Black mat is the common name for an organic-rich layer of soil also called "sapropelic silt," "peaty muds," and "paleo-aquolls." Its content is variable, and its appearance is variable, and it is at the heart of a controversial theory known as the Younger Dryas Impact Hypothesis (YDIH). The YDIH argues that black mats, or at least some of them, represent the remains of a cometary impact thought by its proponents to have kicked off the Younger Dryas.

What is the Younger Dryas?

The Younger Dryas (abbreviated YD), or Younger Dryas Chronozone (YDC), is the name of a brief geological period which occurred roughly between 13,000 and 11,700 calendar years ago (cal BP). It was the last episode of a series of fast-developing climatic changes that occurred at the end of the last Ice Age. The YD came after the Last Glacial Maximum (30,000–14,000 cal BP), which is what scientists call the last time glacial ice covered much of the Northern Hemisphere as well as higher elevations in the south.

Immediately after the LGM, there was a warming trend, known as the Bølling-Ållerød period, during which time the glacial ice retreated. That warming period lasted about 1,000 years, and today we know that it marks the start of the Holocene, the geological period which we are still experiencing today. During the warmth of the Bølling-Ållerød, all kinds of human exploration and innovation developed, from the domestications of plants and animals to the colonization of the American continents. The Younger Dryas was an abrupt, 1,300-year return to the tundra-like cold, and it must have been a nasty shock to the Clovis hunter-gatherers in North America as well as Europe's Mesolithic hunter-gatherers.

Cultural Impact of the YD

Along with a substantial drop in temperature, the sharp challenges of the YD include the Pleistocene megafauna extinctions. The large-bodied animals that disappeared between 15,000 and 10,000 years ago include mastodons, horses, camels, sloths, dire wolves, tapir, and short-faced bear.

The North American colonists at the time called Clovis were primarily—but not exclusively—dependent on hunting that game, and the loss of the megafauna led them to reorganize their lifeways into a broader Archaic hunting-and-gathering lifestyle. In Eurasia, the descendants of hunters and gatherers began domesticating plants and animals—but that's another story.

YD Climate Shift in North America

The following is a summary of the cultural changes that are documented in North America around the time of the Younger Dryas, from most recent to oldest. It is based on a summary compiled by an early proponent of the YDIH, C. Vance Haynes, and it is a reflection of current understanding of the cultural changes. Haynes was never fully convinced that the YDIH was a reality, but he was intrigued by the possibility.

Archaic . 9,000–10,000 RCYBP. Drought conditions prevailed, during which Archaic mosaic hunter-gatherer lifestyles predominate.

. 9,000–10,000 RCYBP. Drought conditions prevailed, during which Archaic mosaic hunter-gatherer lifestyles predominate. Post-Clovis. (black mat layer) 10,000–10,900 RCYBP (or 12,900 calibrated years BP). Wet conditions are in evidence at the sites of springs and lakes. No megafauna except for bison. Post-Clovis cultures include Folsom, Plainview, Agate Basin hunter-gatherers.

(black mat layer) 10,000–10,900 RCYBP (or 12,900 calibrated years BP). Wet conditions are in evidence at the sites of springs and lakes. No megafauna except for bison. Post-Clovis cultures include Folsom, Plainview, Agate Basin hunter-gatherers. Clovis stratum. 10,850–11,200 RCYBP. Drought conditions prevalent. Clovis sites found with now-extinct mammoth, mastodon, horses, camels, and other megafauna at springs and lake margins.

10,850–11,200 RCYBP. Drought conditions prevalent. Clovis sites found with now-extinct mammoth, mastodon, horses, camels, and other megafauna at springs and lake margins. Pre-Clovis stratum. 11,200–13,000 RCYBP. By 13,000 years ago, water tables had fallen to their lowest levels since the Last Glacial Maximum. Pre-Clovis is rare, stable uplands, eroded valley sides.

The Younger Dryas Impact Hypothesis

The YDIH suggests that the climatic devastations of the Younger Dryas were the result of a major cosmic episode of multiple airbursts/impacts about 12,800 +/-300 cal bp. There is no impact crater known for such an event, but proponents argued that it could have occurred over the North American ice shield.

That cometary impact would have created wildfires and that and the climate impact are proposed to have produced the black mat, triggered the YD, contributed to the end-Pleistocene megafaunal extinctions and initiated human population reorganization across the Northern Hemisphere.

The YDIH adherents have argued that black mats hold the key evidence for their cometary impact theory.

What is a Black Mat?

Black mats are organic-rich sediments and soils that form in wet environments associated with spring discharge. They are found throughout the world in these conditions, and they are abundant in Late Pleistocene and Early Holocene stratigraphic sequences throughout central and western North America. They form in a wide variety of soils and sediment types, including organic-rich grassland soils, wet-meadow soils, pond sediments, algal mats, diatomites, and marls.

Black mats also contain a variable assemblage of magnetic and glassy spherules, high-temperature minerals and melt glass, nano-diamonds, carbon spherules, aciniform carbon, platinum, and osmium. The presence of this last set is what the Younger Dryas Impact Hypothesis adherents have used to back up their Black Mat theory.

Conflicting Evidence

The problem is: there is no evidence for a continent-wide wildfire and devastation event. There definitely is a dramatic increase in the number and frequency of black mats throughout the Younger Dryas, but that's not the only time in our geological history when black mats have occurred. Megafaunal extinctions were abrupt, but not that abrupt—the extinction period lasted several thousands of years.

And it turns out the black mats are variable in content: some have charcoal, some have none. By and large, they seem to be naturally-formed wetland deposits, found full of the organic remains of rotted, not burned, plants. Microspherules, nano-diamonds, and fullerenes are all part of the cosmic dust that falls to earth every day.

Finally, what we now know is that the Younger Dryas cold event is not unique. In fact, there were as many as 24 abrupt switches in climate, called Dansgaard-Oeschger cold spells. Those happened during the end of the Pleistocene as the glacial ice melted back, thought to be the results of changes in the Atlantic Ocean's current as it, in turn, adapted to changes in the volume of ice present and water temperature.

Summary

The black mats are not likely evidence of a cometary impact, and the YD was one of several colder and warmer periods during the end of the last Ice Age that resulted from shifting conditions.

What seemed at first like a brilliant and succinct explanation for a devastating climate change turned out on further investigation to be not nearly as succinct as we thought. That's a lesson scientists learn all the time—that science doesn't come as neat and tidy as we can think it to be. The unfortunate thing is that neat and tidy explanations are so satisfying that we all—scientists and the public alike—fall for them every time.

Science is a slow process, but even though some theories don't pan out, we still must pay attention when a preponderance of evidence points us in the same direction.