GE0-3112 Sedimentary processes and productsLecture 12. Deep seaGeoff CornerDepartment of GeologyUniversity of Troms2006Literature:- Leeder 1999. Ch. 26. Oceanic processes and sediments.

ContentsIntroductionCoupled ocean-atmosphere systemSurface oceanic circulationDeep oceanic circulationContental margin sedimentationSumarine canyonsSubmarine fans---------------------------------------------THESE SUBJECTS WILL BE ADDED LATER:Biological and chemical processesPelagic sedimentsPalaeo-oceanography (palaeoceanography) Anoxic eventsHypersaline oceans

Coupled ocean-atmosphere systemOcean-atmosphere heat engine: redistributes heat (from tropicsto poles).

Heating winds wind shear surface drift and (horizontal) gradient currents.Heating heating/cooling and evaporation/precipitation density differences vertical gradient currents.

Lutgens & Tarbuck 2006

Physical forces and processesExternal forcesWind shear surface currents.Wind shear horizontal gradients Ekman transport.Coriolis deflects moving water masses.Tides weak tidal currents (+ pressure differences?).Internal forcesThermohaline density differences deep currents.Suspended particle density differences turbidity currents.Friction.

Surface oceanic circulationComplex in time and space:Latitudinal zonation due to heat engine.Local and regional differences in evaporation/precipitation, glacial meltwater, etc.Local langmuir circulation (horizontal helical eddies).Periodic storms cause movement and mixing to variable depth.

Equatorial currents (trade winds 0-25)Subtropical gyres (trade winds + westerlies, c. 30).

West wind drift.

Intertropical zone of convergent trade windsArctic and Antarctic convergence (polar front).

Subtropical gyresCoriolis-driven Ekman transport raises water surface c. 1.4 m.Generates oblique gradient (geostrophic) currents.

Surface currentsTypically 3 - 4 distinct warm or cold currents encompassing a gyre (e.g. N.Atlantic, Canary, and N.Equatorial current around the N.Atlantic gyre).Flow is intensified on western borders of oceans;warm western boundary currents up to 10x stronger than cool eastern currents (max. vel. >1.4 m/s = 5 km/h)e.g.:N.Atlantic Gulf StreamS.Atlantic Brazil CurrentPacific Kuro Shio (Black tide)Indian Ocean CurrentStronger currents during glacial epochs on e.g. Blake Plateau.

Upwelling and counter currentsIntertropical convergence zone:upwelling of 1 m/day (due to poleward Ekman transport).E-flowing counter current and deeper W-flowing counter-counter current (

ENSOEl-Nio-Southern Oscillation (ENSO)El-Nio = warm water appearance off Peruvian coastS. oscillation = atmosphere-ocean feedback process 1) Normally: trade-wind-driven circulation in S. Pacific piles up warm water in the W.2) During an ENSO event: trade winds weaken relaxation flow (wave) of warm tropical water from W to E warm water replaces cold off S. American coast changes to ocean currents, upwelling and precipitation in Pacific and beyond.Quasi-periodic (every c. 2-5 years), effects last minimum 2 years, with delayed effects farther afield by up to 10 years.Variable in frequency and intensity; 1982-83 was centurys strongest. The southern oscillation tends to switch between two states:El-Nio warm and dryLa Nia cool and wetLake Tarawera, New Zealand

Deep oceanic circulationGlobal oceanic (thermohaline) circulation system:warm Pacific upper waterwarm North Atlantic Driftcold North Atlantic Deep Water (NADW)Circum-Antarctic Undercurrent/ Antarctic Bottom Water (ABW).Circulation takes c. 500 years.

Thermohaline circulation systemDriven by density differences caused by:surface heating (density decreases)evaporative loss (density increases)surface cooling (density increases)runoff and precipitation (density decreases)sea-ice formation (density increases)

Deep oceanic currentsDischarge c. 50 x 106 m3/s (50x worlds rivers).Velocities:normally ~0.05 m/smaximum 0.25 m/s at W ocean margins (boundary currents) and topographic constrictions.Periodic intensification of near-bottom flow during deep-sea storms, i.e. downward transfer of surface eddy energy.Curved paths following submarine topography (contour currents).

Paths and transport rates (in 106 m3/s) of NADW (1.8-4)

Sediment transport by deep currentsBoundary undercurrents cause:transport and deposition contourites comprise alternating thin v.f.sand, silt and bioturbated mud forming km-thick drift.erosion (winnowing) stratigraphic gaps in deep-sea cores.Contourites (unlike distal turbidites) are well sorted due to winnowing. Deep-sea storms ripple-like forms, tractional and current scour features.Nepheloid layers comprise sediment in transit (see below).

Nepheloid layersNepheloid layers high concentrations of suspended sediment.Form at bottom and intermediate depths.Normally 1-200 m thick (>2 km)Mud (

Suspended sediment concentration (nepheloid layer in Atlantic Deep Water)

Continental margin sedimentationThick terrigenous clastic deposits on contintental slope and rise and inner abyssal plain.Some large deltas at the shelf edge (shelf-edge deltas).Steep slopes (~6; max. 30) disturbed by salt diapirs, growth faults and slumps.Submarine fans at the base of slopes.

Progradational and erosional continental margins

Processes affecting graded slope profile.

Resedimentation processesSlope instability caused or enhanced by:Sea-level variations (lowstand-highstand).Development of gas hydrates.Alternating coarse (sandy) and fine (mud) sediments.Pressure fluctuations caused by earthquakes, tsunamis and internal waves.Storms and tides.Slumps, faults and debris flowsTurbidity currents

Dag Ottesen 2006

Debris flows and debris avalanches off Canary Islands

Submarine canyonsOccur on shelves, slopes and fans.Important conduits for sediment from shelf to deep sea.Originate by some or all of following processes:retrogressive slope failure of slump scarsfluvial erosion during s.l. lowstandserosion by turbidity currents Several 100 m deep and kms wide.V-shaped profile ( slumps).Many headless canyons on slope.Downcanyon/turbidity flows (>1m/s) lasting hours/days,triggered by ocean tides, storms,etc.

Submarine fansLocated on the continental slope; large ones extending to the rise and abyssal plain.Fed by submarine canyons and channels; the largest below deltas.Maximum activity during s.l. lowstands; low activity during present (Holocene) highstand.Sensitive to changes in sea-level and runoff, i.e. sediment supply.

Fan morphologyUpper fancontains main feeder channel, usually with leves.debris flow lobes may occur.Middle fanone main, leve-bound, active channel; several older distributary channels.meandering or braided channels.channels terminate or pass into supra-fan lobes. Lower fansmooth or with many small channels.sometimes ending in well-defined terminal fan lobes.Walker 1992, after Normark 1978

Amazon fan morphology and sediments

Channel meanders and cutoff

Low and high sinuosity channels

Fan structure and stratigraphy

Channel-leve complexes (lowstand).Debris flow deposits.Onlapping and draping hemipelagic sediments (highstand).

Turbidite facies on fansTypically thick (100s m) alternating, parallel sandstones and shales.Base sharp and often containing:tool markssole marksorganic marksSandstone bed commonly graded or 'fining-up'Sandstone bed commonly contains complete or partial 'Bouma sequence'.

Terminal fans/suprafansTerminal lobe complex formed by progradation and avulsion Suprafan lobe of the Delgada fan.

Tana delta slope/ submarine fanCorner, unpublished

Further readingAllen, J.R.L. 1970. Physical processes of sedimentation.Chapter 1 covers the same ground as Leeder and explains clearly the principles involved; good supplementary reading for aquiring a sound grasp of the physics of fluid dynamics and sedimentation. Alternatively consult the more encyclopedic:Allen, J.R.L 1984. Sedimentary structures: their character and physical basis. A more encyclopedic alternative to the above if it is unavailable.