15 million years ago, sea levels on Earth plummeted. Where did all that water go?

Today, there is a rapid rise in sea levels worldwide, attributed to glacier melting caused by climate change, as well as the thermal expansion of seawater, reports LiveScience.

However, sea levels have been changing for millions of years due to geological processes reshaping the Earth's ocean basins.

A team of scientists aimed to investigate what occurred between 15 and 6 million years ago when the formation of oceanic crust decreased by 35%. This slowdown in crust formation resulted in the deepening of ocean basins.

As part of the study, researchers established various initial conditions for the area and age of the oceanic crust, as well as its rate of degradation, calculating that the ancient slowdown in seafloor spreading would have led to a drop in sea level of 26–32 meters. This amount is comparable to the sea level change that would occur today if the entire East Antarctic Ice Sheet (the largest ice sheet on Earth) were to melt, but in reverse.

Additionally, the researchers estimated that the heat entering the ocean from the hot mantle beneath it would have decreased by approximately 8% overall from 15 million to 6 million years ago, with an even greater decline (35%) in hydrothermal flow near mid-ocean ridges. They suggest that this drop could have triggered significant changes in ocean chemistry.

In previous work, some of the same researchers proposed that the 35% slowdown in crust formation might have led to a reduction in volcanic emissions of greenhouse gases, thus contributing to global cooling during that period. If this reduction had occurred, sea levels could have fallen by more than 60 meters due to thermal contraction of seawater and a greater volume of water retained by continental ice sheets.

There is only limited evidence of sea level changes in coastal rock layers over the past 15 million years. However, the new calculations align with interpretations of existing stratigraphic data collected in the coastal zone of New Jersey and the shelf of Nova Scotia, the researchers say.

While this is not the first study to evaluate past sea level changes based on tectonic plate movement rates, it covers a later period with higher resolution and greater statistical reliability.

As a reminder, traces of ancient oceans of magma have been found on Earth, extending hundreds of kilometers deep. Geologists have discovered remnants of an ancient ocean of magma in volcanic rocks on the Earth's surface.