Revisiting five decades of Th-234 data: a comprehensive global oceanic compilation

We present here a global oceanic compilation of Th-234 measurements that collects results from researchers and laboratories over a period exceeding 50 years. The origin of the Th-234 sampling in the ocean goes back to 1967, when Bhat et al. (1969) initially studied Th-234 distribution relative to its parent U-238 in the Indian Ocean. However, it was the seminal work of Buesseler et al. (1992) - which proposed an empirical method to estimate export fluxes from Th-234 distributions - that drove the extensive use of the Th-234-U-238 radioactive pair to evaluate the organic carbon export out of the surface ocean by means of the biological carbon pump. Since then, a large number of Th-234 depth profiles have been collected using a variety of sampling instruments and strategies that have changed during the past 50 years. The present compilation is made of a total 223 data sets: 214 from studies published in either articles in refereed journals, PhD theses, or repositories, as well as 9 unpublished data sets. The data were compiled from over 5000 locations spanning all the oceans for total Th-234 profiles, dissolved and particulate Th-234 activity concentrations (in dpm L-1), and POC (VTh)-Th-234 ratios (in mu mol dpm(-1)) from both sediment traps and filtration methods. A total of 379 oceanographic expeditions and more than 56 600 Th-234 data points have been gathered in a single open-access, long-term, and dynamic repository. This paper introduces the dataset along with informative and descriptive graphics. Appropriate metadata have been compiled, including geographic location, date, and sample depth, among others. When available, we also include water temperature, salinity, U-238 data (over 18 200 data points), and particulate organic nitrogen data. Data source and method information (including U-238 and Th-234) is also detailed along with valuable information for future data analysis such as bloom stage and steady-/non-steady-state conditions at the sampling moment. The data are archived on the PANGAEA repository, with the dataset DOI https://doi.org/10.1594/PANGAEA.918125 (Ceballos-Romero et al., 2021). This provides a valuable resource to better understand and quantify how the contemporary oceanic carbon uptake functions and how it will change in future.