Isopycnal mixing controls protactinium and thorium distributions in the Pacific Southern Ocean

Pavia and co-workers (2020, see reference below) determined the physical and chemical speciation as well as the vertical distribution of Protactinium-231 (Pa) and Thorium-230 (Th) at 12 stations across the Southern Pacific Antarctic Circumpolar Current. They confirm that Pa is preferentially adsorbed over Th by biogenic opal. A new finding is the observation of steep gradients of these tracers along neutral density surfaces from north to south, demonstrating the importance of isopycnal mixing in transporting these nuclides to the Southern Ocean. Because these tracers are widely used to constrain scavenging processes and reconstruct past vertical sedimentary fluxes, this sensitivity to physical transport suggests that Th and Pa may be used as sedimentary tracers of eddy mixing in this sector of the Southern Ocean, and invites future modelling efforts to further constrain this sensitivity.

Figure (modified from Pavia et al., 2020): It shows study area (top), including stations sampled for dissolved thorium and protactinium as stars, and locations of Southern Ocean fronts as colored lines. Bottom panel shows section plots of dissolved thorium-230 and protactinium-230 across the study section, with redder colors indicating higher concentrations and bluer colors indicating lower concentrations. Contour lines of constant neutral density are shown in black. Significant gradients exist from north to south on density surfaces between 27.5 and 27.9 kg/m3 for both elements, which allows mixing by eddies to transport both elements southward along lines of constant density. 

Reference:

Pavia, F. J., Anderson, R. F., Pinedo‐Gonzalez, P., Fleisher, M. Q., Brzezinski, M. A., & Robinson, R. S. (2020). Isopycnal Transport and Scavenging of 230Th and 231Pa in the Pacific Southern Ocean. Global Biogeochemical Cycles, 34(12). DOI: https://doi.org/10.1029/2020GB006760

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