LONG AND SHORT TERM CHANGE SINCE THE LAST GLACIAL MAXIMUM IN THE PÁTZCUARO BASIN, MICHOACÁN, MEXICO

Much palaeoenvironmental work in Mexico has focused on the Pátzcuaro Basin. Presented here are results from a series of new cores collected from the northern and south western parts of the basin, covering the last 19,000 years. The cores were collected using both Livingstone and min-Kullenberg cores. The chronology is provided by ¹⁴C dating and tephrochronology. Mini-Kullenberg cores contain the Paricutin tephra (AD 1943) at the surface, although show signs of mixing. The cores have been analysed for diatoms, mineral magnetic properties, sediment geochemistry, LOI and stable isotopes. The chemical composition of tephra layers has been identified using an electron microprobe. As well as reconstructing long term patterns of change, we have also examined the response of the lake system to inputs of tephra.

The pattern of change since the last glacial maximum is generally consistent with that published by Watts and Bradbury (1982) and Bradbury (2000). The lake was relatively deep and clear in the last glacial, but became increasingly turbid and generally shallower through the Holocene. Long term catchment disturbance is indicated by a range of mineral magnetic parameters. The use of an unmixing model allows identification of the contribution of different sediment sources, indicating that inputs of topsoil have been important during the last 3,000 years. Increasing inputs of terrestrial organic matter are also evident from higher C/N ratios. It appears that P is unlikely to have been a limiting nutrient, but available P has increased sharply in recent decades. This is consistent with available limnological data. Over the last 4,000 years, periodic dry phases are indicated by accumulations of ostracods and d¹⁸O values in authigenic carbonate, but the exact timing needs to be established. At least 16 tephra layers have fallen over the basin in the last 19,000 years, but their impact on the long term evolution of the basin appears minimal.

Sediments from the Pátzcuaro Basin record a complex interaction of climate change, human and volcanic impacts. Increasing the range of proxies has enabled us to begin to extract a climate signal for the highly disturbed Late Holocene portion of the record.