HR: 1400h
AN: PP43C-12    [Abstracts]
TI: Implications of Climate Change and Tephra Accumulation on the Lacustrine Environment at an Archaeological Site in Michoacan, Mexico
AU: * Trosper, T A
EM: trosperta@cwu.edu
AF: Department of Geological Sciences, Central Washington University, 400 E. University Way, Ellensburg, WA 98926, United States
AU: Ely, L L
EM: ely@cwu.edu
AF: Department of Geological Sciences, Central Washington University, 400 E. University Way, Ellensburg, WA 98926, United States
AU: Newton, A J
EM: ajn@geo.ed.ac.uk
AF: Department of Geography, University of Edinburgh, University of Edinburgh, Edinburgh, United Kingdom
AU: Gabany-Guerrero, T
EM: t.gabany-guerrero@uconn.edu
AF: Center for Latin American and Caribbean Studies, University of Connecticut, 843 Bolton Rd., Human Development Center #4, Storrs, CT 06269, United States
AU: Hackenberger, S
EM: hackenbe@cwu.edu
AF: Department of Anthropology, Central Washington University, 400 E. University Way, Ellensburg, WA 98926, United States
AB: La Alberca, a volcanic caldera in the active Michoacan-Guanajuato volcanic field of central Mexico, is a site of ancient occupation. The oldest known burial in the state of Michoacan, approximately 5000 years old, was discovered immediately beneath prehispanic cliff paintings on the caldera wall. The site is surrounded by numerous monogenetic cinder cone volcanoes, some of which may be the source of the tephra found throughout the site. The fluvial and lacustrine sediments in the base of the caldera exhibit a bimodal sedimentation pattern of coarse, sandy tephra layers alternating with finer sediments that accumulated in frequent, shallow pools. Driving factors are the timing of local volcanic activity and variations in regional climate. The two deepest trenches contain zones of coarse, dark tephra and cobbles at the base, overlain by a 2-m sequence dominated by laminated or bioturbated fine sand, silt and clay sediments from 2360-530 years BP. A sharp contact divides this middle zone from the upper 2 meters, which is dominated by multiple coarse tephra layers that represent both direct airfall and redeposition within the watershed. Paricutin volcano is the source of some or all of the upper tephra layers. Changes in the sedimentation patterns at La Alberca were compared to regional paleoclimatic reconstructions from nearby lake basins. The zone of fine-grained, laminated sediments in the middle section of the stratigraphy corresponds with drier climatic conditions in the surrounding region of central Mexico, but may represent longer persistence of ephemeral lakes in the lowest portion of the caldera basin because the sediments were less permeable. The zones with a greater influx of coarse tephra correspond in time with wetter climatic periods that may have mobilized the tephra that blankets the watershed. Permeability of the surface substrate increased, likely decreasing the duration of standing water. These variations in the lacustrine and geomorphic environment of the basin could have implications for the types of human uses of the archaeological site within the caldera. Geochemical analyses of the tephras at this site will aid in dating and correlating sediments, determining sources of the tephras and understanding the impact that the influx of tephra has on the lacustrine and fluvial environmental changes in La Alberca. Electron microprobe analysis of a massive, coarse tephra unit beneath the burial in the rockshelter at La Alberca revealed a distinctive, low-silica content geochemistry that will help in its identification and enable a direct link to be made between the archaeological deposit at the rock shelter and the broader paleoenvironment. Additional analyses will be used to directly compare the timing of the stratigraphic variations in the caldera basin with the human uses of La Alberca rock shelter and other archaeological sites in the vicinity of the caldera.
DE: 1616 Climate variability (1635, 3305, 3309, 4215, 4513)
DE: 1825 Geomorphology: fluvial (1625)
DE: 1845 Limnology (0458, 4239, 4942)
DE: 8455 Tephrochronology (1145)
DE: 8499 General or miscellaneous
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Joint Assembly