Editorial: The Oceanic Particle Flux and Its Cycling Within the Deep Water Column – Volume II

Rut Pedrosa Pàmies^1*Maureen H Conte¹Makio Honda²Gerhard Josef Herndl³

• ¹Marine Biological Laboratory (MBL), Woods Hole, United States
• ²Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Japan
• ³Universitat Wien, Vienna, Austria

Mino et al. (2023) investigated mesopelagic nitrogen dynamics in the western North Pacific by applying an isotopic mass-balance framework at two contrasting sites: the subarctic station K2 and the subtropical station S1. The study revealed that non-gravitational pathways, namely the mixedlayer pump and the fragmentation of sinking aggregates, contributed substantially to particulate nitrogen (PN) export, together accounting for approximately 20% of the total flux at both locations. The relative contribution of mixing-driven transport versus fragmentation was elevated at K2 as compared with S1, reflecting the influence of regional hydrography and ecosystem structure on mesopelagic nitrogen transfer. These findings highlight the importance of transformation processes within the water column in modulating export efficiency, challenging the long-standing view that passive gravitational settling of intact particles is the dominant pathway. By quantifying the role of mesopelagic processing, this study expands our understanding of nitrogen budgets across oceanic regimes and underscores the need to incorporate fragmentation and mixing into predictive models of biogeochemical cycling. Kim et al. (2023) present sediment trap observations from the Northwest Pacific during 2017-2018, documenting episodic pulses of sinking particle flux coinciding with high dust deposition events. These flux increases were largely decoupled from surface chlorophyll variability, demonstrating that atmospheric dust deposition can stimulate particle export through both nutrient fertilization and mineral ballasting. With biogenic particles accounting for ~82% of flux, the study shows how dust can act as a powerful driver of export flux, independent of local productivity. These findings highlight the importance of external, episodic forcing in particle cycling, raising critical questions about how changing dust regimes under future climate conditions will reshape deep carbon flux. Salter et al. ( 2023) investigate interannual variability in mesopelagic and bathypelagic particle fluxes in the Fram Strait between 2000 and 2013, a period marked by pronounced shifts in sea-ice extent and meltwater dynamics. Despite increasing surface chlorophyll concentrations, deep carbon export efficiency declined, driven by reduced diatom-derived biogenic silica fluxes and a greater contribution from lithogenic and ice-rafted particles. This apparent decoupling between surface productivity and export underscores the role of sea-ice dynamics and meltwater in shaping the Arctic particle cycling. The study emphasizes that in polar systems, climate-driven changes in ice cover may diminish export efficiency even in the face of enhanced surface productivity, with profound implications for carbon sequestration in polar oceans. Beltran-Perez et al. (2023) present a 22-year sediment trap record (1999-2020) from the Gotland Basin in the Baltic Sea, a marginal basin strongly influenced by eutrophication and seasonal stratification. The study shows clear seasonal maxima in spring, summer, and autumn, with isotopic shifts in exported organic matter linked to transitions between diatom-and cyanobacteriadominated communities. These results demonstrate how community composition can shape both the magnitude and geochemical composition, providing insights relevant to eutrophic systems worldwide. Additionally, the study establishes a valuable baseline for understanding how future ecological changes in coastal and marginal seas will influence carbon and nutrient cycling. Collectively, the six contributions to this Research Topic reinforce several emerging themes. First, community composition exerts a significant control over particle flux processes, from protozoan mediation in the open Atlantic to phytoplankton succession in the Baltic. Second, non-classical pathways, including physical mixing, fragmentation, dust deposition, and ice rafting, provide additional controls of export flux that are often independent of or decoupled from surface production. Third, regional and climatic drivers, ranging from atmospheric forcing to sea-ice dynamics, fundamentally shape export efficiency and flux composition. Finally, isotopic tracers such as barium, carbon, and nitrogen are valuable tools to trace particle sources and transformations, bridging contemporary process studies and paleoceanographic reconstructions. This compilation thus advances our understanding of the biological pump as a highly dynamic and complex system, rather than a linear function of surface productivity. By integrating ecological, physical, and chemical perspectives across a range of environments, these studies point toward a future where predictive models of ocean carbon cycling must incorporate community-specific processes, episodic forcing, and climate-driven variability. Such efforts will be indispensable for projecting the ocean's role in regulating the global carbon cycle under conditions of rapid environmental change.