THE HOLOCENE RECORDS OF THE BASIN OF MEXICO: A HISTORY OF ABRUPT CHANGES

Results of pollen analyses from two sites in the south of the Basin of Mexico in combination with glacial chronology, tephrachronology and magnetic properties provide information of Holocene abrupt climatic changes. The high altitude site, Valle El Marrano at 3850 m is located in the NW side of Iztaccihuatl volcano. Glacial advances occurred between 12 to 10 ka (Mipulco-1) and from 8.4 to 7.4 ka (Mipulco-2) according to the 36Cl chronology. A Mipulco-1 moraine depression was formed and swampy conditions established where sediments began to accumulate. In this moraine depression a 450-cm core was drilled. The sequence elapses the last 9000 years according to the 14C dates and the presence of distinctive pumice with an age of 5000 yr. BP. The pollen record of the Valle El Marrano shows after the retirement of the moraine a gradual colonization by the elements of the alpine grasslands during the early Holocene. After the Mipulco-2 glacial advance a sudden change in vegetal communities is documented; the site is invaded by pine forest at ca. 6000 yr. BP and after this event the alpine grasslands return. For the last 4000 years the expansion of the Pinus harttwegii forests is recorded. In the low altitude site, Chalco lake at 2250 m several cores were taken. The Holocene sequences show important arboreal cover in the area during the early Holocene mainly pine and oak forests. A fir forest expansion occurred between 9000 and 7000 yr. BP. At ca. 6000 yr. BP the pollen assemblages changes and only herbaceous taxa are recorded. Low lake levels and important fires in Chalco area are shown in the sequences. Humid conditions are established during the late Holocene with increase in the forests and higher lake levels. In the Basin of Mexico, a region that can be considered as the northern limit of the tropics in America, proxy records document changing Holocene environments with a period of warming conditions at around 6000 yr. BP.