The colonization of land by plants was the most consequential biological event in Earth's surface history. Plants weathered rock into soil, changed the planet's albedo, restructured the water cycle, buried carbon, and oxygenated the atmosphere. When this happened matters because it determines whether plants caused the Late Ordovician glaciation and mass extinction or merely coincided with it.
Zhao Mingyu and colleagues at the Chinese Academy of Sciences found the answer — not in terrestrial sediments, where the plants lived, but in marine sediments, where they didn't. The team measured organic carbon-to-total phosphorus ratios in marine siliciclastic sediments spanning hundreds of millions of years. Land plants produce organic matter with much higher carbon-to-phosphorus ratios than marine organisms. When rivers carry terrestrial organic matter into the ocean, the C/P ratio in marine sediments rises. The team detected a pronounced increase beginning approximately 455 million years ago — Late Ordovician.
The finding, published in Nature Ecology & Evolution, pushes the timing of significant land plant expansion earlier than many previous estimates. Using a mixing model to separate terrestrial from marine contributions, the team calculated that since the Late Ordovician, terrestrial organic carbon has accounted for approximately 42% of total organic carbon buried in marine sediments — comparable to modern values. The land was already exporting as much carbon to the ocean 455 million years ago as it does now.
The structural insight is about indirect detection. The evidence for a terrestrial event is recorded in a marine archive. This is not a limitation — it is the only way to detect the event at all. Terrestrial sediments from the Ordovician are scarce and poorly preserved. The first land plants left almost no fossils because they lacked the hard structures that survive burial. The plants are invisible in the rock record at the place where they lived. But they changed the chemistry of runoff, and the ocean recorded what the land could not.
Detection through downstream effect inverts the usual relationship between signal and location. The detective does not visit the crime scene; the detective reads the river that flows past it. The method works precisely because the ocean has no reason to produce the signal on its own — elevated C/P ratios in marine sediments have no marine explanation. The anomaly in one system is the signature of a process in another.