Tropical Tectonics May Have Triggered Past Ice Ages

The Earth’s climate has been shown to change widely on geologic timescales, oscillating between distinct periods of glaciation and long spans of more clement temperatures. The question of what drove these periodic temperature cycles has long fascinated scientists. Researchers have proposed that plate tectonic activity in the tropics could actually be a critical component of what drives ice ages. To understand why, we need to talk about rocks.

One of the major temperature-impacting geologic cycles on Earth is known as the carbon-silicate cycle. Rocks like calcium and magnesium can sequester carbon effectively in a process called chemical weathering. The researchers’ theory is that the subduction of oceanic plates in the tropics drove long sutures — areas of contact between the plates — higher, bringing what had been ocean floor into contact with the air for the first time. These areas of former oceanic crust are known as ophiolites. The reason this matters in the tropics, in particular, is because geographically, SE Asia contains a much larger percentage of weatherable rocks than other places where this type of faulting has occurred. The researchers estimate that 10-20 percent of land area is responsible for 50-75 percent of the CO2 sequestration via weathering.

The idea that simple chemical processes between rocks and air could account for an entire ice age might seem fanciful, but there’s a precedent for this kind of interaction. The banded iron formations found all over the world are evidence of a similar sort of interaction between oxygen and iron and may have staved off the Great Oxygen Event until the amount of iron in the Earth’s oceans could no longer absorb the amount of oxygen being produced by photosynthetic cyanobacteria.

According to the researchers, there’s a corresponding “suture” where two tectonic plates collided that aligns with three major historical time periods in which we know an ice age occurred. These sutures were all large, at 10,000km or more, and located in the tropics, where the largest supplies of weatherable material were located. These massive uplifts exposed so much new material to the air, the amount of CO2 in the atmosphere dropped over millions of years, leading to a new ice age.

The three ice ages in question are the Late Ordovician (455 to 440 million years ago), the Permo-Carboniferous (335-280 million years ago) and the Cenozoic (35 million years ago – present). But there’s no evidence of any glacial period in the historical record corresponding to a major non-tropical suture. It’s the uplift of rocks that could sequester carbon, specifically, that are associated with ice ages.

The authors write:

While we acknowledge that volcanic outgassing must have changed through time, and that organic carbon burial also had an impact on long-term climate, our analysis suggests that global weatherability has provided the first-order control on Earth’s climate state. Particularly, arc-continent collisions in the tropics, such as the Indonesian orogenic system today, are ephemeral on geological timescales, and when they drift out of the tropics or exhumation ceases and topography is eroded away, the Earth returns to a non-glacial climate state. Thus, our model accounts for both the initiation and termination of ice ages.

There have even been proposals for grinding up large quantities of ophiolites and using their ability to sequester carbon dioxide chemically to offset human CO2 emissions. Unfortunately, the time scales over which this process typically works makes this concept highly unlikely to scale. If plate tectonics produced meaningful cooling trends on Earth, it still took millions of years for the process to play out. Humanity simply doesn’t have that kind of time.

Feature image courtesy of Wikipedia

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