Trace metal measurements in recent years have revealed a complex distribution of dissolved iron (dFe) in the ocean that models still struggle to reproduce. The GEOTRACES section GA03 across the subtropical North Atlantic was chosen to study the driving processes involved in the Fe cycle in the region. Here, field observations found elevated dFe near the surface under the Saharan dust plume, a strong dFe minimum below the mixed layer depth, a maximum at the oxygen minimum zone near the African shelf, a hydrothermal maximum near the Mid Atlantic Ridge and lower dFe values in the deep eastern basin than in the west. We show that several of these features can be understood and be reproduced in models when they take into account scavenging on dust particles and phytoplankton, a variable ligand concentration and a hydrothermal dFe source. By doing so in a sequence of parameterisation changes, we are able to relate physical and biological processes, as well as internal and external dFe sources to observed features of the dFe distribution. In agreement with the observations, the additional scavenging on dust generates lower dFe concentrations in the deep eastern basin while the new ligand distribution results in a dFe maximum in the intermediate waters in the east basin and moderates the deep dFe gradient between the eastern and western basins.

The ‘Iron Hypothesis’ suggests a fertilization of the Southern Ocean by increased dust deposition in glacial times. This promoted high primary productivity and contributed to lower atmospheric pCO2. In this study, the diatom Pseudo-nitzschia subcurvata, known to form prominent blooms in the Southern Ocean, was grown under simulated glacial and interglacial climatic conditions to understand how iron (Fe) availability (no Fe or Fe addition) in conjunction with different pCO2 levels (190 and 290 μatm) influences growth, particulate organic carbon (POC) production and photophysiology. Under both glacial and interglacial conditions, the diatom grew with similar rates. In comparison, glacial conditions (190 μatm pCO2 and Fe input) favored POC production by P. subcurvata while under interglacial conditions (290 μatm pCO2 and Fe deficiency) POC production was reduced, indicating a negative effect caused by higher pCO2 and low Fe availability. Under interglacial conditions, the diatom had, however, thicker silica shells. Overall, our results show that the combination of higher Fe availability with low pCO2, present during the glacial ocean, was beneficial for the diatom P. subcurvata, thus contributing more to primary production during glacial compared to interglacial times. Under the interglacial ocean conditions, on the other hand, the diatom could have contributed to higher carbon export due to its higher degree of silicification.