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1. Carbon sequestration by multiple biological pump pathways in a coastal upwelling biome

   https://doi.org/10.1038/s41467-023-37771-8  

   Stukel, Michael R.; Irving, John P.; Kelly, Thomas B.; Ohman, Mark D.; Fender, Christian K.; Yingling, Natalia (April 2023, Nature Communications)

   Abstract Multiple processes transport carbon into the deep ocean as part of the biological carbon pump, leading to long-term carbon sequestration. However, our ability to predict future changes in these processes is hampered by the absence of studies that have simultaneously quantified all carbon pump pathways. Here, we quantify carbon export and sequestration in the California Current Ecosystem resulting from (1) sinking particles, (2) active transport by diel vertical migration, and (3) the physical pump (subduction + vertical mixing of particles). We find that sinking particles are the most important and export 9.0 mmol C m⁻²d⁻¹across 100-m depth while sequestering 3.9 Pg C. The physical pump exports more carbon from the shallow ocean than active transport (3.8 vs. 2.9 mmol C m⁻²d⁻¹), although active transport sequesters more carbon (1.0 vs. 0.8 Pg C) because of deeper remineralization depths. We discuss the implications of these results for understanding biological carbon pump responses to climate change. 

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2. Strong and efficient summertime carbon export driven by aggregation processes in a subarctic coastal ecosystem

   https://doi.org/10.1002/lno.12561  

   O'Daly, Stephanie_H; Hennon, Gwenn_M_M; Kelly, Thomas_B; Strom, Suzanne_L; McDonnell, Andrew_M_P (April 2024, Limnology and Oceanography)

   Abstract Sinking marine particles, one pathway of the biological carbon pump, transports carbon to the deep ocean from the surface, thereby modulating atmospheric carbon dioxide and supplying benthic food. Few in situ measurements exist of sinking particles in the Northern Gulf of Alaska; therefore, regional carbon flux prediction is poorly constrained. In this study, we (1) characterize the strength and efficiency of the biological carbon pump and (2) identify drivers of carbon flux in the Northern Gulf of Alaska. We deployed up to five inline drifting sediment traps in the upper 150 m to simultaneously collect bulk carbon and intact sinking particles in polyacrylamide gels and measured net primary productivity from deck‐board incubations during the summer of 2019. We found high carbon flux magnitude, low attenuation with depth, and high export efficiency. We quantitatively attributed carbon flux between 10 particle types, including various fecal pellet categories, dense detritus, and aggregates using polyacrylamide gels. The contribution of aggregates to total carbon flux (41–93%) and total carbon flux variability (95%) suggest that aggregation processes, not zooplankton repackaging, played a dominant role in carbon export. Furthermore, export efficiency correlated significantly with the proportion of chlorophyllain the large size fraction (> 20 μm), total aggregate carbon flux, and contribution of aggregates to total carbon flux. These results suggest that this stratified, small‐cell‐dominated ecosystem can have sufficient aggregation to allow for a strong and efficient biological carbon pump. This is the first integrative description of the biological carbon pump in this region. 

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3. Epipelagic nitrous oxide production offsets carbon sequestration by the biological pump

   https://doi.org/10.1038/s41561-022-01090-2  

   Wan, Xianhui S; Sheng, Hua-Xia; Dai, Minhan; Casciotti, Karen L; Church, Matthew J; Zou, Wenbin; Liu, Li; Shen, Hui; Zhou, Kuanbo; Ward, Bess B; et al (January 2023, Nature Geoscience)

   Abstract The removal of carbon dioxide from the atmosphere by the marine biological pump is a key regulator of Earth’s climate; however, the ocean also serves as a large source of nitrous oxide, a potent greenhouse gas and ozone-depleting substance. Although biological carbon sequestration and nitrous oxide production have been individually studied in the ocean, their combined impacts on net greenhouse forcing remain uncertain. Here we show that the magnitude of nitrous oxide production in the epipelagic zone of the subtropical ocean covaries with remineralization processes and thus acts antagonistically to weaken the radiative benefit of carbon removal by the marine biological pump. Carbon and nitrogen isotope tracer incubation experiments and nitrogen isotope natural abundance data indicate enhanced biological activity promotes nitrogen recycling, leading to substantial nitrous oxide production via both oxidative and reductive pathways. These shallow-water nitrous oxide sources account for nearly half of the air–sea flux and counteract 6–27% (median 9%) of the greenhouse warming mitigation achieved by carbon export via the biological pump. 

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4. Temperature controls carbon cycling and biological evolution in the ocean twilight zone

   https://doi.org/10.1126/science.abb6643  

   Boscolo-Galazzo, Flavia; Crichton, Katherine A.; Ridgwell, Andy; Mawbey, Elaine M.; Wade, Bridget S.; Pearson, Paul N. (March 2021, Science)

   Theory suggests that the ocean’s biological carbon pump, the process by which organic matter is produced at the surface and transferred to the deep ocean, is sensitive to temperature because temperature controls photosynthesis and respiration rates. We applied a combined data-modeling approach to investigate carbon and nutrient recycling rates across the world ocean over the past 15 million years of global cooling. We found that the efficiency of the biological carbon pump increased with ocean cooling as the result of a temperature-dependent reduction in the rate of remineralization (degradation) of sinking organic matter. Increased food delivery at depth prompted the development of new deep-water niches, triggering deep plankton evolution and the expansion of the mesopelagic “twilight zone” ecosystem. 

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5. Drawdown of Atmospheric pCO ₂ Via Variable Particle Flux Stoichiometry in the Ocean Twilight Zone

   https://doi.org/10.1029/2021GL094924  

   Tanioka, Tatsuro; Matsumoto, Katsumi; Lomas, Michael W. (November 2021, Geophysical Research Letters)

   Abstract The strength of the biological soft tissue pump in the ocean critically depends on how much organic carbon is produced via photosynthesis and how efficiently the carbon is transferred to the ocean interior. For a given amount of limiting nutrient, phosphate, soft tissue pump would be strengthened if the carbon (C) to phosphorus (P) ratio of sinking organic matter increases as the remineralization length scale of C increases. Here, we present a new data compilation of particle flux stoichiometry and show that C:P of sinking particulate organic matter (POM) in the ocean twilight zone on average is likely to be higher than the C:P ratio of surface suspended POM. We further demonstrate using a physics‐biology coupled global ocean model combined with a theory from first principles that an increase in C:P export flux ratio in the ocean's twilight zone can lead to a considerable drawdown of atmosphericpCO₂. 

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