Ancient marine sediments have revealed that land plants began sending large amounts of carbon into the oceans about 455 million years ago.
That earlier arrival has pushed back the moment when plants began altering levels of oxygen and carbon dioxide, and global climate, long before forests covered the continents.
Layers of marine mudstone record a sharp rise in carbon relative to phosphorus across widely separated seafloor settings.
Chemical clues in ancient mud
Working across a vast global dataset, Mingyu Zhao at the Chinese Academy of Sciences (CAS) demonstrated that this chemical jump began around 455 million years ago and persisted across changing ocean conditions.
Instead of appearing as a brief anomaly, the elevated signal continued through subsequent intervals, marking a sustained increase in plant-derived carbon that reached the sea.
Because early land fossils are scarce and fragmentary, that enduring sediment record becomes the clearest timeline for when plants began reshaping Earth’s surface systems.
Carbon meets phosphorus
Land plants pack far more carbon into their tissues than ocean algae do, and that imbalance shows up in sediments.
Scientists track a carbon-to-phosphorus ratio – a comparison of buried carbon with buried phosphorus – to spot when land material arrived.
Sturdy cell walls let plants store carbon-rich material without needing much phosphorus, so plant debris skews the ratio upward.
Fossils from early land plants rarely survive, so this chemical trail offers another way to time their spread.
Estimating ancient plant carbon
Instead of guessing from a few fossils, the team estimated how much organic carbon – carbon-rich remains from living things – reached the seafloor.
A two-source calculation treated ocean algae and land plants as the two inputs, then used the ratio to split their contributions.
Across sediments laid down since that time, land-derived material made up about 42 percent, plus or minus 15 percent, of buried carbon.
Modern seafloor samples show a similar 30-57 percent land share, but ancient transport and burial efficiency could have differed.
Carbon burial boosts oxygen
Burying more plant-made carbon changes the air above, because carbon locked away cannot later recombine with oxygen in the form of CO2.
During photosynthesis, plants turn carbon dioxide into biomass, and long-term burial keeps that carbon from turning back into gas. A recent report tied the buried carbon pulse to rising oxygen and falling carbon dioxide.
“Greater organic carbon burial would have promoted atmospheric oxygen accumulation while drawing down carbon dioxide levels,” said Professor Zhao.
Weathering joins in
As plants crept across bare ground, they exposed fresh rock and soil to rainwater, which sped chemical changes.
That process, called silicate weathering – the chemical breakdown of silicate rocks that removes carbon dioxide – also freed phosphorus that fuels new growth.
“These effects may have been further strengthened by intensified silicate and phosphorus weathering linked to rapid land plant diversification,” explained Zhao.
Even with fast colonization, scientists cannot yet pin down how much extra weathering came from plants and how much from changing climate and moving plates.
Early plant spread uneven
One part of the ancient world showed the rising signal first, suggesting that land plants did not spread everywhere at once.
Laurentia, an ancient landmass that included much of North America, sat near the equator and offered broad coastal plains.
Sediments tied to Laurentia show the carbon-heavy pattern appearing earlier than in comparable rocks from other ancient continents.
That uneven start hints that local landscapes, not just global climate, shaped when early plant communities took hold.
Early plants drive carbon swings
Later in the same era, the sediment signal rose in two separate pulses rather than climbing in a straight line.
Those pulses lined up with changes in carbon isotopes – different-weight forms of carbon that leave chemical signatures – preserved in rocks worldwide.
When more carbon gets buried as organic matter, oceans lose lighter carbon, and the remaining pool looks heavier.
Linking both records suggests that early land plants helped drive global carbon cycle swings, though other forces could have joined in.
Cooling and extinction
During the Late Ordovician period, long before dinosaurs were around, Earth slid into an ice age. Most species were marine at that time, and many died out due to the climatic changes.
Cooling locked water into ice sheets, dropped sea levels, and squeezed shallow habitats where much of ocean life had thrived.
An analysis showed that climate change left a clear survival pattern across groups during that extinction.
Plant-driven carbon burial and weathering could have added pressure, yet the study treats plants as one player in a tangled event.
Models test plant timing
Computer models of Earth’s history depend on when plants arrived, because plant growth changes weathering and carbon burial at the same time.
One tool, COPSE, a model linking carbon, oxygen, phosphorus, sulfur, and evolution, let the CAS-led team test their timing.
A paper already suggested small, moss-like plants could boost oxygen far earlier than forests appeared.
Matching COPSE outputs to seafloor chemistry could tighten the oxygen timetable, but the approach still hinges on assumptions about what got buried.
Plants reshape Earth systems
The new sediment signal places early land plants into the center of Earth systems, linking rock chemistry to climate and air.
Future work can test the timing with fossil finds and chemical markers, and it can show where the first plants took root.
The study is published in Nature.
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