Chemical signatures in shale rocks, a consolidated form of mud, point to an increased rate in the rise of land above the ocean 2.4 billion years ago—possibly triggering dramatic changes in climate and life.
In a study published in the journal Nature, researchers from six universities, including the University of Chicago, report that shales sampled from around the world contains archival-quality evidence of fleeting, almost imperceptible traces of rainwater that caused weathering of land as old as 3.5 billion years ago.
The exposure of new land to weathering may have set off a series of glacial episodes and atmospheric changes spawned by the Great Oxygenation Event, in which free oxygen filled the air, said University of Oregon geologist Ilya Bindeman, who led the study.
The evidence is from analyses of three oxygen isotopes, particularly the rare but stable oxygen-17, in multiple shale samples from every continent and spanning 3.7 billion years of Earth's history. Shale rocks are formed by the weathering of crust, so "they tell you a lot about the exposure to air, light and precipitation,” Bindeman said.
Notable changes in the ratios of oxygen-17 and 18 with more common oxygen-16 allowed researchers to read the chemical history in the rocks. In doing so, they were able to establish when the pattern of precipitation on continents switched from near-coastal to more inland, reflecting the transport of moisture over vast swaths of emerged lands as the continents rose above seawater and high-mountain ranges and plateaus were created.
“It is mind-boggling to think that we still find a record of something as evanescent as rainwater in rocks as old as 3.5 billion years old,” said co-author Nicolas Dauphas, head of the University of Chicago Origins Laboratory and professor in the Department of Geophysical Sciences and the Enrico Fermi Institute. “There are a number of challenges to applying this oxygen isotope proxy to ancient rocks, but our study shows that there was a clear change in the pattern of precipitation on continents at a time that coincided with the oxygenation of Earth’s atmosphere approximately 2.4 billion years ago.”
The measurements could help resolve previous arguments whether the emergence of land between 1.1 and 3.5 billion years ago was gradual or stepwise, scientists said. Based on his own previous modeling and other studies, Bindeman said, total landmass on the planet 2.4 billion years ago may have reached about two-thirds of what is observed today.
Chemical weathering on the newly emerged land would have begun to consume carbon dioxide and changed the climate.
“We still need to figure out how everything ties together, but this is a very exciting discovery that opens many avenues of research,” Dauphas said.
Citation: “Rapid emergence of subaerial landmasses and onset of a modern hydrologic cycle 2.5 billion years ago.” Bindeman et al, Nature, May 23, 2018. Doi: 10.1038/s41586-018-0131-1
Funding: National Science Foundation, Natural Sciences and Engineering Research Council of Canada, NASA.