Cooling Element

A researcher collects a continuous sediment column in short increments for platinum analysis along the Pee Dee River in northeastern South Carolina.

Christopher Moore

In contrast to the gradual warming of Earth’s climate over the past 25,000 years or so, a long (1,200-year) period of climate cooling known as the Younger Dryas occurred roughly 12,000 years ago. Precisely what brought on this geological period, during which over 35 genera of North American Pleistocene megafauna became extinct, and when it began, has not been fully understood. New evidence in the form of platinum at dig sites across the United States provides a clue to the onset of the Younger Dryas.

A research team, led by archaeologist Christopher Moore of the University of South Carolina, recently discovered extensive, though microscopic, quantities of platinum, an element that is relatively rare on Earth but abundant in asteroids. The platinum anomaly, as the investigators call it, occurs at depths ranging from 55 centimeters to as much as 10 meters at the 11 widely separated sites they studied. The depths vary due to local geological and environmental conditions, but generally point to deposition around 12,800 years ago.

In 2013, other scientists had found platinum in a Greenland ice core, at a depth that signified deposition 12,800 years ago and led to the Younger Dryas Impact Hypothesis: that a cosmic impact event caused the onset of the cooling period, the extinction of megafauna, a peak in biomass burning, and the end of Clovis culture, a prehistoric Paleoindian culture marked by distinctive stone tools. The new research confirms that there was a sizable impact or airburst at that time. Moore and his colleagues have ruled out volcanic eruptions and other events as possible sources of the platinum.

The new research stops short of determining whether the impact caused the Younger Dryas and its associated extinctions and cultural demise. The discovery of the platinum layer will, however, facilitate understanding of the effects of climate change during the Younger Dryas in other geographic areas where that layer is found. Additionally, the presence or absence of Clovis tools and similar artifacts at the study sites helps date those communities and the timing of their disappearance. (Scientific Reports)