The Huelmo/Mascardi Cold Reversal (HMCR) is the name given to a cooling event in South America between 11,400 and 10,200 ¹⁴C years BP. This cooling began about 550 years before the Younger Dryas cooling in the Northern Hemisphere, and both periods ended at about the same time.

The event was given its name from the Huelmo, Chile, and Lago Mascardi, Argentina (Nahuel Huapi National Park) sites where it was detected and dated.

The HMCR cooling event began with the accentuation of a cooling period which began 12,400 ¹⁴C yr BP. The 11,400 ¹⁴C yr BP ( (13,400 calendar years BP) event onset took place 400-700 years before the onset of the Younger Dryas event. The 10,200 ¹⁴C yr BP (circa 11,500 cal yr BP) end date is difficult to calibrate to a calendar year because it took place during a radiocarbon age plateau. The end of the HMCR also marked the end of the cooling period.

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