reading material
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Reading Material. See class website “Sediments”, from “Oceanography” M.G. Gross, Prentice-Hall. Distribution of Marine Sediments. Lithogenic sediment dominates near continents (shelf, slope, rise) because source from land glacial at high latitudes, fluvial at all latitudes - PowerPoint PPT PresentationTRANSCRIPT
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Reading Material
See class website
“Sediments”, from “Oceanography”M.G. Gross, Prentice-Hall
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Distribution of Marine Sediments
Lithogenic sedimentdominates near continents (shelf, slope, rise)because source from landglacial at high latitudes, fluvial at all latitudes
Biogenic sedimentdominates away from lithogenic sediments, usually away from
continents(exception: calcareous sediment can dominate shallow low-latitude
areas)
calcareous sediment (foraminifera) found on flanks of mid-ocean ridges
because it dissolves in water >4000 m deep
siliceous sediment found where nutrient supply is greatnutrients stimulate marine productivity (diatoms, radiolarians)
Authigenic sediment and red claydominates away from continents, in water depths >4000 m, not
high prodbecause they are overwhelmed everywhere else by lithogenic and
biogenic
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Deep-sea sediments
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Sea-Level Change
Time scales of ~10,000 years
Sea level fluctuates due to climate change
Cold periodsmore precipitation as snow (not rain)more snow remains for multiple years, ice sheets form miles thickevaporation continues from oceans, but return as runoff reducedcold temperatures cause sea water to contractsea level drops
Warm periodsless precipitation as snowglaciers meltwarm temperatures cause sea water to expandsea level rises
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Sea-Level Change Past 40,000 y
Holocene = past 20,000 y, when sea level was rising
Transgression = transfer of shoreline landward
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Sea-Level Rise Past 10,000 y
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Recent Sea-Level Rise
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Example of step-wise sea-level rise
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Flooded river valley on the continental shelf – in the Gulf of Papua (between Australia and New Guinea)
This valley might have been flooded quickly by step-wise sea-level rise
This is a bathymetric chart, cool colors are deep, warm colors are shallow
~35 m deep
~65 m deep
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Holocene Rise in Sea Level
Cold period (ice age) ends ~20,000 years ago
Sea level stood ~130 m below present sea levelat edge of continental shelf (shelf break)
Global sea level rose quickly (~10 mm/y) until ~7000 years ago
Rate of global rise has been slow (~2 mm/y) since then
Sea-level change along any particular coast depends also upon land movement
plate tectonicssediment consolidation (e.g., deltas sink)glacial rebound (weight of glaciers removed, land rises)
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Continental-Margin Sedimentation during Low Sea Level
Rivers and glaciers cross continental shelf to shelf break
Much sediment supplied at top of steep slopecreates unstable sediment
Large storms or earthquakes trigger underwater landslides
Slurry of sediment moves down continental slopeknown as “turbidity currents” and “debris flows”
Erodes seabed on continental slopeforms submarine canyons
Deposits sediment on continental rise and abyssal plainscreates layers known as “turbidites”
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Turbidity Current and resulting
Turbidite
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1929 Grand Banks turbidity current
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Continental-Margin Sedimentation during High
Sea Level
Fluvial and glacial valleys flooded
Sediments trapped in river-mouth estuaries and fjords
If much sediment supplied, estuaries and fjords are filled
deltas formed
Sediment can escape to continental shelfmud winnowed by wavesleaving sand nearshoremud transported to middle shelf
On collision margins (narrow, steep shelf)sediment can escape to continental slope
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Holocene deposits (<20,000 y) on continental shelves
Note: boundary between modern inner-shelf sand and modern mid-shelf mud depends on waves
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Washington continental shelf
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Continental-Margin Sedimentation during High
Sea Level
Fluvial and glacial valleys flooded
Sediments trapped in river-mouth estuaries and fjords
If much sediment supplied, estuaries and fjords are filled
deltas formed
Sediment can escape to continental shelfmud winnowed by wavesleaving sand nearshoremud transported to middle shelf
On collision margins (narrow, steep shelf)sediment can escape to continental slope
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41o00’
40o50’
40o40’
124o40’ 124o30’
700
600
500
400
350
300
250 20
015
0 100
9080
7060
5040
302 0
10
Eel River Drainage BasinEel River Drainage Basin
Eel River Drainage Basin~9000 km2
Study Area
0 m
350
1400
Elevation
CaliforniaEel margin
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Eel Canyon, northern California
Multiple entrants that are presently receiving sediment and experience many turbidity currents each year
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Core
Dep
th0
5
10
15
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25
30
Core
Dep
th
30
35
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45
50
55
60L1C12
DUPLICATE DUPLICATE CORESCORES
L1C13L1C13 L1C12
LarryChannelThalweg
= 137 m
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Sepik Canyon enters the mouth of Sepik River (north coast of New Guinea)
Sediment from the river supplies many turbidity currents each year
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Sepik Canyon turbidites