Deep Carbon Cycle: Global Cooling & Plate Tectonics

Deep Carbon Cycle: Global Cooling

• Thermoregulation prevents a runaway greenhouse effect, acting as an inbuilt planetary thermostat.
• High greenhouse gas concentrations lead to increased evaporation, intensifying the hydrological cycle.
• Increased rainfall enhances silicate weathering, which removes CO2 from the atmosphere, storing it in sediments.
• This thermoregulation process occurs over millions of years and will not mitigate current anthropogenic warming.
• Geoengineering approaches to enhance weathering are costly.

Plate Tectonics and Continental Arrangement

• Antarctica's glaciation started 34 million years ago, influenced by ocean circulation patterns.
• Before glaciation, Antarctica had temperate rainforests and was warmer 55-100 million years ago.
• Plate tectonics significantly contribute to Antarctica's thermal isolation.
• Australia's separation from Antarctica established the Antarctic Circumpolar Current.
• The Antarctic Circumpolar Current deflects warm equatorial currents, preventing them from reaching Antarctica.
• Significant ice sheets appeared 26 million years ago, with current ice sheets several kilometers thick.
• Antarctic glaciation began 34 million years ago, causing a sea level drop of approximately 100 meters.
• Antarctica accounts for 50-60 meters of sea level change.
• The graph shows a pole-to-pole temperature gradient, with equatorial temperatures around 27 degrees Celsius and polar temperatures near zero.
• Plate tectonic rearrangements steepen the temperature gradient between the poles and the equator, contributing to cooling over the last 50 million years.

Chemical and Silicate Weathering

• Continental arrangement impacts climate and sea level.
• Silicate weathering is a major component of this process.
• Rain absorbs CO2 from the atmosphere, becoming slightly acidic (carbonic acid).
• Carbonic acid chemically reacts with silicate rocks, permanently trapping CO2 as bicarbonate or carbonate ions in sediments.
• Bicarbonate and carbonate ions are flushed into the sea, where they are used by marine organisms like coral to build calcium carbonate shells.
• When these organisms die, their shells settle on the sea floor and become sequestered.

Mountain Building and Tectonic Activity

• Mountain building brings fresh rocks to the surface, which are more reactive with carbonic acid.
• Mountain building in humid, equatorial regions is especially effective in this process.
• The India-Eurasia collision, around 40-50 million years ago near the equator, amplified silicate weathering and CO2 drawdown.
• The India-Eurasia collision and other collisions in Southeast Asia caused significant long-term cooling over the last 55 million years.
• The closure of the Tethyan Ocean gateway, which once allowed ocean circulation from the Pacific to the Atlantic across northern India and Arabia, occurred through these collisions.
• Continental collisions create mountain ranges, such as the Himalayas.
• Before the India-Eurasia collision, two subduction zones pumped CO2 into the atmosphere; these shut down sequentially due to the collision.

Biosphere Feedback and Marine Calcifiers

• The India-Eurasia collision led to a long-term cooling trend, dropping temperatures by up to 12 degrees Celsius at the Poles.
• The deep carbon cycle has an interesting feedback with the biosphere.
• Carbonate and bicarbonate ions are flushed into the seas and used by shell-building organisms.
• The evolution of foraminifera and coccolithophores significantly enhanced this system.
• These planktonic organisms photosynthesise and create shells that sink to the sea floor upon death, forming "marine snow."
• The White Cliffs of Dover are made of chalk, which consists of coccolithophores.
• These organisms evolved 200 million years ago and proliferated in the last 150 million years, creating a substantial carbon sink.
• This carbon sink has been active for the last 150 million years influencing the world's carbon and climate systems.
• 15-20% of the carbon captured in the shells becomes permanently stored in the Earth's interior, with a large part stuck in the continental crust.
• The biosphere created a new, dramatic carbon sink, enabling substantial cooling.
• This carbon sink highlights how processes observed now have not always functioned as they do today.
• Geological first principles are best guesses.
• Biosphere modulates climate and habitability.
• Photosynthesising life created the first oxygen on Earth 2.4 billion years ago leading to the great oxygenation event.

Carbon Cycle Components and Climate Change Drivers

• Geology tries to account for CO2 greenhouse gas emissions in different tectonic settings.
• Subduction zones emit and sequester CO2 from erosion and weathering.
• Earth's climate has changed on long geological timescales due to processes like orbital cycles, tectonics rearranging continents, and mantle plume activity.
• Silicate weathering can sequester CO2 from the atmosphere and cause global cooling.
• Marine calcifiers have contributed to long-term cooling over the last 150-200 million years.
• The natural components of Earth's climate system are an important part of the planetary life support system.
• Understanding past signals helps to untangle the anthropogenic signal since industrialisation.