The close of the Permian Period around 250 million years ago saw Earth's biggest extinction ever. At this time large volcanic eruptions were occurring in what is now Siberia. The volcanoes pumped out gases that led to acid rain. Falling on the supercontinent Pangaea, the acid rain killed off end-Permian forests.
The demise of forests led to soil erosion and the production of organic-rich sediments in shallow marine waters.
The sediments are now rocks in cliff faces in the Italian Dolomites, and studying them provides insight into the mechanisms of Permian ecosystem decline. Scientists have proposed a first-ever, organic compound-based, quantitative recorder of acidity for the geological record. Knowledge from the food industry, where vanillin ("vanilla") is used as a flavoring ingredient, shows that oxidation of vanillin to vanillic acid is reduced under acidic conditions.
Ratios of vanillic acid to vanillin in end-Permian organic matter reveal soil acidity close to that of vinegar or lemon juice. Acidification events occurred not once but several times as volcanism hit the land with repeated pulses of acid rain. An acid-induced decline in plant life would have caused a collapse in the food chain, sealing the fate of end-Permian life on land.
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