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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
δ13C and δ18O (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 (δ13C, δ18O and Δ47) 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 δ18O measurements of dinosaur apatite phosphate (δ18Op) and carbonate (δ18Ocarb) and therefore
rely on estimates of meteoric water δ18O (δ18Omw) from δ18Op. Significant shifts in the
hydrologic cycle likely influenced δ18Omw in the region,
complicating these MAAT estimates. Thus, temperature records independent of
δ18Omw (e.g., clumped isotopes or Δ47) are
desirable and required to confirm temperatures estimated with δ18Op and δ18Oc and to reliably determine regional shifts in δ18Omw. Primary carbonate material was
identified using traditional petrography, cathodoluminescence inspection, and δ13C and δ18O subsampling. Our preliminary
Δ47-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 δ18O and Δ47 records suggest shifts in the regional hydrologic cycle (i.e.,
ΔMAP, mean annual precipitation, and Δδ18Omw) that may track
Aptian–Albian climate perturbations (i.e., a drying of Asian continental
climate associated with the cool interval).
