CLIMATIC FLUCTUATIONS USING OXYGEN AND CARBON ISOTOPES FROM LACUSTRINE CARBONATES FROM LAKE CHALCO, MEXICO

Cores extracted from Lake Chalco, located in the Basin of Mexico (BoM; Mexico City), provides access to a tropical climate change record spanning ~400 ka. Sediments from Lake Chalco show continuous accumulation throughout the record, and many facies contain authigenic carbonates. Analysis of stable isotopes of oxygen and carbon permits the preserved carbonate material to be used as a proxy for climate change. As variations in the ratios of these stable isotopes are primarily related to changes in precipitation source, climate, or lake conditions (e.g. hydrologically closed vs open basin). Major influences on lake precipitation-evaporation balance include the position of the northern fringe of the Intertropical Convergence Zone and temperature affecting evaporation rates. On longer timescales these factors are influenced by orbital scale and millennial scale climate signals, and glacial/interglacial cycles. Bulk carbonate oxygen isotope measurements from the core show a range of values from ~ -22‰ to +5‰ (VPDB) with most values (n>100) between -3‰ and +5‰, compared to modern precipitation oxygen isotope values over the BoM with annual average values of ~ -8‰ to -10‰ (VSMOW). Carbon isotope values display a large range, from ~ -29‰ to +20‰ (VPDB). The data are separated into distinct positive and negative clusters in both oxygen and carbon isotope values and display sawtooth pattern fluctuations downcore. More positive oxygen isotope values than modern precipitation are interpreted to reflect evaporation in a closed basin and could also reflect more positive precipitation values in the past. The smaller number of samples from the negative cluster correlate with cold periods including the late LGM as shown by radiocarbon dating and are interpreted to reflect a greater contribution from isotopically depleted westerly frontal precipitation. These westerly storms are unusual at these latitudes and suggest a prominent meridional transport of frontal precipitation during colder periods. Observed changes may also be due to variations in Chalco lake levels and shifts between open and closed basin conditions. Based on these data and the large number of isotopically positive samples relative to modern, evaporation is interpreted to be the primary modification acting on Lake Chalco during periods of carbonate deposition.