Sediments of Lake Patzcuaro, Michoacan, Mexico

The recent sedimentary history of Lake Patzcuaro was investigated by pollen analysis in 1944, and a dry climatic phase, probably hundreds or thousands of years ago, seemed to be indicated by maximal abundance of nonarboreal (grass, chenopod, and composite) pollen. The work of Sears on Lake Texcoco, 250 km east, prompted a reinvestigation of the Patzcuaro pollen sequence and a study of the chemistry and the diatom flora of the sediment cores. On the basis of Sears' Index of Humidity (the ratio of Quercus +Alnus+ Abies pollen to the total arboreal pollen, which is minimal in the driest intervals when Pinus is most abundant), there is clear evidence of a dry phase within the zone of abundant nonarboreal pollen, but the latter now seems to have no direct climatic significance. Instead, since the nonarboreal pollen, particularly its chenopod (+ amaranth?) component, is most abundant during moist phases just preceding and just following the newly defined dry phase, the fluctuations of nonarboreal pollen seem to reflect agricultural practice and, ultimately, demographic history.