Award: OCE-2023108

The ocean's biological carbon pump plays a vital role in controlling atmospheric CO₂ levels by transferring carbon from the surface to the deep ocean, impacting the climate on timescales ranging from seasons to millennia. Recent research has shifted our understanding of this process from one mainly driven by the sinking of particles to a more complex system involving physical ocean dynamics and the vertical migration of animals. However, estimates of the carbon exported by the ocean pump still vary widelyranging from 5 to 15 gigatonnes (Gt) per yeardue to the complexity of the processes and the wide range of spatial and temporal scales involved. Two key components of this pump occuring at fine spatial scale, or eddy scale, are especially poorly understood: the subduction pump -- the process by which surface water is injected deeper in the ocean -- involves ocean dynamics happening on scales smaller than 100 km and timescales shorter than 10 days; and the migrant pump refers to the carbon exported by migrating organisms like zooplankton, which feed at the surface at night and dive deeper during the day to avoid predators. This project was designed to better understand and quantify these two poorly constrained carbon pumps and their contributions to global ocean carbon sequestration. The project was articulated around three main objectives. First, we sought to identify where fine-scale subduction occurs in the ocean. Previous studies had shown this could be detected in specific ocean regions using autonomous floats, but a global picture was still missing. We expanded on this by developing a method to automatically detect subduction events worldwide using the Biogeochemical Argo float database, which contains over 126,000 depth profiles. Our analysis revealed 4,391 subduction events, mostly concentrated in regions with strong and energetic ocean currents, like the Southern Ocean, Kuroshio Extension, and North Atlantic. Our results suggest that certain ocean instabilitiesparticularly symmetric instabilitiesmay play a larger role than other instabilities in driving these subduction events, providing new insights into an ongoing debate in the community. Second, we focused on the "vertical migrant pump," which involves zooplankton moving up and down in the ocean and contributing to carbon export. We developed a high-resolution ocean model of the North Atlantic to capture the seasonal dynamics of the oceans different regions. The model, which includes zooplankton migration, accurately simulates their movement and biomass. Three key findings emerged. First, the "greenness" of the ocean (the amount of phytoplankton and chlorophyll) affects zooplankton migration depths by more than 100 meters. Second, we found that carbon export due to zooplankton migration and ocean currents is mainly driven by dissolved carbon, not particulate carbon as was previously thought. Lastly, migrating zooplankton have a relatively small impact on the functioning of surface ecosystems, they play a crucial role in feeding and providing energy to deeper ecosystems (100-500 meters below the surface). Third, we developed a unifying framework to quantify the various pathways of the biological carbon pump, including settling, physical transport (like subduction), and zooplankton migration. We find that carbon sequestration by the biological carbon pump is mainly controlled by particle settling (~60-75%), then physical transport (~20-35%) and vertical migration (~5-10%). The physical pump dominates sequestration in winter, the gravitational pump dominates the rest of the year, and the migrant pump is maximal in summer. Importantly, most of the carbon sequestered by the biological carbon pump is located at intermediate depths (200-1000m) and not below 1000 m which is the depth commonly used to estimate carbon sequestration. This project was instrumental for the career of four early career scientists: Jessica Garwood who was a postdoc in the early phases of the project and who is now assistant professor at Oregon State University, Maxime Keutgen de Greef and Mathieu Poupon both graduate students in the Atmospheric and Oceanic Program of Princeton University, and Laure Resplandy, the PI of the project who received this funding during her tenure track at Princeton University. All models and codes developed in this project are made publicly available as the results get published, including the gyre and zooplankton migration models. This project advances our understanding of the ocean's biological carbon pumpvital for regulating the Earths carbon cycle, improving climate projections, and assessing climate impacts. By combining high-resolution models and existing observations, weve constrained fine-scale export pumps tied to zooplankton migration and subduction, and quantified the relative contributions of the different export pumps, which are generally considered in isolation. Ultimately, this project offers new insights into how the ocean carbon pump functions, a critical step to explore how they might respond to climate change. Last Modified: 12/17/2024 Submitted by: LaureResplandy