Carbon Cycle and Circulation Change in the North Pacific Ocean at the Initiation of Northern Hemisphere Glaciation Constrained by Boron-Based Proxies in Diatoms

Donald, H. K.; Swann, G. E.A.; Rae, J. W.B.; Foster, G. L.

Abstract

The intensification of Northern Hemisphere glaciation (iNHG) at 2.73Ma is associated with a reorganization of the subarctic Pacific Ocean and abrupt drop in opal mass accumulation rates. Uncertainty, however, remains around the extent to which these changes altered carbon dynamics and contributed to a reduction in atmospheric pCO2 and global temperatures. These issues are addressed here using the boron isotope (δ11B) proxy in diatom frustules to reconstruct past changes in the pH and pCO2 of ambient seawater. Diatom δ11B and [B] indicate a subarctic Pacific surface water increase of 0.3–0.5 pH units over the iNHG. This confirms that delivery of carbon and nutrients into surface waters was reduced at this time, explaining the drop in opal productivity and limiting CO2 outgassing from the ocean interior. We consider two hypotheses to explain this based on potential changes in circulation from the late Pliocene to early Pleistocene: “ventilation to stratification” or “stratification to ventilation.” The ventilation to stratification hypothesis, which posits a switch from vigorous Pacific Meridional Overturning Circulation in the Pliocene to stratification over iNHG, has received more attention in the literature. The stratification to ventilation hypothesis, which posits a modest increase in ventilation, is more consistent with modern and late Pleistocene analogs, the majority of models and δ13C data. These late Pliocene changes in the subarctic Pacific, in conjunction with other external and internal processes including those in the Southern Ocean, would have contributed to a lowering of atmospheric pCO2 and the long-term expansion of ice-sheets across the Northern Hemisphere.

Citation

Donald, H. K., Swann, G. E., Rae, J. W., & Foster, G. L. (2024). Carbon Cycle and Circulation Change in the North Pacific Ocean at the Initiation of Northern Hemisphere Glaciation Constrained by Boron-Based Proxies in Diatoms. Paleoceanography and Paleoclimatology, 39(11), Article e2024PA004968. https://doi.org/10.1029/2024pa004968

• Journal Article Type: Article
• Acceptance Date: Oct 7, 2024
• Online Publication Date: Nov 5, 2024
• Publication Date: 2024-11
• Deposit Date: Nov 5, 2024
• Publicly Available Date: May 6, 2025
• Journal: Paleoceanography and Paleoclimatology
• Print ISSN: 2572-4517
• Electronic ISSN: 2572-4525
• Peer Reviewed: Peer Reviewed
• Volume: 39
• Issue: 11
• Article Number: e2024PA004968
• DOI: https://doi.org/10.1029/2024pa004968
• Keywords: diatom, boron, pliocene, subarctic pacific, northern hemisphere glaciation, carbon
• Public URL: https://nottingham-repository.worktribe.com/output/41544681
• Publisher URL: https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2024PA004968