An extinction event that took place 450 million years ago at the end of the Ordivician period, the second largest in the Earth’s history, may have been caused by the birth of the Appalachian mountain chain, according to new research published in Geology.

Seth Young of Indiana University in Bloomington and colleagues compared the ratio of two isotopes of the element strontium in rock samples from Nevada that were formed on the seafloor during the Ordivician, and found that immediately prior to a well-documented period of cooling, the strontium ratio changed dramatically over a period of around eight to ten million years. The timing of this decline matches that for the rise of the Appalachians, and the researchers suggest that the change in the strontium ratio is the result of weathering of the mountains and the deposition of sediment in the ocean.

The silicate rocks that were uplifted weather quickly, reacting with water and carbon dioxide, the latter being drawn out of the atmosphere and locked up in sediments and dissolved ions. This would have led to a drop in temperature, triggering the ice age that is known to have started around this time. Many animal and plant species will have struggled to cope with the rapid switch from warm temperatures at the start of the Ordivician to glacial conditions at the end, leading to the observed mass extinction.

December 2009