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Water collects on the foams. The first land plants were considered non-vascular, such as mosses. Credit: Katmai Preserve NPS Photo / Russ Taylor
The arrival of plants on Earth around 400 million years ago may have changed the way the Earth naturally regulates its own climate, according to a new study by researchers at UCL (University College London) and Yale .
The carbon cycle, the process by which carbon moves between rocks, oceans, living organisms, 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 Nature, the researchers looked at rock samples spanning the past three billion years and found evidence of a drastic change in how this cycle worked around 400 million years ago, when plants began to colonize Earth.
Specifically, the researchers noted a change in seawater chemistry recorded in the rock that indicates a major change in the overall formation of clay – the “clay mineral factory” – from the oceans to the sea. Earth.
Sampling of Ordovician sediments (450 million years old) by the first author Boriana Kalderon-Asael. Credit: Ashleigh Hood
Since the formation of clay in the ocean (reverse weathering) results in the release of carbon dioxide into the atmosphere, while clay on land is a by-product of chemical weathering which removes carbon dioxide from carbon from the air, this reduced the amount of carbon in the atmosphere, leading to a cooler planet and a sawtooth climate, with alternating ice ages and warmer periods.
The researchers suggested the change was caused by the spread of land plants keeping soils and clays on the land, preventing carbon from being carried into the ocean, and the growth of marine life using silicon for theirs. skeletons and their cell walls, such as sponges, alone – cellular algae and radiolarians (group of protozoa), leading to a decrease in silicon in seawater necessary for the formation of clay.
Lead author Dr Philip Pogge von Strandmann (UCL Earth Sciences) said: “Our study suggests that the carbon cycle has functioned in fundamentally different ways for much of Earth’s history compared to today. .
“The change, which took place gradually between 400 and 500 million years ago, appears to be linked to two major biological innovations of the time: the propagation of plants on land and the growth of marine organisms that extract the silicon from water to create their skeletons and cell walls.
“Before this change, atmospheric carbon dioxide remained high, leading to a stable greenhouse climate. Since then, our climate has rebounded between ice ages and warmer periods. This type of change favors evolution and during this period complex life evolution has accelerated, with the formation of land animals for the first time.
“A lower carbon atmosphere is also more sensitive to change, making it easier for humans to influence the climate by burning fossil fuels.”
First author Boriana Kalderon-Asael, a doctoral student at Yale University, said: “By measuring lithium isotopes in rocks spanning most of Earth’s history, we sought to determine if anything had changed in the functioning of the carbon cycle over a large time scale. We found out that was the case, and this change appears to be linked to the growth of plant life on land and animal life using 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 occurring stable isotopes – one with three protons and three neutrons, and one with three protons and four neutrons.
When clay forms slowly on earth, it strongly favors lithium-6, leaving the surrounding water enriched with the other heavier isotope, lithium-7. By analyzing their samples using mass spectrometry, the researchers found an increase in the levels of lithium isotope-7 in seawater recorded in the rock between 400 and 500 million years ago. , suggesting a major shift in Earth’s clay production coinciding with the spread of plants on land and the emergence of marine life using silicon.
Clay forms on earth as a residue of chemical weathering, the main long-term process by which carbon dioxide is removed from the atmosphere. This happens 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, which flow out. in the ocean. Eventually, the carbon is locked in the rocks at the bottom of the ocean. In contrast, carbon reduction by photosynthesis in plants is reversed once plants decompose and rarely affects carbon dioxide levels over time scales greater than a few hundred years.
When clay forms in the ocean, carbon stays in the water and is eventually released into the air as part of the continuous exchange of carbon that occurs when air meets water.
Reference: “A lithium-isotope perspective on the evolution of carbon and silicon cycles” by Boriana Kalderon-Asael, Joachim AR 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 AE Pogge 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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