CHARACTERIZING AN “ENHANCED HYDROLOGIC CYCLE” FROM CRETACEOUS MEXICO USING CLUMPED AND TRIPLE OXYGEN ISOTOPES OF MARGINAL MARINE CARBONATES

Multi-proxy isotope geochemistry is a developing approach that can be used to accurately reconstruct paleoclimate conditions of key greenhouse periods like the Cretaceous (145-66 Ma). Paired clumped isotope (Δ₄₇) and triple oxygen isotope analyses enable clear identification of altered and pristine carbonate material and unambiguous reconstruction of carbonate formation temperatures and formation water oxygen isotope composition. In this study, we employ these isotopic tools to investigate the Cretaceous Enhanced Hydrologic Cycle (EHC) hypothesis, a proposed mechanism for increasing latent heat transport from the tropics to poles during periods of elevated global temperatures. Sparry calcite and micrite from the Mexican Tlayúa Formation were identified using cathodoluminescence analysis and subsequently subsampled to constrain the differences between primary and secondary geochemical signals. Three sampling schemes were assigned to the samples: dimly-luminescent micrite (MC), luminescent spar (LSP), and non-luminescent spar (NLSP). Calculated Δ₄₇ temperature for MC samples range from 34 °C to 48 °C, while NLSP and LSP Δ₄₇ temperatures range from 29 °C to 34 °C and 28 °C to 36 °C, respectively. Analysis of Δ’¹⁷O vs. δ’¹⁸O reveals that the majority of the Tlayúa limestones precipitated under conditions of isotopic disequilibrium due to high evaporation rates. A single pristine sample indicates that the original formation temperatures were >30 °C. Post-deposition, the bulk of the Tlayúa samples experienced diagenetic alteration at relatively low temperatures (<100 °C). This study sheds light on how paired clumped and triple oxygen isotope analyses can be used to disentangle equilibrium and disequilibrium isotope signals in carbonates to constrain the properties of Earth’s hydrologic system during extreme greenhouse conditions.