The Younger Dryas Boundary Impact Hypothesis is Going Into the Too-Hard Basket

Wow. I give up!
     When my [soon to be ex-] pal Iain Davidson suggested I get in touch with my inner PalaeoIndian and investigate the Younger Dryas (YD) Boundary (YDB) Impact Hypothesis (YDBIH), I stupidly assumed that it would be a question of sedimentary context, or of association and datable material, or something equally mundane. Was I wrong? IMHO, you might say so.
     As it turns out, my spider senses are telling me that this one isn’t going away for a while. Not because there’s inherent merit in the near-earth-object collision idea. Rather, it appears as if there’ll be claims and counterclaims ’til Hell won’t even have ’em. One side says ‘I see nanodiamonds!’ The other side says, ‘I see dumb people who wouldn’t know a nanodiamond if it fell out of the sky and landed smack on their iris.’ One side says, ‘Nyah!’ Then the other side says ‘Nyah, nyah, nyah.’ I’ll admit, this might be an overly simplistic portrait of the circumstances.
     Take nanodiamonds. The proponents of the impact hypothesis claim that they find significant numbers of them wherever they look at the Younger Dryas lower boundary. Great. What’s a nanodiamond? How do you know when you’ve got ’em? Are they truly of extra-terrestrial origin and do they never form on Terra? Good questions all.
     Apparently they’re ubiquitous in space, the result of fullerene (a form of carbon) ‘onions’ that form naturally and that when deformed by collision reform in such a way that nanometer-scale diamonds are created–by the trillions per cc of interstellar dust. [I’ve seen so many abstracts and summaries in the past few days I won’t even try to ferret out the source(s) of this facet *cough* of the YDB question. But the statements I’m making are based on published research from so many journals with arcane titles like Glacial Geology that my poor little brain is spinning faster than a quasar.] Nanodiamonds can be synthesized in the laboratory, and they’ve been found in meteorites. The jury’s still out as to whether nanodiamonds of extra-terrestrial origin could collect in a particular stratum of a column of unconsolidated sediments in the western United States.  
     Someone claims to have found a ton of nanodiamonds at the YDB in the Antarctic ice cap. Someone else says that those ‘yahoos’ sampled a thin profile at the edge of the ice sheet [not unequivocally in primary context, in other words] and don’t even have a well-supported date for the material! Whom to believe? Nanodiamonds may not have spelled doom for the sabertoothed cats. But they’ve sure killed any idea that I might be able to mount a critical examination of the impact hypothesis without about three more graduate degrees.
     The same is true of the claims for peaks of carbon microspherules at the YDB. Carbon microspherules? Yep. Some say they have ’em at the YDB. Others say those people are seeing fungal spores and misidentifying them. someone else says they found one on their residential mailbox. Sheesh! All I know for sure is the objects that are being reported are DEFINITELY spherical!
     ‘He said; she said’ would be preferable to this supposedly empirically based back and forth. How’s a girl to cope? Any judge would throw this out of court before the first recess.
     No big impact crater? No problem. The inferred object hit the Laurentide ice sheet, creating a continent-sized splash and a freshwater flash flood of Armageddon-like proportions (which also helped to reverse the global warming that was occurring at the time in the Northern Hemisphere). Imagine the swimming-pool sized globules of water that would have been emitted from the point of impact (sort of like the peaks of the milk ‘crown’ that you see in those instantaneous photographs of drop-sized splashes). These are what some argue created the elliptical, lake-sized, Carolina Bays (which visible in numbers across the eastern continental plain most densely in the Carolinas, but which are also seen in the southeast and even in Nebraska. Their long axis orientations unambiguously point back to an area just north of the ice margin near the present-day Great Lakes). These, some claim, are the shallow ‘splash’ scars left by the house-sized globules of water thrown up by the ice-centred impact.
     No impact at all? No problem. Simple. It was a north-south vectored near miss that created a continent-sized ‘airburst’ that vaporized everything in its vortex down to the surface (like the Siberian devastation in the early 20th century that resulted from a NEO NOT impacting the surface–a cometary fragment, it’s assumed). Charcoal from the presumed resultant wildfires that would have been produced in vast quantities across the west of North America south of the ice sheets, and putatively ended up in the so-called black mats found at dozens and dozens of archaeological sites at 12.9 ka. Moreover, the cloud of vapour and dust that would have resulted would have lingered, it’s supposed, long enough to have created a short-lived ‘nuclear winter’ called the Younger Dryas.
     Then somebody else says that in fact the YD is effectively invisible in the Antarctic ice sheet, suggesting that this was a Northern Hemisphere phenomenon resulting from the disproportionately large volumes of fresh water emanating from the North’s vast ices sheets, which took some time to equalize and which stalled or reversed the effects of terminal Pleistocene warming for the duration of the YD.
     To my way of thinking, despite all of the back and forth, all of the bluster, and all of the details associated with the various lines of evidence, and regardless of the constituents of those black mats, their undeniable presence at the YDB in site after site across the western US and directly to the south in Mexico is a phenomenon that begs explanation. As I’ve indicated in the preceding paragraphs, it’s still an open question what they contain, and if, at the end of the day, those constituents are found to be the unique result of an encounter with a NEO. But the answers must be found, and the mystery put to rest.
     And I’m sad to ask: ‘Who am I to adjudicate the contrary claims? Especially since my pocket SEM crapped out the other day and I can’t afford a new one! Oh, and, by the way, I’m not a nanogeologist [not many are, I’m guessing]. On the basis of my ‘investigation’ of this matter, I’m calling for an international consortium of scientists possessing the requisite expertise who have NO vested interest to undertake a large-scale investigation of this phenomenon–from, as it were, the ground up.

A Clairvoyant’s Touchstone Thursday: Nanodiamonds are a Sabertoothed Cat’s Worst Enemy

Because I’ve been absent from pixel-land for almost a week, I’ve decided to combine what I was working on before I travelled to Vancouver with Touchstone Thursday. This will entail bringing to your attention not works that have stood the test of time, but rather some recent and fairly recent publications that may very well turn out to be classics in their own right, for any number of reasons. It began with a challenge–to bend my brain in the direction of an issue that’s currently being debated and for which there’s some very up-to-the-minute work that may well be a game breaker in a long-elusive geological and archaeological question. 
     You’ll never guess what the issue is. Go on. Give it a try. Hint: it’s not, strictly speaking, an archaeological issue.

[Final Jeopardy music tinkles in the background]

Alex Trebek [from the podium, as serious and sincere as ever, considering that there are thousands of dollars at stake and millions watching and waiting for the day’s champion to make it a clean sweep]: 

“10 seconds left. tic. tic. tic. tic. tic. tic. tic. tic. tic. … Bob Muckle, what’s the answer to today’s Final Jeopardy question: Black mats that resulted from a close encounter with a near-Earth object are indicative of what?” 

Bob [smiling sheepishly, knowing that he’s nailed it, ’cause he told them on his application that he was a writer of children’s books instead of text books.*]: 

“Uh. What are megafaunal extinctions in North America, Alex?”

[Sound of jackpots being hit, just like in Vegas]

Yup. That’s it. Megafaunal extinctions. Thanks, Dave-oh! Couldn’t have given me low hanging fruit for a change? Could you? Hey, Sysiphus! Wanna gimme a hand? 
     Here’s the crux. It’s my belief that there are only two issues in archaeology and palaeoanthropology that are more hotly debated than megafaunal extinctions: 1) ‘Whose hominid is going to end up being the last common ancestor?’ and 2) ‘Are Neanderthals like us?’ [and all that entails]. Even the question of who came first to North America–the Clovis people or the Clovis people who weren’t yet Clovis people–pales by comparison to the three questions above.
     So, what could I possibly [and I mean it] contribute to the decades-long ‘conversation’ about North America’s geologically recent mass die-off? What, indeed? Perhaps we’ll see. To begin with, I’ll have to set the scene in a way that’s not normally done, by modeling the broad environmental conditions south of the Pleistocene glaciers and what implications they would have had for the earliest human occupations.
     Okay… Well… Here goes… First slide, please. Oops. Sorry! I have the remote! Heh, heh, heh. 
     Some of this will be review for the older students, so bear with your younger colleagues and meet me down around RanchoLaBrean or thereabouts.

All righty. The map above shows the more-or-less official maximum extent of the North American ice sheet at about the time the last deglaciation commenced. This vast accumulation of snow and ice was in fact the coalescence of two great ice sheetsthe Laurentide (centred on what’s now Hudson Bay) and the Cordilleran (aligned with and mantling everything to the left and right of the Rocky Mountains down to about the elevation of the Canadian prairies). To the northwest of the ice sheet lay Beringia–that continental-scale landmass between Asia and North America that comprised a biome that’s been called the Mammoth Steppe [it’s always been termed the ‘Bering Land Bridge,’ but that really obfuscates the reality–it was about 1000 km from north to south. Thus, hardly a ‘bridge’]. Beringia hosted lots of big hairy elephants and other big, most likely hairy, quadrupeds. Directly south of the ice sheet, the Prairies. To the southwest, the Rockies, the Inter-montane Plateau and the Coast Ranges. To the southeast, Appalachia, the Piedmont and the Coastal Plain. For some distance to the south of this ice mass, from the Pacific to the Atlantic coast, periglacial (permafrost) conditions would have been the norm. 

     And what would this mean old periglacial environment look like? Millions of square miles of this when it’s cold out.
And a lot of this when the ‘summer’ arrives.
Or this
This was not a hospitable landscape. 
     And, when you consider that the combined seasonal meltwaters along the middle third of the ice sheet drained into what’s today the gigantic narrowing geographic funnel that is the Mississippi/Missouri River catchment, which narrows southward and becomes the Mississippi River Valley, you can probably imagine the ‘downstream’ extent of the landscape you see in the above illustrations. If you also include the summer runoff from the Rockies via the Arkansas River, you have an enormous affected area. Think of it, the Mississippi River is in reality a misfit (or underfit) stream
     This periglacial environment I’m talking about would have involved much or all of the states of Washington, Oregon, Idaho, Montana, the Dakotas, Nebraska, Iowa, Illinois, Indiana, Ohio, Kentucky, Missouri, Mississippi, Louisiana, Tennessee,West Virginia, Virginia, Pennsylvania, New Jersey, Maryland, and Delaware. 

You can see the affected area better in the map above. I’ve drawn in the red arrows to show the approximate route of the seasonal glacial meltwaters. Remember those desolate outwash plain pictures up above? That’s right. All but the uplands in the middle third of the lower 48 would have looked like the wetter of the two much of the time. 
     So, what can I possibly contribute to the question of when and where the Clovis people became the Clovis people? I think it’s clear that it wasn’t within several hundred kilometres of the ice sheet’s southern margin at 15,000 BP, and not so much in what I’d call ‘The Great Outwash Plain of the Americas’ as long as that area functioned as such, which would have been the case in the first several hundred to a couple of thousand years after the last glacial maximum.
     ‘But what about the megafauna?’ ask the smaller children at the front. ‘You promised you were gonna talk about the big, scary animals!’
     Okay, okay. Listen up, kids. Here’s a picture of one of the Pleistocene faunal communities south of the ice sheets, near Rancho LaBrea, in the western United States. And, yes, those are horses. There were camels, too. Both extirpated during this extinction ‘event.’ The big question has always been ‘Why?’

The explanation has boiled down to two mostly mutually exclusive schools of thought–either it was the climate changes that accompanied the end of the Pleistocene, or the gluttony of the newly arrived people that occupied this vast new territory at the end of the Pleistocene. ‘Debate’ has raged for decades. But lately there’s been a third candidate to explain the die-off: a so-called near-earth object (NEO) that scorched the western U.S. as it ignited the atmosphere and either broke up or ended up in the ocean.
     [If this feels like déjà vu to many of you…yes, a similar set of positions characterized the debate about the dinosaur die-off at the end of the Cretaceous. Odder, still, it played out mostly in the pages of PNAS, too. Hmmm…]
      So, let’s meet the players. In this corner, a heavyweight in the area of megafaunal extinctions: C. Vance Haynes, Jr. and a moderate-sized group of naysayers. In the other corner, a growing number of scientists of various stripes, all of whom are claiming to have found ‘smoking gun’ evidence of a near collision between a NEO and our Earth that scarred the contiguous United States, southern Canada, and central Mexico, around 12,900 years ago. 

Haynes 2008 get url/ref

This controversy seemed to have grown legs just about the time Haynes–one of the most peripatetic and prolific geoarchaeologists on the planet–correlated the so-called ‘black mats’ that occur in numerous geological and archaeological localities with which he was familiar from his extensive fieldwork in North America. The ‘black mat’ sites are shown as filled black circles on the map above, taken from Haynes (2008). Notice the paucity of circles of any kind in the Great Outwash Plain of the Americas? Coincidence? Don’t think so.
     Back to the black mats. It turns out that these charcoal-rich layers are all about 12,900 years old (Haynes 2008), and found across western North America and putatively in the extremely interesting geological features on the eastern coastal plain–the so-called Carolina Bays [which resemble nothing so much as titanic-sized rain-drop scars from a hard, slanting downpour]. The black mats, therefore, are a continent-wide coincidence that begs for an explanation. Not the first to consider the possibility of an extra-terrestrial object, nevertheless Haynes concludes that, while there might well be a connection to cosmic collision, more work needs to be done. He said a mouthful there.
     In the next part of my challenge, I’ll examine: 

1. Firestone RB, et al. (2007) Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. Proc Natl Acad Sci USA 104:16016 –16021.

2. Kennett DJ, et al. (2008) Wildfire and abrupt ecosystem disruption on California’s Northern Channel Islands at Allerød-Younger Dryas (13.0–12.9 ka). Quat Sci Rev 27:2530 –2545.3. Kennett DJ, et al. (2009) Nanodiamonds in the Younger Dryas boundary sediment layer. Science 323:94.

4. Kennett DJ, et al. (2009) Shock-synthesized hexagonal diamonds in Younger Dyras boundary sediments. Proc Natl Acad Sci USA 106:12623–12628.

5. Haynes CV (2008) Younger Dryas ‘‘black mats’’ and the Rancholabrean termination in North America. Proc Natl Acad Sci USA 105:6520 – 6525.

6. Israde-Alcántara, I., et al.  (2012) Evidence from central Mexico supporting the Younger Dryas extraterrestrial impact hypothesis. Proc Natl Acad Sci USA

7. Surovell TA, et al. (2009) An independent evaluation of the Younger Dryas extraterrestrial impact hypothesis. Proc Natl Acad Sci USA 106:18155–18158.

* He was later heard to say: ‘Hell! They looked like kids to me!’