by Kerri Spuller, GSA Science Communication Intern – MS Geoscience Student, Boise State University
Earth’s first 500 million years, named the Hadean after the Greek god of the underworld, was first depicted as a fiery hell with volcanoes and meteorite impacts dominating the landscape. Although it’s hard to imagine life existing under these harsh conditions, evidence for water and an atmosphere during this early period of Earth has raised questions about the processes that shaped our planet, as well as the timing of the emergence of life.
In 2015, Dr. Bell and fellow researchers from UCLA published the first evidence of Hadean life, extending the known record of life on Earth into an eon once thought of as inhospitable. The discovery was part of an on-going project that is using some of the oldest minerals on Earth to understand early geologic, hydrologic, and atmospheric processes. But without a rock record, where do researchers search for Hadean life?
“We are investigating sand grains, extracting information about where they came from and what they’re telling us about early periods of Earth’s history.” The “sand grains” Dr. Bell is referring to are detrital zircons, tiny fragments of minerals that have been preserved in younger rocks. Zircons are some of the strongest minerals on Earth, and therefore also some of the oldest. The zircons examined in this study are from the Jack Hills in western Australia, where about three to five percent of zircons are older than four billion years.
Zircons can also encase and preserve other less resistant minerals. When Dr. Bell, a graduate student at the time, and her colleagues found a piece of graphite, a very soft mineral, fully enclosed in a 4.1-billion-year old zircon grain they were excited to find out what it could tell them about early Earth.
Graphite is composed of carbon, the element found in all life on Earth. Carbon’s radioactive isotope, C-14 (or radiocarbon) is commonly used for dating organic materials. Unlike radiocarbon, carbon isotopes C-12 and C-13 are stable, meaning that they do not decay, and can be used to trace carbon farther back in the geologic record. When plants photosynthesize, they use the lighter C-12 from the environment. Because of this, carbon produced from biogenic sources has lighter isotopic signatures than other sources of carbon, such as meteorites. Researchers can determine the source of carbon by analyzing the graphite’s isotopic composition, but first the tiny grain needed to be removed from the zircon that encased it.
The graphite extraction process was a group effort, with a team of researchers from UCLA and Stanford working with surgical precision on the 10 by 30 micron grain, about “the width of a hair on your head.” The graphite needed to be extracted with a focused ion beam, x-ray imaged, and mounted, all without being contaminated in the process. Once successfully mounted, UCLA researchers were finally able to analyze its composition.
The graphite, once encased in a 4.1-billion-year old zircon, had an isotopically light carbon signature, suggesting that it may have formed from an early life form during the Hadean.
The idea of a hospitable early Earth first began in the early 2000s, with evidence that surface temperatures may have been low enough to sustain water as early as 100 million years into Earth’s history. Although the existence of water suggested the possibility of Hadean life, until this discovery in 2015 the earliest evidence of life was 3.8 billion years old, 200 million years after the end of the Hadean.
The scientific community’s response has been “excited, with a fair amount of skepticism – that right now is warranted,” Dr. Bell describes. This discovery would extend the known record by 300 million years, but with only one data point indicating that there may have been life as early as 4.1 billion years ago, more work is needed to support this hypothesis.
To do this, researchers continue to sift through the several thousand zircons collected from the Jack Hills and other nearby greenstone belts. Currently two more graphite inclusions have been identified, one of which has been dated and is older than 4 billion years. Analysis of additional samples will build a more complete record of carbon isotope ratios, which will be used to rule out alternative hypotheses, such as carbon from meteorites.
Dr. Bell expects to find more Hadean-age zircons in the coming months and years, and in those zircons, inclusions of graphite and other minerals that can be used to understand early Earth. Her current research also includes studying modern zircons as analogs to better understand what else Hadean zircons can tell us. By studying zircons from known depositional and metamorphic settings, she is developing methods for differentiating between primary and secondary mineral inclusions, and determining how well inclusions are preserved during transport, diagenesis, and metamorphism.
Her work is changing our view of early Earth by studying a rock record so old that now it only exists in small fragments. “The earth is very good at turning mountains into sand grains, but I want to take the sand grains and reconstruct the mountains.”
Dr. Bell presented her recent work on the relationships between mineral inclusions and whole rock geochemistry in her presentation “Mineral inclusion assemblage and composition for detrital zircon provenance” at the GSA Annual Meeting in Seattle, October 2017.
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The GSA Science Communication Internship was a program offered at the GSA Annual Meeting in Seattle, WA, designed for student attendees interested in science communication as a possible alternative career path. Interns were paired with GSA’s Science Communication Fellow in order to gain experience in making science clear and exciting, under the tutelage of a professional writer. Students were assigned to conduct interviews with presenters at the meeting and to compile summaries capturing the significance of the presenters’ work for a non-technical audience. Media assignments and mentoring were useful learning experiences and exposure opportunities for students seeking to expand their knowledge into geoscientific reporting.
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