Deep-sea sediment redistribution induced by a meandering Gulf Stream

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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.