Doctoral Dissertation Research: Developing a Cosmogenic Chronology of Tropical Glaciation in the Peruvian Andes

末次盛冰期（LGM）是古气候学家和气候模拟者相当感兴趣的一个时期，因为它代表了一个从现在开始急剧变化的辐射强迫时期。 虽然地球系统各组成部分所施加的异常辐射强迫，如温室气体浓度、行星辐射和海陆分布，受到相对较好的限制，但热带气候对这种强迫的响应却存在广泛的争议。 冰川记录被用来估计末次冰期期间的热带气候，但由于年代控制不佳，热带冰碛是否真的与大约21，000年前的全球末次冰期相关并因此提供相关的气候信息是不确定的。 此外，在末次冰期，热带冰川的增长通常被认为完全是由温度下降驱动的，但其他气候变量（如降水和辐射）的变化很可能也发挥了重要作用，使冰川记录中包含的气候信号复杂化。 本博士论文的研究项目将根据几个冰碛物的10-铍宇宙成因放射性核素测年法，建立一个秘鲁北方安第斯山脉冰川作用的年代学。 博士生将专注于确定该地区末次冰期最大期的年龄，其时间在南美洲和整个热带地区受到很大限制。 这项研究将采用能量平衡-质量平衡建模方法来模拟产生冰川所需的气候条件范围，这些冰川的几何形状和范围与地质记录中推断的冰川相匹配。 位于山谷上游的冰碛物从LGM存款将可能产生一个全新世或冰消千年尺度的气候变化记录。 至少将对五个冰碛进行采样，并在每个冰碛上对大约十个巨石进行采样，以确保在统计上可靠地确定地貌的年龄。 本研究项目中开发的年表将有助于确定低纬度和高纬度气候事件的阶段性。 这一年表对于分离造成全球气候变化的机制非常重要。 用于预测未来全球变暖的相同气候模型通常通过测试其模拟LGM完全不同的气候的能力来验证。 因此，需要精确的重建末次盛冰期气候，为这些模型提供一个目标。 此外，了解热带地区对辐射强迫的敏感性对于评估热带地区如何应对人为温室气体造成的辐射强迫至关重要。 确定热带和高纬度LGM之间的相位关系将有助于深入了解半球间气候变化的性质，并将揭示不同地区对全球气候变化的可变影响。 这项研究还将有助于更好地了解是什么因素控制着热带冰川的物质平衡。 这一知识对于评估热带冰川将如何应对持续的全球变暖至关重要，全球变暖对热带水供应构成了重大威胁。 最后，一个长期存在的争论，在末次盛冰期热带温度低气压的大小，与古海洋学代理一般显示冷却比从古冰川雪线记录推断。 热带冰碛物的宇宙成因年代测定可能会揭示这一主题，确定他们的日期，以末次冰期，从而可以适当地比较末次冰期的海面温度。 该项目的建模策略也可能有助于解开热带雪线降低归因于冷却的部分。 作为博士论文研究改进奖，该奖项还将提供支持，使有前途的学生建立一个强大的独立的研究生涯。

The last glacial maximum (LGM) is an interval of considerable interest to paleoclimatologists and climate modelers because it represents a period of dramatically altered radiative forcing from the present. While the anomalous radiative forcing imposed by various components of the earth system, such as greenhouse gas concentrations, planetary albedo, and land-sea distribution, are relatively well constrained, the tropical climate response to this forcing is widely debated. Glacial records have been used to provide an estimate of tropical climate during the LGM, but because of poor age control, it is uncertain whether tropical moraines actually correlate with the global LGM at roughly 21,000 years ago and thus provide relevant climate information. Additionally, the growth of tropical glaciers during the last glacial period is usually assumed to have been driven entirely by temperature depressions, but changes in other climatic variables like precipitation and radiation quite possibly played an important role as well complicating the climate signal contained in glacial records. This doctoral dissertation research project will develop a chronology of glaciation in the northern Peruvian Andes based on 10-beryllium cosmogenic radionuclide dating of several glacial moraines. The doctoral candidate will focus on determining the age of the last glacial maximum in this region, the timing of which is poorly constrained in South America and throughout the tropics. This study will employ an energy balance-mass balance modeling approach to simulate the range of climatic conditions required to produce glaciers whose geometry and extent matches those inferred from the geologic record. Moraines located up-valley from the LGM deposits will be dated potentially yielding a Holocene or deglacial millennial-scale record of climate variability. At least five moraines will be sampled and approximately ten boulders will be sampled on each moraine to ensure a statistically robust determination of the age of the landform.This doctoral research is significant for several reasons. The chronology developed in this research project will aid in determining the phasing of climate events in the low and high latitudes. This chronology is important for isolating the mechanisms responsible for global climate changes. The same climate models that are used to predict future global warming are often validated by testing their ability to simulate the radically different climate of the LGM. Accurate reconstructions of LGM climate therefore are required to provide a target for these models. Moreover, an understanding of tropical sensitivity to radiative forcing is essential for evaluating how the tropics will respond to the radiative forcing imposed by human-generated greenhouse gases. Determining the phase relationship between the LGM in the tropics and high latitudes will yield insights into the nature of interhemispheric climate change and will reveal the variable impacts of different regions on global climate change. This study also will lead to a better understanding of what factors control the mass balance of tropical glaciers. This knowledge is important for evaluating how tropical glaciers will respond to continued global warming, which poses a major threat to tropical water supplies. Finally, a longstanding debate exists concerning the magnitude of tropical temperature depression during the LGM, with paleoceanographic proxies generally showing less cooling than that inferred from paleoglacier snowline records. Cosmogenic dating of tropical moraines may shed light on this topic by determining if they date to the LGM and thus can be appropriately compared to LGM sea-surface temperatures. The project''s modeling strategy may also help in unraveling the fraction of tropical snowline lowering attributable to cooling. As a Doctoral Dissertation Research Improvement award, this award also will provide support to enable a promising student to establish a strong independent research career.