1. Introduction

The biological carbon pump (BCP) comprises the processes that mediate the transfer of organic carbon from the euphotic zone, where it is produced, to the deep ocean (Volk and Hoffert, 1985). Without the BCP the concentrations of atmospheric CO2 would be ~ 200 ppm higher than present (Parekh et al., 2006). The efficiency of the BCP depends primarily on the gravitational settling of particles, physical advection and mixing of particles, and the active vertical transport of particles due to migrating zooplankton and fish (Boyd et al., 2019). These processes shape the flux of carbon and other elements through the mesopelagic and abyssopelagic zones, where carbon can be locked away from the atmosphere, impacting atmospheric CO2 concentrations over climatologically relevant timescales (Kwon et al., 2009; DeVries et al., 2012). Accurately predicting the response of the ocean carbon storage to already underway and future climate changes requires a mechanistic knowledge of the processes making up the BCP (e.g., Siegel et al., 2023).
The largest component of the BCP is settling particles (Boyd et al., 2019; Nowicki et al., 2022). Particles in the open ocean are mainly produced in the surface ocean and consist of living and dead phytoplankton cells, detritus, carcasses, fecal material, and minerals. Their size range spans between less than a micrometer (typical threshold defined as 0.7 µm in diameter) to many millimeters, with those with a diameter of > 0.5 mm being referred to as “marine snow” (Alldredge and Silver, 1988). The fate and distribution of particles are influenced by the transformation processes that change a particle’s size, composition and, consequently, sinking velocity. Particles can aggregate, disaggregate, solubilize or be remineralized back to inorganic forms via mechanical forcing, bacterioplankton activity, and interaction with zooplankton (Stemmann et al., 2004; Burd et al., 2010; Giering et al., 2014; Collins et al., 2015). When particles aggregate or are repackaged into fecal matter, they have the potential to sink rapidly (~50 to ≥ 2,000 m d-1) and hence more easily escape consumption, fragmentation, and dissolution at shallow depths; thus, bringing organic matter to the deep ocean more efficiently than smaller particles (Alldredge and Silver, 1988; Ebersbach and Trull, 2008).
Particle sinking velocities are typically thought to be largely determined by size. Stokes’ Law, which quantifies the sinking velocity of spherical solid particles under laminar flow conditions, assumes that particle sinking velocity increases as the product of the spherical particle’s diameter squared and the excess density with respect to seawater. The implication for sinking particles in the ocean is that large particles should sink fast and are hence effective vectors for carbon transport to depth, whereas small particles sink slow and are remineralized within the upper mesopelagic, contributing little to BCP-mediated ocean carbon storage (Kriest, 2002; Marsay et al., 2015; Cavan et al., 2017). Nevertheless, the presence of small particles (0.2–20 μm) has been observed at great depths (> 1000 m) (e.g., Dall’Olmo and Mork, 2014; Briggs et al., 2020). Furthermore, recent studies have shown that the downward flux of particulate organic carbon (POC) via small particles (< 100 μm) can be significant in specific ecosystems and seasons, at times constituting the bulk of the total POC flux through the mesopelagic (e.g., Durkin et al., 2015; Giering et al., 2016; Bisson et al., 2020; Dever et al., 2021).
The presence of ballast minerals (biogenic silica from diatoms, particulate inorganic carbon from coccolithophores and foraminifera, and lithogenic material from aeolian and riverine inputs) is also thought to increase particle sinking velocity by increasing particle excess density in respect to seawater (Armstrong et al., 2002; Passow and De La Rocha, 2006; Laurenceau-Cornec et al., 2019; Iversen and Lampitt, 2020; Iversen, 2023). However, cause and effect in the relationship between organic matter and minerals are not clear, and sinking aggregates originating from biological activity in the mixed layer could scavenge and subsequently transport small, suspended mineral particles to depth (Passow, 2004).
The presence of transparent exopolymer particles (TEP; Alldredge et al.,1993) also has the potential to influence the sinking of particles. TEP are largely composed of polysaccharides released by phytoplankton and bacteria as extracellular surface-active exopolymers (Passow, 2002), especially under nutrient-limited conditions (Obernosterer and Herndl, 1995). TEP may act as biological glue and thus enhance particle coagulation rate by increasing particle ”stickiness” (Passow et al., 1994; Jackson, 1995). However, by being positively buoyant, TEP may also reduce aggregates’ sinking velocities especially when aggregates are characterized by high TEP-to-solid particles ratio (Engel and Schartau, 1999; Azetsu-Scott and Passow, 2004). Although TEP has the potential to play an important role in controlling the downward transport of particles, we lack a robust mechanistic understanding of this process (e.g., Mari et al., 2017; Nagata et al., 2021).
Our hypothesis is that TEP play a critical role in determining the sinking velocity of particles, potentially outcompeting the role of ballast minerals. We assess how particle size and composition regulates the partitioning between sinking and suspended particles within the upper mesopelagic in an iron-limited region of the Northeast Pacific Ocean. Particles were collected during the EXPORTS (Export Processes in the Ocean from RemoTe Sensing) field campaign using Marine Snow Catchers (MSC). Finally, we discuss the possible mechanisms driving the formation of small sinking particles in the mesopelagic and highlight the importance of studying small particle characteristics and particle patchiness to enhance our understanding of the functioning of the biological carbon pump.