Global database of oceanic particulate organic carbon to particulate ²³⁴Th ratios

The particulate export of photosynthetically fixed carbon from the surface ocean to the ocean interior is a key component of the biological carbon pump and, by extension, of the global carbon cycle. An extensively applied method to estimate the downward flux of particulate carbon is the ²³⁴Th technique, which uses the deficit of ²³⁴Th with respect to its parent, ²³⁸U, coupled with the C/²³⁴Th ratios measured in sinking. More than two decades have passed since the ²³⁴Th technique was first applied to determine the export flux of particulate carbon but the C/²³⁴Th ratio on sinking particles remains as one of the principal uncertainties of this approach. Here we present a global database of 9110 C/²³⁴Th ratios collected using in situ pumps, sediment traps and bottles. Ratios have been grouped, based on the particle size or the device used for the sampling, as: i) Large particles ii) Small particles, iii) Sediment traps and iv) Bulk. The samples were collected between 1989 and 2016 and are distributed among most of the open ocean Longhurst provinces, with some coastal areas also represented. As expected, the data are not evenly distributed along the water column, with most of the data points found in the upper 100-200 m, although the depth range expands from surface down to >5500 m. Globally, the ratios range from 0.012 to 1778 µmol/dpm. This database shows how particulate C/²³⁴Th ratios vary with time, depth, particle size and location, which is important for the application of the ²³⁴Th approach and the estimate of the global magnitude of the biological carbon pump.