Collaborative Research: Surface Uplift and Climate Change in the Southern Altiplano: Evaluating Mechanisms for Surface Rise and the Effects of Tectonics on Climate

这项研究将结合一种新的碳酸盐环境地温计（基于碳酸盐矿物晶格中碳-12和氧-18的结块），以及对土壤碳酸盐、湖泊碳酸盐和哺乳动物化石牙齿的稳定同位素组成的常规分析，来研究安第斯高原中新世的气候和海拔历史。以前在安第斯高原的研究是基于古植物学数据、沉积碳酸盐的三角洲氧-18和新的块状同位素方法来估计古海拔的。然而，这些研究通过假设一个类似于今天的基本气候，以及今天的海拔、年平均地表温度和大气水体的δ氧-18之间的关系来估计海拔。这种假设是不确定的来源，除非蒸汽源的温度或起始成分可以受到限制。这项研究的一部分将建立一个来自亚安第斯山脉的低海拔气候记录，与来自Altiplano的古海拔记录进行比较。通过这种方法，将根据在安第斯山脉观测到的长期气候变化进行校准，从而获得更准确的海拔记录。重要的是，这种新方法将允许研究整个高原，包括其干旱的南半部，这部分地区一直拒绝尝试仅根据表面碳酸盐的三角洲氧-18来确定古海拔。随着对安第斯山脉地表抬升幅度时空变化的较好分辨率，本研究的更广泛意义将是理解导致山带地表抬升的过程，包括地壳和地幔岩石圈的增厚以及致密、增厚的下岩石圈的移除。最近对北高原抬升史的研究表明，在中新世晚期发生了快速的地表抬升（300万年约3公里），晚于上地壳褶皱和断裂导致的地壳增厚。有证据表明，在超过5度纬度的安第斯山脉宽度上，有类似年龄的地表隆起，这意味着地幔岩石圈的移除是地表隆起的重要机制。然而，独立的数据只预测了南Altiplano和Puna的地幔岩石圈移动。记录安第斯山脉沿走向地表隆起的时间和幅度将揭示这些过程的综合作用以及它们的区域范围。这项工作的其他更广泛的影响是检查造山对区域气候的影响。该项目将考察高原不同纬度地区中新世中期至近期气候的空间变化，以及安第斯山脉东侧从低海拔到高海拔的变化，以评估地表隆起对当地气候的潜在影响。一项对中新世晚期到现代大型依赖水的食草动物的牙釉质的初步研究表明，这些动物摄入的地表水成分与现代降雨相似。这些数据将被扩展到中新世中期，以评估晚中新世到现代的降雨模式是否由区域地表隆起引起。此外，还将评估低海拔和高海拔降水量的时间变化，以验证晚中新世高原地表隆起强迫气候变化的假设，表现为低海拔降雨量增加，同时高海拔降雨量减少。该研究将为通过团块碳-12和氧-18古测温法重建过去温度和常规稳定同位素研究重建降水模式重建过去气候和古海拔提供一个新的和广泛适用的方法模型。

This study will examine the Miocene climate and elevation history of the Andean plateau by combining a new carbonate environmental geothermometer, based on clumping of carbon-12 and oxygen-18 into bonds with each other in the carbonate mineral lattice, with conventional analyses of the stable isotope compositions of soil carbonates, lake carbonates, and fossil mammal teeth. Previous studies in the Andean plateau have estimated paleoelevations based on paleobotanical data, the delta oxygen-18 of sedimentary carbonates, and the new clumped isotope method. However, these studies estimate elevation by assuming a base level climate similar to today, with today's relationships among altitude, mean annual surface temperature, and the delta oxygen-18 of meteoric waters. This assumption is a source of uncertainty unless the temperature or starting composition of the vapor source can be constrained. Part of this study will establish a low elevation climate record from the Subandes against which the paleoelevation records from the Altiplano can be compared. By this approach, a more accurate record of elevation will be extracted by calibrating against secular variations in climate observed in the Subandes. Importantly, this new approach will allow study of the entire plateau, including its arid southern half, which has resisted attempts to determine paleoelevation based on delta oxygen-18 of surface carbonates alone.With a better resolution of the spatial and temporal variation in the magnitude of surface uplift of the Andes, the broader significance of this study will be an understanding of processes that lead to surface uplift of mountain belts, including thickening of crustal and mantle lithosphere and removal of dense, thickened lower lithosphere. Recent studies of the elevation history of the northern Altiplano suggest that rapid surface uplift ( approximately 3 km in 3 million years) occurred in the late Miocene, postdating major crustal thickening by upper crustal folding and faulting. There is evidence of similar-age surface uplift across the width of the Andes for over greater 5 degrees of latitude, implicating the removal of mantle lithosphere as an important mechanism for surface uplift. However, independent data have only predicted mantle lithosphere removal in the southern Altiplano and Puna. Documentation of the timing and magnitude of surface uplift along strike in the Andes will shed light on the combined roles of these processes as well their regional extent.Additional broader impacts of this work are to examine the effects of mountain building on regional climate. This project will examine the spatial variations in climate from middle Miocene to recent over different latitudes in the Altiplano, as well as from low to high elevation along the eastern flank of the Andes, to evaluate the potential influence of surface uplift on local climate. A pilot study of the tooth enamel of late Miocene to modern large, water-dependent grazers shows that these animals were ingesting surface water similar in composition to modern rainfall. These data will be extended back to the middle Miocene to evaluate whether the Late Miocene to modern pattern of rainfall resulted from regional surface uplift. In addition, temporal changes in low elevation and high elevation precipitation amount will be evaluated to test the hypothesis that late Miocene surface uplift of the Altiplano forced climate change that was manifested by an increase in low elevation rainfall amount and a simultaneous decrease in high elevation rainfall amount. This study will provide a model for a new and widely applicable approach to reconstructing past climates and paleoelevation through the clumped carbon-12 and oxygen-18 paleothermometry method to reconstruct past temperatures and conventional stable isotope studies to reconstruct patterns of precipitation.