Tectonic control of CO2 Outgassing

Question 1

Theories of Plate Tectonics and
Glaciation Intervals

Answer 1

• Polar Position Hypothesis
• ice more likely to form when continents are at (or
  near) the poles
• BLAG Spreading Rate Hypothesis
• control of CO2 by rate of sea-floor spreading
• Uplift Weathering Hypothesis
• control of CO2 by rate of chemical weathering

Question 2

Polar Position Hypothesis

Answer 2

The polar position hypothesis focused on latitudinal position as a cause of glaciation of continents. This hypothesis suggested that ice sheets should appear on continents when they are located at polar or near-polar latitudes.

Question 3

Spreading-Rate Hypothesis

Answer 3

The BLAG spreading rate hypothesis Proposes that increased spreading rates lead to increased release of CO2 by magma into the atmosphere. The Uplift Weathering Hypothesis looks at chemical weathering as the active driver of climate change, rather than as a negative feedback that moderates climate change.

• Tectonic control of atmospheric CO2
• Rate of spreading affects the rate of
  CO2 input to the atmosphere & ocean from
  rocks
• Have spreading rates changed over time?

Question 4

Control of CO2 input
by seafloor spreading

Answer 4

CO2 is transferred from the rock
reservoir to the oceanatmosphere system at two
kinds of plate margins: ocean
ridges & subduction zones
1. Margin of convergent plates
where portion of subduction
plate melts and molten magma
rises to the surface in Mountain
belt volcanoes.
2. At the margin of divergent
plates (ocean ridges) where hot
magma carrying CO2 erup

Question 5

BLAG hypothesis

Answer 5

concept that changes in the rate of sea-floor spreading over millions of years
control the rate of delivery of CO2 from the rock reservoir to the Atmosphere-Ocean reservoir &
thus control Earth’s climate.

BLAG hypothesis proposes that
faster rates of spreading at ridge
crests create larger amounts of
new ocean crusts but also more
frequent release of magma
=>more CO2 to ocean.
Faster spreading also causes
more rapid subduction of crusts
& sediment in ocean trenches
=>delivers large volumes of
carbon bearing sediment for
subsequent melting and CO2
release through volcanoes.

(Known age of
ocean-basin
crusts. Faster
spreading rates
in the Pacific
(10x) than
Atlantic.)

Question 6

Spreading-Rate Hypothesis part 2

Answer 6

• Tectonic plates are lithosphere slabs that
  move at 2 cm/year to 4 cm/year at subduction
  zones (today)
• Almost all plates possess both ocean and land areas
• Spreading rate is the primary driver of CO2
  changes over time
• fast spreading ➙ more CO2 input
• slow spreading ➙ less CO2 input
• Chemical weathering as a negative feedback
• stabilizing effect on global temperatures

Question 7

Geologic carbon cycle

Answer 7

• Chemical weathering on land
• CaSiO3 + CO2 ➙ CaCO3 + SiO2
• Silicate rock Atmosphere Plankton
• Melting and transformation in subduction zones
• CaCO3 + SiO2 ➙ CaSiO3 + CO2
• Ocean Sediments Silicate rock Atmosphere

Question 8

Uplift-Weathering Hypothesis

Answer 8

The Uplift Weathering Hypothesis looks at chemical weathering as the active driver of climate change, rather than as a negative feedback that moderates climate change.

Question 9

Effect of fragmentation on surface area

Answer 9

The effects of habitat fragmentation are disruption to organisms that have large territories or who need to travel, increased competition for limited resources, edge-adapted animals spreading into other habitats, and extinction of smaller, vulnerable populations.

Question 10

Effect of exposure time on
weathering rates

Answer 10

The length of exposure often contributes to how vulnerable a rock is to weathering. Rocks, such as lavas, that are quickly buried beneath other rocks are less vulnerable to weathering and erosion than rocks that are exposed to agents such as wind and water.

Question 11

Uplift-Weathering Hypothesis part 2

Answer 11

Continental Uplift
* ➙ More fresh/fragmented rock
* ➙ more weathering
* ➙ climate cooling
* Why?
- mass wasting and erosion
- earthquakes
- steep slopes ➙ more precipitation
- glacial ice

Causes of uplift?
- subduction ➙ mountain formation
‣ constant over time (more-or-less)
- continental collisions
‣ sporadic events
* India/Asia collision at 55 Myrs ago
* Godwana/Eurasia at 325-240 Myrs ago
* Timing of collisions match timing of

Question 12

What controls chemical
weathering?

Answer 12

• Spreading rate hypothesis
• Weathering primarily a negative feedback
  response to climate change
  ‣ climatic control of weathering (warm, wet,
  vegetation)
• Uplift-weathering hypothesis
• Weathering primarily a climate forcing
  ‣ weathering driven by physical processes

Question 13

Amazon
river basin: climate or
physical control of
weathering rates?

Answer 13

80% of weathering occurs in Andes,
20% from lowlands (warmer, wetter,
more vegetation)
Lower Amazon basin ➙ protective layer
of clay, most rock already weathered
Andes ➙ lots of fresh, fragmented rock

Question 14

Feedback or Forcing?

Answer 14

• Forcing:
• uplift ➙ more weathering ➙ CO2 drawdown
• Feedback:
• CO2 drawdown ➙ cooler climate ➙ less
  weathering ➙ slower CO2 drawdown

Question 15

The uplift-weathering hypothesis

Answer 15

Uplift — Steep slopes/ Mass wasting/ Mountain glacier/ slopes precipitation — increased rock fragmentation — increased weatheing and co2 removal — global cooling

Question 16

Negative feedback in
BLAG

Answer 16

Fast seafloor spreading — rapid co2 input — warm greenhouse climate (increased temperature, rain, vegetaition) — increased chemical weathering — increased co2 removal — reduced