The arrival of terrestrial plants 400 million years ago changed the Earth's climate control system

A new study, led by researchers at UCL (University College London) and Yale, shows that the arrival of plants on Earth about 400 million years ago changed the way the Earth naturally regulates its climate.

The carbon cycle, the process by which carbon moves between rocks, oceans, living things, and the atmosphere acts like the Earth’s natural thermostat, regulating its temperature over long periods of time.

In a new study published in the journal natureIn the study, researchers looked at rock samples spanning over the past three billion years and found evidence of a dramatic change in the way this cycle worked about 400 million years ago, when plants began to colonize the Earth.

Specifically, the researchers observed a change in the seawater chemistry recorded in the rock, indicating a significant change in the overall composition of the mud – the “mud factory” – from oceans to land.

Sampling of the Ordovician sediments (which are 450 million years old) by first author Boriana Kalderon-Asael. Credit: Ashley Hood

Since mud that forms in the ocean (reverse weathering) releases carbon dioxide into the atmosphere, while mud on land is a byproduct of chemical weathering that removes carbon dioxide from the air, this reduces the amount of carbon in the atmosphere, resulting in to a cooler temperature. An oscillating planet and climate, with alternating glacial periods and warmer periods.

The researchers suggested that this change resulted from the spread of land plants that sustain soil and mud on land, the halting of carbon drift in the ocean, and the growth of marine life using silicon for their skeletons, and their cell walls, like sponges, outside. Aerated and radioactive algae (a group of protozoa), resulting in low silicon in seawater needed for sludge formation.

Lead author Dr Philipp Pugh von Strandmann (UCL Earth Sciences) said: “Our study indicates that the carbon cycle operated quite differently for most of Earth’s history compared to today.

“The shift, which occurred gradually 400 to 500 million years ago, appears to be linked to two major biological innovations at the time: the multiplication of plants on land and the growth of marine organisms that extract silicon from the water to create their skeletons and cell walls.

Prior to this change, atmospheric carbon dioxide remained elevated, stabilizing global warming. Since then, our climate has bounced back between ice ages and warmer periods. This type of change supports evolution and during this period the evolution of complex life accelerated, with land animals forming for the first time.

“An atmosphere that is less rich in carbon is also more sensitive to change, making it easier for humans to influence the climate by burning fossil fuels.”

“By measuring lithium isotopes in rocks that cover most of Earth’s history, we set out to check whether anything had changed in the way the carbon cycle works over a period of time,” said study first author Boriana Calderon Asyl, a doctoral student at Yale University. long”. University. This change appears to be related to the growth of plant life on land and animal life that uses silicon in the sea.”

In the study, researchers measured lithium isotopes in 600 rock samples taken from many different locations around the world. Lithium has two naturally stable isotopes – one with three protons and three neutrons, and one with three protons and four neutrons.

When clay slowly forms on land, it strongly favors lithium-6, leaving the surrounding water rich in the heavier isotope, lithium-7. When analyzing their samples by mass spectrometry, the researchers found high levels of lithium-7 in seawater recorded in rocks that occurred between 400 and 500 million years ago, indicating a significant change in the production of clay on Earth in conjunction with the spread of plants on land and the emergence of marine life using silicon.

Sludge forms on Earth as the residue of chemical weathering, the main long-term process by which carbon dioxide is removed from the atmosphere. This happens when carbon in the atmosphere combines with water to form a weak acid, carbonic acid, which falls to the ground in the form of rain and dissolves rocks, releasing ions, including calcium ions, that flow into the ocean. Eventually, carbon is trapped in rocks at the ocean floor. In contrast, decarbonization by plant photosynthesis is inactivated once plants decompose, and this rarely affects carbon dioxide levels over time of more than a few hundred years.

When sludge forms in the ocean, the carbon remains in the water and is eventually released into the air as part of the ongoing carbon exchange that occurs when air meets water.

Reference: “The Lithium-Isotope Perspective on the Evolution of Carbon and Silicon Cycles” by Boriana Calderon-Asyl, Joachim AR Kacchinov, Noah J. Blanavsky, Ashley v. S. Hood, Matthew Dillinger, Eric J. Belfreud, David S. Jones, Axel Hoffmann, Frantz USA, Frances A. McDonald, Zhongyang Wang, Terry T. Eason, Jack J. Murphy, John A. Higgins, A. Joshua West, Malcolm W. Wallace, Dan Asel and Philip A. E. Pugh von Strandmann, July 14, 2021, nature.
DOI: 10.1038 / s41586-021-03612-1

The study received support from the European Research Council and NASA.

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