Chapter 4. Seafloor & Ocean Basins. Lecture #5 Week #4. A.K. Morris, Ph.D. We are a species of seafarers. Oceanography Lecture #5

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1 Chapter 4 We are a species of seafarers Seafloor & Ocean Basins Oceanography Lecture #5 1 2 We have become very good at reading the sea surface Despite all that we know historically about the ocean 3 4 Fig. 4.1 Our knowledge of the seafloor pales in comparison. No shortage of ways to sample the ocean bottom these days 5 Seegurke J. Gutt 6

2 Proto bathysphere Made out of leather and metal Gaspard Schott s Curiosa sive Mirabilia Artis 1664 Historic seafloor Mapping 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. 7 UGH! don t t forget your clear markers 8 Historic Seafloor Mapping The deeper you went, the less accurate your measurement was Wind Matthew Fontaine Maury ( ) Father of modern physical oceanography First detailed Atlantic seafloor map (1854) Actually detected the Mid Atlantic ridge, but didn t t know what to make of it R.V. Minnow R.V. Minnow Current 9 10 Matthew Fontaine Maury ( ) Aside: commissioned by the American whaling industry to map whale takes (pink represents sperm whales) Historic Scientific Cruises US Exploring Expedition ( ) H.M.S. Challenger Expedition ( ) 11 12

3 US Exploring Expedition ( ) 1842) American endeavor 6 ships Lt. Charles Wilkes Attempted to map the entire Pacific Smithsonian collection started with this expedition Discovered and named Antarctica! H.M.S. Challenger Expedition ( ) 1876) British endeavor Professor Sir Charles Wyville Thomson Sounding the Bottom Only 300 deep water measurements Thomson found the bottom was as varied as on land Birth of the echo sounder USN invention Uses sound speed in seawater 1922 Birth of the echo sounder Changing the frequency and intensity = new applications! LARGE SCALE: Seismic Reflection Profiling Multi channel seismic systems MEDIUM SCALE: Multi beam echo sounders (side scan scan sonar is most common) FINE SCALE: Chirp Profilers NOAA 17 18

4 Choosing frequency & intensity The car stereo example Seismic Reflection Profiling (large scale) Sound waves fired by an air gun from the ship Sound echoes off different sub-layers Returns to detector at different times Multi channel seismic systems (large scale) Actual multi channel seismic data Same concept using sound waves fired by an air gun Uses many different frequencies, so Much better resolution 21 USGS Gyre Cruise, Bryant Canyon, Louisiana Gulf Coast [Nealon, Dillon, & Twichell, 2000]. USGS Report Echo Sounders (medium scale) Side-scan sonar is very popular today Fan-shaped pulses Works well over relatively flat areas Pluses: Accurate, portable Minuses Difficult over variable bottoms and rough seas Wyse's Ferry Bridge on the bottom of Lake Murray, South Carolina, 160 ft depth. Side scan sonar modern applications Wreck Hunting Frank A. Palmer Both ships collided on Dec 17, 1902 and sank off MA Re-discovered in 2002 Both are still loaded with coal Louise B. Crary 23 24

5 NOAA s s favorite tool these days. Convenience (a rarity in oceanographic sampling) Chirp Profilers (fine scale samplers) Chirp scientific application Sandy Baldwin Chirp applied uses Fig. 4.3 Tacoma, WA 29 30

6 Satellite Altimetry seafloor mapping from space??? USS Alligator video Fig. 4.4 Smith Sandwell Map (based on gravity measurements by satellites and ship board instruments) Sound in the Sea Passive acoustics Active acoustics Passive acoustics Atlantic common dolphin sound: 35 36

7 Active acoustics Most things in the sea resonate Must find their unique frequency Then you can see them Speed of Sound in the Sea approximately 4,921 ft per second Temperature Salinity Pressure SOFAR channel (sound fixing and ranging) Morris, Macauley and Hamner, in press Speed of Sound in the Sea SOFAR channel (sound fixing and ranging) 3 main zones of concern BOTTOM LINE: sound waves get trapped in the SOFAR channel Low Frequency Active Sonar (LFAS) formerly Acoustic Thermometry of Ocean Climate (ATOC) US Navy plans to cover 80% of the ocean with this quiet submarine detection system. VERY low frequencies Low Frequency Active Sonar (LFAS) formerly Acoustic Thermometry of Ocean Climate (ATOC) Low Frequency Active Sonar (LFAS) formerly Acoustic Thermometry of Ocean Climate (ATOC) Growing evidence that it is dangerous/deadly to marine life X OVERTURNED! 41 42

8 Bimodal Crust and Isostasy 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 Earth s hypsographic curve Add more weight, it will sink more Think: glaciers, sedimentation, etc Seafloor Provinces Continental Margins Deep Ocean Basins Oceanic Ridges Hydrothermal Vents Continental Margins Passive versus Active 47 48

9 Fig Active margin: seismically activity, border of plates Pacific-type margin Passive margin: no activity, trailing edge of plate Atlantic-type margin Fig Lesson in Geo anatomy Fig Deep Ocean Basins Abyssal plains mostly featureless expanse Abyssal hills jagged basement covered by many millennia worth of sediment Trenches 53 54

10 Fig Oceanic Ridges Hydrothermal vents Hydrothermal vents 59 60