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.
