Today we are witnessing rapid global sea level rise attributable mostly to climate change–driven melting of ice sheets and glaciers and thermal expansion of seawater. However, sea level change also occurs over millions of years as geological processes gradually reshape Earth’s ocean basins and change their total storage volume.

Colleen Dalton and colleagues home in on a period from 15 million to 6 million years ago, over which, as prior research revealed, ocean crust production dropped by 35%. This reduction, mostly resulting from a global slowdown in seafloor spreading, caused ocean basins to deepen.

In the new work, published in Geochemistry, Geophysics, Geosystems, the researchers considered various possible initial conditions for the area and ages of ocean crust, as well as crust destruction rates, calculating that the ancient seafloor spreading slowdown would have resulted in a sea level drop of 26–32 meters. This amount is comparable to the sea level change that would result today if the entire East Antarctic Ice Sheet (Earth’s largest ice sheet) melted, but in reverse.

In addition, the researchers calculated that heat flowing into the ocean from the hot mantle beneath would have decreased by about 8% overall from 15 million to 6 million years ago, with an even greater drop (35%) in hydrothermal flux near oceanic ridges. They suggest this drop may have caused significant changes in the ocean‘s chemistry.

In previous work, some of the same researchers proposed that the 35% slowdown in crust production could have led to decreased volcanic emissions of greenhouse gases, and thus to global cooling, during the same period. If this decrease occurred, sea level could have fallen by more than 60 additional meters, thanks to thermal contraction of seawater and more water being held in continental ice sheets.

Only limited evidence of sea level changes over the past 15 million years is available from coastal rock layers. Nonetheless, the new calculations are consistent with interpretations of existing sequence stratigraphy data gathered from coastal New Jersey and offshore Nova Scotia, the researchers say.

Though this is not the first study to estimate past sea level changes on the basis of shifting plate tectonic speeds, it covers a more recent period at a finer resolution and with greater statistical certainty than most prior studies, the researchers add.

This story is republished courtesy of Eos, hosted by the American Geophysical Union. Read the original story here.