GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 81-6
Presentation Time: 9:00 AM-5:30 PM


THORNTON, Rachel M., Department of Geology, University of Cincinnati, 2600 Clifton Ave, Cincinnati, OH 45220; Department of Geology, University of Cincinnati, 500 Geology/Physics Building, Cincinnati, OH 45221 and WARD, Dylan, Department of Geology, University of Cincinnati, 500 Geology/Physics Building, Cincinnati, OH 45221,

This study aims to refine understanding of the relative roles of easterly vs. westerly moisture delivery to the Central arid Andes (22-25°S) from the late Pleistocene & Holocene; thus, addressing spatial extents of Northern vs. Southern Hemisphere drivers of South American climate. These drivers are reflected in the position & extent of the South American Arid Diagonal (AD), a NW-trending band of hyper-aridity that represents the boundary between Atlantic-driven circulation systems and the Southern Westerly Winds.

Based on spatiotemporal analysis of regional re-scaled cosmogenic records, Ward et al. (2015) identified ~20°S as the apparent southernmost extent of moraine formation near the northern portion of the AD during the late glacial (10-15 ka). Here we investigate this trend further with the hypothesis that southern tropical glaciers from the eastern flank of the Andes will exhibit a late glacial response making moraine deposits relatively younger from the eastern vs. western Andes. Given the precipitation-sensitivity of the former glaciers, this would imply a time-transgressive loss of moisture on the west flank of the arid Andes from LGM to the late-glacial and a corresponding expansion of the AD.

We present glacial chronologies from 2 field sites using cosmogenic 36Cl exposure ages from moraine deposits in Northern Chile. The sample sites include: (1) lateral moraines from 22°S and (2) nested moraine ridges from the Cordon de Puntas Negras at ~24°S. The southern field site is near Laguna Miscanti, Chile (67° 39’ W), where there are published pluvial lake proxies indicating lake transgressions from ~17-14 ka, ~12-10 ka, and Mid-Holocene (Grosjean et al., 2001). Preliminary moraine mapping from the southern site implies a minimum of 2 stages of glaciation, but it is not known whether these correspond to the wet periods from the lake sediment proxy. Relatively younger moraines are tightly nested & occur >4500m elevation. Older, distal moraines show weaker preservation and occur at elevations ~4200-4500m. Dating these moraines allows comparison with lake proxy & newly-developed independent chronologies from the eastern flank of the Andes, revealing any time-transgressive loss of moisture during the late glacial & constraining changes to the northern boundary of the AD during the late Pleistocene & Holocene.