An Inverse Model Interpretation of the GEOTRACES Aluminum Transects

Aluminum (Al) is delivered to surface ocean waters by aeolian dust, making it a promising tracer to constrain dust deposition rates and the atmospheric supply of trace metal micronutrients. Over recent years, dissolved Al has been mapped along the GEOTRACES transects, providing unapparelled coverage of the world ocean. However, inferring atmospheric input rates from these observations is complicated by a suite of additional processes that influence Al concentrations, including hydrothermal and sediment sources, removal by particle scavenging, and biological uptake by diatoms. Here we employ a data-assimilation model of the oceanic Al cycle that explicitly accounts for these processes, allowing the atmospheric signal to be extracted. We conduct an ensemble of model optimizations that test different dust deposition distributions and consider spatial variations in Al solubility, thereby inferring the atmospheric Al supply that is most consistent with GEOTRACES observations. We find that Al solubility strongly declines with dust loading and that 33-36 Gmol of soluble Al is added to the global ocean by dust each year, comprising 3-5 Gmol/yr in the Southern Ocean, 12-13 Gmol/yr in the Atlantic Ocean, 7-10 Gmol/yr in the Pacific Ocean and 6-9 Gmol/yr in the Indian Ocean. Our model provides additional insights into the oceanic Al cycle, revealing that 5-8 Gmol Al/yr is injected from hydrothermal vents, and that vertical Al redistribution by diatom uptake and remineralization is negligible compared to the effects of reversible scavenging. Furthermore, our results have important implications for the oceanic iron (Fe) budget: based on the observed ratio of soluble Fe to soluble Al in dust, we infer an aeolian Fe input of 3-6 Gmol/yr, which falls at the lower end of previous estimates.