The frozen tropics : palaeoglaciations within northern Peru

Abstract

Tropical glaciers are sensitive indicators of global climate changes, and how these changes influenced the tropics. Recent research has primarily focused on locations where glaciers are still present, or in high elevation locations with recently vacated glacial cirques. Locations that are fully deglaciated, at low elevations (i.e., < 4,000 m asl) are rarely investigated. These are below the traditional latitudinal Last Glacial Maximum snowline elevation, primarily located within a latitudinal data gap between Perú and Ecuador. This thesis reconstructs the ice masses of the Lagunas de Las Huaringas, northern Perú, an area below 4,000 m asl, to assess their extent and nature, and determine the palaeoclimate they may have existed under. This was achieved by: i) remotely sensed geomorphological mapping of palaeoglacial evidence, to enable a first-order reconstruction of glacier extent and temperature cooling, ii) fieldwork to validate remote mapping, and to acquire samples for cosmogenic nuclide dating, and iii) the first use of PISM, a three-dimensional model, over tropical glaciers, to determine their likely climate envelope and response to climate change during advance to, during, and deglaciation from their past extents (38 ka – 16 ka). Research here demonstrates that glaciers were present, and extensive, within the Laguna de Las Huaringas region, with palaeoglacial landforms extending from source area elevations (~3,900 m asl) to their most extensive ice marginal positions downvalley (~2,800 m asl). Temperature cooling estimates from geomorphological evidence and numerical modelling suggest that extensive cooling (-10℃ from present) and a wetter climate (+30% modern) was necessary for extensive glaciation to occur. This is one of the coldest estimates acquired within the tropical Andes. Modelled maximum ice extents, that are assumed to be the region’s Local Last Glacial Maximum (25.4 ka), or its regional maximum advance during the LGM, falls within the range of previously dated tropical Andean LLGM advances, indicating an early-LLGM for the region. This resulted in an ice plateau configuration, with outlet glaciers extending down valley; different from that determined solely by geomorphological mapping of a cirque-to-valley glaciation. The Las Huaringas ice masses were extremely sensitive to temperature changes, with deglaciation being associated with small increases in temperature (e.g., -10℃ to -8.5℃ from present), deglaciating immediately after the termination of the LLGM period (~17 ka). This thesis highlights that relatively low elevation tropical locations, formerly thought not to have supported extensive glaciers due to being below the South American LGM snowline, were likely glaciated during the last glacial cycle. This has important implications for our understanding of the last glacial climate conditions within the tropics, along with the timing of the LGM across the Andes. Future work should focus on cosmogenic dating to determine the timing of glacial advances at the Lagunas de Las Huaringas, along with further low elevational regions within the latitudinal data gap.