Transcript Document

Plate tectonics
Creation and destruction of lithosphere
• Plate tectonics and continent building
– Accretion through collisions
– Recycling of material
– Segregation of melts
The Rock cycle
Evolution of modern plate
tectonics
• Presence moderate temperatures – Venus is too hot
so lithosphere never cool enough to subduct
• Heat removal from mantle through subduction of
cool oceanic lithosphere and upwelling of new
crust
– Drives convection cells
– Allows basalt eclogite transition to be shallow
– Subduction leads to fractional melting of oceanic crust
and segregation to form continental crust
• Presence of water
– Needed for granite formation
– Catalyzes fractional melting in subducting sediments
Archaen-Proterozoic transition
To modern plate tectonics
Present-day plate
tectonics “begins”
period of
rapid crustal
growth
Period of heavy
bombardment
{
1. Early plates became bigger and thicker
2. Continued recycling of oceanic crust
formed large amounts of buoyant
continental crust
•
Continued partial melting/distillation
•
Separation of Si and other elements from
Mg and Fe
•
Conversion of mafic material to felsic
material through rock cycle
3. Decrease in heat production slowed mantle
convection
•
Drove system to larger convection cells
•
Allowed larger plates to travel farther
on the Earth’s surface and cool more
•
Led to subduction rather than collision of
plates
•
Modern plate tectonics
Period of major accretion
(~ 10-30 my)
Present-day plate
tectonics “begins”
Period of heavy
bombardment
{
Period of major accretion
(~ 10-30 my)
Alternative views
• Does life play a role? (Gaia)
• Earth is only planet with life AND plate
tectonics
• Is there a connection? Cause-effect?
• See Lovelock work
• Life affects weathering and calcite
deposition
Since the Archaean
• Intensity of plate tectonics has varied over time
• Wilson cycles – 500 my cycles
– Evidence of supercontinent 600-900 mybp
– Pangea formed ~ 300 mybp
– Causes not well understood
• Periods of rapid sea floor spreading (and vice
versa)
– Sea level rises because large amounts of shallow basalt
form and don’t cool (and subside) much
– High CO2 release – released at spreading centers when
new crust forms and subducting crust has sediment on it
including calcite which releases CO2 when it melts
Age of crustal material
• Continental crust is older because it doesn’t get
subducted
–
–
–
–
Too buoyant
Becomes “core” for accretion
Collisions (closing of basins) mediate accretion
Losses only from weathering and subduction of
sediment
• Oldest rocks are 4.3 – 4.4 by old
• Oceanic crust is young and constantly recycled
(and fractionated)
– Oldest oceanic crust is furthest from spreading centers
near subduction zones
Figure 8.18 Map of a closed
Atlantic Ocean showing the
rifts that formed when Pangaea
was split by a spreading center.
The rifts on today's continents
are now filled with sediment.
Some of them serve as the
channelways for large rivers.
Net result
• Spreading rates at transform faults
– Pacific plate moves NW at 8 cm/yr
– N American plate moves W at 2 cm/yr
– Indian plate moves NE at 12 cm/yr
• Pacific Ocean is shrinking and Atlantic is growing
– Atlantic opened about 200 MY ago so there should be
no rocks older than this in the Atlantic
Most recent episode of
Seafloor spreading:
Pangaea first broke into
2 pieces
Sea opens between N
and S continents and
Between Africa and
Antarctica
India moves North
S Atlantic opens
Antarctica moving S
India moving N
Australia separates
and moves N
50 MY in the future:
1. Africa will move N and close Mediterranean Sea
2. E Africa will detach (Red Sea rift zone) and move to India
3. Atlantic Ocean will grow and Pacific will shrink as it is
swallowed into Aleutian trench.
4. W California will travel NW with the Pacific Plate (LA will
be swallowed into the Aleutian trench in 60 MY).
Tectonic Rock Cycles
Chemical evolution
Creation and destruction of lithosphere
• Rock cycle
• Weathering destroys continental crust
– Materials deposited in sediments
• Some subducted and recycled through melts
• Some added to continents through collisions
• Links to hydrological and biological cycles
The Rock Cycle
Involvement of the hydrologic cycle
and biological processes
Rock cycle linked to ocean
chemistry
• Processes affect ocean chemistry and elemental
cycles
– Seawater circulates through mid-ocean ridges
– Chemical reactions between water and fresh, hot basalt
– Hydrothermal fluids have very different composition
than seawater (loss of Mg2+ and sulfate, addition of
silica and trace metals)
– Major role in cycling of some elements in the oceans
– Balances riverine inputs (Mg2+ and bicarbonate)
• Hydrothermal alteration
More on this later with ocean chemistry
Hydrothermal solutions
• Very acidic – adds protons (H+) to the
oceans and helps remove riverine
bicarbonate
• Titrates bicarbonate back to CO2
• Returns CO2 to the atmosphere
Weathering and erosion processes
• Weathering of continental crust creates soils
– Mechanical weathering
– Chemical weathering
• Cation-rich Al-silicates + protons (H+) 
Cation poor clays + SiO2 + disassociated cations
• Different minerals show different stabilities
• Weathering is a primary source of major ions to
seawater (cations + and anions -)
–
–
–
–
Major role in controlling ocean composition
Source of protons is hydrated atmospheric CO2
Rivers transport bicarbonate to the ocean
Atmospheric CO2 sink
cation-rich Al-silicates + H+ 
cation poor-clays + SiO2 + diss. cations
• protons came from acidic excess volatiles
• left behind their anions (Cl-, S-2 and HCO3-)
• these anions and the cations weathered from
rocks led to an increase in the salt content of the
early oceans.
cation-rich Al silicates + H+ -> cation-poor clays + SiO2 + diss. cations
protons come from the hydration of atm. CO2 - produces bicarbonate (HCO3-)
Weathering transports bicarbonate to the oceans
so it is a CO2 sink
H2O + CO2  H2CO3
CO2 removal
(consumes H+)
Fig. 8-17 Pictorial representation of the carbonate–silicate geochemical cycle.
Role of organisms in weathering
• Plants accelerate weathering
– Mechanical
– Chemical
• Secrete organic acids
• Enhance build-up of CO2 in soils
• In the absence of life, pCO2 would have to be
much higher so that weathering rates
(consumption of CO2) balances CO2 inputs (from
vulcanism, metamorphism and diagenesis)
• Is this Gaia feedback?
Biological involvement in
chemical and mechanical
weathering
Weathering is an important part of ocean/atmosphere CO2 cycle
CaSiO3  2H2CO3  Ca2  2HCO3  SiO2  H2O

Fig. 8-17
CO2 removal
CaSiO3  2H2CO3  Ca2  2HCO3  SiO2  H2O

Ca 2  2HCO3  CaCO3  H 2CO3
Carbonate ppt. (dissolved silica also precipitatesout)
Fig. 8-17
CO2 removal
CaSiO3  2H2CO3  Ca2  2HCO3  SiO2  H2O

Ocean/atm os CO2 exchange
H 2CO3  CO2  H 2O

Ca 2  2HCO3  CaCO3  H 2CO3
Carbonate ppt. (dissolved silica also precipitatesout)
Fig. 8-17
CO2 removal
Net result (of weatheringand biol. ppt in the ocean)
CaSiO3  CO2  CaCO3  SiO2
CaSiO3  2H2CO3  Ca2  2HCO3  SiO2  H2O


Ocean/atm os CO2 exchange
H 2CO3  CO2  H 2O

Ca 2  2HCO3  CaCO3  H 2CO3
Carbonate ppt. (dissolved silica also precipitatesout)
Fig. 8-17
CO2 removal
Weathering
• An acid base reaction
• Anions left behind are Cl-, S-2, HCO3• Weathering produced anions and cations that
increased the salt content of the early oceans
– At present day weathering rates this could have
occurred fairly rapidly (100’s of millions of years)
• As the pH rose above ~7.5, carbonate minerals
(CaCO3) began to precipitate
– Began to buffer the pH of the oceans
• Biological or chemical precipitation - stromatolites
– Led to large drop in atmospheric CO2
– Initial atm likely had higher total CO2
– Most of this CO2 now sequestered in carbonate rocks
• the pH rose above approx. 7.5, carbonate minerals (CaCO3) began to ppt
• began to buffer the pH of the oceans
3.5 by old stromatolite from the
Warrawoona formation in Australia
Estimated size of C reservoirs
(Billions of metric tons)
• Atmosphere
• Soil organic matter
• Ocean
• Marine sediments &
sedimentary rocks
• Terrestrial plants
• Fossil fuel deposits
• 578 (as of 1700) to
766 (in 1999)
• 1500 to 1600
• 38,000 to 40,000
• 66,000,000 to
100,000,000
• 540 to 610
• 4000
The Carbonate-Silicate Cycle and Long-Term
Controls on Atmospheric CO2
CO2
Weathering of
silicate rocks
CO2
Ions (and silica) carried
by rivers to oceans
Ca2+ + 2HCO3-
+(+SiO
SiO2 2[aq])
Organisms build calcareous
(and siliceous) shells
CaCO3 + CO2 + H2O
(+ SiO2(s)]
CO2
Subduction
(increased P and T)
CaSiO3 + 2CO2 + H2O  Ca2+ + 2HCO3- + SiO2
CaCO3 + SiO2  CaSiO3 + CO2
Fig. 8-18 Systems diagram
showing the negative feedback
loop that results from the climate
dependence of silicate–mineral
chemical weathering and its effect
on atmospheric CO2. This
feedback loop is thought to be the
major factor regulating
atmospheric CO2 concentrations
and climate on long time scales.
Negative
Feedback
Tectonic forcing
(addition of CO2)
Negative feedback
on temp. and
lowering of CO2
Incr. solar
luminosity
Present-day
plate tectonics
“begins”
Onset of early “weathering”
(perhaps earlier)
(?)
Period of heavy
bombardment
{
Period of major
accretion (~ 10-30 my)
{
Condensation of water vapor
Accumulation of excess volatiles
(Cl [as HCl]; N [as N2]; S [as H2S]; CO2)
The Sediment Cycle
• Mountains rise
• Rocks erode (water
and wind)
• Sediments are
deposited
• Sediments uplifted or
subducted
• 15 billion metric tons
(16.5 billion tons) of
sediments moved by
rivers each year!
• 100 million metric
tons moved by air
River plumes transport sediments
• Mississippi R and the Gulf of Mexico
• Frazier River
• World’s big rivers
Volcanoes
• Come from ash ejected during eruptions,
carried by winds and rivers.
• Aeolian transport – dust
• Dust and climate – trace metals, cooling,
nuclear winter, asteroid impacts and
extinction events
Dust plumes
• Volcanoes (Mt.
Pinatubo)
• Deserts – Sahara
dust signal
across Atlantic
Dust
• Dust carried in the atmosphere is < 2 mm
• Limit for clean air (US Gov) is 150 mg/m3
(LA is 1250; avg over US cities is 100-125)
• 75% of sediments in the N Pacfic, 64% of
those to the S Atlantic and 30% of those to
the equatorial Atlantic arrive by wind
(mostly from deserts – Mohave and Sahara)
Ice as a transport agent
• Move rocks in glaciers (e.g., morraines,
erratics)
• Find sediments far from their sources
Organisms as transport agents
•
•
•
•
Kelp
Birds
Sea Lions (swallow stones for ballast)
Unpredictable patterns
Early oceans
• With onset of these combined reactions cation
concentrations reached steady state
– Steady state is not chemical equilibrium
– Steady state is just input = output; constant
concentration
• Over last 700 MY concentrations of major ions in
seawater have probably not changed by more than
a factor of 2 (2x or 0.5x present)
– SW composition constrained by distribution of
evaporite minerals in geological record
– Major changes in SW composition would lead to
different evaporite mineral sequence
Early oceans
• Surface waters were much warmer (~50oC)
• Ancient ocean had no dissolved oxygen (no free
O2 in atm)
• Sulfate content much lower and primarily as H2S,
not SO42• CO2 much higher than today so lower pH
– No precipitation yet
• Fe was reduced - Fe (II)
– So soluble, after oxygen concentrations increased this
changed, had Fe(III) which is insoluble
Controls on the chemical concentration
of seawater
• Assume rivers are the predominant source
of materials to the ocean
Early models:
Uni-directional formation of the oceans
igneous rocks + “excess” volatiles  seawater + sediments + air
Gives you a salty ocean…. Okay with respect to concentrations of nonvolatiles
But it does it too quickly (with respect to age of the ocean) !!!!!! (~100
my or so) (see page 92 of text – this gets you the average residence
time of salt in the oceans ~ 100 MY)
Concept of residence time
• Time water (or anything else) spends in any one
reservoir on average.
– Units of time [volume/(volume/time)] or volume/flux
• Larger reservoirs often have longer residence
times
– Residence time in the ocean is long
– Implications for dumping garbage in the ocean!
• But, residence times vary depending on fluxes
– Implications for water quality and planning
Budgets in a nutshell
Inputs
Exports
Concentration
Pool Size
Maintained
Inputs
Exports
Concentration
Accumulates
Inputs
Exports
Concentration
Declines
Recycling?
•
•
•
•
Tectonic and rock cycles
Inputs via weathering reactions
Removal and recycling in the crust
Consistent with previous discussions
But, seawater is not just concentrated river water!
CaCO3 + CO2 + H2O  Ca2+ + 2HCO3cation-rich Al silicates + H+  cation-poor clays + SiO2 + diss. cations
Processes lead to formation of highly alkaline (pH 10) soda lake
e.g., Dead Sea or Great Salt Lake
Sources of major ions to seawater
• Rivers
• Mechanical and chemical weathering
– Breaking
– Reactions
• Dissolution (calcite, halite)
• Acid-base (carbonic acid + igneous rocks)
• Products are cations, diss. silica, clays (Al-silicates),
bicarbonate
• Products of weathering are clay minerals
• A variety of processes are responsible for removal
of these elements to maintain steady state
concentrations
Where do salts come from
• Difference is both in absolute and relative
concentrations
• Composition of salts in water  composition in
crustal rocks
• Composition of salts in water  composition in
rivers or salty lakes
• Principal ions in seawater are Na+ and Cl- while
principal ions in rivers are Ca2+ and HCO3• Where do excess volatiles (constituents that are
not accounted for by weathering of surface rocks)
come from?
We’ll talk about that next time
but…
• Upper mantle – contains more of the substances in
seawater including water
• Hydrothermal alteration
• Excess volatiles include carbon dioxide, chlorine,
sulfur, hydrogen, fluorine, nitrogen and water
• Some constituents are present at concentrations
lower than expected (e.g., magnesium and sulfate)
– mineral deposits, mid-ocean rifts, biological
processes
• Some ocean solutes are hybrids of weathering and
outgassing (e.g., sodium chloride)
Sources and sinks of sea salts & ions
Why isn’t the ocean getting
saltier?
• Some salty lakes do
• Chemical equilibrium – proportion and amounts
dissolved per unit volume are nearly constant
• Inputs must equal exports (remember from lecture
4)
• Steady state ocean
• Idea of residence times of particular salts
– Residence time = total amount of element/rate at which
element is added or removed
Major ions have been constant
• Inputs = outputs
• Concentrations don’t change with time
– dC/dt = 0
• Box model to calculate residence time (t)
 t = AT/(dA/dt)
– Total amount in the ocean AT
– Removal or input rate (dA/dt)
Budgets in a nutshell
Inputs
Exports
Concentration
Pool Size
Maintained
Inputs
Exports
Concentration
Accumulates
Inputs
Exports
Concentration
Declines
Reservoirs, fluxes and residence times
Residence times & fluxes
• The reservoir is the ocean
• Inputs from weathering and outgassing
• Exports due to sedimentation (including biological
particles, adsorption of reactive particles and
precipitation of minerals), subduction
• Residence time depends on chemical activity
• Distribution of element depends on residence time
relative to ocean mixing times (ocean mixing time is
on average 1600 years or so)
• Long residence times ensure complete mixing and is
the foundation for principle of constant proportions
Ocean water
residence time
of about 4100
years based on
precipitation and
evaporation and
the known volume.
Conservative and
nonconservative constituents
• Conservative constituents occur in constant
proportions or change very slowly (long residence
times) – distributions affected by physical mixing
and diffusion
– Include major salts
• Nonconservative constituents are often tied to
biological or seasonal cycles or very short
geological cycles (short residence times)
– Include oxygen carbon dioxide, silica, calcium, iron,
aluminum, nitrogen & phosphorus
• Many trace elements have distributions that are
nonconservative
Reactivity
• Order of reactivity Si>Ca>Na
• Biological processes
• Major ions unreactive so have long
residence times
• Salinity variations caused by evaporation or
precipitation
Salinity map showing areas of high salinity (36 o/oo) in green, medium salinity in blue (35 o/oo),
and low salinity (34 o/oo) in purple. Salinity is rather stable but areas in the North Atlantic,
South Atlantic, South Pacific, Indian Ocean, Arabian Sea, Red Sea, and Mediterranean Sea
tend to be a little high (green). Areas near Antarctica, the Arctic Ocean, Southeast Asia,
and the West Coast of North and Central America tend to be a little low (purple).