Because life began in our planet's oceans, understanding the nature of ancient seas is key to unraveling the many mysteries surrounding Earth's biological infancy.
But getting a solid sense of old oceans — and the early organisms that gradually began to thrive in them — isn't easy. A key question involves determining when dissolved oxygen, an element crucial to life, may have emerged in the primordial deep.
Some scientists say oxygen was likely present from the beginning, or at least from the initial rise of our atmosphere some 2.3 billion years ago. Others, such as MU's Tim Lyons, an associate professor of geological sciences, suspect that oxygen likely entered the deep ocean later, perhaps lagging behind our oxygenated atmosphere by as much as a billion years.
Establishing the time line is important, Lyons explains, because it will help scientists get a handle on when "eukaryotes" — simple organisms whose cells have nuclei containing genetic material — began to emerge. "They were just starting to get a foothold," Lyons says. "Many people would consider the time period of 1.8 to 1.0 billion years ago as being an interval of tremendous importance for eukaryotes as they were establishing themselves on the earth." Eons later these same eukaryotes evolved into higher organisms, humans included, "so it's really critical for us to understand the environment present when these things were first evolving," Lyons adds.
To better explore this environment, Lyons, along with Gail Arnold and Ariel Anbar of the University of Rochester, developed an innovative method of analyzing oxygen levels in the geological record. In the past, researchers could identify ancient deficiencies in seawater oxygen only one location at a time. But the new method provides estimates of oxygen availability throughout the entire ocean by looking at how molybdenum isotopes of different masses are distributed in marine sediments.
Molybdenum is an abundant and biologically essential element that, when dissolved in seawater, can stay in solution for many thousands of years. This makes molybdenum particularly appealing to researchers: Since it is well mixed into all seawater, scientists can use a relatively small set of samples to draw conclusions about ancient oceans as a whole. Previous findings had always been limited to ocean conditions at specific locations.
Molybdenum is eventually removed from seawater through sedimentation. Because variations in the level of oxygenated seawater present during sedimentation are recorded in ratios of the molybdenum isotopes, Lyons and his colleagues are able to use sedimentary rocks to create a window into oceanic oxygen levels both past and present.
The team looked first at samples from the modern seafloor, among them mud from oxygen-deprived, or "anoxic," locations in the depths of the Black Sea. These were then compared to 1.5 billion-year-old shale from northern Australia, which once lay at the bottom of a long-extinct ancient ocean. The data confirmed that much of the ancient ocean was indeed anoxic during this period. The researchers were then able to conclude that mid-Proterozoic seawater probably contained significantly more hydrogen sulfide than do modern oceans. Under these conditions, the researchers reason, only limited areas in the ocean would have supported diverse life at the time eukaryotes began to expand.
"We are providing the environmental context for understanding the ecology of early life," Lyons says. "By interpreting that ecology, we understand what controlled evolution." Their findings were published April 2 in the journal Science.