Loihi Seamount, hydrothermal Helium-3 and dissolved iron sources and their dispersion within the Pacific Ocean

4 May 2020

As part of the GEOTRACES cruise GP15, Jenkins and co-workers (2020, see reference below) observed large water column anomalies in helium isotopes and trace metal concentrations above the Loihi Seamount (~19°N, 154°W) that extends along the GP15 track for hundreds of kilometers. Expanding their data with historical ones, they observe that the Loihi Helium-3 (³He) and dissolved iron (dFe) “signal” was propagating at depth of 1100 m within ∼100 – 1000 km of Loihi, characterized by a distal dFe:³He ratio of ∼4 ×10⁶. Contrastingly, no major methane input was observed. Modeling the regional circulation allowed these authors to estimate a hydrothermal ³He source from Loihi of 10.4 ±4.2 mol a⁻¹ and a corresponding dFe flux of ∼40 Mmol a⁻¹. On a larger scale, they simulate that the Loihi-influenced waters (enriched in He and Fe) eventually upwell along the west coast of North America, also extending around 1000 km into the northwest Pacific Ocean, which could play a role in fertilizing the subpolar North Pacific Ocean.

**Figure:** (a) A map showing the distribution of ³He anomalies at a depth of 1100 m in the subtropical North Pacific (note the eastward plume emanating from Hawaii) and the GEOTRACES GP15 station locations (red stars), (b) the location of the stations closest to the Loihi source, (c) the size of the 1100 m ³He anomaly observed during GP15 (red stars) and previous stations (black dots), and (d) modelled upwelling intensities for the Loihi source.

Reference:

Jenkins, W. J., Hatta, M., Fitzsimmons, J. N., Schlitzer, R., Lanning, N. T., Shiller, A., Buckley, N. R., German, C. R., Lott, D. E., Weiss, G., Whitmore, L., Casciotti, K., Lam, P. J., Cutter, G. A., Cahill, K. L. (2020). An intermediate-depth source of hydrothermal 3He and dissolved iron in the North Pacific. Earth and Planetary Science Letters, 539, 116223. DOI: https://doi.org/10.1016/j.epsl.2020.116223

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