A key part of the hypothesis for the origin of life on Earth is that it developed in the oceans. The conventional model of the early oceans has stipulated that the waters of the middle Archean eon, roughly 4.0 to 3.5 billion years ago, were too hot for (abundant) life. The model also assumed that the composition of the ocean water was more or less the same as it is now. Both of these conditions are challenged by new research.
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.
“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.
This research changes important assumptions about when the Earth’s oceans may have become hospitable to life, pushing the potential timeline further back – possibly into the range of 3.6 – 3.7 billion years. However, the findings have yet to come under sustained scrutiny and verification from further research, which means that the results are tentative.