The causes of the 90-ppm glacial atmospheric CO2 drawdown still strongly debated
Joint Science Highlight with US-Ocean Carbon & Biogeochemistry (US-OCB).
Using an observationally constrained earth system model, S. Khatiwala and co-workers (2019, see reference below) compare different processes that could lead to the 90-ppm glacial atmospheric CO2 drawdown, with an important improvement on the deep carbon storage quantification (i.e. Biological Carbon Pump efficiency). They demonstrate that circulation and sea ice changes had only a modest net effect on glacial ocean carbon storage and atmospheric CO2, whereas temperature and iron input effects were more important than previously thought due to their effects on disequilibrium carbon storage.
Figure: Illustration of the two main mechanisms identified by this study to explain lower atmospheric CO2 during glacial periods. Left: present-day conditions; right: conditions around 19,000 years ago during the Last Glacial Maximum. The obvious explanation for lower CO2 during glacial periods – cooler ocean temperatures (darker blue shade) making CO2 more soluble, much as a glass of sparkling wine will remain fizzier for longer when it is colder – has long been dismissed as not being a significant factor. However, previous calculations assumed that the ocean cooled uniformly and was saturated in dissolved CO2. The model, consistent with reconstructions of sea surface temperature, predicts more cooling at mid latitudes compared with polar regions and also accounts for undersaturation. This nearly doubles the effect of temperature change and accounts for almost half the 90 ppm glacial-interglacial atmospheric CO2 difference. Another quarter is explained in this model by increased growth of marine algae (green blobs and inset) in the waters off Antarctica. Algae absorb CO2 from the atmosphere during photosynthesis and “pump” it into the deep ocean when they die and sink. But their growth in the present-day ocean, especially the waters off Antarctica, is limited by the availability of iron, an essential micronutrient primarily supplied by wind-borne dust. In our model an increased supply of iron to the Southern Ocean, likely originating from Patagonia, Australia and New Zealand, enhances their growth and sucks CO2 out of the atmosphere. This “fertilization” effect was greatly underestimated by previous studies. The study also finds that, contrary to the current consensus, a large expansion of sea ice off Antarctica and reconfiguration of ocean circulation may have played only a minor role in glacial-interglacial CO2 changes. Credit: Illustration by Andrew Orkney, University of Oxford.
Khatiwala, S., Schmittner, A., & Muglia, J. (2019). Air-sea disequilibrium enhances ocean carbon storage during glacial periods. Science Advances, 5(6), eaaw4981. DOI: https://doi.org/10.1126/sciadv.aaw4981
About the decoupled fates of aluminium, manganese, cobalt and lead in the North Pacific Ocean
Did you know that each of these tracers could follow its own marine story, quite decoupled from the others?
This is what is shown and discussed by Zheng and co-workers (2019, see reference below) after having analysed about 500 samples for aluminium (Al), manganese (Mn), lead (Pb) and cobalt (Co) along three sections in the North Pacific Ocean. They demonstrate that the distribution of each element is uniquely related to ocean circulation; that the subsurface Pb maximum has been sustained in the North Pacific Ocean through the growth of anthropogenic sources in Asia and Russia, contrasting with the decrease observed in the Atlantic Ocean (please also read the science highlight from Bridgestock et al., 2016); that the labile fraction of particulate Al is larger than that of particulate lead; and finally that while the Pb enrichment factor confirms its predominant atmospheric origin, those of Mn and Co clearly attest that sources other than the aerosol deposition are more significant contributors to the concentrations of these two tracers.
Figure: Sectional distributions of dissolved metals (dM) and potential density anomaly at depths of 0–1200 m along 160°W (section highlighted in red in the map). Dissolved aluminium (dAl) is high in Equatorial Under Current (EQ, 175 m depth) and North Equatorial Current (20°N, surface). Although dissolved manganese (dMn) and dissolved cobalt (dCo) have a concurrent source at the continental shelf of the Aleutian Islands, dCo is more widely distributed via North Pacific Intermediate Water (NPIW, ~600 m). Dissolved lead (dPb) is concentrated in Subtropical Mode Water and Central Mode Water above the NPIW. Adapted from Zheng et al., 2019. Click here to view the figure larger.
Zheng, L., Minami, T., Konagaya, W., Chan, C.-Y., Tsujisaka, M., Takano, S., Norisuye, K., Sohrin, Y. (2019). Distinct basin-scale-distributions of aluminum, manganese, cobalt, and lead in the North Pacific Ocean. Geochimica et Cosmochimica Acta, 254, 102–121. DOI: http://doi.org/10.1016/J.GCA.2019.03.038
Bridgestock, L., van de Flierdt, T., Rehkämper, M., Paul, M., Middag, R., Milne, A., Lohan, M.C., Baker, A.R., Chance, R.,, Khondoker, R., Strekopytov, S., Humphreys-Williams, E., Achterberg, E.P., Rijkenberg, M.J.A., Gerringa, L. J.A., de Baar, H. J. W. (2016). Return of naturally sourced Pb to Atlantic surface waters. Nature Communications, 7, 12921. doi: http://doi.org/10.1038/ncomms12921