Carolina Bays, near Rex, North Carolina, United States
The Younger Dryas impact
About 12,900 years ago (a cold return, cause open)
The ice age had nearly ended, and then the cold came back and stayed for more than a thousand years. One account blames meltwater and the ocean. Another blames a comet, and points to tiny diamonds, spheres of metal, a spike of platinum, a black soil, and oval bays. The tiny things have been found, and then they have failed to be found again. That failure, together with the answer made to it, is the room.
Why this is extraordinary
About 12,900 years ago the North Atlantic region, already warming its way out of the ice age, dropped back toward glacial cold. The return is called the Younger Dryas, after a tundra flower that shows up in European pollen. It held on until about 11,700 years ago, and the ice cores show the cold arriving quickly. Nobody is disputing the cold. The cause is what stays open.
The leading reading is a change in the way the North Atlantic circulated, fed by meltwater pouring off the ice sheet, so that heat stopped travelling north as it had. A rival reading, in print from 2007 on, says fragments of a comet or an asteroid struck, or burst in the air, at that moment, and that the impact started the cold, ended the Clovis way of life, and finished the mammoths along with the other large animals. The evidence offered is small: magnetic microspherules, nanodiamonds, a peak of platinum, soot, and a dark soil called the black mat. The Carolina Bays, thousands of oval depressions scattered over the Atlantic coastal plain, have been recruited as the craters.
The round structure under the Greenland ice, about 31 km across, dates to 57.99 ± 0.54 million years. It is not the hole for 12,900 years ago. A German eruption dated to 13,006 ± 9 calibrated years sits about 130 years before the start of the cold in Greenland, which those authors say rules out a direct link. An eruption a century earlier can still load the air. It is not the start of the thousand-year stadial on their chronology.
Every one of those has a published reply. An independent sampling at seven sites found no peak of microspherules that belonged only to the boundary. A later paper said the sampling had not followed the protocol, and it reported spherules at three sites. A reply to that paper said the protocol which finds them has never been confirmed on samples other people go out and collect. Nanodiamond identifications have been challenged as other forms of carbon. In one study the isotopes of osmium looked like the Earth’s crust, not like a meteorite. A platinum peak in Greenland ice, sitting at the boundary, is a real measurement. It rises and falls over roughly two decades, a long time for dust to stay aloft and a short time for a climate. No crater of that age has been accepted. Bay rims carry luminescence ages in several clusters spread across more than 100,000 years, and where the rims are nested they get younger toward the middle, which is the way a lake shrinks. The cold is still there. The impact remains a hypothesis with evidence on both sides, and it is not the consensus.
Why it matters
If a comet did start the Younger Dryas, then Clovis and the American megafauna ended in a disaster, and a field of bays is the scar it left. If the comet’s markers do not turn up a second time, the cold still needs a cause, and the reading that uses the ocean and meltwater keeps the job. A marker can be weighed only where someone else has gone looking for it and written down what they found.
A microspherule is a magnetic sphere about the size of a sand grain, a nanodiamond is a crystal of diamond measured in millionths of a millimetre, and platinum and iridium are metals that meteorites often carry as a pair. The black mat is a dark layer left by a wetland, of this same age, seen at Murray Springs and at a great many other sites. A Carolina Bay is an oval, usually running northwest-southeast, rimmed with sand, and common from New Jersey down to Florida. The pin is set on the bay field near Rex, North Carolina, because the bays are the only piece of this hypothesis you can actually see at the scale of a landscape. The hypothesis itself is not a place you can visit.
Photo tour
1 photograph, unedited. On a phone, swipe the row. Choose one to look closely.
- Microspherules, nanodiamonds, and the platinum peak in the ice core are images from a laboratory, not photographs with an open licence that a visitor can be shown here. What you can see is the bays, the landform itself.
What is still unexplained
These sit beside the consensus account, with the same visual weight. None of them is marked solved.
Location and dating
- Location
- The pin on the map is Carolina Bay country around Rex, in Robeson County, North Carolina, the ground the elevation model shows. Ice cores record the cold interval, and so does lake mud across the northern hemisphere. Markers people have claimed turn up in reports from archaeological sites around the continent, the Southwest among them.
- Period label
- Current consensus — open to revision The cold runs from about 12,900 to about 11,700 years ago. The impact claim concerns the first years of that stretch. The bays are landforms, and their histories run much longer than the cold interval does.
- How this dating was done
- Ice cores date the cold, and so does radiocarbon at archaeological sites. Sand in the Bay rims is dated by optically stimulated luminescence, while Microspherules and platinum are not clocks of their own but materials someone then tries to set into a layer that already has a date. When that layer’s date is loose, the marker has no birthday.
- What could change it
- A peak of spherules, a nanodiamond, and a spike of platinum, all in one section, found by a laboratory that did not write the original protocol, and with the samples kept in an archive. Or a crater that is shocked and dated to the boundary. Either result would move the weight, and another failure to repeat the finds would move it the other way.
What is actually there
- Ice cores record the cold coming back, abruptly, at about 12,900 years ago.
- A study of seven sites did not reproduce a microspherule peak that was unique. A study of three sites did, and it blamed the method.
- Ice in Greenland carries a platinum peak at the boundary, lasting on the order of years and out to about two decades.
- No crater that anyone accepts has been dated to the boundary.
- Rims of the Carolina Bays date to several different intervals, and where the rims nest they grow young toward the inside.
- Black mats from many ages carry chemical markers of the same kind.
Open questions
| Question | Status |
|---|---|
| If a comet was not the cause, why did the cold come back? | The leading account is meltwater and a change in North Atlantic circulation. That account has its own debates about which route the water took. It does not depend on spherules. |
| Is the platinum in the ice a small impact, a volcanic pulse, or dust of some other kind? | Open. The peak is measured. The source is not settled by a peak alone. An iron-rich impactor is one reading. It predicts other finds that have not become routine. |
The current narrative
Where publication stands today. It is the best-supported reading, and it is open to revision. It is not a verdict.
The current narrative — open to revision
Meltwater and the ocean, without an impact
The Younger Dryas is a climate turning back on itself, abrupt, driven by changes in the circulation of the North Atlantic, with meltwater off the ice as a plausible way to start it. Clovis and the extinctions overlap that cold, and an overlap is not a comet. The bays are lakes shaped by wind, and they belong to many ages. Microspherules and nanodiamonds have not been confirmed as a layer from an impact.
What supports this
- The cold is real, in ice cores and in pollen, and the same deglaciation holds other examples of climate shifting abruptly.
- Searches made independently did not find a unique peak of spherules, or osmium from a meteorite, or a crater.
- Bay rims are shoreline sand, carrying many luminescence ages, and they grow younger toward the centre where one rim nests inside another.
- Black mats behave as wetlands, and they occur at many different dates.
What pushes back
- Greenland ice holds a platinum peak that sits on the boundary and looks like stratospheric dust.
- Some crews do recover spherules, provided they use the original protocol.
- Cold, cultural change, and extinction falling together is exactly what the impact idea was invented to hold in one hand.
What would settle it
This is how the cold is read by consensus. It does not pretend the trigger is known in any detail, only that a layer from an impact has not been established.
Emerging narratives
Other published readings. Each may be right. None is adopted here unless the evidence carries it.
An emerging narrative
A comet or asteroid fragment at 12,900 years ago
An impact, or an airburst, at the opening of the Younger Dryas brought the cold, the end of Clovis, and the extinction of the megafauna. Spherules, nanodiamonds, platinum, the black mats, and the Carolina Bays are the debris of that event and the scars it left.
What supports this
- A published collection of odd materials, all at the same radiocarbon horizon, and at many sites.
- The platinum peak in Greenland, a brief one, and in the ice study’s reading not a match for an ordinary stony meteorite.
- A shared orientation among the bays, which a glance that is not careful will read as a swarm.
- The black mat spread over the last of the Clovis camps.
What pushes back
- The finds have failed to replicate often enough that the protocol has become the argument.
- Nanodiamond results, and the osmium results, have published readings that do not require an impact.
- There is no crater of the right age.
- The Bay ages are many, and nested rims grow young toward the inside.
- Neither the mats nor their chemistry belong only to the boundary.
What would settle it
It is a coherent story, and the physical layer is the part that has never stabilised. Weight stays with the reading from climate until a second laboratory can find the same spike in a section that has been archived. The story is not mocked. It is unfinished in the way that matters, the materials.
Should the current view be reevaluated?
Open to revision
The interval of cold holds. An impact as the cause of it does not hold as a consensus, because the markers have not replicated cleanly and the bays are the wrong age to be the craters of that impact. Platinum in the Greenland ice is the piece that most deserves to be left open. It is a measurement someone made, not yet a crater, and not yet a layer that other groups recover as a matter of routine.
What drives this
- An abrupt cold, from about 12,900 years ago through to 11,700.
- Spherule studies that conflict, including an argument over the protocol.
- A platinum peak in Greenland ice that is real.
- Bay rims whose dates fall across many intervals of climate.
- Black mats, wetlands, and more than one age for them.
This is the museum’s own weighing of the evidence. It is not a visitor vote, and it is separate from the slider.
Weigh it yourself
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If the contested reading is right
What would follow if a reading that is not the consensus turned out to be true, and what could settle it. This explains the stakes. It does not adopt the reading.
The impact happened, and the negative studies used the wrong protocol
Cold, Clovis, and the extinctions would share one trigger, and an account that uses only the ocean would be missing something. The bays would still need an origin of their own, unless a subset of rims can be shown to share a single age. A crater would still be owed, or else a layer that repeats.
Why it is still worth a test
Splits of the same samples, kept in an archive, run under both protocols, and the counts of spheres published side by side.
The bays are the craters
The coastal plain would be a scar left in 12,900 years ago, and the luminescence ages of the rims would have to be wrong together, the rims that grow young toward the centre included. That revises a great deal of sand.
Why it is still worth a test
A single bay, its many rims, luminescence and radiocarbon kept in stratigraphic order, every result landing on one year.
Sources
- An independent evaluation of the Younger Dryas extraterrestrial impact hypothesis Proceedings of the National Academy of Sciences
- Independent evaluation of conflicting microspherule results Proceedings of the National Academy of Sciences
- Large platinum anomaly in the Greenland ice core Proceedings of the National Academy of Sciences
- Comprehensive review of the Younger Dryas impact hypothesis Earth-Science Reviews
- A round structure under the Greenland ice Science Advances, 2022. About 31 km across, 57.99 ± 0.54 million years. Not the hole for 12,900 years ago.
- A German eruption and the start of the cold Nature, 2021. 13,006 ± 9 calibrated years, about 130 years before the cold in Greenland. Those authors say that rules out a direct link.
- Sedimentology and ages of Carolina Bay sand rims Geological Society of America, southeastern section abstracts
Image credits
- An elevation model of the coastal plain centred on Rex, North Carolina, covering about 600 square kilometres. The colour is height, not a photograph of the ground. These ovals are Carolina Bays, lined up with one another, and a shared alignment is not, by itself, a field of craters.. Michael Davias, Public domain. Shown unedited.