A planet's first pollution crisis
The fells of Pallas-Yllästunturi National Park are the worn-down roots of an ancient mountain range that stood here billions of years ago. These rocks, part of Finland's Karelian Domain, preserve the chemical fingerprints of the single greatest ecological transformation in Earth's history. Around 2.4 billion years ago, during the Paleoproterozoic Era, microscopic organisms called cyanobacteria evolved a new form of photosynthesis. This process released vast quantities of a reactive, corrosive gas as waste: free oxygen.
Before this, Earth's atmosphere was a methane and oxygen-poor haze. The oceans were full of dissolved iron, and life consisted of single-celled anaerobic microbes. For these organisms, oxygen was a deadly poison. The slow, relentless release of oxygen by cyanobacteria over millions of years triggered a mass extinction, wiping out most of the planet's anaerobic inhabitants. This period, known as the Great Oxidation Event, was the world's first large pollution event. The evidence is not in fossils, but is written directly into the chemistry of the ancient bedrock found across Lapland.
The evidence in the stone
The geological formations in this region, known as the Jatulian formations, date to this precise period of planetary change. They provide some of the world's clearest evidence for what geochemists call the Lomagundi-Jatuli Event. This was the most extreme and long-lasting positive carbon isotope excursion in the planet's history, lasting from roughly 2.3 to 2.1 billion years ago.
A carbon isotope excursion refers to a major shift in the ratio of heavy carbon-13 to light carbon-12 in sedimentary rocks. Photosynthesis preferentially uses the lighter carbon-12. When huge quantities of organic matter produced by photosynthetic cyanobacteria were buried on the seafloor, it left the remaining carbon in the seawater and atmosphere enriched with the heavier carbon-13. This isotopic signature was then locked into the carbonate rocks (dolomites) and shales of the Jatulian formations. These formations in Finland record this massive burial of organic carbon, which was directly linked to the unprecedented rise of oxygen in the atmosphere. Analysis of ancient sea salt deposits from a drill core in nearby Russian Karelia confirms that oceanic sulfate concentrations, a product of oxygen reacting with sulfur, were at least 30% of modern levels, far higher than previously thought.