Award Abstract # 1949536
Deep-sea sediment redistribution induced by a meandering Gulf Stream

NSF Org: OCE
Division Of Ocean Sciences
Recipient: WOODS HOLE OCEANOGRAPHIC INSTITUTION
Initial Amendment Date: March 24, 2020
Latest Amendment Date: March 24, 2020
Award Number: 1949536
Award Instrument: Standard Grant
Program Manager: Gail Christeson
gchriste@nsf.gov
 (703)292-2952
OCE
 Division Of Ocean Sciences
GEO
 Directorate for Geosciences
Start Date: April 1, 2020
End Date: March 31, 2023 (Estimated)
Total Intended Award Amount: $682,585.00
Total Awarded Amount to Date: $682,585.00
Funds Obligated to Date: FY 2020 = $682,585.00
History of Investigator:
  • Olivier Marchal (Principal Investigator)
    omarchal@whoi.edu
Recipient Sponsored Research Office: Woods Hole Oceanographic Institution
266 WOODS HOLE RD
WOODS HOLE
MA  US  02543-1535
(508)289-3542
Sponsor Congressional District: 09
Primary Place of Performance: Woods Hole Oceanographic Institution
266 Woods Hole Rd
Woods Hole
MA  US  02543-1535
Primary Place of Performance
Congressional District:
09
Unique Entity Identifier (UEI): GFKFBWG2TV98
Parent UEI:
NSF Program(s): PHYSICAL OCEANOGRAPHY,
Marine Geology and Geophysics
Primary Program Source: 01002021DB NSF RESEARCH & RELATED ACTIVIT
Program Reference Code(s): 1610, 1620
Program Element Code(s): 161000, 162000
Award Agency Code: 4900
Fund Agency Code: 4900
Assistance Listing Number(s): 47.050

ABSTRACT

The abyssal region of the ocean, below a water depth of about 1000 m, is the largest portion of the world's oceans. The original perception of this region as a quiet, almost stagnant, layer of water has been overthrown by observations made with instruments that can withstand the high pressures and corrosive conditions that prevail in the deep sea. In particular, deep oceanic basins underlying strong and variable surface currents witness episodes of high near-bottom velocities and sediment resuspension, leading to the formation of particle-rich layers near the seafloor that are called benthic nepheloid layers (BNLs). Nepheloid refers. Nepheloid refers to the Greek word for "cloud" and indeed BNLs are visually cloudy when disturbed. Although these episodes, called ?benthic storms?, have been discovered about 40 years ago, how they form remains a mystery. In this project, a detailed computer model of ocean circulation and sediment transport will be applied to study the plausibility of two mechanisms responsible for benthic storms in the western North Atlantic. One possible mechanism is that the instability of the Gulf Stream, leading to meanders, rings, and eddies generates the benthic storms. The other possible mechanism is that the passage of atmospheric disturbances, such as tropical storms and hurricanes generates them. Both mechanisms have been postulated to produce a downward transfer of energy throughout the water column and to lead to benthic storms, a process that links the atmosphere, the ocean, and the seafloor sediment. Through detailed model simulations and comparison with observations of the sediment left by benthic storms, the project will advance the understanding of the origin of benthic storms and BNLs. Moreover, because such sediments are used by scientists for studying the paleoclimate of the oceans, the work will provide an assessment of the effect of ocean dynamics on this major geologic archive. The work will also help to evaluate the impact of sediment resuspension on the distribution of particles in the oceans, such as those targeted by the ongoing international oceanographic program GEOTRACES. The project will have a broader impact by involving a Post-Doctoral Investigator (PDI) and two Undergraduate Students (USs). All of the results will be shared with the public by creating a web site and by archiving the computer codes developed for this project at an official NSF-funded software registry. The codes will be produced from an open source platform and will be accompanied by a ?how-to? document for broader public accessibility.

The project will apply an eddy-resolving model of ocean circulation and sediment transport to explore the effects of Gulf Stream meanders, rings, and eddies, as well as the effects of atmospheric disturbances, on the movement of fine sediments at abyssal depths in the western North Atlantic. The work plan will be in three steps. (1) An existing model of ocean circulation and sediment transport will be configured to represent two different domains in the western North Atlantic. A relatively small domain centered on the Nova Scotia Rise where benthic storms have been particularly well documented will be used to produce local but detailed (submesoscale) simulations of sediment transport. A larger domain will be used to produce less detailed but basin-scale simulations of sediment transport in the intense mesoscale eddy field that characterizes the western North Atlantic. (2) Numerical experiments will be conducted with the model for varying atmospheric conditions and sediment characteristics. (3) Model results will be compared to physical observations collected from hydrographic compilations, field programs, and satellite altimetry, to distributions of particle concentration derived from gravimetric, chemical, and optical measurements, and to time series of current velocity and water turbidity obtained from bottom-tethered instruments. From these comparisons, the project will assess the potential of various dynamical phenomena ? deep cyclones, topographic Rossby waves, and internal waves ? to redistribute sediments on the seafloor and produce benthic storms.

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.

PROJECT OUTCOMES REPORT

Disclaimer

This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.

Intellectual Merit:

1. Our detailed computer simulations have shown that a current like the Gulf Stream can become unstable, meander, and produce eddies extending over most of the ocean depth (over more than 4000 m). These deep eddies are the oceanic analogues of the cyclones and anticyclones in the atmosphere which characterize the weather at middle latitudes (between about 20 and 60 degrees of latitude). Results from our project indicate that these deep eddies are a possible mechanism for creating "deep-sea storms", which are episodes of exceptionally strong near-bottom currents and sediment resuspension observed in the abyss under vigorous and variable surface currents.

2. Our computer simulations have also shown that these strong near-bottom currents are associated with the development, above the seafloor, of a layer with a thickness of a few tens of meters where temperature is approximately uniform. This "bottom boundary layer" (BBL) is the oceanic counterpart of the "planetary boundary layer" - the relatively thin layer of the atmosphere where we live. In the BBL, ocean currents are retarded by bottom friction, which leads to large vertical variations in current speed, renders the flow turbulent, and mixes seawater properties like temperature. Our results showed that computer simulations of ocean circulation that do not resolve the BBL may suffer from significant biases. They provide strong incentive for improving the representation of the BBL in ocean general circulation models such as those used in climate studies.

3. The deep cyclones which appear in our computer simulations are characterized by bottom shear stresses that are large enough to entrain sediment from the seafloor and by vertical velocities that are large enough to overcome the effect of gravity and lead to a net upward motion of entrained sediment into the water column. These results suggest a new mechanism for the formation and/or maintenance of "benthic nepheloid layers" - layers with a thickness of a few hundred meters to a thousand meter which are rich in resuspended sediment and which have been observed in the vicinity of strong currents like the Gulf Stream.

4. Our analysis of oceanographic data collected in the western North Atlantic between Bermuda and New England, combined with a mathematical model, have shown that resuspension of sediment particles by strong near-bottom currents could explain a number of prominent features observed in the deep-water column in this region. These features include intense benthic nepheloid layers as well as strong anomalies in the concentration of particle-sensitive metals that have found various applications in oceanography. Our results yield new insight into the influences of sediment resuspension and transport on particle-sensitive substances in the deep sea and highlight the need for considering these processes in their multiple applications.

5. Our analysis of recent observations from the deep-water column in the tropical eastern North Pacific and a mathematical model of vertical sediment transport by turbulence and gravity, point to the importance of resolving the bottom boundary layer in accurate computer simulations of the dispersal of resuspended sediment in this area. These results have important implications for the assessment of the environmental consequences of sediment plumes produced by potential deep-sea mining activities in the German license area of the Clarion-Clipperton Zone in the tropical eastern North Pacific.

Broader Impacts:

1.This project has supported graduate student Si-Yuan (Sean) Chen from the MIT-WHOI Joint Program in Oceanography. As part of this project, Sean has been exposed to, and has applied concepts from, various disciplines, including paleoceanography, marine sedimentology, trace metal marine geochemistry, physical oceanography, geophysical fluid dynamics, and ocean numerical modeling. Sean has been advised by the Principal Investigator (PI) of this project and has benefited from various collaborations both within and outside WHOI. External collaborators include scientists from Texas A&M, USGS, NASA GIS, Alfred Wegner Institute (Germany), and MIT. Overall, Sean has presented the results from his research (i) at 7 scientific conferences, both in the US and abroad, (ii) during two seminars (Lamont Doherty Earth
Observatory and Hong Kong University), and (iii) in 3 first-authored manuscripts, two of which are already published in the peer-reviewed literature.

2. This project has also involved undergraduate Benjamin Angell from the Falmouth Academy (MA) during winter-spring 2023. Benjamin's project was directly related to our main project, as it addressed questions related to the time needed to produce and erode benthic nepheloid layers from sedimenrt resuspension. Benjamin has been advised by the PI of this project and has been exposed to concepts from ocean numerical modeling and marine sedimentology. Benjamin will present the results from his research at the Falmouth Academy later during this year.

3.In addition to publications in the peer-reviewed literature and to presentations at conferences and in seminars, different products from this project have been made publicly available via GitHub - an official NSF-funded software registry.


Last Modified: 05/30/2023
Modified by: Olivier Marchal

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