Aptian-Albian clumped isotopes from northwest China: Cool temperatures, variable atmospheric pCO2 and regional shifts in the hydrologic cycle

Abstract

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 13C and 18O (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 (13C, 18O and 147) and CALMAG 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 18O measurements of dinosaur apatite phosphate (18Op) and carbonate (18Ocarb) and therefore rely on estimates of meteoric water 18O (18Omw) from 18Op. Significant shifts in the hydrologic cycle likely influenced 18Omw in the region, complicating these MAAT estimates. Thus, temperature records independent of 18Omw (e.g., clumped isotopes or 147) are desirable and required to confirm temperatures estimated with 18Op and 18Oc and to reliably determine regional shifts in 18Omw. Primary carbonate material was identified using traditional petrography, cathodoluminescence inspection, and 13C and 18O subsampling. Our preliminary 147-based temperature reconstructions (record mean of 14.9 C), which we interpret as likely being representative of MAAT, match prior estimates from similar paleolatitudes of Asian MAAT (average 15 C) across the Aptian-Albian. This, supported by our estimated mean atmospheric paleo-pCO2 concentration of 396 ppmv, indicates relatively cooler midlatitude terrestrial climate. Additionally, our coupled 18O and 147 records suggest shifts in the regional hydrologic cycle (i.e., 1MAP, mean annual precipitation, and 118Omw) that may track Aptian-Albian climate perturbations (i.e., a drying of Asian continental climate associated with the cool interval).