3 Paleo-climate and -ecosystems of the Southwest and High Plains: From the Paleozoic to the Quaternary

Many Phanerozoic climates and ecosystems serve as potential analogues for the future; others highlight the Earth system response to tectonic and climatic forcings. We invite abstracts that probe the paleoclimate/ecosystems of the US Southwest

Monday, 18 May 2026
9:00 AM - 12:00 PM
Alvarado C

Reconstructing hydroclimate variability in the Basin of Mexico over the past 400 ka using triple oxygen isotopes in Lake Chalco carbonates

Paper number: 3-5
Program: Technical Sessions
Time: 10:40 AM – 11:00 AM

Authors

Abstract

Interpreting lacustrine oxygen isotope records is essential for reconstructing past hydroclimate and precipitation dynamics, but evaporative modification of lake waters often complicates these interpretations. Here, we quantify the evaporation history of Lake Chalco (Basin of Mexico) using triple oxygen isotope measurements (δ18O, Δ′17O) of authigenic carbonates from a 300-m sediment core spanning ~430 ka. Carbonate Δ′¹⁷O values range from −56 to −195 per meg (VSMOW), while δ¹⁸O values vary between −6.7 and 2‰ (VPDB), equivalent to ~24–33‰ VSMOW.

Although modern meteoric precipitation in the Basin of Mexico exhibits δ18O values as low as −7 to −10‰ (VSMOW), reconstructed lake-water δ¹⁸O values are substantially higher (−4 to 1‰ VSMOW), indicating pronounced evaporative enrichment. Application of a Craig–Gordon–type evaporation model shows that carbonate δ18O and Δ′17O values for Marine Isotope Stages (MIS) 2–11, excluding MIS 7, are best explained by steady-state evaporation under low to moderate relative humidity (20–55%) and evaporation fractions (X_E ≈ 40–80%; θdiff = 0.511). These conditions imply a hydrologically open, through-flowing lake system (XE < 1) experiencing substantial but not complete evaporative water loss.

In contrast, sediments assigned to MIS 7 (134.24–167.51 m depth) display elevated Δ′¹⁷O and comparatively lower δ¹⁸O values. Modeling results suggest moderate humidity (42–55%) and significantly reduced evaporation fractions (X_E ~20%), consistent with enhanced through-flow or a chain-of-lakes configuration. Reconstructed meteoric-water compositions (δ¹⁸O: −17 to −5.7‰; Δ′¹⁷O: 20–35 per meg) provide new constraints on precipitation sources and evaporation intensity.

Together, these results establish Δ′¹⁷O as a powerful tracer of evaporative processes and provide a quantitative framework for interpreting Quaternary hydroclimate variability in the Basin of Mexico. The findings further suggest that tropical hydroclimate shifts were closely linked to orbital-scale migrations of the Intertropical Convergence Zone and associated changes in regional moisture balance.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410730

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