Aptian-Albian Clumped Isotopes From Northwest China: Cool Temperatures, Variable Atmospheric Pco(2) And Regional Shifts In The Hydrologic Cycle

The Early Cretaceous is characterized by warm background temperatures (i.e., greenhouse climate) and carbon cycle perturbations that are often marked by ocean anoxic events (OAEs) and associated shifts in the hydrologic cycle. Higher-resolution records of terrestrial and marine delta C-13 and delta O-18 (both carbonates and organics) suggest climate shifts during the Aptian-Albian, including a warm period associated with OAE 1a in the early Aptian and a subsequent "cold snap" near the Aptian-Albian boundary prior to the Kilian and OAE 1b. Understanding the continental system is an important factor in determining the triggers and feedbacks to these events. Here, we present new paleosol carbonate stable isotopic (delta C-13, delta O-18 and Delta(47)) and CAL-MAG weathering parameter results from the Xiagou and Zhonggou formations (part of the Xinminpu Group in the Yujingzi Basin of NW China) spanning the Aptian-Albian. Published mean annual air temperature (MAAT) records of the Barremian-Albian from Asia are relatively cool with respect to the Early Cretaceous. However, these records are largely based on coupled delta O-18 measurements of dinosaur apatite phosphate (delta O-18(p)) and carbonate (18O(carb)) and therefore rely on estimates of meteoric water delta O-18 (delta O-18(mw)) from delta O-18(p). Significant shifts in the hydrologic cycle likely influenced delta O-18(mw) in the region, complicating these MAAT estimates. Thus, temperature records independent of delta O-18(mw) (e.g., clumped isotopes or Delta(47)) are desirable and required to confirm temperatures estimated with delta O-18(p) and delta O-18(c) and to reliably determine regional shifts in delta O-18(mw). Primary carbonate material was identified using traditional petrography, cathodoluminescence inspection, and delta C-13 and delta O-18 subsampling. Our preliminary Delta(47)-based temperature reconstructions (record mean of 14.9 degrees C), which we interpret as likely being representative of MAAT, match prior estimates from similar paleolatitudes of Asian MAAT (average similar to 15 degrees C) across the Aptian-Albian. This, supported by our estimated mean atmospheric paleo-pCO(2) concentration of 396 ppmv, indicates relatively cooler midlatitude terrestrial climate. Additionally, our coupled delta O-18 and Delta(47) records suggest shifts in the regional hydrologic cycle (i.e., Delta MAP, mean annual precipitation, and Delta delta O-18(mw)) that may track Aptian-Albian climate perturbations (i.e., a drying of Asian continental climate associated with the cool interval).