Environmental changes in the Zirahuen Basin, Michoacan, Mexico, during the last 1000 years

Lago de Zirahuen (19degrees 260 N, 101degrees 44' W) lies within a montane basin in highland Michoacan, Mexico at 2075 m a. s. l. The lake basin has high recreational value and is promoted as a tourist attraction. Four short ( lt 1 m), sediment- water interface cores were used to investigate Late Holocene environmental change. The cores span approximately the last 1000 years, based on Pb-210 dating, AMS C-14 dating and tephrochronology. Cesium-137, Americium-241 and tephra layers from Volcan Paricutin (AD 1943 - 1952) and Volcan Jorullo ( AD 1759 - 1774) provide independent age markers. Cores were analysed for diatoms, magnetic susceptibility and heavy metals ( iron, lead, manganese, copper and zinc). Drier climate around 1000 years BP is tentatively inferred from the diatom assemblage and is concordant with other regional records. Increased soil erosion between ca. AD 1100 and AD 1550 may be associated with Pre-Hispanic settlement in the basin, but there is no archaeological evidence to support this. A period of catchment stability occurred during the early Colonial Period, ca. AD 1550 - 1750. The dramatic decline in the indigenous population following the Spanish Conquest in 1521 may have promoted vegetation recovery on the basin slopes. After the mid-18th century, a significant change in diatom species composition is observed along with an increase in soil erosion, as inferred from the magnetic susceptibility profile. Concentrations of copper and lead also increased. These changes are associated with the establishment of a copper smelting industry in the basin and increased Colonial agricultural development. Recently, tourist developments and commercial agriculture have impacted on the basin. The diatom flora has changed dramatically in the last 20 years, apparently in response to the onset of cultural eutrophication. The palaeoenvironmental evidence suggests that the lake is responding rapidly to land use intensification in the basin, which may have implications for future developments