Late Quaternary glaciation of México

Abstract

The glacial chronology of Mexico is based on glacial landforms of Iztaccíhuatl (5,282 m a.s.l., 19°10′N, 98°40′W), Nevado de Toluca (4,558 m a.s.l., 19°08′N, 99°45′W), and La Malinche (4,461 m, 19°14′N, 98°00′W). Morphostratigra-phy, tephrochronology and 81 cosmogenic 36 Cl exposure ages from Iztaccíhuatl reveal new insights into the glacial sequence in Mexico. The most extensive recorded advance (Nexcoalango) occurred during Marine Isotope Stage 6 (MIS 6), probably at 195,000 BP and reached c. 3000 m. Iztaccíhuatl lacks clear evidence of Wisconsinan glaciation prior to 20,000 cal yr BP. The local Late Pleistocene glacial maximum occurred after the climax of the Last Glacial Maximum (LGM). A first pulse (Hueyatlaco 1 advance) peaked at 20,000–17,500 cal yr BP and a second one (Hueyatlaco 2) at c. 17,000–14,000 cal yr BP. Valley glaciers reached to c. 3400–3500 m a.s.l. Recessional moraines developed from 14,000 to 13,000 cal yr BP, followed by a rapid glacier retreat at 13,000–12,000 cal yr BP. Subsequently, glaciers peaked again at c. 12,000 cal yr BP reaching c. 3800 m (Milpulco 1), and built recessional moraines until c. 10,000 cal yr BP. Then between c. 8300–7000 cal yr BP glaciers formed small but distinctive moraines above 4000 m a.s.l. (Milpulco 2). No evidence of glacier expansion has been found between Milpulco 2 deposits and the massive moraines of <1000 cal yr BP (Ayoloco) that occur at 4300–4700 m a.s.l. Possible correlations between the glacial record of Iztaccíhuatl and other glacial sequences (La Malinche, Nevado de Toluca) are presented. The equilibrium line altitudes (ELA) of glaciers reveal ELA depressions for the five late Quaternary advances of 1030 m, 930 m, 730 m, 550 m and 250 m, respectively. The overall pattern of glaciation is similar to those of mid-latitude North America and tropical South America, thus supporting the general synchroneity of major climatic events. A temperature decrease of 5–9°C estimated from Hueyatlaco ELA supports marked cooling over tropical land and oceans during the LGM. Although inconclusive, evidence does not indicate glacier expansion during, but rather immediately following the Younger Dryas Chronozone. However, a global early-to-mid-Holocene event (8200 cal yr event) is coeval with Milpulco 2 advance, and the Ayoloco advance is contemporaneous with the Little Ice Age.