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  • 1
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 118 (2013): 5322–5332, doi:10.1002/jgrc.20379.
    Description: By analyzing global data, we find that over large scales, surfaces of constant nitrate are often better aligned with isopycnals than with isobars, particularly below the euphotic zone. This is unexplained by the movement of isopycnal surfaces in response to eddies and internal waves, and is perhaps surprising given that the biological processes that alter nitrate distributions are largely depth dependent. We provide a theoretical framework for understanding the orientation of isonitrate surfaces in relation to isopycnals. In our model, the nitrate distribution results from the balance between depth-dependent biological processes (nitrate uptake and remineralization), and the along-isopycnal homogenization of properties by eddy fluxes (parameterized by eddy diffusivity). Where the along-isopycnal eddy diffusivity is relatively large, nitrate surfaces are better aligned with isopycnals than isobars. We test our theory by estimating the strength of the eddy diffusivity and biological export production from global satellite data sets and comparing their contributions. Indeed, we find that below the euphotic zone, the mean isonitrate surfaces are oriented along isopycnals where the isopycnal eddy diffusivity is large, and deviate where the biological export of organic matter is relatively strong. Comparison of nitrate data from profiling floats in different regions corroborates the hypothesis by showing variations in the nitrate-density relationship from one part of the ocean to another.
    Description: We acknowledge the support of the National Science Foundation (Grant OCE-0928617) and NASA (Grant NNX- 08AL80G).
    Description: 2014-04-15
    Keywords: Nitrate ; Export ; Mixing ; Isopycnal ; Alignment ; Large-scale
    Repository Name: Woods Hole Open Access Server
    Type: Article
    Format: application/pdf
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  • 2
    Publication Date: 2022-05-26
    Description: © The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Estapa, M., Valdes, J., Tradd, K., Sugar, J., Omand, M., & Buesseler, K. The neutrally buoyant sediment trap: two decades of progress. Journal of Atmospheric and Oceanic Technology, 37(6), (2020): 957-973, https://doi.org/10.1175/JTECH-D-19-0118.1.
    Description: The biological carbon flux from the ocean’s surface into its interior has traditionally been sampled by sediment traps, which physically intercept sinking particulate matter. However, the manner in which a sediment trap interacts with the flow field around it can introduce hydrodynamic biases, motivating the development of neutral, self-ballasting trap designs. Here, the performance of one of these designs, the neutrally buoyant sediment trap (NBST), is described and evaluated. The NBST has been successfully used in a number of scientific studies since a prototype was last described in the literature two decades ago, with extensive modifications in subsequent years. Originated at Woods Hole Oceanographic Institution, the NBST is built around a profiling float and carries cylindrical collection tubes, a feature that distinguishes it from other neutral traps described in the literature. This paper documents changes to the device that have been implemented over the last two decades, including wider trap tubes; Iridium Communications, Inc., satellite communications; and the addition of polyacrylamide gel collectors and optical sedimentation sensors. Information is also provided with the intent of aiding the development of similar devices by other researchers, including the present adaptation of the concept to utilize commercially available profiling float hardware. The performance of NBSTs built around commercial profiling floats is comparable to NBSTs built around customized floats, albeit with some additional operational considerations. Data from recent field studies comparing NBSTs and traditional, surface-tethered sediment traps are used to illustrate the performance of the instrument design. Potential improvements to the design that remain to be incorporated through future work are also outlined.
    Description: Funding supporting this work has come from multiple sources over the years: the NSF Chemical Oceanography and Carbon and Water programs (most recently OCE-1660012 and OCE-1659995), the NASA Ocean Biology and Biogeochemistry and New Investigator programs (80NSSC17K0662 and NNX14AM01G), and the Woods Hole Oceanographic Institution Technology Award.
    Keywords: In situ oceanic observations ; Instrumentation/sensors ; Measurements ; Profilers, oceanic ; Sampling
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 3
    Publication Date: 2023-02-17
    Description: Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 35(3), (2021): e2020GB006764, https://doi.org/10.1029/2020GB006764
    Description: Export of Particulate Organic Carbon (POC) is mainly driven by gravitational sinking. Thus, traditionally, it is thought that larger, faster-sinking particles make up most of the POC export flux. However, this need not be the case for particles whose sinking speeds are comparable to the vertical velocities of a dynamic flow field that can influence the descent rate of particles. Particles with different settling speeds are released in two process-oriented model simulations of an upper ocean eddying flow in the Northeast Pacific to evaluate the impact of (1) ocean dynamics on the respective contribution of the different sinking-velocity classes to POC export, and (2) the particle number size-spectrum slope. The analysis reveals that the leading export mechanism changes from gravitationally driven to advectively driven as submesoscale dynamics become more active in the region. The vertical velocity associated with submesoscale dynamics enhances the contribution of slower-sinking particles to POC export flux by a factor ranging from 3 to 10, especially where the relative abundance of small particles is large (i.e., steep particle size-spectrum slope). Remineralization generally decreases the total amount of biomass exported, but its impact is weaker in dynamical regimes where submesoscale dynamics are present and export is advectively driven. In an advectively driven export regime, remineralization processes counter-intuitively enhance the role of slower-sinking particles to the point where these slower-sinking velocity classes dominate the export, therefore challenging the traditional paradigm for POC export. This study demonstrates that slow-sinking particles can be a significant contribution, and at times, even dominate the export flux.
    Description: The work was funded by NASA grant NNX16AR48 G, to complement the EXport Processes in the global Ocean from RemoTe Sensing (EXPORTS) program.
    Description: 2021-08-17
    Keywords: Export ; Flux ; Particulate organic carbon ; Sinking rates ; Submeso-scales ; Vertical velocities
    Repository Name: Woods Hole Open Access Server
    Type: Article
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