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

Abstract The intensification of Northern Hemisphere glaciation (iNHG) at 2.73 Ma 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 p CO 2 and global temperatures. These issues are addressed here using the boron isotope (δ 11 B) proxy in diatom frustules to reconstruct past changes in the pH and p CO 2 of ambient seawater. Diatom δ 11 B 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 CO 2 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 δ 13 C 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 p CO 2 and the long‐term expansion of ice‐sheets across the Northern Hemisphere.