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1. Particle cycling rates at Station P as estimated from the inversion of POC concentration data

   https://doi.org/10.1525/elementa.2021.00018  

   Amaral, Vinícius J.; Lam, Phoebe J.; Marchal, Olivier; Roca-Martí, Montserrat; Fox, James; Nelson, Norman B. (January 2022, Elementa: Science of the Anthropocene)

   Particle cycling rates in marine systems are difficult to measure directly, but of great interest in understanding how carbon and other elements are distributed throughout the ocean. Here, rates of particle production, aggregation, disaggregation, sinking, remineralization, and transport mediated by zooplankton diel vertical migration were estimated from size-fractionated measurements of particulate organic carbon (POC) concentration collected during the NASA EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) cruise at Station P in summer 2018. POC data were combined with a particle cycling model using an inverse method. Our estimates of the total POC settling flux throughout the water column are consistent with those derived from thorium-234 disequilibrium and sediment traps. A budget for POC in two size fractions, small (1–51 µm) and large (> 51 µm), was produced for both the euphotic zone (0–100 m) and the upper mesopelagic zone (100–500 m). We estimated that POC export at the base of the euphotic zone was 2.2 ± 0.8 mmol m−2 d−1, and that both small and large particles contributed considerably to the total export flux along the water column. The model results indicated that throughout the upper 500 m, remineralization leads to a larger loss of small POC than does aggregation, whereas disaggregation results in a larger loss of large POC than does remineralization. Of the processes explicitly represented in the model, zooplankton diel vertical migration is a larger source of large POC to the upper mesopelagic zone than the convergence of large POC due to particle sinking. Positive model residuals reveal an even larger unidentified source of large POC in the upper mesopelagic zone. Overall, our posterior estimates of particle cycling rate constants do not deviate much from values reported in the literature, i.e., size-fractionated POC concentration data collected at Station P are largely consistent with prior estimates given their uncertainties. Our budget estimates should provide a useful framework for the interpretation of process-specific observations obtained by various research groups in EXPORTS. Applying our inverse method to other systems could provide insight into how different biogeochemical processes affect the cycling of POC in the upper water column. 

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2. Particle Export Fluxes in the Southern Ocean: Importance of Nonheterotrophic Processes in POC Flux Attenuation

   https://doi.org/10.1029/2024JC021607  

   Le_Moigne, Frédéric_A C; Pabortsava, Katsiaryna; Villa‐Alfageme, María; Briggs, Nathan; Baker, Chelsey A; Bouman, Heather A; English, Chance J; Blackbird, Sabena; Henson, Stephanie A; Venables, Hugh; et al (July 2025, Journal of Geophysical Research: Oceans)

   Abstract The ocean contributes to regulating atmospheric CO₂levels via the biological carbon pump (BCP). One critical aspect of the BCP is the depth at which sinking particulate organic carbon (POC) remineralizes in the mesopelagic zone (200–1,000 m). In the Southern Ocean, the circulation is such that the products generated from POC remineralization may have drastically different fates depending on (a) the latitude at which sinking particulate material is produced and (b) the depth at which its remineralization occurs. Here, we assess latitudinal and depth variations of POC export marine aggregate abundance and composition in the Southeast Pacific sector of the Southern Ocean. We show changes in flux attenuation depth horizons in the upper mesopelagic in the subantarctic zone. These correspond to rapid particle accumulation below the depth of the euphotic zone followed by abrupt export. We believe that such rapid changes may be linked to diatom life cycles, including resting cell and spore formation and resulting changes in particle sinking velocities rather than attenuation due to heterotrophic degradation or solubilization in the upper mesopelagic zone. We further discuss the occurrence of such features in the Southern Ocean and at the global scale. Our results highlight the importance of alternative flux attenuation processes, such as sudden changes in particles sinking velocities, in explaining variability in organic carbon sequestration by the ocean's BCP. 

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3. Cycling Rates of Particulate Organic Carbon Along the GEOTRACES Pacific Meridional Transect GP15

   https://doi.org/10.1029/2023GB007940  

   Amaral, Vinícius J.; Lam, Phoebe J.; Marchal, Olivier; Kenyon, Jennifer A. (January 2024, Global Biogeochemical Cycles)

   Abstract Understanding particle cycling processes in the ocean is critical for predicting the response of the biological carbon pump to external perturbations. Here, measurements of particulate organic carbon (POC) concentration in two size fractions (1–51 and >51 μm) from GEOTRACES Pacific meridional transect GP15 are combined with a POC cycling model to estimate rates of POC production, (dis)aggregation, sinking, remineralization, and vertical transport mediated by migrating zooplankton, in the euphotic zone (EZ) and upper mesopelagic zone (UMZ) of distinct environments. We find coherent variations in POC cycling parameters and fluxes throughout the transect. Thus, the settling speed of POC in the >51 μm fraction increased with depth in the UMZ, presumably due to higher particle densities at depth. The settling flux of total POC (>1 μm) out of the EZ was positively correlated with primary production integrated over the EZ; the highest export occurred in the subarctic gyre while the lowest occurred in the subtropical gyres. The ratio of POC settling flux to integrated primary production was low (<5%) along GP15, which suggests an efficient recycling of POC in the EZ in all trophic regimes. Specific rates of POC remineralization did not show clear variations with temperature or dissolved oxygen concentration, that is, POC recycling was apparently controlled by other factors such as microbial colonization and substrate lability. Particle cohesiveness, as approximated by the second‐order rate constant for particle aggregation, was negatively correlated with trophic regime: particles appeared more cohesive in low‐productivity regions than in high‐productivity regions. 

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4. A Mechanistic Yet Simple Model for Dynamic Ocean Particulate Organic Carbon Fluxes, Global Biogeochemical Cycles

   Liang, H; Zakem, E; Lauderdale, J; Ferrari, R (August 2026, Global biogeochemical cycles)

   Biological processes contribute to ocean carbon storage, playing a critical role in the global carbon cycle by maintaining lower atmospheric CO$$_2$$ levels. A major pathway is the gravitational sinking of particulate organic carbon (POC), yet how POC fluxes change in a warming ocean remains uncertain due, in part, to high spatiotemporal variability in observations. Observed POC fluxes, when averaged, typically exhibit a power-law decrease with depth (the ``Martin curve"), yet prognostic models require structural complexity or poorly-constrained, empirically-tuned parameters to reproduce this behavior. Here, we propose considering the Martin curve as an asymptotic equilibrium benchmark that process-based models should converge to in their long-term, time-averaged behavior. We introduce a simple model for oceanic POC fluxes applicable to the entire water column, grounded in the theory that organic matter remineralization rate scales inversely with its age. We derive an analytical formula linking temporal and vertical variability and evaluate the scheme in an idealized 2D Atlantic Meridional Overturning Circulation model with oscillating surface productivity, which generates time-lagged vertical POC fluxes and dynamic water-column responses while converging to the power-law attenuation in the long term. The results demonstrate that much of the POC flux spatiotemporal variability, including its lack of synchronization with surface productivity and between different depths, can result solely from local surface variability, without invoking complex microbial shifts or lateral advection by ocean currents. This POC flux model enables simulations of ocean carbon cycling across diverse timescales and facilitates model-data comparisons, enhancing our mechanistic understanding of ocean carbon dynamics. 

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5. Characterization of Sinking Particulate Organic Carbon Biogeochemical Variability in the Santa Barbara Basin via Solid State 13C NMR

   Chaza, T; Coli, N; Gao, X; Masiello, CA; Benitez-Nelson, CR (February 2026, American Geophysical Union)

   Particulate organic carbon (POC) includes suspended and sinking particles ≥ 0.2 μm, and can consist of microbial detritus, fecal pellets, and terrestrially derived organic material. The bulk chemical characterization of this material has been challenging historically because many approaches are destructive, limiting the number of measurements possible. Here we report data on the seasonal and depth-dependent variability in the POC organic geochemistry in the Santa Barbara Basin (off the coast of California) using solid-state 13C nuclear magnetic resonance (13C NMR), a non-destructive approach. NMR can provide bulk information on the overall chemical characteristics of entire samples. In the case of sinking marine POC, this information can be obtained without chemical pretreatment or destructive analyses of any kind. Here we show that there is significant variation in the bulk chemistry of sinking POC in the Santa Barbara Basin, consistent with changes in surface ecology, decomposition of organic matter, and seasonal delivery of terrestrial material by aerosol and by rivers. This study evaluates how seasonality impacts exported POC by tracking terrestrial inputs during flood or fire events across El Niño-Southern Oscillation (ENSO) phases. The Santa Barbara Basin sediment traps have been deployed since 1993 and offer a long, high-resolution flux record. Using solid-state 13C NMR, we studied sinking POC samples collected bi-weekly from June 2015 to April 2017, a period that included an intense El Niño event. We estimated molecular composition using NMR functional group data and an end-member molecular mixing model (MMM), which reports virtual concentrations of lipids, lignin, protein, carbohydrates, and black carbon. Preliminary MMM results indicate samples have a higher proportion of aromatic C than typical open-ocean POC, indicating more fire-derived black carbon during the El Niño phase. Such detailed molecular insights will impact blue carbon accounting and climate mitigation strategies. 

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