Aptian–Albian clumped isotopes from northwest China: cool temperatures, variable atmospheric <i>p</i>CO<sub>2</sub> and regional shifts in the hydrologic cycle
<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 cla...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2021-08-01
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| Series: | Climate of the Past |
| Online Access: | https://cp.copernicus.org/articles/17/1607/2021/cp-17-1607-2021.pdf |
| Summary: | <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><sup>13</sup></span>C and <span class="inline-formula"><i>δ</i><sup>18</sup></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><sup>13</sup></span>C, <span class="inline-formula"><i>δ</i><sup>18</sup></span>O and <span class="inline-formula">Δ<sub>47</sub></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><sup>18</sup></span>O measurements of dinosaur apatite phosphate (<span class="inline-formula"><i>δ</i><sup>18</sup></span>O<span class="inline-formula"><sub>p</sub></span>) and carbonate (<span class="inline-formula"><i>δ</i><sup>18</sup></span>O<span class="inline-formula"><sub>carb</sub></span>) and therefore
rely on estimates of meteoric water <span class="inline-formula"><i>δ</i><sup>18</sup></span>O (<span class="inline-formula"><i>δ</i><sup>18</sup></span>O<span class="inline-formula"><sub>mw</sub></span>) from <span class="inline-formula"><i>δ</i><sup>18</sup></span>O<span class="inline-formula"><sub>p</sub></span>. Significant shifts in the
hydrologic cycle likely influenced <span class="inline-formula"><i>δ</i><sup>18</sup></span>O<span class="inline-formula"><sub>mw</sub></span> in the region,
complicating these MAAT estimates. Thus, temperature records independent of
<span class="inline-formula"><i>δ</i><sup>18</sup></span>O<span class="inline-formula"><sub>mw</sub></span> (e.g., clumped isotopes or <span class="inline-formula">Δ<sub>47</sub></span>) are
desirable and required to confirm temperatures estimated with <span class="inline-formula"><i>δ</i><sup>18</sup></span>O<span class="inline-formula"><sub>p</sub></span> and <span class="inline-formula"><i>δ</i><sup>18</sup></span>O<span class="inline-formula"><sub>c</sub></span> and to reliably determine regional shifts in <span class="inline-formula"><i>δ</i><sup>18</sup></span>O<span class="inline-formula"><sub>mw</sub></span>. Primary carbonate material was
identified using traditional petrography, cathodoluminescence inspection, and <span class="inline-formula"><i>δ</i><sup>13</sup></span>C and <span class="inline-formula"><i>δ</i><sup>18</sup></span>O subsampling. Our preliminary
<span class="inline-formula">Δ<sub>47</sub></span>-based temperature reconstructions (record mean of
14.9 <span class="inline-formula"><sup>∘</sup></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"><sup>∘</sup></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"><sub>2</sub></span>
concentration of 396 ppmv, indicates relatively cooler midlatitude
terrestrial climate. Additionally, our coupled <span class="inline-formula"><i>δ</i><sup>18</sup></span>O and <span class="inline-formula">Δ<sub>47</sub></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><sup>18</sup></span>O<span class="inline-formula"><sub>mw</sub></span>) that may track
Aptian–Albian climate perturbations (i.e., a drying of Asian continental
climate associated with the cool interval).</p> |
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| ISSN: | 1814-9324 1814-9332 |