STABLE ISOTOPE RECORDS OF LATE HOLOCENE CLIMATE VARIABILITY IN MICHOACÀN, CENTRAL MEXICO

Previous studies indicate that climate has varied in the central Mexican highlands during the last c. 3,000 years. Evidence remains ambiguous, since the climate signal from biological proxies (pollen, diatoms) is partially masked by long-term human impact, which began around 3,500 years ago. Here, we present the results of stable isotope analysis on lake sediment cores from two lakes in Michoacán: Pátzcuaro and Zacapu, which provide more information on Late Holocene climate variability in the region.

Lago de Pátzcuaro has been the focus of much palaeoenvironmental research, including analysis of d¹⁸O values in ostracods (Bridgwater et al., 1999). The modern lake water is enriched in d¹⁸O relative to the GMWL, so it should provide a record of changes in precipitation – evaporation ratios. Results from d¹⁸O analysis of authigenic carbonate in two new sediment cores, one spanning the last 1,000 years, the other covering the last 3,500 years, are presented. Two distinct dry phases are evident in the record. The first occurred between 2,500 and 3,000 years ago, whilst the second occurred several hundred years ago. It has not been possible so far to obtain a precise date for this most recent drought, but it may be related to sustained droughts identified in historical documents from the Colonial Period.

Laguna Zacapu is a spring-fed lake. The stable isotope composition of its modern lake water shows that it lies on the GMWL. As this lake is not evaporated, changes in d¹⁸O values should reflect the influence of temperature, rather than precipitation. No carbonates are present in sediments from Laguna Zacapu, they consist main of diatom valves. It was therefore possible to obtain an oxygen isotope record from the biogenic silica . The record itself is, however, rather complacent, with only minor fluctuations, suggesting that temperatures have not varied significantly over the last 3,000 years. This technique has great potential for further applications in Central Mexico, where lake sediments are often rich in diatoms, but poor in carbonates.

Our results show that by applying a combination of stable isotope techniques with other proxy data, such as diatoms, pollen, magnetic susceptibility and sediment geochemistry,we can begin to disentangle the climatic and anthropogenic signals in lake sediment records.