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Global sea-level fluctuations, depositional environments, and life on Earth

June 16, 2008

I try not to recycle press releases without reading the paper first, but I wanted to point to what looks like an interesting new paper in Nature about the tempo of extinctions and its relationship to global changes in sea level throughout Earth history.

I think the press release is preceding the publication of the paper because I can’t seem to find it … I’ll update this post with a link when it comes out, presumably later today or tomorrow.

Here are a few tidbits from the press release:

Since the advent of life on Earth 3.5 billion years ago, scientists think there may have been as many as 23 mass extinction events, many involving simple forms of life such as single-celled microorganisms. During the past 540 million years, there have been five well-documented mass extinctions, primarily of marine plants and animals, with as many as 75-95 percent of species lost.

For the most part, scientists have been unable to pin down the causes of such dramatic events. In the case of the demise of the dinosaurs, scientists have a smoking gun, an impact crater that suggests dinosaurs were wiped out as the result of a large asteroid crashing into the planet. But the causes of other mass extinction events have been murky, at best.

The abrupt external causes of mass extinctions, such as impacts, certainly deserve the attention they receive from the mainstream press and general public. These infrequent yet important events changed the course of Earth history and evolution in dramatic ways. But, it is nice to see a paper that discusses how the tectonic and surface processes of the Earth have influenced the course of life.

Arnold I. Miller, a palaeobiologist and professor of geology at the University of Cincinnati, says the new study [by Shanan Peters] is striking because it establishes a clear relationship between the tempo of mass extinction events and changes in sea level and sediment: ‘Over the years, researchers have become fairly dismissive of the idea that marine mass extinctions like the great extinction of the Late Permian might be linked to sea-level declines, even though these declines are known to have occurred many times throughout the history of life. The clear relationship this study documents will motivate many to rethink their previous views.’

I saw Shanan Peters give a talk about two years ago about some of this work. That particular talk summarized research he was doing with a huge database of stratigraphic sections. Putting together the time and place of depositional environments over large swaths of Earth history is how we produce paleogeographic maps depicting to what extent and long continents have been flooded by oceans.

As those epicontinental seas drained, animals such as mosasaurs and giant sharks went extinct, and conditions on the marine shelves where life exhibited its greatest diversity in the form of things like clams and snails changed as well.

In other words, sea level goes down, the shallow sea, which we know from the modern world is teeming with life, recedes and species perish.

This does not mean that external events didn’t cause mass extinctions – they certainly did. The point being that understanding the causes of mass extinctions doesn’t require invoking an external event.

The new Wisconsin study, Peters says, does not preclude other influences on extinction such as physical events like volcanic eruptions or killer asteroids, or biological influences such as disease and competition among species. But what it does do, he argues, is provide a common link to mass extinction events over a significant stretch of Earth history.

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