On the Role of Dust‐Deposited Lithogenic Particles for Iron Cycling in the Tropical and Subtropical Atlantic

Abstract

Lithogenic material deposited as dust is one of the major sources of trace metals to the ocean, particularly in the tropical and subtropical Atlantic. On the other hand, it can also act as a scavenging surface for iron. Here we studied this double role of lithogenic material in the marine iron cycle by adding a new scheme for describing particle dynamics into a global biogeochemistry and ecosystem model including particle aggregation and disaggregation of two particle size classes and scavenging on both organic and lithogenic particles. Considering the additional scavenging of iron on lithogenic particles, the modeled dissolved iron concentration is reduced significantly in the tropical and subtropical Atlantic, bringing the model much closer to observations. This underlines the necessity to consider the double role of dust particles as iron source and sink in studies on the marine iron cycle in high dust regions and with changing dust fluxes.

Formation and Maintenance of the GEOTRACES Subsurface‐Dissolved Iron Maxima in an Ocean Biogeochemistry Model

“…The model also overestimates the surface dFe concentration around 10 ∘ N and cannot reproduce the hydrothermal signal around the Central Indian Ridge segment. The overestimation of surface dFe concentration under the high-dust region at 10 ∘ N could indicate the potential role of scavenging by lithogenic particles as suggested by Ye and Völker (2017) for the tropical Atlantic Ocean. Comparing the five model runs in Figure 4, it is clear that the release of ligand and dFe from organic particles is important to form the dFe maximum.…”

“…Thus, in high‐dust regions of the Indian and Atlantic basins, the simultaneous release of ligand and scavenged Fe from organic particles not only supplies dFe to the subsurface waters but also protects dFe from being scavenged, maintaining a high level of subsurface dFe concentration. In fact, the model tends to overestimate the surface dFe in high‐dust regions and likely indicates bias in the representation of processes that remove dFe where dust deposition is high (Ye & Völker, ). This bias may reflect the missing colloidal pumping mechanism for dFe loss in our model, which could be important for high dust deposition regions (Fitzsimmons et al, ).…”

“…Second, Fe' can be scavenged onto inorganic particles, which are not produced by biological processes and may have lithogenic origin (Boyd et al, ; Galbraith et al, ; Tagliabue, Aumont, et al, ). As in Galbraith et al (), the inorganic scavenging is parameterized as a first‐order loss process with a rate coefficient, K inorg , Elevated dust deposition enhances the inorganic scavenging process because of the increase in lithogenic particle concentration under high dust deposition (Ye & Völker, ). Therefore, we scale the rate constant by the dFe flux from atmospheric deposition.…”

“…Thus, we manipulate the solubility of dust Fe for these regions, reducing it by 2 orders of magnitude. We acknowledge the limitation of this approach and are aware of a new approach from Ye and Völker (2017) by explicitly solving for lithogenic particles; however, there is still large uncertainty in the dissolution kinetics of particulate dust Fe (Mahowald et al, 2009) and in the magnitude of dust deposition itself (Anderson et al, 2016).…”

“…Elevated dust deposition enhances the inorganic scavenging process because of the increase in lithogenic particle concentration under high dust deposition (Ye & Völker, 2017). Therefore, we scale the rate constant by the dFe flux from atmospheric deposition.…”