Hydrogen isotope ratios of lacustrine sedimentary n-alkanes as proxies of tropical African hydrology: Insights from a calibration transect across Cameroon
Hydrogen isotope ratios of lacustrine sedimentary n-alkanes as proxies of tropical African hydrology: Insights from a calibration transect across Cameroon
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DOI：
10.1016/j.gca.2011.11.039
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发表时间：
2012-02-15
影响因子：
5
通讯作者：
Sachse, Dirk
中科院分区：
文献类型：
--
作者：
Garcin, Yannick;Schwab, Valerie F.;Sachse, Dirk
利用藻类、细菌和叶蜡等沉积水生和陆生生物标志物的氢同位素值（δ D）记录了中高纬度湖泊环境中古源水（即降水和/或湖水）的同位素特征。然而，在热带地区，与水文系统和植被中同位素分馏相关的两个过程与高纬度地区的过程有很大不同，因此还没有这一替代方法的校准研究。为了填补这一知识空白，我们对喀麦隆11个湖泊的表层沉积物进行了采样，以确定决定水生和陆生脂质生物标志物氢同位素组成的水文气候过程和生理因素。在这里，我们提出了一个强大的框架，化合物特异性氢同位素在热带非洲的应用。结果表明，水体脂质生物标志物n-C-17烷烃的D值与水体D值不相关。碳同位素测量表明，n-C-17烷烃来源于多种来源的生物，这些生物利用不同的氢池进行生物合成。研究表明，n-C-29烷烃的δ D值与地表水（即河水和地下水）的δ D值相关，在大空间尺度上反映了年平均降水的同位素组成。这种关系已经在高纬度地区观察到，支持叶蜡脂δ D代理在半球空间尺度上的稳健性。而n-C-31烷烃的δ D值与蒸发湖水δ D值存在相关性，而与蒸发湖水δ D值存在相关性。这一结果表明，两种叶蜡脂的水源不同，很可能来自两种不同的植物群。这些新发现对解释古湖泊沉积物的长链正构烷烃δ D记录具有重要意义。特别是，对古水文数据的可靠解释需要了解集水区的植被，因为不同的植物可能利用不同的水源。我们的研究结果还表明，碳和氢同位素的组合确实有助于区分生物体的代谢途径和/或生长形式，从而区分脂质生物合成过程中使用的氢的来源。(C) 2011 Elsevier Ltd.版权所有。
Hydrogen isotope values (delta D) of sedimentary aquatic and terrestrial lipid biomarkers, originating from algae, bacteria, and leaf wax, have been used to record isotopic properties of ancient source water (i.e., precipitation and/or lake water) in several mid-and high-latitude lacustrine environments. In the tropics, however, where both processes associated with isotope fractionation in the hydrologic system and vegetation strongly differ from those at higher latitudes, calibration studies for this proxy are not yet available. To close this gap of knowledge, we sampled surface sediments from 11 lakes in Cameroon to identify those hydro-climatological processes and physiological factors that determine the hydrogen isotopic composition of aquatic and terrestrial lipid biomarkers. Here we present a robust framework for the application of compound-specific hydrogen isotopes in tropical Africa. Our results show that the delta D values of the aquatic lipid biomarker n-C-17 alkane were not correlated with the delta D values of lake water. Carbon isotope measurements indicate that the n-C-17 alkane was derived from multiple source organisms that used different hydrogen pools for biosynthesis. We demonstrate that the delta D values of the n-C-29 alkane were correlated with the delta D values of surface water (i.e., river water and groundwater), which, on large spatial scales, reflect the isotopic composition of mean annual precipitation. Such a relationship has been observed at higher latitudes, supporting the robustness of the leaf-wax lipid delta D proxy on a hemispheric spatial scale. In contrast, the delta D values of the n-C-31 alkane did not show such a relationship but instead were correlated with the evaporative lake water delta D values. This result suggests distinct water sources for both leaf-wax lipids, most likely originating from two different groups of plants. These new findings have important implications for the interpretation of long-chain n-alkane delta D records from ancient lake sediments. In particular, a robust interpretation of palaeohydrological data requires knowledge of the vegetation in the catchment area as different plants may utilise different water sources. Our results also suggest that the combination of carbon and hydrogen isotopes does help to differentiate between the metabolic pathway and/or growth form of organisms and therefore, the source of hydrogen used during lipid biosynthesis. (C) 2011 Elsevier Ltd. All rights reserved.