April 26. Silurian deserts?

We know from all that salt in Michigan that the climate during part of the Silurian was hot enough in places to evaporate small seas. Were there deserts?  We don’t really know. There’s a lot of work on Silurian sea-level changes and geochemistry that attempts to unravel the nature of planet earth during the Silurian – and there is still quite a bit of controversy, as far as I can tell.

Scientists studying oxygen and carbon isotopes find that variations in them during the Silurian coincide with some relatively small extinction events. If nothing else, it’s beginning to give a picture of the Silurian as a time with repeated climate changes, in contrast to what I was taught 45 years ago that the Silurian was stable, uniform, and hot. Changes in temperature affect the uptake of different isotopes of carbon and oxygen in rocks and organisms such as the shells of animals – that’s how we use isotope ratios to infer changes in climate. But what is clearly not clear is what was causing those climate changes.

Most of what I have read suggests that indeed, there must have been periods of arid conditions and high temperatures, which fits with those vast salt deposits. So it’s reasonable to suppose that there were deserts somewhere during at least parts of the Silurian Period. But so far as I can tell, there’s little other direct evidence for such deserts – no vast deposits of wind-blown sand, for example.

As usual, research continues.

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Today, April 26, is the birth date of Leopold von Buch, in 1774, at the castle of Stolpe, Brandenburg, Germany. You probably haven’t heard of von Buch, but he was a prominent and important geologist in the early 19th Century. He studied the volcano Vesuvius and recognized the volcanic origin of basalt – at the time, many scientists thought basalt, and virtually everything else, crystallized from water. And von Buch defined the subdivisions of the Jurassic Period of the Mesozoic Era, which we’ll get to in October.

It’s also the birthday of a geophysicist you probably have heard of – Charles Richter, born this day in 1900, in Ohio. He invented the Richter scale, the first real quantitative way of estimating the magnitude of earthquakes. Seismologists use the moment magnitude scale today, but the Richter scale was the principal way of evaluating earthquakes from 1935 into the 1980s.

—Richard I. Gibson

References
Paper by Munnecke et al. 2010
Silurian climate