Collaborative Research: Miocene Climate Change West of the Central Andes: Cause or Consequence of Uplift?

这项工作的主要目标是记录沿中央安第斯山脉西侧大范围(约1000公里)的中新世(2300万年前至500万年前)降水的时空变化，即今天的极度干旱的阿塔卡马沙漠。虽然以前的数据显示，在这段时间间隔内气候发生了重大变化，但这些数据不足以确定区域尺度一级变化模式的规模和时间。最近，有人提出，中安第斯山脉的急剧上升是南美洲西部边缘干旱化的结果，而不是原因。这一假说提出了气候和构造之间的基本地球动力学关系，并通过海沟的沉积物充填对板块界面的力学性质的影响来实现。但是，更全面地纳入板块边界-安第斯系统动力学的相互竞争的模型得出的结论是，与气候有关的现象虽然耐人寻味，但并不具有一级重要性。该项目开发了一段中新世古气候历史，作为对这些相互竞争的假说的严格测试。在这个项目中，古土壤被用作古气候的替代物。用于估计古降水的是各种土壤形态和地球化学特征，包括盐碱地胶结物(硝酸盐、硫酸盐、碳酸盐)的化学成分、粘土层的化学风化比率和土壤碳酸盐的同位素值，所有这些特征都与今天智利0至400毫米/年的降雨量高度相关。这个项目从阿塔卡马沙漠中心的一个试点项目建立到对降水变化发生时间的区域尺度描述，这使人们能够辨别中央安第斯山脉西缘的变化是同步的还是历时的，是在地表抬升之前还是之后，以及气候波动在中新世是否多次发生。除了检验地球动力学模型外，对古气候变化的时空模式的新认识还允许将中央安第斯山脉西部的气候历史与中央安第斯山脉东部边缘的气候历史进行比较，以测试干旱是由地表抬升还是由全球气候变化引起的。区域尺度的气候变化是导致山脉带海拔上升，还是山脉带的抬升导致大陆气候变化？这个“鸡和蛋”问题的解决激发了对智利北部安第斯山脉阿塔卡马沙漠气候和山脉生长历史的研究。气候与山脉和整个山脉的建设之间的正反馈关系是当前地质科学研究的主题。阿塔卡马沙漠提供了一个无与伦比的机会来研究地球气候光谱中非常干燥的一端的反馈过程。将从古代土壤属性中提取的古代降水数据库在记录南美洲内陆的长期历史方面也很有价值，包括具有高度生物多样性的亚马逊盆地是随着全球气候变化还是安第斯山脉抬升引起的气候变化而演变的。这些不确定性与关于今天的气候变化是由自然现象还是人为活动造成的辩论中的不确定性相当。通过美国本科生和研究生的参与，该项目培训下一代地质学家，研究土壤、植被、气候和地形之间的相互关系。通过智利教授和研究生的参与，该项目为学生提供了尊重国际合作的模式和实践，并更有效地实现了项目目标。支持该项目的资金由NSF构造计划和NSF国际科学与工程办公室的美国计划提供。

The primary objective of this work is to document temporal and spatial variations of Miocene (23-5 Million years ago) precipitation along an extensive region (~1000 kilometers) of the western flank of the Central Andes that is today's hyperarid Atacama Desert. Whereas previous data reveal that there was major change in climate over that time interval, the data were insufficient to resolve the magnitude and timing of the regional scale first-order pattern of change. Recently, it was proposed that the dramatic rise of the Central Andes was the consequence of, rather than the cause for, the aridification of the western margin of South America. That hypothesis invokes a fundamental geodynamic relationship between climate and tectonics, coupled through the affect of sediment fill of the trench on the mechanical properties of the plate interface. But competing models that more comprehensively incorporate the dynamics of the plate boundary-Andes system conclude that climate-related phenomena, while intriguing, are not of first-order importance. This project develops a Miocene paleoclimate history that serves as a rigorous test of these competing hypotheses. Paleosols are used in this project as a paleoclimate proxy. Used to estimate paleo-precipitation are a variety of soil morphological and geochemical characteristics, including the chemical composition of saline soil cements (nitrate, sulfate, carbonate), chemical-weathering ratios of argillic horizons, and isotopic values of soil carbonate, all features that are highly correlated with precipitation between 0 and 400 millimeters/year in Chile today. This project builds from a pilot project in the center of the Atacama Desert to a regional-scale description of when precipitation change occurred, which allows one to discern whether the change was simultaneous or diachronous along the western margin of the Central Andes, whether it preceded or followed surface uplift, and whether climate fluctuations occurred multiple times during the Miocene. In addition to testing the geodynamic models, the new knowledge of spatial and temporal patterns of paleoclimate change allow comparison of the western Central Andes climate history to that of the eastern margin of the Central Andes, to test whether the aridification was caused by surface uplift or by global climate change.Does regional-scale climate change cause mountain belts to rise in elevation, or does the rise of mountain belts cause the climate of a continent to change? Resolution of this "chicken-and-egg" question motivates this study of the history of climate and mountain growth in the Atacama Desert of the Andes Mountains of northern Chile. Positive feedback relations between climate and construction of mountain ranges and entire mountain chains are current topics of research in geological sciences. The Atacama Desert affords an unparalleled opportunity to examine the feedback processes at the very dry end of Earth's climate spectrum. The database for ancient precipitation that will be extracted from ancient soil properties is also valuable in documentation of the long-term history of interior South America, inclusive of whether the Amazon basin, with its high degree of biodiversity, evolved in response to global climate change or to climate change induced by uplift of the Andes Mountains. The uncertainties are comparable to those in the debate about whether today's climate changes are caused by natural phenomena or anthropogenic activity. Through participation of U.S. undergraduate and graduate students, the project trains next-generation geologists to study interrelations between soils, vegetation, climate, and topography. Through participation of a Chilean professor and graduate student, the project provides models of and practice in respectful international collaboration for the students, and more efficiently fulfill the project objectives. Funds to support this project are being provided by the NSF Tectonics Program and the America's Program of the NSF Office of InternationalScience and Engineering.