This project aims at dissecting the Biological Carbon Pump, with a focus on mesopelagic processes responsible for the dramatic decrease in C flux with depth. Profiling floats equipped with a newly-developed sensor and existing methods will be used to autonomously quantify these microbially- and zooplankton-mediated degradation processes in 2 contrasting ocean regions: the NW Mediterranean and area around the Kerguelen Islands. These float-derived estimates will be combined with model simulations to better constrain regional mesopelagic C budgets.

The biological carbon pump (BCP) annually supplies 5-10 Gt of carbon to the oceans’ interior, driving long-term carbon sequestration. However, the vast majority of sinking particulate organic carbon (POC) entering the mesopelagic zone (~100-1000 m) is remineralized. This dramatic vertical decrease in POC flux is modulated by multiple processes involving zooplankton and microbes. Despite multi-decadal research, there is no consensus on the controls and latitudinal trends in POC flux attenuation. A better understanding of the factors that control the efficiency of the BCP therefore requires to separate the processes that jointly set POC flux attenuation. To this end, a ground-breaking approach will be developed to autonomously quantify mesopelagic microbial remineralization rates from profiling floats at high temporal resolution and over the full annual cycle. This novel observational approach developed in adhoC and the analysis of the dataset obtained will allow dissecting the BCP for the first time. The newly developed profiling floats will be deployed for >1 year in two contrasting ocean regions: the Northwestern Mediterranean Sea, and the area around the Kerguelen Islands. Over the seasonal cycle, these two regions offer the opportunity to explore wide-ranging ecological, biogeochemical and environmental settings needed to get insights into the drivers of the POC flux attenuation, their relative magnitude and controls. Concurrent estimates of the flux attenuation, and the associated microbial remineralization and zooplankton-mediated particle fragmentation rates will be used to determine the degree of influence (and timing) of these microbial and zooplankton-mediated processes in setting regional patterns in POC flux attenuation. Finally, these float-derived estimates will be combined with numerical model simulations to better constrain regional mesopelagic C budgets and global C sequestration.