Iron Source Attribution and the Age of Dissolved Iron in the Ocean

We quantify the contributions of the aeolian, sediment, and hydrothermal iron sources to the concentration of dissolved iron (dFe) and calculate the mean age of these contributions since their injection into the ocean. Our calculations use estimates of the global iron cycle from a simple inverse model constrained by a data-assimilated global circulation and by available dFe measurements, including the GEOTRACES Intermediate Data Product. We compare our rigorously calculated source contributions with commonly employed source anomalies, i.e., the differences between solutions with and without the source in question. We find that such source anomalies strongly underestimate the true contribution from any given source (by as much as a factor of two for the hydrothermal source) because of the nonlinearity of the iron scavenging. Aeolian iron is the largest contributor in the Southern Ocean euphotic zone, where its mean age reveals that this iron is supplied from depth on average a few hundred years after deposition from the atmosphere. Hydrothermal and sedimentary iron each contribute roughly 20% to the total dFe concentration in the Southern Ocean euphotic zone. Hydrothermal iron tends to have the oldest surface ages except where hydrothermal sources occur near the surface where the scavenging rate is high. The systematics of the source contributions, iron ages, and iron-age distributions are quantified across a family of solutions with a range of aeolian source strengths, all of which are consistent with currently available dFe observations.