Discoveries by a Virginia Tech doctoral student have provided missing information about how oxygen was able to build up in the earth’s atmosphere two billion years ago, and are helping to trace the history of the ocean between Wyoming and Minnesota.
In the Medicine Bow Mountains of Wyoming, geological sciences student Andrey Bekker discovered rocks with a unique carbon isotope composition. These carbonate rocks were once on the edge of the ocean that lapped at the southern margin of the Wyoming craton (WC) — one of the continents that later formed North America. Working with Juha A. Karhu of the Geological Survey of Finland and Jay A. Kaufman, University of Maryland, College Park, Bekker then located the same characteristic isotopic signatures in carbonate rocks in the Hartville Uplift, the easternmost exposure of the WC, near Cheyenne.
Geologists have known of the ocean south and east of Wyoming, but could not reconcile its history with that of the ocean to the east of the WC. “There are only bits of rock left as evidence,” Bekker explains.
But, having studied the carbonates in the Medicine Bow Mountains, the Hartville Uplift, and the Black Hills, S.D., northeast of the Hartville Uplift, the researchers now suggest that the opening of the ocean between the Wyoming craton and the Superior (Minnesota area) craton happened much later than the ocean opening on the southern margin of the Wyoming craton.
The section that Bekker, Karhu, and Kaufman studied contains carbonates that are enriched in 13C (carbon-13) isotope. This enrichment was detected in a number of successions that are between 2.3 and 2.1 billion years old. "What makes this succession particularly interesting is the presence of organic-rich rocks (shales) within these 13C-enriched carbonates," says Bekker. "These organic-rich shales formed under deep water and generally are not found in successions of this age. Without deep-water protection from oxygen, the organic carbon would not have survived because it would have been converted to carbon dioxide when the ancient algal source decayed. Such deposits have long been expected because otherwise oxygen produced during photosynthesis would not have increased in the atmosphere, but would have been recycled back into carbon dioxide when organic matter decayed," he explains.
Geological studies have long indicated that the amount of oxygen in the atmosphere increased dramatically two billion years ago. "The discovery of carbonates of this age provided a missing link as to when and how oxygen began to be introduced into the atmosphere," Bekker says.
"Based on this isotope signature, we can now trace the boundary of the Wyoming craton and learn how it responded to environmental and sea level changes," Bekker says. "Looking at the changes in the chemical characteristics of the carbonates, we can compare the changes in several areas and separate changes of global significance and of local significance. We can use the isotope features as a time marker, to understand the tectonic history of this area, to put events into a time frame, such as when uplifts occurred or oceans formed."
The shoreline location is still general but the isotopic data allow researchers to say that some event on the WC eastern margin is or is not related to an event on the southern margin. For example, "We found that the breakup on the eastern margin that led to the ocean opening affected the southern margin of the WC, similar to what happened when the Atlantic Ocean opened and resulted in subsidence of the older Mediterranean Sea," Bekker explains.
The researchers also demonstrated that the carbonates in the Hartville Uplift are not as old as previously thought, but are the same age as carbonates in the Medicine Bow Mountains. And stromatolites discovered in 1985 in the Hartville Uplift and assumed to be older than 2.5 billion years old, are "only" 2.1 billion years old. Stromatolites are buildups of blue-green algae, one of the earliest forms of life and the engine for converting carbon dioxide to organic carbon and oxygen, resulting in the carbon ration Bekker discovered.
Andrey Bekker received his Ph.D. in 2001. His dissertation is available online.
