Earth's early ocean cooled more than a billion years earlier than thought: Stanford study
The scalding-hot sea that supposedly covered the early Earth may in fact never have existed, according to a new study by Stanford University researchers who analyzed isotope ratios in 3.4 billion-year-old ocean floor rocks. Their findings suggest that the early ocean was much more temperate and that, as a result, life likely diversified and spread across the globe much sooner in Earth's history than has been generally theorized.
It also means that the chemical composition of the ancient ocean was significantly different from today's ocean, which in turn may change interpretations of how the early atmosphere evolved, said Page Chamberlain, professor of environmental earth system science.
When rocks form on the ocean floor, they form in chemical equilibrium with the ocean water, incorporating similar proportions of different isotopes into the rock as are in the water. Isotopes are atoms of the same element that have different numbers of neutrons in the nucleus, giving them different masses. However, because the exact proportion of different isotopes that go into the rock is partly temperature dependent, the ratios in the rock provide critical clues into how warm the ocean was when the rock formed.
Previous studies of similarly aged rocks had looked only at oxygen isotope ratios, which suggested that in the Archean era (about 3.5 billion years ago), the ocean temperature was at least 55 degrees Celsius and may have been as high as 85 C, or 185 F. At a water temperature so perilously close to the boiling point, the only organisms that could have thrived would have been extremophiles – life forms adapted to extreme environments – such as the microbes that live in the intense heat of deep-sea hydrothermal vents or in hot springs such as at Yellowstone National Park.
But isotope ratios recorded in rocks on the ocean floor are also dependent on the chemical composition of the seawater in which those rocks formed, and the past studies assumed the composition of the ancient ocean was essentially what it is today [WB: *searing cry of soul destroying pain* with a blink tag!], which the Stanford study did not.
Using a relatively new approach, Michael Hren and Mike Tice, both Stanford graduate students at the time, analyzed hydrogen isotopes as well as oxygen isotopes in chert, a type of fine-grained sedimentary rock consisting primarily of quartz. The chert they studied was from an ancient deposit, formerly underwater but now on dry land in South Africa.
"By looking at both oxygen and hydrogen in these ancient rocks we were able to put some constraints on how different the ancient ocean composition may have been from today, and then use that composition to try to determine how hot the ancient ocean was," said Hren, who is the lead author of a paper describing the work being published online Nov. 12 by Nature. Tice and Chamberlain are coauthors.
Having data from isotope ratios of two elements allowed the researchers to calculate upper and lower bounds for the range of temperature and composition that could have given rise to the observed ratios. They determined that the ocean temperature could not have been more than 40 C (104 F) – the temperature of a hot tub – and may have been lower in some parts.
"This means that by 3.4 billion years ago, there were at least some places on the surface of the Earth where organisms that could not survive in these hot hydrothermal conditions could exist and thrive," Hren said. "It also suggests that the chemical composition of the ancient ocean was probably not identical to today, as previous studies assumed. It may have been quite different."
The researchers found that the ratio of the two stable isotopes of hydrogen in the chert was tilted away from the heavier of the isotopes – called deuterium.
"The ancient ocean had a lot more hydrogen in it, relative to deuterium, than modern oceans," Chamberlain said.
If the composition of the Archean ocean was significantly different from today, then the atmosphere must have been markedly different, too, owing to the ease with which gases move across the air-water boundary as the ocean and lower atmosphere strive to stay in a rough equilibrium.
That means that sometime during the past 3.4 billion years, the ocean had to lose a lot of hydrogen to the atmosphere to bring the hydrogen isotope ratio in seawater to where it is today. And since oxygen, not hydrogen, has built up in Earth's atmosphere over that same period of time, the atmosphere must have discharged a lot of hydrogen to the only other place it could go: space.
Hren said that some recent models of the early Earth atmosphere suggest that there may have been a prolonged period of hydrogen escaping to space, which would be consistent with the Stanford team's findings.
Oh hohoho! Talk about timing.
Also. Uh. DO NOT CUT N PASTE MODERN VALUES(*) INTO PALEOENVIRONMENTS! FOR CRYING OUT LOUD!
Update: Paper link.
* atmospheric contents, oceanic contents, continental mass, etc.
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