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Free keywords:
cosmogenic nuclides; production rate; cross-calibration; South American Monsoon; Quaternary climate change; moraine
Abstract:
Glacial deposits on the high-altitude, arid southern Central Andean Plateau (CAP), the Puna in northwestern Argentina, document past changes in climate, but the associated geomorphic features have rarely been directly dated. This study provides direct age control of glacial moraine deposits from the central Puna (24°S) at elevations of 3900–5000 m through surface exposure dating with cosmogenic nuclides.
Our results show that the most extensive glaciations occurred before 95 ka and an additional major advance occurred between 46 and 39 ka. The latter period is synchronous with the highest lake levels in the nearby Pozuelos basin and the Minchin (Inca Huasi) wet phase on the Altiplano in the northern CAP. None of the dated moraines produced boulder ages corresponding to the Tauca wet phase (24–15 ka).
Additionally, the volcanic lithologies of the deposits allow us to establish production ratios at low latitude and high elevation for five different nuclide and mineral systems: 10Be, 21Ne, and 26Al from quartz (11 or 12 samples) and 3He and 21Ne from pyroxene (10 samples). We present production ratios for all combinations of the measured nuclides and cross-calibrated production rates for 21Ne in pyroxene and quartz for the high, (sub-)tropical Andes. The production rates are based on our 10Be-normalized production ratios and a weighted mean of reference 10Be production rates calibrated in the high, tropical Andes (4.02 ± 0.12 at g−1 yr−1). These are, 21Neqtz: 18.1 ± 1.2 at g−1 yr−1 and 21Nepx:
at g−1 yr−1 (En
) scaled to sea level and high latitude using the Lal/Stone scheme, with 1σ uncertainties. As 3He and 26Al have been directly calibrated in the tropical Andes, we recommend using those rates.
Finally, we compare exposure ages calculated using all measured cosmogenic nuclides from each sample, including 11 feldspar samples measured for 36Cl, and a suite of previously published production rates.
