Disentangling the sources and transport of iron in the Southern Ocean using a water mass mixing model analysis

21 May 2024

Traill and co-workers (2024, see reference below) used an extended optimum multiparameter analysis (eOMP) water‐mass mixing model to determine the interplay between physical and biological processes, and sources/sinks driving dissolved iron (dFe) distributions along the SR3 cruise (GEOTRACES GS01 section), between Tasmania and Antarctica.

The added value of the eOMP over traditional OMP is to offer i) additional water‐mass tracers but mostly ii) an estimate of the remineralization intensity. Indeed, the addition of a term for remineralization of organic matter (ΔO), normalized to stoichiometric ratios, is used to account for changes in non‐conservative tracer concentrations compared to their predefined “source term” concentrations relative to O₂ consumption. This tool allowed the authors to examine changes in water masses relative to their defining features, and identify regional scale processes over the transect.

The authors could thus establish where and how dFe is supplied across the distinct oceanographic zones of the studied area: in the subtropical zone, external sources from continental and shelf sedimentary Fe were supplemented by remineralized dFe associated with modified mode and intermediate waters. Subantarctic mesopelagic dFe was sustained by remineralization constrained by Antarctic Intermediate Water (AAIW), while deeper waters in this region were likely supplied from hydrothermal inputs into circumpolar deep water (CDW). In the Antarctic zone, strong surface depletion of dFe driven by biological uptake was evident. The dFe distribution in this zone is likely driven by the prevailing CDW distribution and the interactions between hydrothermal Fe inputs with CDW circulation and particle loading.

**Figure 1. a)** Ocean current structures of the Southern Ocean south of Tasmania. The mean position of the Antarctic Circumpolar Current (ACC) fronts from satellite altimetry is shown by the black lines (Park & Durand, 2019), while seasonal ocean currents coming from the north either side of Tasmania are indicated by white lines. b) Samples were collected along GEOTRACES transect GS01 (GO-SHIP section SR3) at stations shown by dots and stars. Surface ocean chlorophyll concentration at the time of sampling (Summer 2018) is also shown to indicate areas of biological productivity, which differ between the indicated oceanographic zones.

**Figure 2.** A schematic representation of the processes that dominate the distribution of dissolved iron over the SR3/GS01 transect. Colored regions denote specific processes, while the black lines show the core of water masses identified using the eOMP. Some processes fit within the water mass structure, indicating physical transport mechanisms, but many are the result of iron sources and sinks, or biological processes that move the distribution of dissolved iron away from the water mass structure.

Reference:

Traill, C. D., Conde‐Pardo, P., Rohr, T., van der Merwe, P., Townsend, A. T., Latour, P., Gault‐Ringold, M., Wuttig, K., Corkill, M., Holmes, T. M., Warner, M. J., Shadwick, E., & Bowie, A. R. (2024). Mechanistic Constraints on the Drivers of Southern Ocean Meridional Iron Distributions Between Tasmania and Antarctica. Global Biogeochemical Cycles, 38. Access the paper: 10.1029/2023gb007856

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