“Time Capsule” Rocks Uncover the Early History of the Earth and the Ocean

two photos of Earth side by side
Zircons that formed 4 billion years ago may hold clues as to what the Earth looked like in its early history and how it went from a fiery ball of lava to the planet we know today. (Images from Wikimedia Commons, compiled by Naomi Greenberg)

Humans have little evidence of what the Earth looked like early in its existence. Today, the planet is covered in a vast ocean, but billions of years ago that may not have been the case. Scientists have long wondered how and when the ocean formed, and they are still trying to piece together that history. When trying to understand the ancient history or “deep time” of our planet, we often look to geology to tell us what the continents, oceans, and atmosphere were like; rocks hold the key to understanding our past. 

But very little physical material has survived from the early period after Earth’s formation 4.5678 billion years ago, making it difficult to paint a picture of this environment. What we do have left from this period is a smattering of small crystals scattered through larger rocks. Some of these microscopic minerals— none of which are larger than a few widths of a human hair— are 4.37 billion years old. All these  4-billion-year-old crystals, called zircons, could fit in the palm of a hand. 

Despite their tiny size, zircons are a large store of information. When they first form from magma, zircons incorporate elements from the chemical environment around them. The amount and type of each element can give scientists clues about when the rocks formed, what temperature and pressure they formed at, and what other minerals might have been present at the time. They are considered a naturally occurring “time capsule” since they are resilient to weathering, even over billions of years. This means that by studying the chemistry of a zircon, scientists today can interpret what the chemistry of the surrounding environment was like when they first formed. 

Preview a striated rock
This piece of the Acasta Gneiss complex, collected by Dr. Samuel A. Bowring, contains tiny zircon crystals that hold information about the Earth’s early history. (Chip Clark, Smithsonian Institution)

“They store most of the Earth’s story,” said Wriju Chowdhury, a Smithsonian researcher who hopes to learn more about what the first 500 million years of Earth’s history looked like.  

Scientists like Chowdhury use a variety of lines of evidence to construct this “story.” At the molecular level, zircon, which is short for zirconium silicate or ZrSiO4, forms a crisscrossing lattice structure that can hold other elements, like vanadium or uranium, within the grid’s pockets. The quantity and chemical characteristics of these other elements can reveal what the world was like when they formed. For example, the ratio of lead to uranium within the zircon indicates the age of the zircon since uranium, a radioactive element, naturally decays into lead at a constant rate. The more lead, the older the zircon. Once the age of the zircon is known, researchers can look at the rocks in and around the zircon to paint a picture of what the earth looked like at that moment in time. They can recreate these conditions in a lab to see if they can give rise to the same type of zircon.  

In 2001, using these methods, a team of scientists found a rock from Jack Hills in Western Australia that contained zircons. These zircons had a specific ratio of the different types, or “isotopes”, of oxygen, that indicated there were clays surrounding the zircon when it formed. And to make clays, you absolutely need one thing: water. 

Preview a molten planet surface
The first 500 million years of Earth’s history were generally thought to be hot, fiery and unfavorable to any form of life. (Artistic Impression by Tim Bertelink, Wikimedia Commons)

The evidence for water in zircons was surprising, as the field had previously speculated that the early Earth was a barren “hellscape” covered by volcanoes, pools of magma, and subject to asteroid bombardment. After a Mars-sized object collided with Earth, forming the moon, this picture of a furious, fiery Earth seemed likely. The name of the first 500 million years of Earth’s history — the “Hadean Eon” even references this notion with a nod to the Greek god of the underworld, Hades. 

Additional experiments on this rock have made scientists even more confident there was water — and perhaps even oceans  — on Earth 4.3 billion years ago. Since titanium levels in zircons are directly related to the temperature at which the zircon formed, scientists were able to measure the titanium content in the zircons and then determined the zircons crystallized at a temperature of 700 degrees Celsius. This temperature, which is actually “cold” relative to the Earth’s mantle, combined with knowledge about how crystals form within cooling magma indicated that the zircons formed within granite saturated with water.  

Finding evidence for early granites was also exciting in and of itself because granites are thought to play a key role in sustaining life on our planet. Granite can experience weathering, releasing sodium — to make saltwater —  calcium — to make hard shells like those of many marine animals —  and phosphorus — to provide cellular energy. When calcium is released, it can form calcium carbonate and thereby decrease the carbon dioxide in the atmosphere, a process thought to be necessary for the origin of life.

Preview two scientists sit at a computer in a lab with scientific equipment
Michael Akerson and Chowdhury analyze samples, such as zircons, in the Mineral Sciences department at the National Museum of Natural History. (Smithsonian Institution)

According to Smithsonian geologist Michael Ackerson, “basically what we pieced together from minerals just from this rock is that the Earth had most of the components to potentially make it habitable, as far back as we can go back in time with the samples that we have.”

Work with zircons ties into answering other questions, like how the first biological molecules might have formed on the surface of clays, or how the planet developed the plate tectonics we see today. Plate tectonics provide an efficient mechanism to recycle material — such as nutrients and carbon dioxide — between the deep Earth to the atmosphere, and are thought to be necessary for life on Earth as we know it today.  

While studying zircons has allowed huge leaps in our understanding of early Earth and the origin of the oceans, there are still so many questions to be answered. “All we can tell you so far is that there were oceans and there was granite. If there are a million things characterizing the early Earth, we can confidently say three or four things,” Chowdhury said.  

Preview a person stands on a rock ledge and looks at the striations in the rock
Ackerson and Chowdhury plan to visit the Acasta Gneiss Complex this summer to in search of – four-billion-year-old zircons. (NTGS)

In the summer of 2024, Chowdhury and Ackerson have planned an excursion to the Acasta Gneiss rock complex in a remote region of the Northwest Territories in Canada. They are hoping to find zircons older than 4 billion years old to help advance the early Earth field. The samples Ackerson and Chowdhury collect from Acasta Gneiss will be housed at the Smithsonian National Museum of Natural History and will be made available to any researcher studying zircons and the early Earth.

This research will support the museum’s “Our Unique Planet” research initiative that seeks to answer fundamental questions about the origins of life, the ocean and the continents on Earth.  

No matter what they find, Ackerson says he is excited about the outcome. “Having another piece of information that adds color to the story of the Earth will help us bring together a picture of what it was like,” Ackerson said. “This is going to be a huge step forward.” 

May 2024