QUATERNARY CLIMATIC AND HYDROLOGIC VARIATIONS IN THE NORTH AMERICAN TROPICS: STABLE ISOTOPE ANALYSIS OF ORGANIC MATTER FROM LAKE CHALCO, MEXICO

Sediment cores from Lake Chalco, located in the Basin of Mexico (BoM) provide a continuous climate record that spans ~400 ka. Today, the BoM lies at the northern margin of the Intertropical Convergence Zone (ITCZ) in N.H. summer, and past orbital and millennial-scale variations likely influenced the position of the ITCZ and the mid-latitude storm track during glacial maxima. These climatic variations likely impacted precipitation-evaporation balance, affecting sediment facies as well as organic matter (OM) properties. Sediments from Lake Chalco cores were analyzed for δ¹³C and δ¹⁵N in OM in addition to TOC, C/N, and lithology. Most of the data (n>200) have C/N between 10-30, indicating mixed algal-terrestrial OM, or a subaquatic macrophyte signal. Samples with C/N <10 (n<200) indicate algal OM. Both sample types are found within diatomaceous lithofacies, although the algal samples occur mostly in volcaniclastic and carbonate facies. Data from the Chalco sediments also show contributions from C3 and C4 plants. The C3 plant material is present throughout the core and is typified by C/N of ≥20 and occurs mostly in organic rich facies. The C4 plant material (C/N >30, higher δ¹³C values) is observed deeper in the core and occurs mainly in carbonate rich facies. Radiocarbon dating at Lake Chalco extends back to ~45 ka and constrains the top ~50 m of the core including Marine Isotope Stages (MIS) 1-3. Sediments from MIS 3 (~42-29 ka) are carbonate rich, indicating shallow evaporative conditions. MIS 2 (~29-14 ka) shows a change from diatomaceous sediments (deep, productive lake) to organic rich sediments with a large terrestrial component (shallow, swampy) during the last glacial and deglacial. Low lake levels persisted into the early Holocene (11-7.5 ka), with continued organic rich sediments. The undated sections of the cores show periods of changing climate with fluctuations between diatomaceous sediments (deep, productive) and carbonate sediments (shallow, evaporative). Comparison of organic proxies with δ¹⁸O show correlation of positive δ¹⁸O values with the shallow water facies, and several negative excursions in the δ¹⁸O data that coincide with organic-rich periods. In general, these data suggest organic periods may correlate with glacial periods and at least reflect reduced precipitation and shallower conditions.