New Insights into the Organic Complexation of Bioactive Trace Metals in the Global Ocean from the GEOTRACES Era

Abstract

The GEOTRACES program has greatly expanded measurements of dissolved trace metal concentrations across ocean basins, but to understand the behavior and cycling of metals and their impacts on primary productivity, we must understand the chemical forms in which they are present in the environment. Organic ligands play a central role in the speciation and cycling of trace metals in the marine environment, controlling their chemical reactivity and bioavailability. Here, we present an overview of the contributions the GEOTRACES program has made to understanding ocean metal speciation through advancing our knowledge of the distribution, sources, and sinks of metal-binding organic ligands across the global ocean, particularly for iron. Detailed assessments and intercalibration of the speciation methods most commonly applied have allowed integration of metal-binding ligand measurements across datasets. Work to characterize specific ligand groups within the wider pool of dissolved organic matter, along with their sources and sinks, is starting to unravel the role of metal-binding organic ligands in global biogeochemical cycles. Recent advances in complementary analytical techniques using liquid chromatography and mass spectrometry present a molecular picture of metal speciation and bioavailability—and also pose new questions. Moving forward, we need to address knowledge gaps in our understanding of how metal speciation and complexation relates to bioavailability in order to recognize the impacts of ocean metal distributions and cycling on marine productivity and the global carbon cycle.

GEOTRACES: Ironing Out the Details of the Oceanic Iron Sources?

“…Our intention here is not to review the entire field of marine Fe research, nor to explore global biogeochemical Fe models, Fe speciation and complexation, the role of internal cycling processes, or marine particles, because this has been aptly done by others (e.g., Boyd and Ellwood, 2010;Tagliabue et al, 2017), or is well covered by other articles in this special issue (Anderson, 2024, in this issue;Twining, 2024;Tagliabue and Weber, 2024;Whitby et al, 2024). Instead, we focus on showcasing the breadth of the GEOTRACES dFe datasets that are publicly available in the latest GEOTRACES data product (IDP2021v2; GEOTRACES Intermediate Data Product Group, 2023) and discuss how our view of the importance and pervasive nature of (non-dust) boundary sources of Fe to the ocean has changed dramatically with the availability of new, high-resolution, geographically distributed dFe and other oceanic TEI datasets since the birth of GEOTRACES.…”