Eddy Mixing of Tracers into the Southern Ocean
The Southern Ocean comprises the waters surrounding Antarctica. Here, there are no continents blocking the flow of waters, allowing for westerly winds to drive a current flowing all the way around Antarctica – aptly named the Antarctic Circumpolar current. This part of the ocean is special. It is one of the only places in the world where the deepest ocean waters, rich in carbon and nutrients, which have been kept away from the atmosphere for hundreds to thousands of years, are drawn up to the sea surface. Once at the surface, they release their natural CO2 to the atmosphere, take up anthropogenic CO2, and deliver nutrients to phytoplankton living at the sea surface, before losing buoyancy and being subducted back to the ocean interior.
Another way dissolved components like nutrients and CO2 (e.g. tracers) are transported to the surface of the Southern Ocean is via mixing by mesoscale eddies. In the Southern Ocean, eddies are especially strong due to the powerful westerly winds – and both the winds and eddy strength are hypothesized to have changed during and following the last glacial maximum 20,000 years ago, potentially allowing for ancient CO2 stored in the deep ocean to be released to the atmosphere, and accelerating the end of the last ice age. However, direct observations of tracer transport by eddies are hard to come by, especially in sedimentary archives used to reconstruct past climate changes.
In 2017 I went on a cruise to the Pacific sector of the Southern Ocean, where we collected seawater samples on a meridional transect at very high spatial resolution. In these samples I measured two radioactive isotopes produced naturally at a constant rate in seawater by uranium decay: thorium-230 and protactinium-231. Since these isotopes are produced uniformly at a known rate, their variable distributions in seawater can be attributed to differential removal by sinking particles, and transport by mixing and currents across concentration gradients.
Our paper from this cruise documenting the importance of eddy mixing for transporting thorium and protactinium into the Southern Ocean was just published in Global Biogeochemical Cycles (link, or email me for a PDF!). We first quantified the capacity of different particle types for adsorbing and removing dissolved thorium and protactinium. Biogenic opal, produced en masse by diatom blooms in the Southern Ocean, has a particularly strong affinity for removing protactinium over thorium relative to other particle types. Thus, the Southern Ocean is a strong sink for protactinium in the ocean.
The exceptional removal of protactinium and thorium by diatom blooms near the Antarctic Polar Front generates large concentration gradients along upwelling waters in the Southern Ocean. These gradients allow for the exceptional transport of protactinium and thorium (especially protactinium!) by mesoscale eddies in this region. We quantify the fluxes of these two isotopes by eddy mixing, and show that the fluxes of protactinium in particular vastly exceed its other sources, suggesting that eddy mixing is extremely important for transporting protactinium to the Southern Ocean.
We end the paper by showing that the role of eddy mixing in transporting thorium and protactinium to the Southern Ocean varies between different basins. These basin to basin differences manifest in sedimentary protactinium to thorium ratios, which are most sensitive to changing opal fluxes in the Pacific Southern Ocean compared to the Atlantic. Could these differing sensitivities and protactinium to thorium ratios be used to reconstruct the intensity of eddy mixing in the recent geologic past? We need some more records to find out!