Multiproxy paleoclimatic record from geochemical analyses of Lake Chalco sediments, a closed basin lake in central Mexico

University of Minnesota M.S. thesis. July 2013. Major: Geological Sciences. Advisor: Dr. Erik Brown. 1 computer file (PDF); vii, 96 pages, appendix p. 96.The Basin of Mexico, located in central Mexico within the Trans Mexican Volcanic Belt, is a high altitude (2250m) closed basin. Lake Chalco, at the southeast corner of the basin near the slopes of the Sierra Nevadas, is a relict lake containing 400 to 500m of lacustrine sediment that record Pleistocene and Holocene paleoclimatic changes. Presented in this study are the results of multiproxy geochemical analyses consisting of x-ray fluorescence (XRF) scanning to determine inorganic elemental concentrations, bulk organic and stable isotope measurements, and n-alkane biomarkers over a period spanning 45ka to 7ka. XRF analysis of Fe, Ca, and Ti indicate high siliciclastic deposition during the early Glacial associated with high precipitation and fluvial erosion rates relative to the mid-Glacial and the Last Glacial Maximum. Organic sediments from the early Glacial also indicate algal dominance associated with greater nutrient runoff and higher lake levels. Sediments indicate a major transition in tropical Mexican climate to an arid LGM with significantly lower lake levels interpreted by a dominance of macrophyte n-alkanes and an expanded littoral zone. Elemental counts and an increase in algal organic matter at 12.5ka onward indicate that conditions become wetter once again, although the prevalence of terrestrial organics suggest lake levels do not return to early or even mid-Glacial levels. The proposed mechanism for climate changes in central Mexico is related to the dynamics of the inter tropical convergence zone (ITCZ). Precipitation patterns in southwestern North America and northern South America are highly dependent on the strength and position of the ITCZ. Fluctuations in northern hemisphere insolation affect the seasonal migration of the ITCZ and have important ramifications on precipitation in the Basin of Mexico