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VARIATIONS IN SALINITYOF
SEAWATER
GEOL 1033
(Lessons 22-23, 25, and parts of Lesson 24)
(ppt file 103-20b)
Salinity Variations with Latitude• Low at “meteorological equator” at about 5 º N lat.
(=rainforests on continents)• Highs at about 25 deg. N and 15 deg. S
(= desert climatic belts on continents)
Salinity Variations with Latitude & Depth• Latitudinal variations with depth• Depth profiles show variations
– Subtropics high at surface– Arctic varies seasonally
• Lower during melting sea ice• Higher during ice formation (dashes)
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Major Gases in the Atmosphere and Seawater• Atmospheric gases (more soluble in colder, deeper water) are
saturated at all depths except oxygen & carbon dioxide
• Oxygen & carbon dioxide are involved in biological processes:– Photosynthesis
– Respiration
– Decay of organic matter
Argon 0.94% 1.4%
(as HCO3-)
Oxygen & Carbon Dioxide in Seawater
• Photic zone consumption of CO2 & production of O2
– Photosynthesis: CO2 + H2O organic matter + O2
• Aphotic zone respiration & decay – Produces CO2 & consumes O2
• High latitude density circulation “sinks” O2 to deep water
Many Factors of Carbon Dioxide Cycle in Seawater
• Photosynthesis• Respiration• Decay of organic matter• Atmospheric gas• Carbonic acid• Bicarbonate ion• Carbonate ion• Mg and Ca carbonates - inorganic precipitation• Calcium carbonate skeletons (forams, clams, corals, etc.)• Loss of carbon in organic matter buried in sediments• C in OM in sediments often generates oil & gas deposits• CCD
The pH Scale
• Average pH of seawater is about 7.8
• Buffered by CO2 – Prevents sudden and wide
changes in pH
– With a pH decrease, CaCO3 in skeletons & sediments dissolves
– With a pH increase, CaCO3 precipitates
Carbon Dioxide in Seawater
Surface Seawater Calcium Carbonate Saturation• CO2 is more soluble in colder
water• It forms carbonic acid
(H2CO3) which dissociates to H+ ion & a bicarbonate ion (HCO3
-)• H2CO3 dissolves CaCO3
• One H+ ion links to the CO32-
carbonate to form another bicarbonate (HCO3
-) ion• This binding of the H+ stops
seawater from becoming more acidic
• Removal of CO2 gives up the H+ in HCO3
- & reprecipitates CaCO3.
• The freed H+ left behind lowers the pH back to normal.
Chlorinity
• “Law of constant proportions”– Major and many minor constituents have a constant ratio
between each other = “conservative” substances– Why?
• Possibly due to the oceans being fairly well-mixed • Because of long residence times, especially sodium & chloride
• Determine Cl- content in g/kg– The units g/kg are equivalent to ppt (o/oo)– Easier than determining all the salts– Cl- Related to the electrical conductivity & temperature
• Multiply this value by the constant 1.80655 to get salinity– Example: 20.00 g/kg x 1.80655 = 36.13 o/oo total salinity– Accurate to + or – 0.005– Determine on shipboard or onshore
Residence Time
• Residence times help to explain why some ions are more abundant than others
Greatest Salinity Variations
• Air-sea interface• Boundaries between different ocean currents• Coastal areas
– Evaporative salt concentration or freshwater dilution
– May 29, 1985:• Blomidon = 24.5 o/00
• Kingsport = 16.3 o/00
• Latitude• Depth
Stopped here
Nutrients
• Less abundant than the dissolved atmospheric gases• Measure in ppm• Many are in limited supply, for example:
– Nitrite – Nitrate, sometimes the “limiting factor”– Ammonium– Phosphate, can be the limiting factor– Silica, SiO2, can be the limiting factor– CaCO3, can be the limiting factor
• Divergences recycle nutrients back to the photic zone:– Equatorial– Polar
• Upwellings also return them to the photic zone
Biological Production of Organic Matter in Present-Day World Oceans
Coastal & continental shelves = highest biological productivity
Upwelling Areas = high
lowest
lowestlowest
low
est lowest
lowest
lowest
Divergences
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Bins (percent)
F D C B A
50 60 70 80
Average = 70%
6 15 28 22 27
• Unit = mL/L• At 1 atm at sea level• STP = standard temperature & pressure
ELEMENT ABUNDANCE IN THE CRUST and WHOLE EARTH• Abundance of elements in the crust by percentage weight:
– Oxygen O 47
– Silicon Si 28
– Aluminum Al 8
– Iron Fe 5
– Calcium Ca 3.6
– Sodium Na 3
– Potassium K 2.6
– Magnesium Mg 2
– all others 1.5
• Whole Earth order of abundance:– Iron Fe 35
– Oxygen O 30
– Silicon Si 15
– Magnesium Mg 13
– Nickel Ni 2.4
– Sulfur S 2
– Calcium Ca 1
– Aluminum Al 1
– all others <1
CHARACTERISTICS OF THE WORLD OCEAN
• Oceans cover 71% of Earth's surface (This is equal to about 361 100 000 km2 or 3.611 x 108 km2)
• Oceans represent about 98% of Earth's surface and near-surface water (1.37 x 109 km3)
• Average depth of the oceans is about 3.8 km (~12,450').• Average temperature of the oceans is about 4 deg. C.
• Average salinity is about 34.482 o/oo (=34.482 g/kg)
HEAT CAPACITY OF COMMON MATERIALS
CROSS SECTIONS OF EARTH• Core (2 layers)
• Mantle (3 layers)• Crust (2 types)
Outer core is liquid
CROSS SECTIONS OF EARTHUpper mantle/crustal layers:
– lithosphere
– asthenosphere
– upper mesosphere
OVERVIEW OF PLATE TECTONIC PROCESSES
Thermal Convection
GL1033x1 Test 2 Results – 9 Nov., 2004 (n=100)
F D C B A4 15 33 31 16
Average = 68%