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  • 1
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    The Oceanic Biological Pump: Rapid carbon transfer to depth at Continental Margins during Winter (2017)
    In:  EPIC3Scientific Reports, 7(1), ISSN: 2045-2322
    Details
    Publication Date: 2018-02-23
    Description: The oceanic biological pump is responsible for the important transfer of CO2-C as POC “Particulate Organic Carbon” to the deep sea. It plays a decisive role in the Earth’s carbon cycle and significant effort is spent to quantify its strength. In this study we used synchronized daily time-series data of surface chlorophyll-a concentrations from the NASA’s MODIS satellite in combination with hourly to daily observations from sea surface buoys and from an Internet Operated Vehicle (IOV) on the seafloor within Barkley Canyon (Northeast Pacific) to investigate the importance of winter processes in the export of fresh phytodetritus. The results indicate that phytoplankton pulses during winter can be as important in POC transfer to depth as the pulses associated with spring and summer blooms. Short winter phytoplankton pulses were observed to disappear from surface waters after low-pressure systems affected the area. Pulses of chlorophyll reached the IOV, at 870 m depth on the canyon seafloor, 12–72 hours later. These observed short pulses of biological carbon production regularly observed in the region from December to March have not been considered a significant component of the biological pump when compared with the denser summer productivity blooms.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , peerRev
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  • 2
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    The landward and seaward mechanisms of fine-sediment transport across intertidal flats in the shallow-water region—A numerical investigation (2012)
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Continental Shelf Research 60 Suppl. (2013): S85-S98, doi:10.1016/j.csr.2012.02.003.
    Description: This study investigates transport of fine sediment across idealized intertidal flats with emphasis on resolving processes at the tidal edge, which is defined as the very shallow region of the land-water interface. We first utilize a two-dimensional, vertical numerical model solving the non-hydrostatic Reynolds-averaged Navier-Stokes equations with a k-ε turbulence closure. The numerical model adopts the Volume of Fluid method to simulate the wetting and drying region of the intertidal flat. The model is demonstrated to be able to reproduce the classic theory of tidal-flat hydrodynamics of Friedrichs and Aubrey (1996) and to predict the turbidity at the tidal edge that is similar, qualitatively, to prior field observations. The Regional Ocean Modeling System (ROMS) is also utilized to simulate the same idealized tidal flat to evaluate its applicability in this environment. We demonstrate that when a small critical depth (hcrit =2 cm) in the wetting and drying scheme is adopted, ROMS is able to predict the main features of hydrodynamics and sediment-transport processes similar to that predicted by the RANS-VOF model. When driving the models with a symmetric tidal forcing, both models predict landward transport on the lower and upper flat and seaward transport in the subtidal region. When the very shallow region of the tidal edge is well resolved, both models predict an asymmetry of tidal velocity magnitude between the flood and the ebb that may encourage landward sediment transport on the flat. Further model simulation suggests that the predicted landward transport of sediment on the flat is mainly due to the settling-lag effect while the asymmetry of tidal velocity magnitude may add a lesser but non-negligible amount. When the bed erosion is limited by the availability of soft mud, the predicted transport direction becomes landward in both the subtidal region and on the flat. These results suggest that the tidal flow generally encourages landward transport while significant seaward transport may be caused by other mechanisms. Comparisons with field observations show similarities in the net landward transport on the flat and enhanced stresses and suspended-sediment concentrations near the very shallow region of the tidal edge. The field results also indicate significant transport of sediment occurs through the channels, as a function of three-dimensional processes, which are not incorporated in the present idealized modeling.
    Description: This study is supported by U.S. Office of Naval Research (Littoral Science and Optics program manager Dr. Thomas Drake) as part of the Tidal Flat DRI (N00014-09-1-0134; N00014-11-1-0270). SNC received partial support from Taiwan's National Science Council under grant NSC 100-2119-M-002 -028.
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
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  • 3
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    How tidal processes impact the transfer of sediment from source to sink : Mekong River collaborative studies (2017)
    Oceanography Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © Oceanography Society, 2017. This article is posted here by permission of Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 30, no. 3 (2017): 22–33, doi:10.5670/oceanog.2017.311.
    Description: Significant sediment transformation and trapping occur along the tidal and estuarine reaches of large rivers, complicating sediment source signals transmitted to the coastal ocean. The collaborative Mekong Tropical Delta Study explored the tidally influenced portion of the Mekong River to investigate processes that impact mud- and sand-sized sediment transport and deposition associated with varying fluvial and marine influences. Researchers participating in this 2014–2015 project found that as sand and mud progress down the tidal portion of the river, sands in suspension can settle during reduced or slack flows as river discharge becomes progressively more affected by tides in the seaward direction. Consequently, deposits on the tidal river bed are connected to sand transport in the channel. In contrast, fine mud particles remain in suspension until they reach an interface zone where waters are still fresh, but the downstream saline estuary nonetheless impacts the flows. In this interface zone, as within the estuary, fine particles tend to settle, draping the sand beds with mud and limiting the connection between the bed and suspended sand. In the Mekong system, the interface and estuarine zones migrate along the distributary channels seasonally, resulting in variable trapping dynamics and channel bed texture. Therefore, the signature of fluvial-sediment discharge is altered on its path to the coastal ocean, and the disconnected mud and sand supply functions at the river mouth should result in distinct offshore depositional signatures.
    Description: This research was funded by the US Office of Naval Research (grant numbers: N00014-15-1-2011, N00014- 13-1-0127, N00014-13-1-0781, N00014-14-1-0145).
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 4
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    Sediment dynamics in the lower Mekong River : transition from tidal river to estuary (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. 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 120 (2015): 6363–6383, doi:10.1002/2015JC010754.
    Description: A better understanding of flow and sediment dynamics in the lowermost portions of large-tropical rivers is essential to constraining estimates of worldwide sediment delivery to the ocean. Flow velocity, salinity, and suspended-sediment concentration were measured for 25 h at three cross sections in the tidal Song Hau distributary of the Mekong River, Vietnam. Two campaigns took place during comparatively high-seasonal and low-seasonal discharge, and estuarine conditions varied dramatically between them. The system transitioned from a tidal river with ephemeral presence of a salt wedge during high flow to a partially mixed estuary during low flow. The changing freshwater input, sediment sources, and estuarine characteristics resulted in seaward sediment export during high flow and landward import during low flow. The Dinh An channel of the Song Hau distributary exported sediment to the coast at a rate of about 1 t s−1 during high flow and imported sediment in a spatially varying manner at approximately 0.3 t s−1 during low flow. Scaling these values results in a yearly Mekong sediment discharge estimate about 65% smaller than a generally accepted estimate of 110 Mt yr−1, although the limited temporal and spatial nature of this study implies a relatively high degree of uncertainty for the new estimate. Fluvial advection of sediment was primarily responsible for the high-flow sediment export. Exchange-flow and tidal processes, including local resuspension, were principally responsible for the low-flow import. The resulting bed-sediment grain size was coarser and more variable during high flow and finer during low, and the residual flow patterns support the maintenance of mid-channel islands.
    Description: Office of Naval Research Grant Numbers: N00014-12-1-0181 , N00014-13-1-0127 , N00014-13-1-0781, and National Defense Science and Engineering
    Description: 2016-03-23
    Keywords: Sediment transport ; Tidal river ; Estuarine dynamics ; Mekong delta ; Tropical river ; Sediment discharge
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 5
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    Wave- and tidally-driven flow and sediment flux across a fringing coral reef : southern Molokai, Hawaii (2007)
    Elsevier B.V.
    Details
    Publication Date: 2022-05-26
    Description: This paper is not subject to U.S. copyright. The definitive version was published in Continental Shelf Research 24 (2004): 1397-1419, doi:10.1016/j.csr.2004.02.010.
    Description: The fringing coral reef off the south coast of Molokai, Hawaii is currently being studied as part of a US Geological Survey (USGS) multi-disciplinary project that focuses on geologic and oceanographic processes that affect coral reef systems. For this investigation, four instrument packages were deployed across the fringing coral reef during the summer of 2001 to understand the processes governing fine-grained terrestrial sediment suspension on the shallow reef flat (h=1 m) and its advection across the reef crest and onto the deeper fore reef. The time–series measurements suggest the following conceptual model of water and fine-grained sediment transport across the reef: Relatively cool, clear water flows up onto the reef flat during flooding tides. At high tide, more deep-water wave energy is able to propagate onto the reef flat and larger Trade wind-driven waves can develop on the reef flat, thereby increasing sediment suspension. Trade wind-driven surface currents and wave breaking at the reef crest cause setup of water on the reef flat, further increasing the water depth and enhancing the development of depth-limited waves and sediment suspension. As the tide ebbs, the water and associated suspended sediment on the reef flat drains off the reef flat and is advected offshore and to the west by Trade wind- and tidally- driven currents. Observations on the fore reef show relatively high turbidity throughout the water column during the ebb tide. It therefore appears that high suspended sediment concentrations on the deeper fore reef, where active coral growth is at a maximum, are dynamically linked to processes on the muddy, shallow reef flat.
    Keywords: Coral reefs ; Waves ; Tides ; Suspended sediment ; Sediment flux ; Cross-shore transport
    Repository Name: Woods Hole Open Access Server
    Type: Article
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