Earth’s Long-Term Habitability Relies on Chemical Cycles. How Can We Better Understand Them?

We, and all other complex life, require stability to evolve. Planetary conditions needed to be benign and long-lived for creatures like us and our multicellular brethren to appear and to persist. On Earth, chemical cycling provides much of the needed stability.

Chemical cycling between the land, atmosphere, lifeforms, and oceans is enormously complex and difficult to study. Typically, researchers try to isolate one cycle and study it. However, new research is examining Earth’s chemical cycling more holistically to try to understand how the planet has stayed in the ‘sweet spot’ for so long.

Earth has supported complex life for hundreds of millions of years, possibly for more than a billion years. This is extremely rare, as far as we can tell. The vast majority of the exoplanets we’ve discovered are not in their stars’ habitable zones. They have very little chance of hosting any life, let alone complex life.

It’s possible that some planets experience a period of stability for much shorter periods of time than Earth. This may describe Mars. It was warm and wet and could’ve hosted simple life, but the planet lost most of its atmosphere and became uninhabitable. Now it’s cold, dry, and dead.

Earth robustly cycles the chemical elements through different systems and has done so for billions of years. Now, about 4.5 billion years after its formation, life is abundant on our precious planet. Biogeochemical cycles like the carbon cycle, the nitrogen cycle, and the methane cycle have allowed the planet to sustain its habitability.

New research published in the Proceedings of the National Academy of Sciences examines these cycles holistically, hoping to better understand the relationships between them. The research is “Balance and imbalance in biogeochemical cycles reflect the operation of closed, exchange, and open sets.” The lead author is Preston Cosslett Kemeny, a University of Chicago TC Chamberlain postdoctoral fellow.

“Overall, this work provides a systematic conceptual framework
for understanding balance and imbalance in global biogeochemical cycles.”

From “Balance and imbalance in biogeochemical cycles reflect the operation of closed, exchange, and open sets.”

Earth’s carbon cycle plays a dominant role in the climate. As carbon accumulates in the atmosphere, the planet warms. As carbon is sequestered into the mantle, the planet cools. Even though it’s been stable for a long time, research shows that small imbalances can upset the system.

What Kemeny and his co-researchers wanted to do was get back to the basics. They wanted to identify a framework for all the reactions, both large and small, that comprise Earth’s chemical cycles. What’s different in their work is that they didn’t specify how they all worked together, if they worked together, or how much they affected one another.

“Our approach provides a new way to identify the fundamental building blocks of stability in the chemical components of Earth’s climate—the underlying ways in which the climate can be stabilized over geological time due to the movement of elements across the ocean, atmosphere, and rock reservoirs,” said Kemeny.

Earth’s long-lasting habitability created the conditions for complex life like us to appear. That habitability is dependent on the complex interplay of chemistry between the ocean, atmosphere and land. This image, captured from the International Space Station 400km above Earth’s surface, shows our planet’s thin atmosphere. Image Credit: NASA

The researchers describe their effort as ‘agnostic’ and explain that it creates “… a systematic and simplified conceptual framework for understanding the function and evolution of global biogeochemical cycles.” They call it agnostic because it doesn’t specify the relationship between environmental conditions and the strength of biogeochemical processes. “By remaining agnostic to the relationships between environmental conditions and the intensity of biogeochemical processes, we sought to recognize and systematize patterns that underly the stability of major element cycles,” Kemeny explains on his website.

“This is an elegant, simplified way to think about an enormous problem, which organizes a lot of previous research on elemental cycles into packages of chemical reactions that can be balanced and understood,” said University of Chicago Assistant Professor Clara Blättler, senior author of the paper.

The complexity of Earth’s cycles makes them difficult to study. They work on long geological timescales, which puts us at a disadvantage. The planet’s carbon cycle illustrates this.

The movement of carbon plays an important role in regulating the planet’s climate. When carbon dioxide accumulates in the atmosphere, the atmosphere traps more heat, which warms up the oceans. However, carbon also creates a weak acid called carbonic acid that breaks rocks down faster. The carbon eventually finds its way to the ocean floor and becomes sequestered in rock. Carbon can also spend some time as part of living things before being sequestered into rock or fossil fuels. This sequestration of carbon eventually cools the planet but takes millions of years. Carbon is eventually returned to the atmosphere by volcanoes and by the burning of fossil fuels.

The Carbon cycle plays a dominant role in moderating Earth’s climate, but other chemical cycles influence it. Image Credit: U.S. DOE, Biological and Environmental Research Information System.

Trying to understand the carbon cycle is made more difficult by its interaction with other cycles. The Earth’s cycles also aren’t static. They change over time, adding to the complexity. There are also missing pieces from the large puzzle of Earth’s cycles. Researchers are forced to make assumptions to fill in the blanks.

Kemeny devotes much of his time to understanding Earth’s cycles, and he and his colleagues hope that their approach can yield better results. “Models of global element cycles seek to understand how biogeochemical processes and environmental conditions interact to sustain planetary habitability,” Kemeny writes on his website. “However, outcomes from such models often reflect specific interpretations of geochemical archives.”

The researchers think their approach may help overcome the obstacles to understanding Earth’s cycles. They employed a mathematical analysis to develop a framework identifying all of the major and minor cycles that contribute to Earth’s long-term habitability by balancing the carbon cycle.

The result was a new, more holistic way to look at Earth. The climate can be represented by a large set of interconnected chemical equations. These equations must balance over certain time periods to keep the carbon cycle stable and the Earth habitable.

The Sulphur Cycle is just one of Earth’s important cycles. It moves sulphur between rocks, water, and living things. Kemeny and his colleagues are trying to understand all of Earth’s cycles holistically rather than in isolation. Image Credit: By Bantle – Own work, CC0,

Kemeny highlights one episode in Earth’s climate history to illustrate the point. The Cenozoic era began about 65.5 million years ago and is the era we live in now. The Cenozoic is a long-term cooling trend in Earth’s history, and the period that preceded it was a greenhouse climate. Kemeny and his colleagues say that their holistic approach can open a window into how the climate changed.

“For example, say that you are considering a hypothesis for why the climate changed in the past – such as the major cooling of the last 65 million years,” Kemeny said. “You can take this framework and use it to say: well, if X process increased or decreased, then it should have also caused Y to happen, or would have needed to be balanced by Z, and that you have to account for those outcomes—so with that prediction we can look for evidence for the joint operation of the whole geochemical system.”

Astrobiology and planetary habitability are key topics in space science. With the help of the JWST and other upcoming observatories and telescopes, scientists are getting a look at the atmospheres of distant exoplanets. But it’s a difficult process, made more difficult by our less-than-complete understanding of our own planet’s habitability. Understanding our own planet can help us better understand exoplanets.

But there’s a certain type of joy in understanding Earth for its own sake, and this new holistic approach should grow our understanding.

“We hope it’s a beautiful way to help understand all the chemistries that are involved in making Earth a safe place for life to evolve,” Blättler said.

“Overall, this work provides a systematic conceptual framework for understanding balance and imbalance in global biogeochemical cycles,” the authors conclude.

Evan Gough

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