LATE PLEISTOCENE AND HOLOCENE GLACIAL CHRONOLOGY AND EQUILIBRIUM LINE ALTITUDES OF IZTACCÍHUATL VOLCANO, CENTRAL MEXICO

There is evidence of late Quaternary glaciation on thirteen mountains of central Mexico (~19°N) peaking above 3850 m. Iztaccíhuatl volcano (5286 m) probably has the most complete record in the region. This work presents the late Pleistocene and Holocene glacial chronology of Iztaccíhuatl, the reconstructed equilibrium line altitude (ELA) of glaciers for different advances, and some paleoclimatic implications. The chronology is based on morphostratigraphy, 81 ³⁶Cl surface exposure ages, and 3 radiocarbon-dated tephras from Popocatépetl volcano. The ELAs for the past 20,000 years were reconstructed using the terminus-to-headwall altitude ratio (THAR=0.4). The most extensive recorded advance reached ~3000 m and falls within MIS 6 (~190 ³⁶Cl ka) (Nexcoalango moraines of White, 1962, GSA Bulletin, 73:935-958). There is no clear evidence of Wisconsin glaciation prior to the LGM. The main advance of the late Pleistocene occurred between 20 and 17.5 ³⁶Cl ka, with a mean ELA of 3940 m (Hueyatlaco-1 moraines of White). An ELA depression (compared to A.D. 1960) of ~1000 m is consistent with similar estimates from North America and northern South America for the LGM. After a short recession, massive moraines formed between 17 and 14 ³⁶Cl ka, with a mean ELA of 4040 m (Hueyatlaco-2 moraines). This substantial late glacial ELA depression (~900 m) might be related to the cooling effect of Laurentide meltwater flow to the Gulf of Mexico. Glacier retreat occurred slowly from 14 to 13 ³⁶Cl ka, then faster between 13 and 12 ³⁶Cl ka. Glaciers expanded (mean ELA: 4240 m) and receded from <12 to ~10 ³⁶Cl ka (Milpulco-1 moraines). There is clear evidence of an advance peaking shortly before 8.0 ³⁶Cl ka, with recession in progress by 7.5 ³⁶Cl ka (Milpulco-2 moraines). This could represent the 8200 cal yr BP cooling event of the North Atlantic. The mean ELA of 4420 m implies a depression of ~50% with respect to the maximum ELA lowering of the late Pleistocene. The younger next advance is dated in <1 ka (based on tephrochronology) and likely corresponds to the Little Ice Age (Ayoloco moraines). The mean ELA (4720 m) was depressed ~250 m, equivalent to 24% of the late Pleistocene maximum. As everywhere in the tropics, the glaciers of Iztaccíhuatl have undergone accelerated retreat since the beginning of the 20th century. The current ELA lies around 5000 m.