Europium reveals billion-year pause in mountain formation

Chemical data from tiny zircon crystals reveal a possible explanation for a time in Earth’s ancient history known as the “boring billion”—a period between 1.8 and 0.8 billion years ago when evolution seemed to stall, producing few new forms of life. The zircon crystals suggest that mountains more than 1–2 km high were scarce during this time (Science 2021, DOI: 10.1126/science.abf1876). With fewer mountains around, there would have been less erosion and a short supply of nutrients flowing into the oceans—a geological famine that may in turn have starved life. Ming Tang of Peking University and colleagues studied lanthanide elements trapped in zircons, tough crystals that preserve a snapshot of the magma chemistry that spawned them. While most lanthanides adopt the +3 oxidation state, europium can exist as +2. This chemical difference depresses the amount of europium in zircons relative to samarium and gadolinium. Zircons that formed under higher pressures in thicker, mountain-forming regions of Earth’s crust include more europium than those made at lower pressures (Geology 2020, DOI: 10.1130/G47745.1). “It had been very difficult to track mountain formation before we invented this new proxy,” Tang says. The researchers compiled published data from about 20,000 zircons spanning 4 billion years of Earth’s history and found a steady decline in crustal thickness during the mid-Proterozoic eon. This suggests very little mountain-forming activity around the time when evolution appears to have stalled.