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

Period of major accretion (~ 10-30 my)

{Period of heavy bombardment

Present-day plate tectonics “begins”

period of rapid crustal growth

Archaen-Proterozoic transitionTo modern plate tectonics

1. Early plates became bigger and thicker2. Continued recycling of oceanic crustformed large amounts of buoyantcontinental crust• Continued partial melting/distillation• Separation of Si and other elements fromMg and Fe• Conversion of mafic material to felsic material through rock cycle3. Decrease in heat production slowed mantleconvection• 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 ofplates• Modern plate tectonics

Period of major accretion (~ 10-30 my)

{Period of heavy bombardment

Present-day plate tectonics “begins”

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 opensAntarctica moving SIndia moving NAustralia separates

and moves N

50 MY in the future:1. Africa will move N and close Mediterranean Sea2. E Africa will detach (Red Sea rift zone) and move to India3. 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

Involvement of the hydrologic cycleand biological processes

The Rock Cycle

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.

protons come from the hydration of atm. CO2 - produces bicarbonate (HCO3-)

cation-rich Al silicates + H+ -> cation-poor clays + SiO2 + diss. cations

Fig. 8-17 Pictorial representation of the carbonate–silicate geochemical cycle.

CO2 removal

Bicarbonate

transport

H2O + CO2 H2CO3

(consumes H+)

Weathering transports bicarbonate to the oceansso it is a CO2 sink

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

Fig. 8-17

CaSiO3 2H2CO3 Ca2 2HCO3 SiO2 H2O

river transport

CO2 removal

Weathering is an important part of ocean/atmosphere CO2 cycle

Fig. 8-17

Ca2 2HCO3 CaCO3 H2CO3

Carbonate ppt. (dissolved silica also precipitates out)

CaSiO3 2H2CO3 Ca2 2HCO3 SiO2 H2O

river transport

CO2 removal

Fig. 8-17

Ca2 2HCO3 CaCO3 H2CO3

Carbonate ppt. (dissolved silica also precipitates out)

CaSiO3 2H2CO3 Ca2 2HCO3 SiO2 H2O

river transport

Ocean /atmos CO2 exchange

H2CO3 CO2 H2O

CO2 removal

Fig. 8-17

CO2 removal

Ca2 2HCO3 CaCO3 H2CO3

Carbonate ppt. (dissolved silica also precipitates out)

CaSiO3 2H2CO3 Ca2 2HCO3 SiO2 H2O

river transport

Ocean /atmos CO2 exchange

H2CO3 CO2 H2O

Net result (of weathering and biol. ppt in the ocean)

CaSiO3 CO2 CaCO3 SiO2

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

CO2

CO2

CaSiO3 + 2CO2 + H2O Ca2+ + 2HCO3- + SiO2

Weathering of silicate rocks

+ SiO2

CaCO3 + SiO2 CaSiO3 + CO2

Subduction(increased P and T)

CO 2

Ions (and silica) carried by rivers to oceans

Ca2+ + 2HCO3-

(+ SiO2[aq])

CaCO3 + CO2 + H2O(+ SiO2(s)]

Organisms build calcareous (and siliceous) shells

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.

Tectonic forcing(addition of CO2)

NegativeFeedback

Incr. solar luminosity

Negative feedback on temp. and lowering of CO2

{Period of heavy bombardment

Present-day plate tectonics “begins”

Period of major accretion (~ 10-30 my)

Accumulation of excess volatiles (Cl [as HCl]; N [as N2]; S [as H2S]; CO2)

Condensation of water vapor

Onset of early “weathering” (perhaps earlier)

(?)

{

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 m• Limit for clean air (US Gov) is 150 g/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 non-volatiles

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

ConcentrationPool Size

Maintained

Inputs Exports

ConcentrationAccumulates

Inputs Exports

ConcentrationDeclines

Inputs Exports

Budgets in a nutshell

Recycling?

• Tectonic and rock cycles

• Inputs via weathering reactions

• Removal and recycling in the crust

• Consistent with previous discussions

CaCO3 + CO2 + H2O Ca2+ + 2HCO3-

cation-rich Al silicates + H+ cation-poor clays + SiO2 + diss. cations

But, seawater is not just concentrated river water!

Processes lead to formation of highly alkaline (pH 10) soda lakee.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 () = AT/(dA/dt)

– Total amount in the ocean AT

– Removal or input rate (dA/dt)

ConcentrationPool Size

Maintained

Inputs Exports

ConcentrationAccumulates

Inputs Exports

ConcentrationDeclines

Inputs Exports

Budgets in a nutshell

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 waterresidence timeof about 4100 years based onprecipitation andevaporation andthe 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).