A 40,000 year geochemical record from Lake Chalco, Mexico.
2012-04
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A 40,000 year geochemical record from Lake Chalco, Mexico.
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2012-04
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Abstract
Water balances of Southwestern North America and of northern South America
are dependent on positions of the North American (Mexican) Monsoon and the ITCZ
respectively. The North American Monsoon leads to significant summer rainfall across a
broad swath of the continent, and constitutes the major source of annual precipitation
over the southwestern United States and northern Mexico. The position of the ITCZ and
the strength of the accompanying monsoon are affected by variability in insolation.
Northern hemisphere cooling results in a southerly displacement of both the ITCZ and
North American Monsoon, whereas northern hemisphere warming results in a more
northward position. The Basin of Mexico can be potentially impacted by both of these
systems. A new geochemical climate record from Lake Chalco, Mexico, which couples
inorganic (X-ray fluorescence) and organic (biomarkers and stable isotopes) geochemical
proxies, reconstructs external forcings of volcanism and aridity over the past 40,000
years, as well as ecosystem responses to these forcings. The Basin of Mexico is a high
altitude closed lacustrine basin (20ºN, 99ºW; 2240 m.a.s.l.) in the Trans Mexican Volcanic Belt. The relict paleolake, Lake Chalco, is located near Mexico City in the
southern sub-basin, and has an area of 120 km2 and a catchment of 1100 km2. Though the
present-day lake has been reduced to a small marsh due to historic diversion of its
incoming rivers; over longer timescales the lake has been a sensitive recorder of
hydroclimatic variations in the neotropics. Low Ca concentrations indicate greater aridity
during the late glacial relative to the last interstadial or early Holocene. Peaks in Fe
concentrations indicate volcanism and the deposition of tephra throughout the last 40ka, with more frequent episodic volcanism occurring in the most recent 23ka. N-alkane
biomarkers, compound specific carbon isotopes, and XRF data provide evidence for
ecosystem responses to both aridity and volcanism. The extent to which both of these
external forcings affected ecosystem is assessed; and the extent to which global or local
external forcings affect the Basin of Mexico is addressed.
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University of Minnesota M.S. thesis. April 2012. Major: Geological sciences. Advisors: Dr. Erik T. Brown and Dr. Josef Werne. 1 computer file (PDF) vii, 127 pages, appendix p. 120.
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Pierce, Megan Leigh. (2012). A 40,000 year geochemical record from Lake Chalco, Mexico.. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/125512.
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