Last glacial maximum and deglaciation of Ampato volcanic complex (Southern Peru) from Late Pleistocene to the Present

Abstract

The aim of this study is to constrain the timing of the deglaciation process since the Last Local Glacial Maximum in HualcaHualca volcano and Patapampa Altiplano, located in the Andean Central Volcanic Zone. Nine 36Cl cosmogenic surface exposure dating of moraine boulders as well as polished and striated bedrock surfaces are presented. The 36Cl cosmogenic exposure ages indicate that the glaciers reached their maximum extent at ~ 17 - 16 ka on the HualcaHualca volcano during the Heinrich 1 event and the Tauca paleolake cycle. Since then glaciers began to retreat until ~ 12 ka, when they went through a phase of readvance or stillstand. The deglaciation of HualcaHualca was constant since ~ 11.5 ka, coinciding with the disappearance of the ice cap from the Patapampa Altiplano. These glacial ages do not corroborate a Last Local Glacial Maximum prior to the global Last Glacial Maximum but they indicate a sensitive reaction of the glacier system to precipitation fluctuations. According to the analysis of cosmogenic exposure ages reported from HualcaHualca, Sajama and Tunupa volcanoes, the onset of deglaciation since Last Local Glacial Maximum occurred at the end of the Heinrich 1 event and the Tauca paleolake cycle in the Andean Central Volcanic Zone. However, the glacier retreat was not continuous because at least one significant readvance or stillstand phase has been reported in most of the volcanoes studied in this region although the ages cannot be clearly related to the Younger Dryas and/or the Antarctic Cold Reversal cold events. After this readvance or stillstand, the glaciers of the Central Volcanic Zone retreated, but at least three clear minor readvances evidence a not homogeneous warm and/or dry climate during the Holocene. Even though in situ cosmogenic exposure provides important glacial chronological data, it is difficult to establish a consistent regional glacial reconstruction and clear connections with the main Late Pleistocene cold episodes due to limitations associated with in situ cosmogenic production rates and the use of different scaling schemes. To reduce the uncertainty and compare the available cosmogenic ages, it would be necessary to determine a precise in situ cosmogenic production rate for each isotope in the Central Andes, a standard scaling scheme and recalculate the published chronological data.