Aptian-Albian clumped isotopes from the Xiagou and Zhonggou Formations, northwest China: Cool summer temperatures and regional shifts in hydrologic cycle
Abstract
The Early Cretaceous is characterized by warm background temperatures (i.e., greenhouse climate), C 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 subsequent "cold snap" near the Aptian-Albian boundary (associated with 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 results (δ13C, δ18O and Δ47) from the Xiagou and Zhonggou Formations (part of the Xinminpu Group in the Yujingzi Basin of NW China) spanning the Aptian-Albian based on C-isotope chemostratigraphy1. Mean annual temperature (MAT) records of the Barremian-Albian from Asia are relatively cool with respect to the Early Cretaceous2. However, these records are largely based on coupled δ18O measurements of dinosaur apatite phosphate (δ18Op) and carbonate (δ18Oc), 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 MAT 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. erOur preliminary Δ47-based temperature reconstructions (averaging in the mid to low 20s°C) are warmer than prior estimates (average ~15°C). However, considering summer biases in soil carbonate formation, they are broadly consistent with, published mid-latitude records of Asian MAT across the Aptian-Albian2. This indicates relatively cooler mid-latitude terrestrial greenhouse climate. Additionally, our coupled δ18O and Δ47 records, suggest shifts in the regional hydrologic cycle (i.e., Δδ18Omw) that may track short-term Aptian-Albian climate perturbations.
1Suarez et al., 2019, J. of Vert. Paleont.; 2Amiot et al., 2011, PNAS.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMPP13C1471H
- Keywords:
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- 1630 Impacts of global change;
- GLOBAL CHANGE;
- 4901 Abrupt/rapid climate change;
- PALEOCEANOGRAPHY;
- 4912 Biogeochemical cycles;
- processes;
- and modeling;
- PALEOCEANOGRAPHY;
- 4948 Paleocene/Eocene thermal maximum;
- PALEOCEANOGRAPHY