Coastal blue carbon ecosystems (seagrass, mangroves, salt marshes) are well-studied carbon sinks, but the open ocean's contribution is vastly underrepresented. The biological carbon pump, where phytoplankton-fixed carbon sinks to the deep ocean via zooplankton fecal pellets and organic matter, is a significant, yet often overlooked, component of this process. This study focuses on Antarctic krill (*Euphausia superba*) as a key player in the Southern Ocean's biological carbon pump. Krill's abundance, rapid fecal pellet sinking rates, and the relatively shallow depth required for long-term sequestration make them a potentially significant carbon sink. The study's importance lies in quantifying krill's carbon sequestration capacity for global comparison with coastal blue carbon systems, bridging the knowledge gap about the open ocean's role in carbon cycling. This methodology can be extended to other pelagic organisms, improving our understanding of their contribution to carbon sequestration. The study addresses the threats to krill populations from climate change (sea ice reduction) and fisheries expansion, highlighting the need for conservation efforts to protect this vital carbon sink.

Previous research has highlighted the importance of the biological carbon pump in sequestering atmospheric CO2. Studies have focused on coastal blue carbon habitats, but the open-ocean contribution remains poorly understood. Prior research on Antarctic krill has shown their fecal pellets dominate Southern Ocean carbon fluxes during the austral growth season. However, these studies often focused on carbon export (flux from surface waters) rather than sequestration (long-term storage). Existing estimates of krill fecal pellet carbon production and export exist, but lack the crucial element of considering sequestration depths and timescales to determine the actual amount of carbon stored. The availability of a circumpolar Antarctic krill density database (KRILLBASE) allowed for the first large-scale assessment of krill's carbon sequestration potential. Previous egestion rate estimates varied considerably, highlighting the need for a more comprehensive and refined approach.

This study used a combined approach to estimate Antarctic krill's circumpolar carbon sequestration flux. First, it utilized the KRILLBASE database to obtain krill density data spanning 90 years (1926-2016). These data were filtered to include only samples from the upper 20m, standardized to January 1st, and projected onto a 2° × 6° grid for spatial analysis. A revised krill fecal pellet egestion rate, derived from multiple independent methods and considering the range of feeding conditions, was determined, resulting in a more conservative estimate compared to previous studies. This rate was then multiplied by the krill density to estimate fecal pellet carbon production flux (FPCprod) at 20m depth. The FPCprod flux was then attenuated to account for pellet degradation with depth due to consumption and bacterial remineralization, using a power-law function ('Martin's b') with a coefficient of -0.3, reflecting the fast-sinking nature of krill pellets. Ocean circulation inverse model (OCIM) data were used to determine the carbon sequestration depth—the depth at which water parcels remain away from the surface for at least 100 years. The FPCprod flux was attenuated to this depth to calculate the krill fecal pellet carbon sequestration flux (FPCflux). The total carbon sequestered was estimated by summing the FPCflux over the study area and time period (October-April). The economic value was calculated using the Social Cost of CO2. Additional analyses were performed to assess the contribution of krill moults and active carbon transport during migrations. To validate the methodology, the model's outputs were compared with existing sediment trap data from South Georgia and Palmer Station. A sensitivity analysis was conducted to evaluate the influence of each parameter (krill density, egestion rate, attenuation rate, sequestration depth) on the total carbon sequestered. An uncertainty analysis using a wider range of parameter values from the literature was also conducted. Finally, an OCIM was used to trace the fate of krill-derived carbon in the global ocean, determining its equilibrium distribution and residence time.

The study's key findings include: 1. Antarctic krill fecal pellets sequester 20 MtC annually during the austral spring and summer. This is comparable to the global totals for salt marshes, mangroves, and seagrass. 2. The mean carbon sequestration depth was 381m, highlighting the relatively shallow depth required for long-term storage. 3. The economic value of this krill-mediated carbon sequestration is estimated between USD$4-46 billion, depending on the Social Cost of CO2. 4. Krill fecal pellets account for approximately 12% of total plankton carbon sequestration in the Southern Ocean. 5. Sensitivity analysis indicated that krill density and egestion rate have the largest influence on total carbon sequestered. Uncertainty analysis highlighted the significant impact of the pellet attenuation rate on the estimate. 6. OCIM analysis showed that most krill-derived carbon remains in the Atlantic sector of the Southern Ocean, with some transported to the Pacific and Indian Oceans, and even reaching the Northern Hemisphere. The average residence time of krill-derived carbon is 219 years. 7. Considering additional carbon sequestration pathways (moults and active transport during migrations), the total krill contribution could reach 66 MtC annually. 8. The study revealed high spatial variability in carbon sequestration, with certain krill hotspots demonstrating particularly efficient carbon export.

The findings address the research question by quantifying the significant contribution of Antarctic krill to ocean carbon sequestration, demonstrating its comparability to well-established coastal blue carbon ecosystems. The magnitude of carbon sequestration by krill highlights its ecological and economic importance. The results are relevant to the field by expanding our understanding of the biological carbon pump, demonstrating that the open ocean plays a vital role in carbon cycling. The vulnerability of krill to climate change and overfishing underscores the need for conservation strategies to protect this crucial carbon sink and the associated ecosystem services. The methodology can be applied to other pelagic organisms to assess their carbon sequestration potential, enriching our knowledge of the ocean's role in climate regulation.

This study demonstrates that Antarctic krill are a major contributor to blue carbon sequestration, comparable to established coastal ecosystems. The substantial economic value of this ecosystem service emphasizes the urgent need for conservation efforts. Future research should focus on refining parameter estimates (especially the pellet attenuation rate), investigating the role of other pelagic organisms in carbon sequestration, and modeling the impact of climate change and fisheries on krill populations and their carbon sequestration capacity. The study’s methodology offers a valuable framework for assessing the carbon sequestration potential of other marine organisms and enhancing our understanding of the ocean's role in the global carbon cycle.

The study's limitations include uncertainties associated with parameter estimates, particularly the krill egestion rate and the attenuation rate of fecal pellets with depth. The spatial resolution of the OCIM might affect the accuracy of sequestration depth estimations. The analysis primarily focused on adult krill, potentially underestimating the contribution of juvenile krill. The economic valuation is dependent on the chosen Social Cost of CO2, which can vary considerably. Future studies using higher-resolution models and improved parameter estimations are needed to address these limitations and refine the findings.