Global chemical weathering dominated by continental arcs since the mid-Palaeozoic
Nature Geoscience volume 14, pages 690–696 (2021)
Earth’s plate-tectonic activity regulates the carbon cycle and, hence, climate, via volcanic outgassing and silicate-rock weathering. Mountain building, arc–continent collisions and clustering of continents in the tropics have all been invoked as controlling the weathering flux, with arcs also acting as a major contributor of carbon dioxide to the atmosphere. However, these processes have largely been considered in isolation when in reality they are all tightly coupled. To properly account for interactions among these processes, and the inherent multi-million-year time lags at play in the Earth system, we need to characterize their complex interdependencies.
Here we analyse these interdependencies over the past 400 million years using a Bayesian network to identify primary relationships, time lags and drivers of the global chemical weathering signal. We find that the length of continental volcanic arcs—the fastest-eroding surface features on Earth—exerts the strongest control on global chemical weathering fluxes. We propose that the rapid drawdown of carbon dioxide tied to arc weathering stabilizes surface temperatures over geological time, contrary to the widely held view that this stability is achieved mainly by a delicate balance between weathering of the seafloor and the continental interiors.
See:
https://www.nature.com/articles/s41561-021-00806-0
How plate tectonics has maintained Earth's 'Goldilocks' climate
26 May 2022
A natural 'carbon conveyor belt' is responsible
Not hothouse, nor icehouse: when tectonic plates move at a moderate speed - not too fast or slow - Earth remains habitable, new University of Sydney research finds.
Hothouse and icehouse climates have existed in the geological past. The Cretaceous hothouse (which lasted from roughly 145 million to 66 million years ago) had atmospheric CO₂ levels above 1,000 parts per million, compared with around 420 today, and temperatures up to 10℃ higher than today.
But Earth’s climate began to
cool around 50 million years ago during the
Cenozoic Era, culminating in an
icehouse climate in which temperatures dropped to roughly 7℃ cooler than today.
What kickstarted this dramatic change in global climate?
Our suspicion was that Earth’s tectonic plates were the culprit. To better understand how tectonic plates store, move and emit carbon, we built a computer model of the tectonic “carbon conveyor belt”.
The carbon conveyor belt
The Earth’s tectonic carbon conveyor belt shifts massive amounts of carbon between the deep Earth and the surface, from mid-ocean ridges to subduction zones, where oceanic plates carrying deep-sea sediments are recycled back into the Earth’s interior. The processes involved play a pivotal role in Earth’s climate and habitability. Author provided
Tectonic processes release carbon into the atmosphere at mid-ocean ridges - where two plates are moving away from each other - allowing magma to rise to the surface and create new ocean crust.
At the same time, at ocean trenches - where two plates converge - plates are pulled down and recycled back into the deep Earth. On their way down they carry carbon back into the Earth’s interior, but also release some CO₂ via volcanic activity.
The Earth’s tectonic carbon conveyor belt shifts massive amounts of carbon between the deep Earth and the surface, from mid-ocean ridges to subduction zones, where oceanic plates carrying deep-sea sediments are recycled back into the Earth’s interior. The processes involved play a pivotal role in Earth’s climate and habitability.
Our model shows that the Cretaceous hothouse climate was caused by very fast-moving tectonic plates, which dramatically increased CO₂ emissions from mid-ocean ridges.
In the transition to the Cenozoic icehouse climate tectonic plate movement slowed down and volcanic CO₂ emissions began to fall. But to our surprise, we discovered a more complex mechanism hidden in the conveyor belt system involving mountain building, continental erosion and burial of the remains of miscroscopic organisms on the seafloor.
The hidden cooling effect of slowing tectonic plates in the Cenozoic
Tectonic plates slow down due to collisions, which in turn leads to mountain building, such as the Himalayas and the Alps formed over the last 50 million years. This should have reduced volcanic CO₂ emissions but instead our carbon conveyor belt model revealed increased emissions.
We tracked their source to carbon-rich deep-sea sediments being pushed downwards to feed volcanoes, increasing CO₂ emissions and cancelling out the effect of slowing plates.
So what exactly was the mechanism responsible for the drop in atmospheric CO₂?
The answer lies in the mountains that were responsible for slowing down the plates in the first place and in carbon storage in the deep sea.
As soon as mountains form, they start being eroded. Rainwater containing CO₂ reacts with a range of mountain rocks, breaking them down. Rivers carry the dissolved minerals into the sea. Marine organisms then use the dissolved products to build their shells, which ultimately become a part of carbon-rich marine sediments.
As new mountain chains formed, more rocks were eroded, speeding up this process. Massive amounts of CO₂ were stored away, and the planet cooled, even though some of these sediments were subducted with their carbon degassing via arc volcanoes.
Rock weathering as a possible carbon dioxide removal technology
The Intergovernmental Panel on Climate Change (IPCC)
says large-scale deployment of carbon dioxide removal methods is “unavoidable” if the world is to reach net-zero greenhouse gas emissions.
The weathering of igneous rocks, especially rocks like basalt containing a mineral called olivine, is very efficient in reducing atmospheric CO₂. Spreading olivine on beaches could
absorb up to a trillion tonnes of CO₂ from the atmosphere, according to
some estimates.
Geological processes, with some human help, may also have their role in maintaining Earth’s “Goldilocks” climate.
This study was carried out by researchers from the University of Sydney’s
EarthByte Group, The University of Western Australia, the University of Leeds and the Swiss Federal Institute of Technology, Zurich using
GPlates open access modelling software. This was enabled by Australia’s National Collaborative Research Infrastructure Strategy (NCRIS) via
AuScope and The Office of the Chief Scientist and Engineer, New South Wales Department of Industry.
This piece was originally published in
The Conversation. Hero image: Ben Turnbull on Unsplash.
See:
https://www.sydney.edu.au/news-opin...-maintained-earth-s--goldilocks--climate.html
The Earth’s tectonic carbon conveyor belt shifts massive amounts of carbon between the deep Earth and the surface, from mid-ocean ridges back to subduction zones, where oceanic plates carrying carbon rich deep-sea sediments are recycled back into the Earth’s interior where some CO2 and other gases are discharged from active volcanoes.
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