Published studies on the latitudinal variability of the biological carbon pump (BCP)’s transfer efficiency –the fraction of exported particulate organic carbon (POC) flux reaching depth– paint an inconsistent global picture. Some report higher transfer efficiency in warm, subtropical oceans, while others suggest that it is higher in cold, high-latitude or equatorial upwelling regions. Resolving this contradiction matters: understanding large-scale and seasonally varying latitudinal patterns of POC flux is central to predicting oceanic carbon sequestration by the BCP and its sensitivity to climate change or the efficacy of CO2 removal strategies.
Here, we argue that the disagreement stems not only from the intrinsic noisiness of the marine environment, but also from how the BCP has been historically observed: sparsely and unevenly using inconsistent sampling frameworks across research projects and without a standardised method to quantify POC flux attenuation with depth. Specifically, we pose the null hypothesis that there is no distinct geographic variation in BCP transfer efficiency. To test it, we compiled in situ POC flux time-series data from sediment traps, radionuclide disequilibria, and instantaneous estimates from UVP5 imagery, alongside fluxes of two ballast minerals (calcium carbonate and biogenic silica), across biogeographically distinct locations. Our analysis reveals that once large local uncertainties are accounted for, no statistically significant geographic variation in BCP transfer efficiency can be identified, an important negative result supporting the null hypothesis.
While these findings do not preclude the existence of distinct spatial variations in transfer efficiency, they raise critical questions about the BCP and its observability. They also underscore the challenges involved and the need for sustained POC flux observations, standardisation of protocols, and intercalibration of technologies if we are to fully understand the processes controlling the BCP and its future evolution under global warming.
