Dataset: 234Th flux in epipelagic waters at Station ALOHA and the Equatorial Pacific from R/V Kilo Moana cruises KM1407, KM1418, & KM1515 during 2014-2015

Total ²³⁴Th 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 ²³⁰Th as a yield tracer and acidified to pH ~ 1 using concentrated nitric acid. After equilibration, ~ 6 hours later, KMnO₄ and MnCl₂ were added and the pH adjusted to ~ 9 with concentrated NH₄OH in order to co-precipitate Th via the formation of MnO₂. After eight hours, total ²³⁴Th samples were filtered onto 25 mm QMA filters, air dried, and frozen.

The ²³⁴Th flux (dpm m⁻² d⁻¹) was calculated from the ²³⁴Th:²³⁸U disequilibrium and assuming a steady state (Umhau et al. 2019). The ²³⁴Th flux at each depth was obtained by integrating the flux for the overlying depths using trapezoidal integration.

Thorium counts were made using a low level RISO beta counter within 2 weeks of sample collection.