Physical Weathering Intensity Controls Bioavailable Primary Iron(II) Silicate Content in Major Global Dust Sources

Abstract

The speciation of iron (Fe) reaching the ocean, for instance in wind‐blown dust and coastal sediments, impacts its bioavailability to phytoplankton and its impact on atmospheric carbon dioxide (CO2) and climate. For dust reaching the Southern Ocean, primary Fe(II) silicates that are physically weathered from bedrock are highly bioavailable compared to more chemically weathered, Fe(III)‐rich species, suggesting that weathering in dust source regions impacts the bioavailable Fe supply. However, this phenomenon has not been studied in other important terrestrial Fe sources, where weathering regimes and source geology vary. Here, we use Fe X‐ray absorption spectroscopy on marine sediment cores to show that major global dust and sediment sources impacted by high physical weathering contain abundant primary minerals and thus are overlooked as a source of highly bioavailable Fe globally. Thus, it is important to consider the role of physical versus chemical weathering in Fe fertilization and biotic CO2 cycling.

Glacial and Hydrothermal Sources of Dissolved Iron (II) in Southern Ocean Waters Surrounding Heard and McDonald Islands

“…However, glacial flour observed at station 24 (van der Merwe et al, 2019) and high concentrations of nonalgal particles (Wojtasiewicz et al, 2019) may indicate that DFe (II) production was the result of physical weathering of underlaying basalt by the glacier (van der Merwe et al, 2019). High silicate concentrations observed at this site associated with lower salinity water (van der Merwe et al, 2019) may also indicate that physical weathering resulted in mobilization of Fe (II) silicates, as has been demonstrated in a previous study of SO marine sediment cores (Shoenfelt et al, 2019). These could be important mechanisms in maintaining the observed DFe (II) and DFe enrichment at Heard Island.…”

“…Ascertaining the exact mechanism of the glacial Fe(II) production was outside the scope of this study. However, glacial flour observed at station 24 (van der Merwe et al, 2019) and high concentrations of non-algal particles (Wojtasiewicz et al, 2019) silicates, as has been demonstrated in previous studies (Shoenfelt et al, 2019). These could be important mechanisms in maintaining the observed DFe(II) and DFe enrichment at Heard…”

The Flux and Provenance of Dust Delivered to the SW Pacific During the Last Glacial Maximum

“…A higher dust flux at the LGM has been suggested to stimulate primary productivity by alleviating Fe limitation in HNLC zones (Martin et al, 1994), with glacially derived dust more bioavailable than other dust sources (Schroth et al, 2009; Shoenfelt et al, 2018, 2019). Although we find an increase in fine‐grained dust, or glacial outwash, delivered to our cores at the LGM, we find no evidence for an associated increase in biological productivity (Figure 3).…”

“…Third, increased glacial activity resulted in an increased supply of dust from easily mobilized fine grained material from glacial outwash plains (Bullard et al, 2016;Delmonte et al, 2017;Lamy et al, 2014;Marx et al, 2018;Shoenfelt et al, 2018;Sugden et al, 2009) which can also be transported in glacial runoff to the open ocean (Bhatia et al, 2013;Hawkings et al, 2014Hawkings et al, , 2018. This glacially derived Fe is thought to be more bioavailable than Fe in lithogenic material produced by arid weathering regimes, as it has undergone less chemical weathering so comprises more leachable, bioavailable Fe (II) (Hawkings et al, 2018;Schroth et al, 2009;Shoenfelt et al, 2017Shoenfelt et al, , 2019.…”

Climatically Modulated Dust Inputs from New Zealand to the Southwest Pacific Sector of the Southern Ocean Over the Last 410 kyr

“…Dust changes the radiative properties of the atmosphere by scattering and absorbing solar and terrestrial radiation, and influences climate both directly and through cloud‐nucleation processes (Köhler et al., 2010; Lohmann & Feichter, 2005; Maher et al., 2010; Masson‐Delmotte et al., 2010; Rohling et al., 2012). Dust deposition onto the ocean surface may also affect climate indirectly by supplying iron as a micronutrient to Fe‐limited waters, thereby fertilizing phytoplankton productivity, which results in atmospheric CO 2 drawdown (Jickells et al., 2005; Kohfeld & Harrison, 2001; Martin et al., 1991; Martínez‐García et al., 2014; Moore et al., 2013; Shoenfelt et al., 2018, 2019; Tagliabue et al., 2017).…”