2012-02-24 22:57:34Was the Younger Dryas cooling triggered by an impact?
John Mason

johntherock@btopenworld...
86.133.202.150

Hi folks,

Just putting a post together but sat surrounded with papers there is one thing I am noticing: papers describing evidence for an impact keep citing that the megafauna extinction and disappearance of the Clovis culture were both abrupt. However, Gill et al in Science 326 (2009) used dung-fungal spore records to show that the megafauna decline was at its most intense from 14,800 to 13,700 years ago and the fauna then remained at low levels through to 11,500 years ago when it finally disappeared, 500 years before the Folsom culture appeared, with spearpoints not dissimilar to Clovis ones. If this is the case, neither an impact nor the Younger Dryas cooling or the Clovis culture saw off the megafauna in its entirety. One can imagine all of them would have created ecological stresses and it's not impossible that an impact caused a cooling due to aerosols: also I note that Murton et al writing in Nature 464 (2010) have identified a main lake Agassiz outburst path that, instead of pouring into the N Atlantic, went down the Mackenzie river-system to enter the Arctic Ocean in the area of the Beaufort Sea. They cite that as the trigger for the Younger Dryas: however, a major freshwater pulse there is a different kettle of fish to one inputting straight into the N Atlantic via the St Lawrence river, for which there is little real evidence: as Broecker himself said in 2006: "our inability to find the path taken by the flood is disconcerting".

The mystery deepens.... anyone know of any studies that correlate with, or fly in the face of, Gill et al?

It's been a while since I read up on this interesting period of change and thinking does seem to have altered considerably. Any feedback at this stage would be very useful.

Cheers - John

2012-02-25 03:12:23
Chris Colose

colose@wisc...
96.249.11.201

John

 

The Younger Dryas comet hypothesis has been falsified with high confidence, and is no longer considered serious by the paleoclimate community (I can say that the IPCC AR5 gave about one sentence to it in their paleoclimate discussion, basically saying the same thing).  There was a paper by Pinter et al. that put most of this to rest, and another one by Melott et al.  This short writeup by Anders Carlson, one of my old professors, is a decent review.

2012-02-25 03:33:58
John Mason

johntherock@btopenworld...
86.133.202.150

Chris, it's still rumbling on! There's a heap of new papers since AR5 - link to the latest below. Cheers - John

Framboidal iron oxide: Chondrite-like material from the black mat, Murray Springs, Arizona

 

2012-02-25 04:01:46
Ari Jokimäki

arijmaki@yahoo...
91.154.109.19

Here are some papers on the YD event. I think the latest studies are against the impact hypothesis, although I couldn't access the paper in John's link (log in required).

2012-02-25 18:32:54
Glenn Tamblyn

glenn@thefoodgallery.com...
58.167.41.21

Important to draw a distinction here. Between the cause of the YD event (bolide, something else), and the consequences of the YD. We need to be sure that discussion of the causation doesn't interfere with discussion of what the consequences mean.

2012-02-25 19:02:14
John Mason

johntherock@btopenworld...
86.133.202.150

The impression I am getting is that the extinction was not caused by any impact, but the YD possibly was, at least partially, and it in turn may have finished off the megafauna.

Ari - thanks for that. I have most of those plus some others from 2008 onwards.

Chris - that's a useful review. It does highlight some of the contradictions I flagged-up in the first post. All in all, the YD remains an enigmatic event.

Not sure why that link doesn't work. Here's the abstract of the new paper. The lead author has sent me the full text.

 

Framboidal iron oxide: Chondrite-like material from the black mat, Murray Springs, Arizona

  • Mostafa Fayeka, Posted Image, Posted Image,
  • Lawrence M. Anovitzb,
  • Lawrence F. Allardc,
  • Sharon Hulld
  • a Department of Geological Sciences, University of Manitoba, 240 Wallace Building, Winnipeg, Canada MB R3T 2N2
  • b Chemical Sciences Division, ORNL, Oak Ridge, TN 37831-6110 USA
  • c Materials Science and Technology Division, ORNL, Oak Ridge, TN 37831 USA
  • d Department of Anthropology, University of Manitoba, Winnipeg, Canada MB R3T 2N2



Abstract
At the end of the Pleistocene a Younger Dryas “black mat” was deposited on top of the Pleistocene sediments in many parts of North America. A study of the magnetic fraction (~ 10,900 ± 50 B.P.) from the basal section of the black mat at Murray Springs, AZ revealed the presence of amorphous iron oxide framboids in a glassy iron-silica matrix. These framboids are very similar in appearance and chemistry to those reported from several types of carbonaceous chondrites. The glass contains iron, silicon, oxygen, vanadium and minor titanium, while the framboidal particles contain calcium as well. The major element chemistry of both the spherules and the glass matrix are consistent with the chemistry of material associated with meteorite impact sites and meteorites. Electron microscopy confirms that the glassy material is indeed amorphous, and also shows that what appear to be individual oxide particles are amorphous as well. The latter appears consistent with their overall morphology that, while euhedral, typically shows significant fracture. Based on these data, we argue that these particles are the product of a hypervelocity impact.

 

Cheers - John

 

2012-03-08 21:58:06
John Mason

johntherock@btopenworld...
81.129.232.193

More:

http://www.pnas.org/content/early/2012/03/01/1110614109.abstract

 

We report the discovery in Lake Cuitzeo in central Mexico of a black, carbon-rich, lacustrine layer, containing nanodiamonds, microspherules, and other unusual materials that date to the early Younger Dryas and are interpreted to result from an extraterrestrial impact. These proxies were found in a 27-m-long core as part of an interdisciplinary effort to extract a paleoclimate record back through the previous interglacial. Our attention focused early on an anomalous, 10-cm-thick, carbon-rich layer at a depth of 2.8 m that dates to 12.9 ka and coincides with a suite of anomalous coeval environmental and biotic changes independently recognized in other regional lake sequences. Collectively, these changes have produced the most distinctive boundary layer in the late Quaternary record. This layer contains a diverse, abundant assemblage of impact-related markers, including nanodiamonds, carbon spherules, and magnetic spherules with rapid melting/quenching textures, all reaching synchronous peaks immediately beneath a layer containing the largest peak of charcoal in the core. Analyses by multiple methods demonstrate the presence of three allotropes of nanodiamond: n-diamond, i-carbon, and hexagonal nanodiamond (lonsdaleite), in order of estimated relative abundance. This nanodiamond-rich layer is consistent with the Younger Dryas boundary layer found at numerous sites across North America, Greenland, and Western Europe. We have examined multiple hypotheses to account for these observations and find the evidence cannot be explained by any known terrestrial mechanism. It is, however, consistent with the Younger Dryas boundary impact hypothesis postulating a major extraterrestrial impact involving multiple airburst(s) and and/or ground impact(s) at 12.9 ka.

 

It seems that the theory is resurrecting itself! I have a partially-completed review of the whole story ongoing. Will post up once it's done.

Cheers - John

2012-03-09 05:53:41
Daniel Bailey
Daniel Bailey
yooper49855@hotmail...
24.213.18.68

Looking forward to it, John!

2012-03-09 06:48:13
Sphaerica

Bob@Lacatena...
72.74.77.39

Perhaps:

http://www.nature.com/nature/journal/v479/n7373/full/nature10574.html

with an overview/synopsis by Gill here:

Climate AND humans? A new study using ancient DNA, fossils, & models contributes to a classic problem in paleoecology

2012-03-10 01:50:20
John Mason

johntherock@btopenworld...
81.129.232.193

Another to add to the collection - thanks!

Cheers - John