Dust, Iron, and Nitrogen Fixers in the South Pacific

Dust, Iron, and Nitrogen Fixers in the South Pacific

The South Pacific Gyre is an oceanographic desert. Inward-spiraling currents push surface waters downward, prevent nutrient-rich waters from reaching the sunlit oceans, and inhibit plankton growth. It’s also about as far from the continents as you can get. So, atmospheric dust inputs are extremely low – and as a result, so are the inputs of micronutrients like iron, which are critical for the few biota that are around to conduct biochemical processes like nitrogen fixation.

In 2015-2016, I sailed on my first oceanographic cruise – the UltraPac expedition on the German research vessel F.S. Sonne. It was a spectacular experience to spend 6 weeks at sea sailing from Chile to New Zealand, and very intense. I spent Christmas, New Years, and my 24th birthday on a ship, in the middle of nowhere, with people I had barely met (though who would eventually become lifelong friends). On the way, I sampled seawater, particles, and sediments across the entirety of the South Pacific Gyre. The goal was to measure long-lived radioactive thorium isotopes – which can be used to track the inputs of continental material to the surface oceans.

Five years after the cruise, our paper on what we’ve learned from these measurements was just published in Global Biogeochemical Cycles (Link, or email me for a PDF!). The first part of the paper walks through some nitty-gritty details about complications and corrections involved in using thorium isotopes to quantify dust fluxes to the surface ocean. It turns out that vertical mixing is much more important for supplying one thorium isotope, Th-230, to the uppermost depths of the ocean than the other thorium isotopes, Th-232.

We then use our dust fluxes computed from Th isotopes to show that global atmospheric models of dust deposition underestimate fluxes by 1-2 orders of magnitude in the South Pacific. That’s a lot! The upshot is that iron inputs from dust deposition are concomitantly underestimated. We revisit the complete budget of iron sources to argue that dust, not diffusive transport from the continental shelves, is the main source of iron to the surface of the South Pacific.

We end the paper with a somewhat speculative section about the implications of our results for nitrogen-fixing biota in the South Pacific. Using our dust fluxes and cellular iron, nitrogen, and carbon stoichiometries of two different diazotrophs, we show that the upper limit of nitrogen fixation that can be supported by the new iron supplied from dust is lower than some measured nitrogen fixation rates in the South Pacific. How can this be? Future studies will have to evaluate how this mismatch persists, but one possibility is that iron recycling, whereby organisms use and re-use iron at the surface ocean over and over again, is critical for sustaining iron stocks for nitrogen fixers.

While using thorium isotopes to calculate dust fluxes is still in its evolving stages, this study is a good example of this method’s potential. Luckily there is still some good stuff in the hopper. Stay tuned for some upcoming work incorporating these estimates into global models, and for some of the first quantifications of modern dust flux to the Pacific Southern Ocean!

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