Earth Sci – Chap 9 – Oceans: Last Frontier
Ocean Water, Sea floor topography, sediments
second half of course…time to move on to the good stuff….Flipper
Oceanography defined – composite science incl bio, chem, phys, and
geol
earth known as the Water Planet…. why?
I. World Ocean
some stats….
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how much of Earth surface covered by it?
break out the hemispheres – Northern has 60% water , southern has
80%
Fig 9.1
vol of ocean basins >> vol of continents (at least the portions of conts
above sea level)
average depth of ocean = ______ feet (hint: deep)
interesting stat: if earth were smooth sphere, ocean would cover it as
a layer 1.2 miles thick
ocean sizes.. Fig 9.2
Pacific is biggest then Atlantic then Indian
II. Seawater composition
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actually a complex sol’n of “salts”, 3.5 weight % dissolved solids
% small but quantity huge cause ocean is so big
another interesting stat:
if all ocean were to evap, blanket of salt ____ feet thick would be left
Terms:
 salinity – amt (by weight) of dissolved solids in pure water, in parts
per thousand (o/oo , not per cent o/o)
example 3.5 % = how many o/oo? (“I can’t do math…”)
ok, so look at Fig 9.3.. of 1000 kilograms of seawater,35 g are diss
solids, 965 g are pure water
- components of salinity
mostly NaCl
some SO4
Mg, Ca, K too ….these 5 represent 99% of diss solids in ocean
Earth Sci wk 9 p.2
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ocean is saturated with these substances, so they are found in same
proportion relative to each other everywhere in ocean (Mediteranean,
Pacific, etc)
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but total salinity can change from place to place…. Fig 9.4
- as pure water content decreases, say due to evap, salinity goes
up..as water content increases (say due to precip), salinity goes
_____
- extreme cases
– restricted seas like Red Sea and Persian Gulf, salinity can rise
to 42 o/oo
- areas w/ heavy precip & rivers,like Baltic Sea, salinity < 10 o/oo
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where do the elements come from?
1) chem wthrg of rocks & mins form continents
2) volcanic outgassing of water and dissolved gas
interesting pt – Cl, Br, S, B are elements that are more abundant in
ocean than in crust….so theory is that maybe they came from source
other than crust, like interior of Earth, and found their way to surface
via volcanoes
III. Layered structure
 temp and salinity vary w/depth, so “layers” result Fig 9.5
- water temperatures at ____ of ocean are warmest due to ____
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the surface “mixed zone” of ocean has fairly even, warm temp
(21-26o C) temp, can be thick, up to 1500 feet
below surface zone, the “thermocline” (as in thermometer) is the
transition zone between warm above and cold below (4o C)
20-25o C
1500 ft
5000 ft
4o C
Earth Sci wk 9 p.3
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similar structure with salinity
- surface zone is more saline, due to evap, transition zone is known
as
______ - cline (hint: think salt, think mineral name for salt)
high salinity
____ - cline
lower salinity
(think “incline” for the slant of these zones)
IV. Earth beneath the sea Fig 9.6
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varied topography due to sea-floor spreading, underwater mts,
trenches, etc
but we didn’t know this until advent of “echo-sounder” in 1920s Fig
9.7
-operation: send a sound signal out, measure time it takes to get
back
- we know the velocity of sound in wtr (5000 feet per second)
- so if it takes one second for a pulse to go round trip from source to
bottom and then back to receiver, how deep is the ocean at that
point??
good exam question, eh?
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3 major topographic units of sea floor.. Fig 9.8
A. continental margins
B. ocean basin floors
C. mid-ocean ridges
sidebar – which surface has been scanned at high resolution MORE –
Venus the planet or ocean basins???
Earth Sci wk 9 p.4
A. continental margins
subdivide into different features – shelf, slope, rise Fig 9.9
1). continental shelf – basically flooded edge of a continent
gentle slope, almost flat (<< 1o slope, 10 feet per mile),
wide on trailing edges away from rifts like east coast US,
narrow when facing subduction zones like west coast S.
america
avg width 50 miles, avg depth at shelf edge about 400 feet
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cont shelves important for resources – petroleum, sand/gravel,
fishing
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once above sea level during Pleistocene, when more water locked up
in glaciers
evidence for this emergence includes:
land-dwelling plant and animal remains have been found
fossilized on shelves now
some of these animals include: _____, _______, and ______.
break in slope at shelf edge known as the “shelf-slope break”, is the
beginning of the continental slope
shelf (10 ft/mi)
continent
slope (400 ft/mi)
rise
20 ft/mi
2). continental slope (essentially the edge of the continent)
leads into v. deep water, avg drop of 370 ft/mile
3.) continental rise (thick prism of seds on the deep ocean floor)
seds deposited as submarine fans by “turbidity currents”
fans grow together to become a blanket
Earth Sci wk 9 p.5
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Submarine Canyons & Turbidity Currents
- steep-sided canyons incised into cont slopes
- some as deep as 10,000 ft
- many line up with present day rivers
- canyons also extend below Pleistocene “base level”, suggesting
that downcutting during Pleistocene by rivers is not the whole story…
- turbidity currents: dense, sediment laden submarine flows may have
done quite a bit of the erosion due to periodic scouring
-t. c.s are thought to originate near shelf-slope break, but they
continue out onto cont rise, cutting channels like rivers in deltas
- eventually tc s come to rest, but they leave deposits in a sorted
manner similar to what rivers leave (remember “critical settling
velocity”?) –
coarser sediments on bottom (first to drop out), followed by
progressively finer sequence toward top. this known as Bouma
sequence (for A.H. Bouma of LSU). also known as a “fining-upward”
sequence. Each sequence thought to represent a single depositional
event….
gravel (bottom of overlying Bouma sequence)
clay
sand
gravel
Bouma sequence
clay (top of underlying Bouma sequence)
B. Ocean Basin Floors
features include:
1. ocean trenches
2. abyssal plains
3. seamounts/guyots
Earth sci wk 9 p.6
1. ocean trenches
deepest part of the ocean (how deep??)
trenches mark subduction zones
trench
Subducting slab
continent
2. abyssal plains
 flat, featureless plains we envision when we think of deep
ocean
example – Argentina – less than 10 feet of relief over 800 miles
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seismic data show that abyssal plains are composed of
sediments that have buried older, irregular features of ocean
floor
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very fine-grained seds, carried far out to sea by turb currents,
and also settling out of ocean water column
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more extensive abyssal plains when trenches not there to
intercept sediment
3. Seamounts
 volcanic origin, some assoc with hotspots, some near ocean
ridges
 if large or elevated enough, these features can break ocean
surface or at least be scoured by waves, become flat-topped
guyots (“gee-ohs”)
 as they move away from ridge or subside, they become
submerged well below wave base
Earth sci wk 9 p.7
C. Mid-Ocean Ridges
- most prominent topographic feature of oceans
- analogous to a “seam on a baseball”
- broad, high features (not usually narrow)
- ridge systems usually quite symmetrical in terms of rates of
spreading
from central rift zone
wide
narrow
Continent
Japan
- but occasionally see assymetry of ocean floor width (eg, Pacific
ocean east of Pacific Rise much narrower than that to west, by
Japan)…this explained by continental plates overriding ocean basin
- rift valley we see along ridges very deep (deeper than Grand
Canyon)
and very similar to continental rift valleys such as E. African Rift
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other topographic features of ocean floor…
- coral reefs / atolls fig 9.11
reefs built from shells of corals and other orgs over 1000s of yrs
most reefs in warm water, where CaCO3 precipitates and where
corals can survive
reef-builders need sunlight, so active reefs grow in < 150 ft of
water
atoll is a particular type of coral island, noted by Darwin in early
1830s
Darwin proposed mechanism of formation to explain apparent
paradox…how can corals, which need sunlight, be found at depths
far below the photic zone??
Earth Sci wk 9 p. 8
Darwin suggested that reefs form on flanks of subsiding volcanic
islands, and as island slowly sinks, corals keep upbuilding into the
photic zone
in this way, atolls and guyots share common subsidence
history…volcanoes start near surface of ocean, end up at great depth
as they travel away from elevated positions into deeper ocean basins
V. Sea Floor Sediments
- thickness of sed blanket varies, from 30,000 ft in some trenches
to zero on fresh mid-ocean ridges
- avg sed accumulations on order of 2,000 ft in Atlantic and
Pacific
- mud size (v. fine gr.) is the most common
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3 main types of seafloor sediments..
1. terrigenous
2. biogenous
3. hydrogenous
1. terrigenous (meaning?) - primarily mineral grains from continents
- very slow rates of deposition – may take _____ for a 1centimeter layer to form
-seds very often stained by iron oxide due to oxidation as particle
travels
through the water column
2. biogenous (meaning?) - primarily shells and skeltons of organisms fig
9.13
- often see a “rain” of tiny foraminifera shells falling through water
column from dead animals…these form a “calcareous ooze” in places
where water is not so cold as to dissolve the CaCO3
- interesting note – below 15,000 ft, calc ooze dissolves, so on
some subsea mts, there is a “snow line” at this depth, where you see
calc ooze above, none below…just like “snow lines” we see in the
Rockies or Alps
ooze-covered
15,000 ft
bare rock
- silicious oozes (SiO2) are made of diatoms and radiolarians, not forams
these are not as susceptible to dissolution in cold water as calc ooze, so
see these at great depths and near the poles
Earth sci wk 9 p. 9
3. Hydrogenous (meaning?) – directly precipitated from seawater
- some limestones
- manganese nodules
some notes on climate change…
organisms change as climate changes, but these orgs sink to sea floor
and become trapped in seds
we study sea-floor seds in order to gain understanding of climate change
by studying the remains of these orgs…
sediment cores taken from drill ships have been extensively studied,
provide us with much new data we use to interpret past climates