LATE QUATERNARY GLACIATION IN CENTRAL MEXICO: TIMING AND COMPARISON WITH NORTH AMERICAN RECORDS

Evidence of glaciation on the mountains of central Mexico indicates marked climatic change in the northern tropics during the late Quaternary, bridging glacial records from the North and South American cordilleras. I used morphostratigraphy, tephrochronology, and cosmogenic exposure ages (³⁶Cl) to develop a glacial chronology for Iztaccíhuatl volcano (5282 m, 19°N), with additional observations on other mountains of the region. On this basis, I explore possible climatic controls and assess the issue of synchroneity of mountain glacial advances across the Americas.

The most extensive recorded advance on Iztaccíhuatl reached ~3000 m and falls within MIS 6 (~190 ³⁶Cl ka). The local late Pleistocene glacial maximum occurred from 20 to 17.5 ³⁶Cl ka (Hueyatlaco-1 advance). Its associated ELA lowering (~1000 m) is consistent with similar estimates from North America and northern South America for the LGM. Massive moraines formed between 17 and 14 ³⁶Cl ka (Hueyatlaco-2). The reconstructed ELA depression (~900 m) is anomalously low for North America at that time and might be related to the cooling effect of Laurentide meltwater flow to the Gulf of Mexico. Glaciers receded slowly from 14 to 13 ³⁶Cl ka, then faster from 13 to 12 ³⁶Cl ka. A re-advance took place between <12 and >10 ³⁶Cl ka (Milpulco-1), i.e. starting by the end of (or after) the Younger Dryas. An advance peaking shortly before 8.0 ³⁶Cl ka (Milpulco-2) could represent the 8200 cal yr BP cooling event of the North Atlantic. Its mean ELA (4420 m) implies a depression of ~50% with respect to the maximum of the late Pleistocene. The younger next advance (Ayoloco) is <1 ka and likely corresponds to the Little Ice Age (LIA). Its ELA depression of ~250 m represents ~25% of the maximum.

Other proxy records from the region indicate dry conditions during the full and late glacial in the region. This suggests that ELA depression was primarily controlled by temperature, although increased precipitation probably contributed to Milpulco-2 advance. The overall pattern of glaciation since 20 ka is similar to those of mid-latitude North America and tropical South America, thus supporting a general synchroneity of major climatic events. However, compared to North America, the central Mexican record shows relatively marked glaciation after 17 ka, around 8 ka and in the LIA.