CONSTRAINING PALEOCLIMATE CONDITIONS OF QUATERNARY LAKE CHALCO USING TRIPLE OXYGEN ISOTOPES

Numerous gaps exist in tropical paleoclimate records, particularly concerning their coverage extending beyond the Last Glacial Maximum (LGM), thus impeding comprehensive understanding of pre-historic climate dynamics. This research focuses on sediment cores procured from Lake Chalco within the Basin of Mexico (BoM), boasting a continuous depositional sequence spanning approximately 300 meters and tentatively dating back around 430,000 years. Methodologically, this study employs triple oxygen isotope analysis of carbonate sediments from Lake Chalco cores as proxies for climate change especially due to the high sensitivity of Δ¹⁷O to evaporation. Variations in isotopic ratios predominantly reflect alterations in precipitation sources, climatic regimes, lacustrine conditions.

In this paleo-lake, evaporation is considered a significant factor since previous δ¹⁸O measurements show very positive values, up to 2 to 3 ‰ (PDB) during glacial times, despite the source precipitation thought to be as low as -8 to -9 ‰(VSMOW). The triple oxygen isotope values of carbonate sediments along the core from Marine Isotope Stage (MIS) 8 (glacial period) gives us a much better view of how much evaporation has taken place and the precipitation change patterns. Our preliminary analysis shows the Δ'¹⁷O values of the carbonates are quite light ranging from ~ -65 to ~ -260 per meg (VSMOW) and δ¹⁸O values ranging from ~25 to ~33‰ (VSMOW). These values clearly indicate evaporation. Using our Δ¹⁷O data in the model for lake evaporation, we reconstruct the meteoric water composition. Different possibilities from the model can be understood by changing the parameters of the evaporation curves, that are, the initial water composition (R_i), relative humidity (h_n) and fraction of evaporation (Xe). From our model, we conclude that the higher Δ'¹⁷O values ranging from ~ -65 to ~ -160 per meg can be explained by steady-state evaporation curves with Xe= 80% and h_n = 55%. The lower Δ'¹⁷O values ranging from ~ -180 to ~ -260 per meg require a non-steady state evaporative system (pan evaporation) to explain the lower Δ¹⁷O values. Initial water δ¹⁸O values are in the range of ~ -13 to ~ -8‰. This information is used to reconstruct the paleoclimate and hydrological conditions of the basin over time, and understand the factors like temperature and precipitation that influenced these changes.