Aptian–Albian clumped isotopes from northwest China: cool temperatures, variable atmospheric <i>p</i>CO₂ and regional shifts in the hydrologic cycle

• Main Authors: D. T. Harper, M. B. Suarez, J. Uglesich, H. You, D. Li, P. Dodson
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-08-01
• Series: Climate of the Past
• Online Access: https://cp.copernicus.org/articles/17/1607/2021/cp-17-1607-2021.pdf
• ISSN: 1814-9324; 1814-9332

<p>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 <span class="inline-formula"><i>δ</i>¹³</span>C and <span class="inline-formula"><i>δ</i>¹⁸</span>O (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 (<span class="inline-formula"><i>δ</i>¹³</span>C, <span class="inline-formula"><i>δ</i>¹⁸</span>O and <span class="inline-formula">Δ₄₇</span>) 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 <span class="inline-formula"><i>δ</i>¹⁸</span>O measurements of dinosaur apatite phosphate (<span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_p</span>) and carbonate (<span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_carb</span>) and therefore rely on estimates of meteoric water <span class="inline-formula"><i>δ</i>¹⁸</span>O (<span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_mw</span>) from <span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_p</span>. Significant shifts in the hydrologic cycle likely influenced <span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_mw</span> in the region, complicating these MAAT estimates. Thus, temperature records independent of <span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_mw</span> (e.g., clumped isotopes or <span class="inline-formula">Δ₄₇</span>) are desirable and required to confirm temperatures estimated with <span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_p</span> and <span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_c</span> and to reliably determine regional shifts in <span class="inline-formula"><i>δ</i>¹⁸</span>O<span class="inline-formula">_mw</span>. Primary carbonate material was identified using traditional petrography, cathodoluminescence inspection, and <span class="inline-formula"><i>δ</i>¹³</span>C and <span class="inline-formula"><i>δ</i>¹⁸</span>O subsampling. Our preliminary <span class="inline-formula">Δ₄₇</span>-based temperature reconstructions (record mean of 14.9 <span class="inline-formula">^∘</span>C), which we interpret as likely being representative of MAAT, match prior estimates from similar paleolatitudes of Asian MAAT (average <span class="inline-formula">∼</span> 15 <span class="inline-formula">^∘</span>C) across the Aptian–Albian. This, supported by our estimated mean atmospheric paleo-<span class="inline-formula"><i>p</i></span>CO<span class="inline-formula">₂</span> concentration of 396 ppmv, indicates relatively cooler midlatitude terrestrial climate. Additionally, our coupled <span class="inline-formula"><i>δ</i>¹⁸</span>O and <span class="inline-formula">Δ₄₇</span> records suggest shifts in the regional hydrologic cycle (i.e., <span class="inline-formula">Δ</span>MAP, mean annual precipitation, and <span class="inline-formula">Δ<i>δ</i>¹⁸</span>O<span class="inline-formula">_mw</span>) that may track Aptian–Albian climate perturbations (i.e., a drying of Asian continental climate associated with the cool interval).</p>