Testing novel isotope approaches to reconstruct past precipitation regimes in the Amazon

南美夏季季风为亚马孙盆地带来了大量降水，为其森林和生计提供了重要的生命线。然而，气候变化和不断增加的二氧化碳水平将如何影响亚马逊地区的降水和森林仍不确定。由于气候模型的预测差异很大，而且我们对亚马逊气候响应的了解仍然有限，因此可以通过重建过去的气候响应来获得对亚马逊水文循环的重大洞察。实现这一目标的一种方法是在降水中使用稳定的同位素。自然界中，水含有不同丰度的轻同位素和重同位素。当水在水循环中流动时，重同位素优先被雨水带走。因此，降水中的同位素比率是降雨量的有用记录。降水中的各种自然同位素档案，如树木年轮和洞穴碳酸盐岩层，已被用来重建过去的气候，但每一种方法都有其缺点。树木年轮具有较高的时间分辨率，但周期较短，还受植物生理过程的影响。相比之下，洞穴覆盖了数千年，但时间分辨率很差。因此，结合洞穴记录和树木年轮记录，可以更真实地重建降水，并提供有关树木对二氧化碳变化的反应的信息。在这项提议中，我们将在英国和巴西两国科学家之间建立新的合作伙伴关系，致力于两个重要的天然替代品：树木年轮和洞穴中的同位素，以实现以下主要目标：i)测试新的(但尚未得到证实的)方法，将树叶蒸发引起的树木年轮中的同位素变化从降水同位素信号中分离出来；ii)组织一个研讨会，汇集亚马逊和邻近地区研究古气候的科学家网络，目的是改进亚马逊气候重建并在空间上整合。最终目的是为了更好地了解与南美洲季风制度有关的亚马逊降水过去和未来的变化。我们将按如下方式进行。我们首先使用新的树木年轮记录来测量纤维素分子中特定位置的同位素比率，并同时测量特定木材成分(例如木质素)的同位素比率。然后，我们将这些信号与研究地点过去七到八年的实际降雨量的水同位素记录进行比较，以确定哪些位置或哪些成分最准确地记录了降雨信号与树叶蒸发信号。这项工作将受益于独特的降水同位素记录(在两个地点)和高分辨率洞穴记录(在一个地点)，这些记录在过去几年中一直并仍在由参与实验室收集。这项工作还将包括新的树木年轮数据收集和对树木水分利用和生长动态的实时监测。拟议的分析方法非常先进，将受益于英国同位素实验室测量化合物比质谱学和使用核磁共振波谱仪的独特特定设施。组建的团队最大限度地利用了集团的专业知识和设施之间的协同效应。英国团队多年来一直致力于树木年轮中的同位素研究，证明这些方法可以用来重建亚马逊降水，而巴西团队则成功地利用洞穴生态系统重建了长时间尺度(过去数千年)的南美洲气候。拟议的试点项目将使人们有可能更真实地重建亚马逊过去的降水模式，也可以重建树叶在树木中的富集史，从而重建树木对高二氧化碳世界的反应。这些都是未来协作的理想主题。

The South American summer monsoon brings vast amounts of precipitation to the Amazon basin, providing an important lifeline to its forest and livelihoods. It remains uncertain however how climate change and increasing CO2 levels will affect Amazonian precipitation and forests. As climate models vary widely in their predictions and as our understanding of Amazon climate responses is still limited, significant insight in the Amazon hydrological cycle can be gained from reconstructions of climate responses in the past. One approach to achieve this is by using stable isotopes in precipitation. Naturally water contains different abundances of light and heavy isotopes. As water moves through the hydrological cycle the heavy isotope is preferentially rained out. Isotope ratios in precipitation are therefore a useful recorder of the amount of rainfall. Various natural archives of isotopes in precipitation, like tree rings and speleothems (cave carbonate formations), have been used to reconstruct past climate, but each of these methodologies have their shortcomings. Tree rings are of high time resolution but cover short periods and are affected also by plant physiological processes. In contrast, speleothems cover thousands of years but time resolution is poor. Thus, combining speleothem records and tree ring records permits more faithful reconstructions of precipitation and provides also information about tree responses to changes in CO2. In this proposal, we will build a new partnership between UK and Brazilian scientists working on two important natural proxies: isotopes in tree rings and speleothems to pursue the following primary aims i) to test new - but not yet proven - methods to separate isotope variation in tree rings due to leaf evaporation from the precipitation isotope signal ii) to organise a workshop to assemble a network of scientists working on paleoclimate in the Amazon and adjacent regions, with the aim to improve and spatially integrate Amazon climate reconstructions. The ultimate aim is to gain a better understanding of past and possibly future variation in Amazon precipitation associated with monsoon regime over South America. We will proceed as follows. We first use new tree ring records to measure isotope ratios at specific positions within the cellulose molecule, and, in parallel, in specific wood constituents (e.g., lignin). We then compare these signals with water isotope records of the actual rainfall from the study site covering past seven to eight years to determine what positions or which constituents most accurately record rainfall signals versus leaf evaporation signals. This work will profit from unique precipitation isotope records (at two sites) and high-resolution speleothem records (at one site) which have been and are still being collected by the participating labs over the last years. The work will also include new tree ring data collections and real-time monitoring of tree water use and growth dynamics. The proposed analytical procedures are highly advanced and will benefit from unique specific facilities of the UK isotope labs to measure compound specific mass spectrometry and the use Nuclear Magnetic Resonance spectrometry. The assembled team makes maximum use of the synergies between the groups' expertise and facilities. The UK team has worked for several years on isotopes in tree rings proving these methods can be used to reconstruct Amazon precipitation, while the Brazilian team has successfully used speleothems to reconstruct South American climate over long time scales (past thousands of years). The proposed pilot project will open the possibility to reconstruct more faithfully past precipitation patterns of the Amazon but also to reconstruct the history of leaf enrichment in trees and thus tree responses to a high CO2 world. These are ideal topics for future collaboration.