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

南美洲夏季季风为亚马逊盆地带来大量降水，为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河流域的降水和森林仍然不确定。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，因此可以从过去的气候响应重建中获得对亚马逊水文循环的重要见解。实现这一点的一种方法是在沉淀中使用稳定同位素。自然水含有不同丰度的轻和重同位素。当水通过水文循环时，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。降水中同位素的各种自然档案，如树木年轮和洞穴沉积物（洞穴碳酸盐地层），已被用于重建过去的气候，但这些方法都有其缺点。树木年轮的时间分辨率高，但覆盖时间短，也受到植物生理过程的影响。相比之下，洞穴沉积物覆盖数千年，但时间分辨率很差。因此，结合洞穴沉积物记录和树木年轮记录允许更忠实的降水重建，并提供有关树木对CO2变化的反应的信息。在这项提案中，我们将在英国和巴西科学家之间建立一个新的伙伴关系，致力于两个重要的自然代理：树木年轮和洞穴沉积物中的同位素，以实现以下主要目标：i）测试新的（但尚未得到证实的）方法，将树木年轮中因树叶蒸发而产生的同位素变化与降水同位素信号分开; ii）组织一次研讨会，召集一个在亚马逊和邻近地区从事古气候研究的科学家网络，目的是改善亚马逊气候重建并在空间上加以整合。最终目的是更好地了解过去和未来可能的变化，亚马逊降水与季风制度在南美洲。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内特定位置的同位素比率，同时，在特定的木材成分中（例如，木质素）。然后，我们比较这些信号与水同位素记录的实际降雨量从研究地点涵盖过去七到八年，以确定什么位置或哪些成分最准确地记录降雨信号与叶蒸发信号。这项工作将受益于独特的降水同位素记录（在两个地点）和高分辨率洞穴沉积物记录（在一个地点），这些记录在过去几年中一直由参与实验室收集并仍在收集。这项工作还将包括新的树木年轮数据收集和树木用水和生长动态的实时监测。拟定的分析方法非常先进，将受益于英国同位素实验室独特的特定设施，以测量化合物特定质谱法和使用核磁共振光谱法。集合的团队最大限度地利用了各小组的专业知识和设施之间的协同作用。英国研究小组多年来一直致力于研究树木年轮中的同位素，证明这些方法可以用来重建亚马逊降水，而巴西研究小组已经成功地利用洞穴沉积物重建了长时间尺度（过去数千年）的南美气候。拟议的试点项目将打开的可能性，更忠实地重建亚马逊过去的降水模式，但也重建树木的树叶富集的历史，从而树的反应高CO2的世界。这些都是未来合作的理想主题。

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.