A 40,000 year geochemical record from Lake Chalco, Mexico.

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.