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1. Impact of a warm anomaly in the Pacific Arctic region derived from time-series export fluxes

   https://doi.org/10.1371/journal.pone.0255837  

   Lalande, Catherine ; Grebmeier, Jacqueline M. ; McDonnell, Andrew M. ; Hopcroft, Russell R. ; O’Daly, Stephanie ; Danielson, Seth L. ( August 2021 , PLOS ONE)

   Incarbona, Alessandro (Ed.)

   Unusually warm conditions recently observed in the Pacific Arctic region included a dramatic loss of sea ice cover and an enhanced inflow of warmer Pacific-derived waters. Moored sediment traps deployed at three biological hotspots of the Distributed Biological Observatory (DBO) during this anomalously warm period collected sinking particles nearly continuously from June 2017 to July 2019 in the northern Bering Sea (DBO2) and in the southern Chukchi Sea (DBO3), and from August 2018 to July 2019 in the northern Chukchi Sea (DBO4). Fluxes of living algal cells, chlorophyll a (chl a ), total particulate matter (TPM), particulate organic carbon (POC), and zooplankton fecal pellets, along with zooplankton and meroplankton collected in the traps, were used to evaluate spatial and temporal variations in the development and composition of the phytoplankton and zooplankton communities in relation to sea ice cover and water temperature. The unprecedented sea ice loss of 2018 in the northern Bering Sea led to the export of a large bloom dominated by the exclusively pelagic diatoms Chaetoceros spp. at DBO2. Despite this intense bloom, early sea ice breakup resulted in shorter periods of enhanced chl a and diatom fluxes at all DBO sites, suggesting a weaker biological pump under reduced ice cover in the Pacific Arctic region, while the coincident increase or decrease in TPM and POC fluxes likely reflected variations in resuspension events. Meanwhile, the highest transport of warm Pacific waters during 2017–2018 led to a dominance of the small copepods Pseudocalanus at all sites. Whereas the export of ice-associated diatoms during 2019 suggested a return to more typical conditions in the northern Bering Sea, the impact on copepods persisted under the continuously enhanced transport of warm Pacific waters. Regardless, the biological pump remained strong on the shallow Pacific Arctic shelves. 

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2. A Visual Tour of Carbon Export by Sinking Particles

   https://doi.org/10.1029/2021GB006985  

   Durkin, Colleen A. ; Buesseler, Ken O. ; Cetinić, Ivona ; Estapa, Margaret L. ; Kelly, Roger P. ; Omand, Melissa ( October 2021 , Global Biogeochemical Cycles)

   To better quantify the ocean's biological carbon pump, we resolved the diversity of sinking particles that transport carbon into the ocean's interior, their contribution to carbon export, and their attenuation with depth. Sinking particles collected in sediment trap gel layers from four distinct ocean ecosystems were imaged, measured, and classified. The size and identity of particles was used to model their contribution to particulate organic carbon (POC) flux. Measured POC fluxes were reasonably predicted by particle images. Nine particle types were identified, and most of the compositional variability was driven by the relative contribution of aggregates, long cylindrical fecal pellets, and salp fecal pellets. While particle composition varied across locations and seasons, the entire range of compositions was measured at a single well‐observed location in the subarctic North Pacific over one month, across 500 m of depth. The magnitude of POC flux was not consistently associated with a dominant particle class, but particle classes did influence flux attenuation. Long fecal pellets attenuated most rapidly with depth whereas certain other classes attenuated little or not at all with depth. Small particles (<100 μm) consistently contributed ∼5% to total POC flux in samples with higher magnitude fluxes. The relative importance of these small particle classes (spherical mini pellets, short oval fecal pellets, and dense detritus) increased in low flux environments (up to 46% of total POC flux). Imaging approaches that resolve large variations in particle composition across ocean basins, depth, and time will help to better parameterize biological carbon pump models.

    

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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)

   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. 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)

   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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5. Sinking carbon, nitrogen, and pigment flux within and beneath the euphotic zone in the oligotrophic, open-ocean Gulf of Mexico

   https://doi.org/10.1093/plankt/fbab001  

   Stukel, Michael R ; Kelly, Thomas B ; Landry, Michael R ; Selph, Karen E ; Swalethorp, Rasmus ( February 2021 , Journal of Plankton Research)

   Dolan, John (Ed.)

   Abstract During two cruises in the oligotrophic oceanic Gulf of Mexico, we deployed sediment traps at three depths: center of the euphotic zone (EZ) (60 m), base of the EZ (117–151 m), and in the twilight zone (231 m). Organic carbon export declined with depth from 6.4 to 4.6 to 2.4 mmol C m−2 d−1, suggesting that net particle production was concentrated in the upper EZ. Net primary production varied from 24 to 29 mmol C m−2 d−1, slightly more than half in the upper EZ. Export ratios varied from 11 to 25%. Trap measurements of chlorophyll and phaeopigments allowed us to quantify fluxes of fresh phytoplankton and herbivorous fecal pellets, respectively, which were both minor contributors to total flux, although their contributions varied with depth. Phytoplankton flux was more important from the upper to lower EZ; fecal pellets were more important at the EZ base and below. C:N elemental ratios and 13C and 15N isotope analyses indicated particle transformations within and beneath the EZ. 234Th-238U disequilibrium measurements varied, likely reflecting the mixing of water from multiple regions over the ~month-long time-scale of 234Th. Our results highlight the complexity of the biological carbon pump in oligotrophic regions. 

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