A line in the dirt
Across North America, from Arizona to South Carolina, archaeologists have found a distinct, dark layer in the soil, often just a few centimeters thick. This "black mat," dated to roughly 12,800 years ago, marks a sharp boundary. Below it, the sediment contains the bones of mammoths, mastodons, and saber-toothed cats, alongside the distinctive stone tools of the Clovis people. In the black mat and above it, this evidence vanishes. The mat itself contains materials that have fueled a contentious scientific debate. Proponents of the Younger Dryas Impact Hypothesis point to the presence of materials of an extraterrestrial event. These include magnetic microspherules (tiny metallic beads), elevated levels of platinum and iridium, and specific carbon structures like nanodiamonds.
The platinum anomaly is especially telling; this element is rare in Earth's crust but more common in asteroids and comets. Spikes in platinum concentration dating to the Younger Dryas boundary have been found at over 25 sites globally. Researchers also identify high concentrations of soot within the layer, suggesting widespread biomass burning across continents. The theory is that one or more large cometary fragments exploded over the Laurentide Ice Sheet, which still covered much of North America. Such an airburst would not leave a crater but would unleash a thermal blast and shockwave capable of igniting continent-spanning wildfires and melting vast quantities of ice.