Chlorine-36 Surface Exposure Dating and Glacial Sensitivity Analysis of late-Holocene Moraines, South-Central Chilean Andes (38S)

Here we present the first 36Cl ages of late-Holocene moraines from the South-Central Chilean Andes to compare the timing of southern hemisphere mid-latitude glacial variability with low and high latitude regions. With peak elevations rarely exceeding 3500 m, mountain icecaps and small cirque glaciers dominate the modern glacial landscape. A series of concentric moraine ridges were identified in a cirque valley on Monte Sierra Nevada in the Araucanía region, demarcating progressive phases of glacial retreat over the late-Holocene. Chlorine-36 concentrations were measured on whole-rock samples from stable basaltic boulders (n=10) located atop moraine crests, establishing a high-precision chronology of late-Holocene ice retreat. Moraine abandonment of the oldest and most distal ridge in the valley commenced by ~3.7 ka. The glacier then retreated to a stable up valley position by ~2.6 ka. A final readvance of the ice margin occurred, roughly coincident with the Southern Hemisphere expression of the Little Ice Age. A final pulsed recession formed a number of moraines over the last ~350 years. Additionally, cores from Araucaria araucana trees (n=35) were collected, providing an independent, high-resolution chronology to infer climate fluctuations, as well as a regional record of temperature and moisture availability. The oldest tree-ring record, collected from the crest of the outermost moraine, dates to 1650 AD, indicating that several generations of A. araucaria have colonized that particular landform. Regional glacier-climate sensitivity since the Little Ice Age, was analyzed using the Open Global Glacier Model to quantify an envelope of possible forcing parameters (e.g. +/- 10% precipitation and +/- 1°C), suggesting that local glacial fluctuations are sensitive to variability of both temperature and precipitation. These records point to a near-coeval record of late-Holocene climate variability between the low and high southern latitudes.