Ancient oceans may have been split for nearly 2 billion years into two distinct layers that resemble conditions in the modern Black Sea, able to support life near the surface but almost devoid of life in deep water, a new study suggests.
The key difference between the layers, according to Harvard University researchers, was the level of oxygen dissolved in the water.
“The pattern of chemistry that we saw was a little surprising -- what it tells us is not only was the oxygen level in the ocean a good deal lower than it is today, it was maybe only a few percent of modern levels,” said biologist Andrew Knoll, co-author of the study published in the June 5 issue of Nature.
Oxygen is fairly evenly distributed in modern oceans, even at great depths, allowing sea life to flourish even thousands of feet below the surface. In ancient oceans, however, oxygen probably was plentiful only in shallow water -- much like the Black Sea, according to the study.
Knoll and Harvard colleague Yanan Shen determined the ancient oxygen levels not by looking in today’s oceans, but by comparing iron and sulfur concentrations in rock samples. The cores, about 1.5 billion years old, were taken from the Roper River basin in Australia.
The rock samples indicated the relative levels of oxygen needed to interact with iron and sulfur to form different chemical compounds at various depths in ancient seas -- an innovative approach to estimating oxygen levels that has been praised by other researchers.
“Until now, there has been no concrete evidence for the existence of two chemically distinct water masses,” said Matthew Hurtgen, a Penn State researcher who wrote an accompanying commentary on the study for Nature.
Scientists believe that there was a major increase in oxygen levels beginning about 2.4 billion years ago, followed by a long stretch called the Proterozoic era before oxygen levels started to rise again roughly 800 million years ago. That increase occurred just before the emergence of large animals and the explosion of great numbers of different species of all sizes.
The Harvard study adds evidence to support the evolutionary time frame because it also estimated the sulfur concentration in the ocean -- an important indicator of atmospheric oxygen vital to the evolution of modern plant and animal life.
The uneven oxygen levels in the two-layer oceans for nearly 2 billion years -- almost half the 4.5-billion-year age of the Earth -- “might have put some very strong brakes on evolution,” Knoll said.
David Des Marais, geochemist at NASA’s Ames Research Center, said the study will help test theories about how ocean chemistry affected evolution.
“The hypothesis that oxygen levels had a lot to do a lot with the timing of the rise of plants and animals is just that, a hypothesis,” Des Marais said.
“But this gives us a handle on another approach to explore the levels of oxygen and how it might translate to biological events -- that’s where we want to go eventually.”