Global database of oceanic particulate organic carbon to <sup>234</sup>Th ratios: Improving estimates of the biological carbon pump
Abstract. The ocean's biological carbon pump (BCP) plays a major role in the global earth carbon cycle. A fraction of the photosynthetically fixed organic carbon produced in surface waters is exported below the sunlit layer in the form of settling particles (e.g. marine snow). Since the seminal works on the BCP, global estimates of the global strength of the BCP have improved but large uncertainties remain (from 5 to 20 GtC yr−1 exported below the euphotic zone or mixed layer depth). The 234Th technique is widely used to measure the downward export of particulate organic carbon (POC). This technique has the advantage to allow a downward flux to be determined by integrating the deficit of 234Th in the upper water column and coupling it to an estimate of the POC/234Th ratio in sinking matter. However, the factors controlling the regional, temporal and depth variations of POC/234Th ratios are poorly understood. We present a database of 9110 measurements of the POC/234Th ratio in the ocean, from the surface down to > 5500 m, sampled on three size fractions (~ > 0.7 μm, ~ 1–50 μm, ~ > 50 μm), collected with in situ pumps and bottles, and also from bulk particles collected with sediment traps. The dataset is archived on the data repository PANGAEA® (https://www.pangaea.de/) under https://doi.pangaea.de/10.1594/PANGAEA.902103 (Puigcorbé, 2019). The samples presented in this dataset were collected between 1989 and 2016 and the data have been obtained from published papers and open datasets available online. Unpublished data has also been included. Most of the open ocean provinces are represented by multiple measurements. However, there is an uneven distribution of the data, with some areas highly sampled (e.g, China Sea, Bermuda Atlantic Time Series station) compared to some others that are not well represented, such as the southeastern Atlantic, the south Pacific and the south Indian oceans. Some costal areas, although in a much smaller number, are also included in this global compilation. Globally, based on different depths horizons and climate zones, the median POC/234Th ratios have a wide range, from 0.6 to 18 μmol dpm-1.