What do marine particle characteristics and dynamics tell us about the efficiency of the Biological Carbon Pump?

Gravitational sinking of particles is a key pathway for the transport of particulate organic carbon (POC) into the deep ocean. The sinking of POC and its remineralization directly impact ocean carbon storage on climatologically relevant timescales. Particle size, density and composition influence particle sinking velocity and the remineralization length scale i.e., the depth where POC is converted to suspended organic carbon or inorganic carbon. However, the factors affecting this relationship and their spatiotemporal variability are not well understood. Here, we use data collected from Marine Snow Catchers to characterize profiles of both suspended and sinking particles. Particle size, biogeochemical composition, and microbial activity are used to understand the controls and fate of suspended and sinking POC in the upper ocean. My first chapter shows that during the late summer in the subarctic Pacific POC fluxes were low, marine snow-sized aggregates (d > 0.5 mm) were rare and small, suspended particles differed from small sinking particles by their higher TEP (transparent exopolymer particles) content. This work provides the first in situ data to support the hypothesis by Xavier et al. (2017) that the ratio between TEP-C and POC determines, at least partially, the efficiency of the biological carbon pump (i.e., POC flux 100 m below reference depth/flux at reference depth).My second chapter focuses on the decline of the spring diatom bloom in the NE Atlantic and shows that turbulent disaggregation and changes in the mixed layer depth due to four strong storms delayed the formation and sinking of POC-rich marine snow-sized aggregates (d > 0.1 mm) while small, slow-sinking silica-rich particles sank from the mixed layer, creating an inefficient POC export event. After the last storm passed, we observed the sinking of marine snow aggregates, which resulted in vertical flux of a mixed post-bloom plankton community out of the euphotic zone.\nMy third chapter focuses on a study conducted in the Labrador Sea, where microbial degradation of total organic carbon (TOC) differed between suspended and sinking particles and across different stages of a spring Phaeocystis bloom. Carbon removal rates for fast-sinking and slow-sinking particles were similar but were on average one order of magnitude larger than those found for the suspended fraction. However, at the station characterized by abundant free-floating Phaeocystis colonies, the suspended fraction sustained a TOC removal rate similar to the ones for sinking particles. This observation suggests that the presence of Phaeocystis colonies may have enhanced the rate of microbial degradation of TOC. \nOverall, this work highlights the importance of investigating characteristics and dynamics of suspended and sinking particles to understand which factors affect the efficiency of the biological carbon pump in different open ocean systems.\n

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