地减少。根据这些经验关系，盐度和生长水的2H/1H比可以通过测量taraxerol的2H/1H和13C/12C比率来确定，taraxerol是一种由Rhizophora（红）红树林产生的叶脂。这些数据将用于计算降水的2H/1H比，由于降水的2H/1H比与热带地区的降雨量之间存在反相关关系，由此可以确定降水率。Taraxerol将从先前收集的红树林沼泽沉积物中提取，这些沉积物来自6个美国附属密克罗尼西亚群岛，横跨2800公里的海洋，现代平均年降雨量为6至14毫米d-1。降水、盐度和降雨率的2H/1H比值将确定：(1)过去10-20年的0-2 cm沉积物，并与仪器资料进行比较；(2)20世纪的2 - 10 cm沉积物，以检验1990年以来卫星获得的降水是否高于20世纪的平均值；(3)小冰期早期（公元1400-1600年）和晚期（公元1600-1800年），以检验在此期间西热带北太平洋的降水是否较低。

Global rainfall patterns have changed over the last 35 years, particularly in the tropics, where high-precipitation regions have become wetter, and low-precipitation regions have become drier. In addition, subtropical dry zones have expanded poleward by some 125-250 miles. The underlying causes of these changes are not well understood, owing to the complexity of the climate system and an imperfect understanding of tropical precipitation mechanisms. However, both heuristic arguments and state-of-the-art climate models suggest that these trends will continue if greenhouse gases continue to accumulate in the atmosphere faster than they can be absorbed by the ocean and land biosphere. Confidence in these projections could be improved with a longer precipitation record, particularly over the low-latitude oceans. Unfortunately, the start of the precipitation record over the ocean began just 35 years ago, coinciding with the satellite weather era. Thus the only way to extend the tropical rainfall record back in time is through indirect means. The purpose of this project is to develop a new approach to reconstruct tropical and subtropical rainfall rates from hydrogen and carbon isotope ratios in lipids produced by mangrove trees inhabiting intertidal regions throughout the tropics and subtropics. The research team will continue to engage local government agencies and non-governmental organizations in rapid ecological assessments of pristine and anthrpogenically-impacted systems, and will conduct workshops on mangroves and climate for Micronesian school teachers. Funding also supports education and training of a PhD student and provides research and field experiences for several undergraduates at the University of Washington.This new method for determining rainfall is based on the sensitive biochemical and biophysical responses of mangrove trees to salinity, and the resultant impact on the hydrogen and carbon isotope composition of lipids in mangrove leaves. Recent studies have shown that when salinity increases, 2H/1H fractionation in mangrove lipids systematically increases, and 13C/12C fractionation systematically decreases. From these empirical relationships, salinity and the 2H/1H ratio of growth water can be determined from measurements of the 2H/1H and 13C/12C ratios of taraxerol, a leaf lipid produced by Rhizophora (red) mangroves. Those data will be used to calculate the 2H/1H ratio of precipitation, from which precipitation rates can be determined, owing to the anti-correlation between precipitation 2H/1H ratios and rainfall amount in the tropics. Taraxerol will be extracted from previously-collected mangrove swamp sediments from six US-affiliated Micronesian islands across 2,800 km of ocean that span modern mean annual rainfall rates of 6 to 14 mm d-1. The 2H/1H ratio of precipitation, salinity, and rainfall rates will be determined for: (1) the last 10-20 years from 0-2 cm sediments and compared to instrumental data, (2) the 20th century from 2~10 cm sediments to test whether satellite-derived precipitation since ~1990 is higher than the 20th century mean, and (3) the early (1400-1600 AD) and late (1600-1800 AD) Little Ice Age to test whether precipitation was lower in the western tropical North Pacific during that time.