CARBON AND NITROGEN ISOTOPES FROM ORGANICS IN LAKE CHALCO, MEXICO: A RECORD OF QUATERNARY CLIMATE AND ENVIRONMENTAL CHANGE IN NEOTROPICAL NORTH AMERICA

The Mexidrill Project recovered four overlapping sediment cores from Lake Chalco in the Basin of Mexico, near Mexico City, to reconstruct this region’s paleoclimatic history. A composite core dated with a mixture of preliminary radiocarbon dates and tephrochronology suggests that Lake Chalco was present for several hundred thousand years at this location. Bulk organic matter δ¹³C, δ¹⁵N, carbon/nitrogen ratios and total organic carbon in lake sediments may correlate with periods of increased/decreased lake level and suggest changes in watershed vegetation related to regional climate. Results from the upper section of the composite core suggests that the Last Glacial Maximum (~20-18 kya) at Lake Chalco was a period of higher lake level, due to increased precipitation and/or reduced evaporation. The δ¹³C values and C/N ratios become lower through this interval, suggesting an increase in terrestrial C3 plant matter carried into the lake by runoff. The δ¹⁵N isotopes become higher, possibly due to lake stratification supporting more denitrification in anoxic bottom water. The bulk organic matter samples from this interval are sapropels with high total organic carbon (17-40%), suggesting that anoxic bottom water promoted increased organic matter preservation in sediments. The δ¹³C isotopes become higher in the latest Pleistocene (~14 kya) before becoming lower again in the early Holocene (~7 kya). This suggests that C4 vegetation in the lake watershed expanded in the latest Pleistocene, possibly due to increased aridity. The lower δ¹³C and C/N values from early Holocene sapropel sediments suggest a return to wetter conditions and expanded C3 watershed vegetation that may correspond to the insolation maximum strengthening the Mexican Monsoon. Further analysis down-core will establish whether these initial trends persist during earlier glacial-interglacial cycles.