There's a lot of talk about rising sea levels due to climate change. But that's not the only thing that can change ocean height. Over millions of years, geology also plays a role. And not just a small one.
Less crust, more depth
This is what a study by researcher Colleen Dalton and her team, published in the journal Geochemistry, Geophysics, Geosystems, shows. The scientists took a close look at what happened between 15 and 6 million years ago. During that time, the production of oceanic crust—basically, the renewal of the ocean floor—slowed by 35%. A huge number.
When tectonic plates slow down, there is less young, warm crust to "bulge" the ocean floor. As a result, ocean basins become deeper. The researchers estimate that this phenomenon would have been enough to lower global sea levels by 26 to 32 meters. To give you an idea, this is the opposite effect of what would happen if the entire East Antarctic ice sheet were to melt today.
But that's not all. By slowing down, the plates have also reduced heat exchange between the Earth's mantle and the oceans. According to the authors' calculations, this loss of heat flow represents about 8% less than before – and even 35% near mid-ocean ridges, these crust-forming zones. This could have changed the chemistry of the oceans, or even contributed to global cooling.
Indeed, other work suggests that a slowdown in tectonics can lead to a decrease in volcanic emissions of greenhouse gases, such as CO₂. Fewer gases in the atmosphere potentially means less of a greenhouse effect… and therefore a cooling climate. In this scenario, the oceans contract (cold water takes up less space) and more water remains trapped as ice. As a result, sea levels could have fallen by more than 60 meters in total.
Even though the geological data from this period are fragmentary, some observations made on the New Jersey coast or off the coast of Nova Scotia point in the same direction. This study therefore does not just go back in time: it reminds us that the internal movements of the Earth sometimes have effects on the surface.
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