Evaluation of the Particulate Inorganic Carbon Export Efficiency in the Global Ocean

Abstract. The oceanic carbonate pump corresponds to the production and the sinking of particulate inorganic carbon (PIC) thanks to calcified planktonic organisms. In this study, the global PIC production from ocean colour satellite observations were combined with PIC flux observation from short-term sediment traps deployed during the last decades covering the global ocean. Coccolithophores are the main planktonic calcified group in the euphotic zone, with an important phenological blooming pattern and an important latitude dependant seasonal response. The present study highlights that the PIC production in the euphotic zone and the pelagic PIC flux varied among oceanic regions, depth and season. Based on a geographic matchup between the PIC flux from sediment traps and remote sensing climatology observation, correlation between net primary production (NPP) of particulate organic carbon (POC) in the euphotic zone and PIC flux is revealed. However, PIC production in the euphotic zone is not correlated with PIC flux at global scale, but only for delimited ocean basin such as in the North Atlantic and the Southern Ocean. Despite lower PIC production and PIC/POC ratios in the euphotic zone, temperate and subpolar areas are more efficient to export PIC compared to equatorial and subtropical areas (higher PIC production and PIC/POC ratios in the euphotic zone). The plankton phenology seems to be an important driver of PIC export efficiency (PE_eff) and PIC transfer efficiency (T_eff). Pelagic PIC dissolution is assumed to be responsible for the decrease in PE_eff and T_eff. In a context of climate change, any modification of plankton network community as well as modification PIC T_eff is expected to have consequences on surface alkalinity balance and CO₂ exchange between ocean and atmosphere. This study suggests that the 'packaging factor' corresponding to the vehicle of the biological carbon pump (marine snow aggregates, fecal pellets) and the plankton network (e.g. zooplankton community, microbial loop) may affect the PIC export efficiency and the PIC transfer efficiency.