Ocean Sediments: Why do we care? Marine

Ocean Sediments: Why do we care?
Marine sediments
record much of
Earth’s history
Marine Sediments: Why we care
• Modern marine
sediments
– One of two ‘complete’
records of ‘recent’ past
– Small changes in
record imply
significant events in
earth history
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Marine Sediments: Why we care
• Ancient & ‘modern’
marine sediments
– Host much of Earth’s
fossil fuels
– Marine sediments are
the source of OIL
– Potential ore deposits
too!
Two environments of deposition:
• Neritic vs. Pelagic
• Size of sediment
(grainsize)
– Neritic - range of sizes
– Pelagic - clay sizes
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Neritic = near coast
• Derived from continents (15 million metric tons/year)
– 25% from Tibetan Plateau (most out Ganges into Bay of Bengal)
– Delivered by river drainages
Drainage of Tibetan Plateau
• 25% of all terrigenous sediment in oceans is from
Tibetan Plateau (most out Ganges into Bay of Bengal)
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Sediment Maturity
In general, size decreases and maturity increases with
distance from source/time of transport.
Sediment size: sorting & energy
• Sorting = degree of uniform grain size
• Determined by sustained Energy
– E.g. wave action vs storm event
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Second ‘Environment’: Pelagic
• Pelagic = settled from water
– Wind blown or skeletal - near-shore deposits
‘swamped’ by neritic deposits
– Fine grained
– Slow accumulation (~0.5 cm/1000 yrs)
– Thick 500 -600 m avg.
Size and transport
• Look at clay size - this is common size of wind
blown dust and pelagic organisms
• See how long it takes to sink! 130 years
• Could travel 5 times around Earth in that time
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Types of Sediment & Sources:
Lithogenous, Biogenous, Hydrogenous, Cosmogenous
Lithogenous = from rock
Pelagic
• Neritic & pelagic
• Near and off-shore (everywhere)
• Sources...
Neritic
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Lithogenous sources: Continental erosion
• Neritic
– Delivered to shelf by rivers
– Distributed along shelf by
currents
– Trubidites transport to the Rise
Lithogenous source: volcanic ash
Pelagic & neritic
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Lithogenous source: airborne dust
Pelagic & neritic
Distribution of Lithogenous Pelagic sediment
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Biogenous Sediment: from organisms
• Neritic, pelagic, and benthic (everywhere)
• Neritic Example: Coral = limestones
More neritic biogenous…atolls of
Maldives
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Limestones (CaCO3)
Produced in warm, shallow seas
Pelagic Biogenous Sediment
• Microscopic
Photosynthetic organisms
– E.g. diatoms and forams
– Require sunlight and
nutrients (N & P)
• Secrete outer shell
– Die and shells accumulate
on seafloor
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pelagic ooze: > 30% biogenous sediment
• Siliceous ooze
– SiO2 shells
– diatomes, radiolaria
• Calcareous ooze
– Calcite (CaCO3) shells
– cocccolithophores,
formaminfera
Preservation of Siliceous Ooze
• Ocean is not saturated with SiO2, so…
• Siliceous sediment dissolves - just not very fast
• Where productivity is high, sileceous ooze can accumulate
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Preservation of Calcareous Ooze
• Solubility of CaCO3 varies with water Temperature
– Shallow warm ocean is saturated with CaCO3 - thus, no dissolution
– Deep cold water is rich in CO2 (acidic) and dissolves CaCO3
• Carbonate Compensation Depth = depth of dissolution, avg. 4500 m
– CCD in Atlantic can be as shallow as 4000 m,
– CCD in Pacific can be as shallow as 3500 m
– Depends on Productivity: where it is high, CCD is deep b/c cannot dissolve all the
CaCO3 (buffered solution).
Preservation of Calcareous Ooze
• Can accumulate Calcareous ooze in shallow region (above CCD)
• Cover it with clay or Silica ooze and preserve it in deeper regions
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Hydrogenous Sediment Precipitates from seawater
• Evaporites (restricted basins)
• Mined for Salt
Hydrogenous sediment: hydrothermal
• Hot mineral laden water precipitates minerals as it
mixes with cold ocean water.
• Ancient deposits mined for copper - Cyprus
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Hydrogenous: Mn Nodules
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•
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Slow growth (pcpt from seawater) 1-200 mm/yr.
Requires that no other sediment or current present to disrupt growth.
Possible economic source of Manganese and CHROMIUM!
Where abundant?...
Mn nodules:
• Abundant in Pacific - remember, trenches trap
turbidite deposits
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Cosmogenous Sediment
Cosmic dust
• Derived from space
• 10% of meteors hit the surface
Meteorites
Now, are layers significant?
• Layers in ocean sediment
caused by…
• Changes in sediment
source & size
• Thus produced by
changes in Climate &
Tectonics
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Seafloor Mining: Ores
• Most mining on
Continental Shelf
– Primarily for Sand,
gravel.
– Shells for lime and
cement
– P for fertilizers
– Minor coal - Japan,
Scottland
– Sulfur - diminishing
– Mn Nodules - Cr
• Impact of seafloor
mining?
This is just a little
science dredge, not a
mining technique
Seafloor mining: Oil and Gas
• Oil & gas = 95% of seafloor mining
• Proportion of US oil >30%
• Deepest wells >2600 mbsl
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Oil and Gas: Where do we find it in oceans?
• On continental margins: e.g. Gulf, Persian G., N. Australia, S.
coast CA, W. Africa
• New areas: Nova Scotia, Indonesia, N Alaska, Caspian
A new ‘find’: Gas Hydrates
• Gas Hydrate = ice with
trapped methane
• Stable solid at high
pressure, low
temperature
– >300 mbsl
– under shelf!
• Methane = natural gas!
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Where is Gas Hydrate?
• ‘imaged’ seismically on most shelves and in arctic tundra
How abundant is gas hydrate?
• Estimated Carbon
abundance >2x all
other fossil fuels
• Possible future
source of fuel???
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Gas Hydrate and Climate Change
• Could Release of
methane into
atmosphere cause
global warming?
• We don’t know
yet.
• But, estimated
methane in GH is
3000x atmosphere
Gas Hydrate and Turbidity Currents:
• ‘melt’ G-H and ‘weaken’ sediment on shelf/slope and cause failure (avalanche)
• Turbidity trigger??
– Many slump scars on the shelf are at the depth of GH stability.
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