NSF Award
2421092
Marine algae take up carbon dioxide as they grow in surface ocean waters, building carbon into organic matter that then sinks to the deep ocean as particles. This process shapes global climate by trapping carbon in the deep ocean, and the amount of carbon that is trapped depends on how deep the organic particles can sink before they break down. In ocean regions with very little oxygen, these particles reach much deeper than elsewhere, but the reasons remain unclear. We will visit the ocean’s largest low-oxygen zone, the Eastern Tropical North Pacific, to measure particle abundance, size, and sinking rates, as well as how quickly and where particles are broken down. We will use these measurements with computer models to test two ideas. Low oxygen could either exclude the tiny animals that break up particles as they feed, or it could slow the growth of bacteria that consume organic matter, especially inside very large particles where oxygen runs out completely. Our findings from this work will improve our understanding of changes to the carbon cycle that could occur as the ocean loses oxygen in a warming climate.<br/><br/>The proposed work aims to improve our understanding of the Biological Carbon Pump (BCP) under low oxygen concentrations ([O2]). Sinking particulate organic carbon (POC) is more efficiently transferred to depth in oxygen minimum zones (OMZs) than in well-oxygenated regions, but the [O2] thresholds and the mechanisms of this transfer are poorly understood. This project will combine new observations and models to test three specific hypotheses (H1-H3), each of which predict unique changes in the POC particle size distribution (PSD) in low oxygen waters: (H1) Low water-column [O2] inhibits microbial respiration of POC, preserving all particle sizes but especially small slow-sinking particles that spend the longest time in the low [O2] layer; (H2) Remineralization slows because anoxic and even euxinic microenvironments develop in particle interiors, preferentially preserving the largest particles; (H3) Reduced zooplankton activity and migration depth at low [O2] curtails particle disaggregation, preserving large particles while preventing small particle production. We propose a research cruise in the Eastern Tropical North Pacific (ETNP) OMZ, sampling across the oxic-anoxic transition. Primary datasets will include POC flux profiles from sediment traps and thorium-based reconstructions, POC remineralization rates from in-situ incubation, PSD and zooplankton images from an Underwater Vision Profiler (UVP), and size-fractionated [POC] from large volume filtration. These data will be incorporated into a mechanistic particle flux model that includes sinking, remineralization, zooplankton-mediated disaggregation, and microenvironment formation across hundreds of POC size classes. The model-data synthesis will determine which combination of hypotheses H1-H3 best explains variations in POC flux, remineralization, PSD, and zooplankton abundance across the oxygen gradient.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
