Extraterrestrial impact kills megafauna?

  • Thread starter Andre
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  • #26
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At least the tephra and ice-core layers are related. Hopefully, the eruption was sufficiently singular, rather than extended. It would not do to have timber washed into the lake after laying for indeterminate time(s) en-route...
 
  • #27
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So the counters of the annual sedimentation layers of the Caracio basin counted 200 layers too many maybe? or maybe they corrolated it to other chronologies like the GISPII ice core, firmly assuming it to be right. Only they can tell and there could be other explanations as well.
So guessing that an ice core can contain layers of a vastly different mass when compared to sedimentary basin rock and it is calibrated correctly.

Would isotope levels in the rock be a different result, compared to the ice core due to the geography of the land? Perhaps the sedimentary rock was run off from a mountain where radio carbon was swept down to a basin and collected, increasing in volume? While the ice core was just steady precipitation? Or perhaps the ice core is different due to an uneven precipitation?

By combining different data and techniques they find that the eruption took place in the late spring of 12,916 Cal years BP and...
I do trust that professionals are doing the work correctly or to the best available methods, my real question is just trying to understand the difference in the results. Is it an indication of something significant or unknown, or is it easily explainable by another means? Sorry... I am not up to speed obviously.
 
  • #28
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At least the tephra and ice-core layers are related. Hopefully, the eruption was sufficiently singular, rather than extended. It would not do to have timber washed into the lake after laying for indeterminate time(s) en-route...
I believe that maar-type explosive eruptions are believed to be singelar.

Also if older timber was used to date the event, obviously we would date only the timber and that would make the eruption even younger, farther away from the original assumptions. However I believe that Baales et al 2002 mention that dating is done on in situ buried standing trunks.
 
  • #29
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These questions require some elaborations, unfortunately I am helping relatives move so I get bac to that later.
 
  • #30
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Anyway, yes let's talk ice cores. Obviously the difference between the different Greenland ice cores (DYE-3, GRIP, GISP-II and NGRIP) especially considering the Younger Drays boundaries was reason for a thorough investigation, resulting in a new Greenland Ice Core Chronology 2005 (GICC05) Rasmussen et al, 2006 (draft here).

The result is rather remarkable, in table 4 (page X-12) the date ("Age B2K") of 12,896 years +- 138 is given. B2K is obviously before 2000AD, while the conventional "BP' - Before Present means before 1950AD. Hence we are looking at 12,846 years BP. Are we back to the 12.9 Ka Boundary? Certainly a very interesting development inviting a more thorough look.
 
  • #31
Dotini
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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
 
  • #32
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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.
 
  • #33
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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:

2mcu7iv.jpg


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.
 
  • #34
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Meanwhile, resurrecting the thread, due to a new publication,

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

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.
 
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