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The physical environment of cobalt-rich ferromanganese crusts deposits, the potential impact of exploration and mining on this environment, and data required to establish environmental baselines (HYDRODYNAMIC PROCESSES AT SEAMOUNTS) Aike Beckmann Division of Geophysics University of Helsinki

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OUTLINE 1 Introduction seamounts in the world ocean, hydrodynamics of flow past a cylinder 2 Processes and Phenomena - seamounts in (quasi-) steady flows - seamounts in alternating (tidal) flows - seamount induced turbulence 3 Consequences for material transport - closed circulation cells, retention, sediment patterns 4 Parameter dependencies - geometry, rotation, stratification, forcing 5 Monitoring requirements - observational and modeling (main examples: Fieberling Guyot and Great Meteor Seamount)

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INTRODUCTION seamounts in the world ocean: > in the Pacific alone Foundation Seamount Chain

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HYDRODYNAMICS OF FLOW PAST A CYLINDER in a “technical” settingin a geophysical setting (without rotation) (with rotation) closed circulation cell

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THE GEOPHYSICAL PROBLEM rotation subsurface summit steep (>30%) but finite slopes stratification variable currents

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PROCESSES AND PHENOMENA seamounts in (quasi-) steady flows: Taylor columns, generation of meanders, eddy shedding seamounts in alternating (tidal) flows: seamount trapped waves, resonant amplification, rectified flows, radiating waves seamount induced turbulence: internal waves, internal tides, vertical mixing consequences for material transport: closed circulation cells, retention, sediment patterns seamount chains, seamount clusters

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SEAMOUNTS IN (QUASI-) STEADY FLOWS “SPIN-UP” (numerical simulation) (a)trapped wave (b) vortex generation (c) eddy shedding

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SEAMOUNTS IN ALTERNATING (TIDAL) FLOWS seamount trapped waves, resonant amplification rectified flows, radiating waves horizontal currents easily reach 0.5m/s, vertical currents 1000 m/day

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OVERVIEW: SEAMOUNT FLOW REGIMES “butterfly patterns” closed circulations seamount trapped waves rectified flows overturning motion

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SEAMOUNT-INDUCED TURBULENCE internal waves, internal tides, vertical mixing fluctuations diffusivity Ampere Seamount

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SEAMOUNT-INDUCED TURBULENCE vertical mixing turbulent dissipation in a relatively thin bottom boundary layer diffusivities averaged over the summit of Ampere Seamount are as high as (1-2) m 2 /s the averaged mixing rate in the region surrounding Ampere is times the far field value significant contribution to global vertical mixing

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CONSEQUENCES FOR MATERIAL TRANSPORT closed circulation cells, retention, strong currents, upwelling, mixing chlorophyll a at Great Meteor Bank (M. Kaufmann, pers. comm.) observed biological fields

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METEOR SEAMOUNT TRACER STUDIES (I) passive tracer at Great Meteor Bank (combined tidal mean flow forcing) numerical simulation of a summit source (Mohn and Beckmann, 2002) after 50 days: substantial retention despite strong currents, especially below the surface mixed layer

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METEOR SEAMOUNT TRACER STUDIES (II) passive tracer at Great Meteor Bank (combined tidal mean flow forcing) numerical simulation of a surface source (Mohn and Beckmann, 2002) after 50 days: substantial upwelling above the seamount, mixed layer thickness (air-sea interaction) change

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METEOR SEAMOUNT PARTICLE STUDIES particles at Great Meteor Bank (combined tidal mean flow forcing) numerical simulation of a surface layer trajectories (Beckmann and Mohn, 2002) 50 days tracks: indication of retention for passive, less for active particles

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CONSEQUENCES FOR SEDIMENT TRANSPORT characteristic deep sea sedimentation patterns seamount summit areas are often sediment depleted (due to strong currents) high sediment loads may lead to downslope turbidity currents

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SEAMOUNT CHAINS AND CLUSTERS interaction between neighboring seamounts is small, as long as distance is larger than diameter (exception: circulation near the seamount base) not investigated in detail:

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PARAMETER DEPENDENCIES the ingredients are the same, their relative contribution depends on: SEAMOUNT geometry o height o diameter o steepness o base shape o degree of symmetry o smoothness o … AMBIENTE rotation and stratification forcing

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RESONANCE AS A FUNCTION OF SEAMOUNT GEOMETRY resonance phenomena: seamount trapped waves rectified (time-mean) flow given a particular environmental setting, resonances occurs for certain seamount geometries upper curves: wave amplitude (current strength); lower curves: time-mean flow (retention potential)

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RESONANCE AS A FUNCTION OF STRATIFICATION/ROTATION Burger number S = NH/fL N stability frequency f Coriolis parameter H water depth L seamount diameter forcing frequency

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OBSERVATIONAL/MONITORING REQUIREMENTS observational array during the TOPO project (1991) current meter moorings: summit plain, upper flanks, far-field reference large number of XBTs on radial transetcs duration: several months (background fields) plus 1-2 weeks intense survey

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FIEBERLING GUYOT NUMERICAL MODEL STUDY high resolution ocean circulation model (Beckmann and Haidvogel, 1997) (dominating tidal forcing) model domain topography, mooring locations

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FIEBERLING GUYOT MODELING RESULTS excellent quantitative agreement time-mean flow week-long velocity time-series

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SUMMARY AND CONCLUSIONS the physical environment at seamounts is quite different from the ambient ocean there is a strong dominance of a number of physical processes and phenomena strong horizontal flows due to large amplitude seamount trapped waves, substantial up- and downwelling over the upper flanks, radial density gradients, and corresponding closed circulation cells, and a significantly enhanced level of turbulent vertical mixing. some consequences for marine ecosystems are known (nutrient supply, retention, endemisms), though not fully understood in all cases determination of the regime and subsequent monitoring requires limited time observational arrays accompanied by numerical simulations the relative contribution of each of the above processes depends strongly on seamount geometry (height, diameter, steepness of slopes), geographical latitude, stratification of the water column, and ambient ocean currents; the physical regime for each seamount has to be determined separately from an array of density and current measurements at various locations both on top of the seamount. These and additional observations from the surrounding deep ocean can then be used in numerical experiments to obtain a more complete view of the physical environment, its sensitivity and consequences

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