A thin sediment layer found across North America containing microscopic diamonds and exotic metals that some scientists believe came from a comet impact 12,800 years ago. The impact theory remains controversial but could explain a sudden return to ice age conditions.
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The black mat
At the Murray Springs archaeological site, a distinct, dark layer of earth cuts across the exposed walls of an arroyo. Known as the "black mat," this thin stratum of organic-rich clay and silt marks a moment in prehistoric North America. Radiocarbon dating places the base of this layer at approximately 12,900 years ago, a date that coincides with the start of a sudden and intense cold snap called the Younger Dryas. This climatic shift interrupted the warming trend at the end of the last major ice age, plunging parts of the Northern Hemisphere back into near-glacial conditions for over a thousand years.
The black mat itself covers a prehistoric terrain. At Murray Springs, archaeologists have excavated Clovis-era artifacts, a fire pit, and the skeletons of megafauna like mammoths and bison from directly beneath it. Importantly, no Clovis artifacts or the remains of extinct large mammals are found in or above the mat, indicating their abrupt disappearance from this location. Proponents of the Younger Dryas impact hypothesis argue this boundary layer contains evidence of an extraterrestrial event.
An impact under debate
The Younger Dryas Impact Hypothesis posits that a fragmented comet or asteroid exploded over North America, triggering continent-wide wildfires and injecting enormous amounts of dust and soot into the atmosphere. This would have blocked sunlight, causing an "impact winter" and the rapid onset of the Younger Dryas cooling.
Evidence cited for this event includes materials found within the boundary layer at various sites. These include magnetic microspherules, elevated levels of platinum and iridium, and nanodiamonds. A type of nanodiamond, lonsdaleite, is important because it is known to form under the extreme shock pressures of a cosmic impact. Platinum concentrations in Greenland ice cores show a significant spike around this time, which some researchers link to an extraterrestrial source.
The hypothesis, however, is a subject of scientific debate. Many researchers have been unable to replicate the findings of impact markers, and some suggest the materials have been misinterpreted. For example, some studies argue that the particles identified as nanodiamonds are actually more common graphene and graphane-oxide aggregates. Alternative explanations for the Younger Dryas cooling point to a massive influx of glacial meltwater into the North Atlantic, which could have shut down ocean heat transport. Recent research also suggests that the platinum spike in Greenland ice might be from a large volcanic eruption and that it occurred decades after the cooling began, making it a consequence, not a cause.
💡Fun Facts
The hexagonal nanodiamonds called lonsdaleite, reported in the boundary layer, are only known to form on Earth in meteorites and at impact craters.
At Murray Springs, excavations revealed a mammoth skeleton but also hundreds of mammoth footprints preserved in the sand, filled in and covered by the black mat.
The platinum spike found in Greenland ice cores from this period is one of the most significant of the last 100,000 years, with concentrations far exceeding background levels.
The Younger Dryas cooling event saw temperatures in Greenland drop by more than 15°C colder than today.
A waterless toilet is available at the parking area. The self-guided interpretive trail is approximately 1/3 mile long. The trail surface is natural and unpaved, which may present challenges for visitors with mobility issues.
