Lecture #5
Week #4
We are a species of seafarers
Chapter 4
Seafloor & Ocean Basins
Oceanography
Lecture #5
A.K. Morris, Ph.D.
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We have become very good at “reading”
reading”
the sea surface
Despite all that we know
historically about the ocean
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Fig. 4.1
Our knowledge of the seafloor
pales in comparison.
No shortage of ways to
sample the ocean bottom
these days
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Seegurke J. Gutt
A.K. Morris, Ph.D.
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Lecture #5
Week #4
Historic seafloor Mapping
Proto‐bathysphere
• “Heaving the lead”
• Line is “clearly” marked
– 2 fathoms from the lead ‐ 2 strips of leather.
– 3 fa ‐ 3 strips of leather.
– 5 fa‐ a piece of white cloth.
– 7 fa‐ a piece of red cloth.
– 10 fa‐ a piece of leather with a hole in it.
– 13 fa‐ a piece of blue cloth.
– 15 fa‐ same as for 5 fathoms.
– 17 fa‐ same as for 7 fathoms.
– 20 fa‐ small line with 2 knots.
– 25 fa‐ small line with 1 knot.
– 30 fa‐ small line with 3 knots.
– 35 fa‐ small line with 1 knot.
– 40 fa‐ small line with 4 knots.
•Made out of leather and metal
•Gaspard Schott’
Schott’s
“Curiosa sive Mirabilia Artis”
Artis” 1664
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Matthew Fontaine Maury (1806‐1873)
Historic Seafloor Mapping
Wind
• The deeper you went, the less accurate your measurement was…
measurement was…
R.V. Minnow
UGH! – don’
don’t forget your “clear”
clear” markers
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Father of modern physical oceanography
First detailed Atlantic seafloor map (1854)
Actually detected the Mid‐
Actually detected the Mid‐Atlantic ridge, but didn’
but didn’t know what to make of it
R.V. Minnow
Current
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Matthew Fontaine Maury (1806‐1873)
• Aside: commissioned by the American whaling industry to map whale takes (pink represents sperm whales)
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A.K. Morris, Ph.D.
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Historic Scientific Cruises
• US Exploring Expedition (1838‐1842)
• H.M.S. Challenger Expedition (1872‐1876)
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Lecture #5
Week #4
US Exploring Expedition
H.M.S. Challenger Expedition
(1838‐
(1838‐1842) (1872‐
(1872‐1876) •
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American endeavor – 6 ships
Lt. Charles Wilkes
Attempted to map the entire Pacific
Smithsonian collection started with this expedition
Discovered and named Antarctica!
• British endeavor
• Professor Sir Charles Wyville Thomson
1865
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Sounding the Bottom
• Only 300 deep water measurements
• Thomson found the bottom was “as varied as on land”
as varied as on land”.
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1922 – Birth of the “echo sounder”
1922 – Birth of the “echo sounder”
• USN invention
• Uses sound speed in seawater
• Changing the frequency and intensity = new applications!
LARGE SCALE:
• Seismic Reflection Profiling
• Multi‐
Multi‐channel seismic systems
MEDIUM SCALE:
• Multi‐
Multi‐beam echo sounders (side‐
beam echo sounders (side‐scan sonar is most common)
FINE SCALE:
• Chirp Profilers NOAA
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A.K. Morris, Ph.D.
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Lecture #5
Week #4
Seismic Reflection Profiling (large scale)
Choosing frequency & intensity
• Sound waves fired by an air gun from the ship
• Sound echoes off different subsub-layers
• Returns to detector at different times
• The “car stereo” example
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Actual multi‐channel seismic data Multi‐
Multi‐channel seismic systems (large scale)
• Same concept using sound waves fired by an air gun
• Uses many different frequencies, so
• Much better resolution
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USGS Gyre Cruise, 1997. Bryant Canyon, Louisiana Gulf Coast [Nealon, Dillon, & Twichell, 2000]. USGS Report 99-449.
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Side‐
Side‐scan sonar modern applications
Echo Sounders (medium scale)
• Wreck Hunting
© Louise B. Crary
• SideSide-scan sonar is very popular today
• FanFan-shaped pulses
• Works well over relatively flat areas
Pluses:
•Accurate, portable
Minuses
•Difficult over variable bottoms and rough seas
© Frank A. Palmer
Wyse's Ferry Bridge on the bottom of Lake Murray, South Carolina, 160 ft depth.
•Both ships collided on
Dec 17, 1902 and sank off MA
•Re-discovered in 2002
•Both are still loaded with coal
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A.K. Morris, Ph.D.
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Lecture #5
Week #4
NOAA’
NOAA’s favorite tool these days…
days….
Convenience (a rarity in oceanographic sampling)
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Chirp Profilers
Chirp – scientific application
(fine scale samplers) © Sandy Baldwin
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Fig. 4.3
Chirp – applied uses
Tacoma, WA
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A.K. Morris, Ph.D.
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Lecture #5
Week #4
Satellite Altimetry
seafloor mapping from space???
USS Alligator video…
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Fig. 4.4
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Smith‐Sandwell Map
(based on gravity measurements by satellites and ship‐board instruments) 33
Sound in the Sea
Passive acoustics
• Passive acoustics
• Active acoustics
Atlantic common dolphin sound:
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Lecture #5
Week #4
Speed of Sound in the Sea
Active acoustics
approximately 4,921 ft per second
• Temperature
• Salinity
• Pressure
• Most things in the sea resonate
• Must find their unique frequency
• Then you can “see” them
SOFAR channel (sound fixing and ranging)
Morris, Macauley and Hamner, in press
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Low‐Frequency Active Sonar (LFAS)
Speed of Sound in the Sea
formerly Acoustic Thermometry of Ocean Climate (ATOC)
SOFAR channel (sound fixing and ranging)
• 3 main zones of concern
•US Navy plans to cover 80% of the ocean with this
“quiet submarine” detection system.
• BOTTOM LINE: sound waves
get trapped in the SOFAR channel
• VERY low frequencies
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Low‐Frequency Active Sonar (LFAS)
Low‐Frequency Active Sonar (LFAS)
formerly Acoustic Thermometry of Ocean Climate (ATOC)
formerly Acoustic Thermometry of Ocean Climate (ATOC)
Growing evidence that it is dangerous/deadly to marine life…
OVERTURNED !
X
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Lecture #5
Week #4
Bimodal Crust and Isostasy
Earth’s hypsographic curve
• Bimodal – 2 states of earth crust
– Ocean basins
– Continents
• Isostasy – steady buoyant state
• Archimedes principle – an objects buoyancy determined by weight of “stuff” it displaces
– In this case, balance between weight of crust and weight of fluid mantle it displaces
Add more weight, it will sink more
Think: glaciers, sedimentation, etc.
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Seafloor Provinces
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Continental Margins
Deep Ocean Basins
Oceanic Ridges
Hydrothermal Vents
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Continental Margins
• Passive versus Active
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Lecture #5
Week #4
Fig. 4.10
•Active margin: seismically activity, border of plates
“PacificPacific-type”
type” margin
•Passive margin: no activity, trailing edge of plate
“AtlanticAtlantic-type”
type” margin
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Fig. 4.11
Lesson in Geo‐anatomy
Fig. 4.12
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Deep Ocean Basins
• Abyssal plains – mostly featureless expanse
• Abyssal hills – jagged basement covered by many millennia‐worth of sediment
• Trenches
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Lecture #5
Week #4
Fig. 4.16
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Oceanic Ridges
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www.marum.de
Hydrothermal vents
Hydrothermal vents
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