Mixing-Driven Mean Flows and Submesoscale Eddies over Mid-Ocean Ridge Flanks and Fracture Zone Canyons

AbstractTo close the abyssal overturning circulation, dense bottom water has to become lighter by mixing with lighter water above. This diapycnal mixing is strongly enhanced over rough topography in abyssal mixing layers, which span the bottom few hundred meters of the water column. In particular, mixing rates are enhanced over mid-ocean ridge systems, which extend for thousands of kilometers in the global ocean and are thought to be key contributors to the required abyssal water mass transformation. To examine how stratification and thus diabatic transformation is maintained in such abyssal mixing layers, this study explores the circulation driven by bottom-intensified mixing over mid-ocean ridge flanks and within ridge-flank canyons. Idealized numerical experiments show that stratification over the ridge flanks is maintained by submesoscale baroclinic eddies and that stratification within ridge-flank canyons is maintained by mixing-driven mean flows. These restratification processes affect how strong a diabatic buoyancy flux into the abyss can be maintained, and they are essential for maintaining the dipole in water mass transformation that has emerged as the hallmark of a diabatic circulation driven by bottom-intensified mixing.

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Restratification of Abyssal Mixing Layers by Submesoscale Baroclinic Eddies

Journal of Physical Oceanography ◽

10.1175/jpo-d-18-0082.1 ◽

2018 ◽

Vol 48 (9) ◽

pp. 1995-2010 ◽

Cited By ~ 11

Author(s):

Jörn Callies

Keyword(s):

Large Scale ◽

Water Mass ◽

Mixing Layer ◽

Mixing Layers ◽

Small Scale ◽

Surface Mixed Layer ◽

Overturning Circulation ◽

Topographic Slope ◽

Abyssal Mixing ◽

Water Mass Transformation

AbstractFor small-scale turbulence to achieve water mass transformation and thus affect the large-scale overturning circulation, it must occur in stratified water. Observations show that abyssal turbulence is strongly enhanced in the bottom few hundred meters in regions with rough topography, and it is thought that these abyssal mixing layers are crucial for closing and shaping the overturning circulation. If it were left unopposed, however, bottom-intensified turbulence would mix away the observed mixing-layer stratification over the course of a few years. It is proposed here that the homogenizing tendency of mixing may be balanced by baroclinic restratification. It is shown that bottom-intensified mixing, if it occurs on a large-scale topographic slope such as a midocean ridge flank, not only erodes stratification but also tilts isopycnals in the bottom few hundred meters. This tilting of isopycnals generates a reservoir of potential energy that can be tapped into by submesoscale baroclinic eddies. The eddies slide dense water under light water and thus restratify the mixing layer, similar to what happens in the surface mixed layer. This restratification is shown to be effective enough to balance the homogenizing tendency of mixing and to maintain the observed mixing-layer stratification. This suggests that submesoscale baroclinic eddies may play a crucial role in providing the stratification mixing can act on, thus allowing sustained water mass transformation. Through their restratification of abyssal mixing layers, submesoscale eddies may therefore directly affect the strength and structure of the abyssal overturning circulation.

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Provenance, Stratigraphic Architecture, and Hydrogeologic Influence of Turbidites on the Mid-Ocean Ridge Flank of Northwestern Cascadia Basin, Pacific Ocean

Journal of Sedimentary Research ◽

10.2110/jsr.2005.012 ◽

2005 ◽

Vol 75 (1) ◽

pp. 149-164 ◽

Cited By ~ 48

Author(s):

M. B. Underwood ◽

K. D. Hoke ◽

A. T. Fisher ◽

E. E. Davis ◽

E. Giambalvo ◽

...

Keyword(s):

Pacific Ocean ◽

Stratigraphic Architecture ◽

Mid Ocean Ridge ◽

Ocean Ridge ◽

Ridge Flank

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Evaluation of an operational ocean model configuration at 1/12° spatial resolution for the Indonesian seas – Part 1: Ocean physics

Geoscientific Model Development Discussions ◽

10.5194/gmdd-8-6611-2015 ◽

2015 ◽

Vol 8 (8) ◽

pp. 6611-6668 ◽

Cited By ~ 5

Author(s):

B. Tranchant ◽

G. Reffray ◽

E. Greiner ◽

D. Nugroho ◽

A. Koch-Larrouy ◽

...

Keyword(s):

Ocean Circulation ◽

Satellite Data ◽

Water Mass ◽

Ocean Model ◽

Internal Tides ◽

Global Ocean ◽

Open Boundary ◽

Indonesian Throughflow ◽

The South ◽

Water Mass Transformation

Abstract. INDO12, a 1/12° regional version of the NEMO physical ocean model covering the whole Indonesian EEZ has been developed and is now running every week in the framework of the INDESO project (Infrastructure Development of Space Oceanography) implemented by the Indonesian Ministry of Marine Affairs and Fisheries. The initial hydrographic conditions as well as open boundary conditions are derived from the operational global ocean forecasting system at 1/4° operated by Mercator Ocean. Atmospheric forcing fields (3 hourly ECMWF analyses) are used to force the regional model. INDO12 is also forced by tidal currents and elevations, and by the inverse barometer effect. The turbulent mixing induced by internal tides is taken into account through a specific parameterization. In this study we evaluate the model skill through comparisons with various datasets including outputs of the parent model, climatologies, in situ temperature and salinity measurements, and satellite data. The simulated and altimeter-derived Eddy Kinetic Energy fields display similar patterns and confirm that tides are a dominant forcing in the area. The volume transport of the Indonesian ThroughFlow is in good agreement with the INSTANT current meter estimates while the transport through Luzon Strait is, on average, westward but probably too weak. Significant water mass transformation occurs along the main routes of the Indonesian Throughflow (ITF) and compares well with observations. Vertical mixing is able to erode the South and North Pacific subtropical waters salinity maximum as seen in TS diagrams. Compared to satellite data, surface salinity and temperature fields display marked biases in the South China Sea. Altogether, INDO12 proves to be able to provide a very realistic simulation of the ocean circulation and water mass transformation through the Indonesian Archipelago. A few weaknesses are also detected. Work is on-going to reduce or eliminate these problems in the second INDO12 version.

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Magnetostratigraphic effects and artifacts of an inverse redox zonation in bottom-up oxygenated East Pacific mid-ocean ridge flank sediments

10.5194/egusphere-egu21-9458 ◽

2021 ◽

Author(s):

Adrian Höfken ◽

Tilo von Dobeneck ◽

Sabine Kasten

Keyword(s):

Pore Water ◽

Planetary Science ◽

Ex Situ ◽

Sediment Geochemistry ◽

Magnetic Minerals ◽

Diagenetic Alteration ◽

Mid Ocean Ridge ◽

Redox Zonation ◽

Ocean Ridge ◽

Ridge Flank

Shipborne ex-situ oxygen measurements in mid-ocean ridge flank sediment cores from the eastern low-latitude North Pacific (Clarion-Clipperton Zone) revealed a downward increase of pore-water oxygen above the sediment-crust interface (Mewes et al., 2016, Kuhn et al., 2017). This inverse redox zonation is caused by an upward diffusion of oxygen (and other solutes) from fluids circulating through the underlying 20 Mio. Year old and still cooling ocean crust. In consequence, these sediments experience a cyclic change in redox-conditions from oxic seafloor conditions at the top through mostly suboxic conditions throughout the sediment column back to oxygen-rich pore water in the last few sediment meters above the rock basement. We studied paleomagnetic records and bulk magnetic properties of three gravity cores from such settings that were collected during RV Sonne expedition SO-240 in 2015 and obtained high-quality magnetostratigraphic records covering the past 3.2 Ma. The generally very good preservation and interpretability of our reversal and RPI records, however, conflicts with a well-defined, but irregular &#8216;ghost event&#8217; of normal polarity within the upper Gilbert reversed C2Ar section. This magnetic polarity and intensity artifact cannot be explained by sediment tectonics, but coincides with the present depth of the lower suboxic-to-oxic redox boundary. Although chemical overprinting could be considered as an obvious explanation of such findings, bulk magnetic analyses (FORCs, thermomagnetics) infer no diagenetic alteration of the magnetic minerals. Over the entire paleomagnetic record, bacterial magnetite appears to be the predominant NRM carrier. We therefore introduce a novel conceptual model of secondary biogenic magnetite formation at crustal depth, hypothesizing that microaerophilic magnetotactic bacteria live and biomineralize not only in the shallow subsurface, but also near the deep oxygen above the sediment-crust interface.&#160;References Mewes, K., Mogoll&#243;n, J.M., Picard, A., R&#252;hlemann, C., Eisenhauer, A., Kuhn, T., Ziebis, W., Kasten, S., 2016. Diffusive transfer of oxygen from seamount basaltic crust into overlying sediments: An example from the Clarion-Clipperton Fracture Zone. Earth and Planetary Science Letters 433, 215-225.Kuhn, T., Versteegh, G.J.M., Villinger, H., Dohrmann, I., Heller, C., Koschinsky, A., Kaul, N., Ritter, S., Wegorzewski, A.V., Kasten, S., 2017. Widespread seawater circulation in 18-22 Ma oceanic crust: Impact on heat flow and sediment geochemistry. Geology 45, 799-802.&#160;&#160;&#160;

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Reply to “Comments on ‘Diathermal Heat Transport in a Global Ocean Model’”

Journal of Physical Oceanography ◽

10.1175/jpo-d-19-0139.1 ◽

2019 ◽

Vol 49 (8) ◽

pp. 2195-2197 ◽

Cited By ~ 2

Author(s):

Ryan M. Holmes ◽

Jan D. Zika ◽

Matthew H. England

Keyword(s):

Water Mass ◽

Control Volume ◽

Internal Heat ◽

Heat Content ◽

Boussinesq Approximation ◽

Ocean Model ◽

Heat Fluxes ◽

Global Ocean ◽

Volume Flux ◽

Water Mass Transformation

AbstractHochet and Tailleux (2019), in a comment on Holmes et al. (2019), argue that under the incompressible Boussinesq approximation the “sum of the volume fluxes through any kind of control volume must integrate to zero at all times.” They hence argue that the expression in Holmes et al. (2019) for the change in the volume of seawater warmer than a given temperature is inaccurate. Here we clarify what is meant by the term “volume flux” as used in Holmes et al. (2019) and also more generally in the water-mass transformation literature. Specifically, a volume flux across a surface can occur either due to fluid moving through a fixed surface, or due to the surface moving through the fluid. Interpreted in this way, we show using several examples that the statement from Hochet and Tailleux (2019) quoted above does not apply to the control volume considered in Holmes et al. (2019). Hochet and Tailleux (2019) then derive a series of expressions for the water-mass transformation or volume flux across an isotherm in the general, compressible case. In the incompressible Boussinesq limit these expressions reduce to a form (similar to that provided in Holmes et al. 2019) that involves the temperature derivative of the diabatic heat fluxes. Due to this derivative, can be difficult to robustly estimate from ocean model output. This emphasizes one of the advantages of the approach of Holmes et al. (2019), namely, does not appear in the internal heat content budget and is not needed to describe the flow of internal heat content into and around the ocean.

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Hydrothermal flow through the Mariana Mounds: Dissolution of amorphous silica and degradation of organic matter on a mid-ocean ridge flank

Geochimica et Cosmochimica Acta ◽

10.1016/0016-7037(94)90024-8 ◽

1994 ◽

Vol 58 (11) ◽

pp. 2461-2475 ◽

Cited By ~ 24

Author(s):

C.Geoffrey Wheat ◽

Russell E. MeDuff

Keyword(s):

Organic Matter ◽

Amorphous Silica ◽

Mid Ocean Ridge ◽

Flow Through ◽

Ocean Ridge ◽

Ridge Flank ◽

Hydrothermal Flow

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Germanium in mid-ocean ridge flank hydrothermal fluids

Geochemistry Geophysics Geosystems ◽

10.1029/2007gc001892 ◽

2008 ◽

Vol 9 (3) ◽

pp. n/a-n/a ◽

Cited By ~ 17

Author(s):

C. Geoffrey Wheat ◽

James McManus

Keyword(s):

Hydrothermal Fluids ◽

Mid Ocean Ridge ◽

Ocean Ridge ◽

Ridge Flank

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Evaluation of an operational ocean model configuration at 1/12° spatial resolution for the Indonesian seas (NEMO2.3/INDO12) – Part 1: Ocean physics

Geoscientific Model Development ◽

10.5194/gmd-9-1037-2016 ◽

2016 ◽

Vol 9 (3) ◽

pp. 1037-1064 ◽

Cited By ~ 19

Author(s):

Benoît Tranchant ◽

Guillaume Reffray ◽

Eric Greiner ◽

Dwiyoga Nugroho ◽

Ariane Koch-Larrouy ◽

...

Keyword(s):

Satellite Data ◽

Water Mass ◽

Ocean Model ◽

Internal Tides ◽

Global Ocean ◽

Open Boundary ◽

Biogeochemical Model ◽

The South ◽

Medium Range Weather Forecast ◽

Water Mass Transformation

Abstract. INDO12 is a 1/12° regional version of the NEMO physical ocean model covering the whole Indonesian EEZ (Exclusive Economic Zone). It has been developed and is now running every week in the framework of the INDESO (Infrastructure Development of Space Oceanography) project implemented by the Indonesian Ministry of Marine Affairs and Fisheries. The initial hydrographic conditions as well as open-boundary conditions are derived from the operational global ocean forecasting system at 1/4° operated by Mercator Océan. Atmospheric forcing fields (3-hourly ECMWF (European Centre for Medium-Range Weather Forecast) analyses) are used to force the regional model. INDO12 is also forced by tidal currents and elevations, and by the inverse barometer effect. The turbulent mixing induced by internal tides is taken into account through a specific parameterisation. In this study we evaluate the model skill through comparisons with various data sets including outputs of the parent model, climatologies, in situ temperature and salinity measurements, and satellite data. The biogeochemical model results assessment is presented in a companion paper (Gutknecht et al., 2015). The simulated and altimeter-derived Eddy Kinetic Energy fields display similar patterns and confirm that tides are a dominant forcing in the area. The volume transport of the Indonesian throughflow (ITF) is in good agreement with the INSTANT estimates while the transport through Luzon Strait is, on average, westward but probably too weak. Compared to satellite data, surface salinity and temperature fields display marked biases in the South China Sea. Significant water mass transformation occurs along the main routes of the ITF and compares well with observations. Vertical mixing is able to modify the South and North Pacific subtropical water-salinity maximum as seen in T–S diagrams. In spite of a few weaknesses, INDO12 proves to be able to provide a very realistic simulation of the ocean circulation and water mass transformation through the Indonesian Archipelago. Work is ongoing to reduce or eliminate the remaining problems in the second INDO12 version.

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