Surface Ocean Biogeochemistry Regulates the Impact of Anthropogenic Aerosol Fe Deposition on the Cycling of Iron and Iron Isotopes in the North Pacific

Abstract

Distinctively‐light isotopic signatures associated with Fe released from anthropogenic activity have been used to trace basin‐scale impacts. However, this approach is complicated by the way Fe cycle processes modulate oceanic dissolved Fe (dFe) signatures (δ56Fediss) post deposition. Here we include dust, wildfire, and anthropogenic aerosol Fe deposition in a global ocean biogeochemical model with active Fe isotope cycling, to quantify how anthropogenic Fe impacts surface ocean dFe and δ56Fediss. Using the North Pacific as a natural laboratory, the response of dFe, δ56Fediss, and primary productivity are spatially and seasonally variable and do not simply follow the footprint of atmospheric deposition. Instead, the effect of anthropogenic Fe is regulated by the biogeochemical regime, specifically the degree of Fe limitation and rates of primary production. Overall, we find that while δ56Fediss does trace anthropogenic input, the response is muted by fractionation during phytoplankton uptake, but amplified by other isotopically‐light Fe sources.

Aerosol Iron from Metal Production as a Secondary Source of Bioaccessible Iron

“…However, this hypothesis requires careful evaluation of other Fe sources and isotope fractionation during phytoplankton uptake. 57 The southeastern region of Madagascar exhibits a major sporadic phytoplankton bloom, the South-East Madagascar Bloom, in austral summer and fall. 58 The South-East Madagascar Bloom in winter is a recurrent and regular phenomenon of the phytoplankton phenology but is weaker than in summer.…”

“…The light isotopic signatures might support the dominant roles of anthropogenic aerosols in bioaccessible Fe deposition to the tropical south-east Pacific with the higher estimates of smelting Fe emission factors. However, this hypothesis requires careful evaluation of other Fe sources and isotope fractionation during phytoplankton uptake …”

Upper Ocean Biogeochemistry of the Oligotrophic North Pacific Subtropical Gyre: From Nutrient Sources to Carbon Export

“…A recent model effort incorporating both seawater and aerosol Fe isotope ratios in the PISCES biogeochemical ocean model successfully disentangled the aerosol Fe sources from dust, fires, and anthropogenic activities, and it examined the impacts of anthropogenic aerosols on the Fe cycles and primary production across the different biogeochemical regimes in the North Pacific Ocean (König et al., 2022). While dust‐derived soluble Fe shows the highest deposition during spring and early summer, combustion aerosols contribute more soluble Fe deposition during winter and spring than other seasons, and wildfires cause elevated soluble Fe deposition in spring (König et al., 2022). Nevertheless, more observational and model investigations are needed to precisely decipher the contribution and impacts of anthropogenic aerosol Fe in addition to dust on the biogeochemistry and ecological responses of the North Pacific Ocean under the ongoing variability of anthropogenic activities and climate.…”

Anthropogenic Iron Invasion into the Ocean: Results from the East Sea (Japan Sea)

“…However, the isotopic signature of Fe also has large uncertainties in quantifying anthropogenic Fe in the ocean because of isotopic fractionation associated with biological production, scavenging, and ligand complexation. 30 Thus, in this study, we used various tracers (Al, K, V, Ni, Pb, and 210 Pb) to identify the main sources of anthropogenic Fe in 72 aerosol samples collected over one year in the eastern coastal region of Korea, located in the northwestern Pacific Ocean (Figure 1). In addition, we used 210 Pb as a tracer of the atmospheric fallout flux of anthropogenic Fe.…”

“…In the real-world ocean, to our knowledge, Pinedo-González et al is the only study that provides in situ evidence of anthropogenic Fe contribution (20–60%) to the dissolved Fe in the surface layer of the North Pacific Ocean using isotopic mass balance. However, the isotopic signature of Fe also has large uncertainties in quantifying anthropogenic Fe in the ocean because of isotopic fractionation associated with biological production, scavenging, and ligand complexation …”