Quantitative Salinity & Water D/H from Paired H & C Isotopes in Mangrove Lipids

在过去的半个世纪里，全球降雨模式一直在发生变化，影响着世界各地人民的生活。对这些变化的充分了解还没有出现，但它们与大气中温室气体积聚导致的全球气温变暖有关。在更广泛的降水变化中，热带地区发生了变化，那里的潮湿地区变得越来越潮湿，而干燥地区变得更加干燥。从数百年或数千年的时间尺度来看，过去30年观察到的热带降雨量变化是否不寻常，目前尚不得而知。因此，到目前为止还不可能说最近的降水趋势是否超出了自然变化的范围。任何这样的结论都需要记录一千年或更久以前热带地区的降雨量。但目前，在雨量计和卫星出现之前，几乎没有任何方法可以准确地重建地球上大多数地区的降雨量，特别是在热带海洋地区。该项目的目标是开发一种方法来确定水的盐度和重氢比，这两个参数与热带地区的降雨量密切相关，可以从海洋和湖泊沉积物岩心中提取红树林类脂的化学成分。红树林是适应咸水生活的非凡树木。它们生活在全球热带和亚热带的潮间带。建议的研究方法是基于研究人员最近的观察，即红树林树叶的脂肪中的氢(2H)/氢(1H)和碳-13(13C)/碳-12(12C)的比率随着它们生活的水的盐度的变化而发生系统的变化。这项研究将通过在实验室中在不同盐度范围内种植红树林来校准多种红树林的反应。然后，研究人员将测试这些校准在应用于多种红树林物种时是否一致，这些物种居住在热带帕劳岛盐碱湖的海岸线上，这些盐度跨越了广泛的盐度。由于红树林类脂的氢同位素组成包含了红树林生活的水的氢与氢的比率，以及红树林生活的水的盐度，而碳同位素只对盐度作出反应，因此可以同时计算近岸水域的盐度和氢同位素比率。从这两个参数可以很好地估计降雨量，因为随着降雨量的增加，沿岸水的盐度和重氢比都会降低。当对从沿海环境和海洋湖泊的沉积物岩芯中提取的红树林类脂进行测量时，将有可能重建过去的降雨量估计，从而提供所需的长期观点，以得出热带降雨的最近趋势是否超出自然变化范围。该项目由美国国家科学基金会海洋科学部门的海洋地质和地球物理计划联合支持。

Global patterns of rainfall have been changing over the last half-century, impacting the lives of people worldwide. A full understanding of these changes has not yet emerged, but they have been linked to a warming of global temperatures in response to the buildup of greenhouse gases in the atmosphere. Among the more widespread changes in precipitation have occurred in the tropics, where wet regions have been getting wetter and dry regions drier. It remains unknown whether the changes in tropical rainfall observed over the last 30 years are unusual when viewed on a multi-century or multi-millennial time scale. As yet it is therefore not possible to say whether recent precipitation trends are outside the range of natural variability. Any such conclusion will require records of rainfall from the tropics extending back in time a thousand years or more. But at present there are few, if any methods to accurately reconstruct rainfall in most locations on the planet prior to the advent of rain gauges and satellites, especially in the maritime tropics. The goal of this project is to develop a method for determining the salinity and deuterium-to-hydrogen ratio of water, two parameters very closely linked to rainfall in the tropics, from the chemistry of mangrove lipids that can be recovered from ocean and lake sediment cores.Mangroves are remarkable trees that have adapted to living in saltwater. They inhabit the intertidal zones of the global tropics and subtropics. The approach of the proposed study is based upon the investigator's recent observation that the ratio of both deuterium (2H) to hydrogen (1H) and carbon-13 (13C) to carbon-12 (12C) in the lipids of mangrove leaves changes systematically when the salinity of the water in which they live changes. This study will calibrate that response in multiple species of mangroves by growing them in the laboratory at a range of different salinities. The investigator will then test whether those calibrations are consistent when applied to multiple mangrove species inhabiting the shorelines of saline lakes on the tropical island of Palau that span a wide range of salinities. Because the hydrogen isotope composition of mangrove lipids incorporates the deuterium-to-hydrogen ratio of the water in which the mangroves live, as well as the salinity of that water, and the carbon isotopes only respond to the salinity, it is possible to calculate both the salinity and the hydrogen isotope ratio of the coastal water. From these two parameters one can make good estimates of rainfall, since both the salinity and the deuterium-to-hydrogen ratio of coastal water decreases when rainfall increases. When measured on mangrove lipids extracted from sediment cores taken in coastal settings and maritime lakes it will be possible to reconstruct rainfall estimates back in time, thus providing the long-term view needed to conclude whether recent trends in tropical rainfall are outside the range of natural variability.This project is supported jointly by the Marine Geology and Geophysics Program in the NSF Division of Ocean Sciences.