Clues to Archean Skies in Ancient Carbon

Xenon is a noble gas found in tiny amounts in the atmosphere, but this unreactive element has had a turbulent history on Earth. Much of the original atmospheric xenon likely came from a mixture of meteors and comets during the final stages of Earth’s accretion. Later, in the Archean eon, most of the primordial xenon escaped to space as the atmosphere changed and evolved. New research finds evidence of these atmospheric events, and the transition from primordial to modern xenon compositions, preserved inside ancient carbonaceous compounds.

Clues to Archean Skies Trapped in Ancient Carbon

In a new paper in Science Advances [1], David Bekaert, with DCO Reservoirs and Fluxes Community members Michael Broadley and Bernard Marty, (all at Centre de Recherches Pétrographiques et Géochimiques, France), along with colleagues at Sorbonne Université and the California Institute of Technology, analyzed xenon trapped in ancient organic matter, called kerogen. By determining the isotopic composition of xenon in the sample, which is the ratio of atoms with different numbers of neutrons, researchers can estimate the deposition age of the organic material. This technique will enable researchers to date early cells and biologically relevant compounds deposited with Archean formations, independently of the surrounding host rock. The knowledge may refine our estimates of when and where life first emerged on Earth.

Previous studies that looked at tiny bubbles of Archean atmosphere confined inside minerals yielded some clues to xenon’s changing isotopic composition from the Archean eon, 4 to 2.5 billion years ago. These bubbles, however, are rare and hold very little gas for analysis. Researchers have shown previously that organic material tends to hold on to atmospheric noble gases like xenon. Xenon consists of nine long-lived isotopes, and the mix of these different isotopes in the atmosphere has changed over time. In the current study, researchers analyzed the mix of xenon isotopes stuck in ancient carbon to gain a clearer understanding of the Archean atmosphere.

“Xenon’s isotopic composition changed from the very first stages of atmosphere building,” said Bekaert. “It’s an exceptional tool for probing the evolution of ancient Earth's atmosphere.

The researchers isolated kerogen from the 3.0 billion-year-old Farrel Quartzite collected from the Pilbara Craton, an incredibly old chunk of Earth’s crust located in Western Australia. They employed two complementary techniques to analyze the kerogen. First, they used Raman spectroscopy, which uses lasers to determine the molecules present in a sample. In this case, it was useful to show whether a sample was in pristine condition, or if hydrothermal fluids had contaminated the rock with newer carbon compounds, which could make the kerogen appear younger than it really is. Once they certified the kerogen as pristine, they determined the isotopic composition of its xenon through noble gas analysis. Finally, they matched up the xenon profile in the kerogen with the evolution curve of atmospheric xenon isotopes to calculate when the carbon compounds formed.

The analyses showed that the kerogen in the ancient rock was unadulterated, and that the trapped xenon was 2.98 billion years old, in excellent agreement with the age of the surrounding rock.  “We were very surprised to find such a beautiful Archean signature,” said Bekaert.

This technique will enable researchers to determine the age of organic material independently of its host rock, and has the potential to clear up disputes over the age of purported ancient fossil cells. It also can help researchers nail down the timing of when Earth began recycling surface carbon into the interior through subduction, based on the isotopic profiles of xenon trapped in mantle rocks and diamonds. One limitation of the technique, however, is that it can only be applied to ancient carbon because xenon’s isotopic composition has not changed in the last 2 billion years.

Changes in the isotopic composition of xenon likely are linked to the early atmosphere’s hydrogen content. Hydrogen gradually escaped into space leading up to the Great Oxygenation Event, when atmospheric oxygen levels began to rise, starting around 2.5 billion years ago. Atmospheric hydrogen and oxygen levels are important because a less oxidized atmosphere is more conducive to producing the organic compounds necessary to give rise to the earliest cells.

Now, the researchers are using this technique to date organic carbon from additional samples, including some that are even older than the Farrel Quartzite. They hope to form a clearer picture of how xenon has evolved in the atmosphere through time and to improve their ability to date ancient kerogens.

Image: Xenon gas trapped by organic matter in primeval rocks from Pilbara Craton in Western Australia provides a new way of exploring Earth’s early atmosphere. Credit: Bernard Marty

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