Glacial chronology of Iztaccihuatl Volcano, Central Mexico, a record of environmental change on the border of the tropics

Glacial landforms of Iztaccíhuatl (5282 m, 19°10′ N) reveal evidence for marked climatic change in the tropics during the Late Quaternary. Morphostratigraphy, tephrochronology, and 81 cosmogenic 36Cl exposure ages reveal new insights about the glacial chronology of lztaccíhiuatl. The most extensive recorded advance (Nexcoalango) occurred during early oxygen isotope stage 6, probably at 190–200 ka, and reached ∼3,000 m. Iztaccíhuatl lacks clear evidence of Wisconsin glaciation prior to ca. 20 ka (calendar years). The local Late Pleistocene glacial maximum occurred after the climax of the Last Global Glacial Maximum (LGGM). A first pulse (Hueyatlaco-1 advance) peaked at 19–18 ka and a second one (Hueyatlaco-2) at ca. 17–14 ka; valley glaciers reached ∼3400 m and ∼3500 m, respectively. Recessional moraines developed from 14 to 13.3 ka, followed by rapid glacier retreat at 13-12 ka. Glaciers peaked again at ∼11.8 ka reaching ∼3,800 m (Milpulco-1), and built recessional moraines until ca. 10 ka. Between ∼8.3–7.3 ka glaciers formed small but distinctive moraines above 4000 m (Milpulco-2). No evidence of glacier expansion was found between Milpulco-2 deposits and the massive moraines of <1 ka (Ayoloco) located at 4300–4700 m. The equilibrium line altitude (ELA) of glaciers, calculated using the toe-to-headwall altitude ratio method, reveals ELA depressions for the five late Quaternary advances of 1030 m, 930 m, 730 m, 550 m and 250 m, respectively. Local and regional paleoenvironmental records suggest that temperature lowering caused each ELA depression, with the probable exception of increased precipitation during Milpulco-2. Iztaccíhuatl volcano bridges glacial records in North and South American cordilleras. The overall pattern of glaciation is similar to those of mid-latitude North America and tropical South America, thus supporting a 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 LGGM. Although inconclusive, evidence does not indicate glacier expansion during (but rather immediately after) the Younger Dryas. However, a global early-to-mid Holocene atmospheric event is coeval with Milpulco-2 advance, and the Ayoloco advance is contemporaneous with the Little Ice Age in Europe.