Decreased Deep Water Oxygenation in the Western North Pacific During the Plio‐Pleistocene Transition

The deep North Pacific, a vast respired carbon reservoir, likely plays a key role in the Plio‐Pleistocene CO 2 decline and global cooling, but evidence for this is lacking. Here, oxygenation change in the upper deep western North Pacific between 5.3 and 1.0 million years ago (Ma) is reconstructed using multiple organic and inorganic proxies from the South China Sea. Results show higher oxygenation during the middle Pliocene, followed by continuous deoxygenation from 3.6 to 2.0 Ma. This deoxygenation, suggesting increased carbon sequestration and thus promoting CO 2 decline, coincides with an expansion of oxygen‐deficient Pacific deep water in the western North pacific, which could be due to progressive tectonic deformation of the northwestern Pacific basin. This study demonstrates a close connection between tectonics, ocean currents and carbon cycles.