Palaeomagnetic and palaeoenvironmental studies in the southern basin of Mexico - II Late Pleistocene - Holocene Chalco lacustrine record

Abstract

Results of a detailed palaeomagnetic and microfossil study of the first twelve and eigth meters, respectively, of the lacustrine sedimentary sequence from Chalco Lake, southem Basin of Mexico are reported. The study is based on four cores recovered from the central part of the lake. Radiocarbon dating indicates an age for the bottom of the section (12 m) of about 25,000 yr B.P. Sediments are characterized by high magnetic susceptibility values, related to the intensive and widespread volcanic and tectonic activity in and around the lake. The palaeoflora was mainly constituted by coniferous forests in the highlands and swamp vegetation closer to the lake. Palaeolimnological data document fluctuations in lake levels, with changes in salinity and alkalinity. Diminution or absence of palaeofossil populations in the sequence show a correlation with the tephra (as indicated by high magnetic susceptibilities and remanence intensities). Volcanic activity affects microfossil abundance and distribution both directly (tephra fallout in the lake arca) and indirectly (e.g., through enhanced erosion, tectonism and hydrothermal alteration). The characteristic declination shows severa! anomalies; large swings are observed at 5,000 and 14,000 yr B.P. The inclination varíes around a mean value of 32.9°, with an apparent cyclic pattem. There are no large swings in the inclination data as compared with the declination, but the inclination anomaly shows a peak at about 14,000 yr B.P. that correlates with the declination swing. Intensity data show periodicities from 650 to 2,900 yr. Declination data show similar dominant periods in the order of 850 to 2,900 yr. lnclination data present periodicities in the range from 1,000 to 3,800 yr.