Hydrothermal Iron from Western Pacific Arc Systems: Fertilization of Phytoplankton Blooms and Potential Feedbacks on Sea Surface Temperature

Prepared for The Gradient Series / Convergence Series — July 2026

Executive Summary

Hydrothermal systems linked to subduction arcs and back-arc basins in the western Pacific (Philippine Sea Plate, Mariana region, and connections to Indonesian convergence) release iron (Fe). Through speciation, ligand stabilization, and isotope-traced dispersal, this Fe can fertilize phytoplankton blooms. These blooms exert biophysical effects on sea surface temperature (SST), including local warming via light absorption and stratification. This interacts with 2026 El Niño dynamics observed in Niño 3.4 and ARMOR3D data.

1. Tectonic Sources of Iron

The Mariana Arc and Trough exemplify active settings where subduction and spreading drive hydrothermal circulation. Arc vents (shallower, acidic, volatile-rich) and back-arc spreading centers release Fe-rich fluids. These are part of the broader PSP–Sunda Plate convergence zone near Indonesia.

2. Fe Speciation, Bioavailability & Isotope Tracing

Fe is released primarily as Fe(II). In plumes, oxidation and particle formation occur, but organic ligands stabilize dissolved Fe (dFe), making it bioavailable. Key data (Wang et al. 2021/2022 on Mariana/Lau back-arc plumes):

• dFe/TFe ratios up to ~82% in low-temperature/diffuse flow.
• Organic ligands ~29% of dFe on average.
• Isotopic shifts (δ⁵⁶dFe) from sulfide/oxyhydroxide precipitation and oxidation (e.g., from -4.08‰ to +0.22‰ in diluted plumes).

3. Natural Iron Fertilization and Blooms

Shallow arc vents facilitate Fe delivery to the photic zone. Evidence includes Tonga Arc (diazotroph blooms) and Southern Ocean examples (massive recurring blooms from upwelled hydrothermal Fe).

4. Blooms and SST Feedbacks

Blooms can raise local SST via enhanced solar absorption and stratification. Complex effects include DMS/cloud formation (cooling potential) and carbon export. In ENSO contexts, these feedbacks modulate regional patterns.

5. Relevance to 2026 El Niño

SST reveals subsurface anomalies consistent with Kelvin wave/heat advection. Hydrothermal Fe inputs and bloom responses interact with these dynamics in the western Pacific warm pool.

Conclusions

Tectonic Fe sources provide a traceable nutrient input that can influence blooms and SST in tectonically active regions. This adds nuance to primary wind-driven ENSO mechanisms. Full details and references in the companion PDF.

References include Wang et al. (Fe isotopes/speciation), Bonnet et al. (Tonga fertilization), Ardyna/Schine et al. (Southern Ocean blooms), and GEOTRACES hydrothermal Fe studies.