Plate Tectonics Can Lead to Ice Ages

The Ross Ice Shelf at the Bay of Whales

The chemical alteration of silicate rocks is a pivotal stage in the terrestrial carbon cycle. By producing carbonates, this process facilitates a reduction in atmospheric CO2 levels, thus contributing to climate regulation. A recent study indicates that another mineral, whose production is associated with significant tectonic events, may also play a crucial role.

The established link between climate and certain geological processes is well acknowledged. The amount of carbon dioxide in the atmosphere is primarily responsible for controlling global temperatures. Setting aside momentarily the emissions induced by human activities, the key natural factors influencing atmospheric CO2 levels are volcanic activity, silicate weathering processes, and the burial of organic carbon. While the former is a CO2 emitter, the latter two can, conversely, remove carbon from the atmosphere by storing it over extended periods. However, geological history reveals that carbon fluxes, in both directions, have not remained constant over time. With variations in orbital parameters, it is this variability that accounts for the climatic fluctuations Earth has experienced.

Tectonics and Climate Coincide Over Time

The Chenaillet ophiolites represent the remains of ancient oceanic crust transported to the continent during the Alpine collision
The Chenaillet ophiolites represent the remains of ancient oceanic crust transported to the continent during the Alpine collision © Florent Figon, Flickr, CC by-sa 2.0

It has been shown that ice ages are linked to an increase in carbonate flux to the oceans, resulting from periods of increased chemical alteration. However, it appears that the outcropping of particular types of rock under warm, humid climatic conditions favors weathering processes.

Previous studies have shown a temporal coincidence between certain tectonic events in tropical regions and a notable cooling of the climate. These tectonic events include obduction episodes that occur in collision contexts.

When two continents converge, the ocean typically situated between them closes through subduction. In the final stages of this closure, just before continental collision, it sometimes happens that scales of oceanic crust are torn off and accreted at the orogeny in formation. These fragments of oceanic crust, composed of mafic rocks (basalts, gabbros) and ultra-mafic rocks (peridotites), form what is known as ophiolites. The Chenaillet in the Alps is a typical example of this process.

Minerals as Carbon Traps

The various processes involved in the carbon cycle
The various processes involved in the carbon cycle, including the burial of organic carbon on the ocean floor. R. Dasgupta, Rice University.

The alteration of ophiolitic rocks by rainwater charged with CO2 will primarily generate bicarbonates, a reaction that consumes a significant amount of atmospheric CO2. These bicarbonates will then ultimately settle on the ocean floor, forming carbonate rocks and thereby contributing to Earth’s climate regulation. However, a recent study published in Nature Geoscience reveals that this reaction is not the sole contributor to CO2 sequestration.

In addition to bicarbonates, the alteration of ophiolites also produces clays, particularly smectite, a mineral with the capacity to influence the climate in a distinct manner.

Smectites Produced by the Weathering of Ophiolites Can Influence Climate

The structure of smectite
The structure of smectite (seen here under an electron microscope) enables it to capture large quantities of organic carbon. Anthony Priestas, Boston University.

Through simulations, two researchers from the Massachusetts Institute of Technology (MIT) have observed that large quantities of smectites are produced following episodes of obduction. The structure of these clays allows them, once they reach the oceanic environment, to incorporate organic carbon from the decomposition of dead organisms. Normally, certain bacteria consume organic carbon and then release CO2 back into the atmosphere. This is the organic carbon cycle. However, by trapping organic carbon, smectites protect it from bacteria. Instead of quickly returning to the atmosphere, this carbon is stably stored within oceanic sediments. When viewed over millions of years, this process has the potential to influence the Earth’s climate.

The study reveals that over the past 500 million years, each major tectonic event involving obductions has led to a sufficient production of smectites, resulting in a global cooling of the climate. This cause-and-effect relationship is observable in geological records. This is the first study to clearly demonstrate that plate tectonics can induce ice ages through the production of smectites.