This open pit mine in Anshan, China, contains rocks from the Archean Eon, 4 billion to 2.5 billion years ago, that help pinpoint when rivers began operating on a global scale.
Second Geological Brigade/Hebei Bureau of Geology and Mineral Resources, Luanxian Sijiaying Iron Mine Co., Ltd./Hebei Iron and Steel Group
Rivers may have operated on a global scale around 3.5 billion years ago.
The new find comes courtesy of ancient rocks in China and South Africa. A change in rock chemistry around that time provides the earliest known chemical evidence for the weathering of Earth’s continents and the subsequent delivery of nutrients from land to ocean, geobiologist Kurt Konhauser and colleagues report December 12 in Geology.
Water chips away at rocks on land, removing minerals and washing them away. “As soon as you get weathering, you’ve got a nutrient influx to the oceans, which can lead to … life thriving in coastal waters,” says Konhauser, of the University of Alberta in Edmonton, Canada.
Konhauser’s team analyzed a kind of rock called a banded iron formation. These rocks, which can display ornate patterns of red and dark alternating stripes, store snapshots of the chemistry of oceans that they once lay beneath.
The banded iron formations the team studied record a dramatic change in the relative amounts of germanium and silicon about 3.5 billion years ago. That change could have happened only if rivers were transporting weathered material from land to the ocean.
Earth formed about 4.54 billion years ago. The emergence of weathering about a billion years later gives evidence “for the rise and exposure of continental landmasses really early on in Earth’s history,” says study coauthor Leslie Robbins, a geochemist at the University of Regina in Canada.
Pinpointing when continents emerged is a subject of debate, in part because the planet’s rock record gets spotty far back in time. Other studies have placed the emergence of continents at around 3 billion years ago, Konhauser says. The new finding “is just pushing back the emergence of continental landmasses quite a bit relative to what the general view has been.”
The presence of continents is crucial for life to take hold on their margins, says Eva Stüeken, a geochemist at St. Andrews University in Scotland who was not involved in the research. “It provides a platform for life in shallow water,” she says. “Most marine life today is located in these shallow marine habitats.”
The finding doesn’t mean rivers weren’t active before this time, Stüeken says. “I’m sure there were little islands and small rivers beforehand, but at this point we’re seeing this change where rivers are starting to majorly impact the chemistry of the ocean.”
The team wants to see if their geochemical data can tell them anything about the size of ancient continents. “What does that actually mean in terms of how much continental crust there was? That’s the next thing,” Konhauser says. “Were they similar in size as they are today? Unlikely, but we don’t really have an idea.”
