The Devonian Land Plants Arrival And How It Shaped The Earth’s Climate Control System

A group of Researchers at the University College London (UCL) and Yale has in a new study discovered how, the coming of plants on the land surface 400 million years ago changed the way the Earth regulates.

The new study, which received support from the European Research Council and NASA, was first published in the ‘Nature’ journal, had the researchers examining the samples of rocks in the last three billion years and a sensational change in how the cycle functioned when plants got on land 400 million years ago was found.

The researchers specifically noted a change in the chemistry of seawater recorded in the rock, an indication of a major shift in the global formation of clay – the “clay mineral factory” – from the oceans to the land.

In cognizance of the fact that reverse weathering leads to the release of carbon dioxide into the atmosphere, clay on land is a by-product of ocean clay forming, thereby reducing the amount of carbon in the atmosphere, with the resulting effect of having a seesawing climates that have warmer periods with alternating ice ages.

From observation, the researchers discovered that the switch came about due to the spread of land plants keeping soils and clays on land, preventing carbon from being washed into the ocean, and by the growth in marine life using silicon for their skeletons and cell walls, such as sponges, single-celled algae and radiolarians (a group of protozoa), leading to a drop in silicon in the seawater required for clay formation.

Dr. Philip Pogge von Strandmann, from UCL department of Earth Sciences, and a senior author said of the study talked about the shift when he said:

“Our study suggests that the carbon cycle operated in a fundamentally different way for most of Earth’s history compared to the present day.

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

“Before this change, atmospheric carbon dioxide remained high, leading to a stable, greenhouse climate. Since then, our climate has bounced back and forth between ice ages and warmer periods. This kind of change promotes evolution and during this period the evolution of complex life accelerated, with land-based animals forming for the first time.

“A less carbon-rich atmosphere is also more sensitive to change, allowing humans to influence the climate more easily through the burning of fossil fuels.”

Boriana Kalderon-Asael, a PhD student at Yale University and the first author of the study said: “By measuring lithium isotopes in rocks spanning most of Earth’s history, we aimed to investigate if anything had changed in the functioning of the carbon cycle over a large time scale. We found that it had, and this change appears to be linked to the growth of plant life on land and silicon-using animal life in the sea.

Lithium isotopes grouped in 600 samples of rocks taken from different locations around the world were measured.  The element has two stable isotopes; one Isotope has three protons and three neutrons while the other one has three protons and four neutrons.

As clay slowly forms on the land, lithium-6 is favoured, while the heavier lithium-7 isotope gets enriched with the surrounding water.

The researchers, using mass spectrometry in analysing the samples, discovered  a seawater leap in the lithium isotope-7, recorded in the rang about 400 and 500 million years ago, implying a large scale shift in the production of Earth’s clay, which also coincided with the spread of plants on land and emergence of silicon-using marine life.

The removal of carbon dioxide from the atmosphere, a process of clay-forming  occurs when atmospheric carbon combines with water to form a weak acid, carbonic acid, which falls to the ground as rain and dissolves rocks, releasing ions including calcium ions that flow into the ocean.

Carbon is eventually locked up in the ocean floor rocks while in  contrast, carbon by plant photosynthesis is overcome as the plants gets decayed and doesn’t really affect carbon dioxide levels on timescales longer than a few hundred years.

With the formation of clay in the ocean, carbon remains in the water and later released in the air due to the consistent process of carbon exchange when air comes in contact with water.

Reference: “A lithium-isotope perspective on the evolution of carbon and silicon cycles” by Boriana Kalderon-Asael, Joachim A. R. Katchinoff, Noah J. Planavsky, Ashleigh v. S. Hood, Mathieu Dellinger, Eric J. Bellefroid, David S. Jones, Axel Hofmann, Frantz Ossa Ossa, Francis A. Macdonald, Chunjiang Wang, Terry T. Isson, Jack G. Murphy, John A. Higgins, A. Joshua West, Malcolm W. Wallace, Dan Asael and Philip A. E. Pogge von Strandmann, 14 July 2021, Nature.
DOI: 10.1038/s41586-021-03612-1