Perspective on identifying and characterizing the processes controlling iron speciation and residence time at the atmosphere-ocean interface

Abstract

It is well recognized that the atmospheric deposition of iron (Fe) affects ocean productivity, atmospheric CO 2 uptake, ecosystem diversity, and overall climate. Despite significant advances in measurement techniques and modeling efforts, discrepancies persist between observations and models that hinder accurate predictions of processes and their global effects. Here, we provide an assessment report on where the current state of knowledge is and where future research emphasis would have the highest impact in furthering the field of Fe atmosphere-ocean biogeochemical cycle. These results were determined through consensus reached by diverse researchers from the oceanographic and atmospheric science communities with backgrounds in laboratory and in situ measurements, modeling, and remote sensing. We discuss i) novel measurement methodologies and instrumentation that allow detection and speciation of different forms and oxidation states of Fe in deliquesced mineral aerosol, cloud/rainwater, and seawater; ii) oceanic models that treat Fe cycling with several external sources and sinks, dissolved, colloidal, particulate, inorganic, and organic ligand-complexed forms of Fe, as well as Fe in detritus and phytoplankton; and iii) atmospheric models that consider natural and anthropogenic sources of Fe, mobilization of Fe in mineral aerosols due to the dissolution of Fe-oxides and Fe-substituted aluminosilicates through proton-promoted, organic ligand-promoted, and photo-reductive mechanisms. In addition, the study identifies existing challenges and disconnects (both fundamental and methodological) such as i) inconsistencies in Fe nomenclature and the definition of bioavailable Fe between oceanic and atmospheric disciplines, and ii) the lack of characterization of the processes controlling Fe speciation and residence time at the atmosphere-ocean interface. Such challenges are undoubtedly caused by extremely low concentrations, short lifetime, and the myriad of physical, (photo)chemical, and biological processes affecting global biogeochemical cycling of Fe. However, we also argue that the historical division (separate treatment of Fe biogeochemistry in oceanic and atmospheric disciplines) and the classical funding structures (that often create obstacles for transdisciplinary collaboration) are also hampering the advancement of knowledge in the field. Finally, the study

An explicit estimate of the atmospheric nutrient impact on global oceanic productivity

“…Iron is primarily utilized by marine phytoplankton in its dissolved form, although the actual bioavailability may substantially differ from the soluble forms of the deposited nutrients into the ocean (Meskhidze et al, 2019). For example, Rubin et al (2011) showed that in the tropical and subtropical Atlantic Ocean, some marine organisms (e.g., the Trichodesmium) can directly access the mineral (insoluble) particulate iron.…”

Evaluation of aerosol iron solubility over Australian coastal regions based on inverse modeling: implications of bushfires on bioaccessible iron concentrations in the Southern Hemisphere

“…The operational definition of L Fe could be applied to measurements of both the deliquesced aerosol solution and seawater. To clarify the differences of chemical and physical forms in between aerosol water and seawater, "bioaccessible" Fe in aerosol water is defined as potentially "bioavailable" forms of Fe in seawater (Meskhidze et al 2019). To avoid any confusion in comparing model estimates to observations in this study, we consider L Fe as bioaccessible Fe (Myriokefalitakis et al 2018;Ito et al 2019;Perron et al 2020a).…”

“…The solubility of aeolian Fe will exert a key control on the impact of atmospheric supply on open ocean ecosystems (Baker and Croot 2010;Meskhidze et al 2019). The solubility of aerosol Fe is initially characterized by the nature of the emission source (Schroth et al 2009;Sholkovitz et al 2012;Ito 2013).…”

On mineral dust aerosol hygroscopicity

“…Mineral dust can also change the abundance of reactive trace gases as well as aerosol compositions via heterogeneous reactions (Usher et al, 2003;Dupart et al, 2012;He et al, 2014;Tang et al, 2017;Yu and Jang, 2019). Furthermore, the deposition of mineral dust will bring substantial amounts of nutrients (e.g., Fe and P) into some marine and terrestrial ecosystems, thereby largely affecting biogeochemistry in these regions (Jickells et al, 2005;Okin et al, 2011;Schulz et al, 2012;Li et al, 2017;Tagliabue et al, 2017;Meskhidze et al, 2019).…”