If the entire East Antarctic Ice Sheet, including the Denman Glacier shown here, were to melt today, sea levels would rise by about 36 meters. Between 15 and 6 million years ago, the deepening of Earth's ocean basins could have caused sea levels to change by a similar amount, but in the opposite direction. (Image credit: NASA)
We are currently experiencing a significant rise in global sea levels, largely due to the melting of ice caps and glaciers as a result of climate change, as well as the thermal expansion of seawater. However, sea levels also change over millions of years as geological processes gradually alter the shape of the Earth's ocean basins and their overall storage volume.
Dalton and his colleagues zeroed in on a time span from 15 million to 6 million years ago, during which previous studies have shown that ocean crust production declined by 35%. That decline, largely due to a global slowdown in seafloor spreading, led to the deepening of ocean basins.
In the new paper, the researchers analyzed different potential initial conditions for the area and age of oceanic crust, as well as the rate of crustal destruction, and calculated that slowing the spreading of ancient seafloor could have led to a sea level drop of 26 to 32 meters. This range is comparable to the sea level change that would occur today if the entire East Antarctic Ice Sheet (Earth’s largest ice sheet) melted, but in the opposite direction.
The researchers also estimated that the amount of heat entering 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 larger drop (35%) in hydrothermal flow near ocean ridges. They suggest that this drop could have caused significant changes in ocean chemistry.
In previous work, some of the same researchers suggested that a 35% slowdown in crustal formation could have led to a decrease in volcanic greenhouse gas emissions and, therefore, to global cooling during the same period. If this reduction had occurred, sea levels could have fallen by another 60 meters or more due to thermal contraction of seawater and an increase in the amount of water trapped in continental ice sheets.
Only limited data on sea level changes over the past 15 million years are available from coastal rock layers. However, the new calculations are consistent with interpretations of existing sequence stratigraphy data collected from the New Jersey coast and offshore Nova Scotia, the researchers say.
The researchers also add that while this is not the first study to estimate historical sea level changes based on the rates of tectonic plate shifts, it covers a more recent period with higher resolution and greater statistical power than most previous studies.
This article was originally published on Eos.org. Read the original article.
Sarah StanleyNavigate Social LinksFreelance Writer
Sarah Stanley has a background in environmental microbiology, but she also covers a wide range of scientific topics for a variety of audiences. She has also written for PLOS, the University of Washington, Kaiser Permanente, Stanford Medicine, the Gladstone Institutes, and Cancer Commons, a nonprofit organization serving cancer patients.
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