A ROCK MAGNETISM RECORD FROM LAKE CHALCO, CENTRAL MEXICO

The Lake Chalco is located in the Transmexican Volcanic Belt active continental arc. It is a high altitude tropical site 2,250 m asl, that had an extensive lake system, sensitive to climatic fluctuations during the past times. A lacustrine sedimentary sequence 122 m long from Lake Chalco, basin of Mexico (central Mexico) has been subjected to sedimentological, geochemical, bioproxies and rock magnetic analyses. Radiocarbon dates available from the upper part and a Th/U age from the middle, suggest that the122 m record could encompass the last ca. 150 kyr. Rock magnetism analyses will allow understanding the link between detrital input, biogenic productivity and diagenesis; and in turn these will allow us to establish the link between magnetic mineralogy and paleoclimate, and to validate the paleomagnetic variations recorded in Chalco sediments. Rock magnetism parameters characterize the magnetic components in terms of mineralogy, grain size distribution and concentration. The analyses include identification of magnetic mineral phases by Curie temperatures and FORC analyses, and measurement of magnetic susceptibility, NRM, ARM, IRM, hysteresis parameters Ms, Mr, Hc, Hcr; and the ratios S300 and HIRM300.

The results show a correspondence between rock magnetism parameters with the sedimentary facies. Rock magnetic analyses revealed that the main magnetic minerals are Ti-magnetites/maghemites in the PSD domain range of detrital origin. Sharp rock magnetic variations occur at 107, 79 and 57 m depth linked to shifts towards higher coercivity and grain domain. These boundaries are close to the transition of MIS6-MIS5, MIS5e and MIS5-MIS4, respectively. In the warmer periods, magnetic minerals are characterized by a mixture of MD-PSD Ti-magnetites and a significant contribution of hematite/goethite.