Volcanism‐Triggered Climatic Control on Late Cretaceous Oceans

• Published in: Geochemistry, geophysics, geosystems : G3 Vol. 23; no. 4
• Main Authors: Sun, Ruiyang, Yao, Hanwei, Deng, Changzhou, Grasby, Stephen E., Wang, Chengshan, Chen, Xi, Yin, Runsheng
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01-04-2022; Wiley
• Subjects: Anoxia; black shale; Climate; Climate control; climate/ocean dynamics; Cretaceous; Dynamics; Isotope composition; Isotopes; Late Cretaceous; Mercury; Ocean dynamics; oceanic red beds; Oceans; Oxic conditions; Runoff; Sedimentary rocks; Shale; Shales; Volcanism
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2021GC010292

During the Late Cretaceous, Earth's climate oscillated between warm and cool states, and global oceans changed between anoxic and oxic conditions, resulting in black/gray shales and oceanic red beds (ORBs) deposition, respectively. To understand such climate/ocean dynamics, this study investigated bulk Hg and Hg isotopes, as well as Fe3+/Fe2+ in Upper Cretaceous sediments deposited in southern Tibet and the North Atlantic. In both areas, black/gray shales show much higher Hg concentrations than ORBs, indicating enhanced Hg flux to global oceans during time of black/gray shale deposition. Black/gray shales show lower Fe3+/Fe2+ and positive Δ199Hg, suggesting a significant input of Hg into the anoxic/dysoxic ocean via atmospheric deposition. The isotope values are consistent with a volcanic source for this excess Hg. ORBs show high Fe3+/Fe2+ and negative shifts of Δ199Hg, suggesting that the dominant source of Hg into the oxic oceans was via terrestrial runoff. This study suggests that volcanism was an important driver of the climate/ocean dynamics during the Late Cretaceous.
Plain Language Summary
The Late Cretaceous ocean developed widespread black/gray shales and oceanic red beds which were deposited in warm‐anoxic/dysoxic and cool‐oxic oceanic conditions, respectively. The driving force for such climate/ocean dynamics remains unclear. This study demonstrates distinct Hg concentrations and isotopic composition in black/gray shales and oceanic red beds, which suggest that volcanism was a major trigger of the climate/ocean dynamics in the Late Cretaceous.
Key Points
The Late Cretaceous black/gray shales and oceanic red beds display opposing mass‐independent fractionation of Hg isotopes
Changes in climate/ocean conditions had strong control on the sources of Hg in the Late Cretaceous ocean
Volcanism served as an important driving force of the climate/ocean dynamics during the Late Cretaceous