Predicting and monitoring the effects of large-scale ocean iron fertilization on marine trace gas emissions

Large-scale (> 40 000 km(2), >1 yr) ocean iron fertilization (OIF) is being considered as an option for mitigating the increase in atmospheric CO2 concentrations. However OIF will influence trace gas production and atmospheric emissions, with consequences over broad temporal and spatial scales. To illustrate this, the response of nitrous oxide (N2O) and dimethylsulphide (DMS) in the mesoscale iron addition experiments (FeAXs) and model scenarios of large-scale OIF are examined. FeAXs have shown negligible to minor increases in N2O production., whereas models of long-term OIF suggest significant N2O production with the potential to offset the benefit gained by iron-mediated increases in CO2 uptake. N2O production and emission will be influenced by the magnitude and rate of vertical particle export, and along-isopycnal N2O transport will necessitate monitoring over large spatial scales. The N2O-O-2 relationship provides a monitoring option using oxygen as a proxy, with spatial coverage by Argo and glider-mounted oxygen optodes. Although the initial FeAXs exhibited similar increases (1.5- to 1.6-fold) in DMS, a subsequent sub-arctic Pacific experiment observed DMS consumption relative to unfertilized waters, highlighting regional variability as a complicating factor when predicting the effects of large-scale OIF. DMS cycling and its influence on atmospheric composition may be studied using naturally occurring blooms and be constrained prior to OIF by pre-fertilization spatial mapping and aerial sampling using new technologies. As trace gases may have positive or negative synergistic effects on atmospheric chemistry and climate forcing, the net effect of altered trace gas emissions needs to be considered in both models and monitoring of large-scale OIF.