12.1 Plate tectonics and the carbon cycle

Plate tectonics plays a crucial role in the carbon cycle, influencing CO2 levels over millions of years. Through processes like subduction and volcanism, it affects carbon storage and release, impacting Earth's climate and habitability.

The balance between CO2 release from volcanoes and consumption through weathering regulates atmospheric levels. Understanding these processes helps explain past climate changes and predict future trends in Earth's carbon cycle.

Plate Tectonics and the Carbon Cycle

Tectonic Processes and Carbon Cycling

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• Plate tectonics drive the long-term carbon cycle through processes of subduction, volcanism, and mountain building affect carbon storage and release over geological timescales
• Seafloor spreading at mid-ocean ridges releases CO2 from the mantle contributes to atmospheric carbon levels and oceanic dissolved inorganic carbon
• Subduction zones facilitate the transport of carbon-rich sediments and altered oceanic crust into the Earth's interior effectively removes carbon from the surface reservoir
• Orogenic processes expose fresh silicate rocks to weathering enhances CO2 consumption through chemical weathering reactions
• Metamorphism of carbonate rocks in subduction zones can release CO2 through decarbonation reactions contributes to volcanic and hydrothermal emissions
  • Example: Calcium carbonate (CaCO3) metamorphoses to form calcium silicate (CaSiO3) and CO2
• Tectonic uplift increases the exposure of rocks to weathering potentially accelerates the drawdown of atmospheric CO2 through enhanced silicate weathering
  • Example: Himalayan orogeny increased global weathering rates and CO2 consumption

Carbon Cycle Regulation

• The balance between CO2 release from volcanic activity and CO2 consumption through weathering regulates atmospheric CO2 levels over geological time scales
• Tectonic processes influence both sources and sinks of carbon in the Earth system
  • Sources: Volcanic emissions, metamorphic decarbonation
  • Sinks: Subduction of marine sediments, enhanced silicate weathering
• Changes in tectonic activity can lead to long-term shifts in atmospheric CO2 concentrations
  • Example: Increased volcanic activity during the Cretaceous period led to higher atmospheric CO2 levels and warmer global temperatures
• The carbon cycle's response to tectonic changes operates on timescales of millions of years
• Feedback mechanisms between tectonics, climate, and weathering help maintain Earth's habitability over geological time

Subduction and Carbon Sequestration

Subduction Zone Dynamics

• Subduction zones act as carbon sinks by transporting carbon-rich marine sediments and altered oceanic crust into the Earth's mantle
• Carbonate minerals in subducted sediments undergo decarbonation reactions at depth releases CO2 that can be stored in the mantle or returned to the surface through volcanism
• The efficiency of carbon subduction depends on factors such as the thermal structure of the subduction zone, slab dip angle, and the composition of subducted materials
  • Example: Cold subduction zones (steep slab angles) tend to subduct more carbon than hot subduction zones (shallow slab angles)
• Some subducted carbon remains in the mantle for long periods effectively sequesters it from the surface carbon cycle for millions to billions of years
• Subduction of organic carbon in marine sediments can lead to its long-term storage in the mantle or its transformation into graphite or diamond at high pressures and temperatures
  • Example: Formation of microdiamonds in ultra-high-pressure metamorphic rocks

Carbon Fate and Implications

• The fate of subducted carbon (whether it is stored in the mantle or returned to the surface) has significant implications for long-term climate regulation and the global carbon budget
• Quantifying the amount of carbon subducted versus the amount returned to the surface through arc volcanism determines the net effect of subduction on the carbon cycle
• Variations in subduction efficiency over geological time can influence atmospheric CO2 levels and global climate
  • Example: Changes in global subduction rates during supercontinent cycles may affect long-term climate trends
• The deep carbon cycle, driven by subduction, interacts with the surface carbon cycle on million-year timescales
• Understanding subduction-related carbon sequestration helps in reconstructing past climate conditions and predicting future long-term climate trends

Volcanic Activity and Atmospheric CO2

Volcanic CO2 Emissions

• Volcanic eruptions release significant amounts of CO2 into the atmosphere serve as a primary natural source of atmospheric carbon dioxide
• The composition and style of volcanic eruptions (explosive vs. effusive) influence the quantity and rate of CO2 release into the atmosphere
  • Example: Effusive basaltic eruptions (Hawaii) tend to release more CO2 than explosive silicic eruptions (Mount St. Helens)
• Mid-ocean ridge volcanism contributes a steady flux of CO2 to the ocean-atmosphere system while subduction zone volcanism can produce more variable and potentially larger CO2 emissions
• Large igneous province (LIP) eruptions can release massive amounts of CO2 over geologically short time periods potentially triggers global climate changes and mass extinction events
  • Example: Siberian Traps eruption at the end of the Permian period contributed to the largest mass extinction in Earth's history

Climate Impacts and Measurement

• The cooling effect of volcanic aerosols can temporarily mask the warming effect of CO2 emissions complicates the short-term climate impact of volcanic eruptions
• Quantifying volcanic CO2 emissions presents challenges but remains crucial for understanding natural variability in the carbon cycle and for distinguishing between anthropogenic and natural sources of atmospheric CO2
  • Methods include direct gas sampling, satellite observations, and isotopic analysis of volcanic gases
• The balance between volcanic CO2 emissions and CO2 consumption through weathering processes plays a key role in regulating atmospheric CO2 levels over geological time scales
• Volcanic activity influences both short-term climate variability and long-term climate trends
  • Example: The 1991 Mount Pinatubo eruption caused global cooling of about 0.5°C for several years
• Studying past volcanic events helps in understanding potential future impacts of large-scale volcanism on climate and ecosystems

Plate Tectonics vs. Silicate Weathering

Tectonic Influence on Weathering

• Plate tectonic processes, particularly mountain building (orogeny), expose fresh silicate rocks to the atmosphere enhances chemical weathering rates
• Silicate weathering consumes atmospheric CO2 through reactions that convert silicate minerals into clay minerals and dissolved ions acts as a long-term carbon sink
  • Example: Weathering of feldspars to form kaolinite clay consumes CO2
• The rate of silicate weathering depends on factors such as temperature, precipitation, and the surface area of exposed rocks all of which can be affected by tectonic processes
• Tectonic uplift in orogenic belts increases the potential for physical erosion exposes fresh rock surfaces and accelerates chemical weathering rates
  • Example: The uplift of the Tibetan Plateau significantly increased regional and global weathering rates

Weathering Feedback and Climate Regulation

• The coupling between tectonic uplift, erosion, and silicate weathering forms a negative feedback loop helps regulate atmospheric CO2 levels and global climate over geological time scales
• The spatial distribution of mountain ranges, influenced by plate tectonics, affects regional and global weathering patterns and their impact on the carbon cycle
  • Example: The Andes mountain range influences South American climate and weathering patterns
• Changes in continental configuration due to plate tectonics can alter global weathering rates by affecting climate patterns and the distribution of rainfall over continents
• Silicate weathering acts as a thermostat for Earth's climate system increases in atmospheric CO2 lead to increased weathering rates, which in turn reduce CO2 levels
• The efficiency of the silicate weathering feedback depends on the availability of fresh silicate rocks and the presence of liquid water on Earth's surface
  • Example: During Snowball Earth events, the reduction in liquid water may have weakened the silicate weathering feedback