Total \u00b2\u00b3\u2074Th was measured in samples of seawater collected at discrete depths using Niskin bottles as described in Umhau et al. (2019). Briefly, 2 L of seawater was collected at each selected depth. Samples were spiked with \u00b2\u00b3\u2070Th as a yield tracer and acidified to pH ~ 1 using concentrated nitric acid. After equilibration, ~ 6 hours later, KMnO\u2084 and MnCl\u2082 were added and the pH adjusted to ~ 9 with concentrated NH\u2084OH in order to co-precipitate Th via the formation of MnO\u2082. After eight hours, total \u00b2\u00b3\u2074Th samples were filtered onto 25 mm QMA filters, air dried, and frozen.
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The \u00b2\u00b3\u2074Th flux (dpm m\u207b\u00b2 d\u207b\u00b9) was calculated from the \u00b2\u00b3\u2074Th:\u00b2\u00b3\u2078U disequilibrium and assuming a steady state (Umhau et al. 2019). The \u00b2\u00b3\u2074Th flux at each depth was obtained by integrating the flux for the overlying depths using trapezoidal integration.
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Thorium counts were made using a low level RISO beta counter within 2 weeks of sample collection.