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Early plants slowed mud from flowing into sea

For much of Earth’s history, when it rained, silt and mud simply washed out to sea. Geologists have long suspected that the evolution of plants halted this trend. Now, a first-of-its-kind analysis that includes geological data from every one of the more than 700 known rock formations deposited by flowing water on land provides strong evidence of the mud-trapping power of plants. In sedimentary formations laid down between 2.5 billion and 4 billion years ago, researchers found that the median proportion of mud, silt, and clay in those rocks was a mere 1%. But starting about 460 million years ago, about the time that land plants started to spread over the landscape in large quantities, the overall fraction of mud, silt, and clay in such rocks began to rise steadily. In the last 60 million years of the period the scientists studied, the median percentage of fine-grained sediments in the rocks was 26.2%—and in some formations tallied a whopping 90%—the researchers report today in Science. The dramatic—and geologically recent—rise of fine-grained sedimentary rocks deposited on land doesn’t seem to be linked to ancient episodes of mountain building, the breakup and reassembly of supercontinents, or repeated swings between hot and frigid climates worldwide. Instead, the evolution of land plants likely triggered the boom in these so-called “mudrocks”: Their roots chemically and physically break rock apart, they trap large amounts of material before it can wash away, and they even help trap sediment by slowing down sediment-laden waters and winds flowing around them. The implications of this trend for ocean chemistry aren’t clear, the researchers note; although plants robbed the seas of an increasingly larger fraction of fine-grained material from which minerals could dissolve, their rock-busting action easily might have boosted the overall amount of mud, silt, and clay that ended up in the ocean.