Collaborative Research: Investigating Inter-Hemispheric Phasing of Tropical Andean Hydroclimate in Response to Holocene Orbital Forcing

本研究的主要目的是探讨最近10000年（全新世）热带南美洲水文气候的变化机制。二十多年来，热带雨带（也称为热带辐合带；ITCZ）位置的纬度（南北）变化对地球表面太阳能分布变化的响应被视为热带南美洲水文气候的主要驱动因素。然而，越来越多的热带安第斯山脉的湖泊水位重建表明，安第斯山脉水文气候的变化并没有密切跟踪太阳能的长期变化。相反，干旱和雨（湿阶段）之间的突变和显著变化持续了几个世纪到几千年。全球太阳能分布的逐渐变化如何导致热带安第斯山脉水文气候的突然而持久的变化仍然是一个悬而未决的问题。确定过去造成安第斯山脉气候突变的机制，对于预测未来气候变化对该地区的影响方式具有深远意义，该地区为11个国家的3.5亿多人提供淡水资源。由于热带气候变率直接影响到包括北美在内的中纬度地区，这项研究将进一步提供对大规模气候系统相互作用的关键见解，这将提高我们对中纬度气候未来如何变化的理解。研究人员将通过对哥伦比亚安第斯山脉东部科迪勒拉的高海拔湖泊Tota湖的沉积物进行生物、地球化学和沉积学分析，调查全新世热带安第斯水文气候。研究人员将利用这些数据开发年平均温度（MAT）、年平均降水（MAP）、降水和湖水的氢同位素组成以及湖泊水位的代理记录。之所以特别选择托塔湖进行这项工作，是因为之前的地球物理和沉积物岩心研究表明，它包含了一层厚厚的全新世沉积物序列，这些沉积物记录了上述水文气候代用物，可以用辐射测量技术准确地确定年代。这些数据将提供对未被充分研究的南美洲ITCZ核心区域的水文气候对外部强迫的响应的见解。将代理数据与湖泊和气候模式模拟以及现有的古气候记录相结合，研究人员将验证全新世安第斯山脉有效水分（降水和蒸发之间的平衡）是由MAP（受ITCZ动力学控制）和年平均蒸发量（受MAT控制）对半球夏季日照分布的响应驱动的假设，以及北半球（NH）安第斯山脉有效水分通常与南半球（SH）安第斯山脉反相的假设。在全新世早期，当ITCZ处于最北端时，安第斯山脉的MAP值略低，而在全新世中期早期，随着ITCZ向南迁移，MAP值升高，随着ITCZ进一步向南迁移，MAP值普遍降低，夏季日照在NH减少，SH增加。MAE的变化，在MAT的调节下，减缓或增强了ITCZ驱动的MAP值的变化，导致了非日照样的有效湿度趋势。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The primary objective of this research is to investigate the mechanisms of tropical South American hydroclimate variably during the last 10,000 years (the Holocene). For more than two decades, latitudinal (north-south) changes in the position of the tropical rain belt (also referred to as the Intertropical Convergence Zone; ITCZ) in response to variations in the distribution of solar energy across the Earth’s surface have been viewed as the primary driver of tropical South American hydroclimate. A growing number of lake-level reconstructions from the tropical Andes, however, show that changes in Andean hydroclimate did not closely track long-term changes in solar energy. Instead, there were abrupt and marked changes between drought and pluvial (wet phases) conditions that lasted for centuries to millennia. How gradual changes in the global distribution of solar energy produced abrupt and long-lasting changes in tropical Andean hydroclimate remains an open question. Determining the mechanisms responsible for abrupt Andean climate changes in the past has profound implications for predicting the ways in which future climate change will impact this region which provides freshwater resources to over 350 million people across 11 countries. Because tropical climate variability directly impacts mid-latitude regions, including North America, this research will additionally provide critical insight into large-scale climate system interactions that will improve our understanding of how mid-latitude climate will change in the future.The researchers will investigate Holocene tropical Andean hydroclimate by conducting biological, geochemical, and sedimentological analyses of sediment from Lake Tota, a high-altitude lake in the eastern cordillera of the Colombian Andes. The researchers will use these data to develop proxy records of mean annual temperature (MAT), mean annual precipitation (MAP), the hydrogen isotopic composition of precipitation and lake water, and lake levels. Lake Tota was specifically selected for this work because previous geophysical and sediment core research shows that it contains a thick sequence of Holocene sediments that archive the aforementioned hydroclimate proxies and can be accurately dated with radiometric techniques. These data will provide insight on hydroclimate responses to external forcing in the understudied core ITCZ region of South America. Integrating the proxy data with lake and climate model simulations and existing paleoclimate records, the researchers will test the hypothesis that Holocene Andean effective moisture (the balance between precipitation and evaporation) was driven by MAP (as controlled by ITCZ dynamics) and mean annual evaporation (as controlled by MAT) responses to the hemispheric distribution of summer insolation and that Northern Hemisphere (NH) Andean effective moisture was generally anti-phased with the Southern Hemisphere (SH) Andes. For the NH, it is hypothesized that Andean MAP was slightly lower during the early Holocene when the ITCZ was at its northerly most position, increased during the early middle Holocene as the ITCZ migrated southward, and generally lessened as the ITCZ migrated further southward as summer insolation decreased in the NH and increased in the SH. Changes in MAE, as moderated by MAT, mitigated or enhanced changes in ITCZ-driven MAP, resulting in non-insolation-like effective moisture trends.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.