The evolution of land plants took place about 430 million years ago (Silurian period), when North America and Europe were conjoined in a landmass called Pangaea, and transformed Earth’s biosphere. Because the surface and interior of our planet are linked by tectonic processes, the linked evolution of the biosphere and sedimentary rocks should be recorded as a near-contemporary shift in the composition of the continental crust. To test this hypothesis, University of Southampton researchers assessed the isotopic signatures of zircons formed at subduction zones where marine sediments are transported into the mantle.
Schematic model of fluvial systems both before and after the development of land plants: before the Devonian period, minimal land plants, rapid channel migration and river systems dominated by braided rivers led to rapid sediment transfer, minimal development of mud-sized particles and minor deposition of mudrocks along continental margins; in contrast, after the Devonian, the expansion of vascular land plants led to the development of meandering rivers and the dramatic slowing of sediment transfer to marine environments; the red dashed line represents a generalized sediment transport path that is more sinuous post-Devonian and less sinuous pre-Devonian; these diametric depositional systems influenced the isotopic composition of sediment available for reworking and melting in magmatic systems along destructive plate boundaries. Image credit: Spencer et al., doi: 10.1038/s41561-022-00995-2.
Plants are the dominant kingdom of life on Earth, accounting for 450 Gt of a total 550 Gt of living biomass, and successfully colonizing 84% surface area of the presently subaerially exposed continental crust.
Yet Earth’s status as a ‘green planet’ is geologically recent, and other than some millimeter-thick microbial mats, land vegetation was absent for approximately 90% of the planet’s history.
“Plants caused fundamental changes to river systems, bringing about more meandering rivers and muddy floodplains, as well as thicker soils,” said Dr. Christopher Spencer, a researcher at Queen’s University.
“This shift was tied to the development of plant rooting systems that helped produce colossal amounts of mud — by breaking down rocks — and stabilized river channels, which locked up this mud for long periods.”
According to Dr. Spencer and colleagues, Earth’s surface and deep interior are linked by plate tectonics — rivers flush mud into the oceans, and this mud then gets dragged into the Earth’s molten interior (or mantle) at subduction zones where it gets melted to form new rocks.
“When these rocks crystallize, they trap in vestiges of their past history,” said University of Southampton’s Dr. Tom Gernon.
“So, we hypothesized that the evolution of plants should dramatically slow down the delivery of mud to the oceans, and that this feature should be preserved in the rock record — it’s that simple.”
To test this idea, the authors studied a database of over five thousand zircon crystals formed in magmas at subduction zones — essentially ‘time capsules’ that preserve vital information on the chemical conditions that prevailed on Earth when they crystallized.
They uncovered compelling evidence for a dramatic shift in the composition of rocks making up Earth’s continents, which coincides almost precisely with the onset of land plants.
Notably, they also found that the chemical characteristics of zircon crystals generated at this time indicate a significant slowing down of sediment transfer to the oceans, just as they had hypothesized.
They show that vegetation changed not only the surface of the Earth, but also the dynamics of melting in Earth’s mantle.
“It is amazing to think that the greening of the continents was felt in the deep Earth,” Dr. Spencer said.
“Hopefully this previously unrecognized link between the Earth’s interior and surface environment stimulates further study.”
The research is described in a paper published this month in the journal Nature Geoscience.
C.J. Spencer et al. Composition of continental crust altered by the emergence of land plants. Nat. Geosci, published online August 29, 2022; doi: 10.1038/s41561-022-00995-2