Sedimentation rates in Lake Chapala (Western Mexico): Possible active tectonic control

Abstract

Lake Chapala, a major natural freshwater reservoir, belongs to the watershed of the Rio Lerma, which originates from the Mexico City area, 450 km eastwards. The east-west-trending Lake Chapala basin is located on the active tectonic feature Citala rift. Sediments of cores collected in the lake were studied for their 210Pb, 226Ra, 137Cs and 239-240Pu activities in order to evaluate sedimentation rates. Profiles of magnetic susceptibility allowed stratigraphic correlations to be made since some fine levels display more or less high susceptibility values. This allows an average sediment accumulation rate to be calculated. The sediment accumulation rate was calculated assuming that, the activity at the interface of a defined site is constant in the course of time, since any global decrease or increase in the rates could not be observed. The direct flux of atmospheric 210Pb was evaluated from the inventory in a horizontal soil: 0.25-0.3 dpm g-1. Except from two cores at the west, the fluxes to sediment were higher than the direct atmospheric flux. The sedimentation rate's temporal variations along the cores seem to be mainly related to annual precipitation changes, with rates lower during the dryness epochs (1945-1957 and 1979-1991). The rates measured in the western part of the lake range approximately from 1 to 2 mm year-1. The lowest rates in the central and eastern areas are higher than 2.4-2.5 mm year-1 and exceed 3.5 mm year-1 (0.066 g cm-2 year-1) near the Rio Lerma prodelta. As the lake is not deeper eastward than westward, it can be suggested that relative subsidence, related to the active tectonic framework, compensates the higher sedimentation rate. © 2001 Elsevier Science B.V. All rights reserved.