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Have plate tectonics, the stage for life on Earth?

Plate tectonics may have helped to promote the development of life on Earth. Credit: NASA

A new study suggests that a rapid cooling in the Earth’s mantle by plate tectonics played an important role in the development of the first forms of life, which in turn led to the oxygenation of the atmosphere of the Earth. The study was published in the March 2018 issue of Earth and Planetary Science Letters.

Scientists at the University of Adelaide and Curtin University in Australia, and the University of California, Riverside, California, USA, collected and analyzed data on igneous rocks of the geological and geochemical data repositories in Australia, Canada, New Zealand, Sweden and the United States. They found that more than the 4.5 billion years of the Earth development, rocks, and rich in phosphorus build up in the earth’s crust. They looked at the relationship of this accumulation to that of oxygen in the atmosphere.

Phosphorus is essential for life as we know it. Phosphates, compounds containing phosphorus and oxygen, is a part of the backbones of DNA and RNA, as well as the membranes of cells and help control cell growth and function.

To find out how the level of phosphorus in the earth’s crust has increased over time, the scientists have studied how the rock formed as the Earth’s mantle cooled. They are modelling carried out to find out how mantle-derived rocks changed composition as a result of the long-term cooling of the mantle.

Their results suggest that during an early warmer period in the history of the Earth – the Archaean period between four and 2.5 billion years ago – there was a greater amount of molten mantle. Phosphorus would have been too dilute in these rocks. However, after a period of time, the Earth cooled sufficiently, helped by the beginning of the plate tectonics, in which the colder outer crust of the planet is subducted back into the hot mantle. With this cooling, partial mantle melts became smaller.

Dr. Grant Cox, an earth scientist at the University of Adelaide and co-author of the study, explains, the result is that “phosphorus will be concentrated in a small percentage of melt, so if the mantle of the earth cools down, the amount of melt your extract is smaller, but that melts the higher concentrations of phosphorus.”

Phosphorus’ role in the oxidation of the Earth

The phosphorus was concentrated and crystallized in a mineral called apatite, which became part of the igneous rocks that are made from the cooled mantle. Ultimately, these rocks reached the surface of the Earth and formed a large part of the crust. When the minerals phosphorus is derived from the crust is mixed with the water in the lakes, rivers and oceans, apatite broke in phosphates, which became available for the development of and the power of the primitive life.

The scientists estimate the mixing of elements from the earth’s crust with seawater in time. They found that a higher level of bio-essential elements in parallel by a large increase of the oxygen content in the Earth’s atmosphere: the Great Oxidation Event (GOE) about 2.4 billion years ago, and the Neoproterozoic Oxygen Event, 800 million years ago, after which oxygen levels were thought to be high enough to support multicellular life.

Even before the GOE, from approximately 3.5 to 2.5 billion years ago, some of the earliest forms of life generated oxygen by photosynthesis. However, during that time, most of this oxygen will react with iron and sulfur in igneous rocks. To understand how these reactions affected the oxygen content in the atmosphere over a period of four billion years, the scientists measured the amounts of sulphur and iron, in igneous rocks, and figured out how much oxygen had reacted. They are facing all of these events with changes in the levels of oxygen in the air. The scientists found that the decreases in sulphur and iron, together with an increase of phosphorus, parallel with the Great Oxidation Event and the Neoproterozoic Oxygen Event.

An explosion of life

All of these events support a scenario in which the cooling of the Earth’s mantle led to the increase of the phosphorus-rich rocks in the earth’s crust. These rocks are then mixed with the oceans, where phosphorus-containing minerals broken up and leached in water. Once phosphorus levels in the water were high enough, primitive forms of life thrived and their numbers increased so that they could generate enough oxygen that most of the reach of the atmosphere. Oxygen reached levels sufficient to support multicellular life.

Dr. Peter Cawood, a geologist at the Monash University in Melbourne, Australia, comments, Astrobiology Magazine that, “it is intriguing to think that the [oxygen] on which we depend for life owes its ultimate origin in the secular decline in mantle temperature, that are believed to have fallen from some of 1,550 degrees Centigrade for about three billion years ago to about 1,350 degrees Celsius today.”

Would a similar scenario be playing out on a possible exo-Earth? With the Kepler discoveries of a growing number of possible Earth-like planets, could any of these support life? Cawood suggests that the finding of potential importance for the development of aerobic life (i.e. the life that develops in an oxygen rich environment) on exoplanets. “This is provided that [phosphorus] within the igneous rocks on the planet’s surface is subject to weathering to ensure that the bio-availability,” says Cawood. “It is important that the phosphorus content of igneous rocks is the highest in the rocks low in silica [rocks formed by rapid cooling] and the rocks of this composition dominate the crust of Venus and Mars, and probably also on exoplanets.”

Cox concludes by saying that “This relationship [rising oxygen levels and the jacket cooling] has implications for terrestrial planet. All the planets will cool, and people with efficient plate tectonics convection cools faster. We concluded that the speed of cooling can influence the speed and pattern of biological evolution on a potentially habitable planet.”

The research was supported by the NASA Astrobiology Institute (NAI) element of the NASA Astrobiology Program, as well as the National Science Foundation Frontiers in Earth System Dynamics Program and the Australian Research Council.

This story was provided by Astrobiology Magazine, a web-based publication sponsored by the NASA astrobiology program. This version of the story published on Space.com.

 

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