Extraterrestrial impact kills megafauna?

[Dotini]

Anyway, yes let's talk ice cores.

Andre, is it conceded that ice core records can be muddled by layers formed during a season of melts/refreezing due to weather events?

Respectfully,
Steve

 

[Andre]

Sort of, Rasmussen et al are explaining elaborately about the uncertanties of what constitutes a year. One needs to read the paper in toto especially part 4, identification of annual layers. They use several proxies in order to find back an annual signal and provide an optical example (fig 3) that seems to be okay. However, in how many cases there was doubt is not really clear.

What is clear however is that Rasmussen et al do not waste a single word comparing their timescale with the milestones to other chronologies. That's where it gets very interesting.

More later.

 

[Andre]

The first check that Rasmussen et al could have done is verifying the dating of another most important volcanic tephra layer, the Vedde Ash. It has been radiocarbon dated extensively to an average of 10,310 +/- 50 14C years

Calibrated with Intcal09 this gives a range of 12,039 - 12,137 Cal BP

Rasmussen et al count the Vedde Ash in table 4 on 12,121 Cal BP (12.171 B2K) +/- 114 years hence a range of 12,007 - 12,235 years Cal BP. We see that both ranges overlap nicely abeit that Rasmussen puts the mean weight a few decades later.

Next we could also compare the onset and termination of the Younger Dryas as summarized by Baales et al 2002 in table 4 (p285) with the timescale of rasmussen, GICC05:

See for caption Baales et al 2002, note that the dating in there for GISP-II (Alley et al., 1993) and GRIP (Johnsen et al., 1992) are early versions, superseded later. The lower two rows are depicting the dating of the Younger Dryas, based on their layer counting between the Laacher See Tephra and the onset of the Younger Dryas and then the termination, by adding the counted duration of the YD. Note that Brauer et al 1999, (in the column "MFM*") the only varve layer counting chronology from year zero, arrives at a slightly younger age.

Now see that all proxies agree closely to the termination of the Younger Dryas, only a few years away from the average of 11,570 years Cal BP. Rasmussen et al, (GICC05) however are now way off, all of a sudden with 11,653 years Cal BP, the difference increasing at the onset of the Younger Dryas all records being generally well within two decades of 12,712, against 12,846 for GICC05.

Seems indeed that some scrutiny would be in order.

 

[Andre]

Meanwhile, resurrecting the thread, due to a new publication,

http://www.pnas.org/content/early/2012/06/14/1204453109.full.pdf

Abstract

It has been proposed that fragments of an asteroid or comet impacted Earth, deposited silica-and iron-rich microspherules and other proxies across several continents, and triggered the Younger Dryas cooling episode 12,900 years ago. Although many independent groups have confirmed the impact evidence, the hypothesis remains controversial because some groups have failed to do so. We examined sediment sequences from 18 dated Younger Dryas boundary (YDB) sites across three continents (North America, Europe, and Asia), spanning 12,000 km around nearly one-third of the planet. All sites display abundant microspherules in the YDB with none or few above and below. In addition, three sites (Abu Hureyra, Syria; Melrose, Pennsylvania; and Blackville, South Carolina) display vesicular, high-temperature, siliceous scoria-like objects, or SLOs, that match the spherules geochemically. We compared YDB objects with melt products from a known cosmic impact (Meteor Crater, Arizona) and from the 1945 Trinity nuclear airburst in Socorro, New Mexico, and found that all of these high-energy events produced material that is geochemically and morphologically comparable, including: (i) high-temperature, rapidly quenched microspherules and SLOs; (ii) corundum,mullite, and suessite (Fe3Si), a rare meteoritic mineral that forms under high temperatures; (iii) melted SiO2 glass, or lechatelierite, with flow textures (or schlieren) that format >2,200 °C; and (iv) particles with features indicative of high-energy interparticle collisions. These results are inconsistent with anthropogenic, volcanic, authigenic, and cosmic materials, yet consistentwith cosmic ejecta, supporting the hypothesis of extraterrestrial airbursts/impacts 12,900 years ago. The wide geographic distribution of SLOs is consistent with multiple impactors.

The article is very elaborate and puts the ball back in the field of the sceptics. One can follow the dispute on wiki.

I repeat my take, since, as far as I know, the complete fingerprint of the Younger Dryas in all proxies (like the ice cores - including deuterium excess, but also in oceanic sediment cores) is about equal to the fingerprints of the Dansgaard Oeschger (D-O) events, it's hard to see why we need impacts to cause that, where the D-O events did without.

Nevertheless impacts do happen, and I'm more than happy to accept that there was/ were (multiple) impact(s) at the onset of the Younger Dryas, the problem is that cause and effect may be a lot more complicated.