Marine Sedimentation

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Marine Sedimentation
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• Sediment Defined
•  unconsolidated organic and inorganic particles
  that accumulate on the ocean floor
• originate from numerous sources
• weathering and erosion of the continents
• volcanic eruptions
• biological activity
• chemical processes within the oceanic crust and
  seawater
• impacts of extra-terrestrial objects
•  classified by size according to the Wentworth
  scale

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• grain size indicates condition under which
  sediment is deposited
• high energy environments characteristically yield
  sediments larger in size
• small particles (silts, clays) indicate low
  energy environments
• considered well-sorted if most particles appear
  in the same size classification
• poorly sorted sediments comprised of multiple
  sizes
•  sediment maturity is indicated by several
  factors
• decreased silt and clay content
• increased sorting
• increased rounding of grains, as a result of
  weathering and abrasion
• particle transport is controlled by grain size
  and velocity of transporting medium

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4-1
Sediment in the Sea

• Average grain size reflects the energy of the
  depositional environment.
• Hjulstroms Diagram graphs the relationship
  between particle size and energy for erosion,
  transportation and deposition.

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Classification of marine sediments can be based
upon size or origin.
4-1
Sediment in the Sea

• Size classification divides sediment by grain
  size into gravel, sand and clay.
• Mud is a mixture of silt and clay.
• Origin classification divides sediment into five
  categories Terrigenous sediments, Biogenic
  sediments, Authigenic sediments, Volcanogenic
  sediments and Cosmogenic sediments.

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• Terrigenous (or Lithogenous Sediments)
• derived from weathering of rocks at or above sea
  level (e.g., continents, islands)
• two distinct chemical compositions
• ferromagnesian, or iron-magnesium bearing
  minerals
• non-ferromagnesian minerals e.g., quartz,
  feldspar, micas
• largest deposits on continental margins (less
  than 40 reach abyssal plains)
• transported by water, wind, gravity, and ice
• transported as dissolved and suspended loads in
  rivers, waves, longshore currents

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• (LANDSAT images adapted from Geospace Images
  catalog).
• sediment delivered to the open-ocean by wind
  activity as particulate matter (dust)
• primary dust source is deserts in Asia and North
  Africa
• comprise much of the fine-grained deposits in
  remote open-ocean areas (red clays)
• volcanic eruptions contribute ash to the
  atmosphere which settles within the oceans

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• sediment also transported to the open-ocean by
  gravity-driven turbidity currents
• dense 'slurries' of suspended sediment moved as
  turbulent underflows
• typically initiated by storm activity or
  earthquakes 
• first identified during 1929 Grand Banks
  earthquake
• seismic activity triggered turbidity current
  which severed telegraph lines
• initial flow often confined to submarine canyons
  of the continental shelf and slope
• form deep-sea fans where the mouth of the canyon
  opens onto the continental rise

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20 m s-1 near Grand Banks
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• boulder to clay size particles also eroded and
  transported to oceans via glacial ice
• glacier termination in circum-polar oceans
  results in calving and iceberg formation
• as ice (or icebergs) melt, entrained material is
  deposited on the ocean floor
• termed 'ice-rafted' debris

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• Biogenous Sediments
• composed primarily of marine microfossil remains
• shells of one-celled plants and animals, skeletal
  fragments
• median grain size typically less than 0.005 mm
  (i.e., silt or clay size particles)
• characterized as CaCO3 (calcium carbonate) or
  SiO2 (silica) dominated systems
• sediment with biogenic component less than 30
  termed calcareous, siliceous clay
• calcareous or siliceous 'oozes' if biogenic
  component greater than 30 

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• siliceous oozes (primarily diatom oozes) cover
  15 of the ocean floor
• distribution mirrors regions of high productivity
  
• common at high latitudes, and zones of upwelling
• radiolarian oozes more common in equatorial
  regions

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• calcareous oozes (foraminifera, coccolithophores)
  cover 50 of the ocean floor
• distribution controlled largely by dissolution
  processes
• cold, deep waters are undersaturated with respect
  to CaCO3
• deep water is slightly acidic as a result of
  elevated CO2 concentrations
• solubility of CaCO3 also increases in colder
  water and at greater pressures
• CaCO3 therefore readily dissolved at depth 
• level below which no CaCO3 is preserved is the
  'carbonate compensation depth'
• typically occurs at a depth of 3000 to 4000 m

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Microfossils in Paleoclimatology/Paleoceanography
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• DissolutionCalcium carbonate dissolves better in
  colder water, in acidic water, and at higher
  pressures. In the deep ocean, all three of these
  conditions exist. Therefore, the dissolution rate
  of calcium carbonate increases greatly below the
  thermocline. This change in dissolution rate is
  called the lysocline.Below the lysocline, more
  and more calcium carbonate dissolves, until
  eventually, there is none left. The depth below
  which all calcium carbonate is dissolved is
  called the carbonate compensation depth or CCD.

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• Hydrogenous (or Authigenic) Sediments
• produced by chemical processes in seawater
• essentially solid chemical precipitates of
  several common forms
• non-biogenous carbonates
• form in surface waters supersaturated with
  calcium carbonate
• common forms include short aragonite crystals and
  oolites
• phosphorites
• phosphate crusts (containing greater than 30
  P2O5) occurring as nodules
• formed as large quantities of organic phosphorous
  settle to the ocean floor
• unoxidized material is transformed to phosphorite
  deposits
• found on continental shelf and upper slope in
  regions of high productivity

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• manganese nodules
• surficial deposits of manganese, iron, copper,
  cobalt, and nickel
• accumulate only in areas of low sedimentation
  rate (e.g., the Pacific)
• develop extremely slowly (1 to 10 mm/million
  years)

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• The term evaporites is used for all deposits,
  such as salt deposits, mainly chemical sediments
  that are composed of minerals that precipitated
  from saline solutions concentrated by
  evaporation. Evaporite deposits are composed
  dominantly of varying proportions of halite (rock
  salt) (NaCl), anhydrite (CaSo4) and gypsum
  (CaSo4.2H2O). Evaporites may be classified as
  chlorides, sulfates or carbonates on the basis of
  their chemical composition (Tucker, 1991).

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evaporites ('salt' deposits') occur in regions
of enhanced evaporation (e.g., marginal seas)
evaporative process removes water and leaves a
salty brine e.g., Mediterranean 'Salinity
Crisis' between 5 and 6 million years ago
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• Cosmogenous Sediments
• sediments derived from extraterrestrial
  materials 
• includes micrometeorites and tektites
• tektites result from collisions with
  extraterrestrial materials
• fragments of earth's crust melt and spray outward
  from impact crater
• crustal material re-melts as it falls back
  through the atmosphere
• forms 'glassy' tektites

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• Distribution of Marine Sediments
• sediments thickest along continental margins,
  thin at mid-ocean ridges 
• coastlines
• dominated by river-borne and wave reworked
  terrigenous sediments
• shelf and slope characterized by turbidites and
  authigenic carbonate deposits
• glacial deposits and ice-rafted debris common at
  high latitudes
• high input of terrigenous sediments 'dilutes'
  biogenous components
• deep-sea (pelagic) basins
• abyssal clays (wind blown deposits) common
• lower quantities of biogenic material
• distribution of biogenous sediments dependent
  upon three primary factors
• production in surface waters
• dissolution in deep waters
• dilution by other sediments types

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• high productivity in zones of upwelling and
  nutrient-rich high latitude waters
• calcareous oozes more common in warmer or
  shallower water
• siliceous oozes more common in colder or deeper
  water
• terrigenous sedimentation rates range from 1 mm
  to 10's cm/1000 years
• biogenous sedimentation rates typically 1
  cm/1000 years

Nearshore sediments, turbiditesUp to km/my
(kilometers/million years)Hemipelagic deposits
Tens to hundreds of m/myDrift deposits40-400
m/myMid-latitude eolian deposits 3 to 10
m/myIce rafted material 10 m/myCarbonate
oozes Up to 50 m/mySiliceous oozes Up to 10
m/myHydrothermal deposits (off ridge axes)About
0.5 m/myHydrogenous sediments Rarely exceed 0.2
m/myFerromanganese nodules 0.0002 to 0.005 m/my
(0.2 to 5 mm/my)
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Shelf sedimentation is strongly controlled by
tides, waves and currents, but their influence
decreases with depth.
4-2
Sedimentation in the Ocean

• Shoreline turbulence prevents small particles
  from settling and transports them seaward where
  they are deposited in deeper water.
• Particle size decreases seaward for recent
  sediments.
• Past fluctuations of sea level has stranded
  coarse sediment (relict sediment) across the
  shelf including most areas where only fine
  sediments are deposited today.

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Geologic controls of continental shelf
sedimentation must be considered in terms of a
time frame.
4-2
Sedimentation in the Ocean

• For a time frame up to 1000 years, waves,
  currents and tides control sedimentation.
• For a time frame up to 1,000,000 years, sea level
  lowered by glaciation controlled sedimentation
  and caused rivers to deposit their sediments at
  the shelf edge and onto the upper continental
  slope.
• For a time frame up to 100,000,000 years, plate
  tectonics has determined the type of margin that
  developed and controlled sedimentation.

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60 of the worlds shelves are covered with
relict sediments that were formed about 15,000 y
BP under a different energy regime.
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• Gas Methane Hydrates (Clathrates)
• Hydrates store immense amounts of methane, with
  major implications for energy resources and
  climate, but the natural controls on hydrates and
  their impacts on the environment are very poorly
  understood
• The worldwide amounts of carbon bound in gas
  hydrates is conservatively estimated to total
  twice the amount of carbon to be found in all
  known fossil fuels on Earth (USGS).
• Methane bound in hydrates amounts to
  approximately 3,000 times the volume of methane
  in the atmosphere.

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