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.

The sequence as now interpreted correlates remarkably well with that from the Valley of Mexico, and, using the archaeologic dates that Sears established there, there are: zone D, pre-Archaic (before 1500 B.C.), dry; zone C, early and middle Archaic (1500–500 B.C.), moist; zone B, late Archaic-Teotihuacan (500 B.C.–A.D. 900), climate fluctuating, but with at least one markedly dry episode; zone A, Nahua (A.D. 900–1521), moist. The few archaeologic data and the folklore from the Patzcuaro region tend to confirm this interpretation. There are some differences between the Patzcuaro and the Texcoco pollen sequences, notably in the less well marked character of the last (moist) phase in Patzcuaro.

Of particular interest is the evidence of relatively intense aridity shown in the cores at the inferred late Archaic or Teotihuacan level: a minimum of Sears' Index corresponds to a strong increase in the calcium content of the sediments, and to a zone having only shallow-water benthic and littoral diatoms, above and below which the normal planktonic diatom flora is recorded. The two independent evidences for increased evaporation or reduced rainfall suggest that the dry phase was due to climatic causes and not to culture or volcanism.

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