137-6 Reconstructing Hydroclimate Variability in the Basin of Mexico Over the Past 400 ka Using Triple Oxygen Isotopes in Lake Chalco Carbonates

Session: Joint SGD-SEPM-IAS Focus on the Sedimentary Record of Climate Change

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Abstract:

Tropical paleoclimate records extending beyond the Last Glacial Maximum remain limited, constraining our ability to resolve long-term hydroclimate variability in low-latitude regions. To address this gap, we reconstruct the evaporation history of Lake Chalco in the Basin of Mexico using triple oxygen isotope (δ′¹⁸O, Δ′¹⁷O) measurements from authigenic carbonates preserved in a continuous sediment core spanning ~400 ka and ~300 meters in depth (Hodgetts et al., 2025).

Measured carbonate Δ′¹⁷O values range from –15 to –195 per meg (VSMOW), while δ′¹⁸O values range from ~24 to 33‰ (VSMOW). Despite modern precipitation in the region having δ′¹⁸O values as low as –7 to –10‰ (VSMOW), carbonate δ′¹⁸O values remain unexpectedly high (2–3‰ VPDB or ~33–34‰ VSMOW), indicating strong and persistent evaporative enrichment. These enrichments persist across multiple glacial–interglacial cycles (MIS 11 to MIS 2), underscoring the lake’s sensitivity to evaporation-driven isotope effects through time.

Using a Craig-Gordon–based lake evaporation model, we assessed the influence of initial meteoric water δ′¹⁸O, relative humidity, and the fraction of lake evaporation on the observed isotopic signals. We find that Δ′¹⁷O carbonate values for all glacial-interglacial MIS stages except MIS 7 are consistent with steady-state evaporation, assuming θ_diff of 0.511 for diffusive fractionation (Angert et al., 2004), under moderate relative humidity (30-55%) and evaporation fractions of ~40-80%. The reconstructed meteoric water δ′¹⁸O values (–17‰ to –5.7‰) and Δ′¹⁷O values (20-35 per meg) help constrain past precipitation sources and atmospheric circulation patterns. 

MIS 7 interglacial stage stands out as a notable exception, with moderate δ′¹⁸O ~24 to 28‰ (VSMOW) and more positive Δ′¹⁷O values ranging from -56 to -100 per meg, indicate reduced evaporation and a greater contribution from summer precipitation. This suggests wetter conditions and potentially intensified monsoonal input during this period.

These findings demonstrate the utility of triple oxygen isotope measurements for disentangling evaporation dynamics in closed-basin lakes and provide new insights into tropical hydroclimate evolution in central Mexico over the past 400 ka.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9712

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