GLACIAL HISTORY OF S CORDON DE PUNTAS NEGRAS, CHILE 24.3°S USING IN SITU COSMOGENIC 36CL

This study is motivated by interest in variable moisture delivery to the arid Central Andes (22-25°S) during the last glacial termination. The work of Ward et al. (2015) identified a potential loss of glacial response from lake highstand Tauca Phase (17-14 ka) through Lateglacial (15-10 ka) ~20°S, thus a new cosmogenic ³⁶Cl record was generated from previously undated moraines in the South Cordón de Puntas Negras, Chile (23.4°S). Data will be considered in a spatiotemporal analysis of the Arid Diagonal (AD) region of N Chile and will contribute a new record in an area with scarce chronologic data. Migrating boundaries of the AD, a NW-SE trending band of hyper-aridity, recorded in the glacial record may help refine understanding of climatic conditions from the late Pleistocene to Holocene.

Two phases of glaciation have been identified at the South Puntas Negras (SPN) site. The ice-distal phase (4500 m) was characterized by undulating, sinuous moraines with weaker preservation mantled more densely with boulders relative to inset moraines. Outer moraines also sustained more vegetation than those up-valley. The relatively younger moraines (4800 m) were tightly nested with polygonal patterned ground separating the terminal from intermediate moraines. On the eastern valley wall is a distinct set of lateral moraines and a glacial trimline. Twenty-one samples for cosmogenic exposure dating were collected from boulders bracketing the oldest & youngest accessible moraines in the summer of 2016 and are being processed to reconstruct the SPN glacial history. The chronology will be compared to a ¹⁴C dated lake core from adjacent Laguna Miscanti to test whether peak glacial advances are concurrent with lake highstands.

Samples were collected in 2014 near the Cordillera del Tatio ~22°S from opposite sides of lateral moraines extending down the valley. They connect in a series of inset stages as well. Eight samples were collected and dated (2 outliers omitted). On the N side of the valley there is a sharp and prominent outermost moraine resulting in ages of 34.6±3.3 and 40.9±4.6 ka. This is comparable to other regional records. Another inset, stratigraphically younger moraine yielded ages of 19.8±0.9, 24.9±2.3, 26±3, and 27±2 ka, thus precede or are synchronous with global LGM.