Chemists Retrieve Clues to Ancient Ocean Chemistry and Global Greenhouse From Cretaceous Sediments

Global warming. Rising sea levels. Massive volcanic activity around the world. Widespread erosion.

It's not a scene from the latest Hollywood disaster film, "The Day After Tomorrow," but Earth as it appeared during the mid- to late-Cretaceous geological period, 135 million to 65 million years ago, when the largest dinosaurs ruled the planet.

Scientists have long sought clues to Earth's ancient climate from ice cores that go back hundreds of thousands of years. Now, chemists at the University of California, San Diego and Stanford University report in the June 11 issue of Science that they have extended their glimpse of Earth's oceanic and atmospheric past to 130 million years, during one of its greatest upheavals of climatic change.
Their results, the first high-resolution record of changes in seawater sulfate, provide a portrait of the interactions between Earth and its atmosphere during the Cretaceous that should help scientists improve their predictions of how our climate might change as the accumulation of carbon dioxide and other greenhouse gases from human activities warm the planet.

"The planet during the Cretaceous was very different than it is today," says Adina Paytan, an assistant professor of geological and environmental sciences at Stanford and the first author of the paper. "Not only were dinosaurs present, but the climate was extremely warm and global sea levels were significantly higher than they are today. Understanding how the atmosphere, land and ocean system interacted while in this global greenhouse mode is very relevant if we want to understand the fate of our future climate."

"This was a time when there were no glaciers in either the Arctic or Antarctic," says Miriam Kastner, a professor of earth sciences at UCSD's Scripps Institution of Oceanography and a co-principal investigator of the study. "So the record that we have obtained is an unusual portrait of an extreme climate in the Earth's past that will help us develop better predictive models in the future."

"If we can explain the major excursions that occurred 100 million years ago, we can develop good models of what is going to happen in the future," says Mark H. Thiemens, dean of UCSD's Division of Physical Sciences and a co-author of the paper. "This was a period of extremes."

The scientists obtained their high-resolution record of climatic changes during the Cretaceous from sulfur deposited over millions of years in ocean sediments in the mineral barite and retrieved from deep ocean cores. Sulfur in its various chemical forms can provide an uninterrupted record of large scale geochemical processes on Earth's land masses and its oceans, as well as an indirect measure of its atmosphere.

For example, variations in the four most common isotopes, or forms, of sulfur incorporated into sulfide and sulfate minerals give an indirect measure of how much oxygen was present in the atmosphere at the time. Continental weathering, geothermal activity and volcanism have other distinct sulfur isotope signatures, allowing the scientists to infer at specific points in Earth's history what processes dominated the land masses and how they might have affected the atmosphere and climate. Because much of these chemicals eventually made their way to the oceans, the seawater sulfur deposited in ocean floor sediments has proven to be a particularly good way to glimpse geological and atmospheric changes over periods of millions of years.

"The sulfur isotopes provide an insight into carbon cycling in the oceans that includes fluctuations in productivity or organic matter burial," says Kastner. "The intimate coupling between the global biogeochemical cycles of sulfur and carbon constitutes a major control on the level of atmospheric oxygen. Therefore, the study of these cycles and how they have varied with time is important for the history of Earth's environment."

Kastner, Thiemens and Paytan, a former graduate student and postdoctoral fellow in Kastner's laboratory, discovered from their sulfur record that much less sulfur-34 isotope was deposited during the Cretaceous than in the previous 45 million year period of Earth's history, "most likely as a result of extensive volcanic activity," they write in their paper.

The scientists found that from 120 million years ago to 105 million years ago, then again from 95 million years ago to 80 million years ago, the fraction of sulfur-34 dipped even more precipitously, suggesting a sharp reduction in the amount of organic matter buried in the ocean and used by sulfate-reducing bacteria. These changes in the productivity of ocean life suggest that Earth's atmosphere may have gone through fluctuations in the amount of available oxygen, the scientists conclude in their paper.

"One thing that we can learn from this record is that there might have been more rapid changes in the atmosphere of the Cretaceous than we knew about," says Paytan. "Some relatively rapid changes can happen on Earth. So we have to be prepared."

Douglas Campbell, an undergraduate student at UCSD working in Kastner's laboratory also contributed to the study. The research project was funded by the National Science Foundation, which also operates the Deep Sea Drilling Project and Ocean Drilling Program, which made available the sediment cores.