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Lecture 12. Deep sea. GE0-3112 Sedimentary processes and products. Geoff Corner Department of Geology University of Tromsø 2006. Literature: - Leeder 1999. Ch. 26. Oceanic processes and sediments. Contents. Introduction Coupled ocean-atmosphere system Surface oceanic circulation
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Lecture 12. Deep sea GE0-3112 Sedimentary processes and products Geoff Corner Department of Geology University of Tromsø 2006 Literature: - Leeder 1999. Ch. 26. Oceanic processes and sediments.
Contents • Introduction • Coupled ocean-atmosphere system • Surface oceanic circulation • Deep oceanic circulation • Contental margin sedimentation • Sumarine canyons • Submarine fans --------------------------------------------- • THESE SUBJECTS WILL BE ADDED LATER: • Biological and chemical processes • Pelagic sediments • Palaeo-oceanography (palaeoceanography) • Anoxic events • Hypersaline oceans
Coupled ocean-atmosphere system • Ocean-atmosphere heat engine: redistributes heat (from tropics to poles). • Heating winds wind shear surface drift and (horizontal) gradient currents. • Heating heating/cooling and evaporation/precipitation density differences vertical gradient currents.
Physical forces and processes • External forces • Wind shear surface currents. • Wind shear horizontal gradients Ekman transport. • Coriolis deflects moving water masses. • Tides weak tidal currents (+ pressure differences?). • Internal forces • Thermohaline density differences deep currents. • Suspended particle density differences turbidity currents. • Friction.
Surface oceanic circulation • Complex 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 winds • Arctic and Antarctic convergence (polar front).
Subtropical gyres • Coriolis-driven Ekman transport raises water surface c. 1.4 m. • Generates oblique gradient (geostrophic) currents.
Surface currents • Typically 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 Stream • S.Atlantic Brazil Current • Pacific Kuro Shio (’Black tide’) • Indian Ocean Current • Stronger currents during glacial epochs on e.g. Blake Plateau.
Upwelling and counter currents • Intertropical convergence zone: • upwelling of 1 m/day (due to poleward Ekman transport). • E-flowing counter current and deeper W-flowing counter-counter current (<1 m/s) (also causes upwelling and eddy mixing). • Antarctic (and Arctic convergence): • descent of cold water accompanied by upwelling. • Upwelling where convergent winds cause water flow divergence: • cf. intertropical convergence zone and elsewhere. • Coastal upwelling occurs where flow is away from the coast (Ekman/Coriolis transport to left or right): • Peru • California • NW and SW Africa
ENSO • El-Niño-Southern Oscillation (ENSO) • El-Niño = warm water appearance off Peruvian coast • S. 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 century’s strongest. • The southern oscillation tends to switch between two states: • El-Niño – warm and dry • La Niña cool and wet Lake Tarawera, New Zealand
Deep oceanic circulation • Global oceanic (thermohaline) circulation system: • warm Pacific upper water • warm North Atlantic Drift • cold North Atlantic Deep Water (NADW) • Circum-Antarctic Undercurrent/ Antarctic Bottom Water (ABW). • Circulation takes c. 500 years.
Thermohaline circulation system • Driven 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 currents • Discharge c. 50 x 106 m3/s (50x world’s rivers). • Velocities: • normally ~0.05 m/s • maximum 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’).
Sediment transport by deep currents • Boundary 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 layers • Nepheloid layers – high concentrations of suspended sediment. • Form at bottom and intermediate depths. • Normally 1-200 m thick (>2 km) • Mud (<12 μm: clay-fine silt) • Concentrations: <500-5000. • Produced by: • resuspension by deep-sea ’storms’ • enhanced thermohaline currents • distal turbidites.
Suspended sediment concentration (nepheloid layer in Atlantic Deep Water)
Continental margin sedimentation • Thick 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.
Resedimentation processes • Slope 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 flows • Turbidity currents
Submarine canyons • Occur 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 scars • fluvial erosion during s.l. lowstands • erosion by turbidity currents • Several 100 m deep and km’s 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 fans • Located 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 morphology • Upper fan • contains main feeder channel, usually with levées. • debris flow lobes may occur. • Middle fan • one main, levée-bound, active channel; several older distributary channels. • meandering or braided channels. • channels terminate or pass into ‘supra-fan lobes’. • Lower fan • smooth or with many small channels. • sometimes ending in well-defined terminal fan lobes. Walker 1992, after Normark 1978
Fan structure and stratigraphy • Channel-levée complexes (lowstand). • Debris flow deposits. • Onlapping and draping hemipelagic sediments (highstand).
Turbidite facies on fans • Typically thick (100s m) alternating, parallel sandstones and shales. • Base sharp and often containing: • tool marks • sole marks • organic marks • Sandstone bed commonly graded or 'fining-up' • Sandstone bed commonly contains complete or partial 'Bouma sequence'.
Suprafan lobe of the Delgada fan. • Terminal fans/suprafans Terminal lobe complex formed by progradation and avulsion
Tana delta slope/ submarine fan Corner, unpublished
Further reading • Allen, 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.