Quantifying the Cenozoic Oxygen Isotopic Variability of Precipitation on the Andes: A Test of Stable Isotope Paleoaltimetry and Plateau Uplift

安第斯高原的形成还没有被很好地理解，部分原因是高原隆起的时间和速率没有得到很好的约束。基于古气候和温度计数据的估计与大约1300万年前形成的现代宽度的高原是一致的。相比之下，基于稳定同位素古测高法的估计（一种相对较新的技术，利用古土壤碳酸盐的氧同位素浓度（δ - 18o）来推断过去的海拔变化）表明，大约在1000万到700万年前出现了一次快速上升。同位素古海拔测量是一种很有前途的量化海拔变化的新工具，但一些突出的问题可能使其解释复杂化。特别是近4000万年来地表温度、大气环流、降水速率和蒸汽源的变化可能影响古代土壤碳酸盐的氧同位素浓度。这个项目的目的是评估控制安第斯山脉三角洲- 18o降水的过程，并检验安第斯高原在过去2000万年里抬升实际上是稳定的假设。中新世晚期高原的明显快速上升可能是新生代气候和大气环流变化的产物，这些变化导致了安第斯降水的减少。具有预测氧和氘同位素输运和分等能力的全球和区域气候模式为量化安第斯高原上的这些影响提供了一个工具。然而，在这些模型能够可靠地使用之前，它们必须通过对降水中氧同位素的现代观测进行验证，这些观测目前在安第斯山脉很少得到。在这个项目中，一项横跨秘鲁中部和玻利维亚南部的多年采样活动测量了降水中氧同位素组成的月度变化。当这些数据与现有的测量、气象数据和回溯分析相结合时，将为评估气候模式对降水氧同位素组成的预测提供关键数据，并评估瑞利定律是否能充分预测安第斯山脉的同位素分选。验证的区域和全球气候模式用于预测安第斯三角洲- 18o组成的过去变化，并评估过去气候变化对用于古高程研究的土壤碳酸盐降水三角洲- 18o的影响。安第斯高原是地球上最引人注目的地形特征之一。尽管它的性质令人印象深刻，但人们对其形成的机制和速度仍然知之甚少。人们提出了一系列相互竞争的地球动力学模型来解释高原的形成。为了区分这两种模式，有必要了解与高原及其边缘冲断带有关的变形和侵蚀历史、现今地壳和岩石圈结构以及高原的古高程历史。其中，目前对安第斯古海拔的了解可以说是最不确定的，但却为高原的形成提供了很好的见解。古海拔历史可以用来推断造山的速度和时间，并最终推断控制造山的地球动力学过程。该项目的研究成果将对同位素古高程技术进行重要评价，并对安第斯高原地表隆升历史的计算提供改进。

The formation of the Andean Plateau is not well understood partly because the timing and rates of plateau uplift are poorly constrained. Estimates based on paleoclimate and thermochronometer data are consistent with formation of a plateau of modern width by about 13 million years ago. In contrast estimates based on stable isotope paleoaltimetry, a relatively new technique that employs the oxygen isotope concentration (delta-18O) of ancient soil carbonates to infer past elevation change, suggest a rapid uplift about 10 to 7 million years ago. Isotope paleoaltimetry is a promising new tool for quantifying elevation change, but several outstanding issues may complicate its interpretation. In particular, changes over the last 40 million years in surface temperature, atmospheric circulation, precipitation rate, and vapor source may affect the oxygen isotopic concentration of ancient soil carbonates. If substantial, these effects could compromise paleoelevation inferences based on the isotopes The objective of this project is to evaluate the processes that control delta-18O of precipitation in the Andes and to test the hypothesis that Andean Plateau uplift was in fact steady over the last 20 million years. The apparent rapid rise of the plateau in the late Miocene may be an artifact of changes in Cenozoic climate and atmospheric circulation that caused depletion of Andean precipitation 18O. Global and regional climate models with the capability to predict oxygen and deuterium isotope transport and fractionation provide a tool for quantifying these effects over the Andean Plateau. However, before these models can be used with any confidence, they must be validated against modern observations of oxygen isotopes in precipitation, which are currently sparsely available in the Andes. In this project, a multi-year sampling campaign across central Peru and southern Bolivia measures monthly variations in the oxygen isotope composition of precipitation. These data, when integrated with existing measurements, meteorological data, and backtracking analyses will provide critical data for assessing climate model predictions of precipitation oxygen isotopic composition and evaluate whether a Rayleigh law adequately predicts isotope fractionation on the Andes. The validated regional and global climate models are used to predict past changes in Andean delta-18O composition and to assess the influence of past climate changes on precipitation delta-18O used in soil carbonates for paleoaltimetry studies.The Andean Plateau is one of the most dramatic topographic features on Earth. Despite its impressive nature, the mechanisms and rates of its formation remain poorly understood. A range of competing geodynamic models have been proposed to explain the formation of plateaus. To distinguish between these models it is necessary to understand the deformation and erosion history associated with the plateau and its marginal thrust belts, the present day crustal and lithospheric structure, and the paleoelevation history of the plateau. Of these, current understanding of Andean paleoelevation is arguably the most uncertain, and yet offers great insights into the formation of the plateau. The paleoelevation history can be used to make inferences about the rate and timing of mountain building, and ultimately the geodynamic processes that govern mountain building. The results of this project will provide an important evaluation of the isotope paleoaltimetry technique and provide improved calculations of the surface uplift history of the Andean Plateau.