The Experts below are selected from a list of 1134 Experts worldwide ranked by ideXlab platform
John A. Goff - One of the best experts on this subject based on the ideXlab platform.
-
Impact of synthetic Abyssal Hill roughness on resolved motions in numerical global ocean tide models
Ocean Modelling, 2017Co-Authors: Patrick G. Timko, John A. Goff, Brian K. Arbic, Angelique Melet, Walter H. F. Smith, Joseph K. Ansong, Alan J. WallcraftAbstract:Abstract Global models of seafloor topography have incomplete and inconsistent resolution at horizontal wavelengths less than about 10–20 km, notably due to their inability to resolve Abyssal Hills in areas unsurveyed by ships (that is, about 90% of the global seafloor). We investigated the impact of this unresolved bottom roughness on global numerical simulations of the HYbrid Coordinate Ocean Model (HYCOM) that are forced exclusively by the M2 and K1 internal tides. Simulations were run with horizontal resolutions of 0.08° and 0.04°, 10 isopycnal layers in the vertical direction, and two versions of bathymetry: one derived from the SRTM30_PLUS global bathymetry model, and one merging SRTM30_PLUS with a synthetic fractal surface simulating the expected roughness of Abyssal Hills in the 2–10 km horizontal wavelength band. Power spectra of the two bathymetry versions diverge at wavenumbers of order 4*10–4 radians/m and higher (wavelengths of order 15 km and lower), with more pronounced differences evident on the 0.04° grid, as the 0.08° grid has a more limited ability to capture bathymetric details at the Abyssal Hill scale. Our simulations show an increase in the amount of kinetic and potential energy in higher vertical modes, especially in the 0.04° simulation, when the synthetic roughness is added. Adding Abyssal Hills to the 0.04° simulation increases the M2 kinetic energy for modes 3 and 4 by 12–18% and the potential energy by 5 – 15%. Adding Abyssal Hills to the 0.08° simulation yields a reduced, though still measurable, impact on simulated baroclinic tidal energies. Baroclinic tidal energy conversion rates increase by up to 16% in regions of high roughness, and by up to 3.4% in the global integral. The 3.4% increase in global conversion rates in the numerical simulations is less than the 10% increase computed from a linear analysis on a 0.008° grid because of the resolution limitations of the numerical simulations. The results obtained in the present study, though limited by the horizontal and vertical resolutions of the simulations, are consistent with those of previous studies indicating that Abyssal Hills on the seafloor transfer energy into higher vertical mode internal tides. The method employed here to add synthetic roughness could easily be replicated in other models, with higher resolution and/or more complex forcing.
-
LONG-TERM GOALS
2016Co-Authors: John A. Goff, Brian K. ArbicAbstract:The small-scale roughness properties of the seafloor are increasingly being recognized as critical parameters in determining important processes in physical oceanography. For instance, in situ observations (e.g., Polzin et al., 1997) find that mixing levels are greatly elevated in regions of rough topography. Gille et al. (2000) demonstrate that mesoscale eddy energy tends to be lower in areas where the bottom is rough (suggesting the possibility that dissipation of eddy energy takes place in such areas), and Egbert and Ray (2003) show that substantial tidal dissipation occurs in such areas. The dissipation is generally thought to arise from the breaking of internal waves generated by flows over the rough seafloor. On the time scales of internal waves, mesoscale eddies and the general circulation can be regarded as steady, while tides are oscillatory. The physics of linear internal wave generation is different for these two classes of motions (e.g., Bell 1975), but for both types of flows the wave generation is strongly dependent on the horizontal and vertical scales inherent in the bottom topography. Using the classical formulation for lee waves (e.g., Cushman-Roisin, 1994, St. Laurent, 1999), one can argue that horizontal wavelengths ranging from ~60 m to 6 km generate internal waves when forced by steady flows. Features typical of Abyssal Hill morphology (e.g., 50 m height over 1 km horizontal scale) will generate a significant vertical internal wave energy flux. High-resolution regiona
-
comment on glacial cycles drive variations in the production of oceanic crust
Science, 2015Co-Authors: John A. GoffAbstract:Crowley et al. (Reports, 13 March 2015, p. 1237) propose that Abyssal Hill topography can be generated by variations in volcanism at mid-ocean ridges modulated by Milankovitch cycle–driven changes in sea level. Published values for Abyssal Hill characteristic widths versus spreading rate do not generally support this hypothesis. I argue that Abyssal Hills are primarily fault-generated rather than volcanically generated features.
-
internal tide generation by Abyssal Hills using analytical theory
Journal of Geophysical Research, 2013Co-Authors: Angelique Melet, Brian K. Arbic, Maxim Nikurashin, Caroline Muller, S. Falahat, Jonas Nycander, Patrick G. Timko, John A. GoffAbstract:[1] Internal tide driven mixing plays a key role in sustaining the deep ocean stratification and meridional overturning circulation. Internal tides can be generated by topographic horizontal scales ranging from hundreds of meters to tens of kilometers. State of the art topographic products barely resolve scales smaller than � 10 km in the deep ocean. On these scales Abyssal Hills dominate ocean floor roughness. The impact of Abyssal Hill roughness on internal-tide generation is evaluated in this study. The conversion of M2 barotropic to baroclinic tidal energy is calculated based on linear wave theory both in real and spectral space using the Shuttle Radar Topography Mission SRTM30_PLUS bathymetric product at 1/120 � resolution with and without the addition of synthetic Abyssal Hill roughness. Internal tide generation by Abyssal Hills integrates to 0.1 TW globally or 0.03 TW when the energy flux is empirically corrected for supercritical slope (i.e., � 10% of the energy flux due to larger topographic scales resolved in standard products in both cases). The Abyssal Hill driven energy conversion is dominated by mid-ocean ridges, where Abyssal Hill roughness is large. Focusing on two regions located over the Mid-Atlantic Ridge and the East Pacific Rise, it is shown that regionally linear theory predicts an increase of the energy flux due to Abyssal Hills of up to 100% or 60% when an empirical correction for supercritical slopes is attempted. Therefore, Abyssal Hills, unresolved in state of the art topographic products, can have a strong impact on internal tide generation, especially over mid-ocean ridges.
-
Internal tide generation by Abyssal Hills using analytical theory
Journal of Geophysical Research. Oceans, 2013Co-Authors: Angelique Melet, Brian K. Arbic, Maxim Nikurashin, Caroline Muller, S. Falahat, Jonas Nycander, Patrick G. Timko, John A. GoffAbstract:Internal tide driven mixing plays a key role in sustaining the deep ocean stratification and meridional overturning circulation. Internal tides can be generated by topographic horizontal scales ranging from hundreds of meters to tens of kilometers. State of the art topographic products barely resolve scales smaller than approximate to 10 km in the deep ocean. On these scales Abyssal Hills dominate ocean floor roughness. The impact of Abyssal Hill roughness on internal-tide generation is evaluated in this study. The conversion of M-2 barotropic to baroclinic tidal energy is calculated based on linear wave theory both in real and spectral space using the Shuttle Radar Topography Mission SRTM30_PLUS bathymetric product at 1/120 degrees resolution with and without the addition of synthetic Abyssal Hill roughness. Internal tide generation by Abyssal Hills integrates to 0.1 TW globally or 0.03 TW when the energy flux is empirically corrected for supercritical slope (i.e., approximate to 10% of the energy flux due to larger topographic scales resolved in standard products in both cases). The Abyssal Hill driven energy conversion is dominated by mid-ocean ridges, where Abyssal Hill roughness is large. Focusing on two regions located over the Mid-Atlantic Ridge and the East Pacific Rise, it is shown that regionally linear theory predicts an increase of the energy flux due to Abyssal Hills of up to 100% or 60% when an empirical correction for supercritical slopes is attempted. Therefore, Abyssal Hills, unresolved in state of the art topographic products, can have a strong impact on internal tide generation, especially over mid-ocean ridges.
Angelique Melet - One of the best experts on this subject based on the ideXlab platform.
-
Impact of synthetic Abyssal Hill roughness on resolved motions in numerical global ocean tide models
Ocean Modelling, 2017Co-Authors: Patrick G. Timko, John A. Goff, Brian K. Arbic, Angelique Melet, Walter H. F. Smith, Joseph K. Ansong, Alan J. WallcraftAbstract:Abstract Global models of seafloor topography have incomplete and inconsistent resolution at horizontal wavelengths less than about 10–20 km, notably due to their inability to resolve Abyssal Hills in areas unsurveyed by ships (that is, about 90% of the global seafloor). We investigated the impact of this unresolved bottom roughness on global numerical simulations of the HYbrid Coordinate Ocean Model (HYCOM) that are forced exclusively by the M2 and K1 internal tides. Simulations were run with horizontal resolutions of 0.08° and 0.04°, 10 isopycnal layers in the vertical direction, and two versions of bathymetry: one derived from the SRTM30_PLUS global bathymetry model, and one merging SRTM30_PLUS with a synthetic fractal surface simulating the expected roughness of Abyssal Hills in the 2–10 km horizontal wavelength band. Power spectra of the two bathymetry versions diverge at wavenumbers of order 4*10–4 radians/m and higher (wavelengths of order 15 km and lower), with more pronounced differences evident on the 0.04° grid, as the 0.08° grid has a more limited ability to capture bathymetric details at the Abyssal Hill scale. Our simulations show an increase in the amount of kinetic and potential energy in higher vertical modes, especially in the 0.04° simulation, when the synthetic roughness is added. Adding Abyssal Hills to the 0.04° simulation increases the M2 kinetic energy for modes 3 and 4 by 12–18% and the potential energy by 5 – 15%. Adding Abyssal Hills to the 0.08° simulation yields a reduced, though still measurable, impact on simulated baroclinic tidal energies. Baroclinic tidal energy conversion rates increase by up to 16% in regions of high roughness, and by up to 3.4% in the global integral. The 3.4% increase in global conversion rates in the numerical simulations is less than the 10% increase computed from a linear analysis on a 0.008° grid because of the resolution limitations of the numerical simulations. The results obtained in the present study, though limited by the horizontal and vertical resolutions of the simulations, are consistent with those of previous studies indicating that Abyssal Hills on the seafloor transfer energy into higher vertical mode internal tides. The method employed here to add synthetic roughness could easily be replicated in other models, with higher resolution and/or more complex forcing.
-
A three‐dimensional map of tidal dissipation over Abyssal Hills
Journal of Geophysical Research: Oceans, 2015Co-Authors: Adrien Lefauve, Caroline Muller, Angelique MeletAbstract:The breaking of internal tides is believed to provide a large part of the power needed to mix the Abyssal ocean and sustain the meridional overturning circulation. Both the fraction of internal tide energy that is dissipated locally and the resulting vertical mixing distribution are crucial for the ocean state, but remain poorly quantified. Here we present a first worldwide estimate of mixing due to internal tides generated at small-scale Abyssal Hills. Our estimate is based on linear wave theory, a nonlinear parameterization for wave breaking and uses quasi-global small-scale Abyssal Hill bathymetry, stratification, and tidal data. We show that a large fraction of Abyssal-Hill generated internal tide energy is locally dissipated over mid-ocean ridges in the Southern Hemisphere. Significant dissipation occurs above ridge crests, and, upon rescaling by the local stratification, follows a monotonic exponential decay with height off the bottom, with a nonuniform decay scale. We however show that a substantial part of the dissipation occurs over the smoother flanks of mid-ocean ridges, and exhibits a middepth maximum due to the interplay of wave amplitude with stratification. We link the three-dimensional map of dissipation to Abyssal Hills characteristics, ocean stratification, and tidal forcing, and discuss its potential implementation in time-evolving parameterizations for global climate models. Current tidal parameterizations only account for waves generated at large-scale satellite-resolved bathymetry. Our results suggest that the presence of small-scale, mostly unresolved Abyssal Hills could significantly enhance the spatial inhomogeneity of tidal mixing, particularly above mid-ocean ridges in the Southern Hemisphere.
-
A three-dimensional map of tidal dissipation over Abyssal Hills
Journal of Geophysical Research. Oceans, 2015Co-Authors: Adrien Lefauve, Caroline Muller, Angelique MeletAbstract:The breaking of internal tides is believed to provide a large part of the power needed to mix the Abyssal ocean and sustain the meridional overturning circulation. Both the fraction of internal tide energy that is dissipated locally and the resulting vertical mixing distribution are crucial for the ocean state, but remain poorly quantified. Here we present a first worldwide estimate of mixing due to internal tides generated at small-scale Abyssal Hills. Our estimate is based on linear wave theory, a nonlinear parameterization for wave breaking and uses quasi-global small-scale Abyssal Hill bathymetry, stratification, and tidal data. We show that a large fraction of Abyssal-Hill generated internal tide energy is locally dissipated over mid-ocean ridges in the Southern Hemisphere. Significant dissipation occurs above ridge crests, and, upon rescaling by the local stratification, follows a monotonic exponential decay with height off the bottom, with a nonuniform decay scale. We however show that a substantial part of the dissipation occurs over the smoother flanks of mid-ocean ridges, and exhibits a middepth maximum due to the interplay of wave amplitude with stratification. We link the three-dimensional map of dissipation to Abyssal Hills characteristics, ocean stratification, and tidal forcing, and discuss its potential implementation in time-evolving parameterizations for global climate models. Current tidal parameterizations only account for waves generated at large-scale satellite-resolved bathymetry. Our results suggest that the presence of small-scale, mostly unresolved Abyssal Hills could significantly enhance the spatial inhomogeneity of tidal mixing, particularly above mid-ocean ridges in the Southern Hemisphere.
-
internal tide generation by Abyssal Hills using analytical theory
Journal of Geophysical Research, 2013Co-Authors: Angelique Melet, Brian K. Arbic, Maxim Nikurashin, Caroline Muller, S. Falahat, Jonas Nycander, Patrick G. Timko, John A. GoffAbstract:[1] Internal tide driven mixing plays a key role in sustaining the deep ocean stratification and meridional overturning circulation. Internal tides can be generated by topographic horizontal scales ranging from hundreds of meters to tens of kilometers. State of the art topographic products barely resolve scales smaller than � 10 km in the deep ocean. On these scales Abyssal Hills dominate ocean floor roughness. The impact of Abyssal Hill roughness on internal-tide generation is evaluated in this study. The conversion of M2 barotropic to baroclinic tidal energy is calculated based on linear wave theory both in real and spectral space using the Shuttle Radar Topography Mission SRTM30_PLUS bathymetric product at 1/120 � resolution with and without the addition of synthetic Abyssal Hill roughness. Internal tide generation by Abyssal Hills integrates to 0.1 TW globally or 0.03 TW when the energy flux is empirically corrected for supercritical slope (i.e., � 10% of the energy flux due to larger topographic scales resolved in standard products in both cases). The Abyssal Hill driven energy conversion is dominated by mid-ocean ridges, where Abyssal Hill roughness is large. Focusing on two regions located over the Mid-Atlantic Ridge and the East Pacific Rise, it is shown that regionally linear theory predicts an increase of the energy flux due to Abyssal Hills of up to 100% or 60% when an empirical correction for supercritical slopes is attempted. Therefore, Abyssal Hills, unresolved in state of the art topographic products, can have a strong impact on internal tide generation, especially over mid-ocean ridges.
-
Abyssal Hill roughness impact on internal tide generation: linear theory
2013Co-Authors: Caroline Muller, Angelique Melet, Maxim NikurashinAbstract:Internal tide driven mixing plays a key role in sustaining the deep ocean stratification and meridional overturning circulation. Internal tides can be generated by topographic horizontal scales ranging from hundreds of meters to tens of kilometers. State of the art topographic products hardly resolve scales smaller than ~10 km in the deep ocean, over which Abyssal Hills are the dominant ocean floor roughness fabric. An evaluation of the impact of Abyssal Hill roughness on internal-tide generation is presented in this study.
Robert A. Pockalny - One of the best experts on this subject based on the ideXlab platform.
-
Bathymetric gradients of lineated Abyssal Hills: Inferring seafloor spreading vectors and a new model for Hills formed at ultra-fast rates
Earth and Planetary Science Letters, 2006Co-Authors: Kelly A. Kriner, Robert A. Pockalny, Roger L. LarsonAbstract:Abstract Abyssal Hill morphology provides a preliminary measure of the direction and rate of seafloor spreading, however, additional information (e.g., magnetic anomaly data or a nearby mid-ocean ridge) is usually required to verify these estimates. Previous attempts to identify a unique spreading rate proxy from Abyssal Hill dimensions (e.g., height, length, width) have largely failed due to the relatively large scatter of data or the non-linear character of spreading rate trends. We present a new, stand-alone method of determining both spreading rate and spreading direction using the distribution of azimuths for slopes facing toward and away from the ridge axis. The spreading rate is determined with the Δ peak height parameter, defined as the difference in the height (maximum frequency) of the two dominant modes observed in the azimuthal histograms. This parameter exhibits a clear, nearly linear spreading rate trend and allows half spreading rates to be estimated to within 10–20 km/Myr. The spreading direction is determined with the Δ peak width parameter, which compares the average width of the two dominant modes in the azimuthal histograms. The wider distribution of slope azimuths is oriented away form the paleo-ridge axis for all spreading rates, and thus spreading direction can be determined. The trends in the peak height and width parameters are used to constrain a new model of Abyssal Hill formation at ultra-fast spreading rates, which require greater off-axis extensional faulting resulting in a few large-throw faults on the outward-facing Hillsides, and many smaller throw faults on the inward-facing Hillsides.
-
Tectonic evolution of the Pacific Phoenix Farallon triple junction in the South Pacific Ocean
Earth and Planetary Science Letters, 2005Co-Authors: Richard F. Viso, Roger L. Larson, Robert A. PockalnyAbstract:Analysis of multibeam and gravity data reveals the tectonic history of the mid-Cretaceous (119–107 Ma) Penrhyn basin in the equatorial south Pacific Ocean. The basin formed during a period of heightened geodynamic activity and cessation of magnetic reversals. Similarities in the geometry of the Tongareva triple junction and the Rodriguez triple junction in the Indian Ocean make this study an interesting comparison between modern and ancient tectonics. Changes in Abyssal Hill trends during the formation of the basin suggest either a change in the location of the Euler pole describing the relative motion between the Pacific and Farallon plates, or a significant period of oblique spreading. Interaction between the local stress field associated with the break-up of the Manihiki plateau and the regional stress field controlling major plate motions complicated the tectonic evolution of the Penrhyn basin. Construction of velocity triangles from Abyssal Hill trends and measurements of the triple junction trace suggests that the triple junction oscillated between ridge–ridge–ridge and ridge–ridge–fault configurations. At least two reorganizations in the geometry of the triple junction occurred within 10 Ma of the initial rifting of the Manihiki plateau. Both changes in triple junction geometry coincide with discontinuities in the triple junction trace and result in right-lateral displacements of the triple junction trace. Changes in the bathymetric expression of the triple junction trace suggest a period of triple junction propagation controlled by rift propagation shortly after the change in Euler pole location.
-
ANOMALOUSLY ROTATED Abyssal HillS ALONG ACTIVE TRANSFORMS : DISTRIBUTED DEFORMATION OF OCEANIC LITHOSPHERE
Geology, 1999Co-Authors: Leslie J. Sonder, Robert A. PockalnyAbstract:Abyssal Hills within 20 km of the Clipperton and Pitman transforms deviate from regional trends by as much as 15°–20°. We use a thin viscous sheet model with power-law rheology to test the hypothesis that these anomalous trends result from distributed deformation of oceanic lithosphere, probably associated with periods of transpression. The variation of Abyssal-Hill trends with distance from the transform matches model predictions, with power-law exponents consistent with lithospheric rheology controlled by a combination of slip on faults at shallow depths and ductile creep in deeper parts. Predicted displacements are less than actual slip along the transforms, suggesting that only part of relative plate motion is taken up by distributed deformation. These results suggest that Abyssal-Hill rotation is sporadic, occurring perhaps when physical conditions (e.g., pore-fluid pressure, hydrothermal alteration, or transpression) are particularly favorable.
-
Abyssal Hill topography as an indicator of episodicity in crustal accretion and deformation
Earth and Planetary Science Letters, 1990Co-Authors: Alberto Malinverno, Robert A. PockalnyAbstract:Abstract Recent studies of seafloor morphology suggest that Abyssal Hills are created at the ridge axis by the interplay of magmatic and tectonic processes; therefore, Abyssal Hills should be ideal indicators of episodicity. In this study, we use Abyssal Hill topography created near the ridge axis and subsequently transported onto the ridge flanks as a time series reflecting the episodicity in magmatism/tectonism at the ridge axis. If the creation of an Abyssal Hill is viewed as a discrete event (i.e., a pulse of magmatism or faulting), we can parameterize episodicity by considering a sequence of events of magnitude M (proportional to the relief or cross-sectional area of a Hill) separated by time intervalsΔt (proportional to the spacing between Hills). This analysis will allow us to decide if Abyssal Hills are periodic or if they are randomly distributed in time as a Poisson process. Ten profiles 200–1000 km long of unaveraged centerbeam Sea Beam, located along flowlines and covering a range of spreading rates (15–60 km/Ma half-rate), were used to measure locations and characteristics of the relief (cross-sectional area and height) of Abyssal Hills. The principal results of the analysis are that: (1) Abyssal Hills are clearly not periodic, and their distribution in time corresponds to a Poisson process with a characteristic frequency λ; the characteristic frequency λ is roughly proportional to the square of the spreading rate, and varies from 12.8 events/Ma at fast rates to 1.4 events/Ma at slow rates; and (2) while the magnitudes M of Abyssal Hills (cross-sectional areas and reliefs) are positively correlated with the time elapsed since the formation of the previous Hill, there is no correlation between M and the amount of time to the next event. These results agree with a scenario in which magma or stress accumulate at a steady rate at the ridge axis, and are episodically released in discrete events of volcanism or faulting whose magnitude is proportional to the time elapsed since the last event.
Rachel M Haymon - One of the best experts on this subject based on the ideXlab platform.
-
A.: Evidence for hydrothermal Archaea within the basaltic flanks of the East Pacific Rise, Environ
2015Co-Authors: Christopher J. Ehrhardt, Rachel M Haymon, Michael G. Lamontagne, Patricia A. HoldenAbstract:Little is known about the fluids or the microbial com-munities present within potentially vast hydrothermal reservoirs contained in still-hot volcanic ocean crust beneath the flanks of the mid-ocean ridge. During Alvin dives in 2002, organic material attached to basalt was collected at low, near-ambient tempera-tures from an Abyssal Hill fault scarp in 0.5 Ma litho-sphere on the western ridge flank of the East Pacific Rise. Mineral analysis by X-ray diffractometry and scanning electron microscopy revealed high-temperature (> 110°C) phases chalcopyrite (Cu5FeS4) and 1C pyrrhotite (Fe1-xS) within the fault scarp materials. A molecular survey of archaeal genes encoding 16S rRNA identified a diverse hyperthermo-philic community, including groups within Crenarcha-eota, Euryarchaeota, and Korarchaeota. We propose that the sulfide, metals and archaeal communities originated within a basalt-hosted subseafloor hydro-thermal habitat beneath the East Pacific Rise ridge flank and were transported to the seafloor during a recent episode of hydrothermal venting from the Abyssal Hill fault. Additionally, inferred metabolisms from the fault scarp community suggest that an eco-logically unique high-temperature archaeal biosphere may thrive beneath the young East Pacific Rise ridge flank and that Abyssal Hill fault scarps may present new opportunities for sampling for this largely unex-plored microbial habitat
-
Evidence for hydrothermal Archaea within the basaltic flanks of the East Pacific Rise
Environmental microbiology, 2007Co-Authors: Christopher J. Ehrhardt, Rachel M Haymon, Michael G. Lamontagne, Patricia A. HoldenAbstract:Little is known about the fluids or the microbial communities present within potentially vast hydrothermal reservoirs contained in still-hot volcanic ocean crust beneath the flanks of the mid-ocean ridge. During Alvin dives in 2002, organic material attached to basalt was collected at low, near-ambient temperatures from an Abyssal Hill fault scarp in 0.5 Ma lithosphere on the western ridge flank of the East Pacific Rise. Mineral analysis by X-ray diffractometry and scanning electron microscopy revealed high-temperature (> 110 degrees C) phases chalcopyrite (Cu(5)FeS(4)) and 1C pyrrhotite (Fe(1-x)S) within the fault scarp materials. A molecular survey of archaeal genes encoding 16S rRNA identified a diverse hyperthermophilic community, including groups within Crenarchaeota, Euryarchaeota, and Korarchaeota. We propose that the sulfide, metals and archaeal communities originated within a basalt-hosted subseafloor hydrothermal habitat beneath the East Pacific Rise ridge flank and were transported to the seafloor during a recent episode of hydrothermal venting from the Abyssal Hill fault. Additionally, inferred metabolisms from the fault scarp community suggest that an ecologically unique high-temperature archaeal biosphere may thrive beneath the young East Pacific Rise ridge flank and that Abyssal Hill fault scarps may present new opportunities for sampling for this largely unexplored microbial habitat.
-
Hydrothermal mineral deposits and fossil biota from a young (0.1 Ma) Abyssal Hill on the flank of the fast spreading East Pacific Rise: Evidence for pulsed hydrothermal flow and tectonic tapping of axial heat and fluids
Geochemistry Geophysics Geosystems, 2006Co-Authors: Sara B Benjamin, Rachel M HaymonAbstract:Heat flow data indicate that most hydrothermal heat loss from ocean lithosphere occurs on the flanks of the mid-ocean ridge, but few ridge flank hydrothermal sites are known. We describe the first nonseamount, Abyssal Hill hydrothermal mineral deposits to be recovered from the fast spreading East Pacific Rise (EPR) flanks. Deposits were sampled at two sites on an Abyssal Hill ∼5 km east of the EPR axis, just north of Clipperton Fracture Zone at 10°20′N, on ∼0.1 Ma lithosphere. “Tevnia Site” is on the axis-facing fault scarp of the Hill, and “Ochre Site” is located ∼950 m farther east near the base of the outward-facing slope. Clusters of fragile, biodegradable Tevnia worm tubes at both sites indicate that hydrothermal fluids carried sufficient H2S to sustain Tevnia worms, and that fluid flow waned too recently to allow time for tube destruction. Presence of microbial mats and other biota also are consistent with recent waning of flow. The deposits are mineralogically zoned, from nontronite-celadonite to hydrous Fe-oxide+opaline silica to Mn-oxide (birnessite and todorokite). This places them into a distinctive class of Fe-Si-Mn hydrothermal deposits found along tectonic cracks and faults in young oceanic crust, and suggests that (1) deposits precipitated along an O2 gradient between ambient seawater and hydrothermal fluid; (2) fluid temperatures were
-
hydrothermal mineral deposits and fossil biota from a young 0 1 ma Abyssal Hill on the flank of the fast spreading east pacific rise evidence for pulsed hydrothermal flow and tectonic tapping of axial heat and fluids
Geochemistry Geophysics Geosystems, 2006Co-Authors: Sara B Benjamin, Rachel M HaymonAbstract:Heat flow data indicate that most hydrothermal heat loss from ocean lithosphere occurs on the flanks of the mid-ocean ridge, but few ridge flank hydrothermal sites are known. We describe the first nonseamount, Abyssal Hill hydrothermal mineral deposits to be recovered from the fast spreading East Pacific Rise (EPR) flanks. Deposits were sampled at two sites on an Abyssal Hill ∼5 km east of the EPR axis, just north of Clipperton Fracture Zone at 10°20′N, on ∼0.1 Ma lithosphere. “Tevnia Site” is on the axis-facing fault scarp of the Hill, and “Ochre Site” is located ∼950 m farther east near the base of the outward-facing slope. Clusters of fragile, biodegradable Tevnia worm tubes at both sites indicate that hydrothermal fluids carried sufficient H2S to sustain Tevnia worms, and that fluid flow waned too recently to allow time for tube destruction. Presence of microbial mats and other biota also are consistent with recent waning of flow. The deposits are mineralogically zoned, from nontronite-celadonite to hydrous Fe-oxide+opaline silica to Mn-oxide (birnessite and todorokite). This places them into a distinctive class of Fe-Si-Mn hydrothermal deposits found along tectonic cracks and faults in young oceanic crust, and suggests that (1) deposits precipitated along an O2 gradient between ambient seawater and hydrothermal fluid; (2) fluid temperatures were <150°C; and (3) undiluted fluids were Mg-depleted, and Fe-, K-, Si- and Mn-enriched. These fluids may derive from high temperature seawater-basalt interaction ± phase separation proximal to the axial melt zone, and lose Cu and Zn before venting due to conductive cooling and/or pH increase. Ochre Site samples are purely hydrothermal; however, Tevnia Site samples incorporate volcanic, sedimentary, and fossil components, and exhibit at least three generations of fracturing and hydrothermal cementation. The Tevnia Site breccias accumulated on the exposed fault scarp, possibly during multiple slip events and hydrothermal pulses as the Abyssal Hill was uplifted. We hypothesize that frequent earthquakes rejuvenate young Abyssal Hill hydrothermal systems episodically over 104–105 years, tapping axial heat and hydrothermal fluids, sustaining biota, and likely helping to cHill the margins of the axial melt zone.
-
Manifestations of hydrothermal discharge from young Abyssal Hills on the fast-spreading East Pacific Rise flank
Geology, 2005Co-Authors: Rachel M Haymon, Sara B Benjamin, Ken C. Macdonald, Christopher J. EhrhardtAbstract:Spectacular black smokers along the mid-ocean-ridge crest represent a small fraction of total hydrothermal heat loss from ocean lithosphere. Previous models of measured heat flow suggest that 40%–50% of oceanic hydrothermal heat and fluid flux is from young seafloor (0.1–5 Ma) on mid-ocean-ridge flanks. Despite evidence that ridge-flank hydrothermal flux affects crustal properties, ocean chemistry, and the deep-sea biosphere, few ridge-flank vent sites have been discovered. We describe the first known seafloor expressions of hydrothermal discharge from tectonically formed Abyssal Hills flanking a fast-spreading ridge. Seafloor manifestations of fluid venting from two young East Pacific Rise Abyssal Hills (0.1 Ma at 10°20′N, 103°33.2′W; 0.5 Ma at 9°27′N, 104°32.3′W) include fault-scarp hydrothermal mineralization and macrofauna; fault-scarp flocculations containing hyperthermophilic microbes; and Hilltop sediment mounds and craters possibly created by fluid expulsion. These visible features can be exploited for hydrothermal exploration of the vast Abyssal Hill terrain flanking the mid-ocean ridge and for access to the subseafloor biosphere. Petrologic evidence suggests that Abyssal Hills undergo repeated episodes of transitory fluid discharge, possibly linked to seismic events, and that fluid exit temperatures can be briefly high enough to transport copper (≥250 °C).
Caroline Muller - One of the best experts on this subject based on the ideXlab platform.
-
A three‐dimensional map of tidal dissipation over Abyssal Hills
Journal of Geophysical Research: Oceans, 2015Co-Authors: Adrien Lefauve, Caroline Muller, Angelique MeletAbstract:The breaking of internal tides is believed to provide a large part of the power needed to mix the Abyssal ocean and sustain the meridional overturning circulation. Both the fraction of internal tide energy that is dissipated locally and the resulting vertical mixing distribution are crucial for the ocean state, but remain poorly quantified. Here we present a first worldwide estimate of mixing due to internal tides generated at small-scale Abyssal Hills. Our estimate is based on linear wave theory, a nonlinear parameterization for wave breaking and uses quasi-global small-scale Abyssal Hill bathymetry, stratification, and tidal data. We show that a large fraction of Abyssal-Hill generated internal tide energy is locally dissipated over mid-ocean ridges in the Southern Hemisphere. Significant dissipation occurs above ridge crests, and, upon rescaling by the local stratification, follows a monotonic exponential decay with height off the bottom, with a nonuniform decay scale. We however show that a substantial part of the dissipation occurs over the smoother flanks of mid-ocean ridges, and exhibits a middepth maximum due to the interplay of wave amplitude with stratification. We link the three-dimensional map of dissipation to Abyssal Hills characteristics, ocean stratification, and tidal forcing, and discuss its potential implementation in time-evolving parameterizations for global climate models. Current tidal parameterizations only account for waves generated at large-scale satellite-resolved bathymetry. Our results suggest that the presence of small-scale, mostly unresolved Abyssal Hills could significantly enhance the spatial inhomogeneity of tidal mixing, particularly above mid-ocean ridges in the Southern Hemisphere.
-
A three-dimensional map of tidal dissipation over Abyssal Hills
Journal of Geophysical Research. Oceans, 2015Co-Authors: Adrien Lefauve, Caroline Muller, Angelique MeletAbstract:The breaking of internal tides is believed to provide a large part of the power needed to mix the Abyssal ocean and sustain the meridional overturning circulation. Both the fraction of internal tide energy that is dissipated locally and the resulting vertical mixing distribution are crucial for the ocean state, but remain poorly quantified. Here we present a first worldwide estimate of mixing due to internal tides generated at small-scale Abyssal Hills. Our estimate is based on linear wave theory, a nonlinear parameterization for wave breaking and uses quasi-global small-scale Abyssal Hill bathymetry, stratification, and tidal data. We show that a large fraction of Abyssal-Hill generated internal tide energy is locally dissipated over mid-ocean ridges in the Southern Hemisphere. Significant dissipation occurs above ridge crests, and, upon rescaling by the local stratification, follows a monotonic exponential decay with height off the bottom, with a nonuniform decay scale. We however show that a substantial part of the dissipation occurs over the smoother flanks of mid-ocean ridges, and exhibits a middepth maximum due to the interplay of wave amplitude with stratification. We link the three-dimensional map of dissipation to Abyssal Hills characteristics, ocean stratification, and tidal forcing, and discuss its potential implementation in time-evolving parameterizations for global climate models. Current tidal parameterizations only account for waves generated at large-scale satellite-resolved bathymetry. Our results suggest that the presence of small-scale, mostly unresolved Abyssal Hills could significantly enhance the spatial inhomogeneity of tidal mixing, particularly above mid-ocean ridges in the Southern Hemisphere.
-
internal tide generation by Abyssal Hills using analytical theory
Journal of Geophysical Research, 2013Co-Authors: Angelique Melet, Brian K. Arbic, Maxim Nikurashin, Caroline Muller, S. Falahat, Jonas Nycander, Patrick G. Timko, John A. GoffAbstract:[1] Internal tide driven mixing plays a key role in sustaining the deep ocean stratification and meridional overturning circulation. Internal tides can be generated by topographic horizontal scales ranging from hundreds of meters to tens of kilometers. State of the art topographic products barely resolve scales smaller than � 10 km in the deep ocean. On these scales Abyssal Hills dominate ocean floor roughness. The impact of Abyssal Hill roughness on internal-tide generation is evaluated in this study. The conversion of M2 barotropic to baroclinic tidal energy is calculated based on linear wave theory both in real and spectral space using the Shuttle Radar Topography Mission SRTM30_PLUS bathymetric product at 1/120 � resolution with and without the addition of synthetic Abyssal Hill roughness. Internal tide generation by Abyssal Hills integrates to 0.1 TW globally or 0.03 TW when the energy flux is empirically corrected for supercritical slope (i.e., � 10% of the energy flux due to larger topographic scales resolved in standard products in both cases). The Abyssal Hill driven energy conversion is dominated by mid-ocean ridges, where Abyssal Hill roughness is large. Focusing on two regions located over the Mid-Atlantic Ridge and the East Pacific Rise, it is shown that regionally linear theory predicts an increase of the energy flux due to Abyssal Hills of up to 100% or 60% when an empirical correction for supercritical slopes is attempted. Therefore, Abyssal Hills, unresolved in state of the art topographic products, can have a strong impact on internal tide generation, especially over mid-ocean ridges.
-
Abyssal Hill roughness impact on internal tide generation: linear theory
2013Co-Authors: Caroline Muller, Angelique Melet, Maxim NikurashinAbstract:Internal tide driven mixing plays a key role in sustaining the deep ocean stratification and meridional overturning circulation. Internal tides can be generated by topographic horizontal scales ranging from hundreds of meters to tens of kilometers. State of the art topographic products hardly resolve scales smaller than ~10 km in the deep ocean, over which Abyssal Hills are the dominant ocean floor roughness fabric. An evaluation of the impact of Abyssal Hill roughness on internal-tide generation is presented in this study.
-
Internal tide generation by Abyssal Hills using analytical theory
Journal of Geophysical Research. Oceans, 2013Co-Authors: Angelique Melet, Brian K. Arbic, Maxim Nikurashin, Caroline Muller, S. Falahat, Jonas Nycander, Patrick G. Timko, John A. GoffAbstract:Internal tide driven mixing plays a key role in sustaining the deep ocean stratification and meridional overturning circulation. Internal tides can be generated by topographic horizontal scales ranging from hundreds of meters to tens of kilometers. State of the art topographic products barely resolve scales smaller than approximate to 10 km in the deep ocean. On these scales Abyssal Hills dominate ocean floor roughness. The impact of Abyssal Hill roughness on internal-tide generation is evaluated in this study. The conversion of M-2 barotropic to baroclinic tidal energy is calculated based on linear wave theory both in real and spectral space using the Shuttle Radar Topography Mission SRTM30_PLUS bathymetric product at 1/120 degrees resolution with and without the addition of synthetic Abyssal Hill roughness. Internal tide generation by Abyssal Hills integrates to 0.1 TW globally or 0.03 TW when the energy flux is empirically corrected for supercritical slope (i.e., approximate to 10% of the energy flux due to larger topographic scales resolved in standard products in both cases). The Abyssal Hill driven energy conversion is dominated by mid-ocean ridges, where Abyssal Hill roughness is large. Focusing on two regions located over the Mid-Atlantic Ridge and the East Pacific Rise, it is shown that regionally linear theory predicts an increase of the energy flux due to Abyssal Hills of up to 100% or 60% when an empirical correction for supercritical slopes is attempted. Therefore, Abyssal Hills, unresolved in state of the art topographic products, can have a strong impact on internal tide generation, especially over mid-ocean ridges.
