Settling of porous spheres, as a proxy for marine snow, through density stratification Public Deposited

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Last Modified
  • March 21, 2019
Creator
  • Yu, Sungduk
    • Affiliation: College of Arts and Sciences, Department of Marine Sciences
Abstract
  • The settling of marine snow, which is a dominant form of settling particulate organic carbon (POC), is a major pathway for carbon transport from the surface to the deep ocean. Although there have been many studies to estimate the global POC flux, the physical settling behavior of POC at an individual level has not been well investigated. Because marine snow is a hotspot for microbial activity, most POC is remineralized while sinking through the upper water column, limiting the total carbon export to the deep ocean. Thus, an understanding of the competing timescales of physical sinking vs. remineralization can lead to a better understanding of vertical carbon flux. Accordingly, the time scale of delayed settling of porous particles at the stratified region (residence time, τ?r?) is the key variable in this study. Here we present experimental results for the settling of a single and a cloud of porous spheres, as a proxy for marine snow, through water columns with various stratification regimes, e.g. homogeneous, 2-layered, and linearly stratified. In addition, the experimental results were compared with the results from numerical models formulated both for a single and a cloud of spheres. We found that the settling of porous spheres can be characterized by two regimes depending on their sizes -- when sphere sizes are small, their settling behavior at a density interface is governed by their settling rate (settling regime), and when sphere sizes are large, their settling behavior at a density interface is governed by molecular diffusion (diffusion regime). In the settling regime, τ?r? decreases with sphere size, while in the diffusion regime, τ?r? increases with sphere size. The numerical models could predict the overall tendency of τ?r? over the sphere sizes (e.g. the settling and diffusion regimes), but the τ?r? from the numerical models were underestimated compared to the laboratory experimental results. However, the modified numerical model, which included the entrained fluid shell around a sphere, was able to return τ?r? similar to the laboratory experimental results. Considering that the thin layers in the ocean are usually observed near density discontinuities, the prolonged retention of porous spheres within density stratification we observed could be a possible mechanism of thin layer formation.
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  • In Copyright
Advisor
  • White, Brian
Degree
  • Master of Science
Degree granting institution
  • University of North Carolina at Chapel Hill
Graduation year
  • 2013
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