Tracing and constraining anthropogenic aerosol iron fluxes to the North Atlantic Ocean using iron isotopes

Tim M Conway,Natalie M Mahowald,Seth G John,Ana M Aguilar-Islas,William M Landing,Rachel U Shelley,Douglas S Hamilton

doi:10.1038/s41467-019-10457-w

Abstract

Atmospheric dust is an important source of the micronutrient Fe to the oceans. Although relatively insoluble mineral Fe is assumed to be the most important component of dust, a relatively small yet highly soluble anthropogenic component may also be significant. However, quantifying the importance of anthropogenic Fe to the global oceans requires a tracer which can be used to identify and constrain anthropogenic aerosols in situ. Here, we present Fe isotope (δ56Fe) data from North Atlantic aerosol samples from the GEOTRACES GA03 section. While soluble aerosol samples collected near the Sahara have near-crustal δ56Fe, soluble aerosols from near North America and Europe instead have remarkably fractionated δ56Fe values (as light as −1.6‰). Here, we use these observations to fingerprint anthropogenic combustion sources, and to refine aerosol deposition modeling. We show that soluble anthropogenic aerosol Fe flux to the global surface oceans is highly likely to be underestimated, even in the dusty North Atlantic.

Highlights

Atmospheric dust is an important source of the micronutrient Fe to the oceans
Based on published HYSPLIT back-trajectory modeling of air masses sampled on the cruises[43], we chose a subset of samples (Fig. 1) from defined air masses, including areas strongly influenced by the mineral dust plume emanating from the Sahara desert (Saharan air masses) as well as regions near North America (North American air masses) and Europe (European air masses)
We found that GA03 aerosols considered to be dominated by the Saharan dust plume were characterized by near-crustal δ56Fe in both the bulk and water-soluble phases (+0.12 ± 0.03‰ and +0.09 ± 0.02‰, respectively), confirming the observation that natural aerosol Fe is defined by near-crustal δ56Fe (Table 1; Fig. 2a32,33)

Summary

Introduction

Atmospheric dust is an important source of the micronutrient Fe to the oceans. relatively insoluble mineral Fe is assumed to be the most important component of dust, a relatively small yet highly soluble anthropogenic component may be significant. While soluble aerosol samples collected near the Sahara have near-crustal δ56Fe, soluble aerosols from near North America and Europe instead have remarkably fractionated δ56Fe values (as light as −1.6‰) We use these observations to fingerprint anthropogenic combustion sources, and to refine aerosol deposition modeling. High Fe solubilities (up to 19%) in marine aerosols collected near Bermuda have been linked to anthropogenic processes[14,15], while mineral Fe within natural desert dust typically has very low solubility (1%)[18] Such studies demonstrate that aerosols can carry large amounts of soluble Fe, it is not possible to determine whether solubility is directly related to the presence of an anthropogenic component or whether Fe solubility has been enhanced due to physical and/or chemical atmospheric processing. Modeling suggests that a small but highly soluble flux of combustion-sourced Fe has a major impact on the total estimated atmospheric soluble Fe delivery to the oceans worldwide[24,25]

Methods

Results

Conclusion