Partitioning Particulate Carbon Export into Three Distinct Pathways Using Biogeochemical Argo Floats

At high latitudes, the export of organic matter from the surface to the ocean interior, the so-called Biological Carbon Pump (BCP), has traditionally been attributed to the gravitational sinking of particulate organic carbon following the spring phytoplankton bloom. Yet, advanced observational techniques such as BGC-Argo floats have confirmed the widespread importance of previously identified physically-mediated pathways through which organic matter can exit the surface mixed layer. These pathways are referred to as particle injection pumps (PIPs). For example, recent studies have shown that export occurs through localized eddy-driven water subduction, a process known as the eddy pump. BGC-Argo studies have shown that the succession of deep mixing and re-stratification events during the winter-to-spring transition can also act as a physical pump, the so-called mixed layer pump. Sinking of particles, termed the biological gravitational pump (BGP), the eddy pump and mixed layer pump seem to dominate at different times of the seasonal cycle. Previous work has only focused on a single mechanism due to the technical difficulties in discerning the three mechanisms (BGP and 2 PIPs) throughout a complete seasonal cycle. In this work we show how BGC-Argo floats can be used to simultaneously study the three export mechanisms thanks to recent advances in optical signal analysis and strategic sampling. This approach allowed us to study four different bloom cycles and BCP seasonality in the Southern Ocean. Our results show how location and season but also phytoplankton composition, determine the relative contribution of each mechanism. This work presents a novel and relevant application of BGC-Argo observations, having technical implications on the optimal choice for vertical resolution and sampling frequencies.