Early water on Earth
Geologists have long thought that Earth’s first 500 million years were as hot as Hades, dubbing this time frame the Hadean. The high temperatures would have prevented liquid water from condensing on the surface. But new findings on zircon grains, Earth’s oldest known terrestrial materials, suggest that the Hadean might have hosted liquid water.
Recovered from the metamorphosed sediments of the Jack Hills in western Australia, the zircon grains are dated to be more than 4 billion years old and are the only geological evidence available to provide insight into the first 500 million years of Earth’s history.“This period is also considered the dark ages of the Earth because there are no known rocks that have been preserved,” says John Valley from the University of Wisconsin-Madison, lead investigator in the Jack Hills zircon research. Presenting at the Geological Society of America’s annual meeting in Denver last October, Valley and his colleagues suggest geochemical clues in the zircons provide the surprising news that early Earth was cool enough for liquid water to exist.
Eranondoo Hill in the Jack Hills of western Australia contains zircon grains as old as 4.4 billion years. A team led by John Valley of the University of Wisconsin-Madison (left) found surprising clues in the grains. Also pictured are Aaron Cavosie of the University of Wisconsin-Madison and Simon Wilde of Curtin University. Photo by David Valley.
In the late 1990s, Valley and his co-workers realized that zircons accurately preserve their original oxygen isotope values, and they decided to document zircons through all of geological time. This realization prompted their discovery of the oldest Jack Hills zircon, a 4.4-billion-year-old detrital grain. The Hadean hypothesis holds that Earth had not yet developed any source materials other than molten magma generated from the interior or from a meteorite bombardment. When the team first placed the grain on the ion microprobe, they expected it to have oxygen isotope ratios of a zircon crystallized in a rock that would in turn have mantle geochemical signatures. But the values of the grain’s oxygen isotopes were much higher than they expected.
“Rocks that have zircons with higher oxygen isotope values indicate a source that has interacted with water at low temperatures,” says Aaron Cavosie, also from the University of Wisconsin-Madison. The new isotope values have two implications, Cavosie says. First, they suggest that water existed as early as 4.4 billion years ago. Because water is a prerequisite for life, this research potentially pushes back the time that life could have originated. Prior to this research, the first known rock evidence for water on Earth was from the Isua metasediments in Greenland, which date to 3.8 billion years ago (Geotimes, July 2002).
Second, the data suggest that the zircons formed in a source rock that was contaminated with material that had interacted with water at low temperatures — that is, rock close to the surface. The trace element geochemistry and quartz inclusions of the zircons corroborate the oxygen isotope data, and point to an evolved rock, much like a granite, indicating that continental crust probably existed on early Earth.
“These are the first real data that suggest that there was supracrustal material that early on Earth,” says co-author William Peck of Colgate University. “In addition, the possibility of liquid water — and perhaps oceans — this early in the planet’s formation is really exciting. It has implications for when catastrophic bombardment of the Earth by meteorites likely waned.” Meteorite bombardments are heat sources, so the idea that early Earth was cool enough for water raises questions regarding the timing and intensity of bombardments.
Alex Halliday, a geochemist from the Institute of Isotope Geology and Mineral Resources in Zurich, suggests caution about the findings. “They are not being outrageous or even factually incorrect. It is just that there are inevitable, fundamental assumptions involved in scaling up a portion of a single grain of zircon to the existence of continents and oceans.”
Valley and his colleagues agree. “The cool early Earth hypothesis is controversial, and we are working very hard to test it,” Valley says. Whereas in 2001 they had only one zircon older than 4.3 billion years, they now have several, which they are using to test their interpretations.
The Jack Hills zircons are proving to be a treasure trove for understanding early Earth. “The most important aspect is that we now have an archive to work on, albeit limited,” Halliday says. “The past 10 years have seen major revisions in our understanding of the earliest Earth, and this field has been viewed as one of the most interesting and rewarding today in terms of significant discoveries.”