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Sea level on Earth has been rising and falling ever since there was water on the planet. Scientists were already able to use sediments and fossils to roughly reconstruct how sea levels changed over time steps of a million years or more.

Now, for the first time, scientists from Utrecht, the UK and the US have been able to determine how sea levels must have varied on thousand-year timescales during the last 540 million years. This new research was on 3 July in Earth and Planetary Science Letters.

"Taking these rapid sea level variations into account is important for understanding the structure of the subsurface, and the applications to green energy resources," says Dr. Douwe van der Meer, guest researcher at Utrecht University and lead author of the study.

How high or low sea level is depends largely on two things: plate tectonics determine , and the amount of land ice determines how much water is in that bathtub.

"In time steps of about a million years, you can derive an average sea level for as far back as there are fossils, about 540 million years," says Dr. Van der Meer. "That varied by as much as 200 meters. We suspected that sea level could go up and down enormously in much shorter periods as well, but there is not enough data to make those shorter time steps."

Geological tree rings

To estimate sea level variations in shorter periods, scientists can learn by looking at rocks from the last few million years. Like growth rings in trees, they read sedimentary deposits of, for example, sandstone and claystone. Claystone forms mainly when an area is under deep water, and sandstone when the sea is closer to the coast.

"We see alternations, especially when the global climate is cold, and there is ice on the poles. Then, the rhythmic wobbling of Earth's spin axis creates ice ages that last only tens of thousands of years, during which the sea level can go up and down by as much as 100 meters."

Until now, the effects of such short sea level changes could not be determined in the distant geological past. But Van der Meer and his colleagues devised a trick: the researchers calculated the relationship between Earth's climate and the size of ice sheets for the last tens of millions of years of when good data is available. "This gave us the most accurate information, where we have warm periods without ice, and cold periods with ice—as well as ice ages. We used that relationship to determine short-term variability further back in time, to 540 million years ago."

The new sea level variations that the researchers reconstructed proved to be much more consistent with fossil-based reconstructions. "This is the first time we have been able to quantify sea level in a consistent way for such short timescales," says Van der Meer.

Over the past few million years, we have seen the coming and going of ice ages, with sea level changes of up to 100 meters. But during the time of the dinosaurs in the Jurassic and Cretaceous, sea level change was much less pronounced due to a lack of land ice. On the contrary, in the late Carboniferous, when giant dragonflies flew over enormous marshes in the Netherlands, there were very large sea level variations due to a large ice cap in the southern hemisphere.

Applications for underground storage

Because we now know much more precisely what happened to sea levels in the past, researchers are able to make better maps of Earth for all time periods. They can use these for climate and evolution models, and their response to sea level change. "High or low sea levels, it's all happened before in the geological past," Van der Meer says.

Knowledge of sea levels in the geological past has many different applications. Today, we seek methods for underground CO₂ and hydrogen storage, or geothermal energy. Sandstone, deposited at low sea level, is important here because it can be used as a reservoir. Claystone, deposited at high sea level, acts as a seal through which water or CO₂ cannot easily pass.

Currently, suitable sites for storing radioactive waste are also being sought in these kinds of strata. "If we know that at a certain time global sea level was high, we also know that a relatively continuous layer of claystone would have been deposited. We can use that information to create a global layer map of sand and claystone, which helps us in the safe use of the subsurface," says Van der Meer.