Enhanced upwelling and CO2 degassing from the subpolar North Pacific during a warm event 14,000 years ago may have helped keep atmospheric CO2 levels high enough to propel the Earth out of the last ice age.
Two of the high-latitude ocean regions, the North Atlantic and Southern oceans, have long been given starring roles in ice-age cycles due to their impacts on deep-ocean carbon storage1. But the subarctic Pacific Ocean is rarely considered, in part because deep waters formed in the basin make up a negligible proportion of the deep-ocean volume. Yet, this region has undergone major changes across glacial cycles2 and transitions between glacial and interglacial states3,4 that could have affected the global carbon cycle. Writing in Nature Geoscience, Gray and colleagues5 document a potentially important supporting role for the third polar ocean: the upwelling and release of CO2 from the subarctic North Pacific during the Bølling–Allerød warm interval of the last deglaciation.
Unlike the North Atlantic and Southern oceans, the modern subarctic Pacific is permanently capped by a freshwater lid that limits the large-scale sinking of surface waters to just a few hundred metres depth6. The subarctic Pacific is also characterized by the regional upwelling of nutrient- and CO2-charged subsurface waters, which fuel high rates of biological production despite strong iron limitation (Fig. 1). During the last deglaciation, this region experienced a dramatic decrease of subsurface oxygen concentrations3, and an increased accumulation of biogenic particles in sediments; the change occurred quite abruptly at the start of the Bølling–Allerød warm interval4.
Gray and colleagues5 report boron isotope data measured on surface-dwelling foraminifera from a marine sediment core retrieved from the deep northwest subarctic Pacific. Boron isotopes are challenging to measure, but provide unique constraints on surface pH at the time of shell growth, allowing past changes in seawater CO2 concentrations to be reconstructed7. The results indicate a general increase in near-surface ocean CO2 concentrations from the Last Glacial Maximum to the Holocene, consistent with the rise of atmospheric CO2 levels recorded in ice cores. However, unlike the ice core records, these data show a dramatic rise in surface CO2 concentrations during the Bølling–Allerød. The Bølling–Allerød values stand in stark contrast to the immediately preceding Heinrich Stadial 1, when the surface pCO2 was about 75 ppm lower.
Heinrich Stadial 1 was a cold period in the Northern Hemisphere, lasting from about 18,000 to 15,000 years ago. During this interval, atmospheric CO2 concentrations rose, which has been attributed to outgassing from the Southern Ocean. This cold period was followed by the Bølling–Allerød warming, when ice core CO2 jumped up to a stable plateau. A host of differences have been found between Heinrich Stadial 1 and the Bølling–Allerød throughout the world. Most have been linked to the formation of North Atlantic Deep Water and the associated Atlantic meridional overturning circulation, which were markedly reduced during Heinrich Stadial 1 and came churning back during the Bølling–Allerød8.
The increase in North Pacific surface CO2 concentrations during the Bølling–Allerød indicates that the supply of CO2 exceeded its uptake by phytoplankton; as such, the local efficiency of the biological carbon pump was reduced. The subarctic Pacific was therefore a local source of CO2 to the atmosphere, joining the eastern equatorial Pacific in outgassing at a time when the Southern Ocean was apparently taking a breather from its deglacial carbon release9. As such, the North Pacific could have helped to maintain high CO2 concentrations, keeping up the greenhouse momentum required to push the climate system out of the last glacial period.
This begs the question of what underlying mechanism could have caused these deglacial events in the subarctic Pacific. Pointing to the Paleoclimate Model Intercomparison Project multi-model ensemble, Gray and colleagues suggest that the presence of large North American ice sheets deepened the Aleutian Low pressure system, thereby increasing cyclonic wind stress over the subarctic Pacific and enhancing the upwelling intensity. A concomitant shallowing of the surface mixed layer could also have allowed the nutrient and CO2 reservoir to shoal, where it could be entrained by the strong Ekman pumping driven by the increased wind stress.
Gray et al. argue that it was the juxtaposition of a large North American ice sheet with a strong Atlantic meridional overturning circulation that led to the extreme characteristics of the Bølling–Allerød, relative to the Holocene, in the subarctic Pacific. But this is not necessarily the whole story. Other factors, such as changes in nutrient inventories or temperature-driven changes in remineralization10, may have also played significant roles. In addition, the accumulation rate of biogenic detritus at the seafloor, used in this and other studies as an indicator of export production from the sea surface, may have a more complex interpretation than previously realized. The efficiency with which export production is transferred to the seafloor can vary geographically11, meaning that the preserved sedimentary fluxes can become decoupled from the export from the surface through time. Thus, the dramatic changes recorded in sediments may have as much to do with ecosystem structure and function as with primary production12.
Gray et al.5 have provided important new insight into the dramatic changes that rippled through the North Pacific at the onset of the Bølling–Allerød. Further work to resolve how these changes altered the marine ecosystem in this under-studied region, and their global biogeochemical impacts, is sure to reveal more surprises in future.
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Jaccard, S.L., Galbraith, E.D. Push from the Pacific. Nature Geosci 11, 299–300 (2018). https://doi.org/10.1038/s41561-018-0119-3