The arrival of plants on land some 400 million years ago could have changed the way the earth naturally regulates its own climate, according to a new study led by researchers at UCL (University College London) and Yale.
The arrival of plants on land some 400 million years ago could have changed the way the earth naturally regulates its own climate, according to a new study led by researchers from UCL and Yale.
The carbon cycle, the process by which carbon moves between rocks, oceans, living organisms, and the atmosphere, acts as the earth’s natural thermostat, regulating its temperature over long periods of time.
In a new study published in the journal nature The researchers examined rock samples from the past three billion years and found evidence of a dramatic change in how this cycle worked about 400 million years ago when plants began to colonize land.
Specifically, the researchers determined a change in the chemistry of the seawater recorded in the rock, which indicates a strong shift in global clay formation – the “clay mineral factory” – from the oceans to the land.
Since clay formation in the ocean (reverse weathering) causes carbon dioxide to be released into the atmosphere, while clay on land is a by-product of chemical weathering that removes carbon dioxide from the air, this reduces the amount of carbon in the atmosphere, leading to a cooler planet and a fluctuating climate with alternating ice ages and warmer periods.
The researchers hypothesized that the shift was caused by the spread of land plants that hold soils and clays on land that prevent carbon from being washed into the ocean, and the growth of marine life that use silicon for their skeletons and cell walls, such as sponges, single cell algae, and radiolarians (a group of protozoa), all of which lead to a drop in the amount of silicon in seawater that is necessary for clay formation.
Senior author Dr. Philip Pogge of Strandmann (UCL Earth Sciences) said, “Our study suggests that for most of Earth’s history the carbon cycle worked fundamentally differently than it does today.
“The shift, which was gradual 400 to 500 million years ago, appears to be linked to two major biological innovations at the time: the proliferation of plants on land and the growth of marine organisms that extract silicon from the water to create their skeletons form, and cell walls.
“Before this change, the carbon dioxide levels in the atmosphere remained high, which resulted in a stable greenhouse climate. Since then, our climate has moved back and forth between ice ages and warmer periods. This type of change promotes evolution and during this time the evolution of complex life accelerated, with land animals forming for the first time.
“A less carbon-rich atmosphere is also more sensitive to changes, so that humans can more easily influence the climate by burning fossil fuels.”
First author Boriana Kalderon-Asael, a PhD student at Yale University, said, “By measuring lithium isotopes in rocks that span most of the Earth’s history, we wanted to see if something changed in the way the carbon cycle worked over a long period of time Has. We found this to be the case, and this change appears to be related to the growth of plants on land and silicon-using wildlife in the ocean. ”
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 slowly forms on land, it strongly favors lithium-6, so that the surrounding water is enriched with the other, heavier isotope, lithium-7. When analyzing their samples with mass spectrometry, the researchers found an increase in lithium isotope-7 content in seawater recorded in rocks 400 to 500 million years ago, suggesting a large shift in earth’s clay production coinciding with the spread of plants on land and the emergence of silicon-using marine life.
Clay forms on land as residual chemical weathering, the primary 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, that flow into the ocean. Eventually, the carbon becomes trapped in rocks on the ocean floor. In contrast, carbon degradation by plant photosynthesis is negated as soon as the plants decay and rarely affects carbon dioxide levels on time scales longer 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 carbon exchange that occurs when air meets water.
The study was supported by the European Research Council and NASA.