Carbon dynamics in the subarctic Pacific Ocean - diatom δ11B insights into the past

The intensification of Northern Hemisphere glaciation (iNHG) at 2.73 Ma is associated in the subarctic Pacific Ocean with an abrupt drop in opal mass accumulation rates and the development of a halocline stratification. Uncertainty, however, remains around the extent to which these changes: 1) altered carbon dynamics in the water column; and 2) contributed to a reduction in atmospheric pCO₂ over this period. These issues can be resolved by applying the boron isotope (δ¹¹B) proxy to quantify past changes in the pH and pCO₂ of ambient seawater. By analysing δ¹¹B in diatom frustules, we document a surface water increase of 0.3 to 0.5 pH units in the subarctic Pacific Ocean over the iNHG. This confirms that the development of the halocline at 2.73 Ma both reorganised the vertical water column structure and limited CO₂ outgassing from the ocean interior. In conjunction with other external and internal processes, including changes in the Southern Ocean, these events would have contributed to a lowering of atmospheric pCO₂ and the long-term expansion of ice sheets across the Northern Hemisphere.

This research demonstrates the potential for diatom δ¹¹B measurements to be used to reconstruct changes in pCO₂, particularly in high-latitude settings where diatom frustules are readily preserved and dominate the sediment record. Such analyses have the potential to complement δ¹¹B measurements in foraminifera and so provide further insights into the carbon dynamics of the “warm Pliocene” and other periods of interest.