Earth’s climate has remained within a fairly narrow temperature range throughout its history when compared to the inhospitable heat and cold elsewhere in our solar system. This relative stability is maintained by a complex system of interactions. On geological timescales, the chemical trade between the atmosphere and the rock of the Earth’s crust acts like a thermostat. The weathering of common minerals involves a reaction that removes CO2 from the atmosphere. High temperatures (caused by higher CO2) mean faster weathering, which gradually introduces CO22 and temperature down again. It is a moderating influence.
But plate tectonics also plays with the dial on that thermostat. Arches of volcanoes along subduction zones (where one plate dips under another) provide a constant source of CO2and subduction zones come and go over time. Research using hard zircon crystals as records of volcanic arcs has found a connection with climate over geological time. In fact, a new study published this week in Science extends that equation over the past 720 million years by finding evidence that volcanic activity rises and falls with major fluctuations in Earth’s climate.
A second study, published in the Procedures of the National Academy of Sciences and led by MIT’s Oliver Jagoutz – looks at the flip side of the equation: the ability of plate tectonics to amplify the weathering feedback that eats CO2. While climate change can increase or decrease the rate of weathering, the amount of exposed and easily weatherable rock makes a huge difference. For example, the igneous rocks that make up the oceanic crust make excellent CO2 sponges – or at least they would, if they weren’t at the bottom of the sea.
Over there Are situations where plate tectonics pushes oceanic crust onto continents. Over the past 100 million years, Africa, Arabia and India have all docked with the Eurasian continent, closing an ocean basin we call the Tethys. In addition to subduction zones along the continental boundaries, there were oceanic subduction zones with arcs of volcanic islands in the middle of the Tethys. As this mess coalesced, huge amounts of oceanic crust piled up on dry land. This area was in the tropics at the time, where warmth and miraculous rainfall combine for the strongest weathering in the world.
Between 90 and 40 million years ago, the Earth’s climate underwent major shifts. (Also, some creatures called “dinosaurs,” you may have heard of, are gone.) Earth was a sultry “hothouse” 90 million years ago, with sea levels so high that the central part of what is now the U.S. is a seaway was. Over the next 20 million years, it cooled significantly. The trend then reversed to warming between 70 and 50 million years ago, although temperatures remained below the previous greenhouse peak. And from 50 to 40 million years ago, it cooled down again – soon after, the Antarctic Ice Sheet formed.
So how does the slow death of the Tethys Ocean correspond to this sequence of cooling, warming and cooling? During the first phase of cooling, the African and Arabian plates abutted the western end of the oceanic subduction zone. That closed off those volcanoes and pushed a 4,000-kilometer stretch of oceanic crust up to 500 kilometers into the tropical continent. The same happened on the east side of the subduction zone, which collided with Eurasia. The resulting weathering could have removed a huge amount of CO2 from the atmosphere as that exposed rock crumbled and eroded.
During the warming phase, India moved north at full steam, driving a powerful subduction zone that consumed the oceanic crust and spewed out CO2 in the air. That more than made up for the volcanoes that stopped elsewhere in the Tethys. The huge areas of stranded oceanic crust would have begun to shrink by then as erosion took its toll, weakening the power of the CO2 sponge.
About 50 million years ago — the beginning of its final cooling phase — the Indian plate reached the middle part of the oceanic subduction zone, once again capping off volcanoes and accumulating its own slice of oceanic crust on the surface. That rejuvenated the removal of CO2 due to weathering and slowed down the volcanic source.
The researchers used a model of all these processes to add up the CO2 deposits and withdrawals over time. By comparing the results to records of deep ocean temperatures, they found that the timing matches. More work is needed to find out exactly how much CO is in the atmosphere2 concentrations changed and how much of the recorded temperature changes could explain subduction. But the hypothesis that plate tectonic interactions played a major role is plausible.
The researchers found that it is only plausible because all this took place in the tropics. Simulations with a mid-latitude climate couldn’t get the job done because the rocks didn’t weather fast enough. Looking through history, the researchers note that there are other examples of similar plate collisions. Some occurred in the tropics and coincidentally coincide with periods of major cooling. But those that occurred at higher latitudes were not associated with cooling.
In the dizzying expanse of geologic time, even oceans are born and die. A continent tears apart, creating a larger ocean basin. At some point, the separation splashes out and the basin shrinks until the continents come back together — it’s a process geologists call the Wilson cycle. The researchers write, “This close link between tectonism and global climate represents an extension of the Wilson cycle from the solid Earth to the oceans and atmosphere.” No part of the Earth system stands alone.
PNAS2016. DOI: 10.1073/pnas.1523667113 (About DOIs).