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Biogeochemistry

Gassing Earth Out of the Ice Age: the North Pacific

Paper: Gray, William R., et al. 2018. Deglacial upwelling, productivity and CO2 outgassing in the North Pacific Ocean. Nature Geoscience, v.11: 340–344 

The polar North Atlantic and Southern oceans have important roles in ice-age cycles due to their influence on deep-ocean carbon storage. But the subarctic Pacific Ocean (just below the Bering Sea and the Aleutian Islands) is rarely considered as important because deep waters formed in this basin make up a tiny proportion of the deep-ocean volume. However, this region has undergone some major changes across glacial and interglacial states that could have affected the global carbon cycle.

The interaction between ocean circulation and biological productivity affects atmospheric carbon dioxide (CO2) levels and ocean oxygen concentrations. Today’s subarctic Pacific is capped by a freshwater layer that limits the sinking of surface waters and thus limits the transport of carbon from the surface to the deep ocean. But it is also characterized by the regional upwelling of nutrient- and CO2-rich waters, which fuels biological production.

Fig 1. (a) Annual surface ocean–atmosphere CO2 flux, with contours of surface ocean phosphate (PO4). The location of the studied core is indicated by a star. (b) Upper water column profiles of nutrients (DIC and phosphate) in the western subpolar North Pacific. (Credit: Gray et al. 2017)

The transition from cold to warm occurred abruptly at the beginning of the Bølling–Allerød period. The Bølling-Allerød was an abrupt interstadial period (a minor period of less-cold climate during a glacial period) that occurred during the final stages of the last glacial period. It ran from about 14,700 to 12,700 years ago. Stadials and interstadials are phases dividing the Quaternary period (the last 2.6 million years) – Stadials are periods of colder climate while interstadials are periods of warmer climate. During the warming of the last deglaciation, the North Pacific experienced a peak in biological productivity and widespread hypoxia (oxygen deficiency) (Fig. 1) in subsurface waters, potentially caused by changes in circulation, iron supply, and light limitation.

Fig 2. Foraminifera (plankton) shells like these are preserved in sediment cores and can tell us about ocean conditions at the time that they were grown. (Credit: Wikicommons)

Gray and fellow researchers studied the boron-isotope composition of ancient plankton (foraminifera, Fig. 2) from a sediment core in the deep, western North Pacific to reconstruct pH and dissolved CO2 concentrations from 24,000 to 8,000 years ago. Boron isotopes provide unique constraints on surface pH (how acidic the water is) at the time of shell growth and allow the reconstruction of past changes in seawater CO2 concentrations.

The results showed that the plankton productivity peak during the Bølling–Allerød interval was associated with a decrease in near-surface pH (more acidic) and an increase in CO2. This suggests that CO2 was being produced faster than it was being used by plankton at the surface. Therefore, the subarctic Pacific was a local source of COto the atmosphere, and could have helped maintain high CO2 concentrations. In turn, this would maintain the greenhouse gas momentum required to propel Earth’s climate out of the last glacial period.

How the Subarctic Pacific Got its CO2 Boost

Gray and his colleagues used the Paleoclimate Model Intercomparison Project (a project to coordinate atmospheric circulation models and to assess their ability to simulate large climate changes) to suggest that the presence of large North American ice sheets enhanced the Aleutian Low (a semi-permanent low-pressure system located near the Aleutian Islands in the Bering Sea, which can trap and intensity storms and wind). This would then increase wind over the subarctic Pacific during the Bølling-Allerød and enhance the upwelling intensity by Ekman transport. Ekman transport refers to the wind-driven net transport of the surface layer of a fluid that occurs at 90° to the direction of the surface wind (Fig. 3). Ekman transport is a factor in coastal upwelling which provide the nutrient supply for some of the largest fishing markets on the planet.

Fig. 3. Mechanisms of Ekman transport. Wind along coast pushes surface water away from coast, allowing deep water to upwell and bring nutrients to surface ocean. (Credit: Wikipedia)

The researchers thus argue that it was the juxtaposition of a large North American ice sheet with a strong Atlantic meridional overturning circulation 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 the accumulation rate of biogenic particles on the seafloor, used as an indicator of export production from the sea surface, may have a more complex interpretation than previously thought. The efficiency with which export production is transferred to the seafloor can vary geographically, 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 production.

Summary

In a climate model ensemble, the presence of large ice sheets over North America results in high rates of wind-driven upwelling within the subarctic North Pacific. The authors suggest that this process, combined with ocean circulation patterns at the onset of the Bølling–Allerød, led to high rates of upwelling of water rich in nutrients and CO2, and supported the peak in biologic productivity recorded in sediment isotopes. The respiration of this organic matter, along with poor circulation, probably caused the regional hypoxia. This study suggests that CO2 outgassing from the North Pacific helped to maintain high atmospheric CO2 concentrations during the Bølling–Allerød and contributed to the deglacial CO2 rise.

This study has provided important 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 impacts, is sure to reveal more surprises in future.

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