How Hot Is Too Hot? Disentangling Mid-Cretaceous Hothouse Paleoclimate from Diagenesis

Abstract

The North American Newark Canyon Formation (~113–98 Ma) presents an opportunity to examine how various terrestrial carbonate facies reflect different aspects of paleoclimate during one of the hottest periods of Earth’s history. We combined carbonate facies analysis with δ13C, δ18O, and Δ47 datasets to assess which palustrine and lacustrine facies preserve stable isotope signals that are most representative of climatic conditions. Type section palustrine facies record the heterogeneity of the original palustrine environment in which they formed. Using the pelmicrite facies that formed in deeper wetlands, we interpret a lower temperature zone (35–40°C) to reflect warm season water temperatures. In contrast, the mottled micrite facies reflects hotter temperatures (36–68°C). These hotter temperatures preserve radiatively heated “bare-skin” temperatures that occurred in a shallow depositional setting. The lower lacustrine unit has been secondarily altered by hydrothermal fluids while the upper lacustrine unit likely preserves primary temperatures and δ18Owater of catchment-integrated precipitation. Based on this investigation, the palustrine pelmicrite and lacustrine micrite are the facies most likely to reflect ambient climate conditions, and therefore, are the best facies to use for paleoclimate interpretations. Average warm season water temperatures of 41.1±3.6°C and 37.8±2.5°C are preserved by the palustrine pelmicrite (~113–112 Ma) and lacustrine micrite (~112–103 Ma), respectively. These data support previous interpretations of the mid-Cretaceous as a hothouse climate. Our study demonstrates the importance of characterizing facies for identifying the data most representative of past climates.