The importance of the permanent thermocline to the cold-water coral ...

The importance of the permanent thermocline tothe cold-water coral carbonate mounddistribution in the NE AtlanticMartin White 1 and Boris Dorschel 21Earth & Ocean Sciences, School of Natural Sciences, NUI, Galway2Department of Geology / Environmental Research Institute, UCC

Content:‣ Review of depth distribution of carbonate mounds‣ Define permanent thermocline and associatedhydrography‣ Highlight importance of baroclinic motions associatedwith thermoclineFrom: van Weering et al. (2003)

Carbonate mound on the Irish seabed• 1013 individual mounds• ~7 distinct mound provinces• 10s - 100s meter high• up to 5km base diameter• Mound provinces occur on theupper continental slopes

Depth distribution of carbonate mounds• Peak in mound abundancebetween 640-650 m• 94 % of mounds occurshallower than 1000 m bsl• 36 % between 600-700 m• skewed distribution with atail at greater depthsNumber of mound features detected withinINSS dataset:A) in 10 m depth intervals andB) in 100 m intervals as a % of total

Location of (left) Mounds and (right) long term observations of currents (.)and (x) example water column CTD profile

The Permanent thermoclineBase of the winter mixed layer: deepest extent of direct atmosphereRelatively sharp change in Temp hence Density between 600-1000 mWMLSeasonalthermoclinePermanentthermoclineFebOctDensityN = buoyancy freq.- Vertical density grad

Near seabed current strength• Mean vector speed (residual flow)at 8 - 50 mab• (8 m is assumed to be outside of mainfrictional bottom layer)• All depths have a typical range 1-5 cms -1• Between 600-1000 m – residuals up to20 cm s -1• Highest residual flow speed oversteepest part of continental slopeResidual current speed (mean flow) vsdepth for ~40 locations in NE AtlanticO = individual obs; 100m mean

Internal or baroclinic motionsMIXEDWEAKSTRONGWEAKInternal oscillations of the density layers - can take two forms• Freely propagating internal waves• Longer period waves trapped to topographic features, such as thecontinental slope. Long period – means greater than the inertial period(related to Coriolis frequency)Propagation characteristics depend on wave period, Coriolis force and N

K1 tide –amplitudeTrapped baroclinic waves: - diurnal tides• Diurnal tidal periods (24-25 hrs) longer thanthe local inertial period (2π/f) ~ 15 hrs• Tidal motion across margin may be rectified,amplified and trapped to the slope region• Amplification possibility is controlled by valueof N and bottom slope () and wave period.• Possible resonance at Nsin() = maximum• Resonance requires a matching of wave periodand natural period of water column• Nsin() is also the minimum wave period forresonance top occur

Trapped Wave conditions at different carbonate mound locations25hrsNW PorcRockallHovlandNW PorcRockallHovlandMaximum ~ 850 m for alllocations except HovlandprovinceMinimum period < forcing period(24-25hrs) between 600-1200 m(except at Hovland province).Therefore resonance may occur

Diurnal Tides are amplified at Belgica and Logachev (S Rockall Bank) moundsCurrents at Belgica moundsStrong tidal currents(> 50 cm/s) withdiurnal signal andspring-neap cycleDaily averagedcurrents show largealong slope residualswhich vary instrength on S-Ncycle‘rectification’ of thetideWhite et al, GML, 2007Diurnal amplification doesnot occur at Porcupine BankmoundsMis-match of N and profiles?Internal waves are importantat Porcupine Bank however.

Freely propagating internal wavesCgCpCpCgEnergy propagates at an angle depending on period and NGenerated or resonance where Enhanced energy where waves hitseabed where

Internal waves and N Porcupine BankResonance0 mSuspendedsediment conc.Energyenhanced500 mResonance1000 mRelationship between and • Resonance about 600 m. Evidencefor internal waves at that depth hasbeen observed• Energy enhancement 600-1200m1500 m2000 mStrongsuspendedsedimentsignal between500-600 m

Summary• Carbonate mounds found between 500-1000 m water depth• Depth range spans the upper end of the permanent thermocline• The thermocline (with steep margin slope) acts to focus wave energy &enhance periodic and residual current strengthResidual currentsShelf EdgeProductionxxEnhanced long-term, alongslopesediment fluxesActs to draw water intobottom layerWave motionsPermanentthermoclinexIncreased currents andlarge cross-slope movement= large periodic fluxes oforganic matter