Impacts of dynamic plankton iron quotas on carbon cycle sensitivity to atmospheric iron deposition

Ocean biogeochemistry plays a key role in the global carbon cycle through uptake of atmospheric carbon dioxide (CO2) and long-term storage via the biological and solubility pumps. Phytoplankton help drive the biological pump by taking up dissolved CO2 and converting it to organic matter via primary productivity. This carbon can then be stored in the deep ocean through sinking organic matter.

Dissolved iron (dFe) plays a particularly important role in regulating biogeochemical cycles. In high nutrient low chlorophyll (HNLC) regions, which includes > 33% of the global ocean surface, iron is the key limiting nutrient. It also can regulate nitrogen fixation by diazotrophs in low-latitude regions, and theoretically controls up to half of global ocean productivity. The link between iron availability and carbon export is strongly dependent on the phytoplankton iron quota, or Fe:C ratio. This ratio can vary by more than an order of magnitude and seems to be correlated with ambient dissolved iron concentration in sparse observations.

The Community Earth System Model Biogeochemical Elemental Cycling (CESM-BEC) model contains three explicit phytoplankton groups: diatoms, nanophytoplankton, and diazotrophs, with dynamic iron quotas (Fe:C) that vary as a function of ambient iron. Here we present new limits for the phytoplankton iron quotas for each group. We evaluated the new simulated Fe:C ratios and the resulting nutrient distributions when spun up to modern values against limited field data in order to improve the representation of this key Fe-C link in the model. We also found that properly accounting for these plankton iron quotas weakens marine carbon cycle sensitivity to variations in atmospheric iron inputs, indicating that fluctuations in atmospheric iron deposition from pyrogenic (fossil fuel) sources with changing emissions scenarios will have minimal impact on the biological pump and air-sea CO2 exchange.