As two rovers scour Mars for signs of water and the precursors of life, University of Rochester geochemists have uncovered evidence that Earth’s own ancient oceans were much different from today’s. The research, published in the journal Science, cites new data that shows that Earth’s life-giving oceans had less oxygen dissolved in them than today’s and could have been nearly devoid of oxygen for a billion years longer than previously thought. These findings may help explain why complex life barely evolved for billions of years after it arose.
The Rochester team has pioneered a new method that reveals how ocean oxygen might have changed globally. Most geologists agree that there was virtually no oxygen dissolved in the oceans until about 2 billion years ago, and that they were oxygen-rich during most of the last half billion years—but there has always been a mystery about the period in between. Geochemists developed ways to detect signs of ancient oxygen in particular areas, but not in the Earth’s oceans as a whole. The Rochester team’s method, however, can be extrapolated to grasp the nature of all oceans around the world.
“This is the best direct evidence that the global oceans had less oxygen during that time,” says Gail Arnold, a doctoral student of earth and environmental sciences at the University of Rochester and lead author of the research.
Arnold examined rocks from northern Australia that were at the floor of the ocean over a billion years ago, using the new method developed by her and her University of Rochester coauthors, Jane Barling of the University of British Columbia, and Ariel Anbar, associate professor of earth and environmental sciences. Previous workers had drilled down several meters into the rock and tested its chemical composition, confirming it had kept original information about the oceans safely preserved. The Rochester team members brought these rocks back to their labs where they used newly developed technology—called a Multiple Collector Inductively Coupled Plasma Mass Spectrometer—to examine the chemistry of molybdenum’s isotopes within the rocks.
Molybdenum is an element that enters the oceans through river runoff, dissolves in seawater, and can stay dissolved for hundreds of thousands of years. By staying in solution so long, molybdenum mixes well throughout the oceans, making it an excellent global indicator. It is then removed from the oceans into two kinds of sediments on the seafloor: those that lie beneath waters that are oxygen-rich, and those that are oxygen-poor. Working with coauthor Timothy Lyons of the University of Missouri, the Rochester team examined samples from the modern seafloor, including the rare locations that are oxygen-poor today. They learned that the chemical behavior of molybdenum’s isotopes in sediments is different depending on the amount of oxygen in the overlying waters, and as a result that the chemistry of molybdenum isotopes in the global oceans depends on how much seawater is oxygen-poor. They also found that the molybdenum in certain kinds of rocks records this information about ancient oceans. Compared to modern samples, measurements of the molybdenum chemistry in the rocks from Australia points to oceans with much less oxygen.
How much less oxygen is the next question. A world full of anoxic oceans could have serious consequences for evolution. Eukaryotes, the kind of cells that make up all organisms except bacteria, appear in the geologic record as early as 2.7 billion years ago. But eukaryotes with many cells—the ancestors of plants and animals- did not appear until much later- about the time that the oceans became rich in oxygen, around half a billion years ago. With paleontologist Andrew Knoll of Harvard University, Anbar previously advanced the hypothesis that an extended period of anoxic oceans may be the key to why the more complex eukaryotes barely eked out a living while their prolific bacterial cousins thrived. Arnold’s study is an important step in testing this hypothesis.
“It’s remarkable that we know so little about the history of our own planet’s oceans,” says Anbar. “Whether or not there was oxygen in the oceans is a really straightforward chemical question that you’d think would be easy to answer. It shows just how hard it is to tease information from the rock record and how much more there is for us to learn about our origins.”
Figuring out just how much less oxygen was in the oceans in the ancient past is the next step for the Rochester researchers. They plan to continue studying molybdenum chemistry to answer this question, with continuing support from the National Science Foundation and NASA, the agencies that supported the initial work. The information will not only shed light on our own evolution, but may help us understand the conditions we should be looking for as we search for life beyond Earth.