The Slow Carbon Cycle: Long-Term Climate Thermostat
The formation of carbonate sediments and rocks is a slow process that sequesters massive quantities of carbon in the geosphere. Unfortunately, it operates too slowly to help us out this century. Forests are a faster, more immediate atmospheric carbon sink.
Let’s delve into the slow carbon cycle!
Fluxes include: respiration and photosynthesis (between the biosphere, hydrosphere, and atmosphere), sedimentation and metamorphosis, (between the biosphere and lithosphere), weathering, erosion, volcanism, and fossil fuel combustion (between the lithosphere and atmosphere), dissolution and outgassing (between the hydrosphere and the atmosphere), and precipitation, melting, evaporation, and sublimation (between the cryosphere, hydrosphere, and atmosphere).
When plants photosynthesize, they “inhale” CO2 and combine it with sunshine to produce sugars and oxygen. 6CO2 + 6H2O —> C6H12O6 + 6O2. (Plankton in the ocean create calcium carbonate (CaCO3) shells). Photosynthesizers literally build plant matter and tiny shells out of atmospheric CO2, storing it in their biomass. How neat is that? Plants, trees, and plankton are therefore carbon sinks, meaning they remove atmospheric CO2 and store it in a woody/leafy/carbonate shell form. When plants, trees, and plankton die, they degrade and decompose, thus succumbing to the reverse process and releasing CO2 back into the atmosphere.
Think of a carbon sink like a literal sink: the only way to drain the atmospheric bathtub of CO2 is to open a drain, or to store the carbon somewhere that is not the atmosphere. Biogeochemical cycles are the fluxes to other reservoirs.
If biomass becomes buried (undergoes sedimentation) and is subjected to intense heat and pressure (undergoes metamorphosis) instead of respiring into the atmosphere, it can become: limestone rock that precipitates from the sediments produced by dead plankton, oil or natural gas(plankton metamorphosing under different temperature and pressure regimes and in different geologic formations), or coal (buried trees, usually having decayed anoxically in ancient swamps). Technically, limestone precipitates from the biogenic material in seawater, and quick note, marble is metamorphosed limestone.
Metamorphosis occurs deep within the lithosphere, specifically the aesthenosphere: the boundary between crust and mantle. The lithosphere is therefore also a carbon sink, as sedimentation, metamorphosis, weathering, and erosion are the fluxes that take up to hundreds of millions of years, to unfold, but allow the geosphere to store the largest amount of carbon. An estimated 65,500 billion metric tons (a billion in scientific notation is 1 x 10^9) are stored in the crust and aesthenosphere as sediments, and sedimentary and metamorphic rocks (https://earthobservatory.nasa.gov/features/CarbonCycle).
Weathering and erosion (the mechanical and chemical breakdown of rocks), transports carbon from the atmosphere to the lithosphere via a brief stop in the hydrosphere! Rainwater dissolves CO2, forming a weak carbonic acid that erodes the lithosphere over many hundreds of millions of years. Geoscientists call this “atmospheric scrubbing” because the more CO2 in the atmosphere, the more acidic the rainwater. And remember, more CO2 —> higher temperatures —> more active hydrological cycle —> more atmospheric carbon scrubbing. This is an example of a negative feedback loop, meaning the increase of CO2 leads to an increase in weathering, which dampens the effect of warming caused by the initial increase in CO2. Chemical weathering is one of the greatest, long-term (again, HUNDREDS of MILLIONS of years, an astronomically long time) correcting mechanisms for when Earth’s atmosphere finds itself with excess CO2. But unfortunately for humans, it doesn’t operate on a timescale that can benefit us and our survival as a species this upcoming century. Additionally, mountain building events, like the collision of the India subcontinent and the Eurasian plate causing the uplifting of the Himalayas, are needed to spur on weathering by providing fresh rock surfaces. Plate tectonics is also, you guessed it! an extraordinarily slow geologic process.
Dissolution and outgassing are intuitive. Gases can either dissolve into a solution (like water), or they can be released from a material in which they were frozen, dissolved, trapped, or absorbed. We know the hydrological cycle quite well, so won’t waste more time here.
The slow carbon cycle (metamorphosis, weathering, erosion, sedimentation) acts as a climate thermostat by keeping the atmospheric amount of CO2 relatively stable, within a certain range of values.
We’re piecing it all together . . .
Quick thank you to Dr. Tom Brandes and Trinity Bob for mentioning Eunice Foote’s work in the 1850s discovering the heat-trapping properties of carbon dioxide!
Image sourced from: http://euanmearns.com/the-carbon-cycle-a-geologists-view/