Modelling of Earth's ancient carbon cycle reveals that volcanic arcs only became a dominant source of greenhouse gas emissions towards the end of the dinosaur era. This shift occurred around 100 million years ago, driven by the emergence of certain ocean plankton 150 million years prior. Previously, most carbon was released through rifting processes rather than volcanic activity.
Earth's climate has long swung between icehouse and greenhouse states, influenced by atmospheric levels of carbon dioxide and other greenhouse gases. Volcanic arcs, such as those in Japan where tectonic plates converge, have historically contributed to natural carbon emissions by releasing CO2 from the planet's interior. However, new research indicates this role intensified relatively recently in geological terms. Ben Mather at the University of Melbourne, Australia, led a study modelling the past 500 million years of plate tectonics and the carbon cycle, published in Nature Communications Earth and Environment. The findings show that volcanic arcs emerged as the primary carbon source around 100 million years ago, near the close of the dinosaur age. This change traces back to about 150 million years ago, when phytoplankton bearing calcium carbonate scales appeared in the oceans. Upon dying, these organisms deposited vast layers of calcium carbonate on the deep-sea floor. As oceanic plates subduct—slipping beneath others and recycling into the mantle—this stored carbon is carried deep into Earth. Mather explains, “Most of the carbon from the plankton that leaves the subducting oceanic plate will get mixed into the molten interior, but a portion of that will get emitted via volcanic-arc volcanoes.” Before these plankton, emissions from volcanic arcs contained comparatively little CO2. Throughout much of Earth's history, carbon was instead liberated through rifting, where continents split apart over geological timescales, as seen in the East African Rift or mid-ocean ridges. “When tectonic plates are being spread apart, essentially, what you’re doing is ‘unroofing’ some of the molten interior of the Earth,” Mather notes. This process forms new crust and releases carbon, with emissions depending on rift length and spreading speed. In the last 100 million years, volcanic arc emissions have risen sharply, now two-thirds higher than 150 million years ago due to the seafloor carbon reservoir from plankton. Today, Earth sits in an interglacial phase of a larger ice age that began 34 million years ago. Phytoplankton continue to sequester more carbon on the seafloor than volcanoes release, aiding the current cooler climate. Alan Collins at the University of Adelaide praises such models for illuminating how volcanism and tectonics have shaped climate over time, noting changes in ocean sediment composition from evolving marine life like calcium carbonate zooplankton.