Holocene climate change and human environmental impacts in Guanajuato, Mexico

This dissertation presents the results of the analyses (pollen, microscopic charcoal, fungus spores, magnetic susceptibility, and sediment chemistry) of cores from two maar lakes in the Valle de Santiago, Guanajuato, Mexico, Hoya San Nicolas and Hoya Rincon de Parangueo. Collectively the cores cover most of the last 11,000 years.; The San Nicolas site is well known to students of Mexican environmental history because it was the main focus of an earlier palynological study by Brown (1984). Brown interpreted changes in pine pollen frequencies as indicating vegetation change due to human disturbance. The conclusions reached in this study are very different. The changes in pine pollen frequencies are seen as being primarily the result of selective destruction of non-pine pollen during desiccation episodes.; The lake at San Nicolas was shallow (<10m) and desiccated several times during the Holocene whereas the lake at Rincon was relatively deep (>30m). Because of the physical differences between the two lakes their pollen and sediment chemistry records are different. However their data show the same general pattern of Holocene climate change.; If we characterize climate change in terms of variation around the Holocene mean, the evidence from the two sites indicates that during the earliest Holocene (ca. 10,500 to 8300 cal yr B.P.) there were short term oscillations of wetter and drier climate; during the early Holocene (ca. 8300 to 5500 cal yr B.P.) it was on average wetter; during the middle Holocene (ca. 5500 to 3700 cal yr B.P.) drier; and during the middle to late Holocene (ca. 3700 to 2200 cal yr B.P.) wetter. During the late Holocene increasing human disturbance of environment obscures the climatic record at both sites.; At San Nicolas agricultural activities appear to have been especially intensive during the pre-Classic and Classic periods. Unfortunately the 2001 core is not well dated in this section so precise reconstruction is not possible. The Rincon 2004 core is much better dated with 4 AMS radiocarbon dates and several historical marker horizons.; The Rincon 2004 record (pollen, microscopic charcoal, and Sporormiella ) clearly shows two major periods of agricultural activity. The first dates from ca. 2400 to 1100 cal yr B.P. and involved several cycles of increased and decreased activity. The second period dates from the late 16th century to the present. Here the evidence points to significant changes in vegetation. A decrease in grass pollen and a corresponding increase in dung fungus spores (Sporormiella) all indicate that the arrival of domesticated animals in the area after the Conquest brought about far reaching changes in vegetation. Our interpretation is that overgrazing led to a reduction in grass fires and, most likely, an increase in trees and shrubs such as mesquite (Prosopis laevigata). In brief, a key finding of the research is that human impact on the environment was less important before the Conquest than it was afterwards.