Abstract
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).
Highlights
Cretaceous climate is characterized by a warm background greenhouse climate state and perturbations to climate and the carbon cycle associated with shifts in global δ13C, including Cretaceous ocean anoxic events (OAEs; Föllmi, 2012; Hay, 2016; Jenkyns, 2018)
mean annual air temperature (MAAT) estimates from δ18O of dinosaur tooth enamel phosphate (δ18Op) hinge on the relationship between mean annual temperature, latitude and the δ18O of meteoric water or δ18Omw (Amiot et al, 2004). δ18Omw is influenced by other parameters in addition to temperature and latitude and is further complicated as the intensity of poleward moisture transport is altered by greenhouse climate conditions
New continental Asia midlatitude multi-proxy records of the Aptian–Albian carbon cycle, climate and hydrologic cycle suggest cool conditions in Early Cretaceous midlatitudes relative to background Cretaceous greenhouse warmth, consistent with our estimated atmospheric pCO2 calculated using carbon isotopes in pedogenic carbonates and previous regional MAAT observations (Amiot et al, 2011)
Summary
Cretaceous climate is characterized by a warm background greenhouse climate state and perturbations to climate and the carbon cycle associated with shifts in global δ13C, including Cretaceous ocean anoxic events (OAEs; Föllmi, 2012; Hay, 2016; Jenkyns, 2018). Estimates of Aptian–Albian atmospheric paleopCO2, while highly uncertain, tend to suggest low (less than 1000 to 1500 ppmv background greenhouse climate conditions; Franks et al, 2014) concentrations at the Aptian– Albian consistent with a cooler climate (Ekart et al, 1999; Wallmann, 2001; Fletcher et al, 2005; Aucour et al, 2008; Passalia, 2009; Haworth et al, 2010; Du et al, 2018) This C10 interval has been identified on land using stable isotopes in terrestrial paleosol carbonates and organic carbon from the continental interiors of North America (Ludvigson et al, 2010; Suarez et al, 2014) and Asia (Suarez et al, 2018). Confirming these temperatures with a secondary geochemical proxy is warranted
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