micatala wrote:Here is an article on visual stratigraphy from Greenland.
http://www.gfy.ku.dk/~www-glac/ngrip/pa ... fs/206.pdf
It has a 2005 publication date. The article includes quite a number of good graphics, including a sample of 9 fairly long ice core samples from varying depths.
This will take some time to wade through, but does seem very relevant to the discussion. It specifically mentions how deep the visual layers can be discerned.
To paraphrase the hawkers of reading material of various kinds, "if you only read one article on ice cores this week, this is the one!"
OK. Let's see what this article has to say:
A continuous high-resolution record of digital images has been obtained from the
North Greenland Ice Core Project (NorthGRIP) ice core (75.1N, 42.3W) in the depth interval from 1330 m to the bedrock at 3085 m. The ice core stratigraphy is clearly visible throughout the glacial period with the most frequent and brightest visible layers appearing during the coldest events. Down to a depth of 2600 m the horizontal layering is very regular; below this depth, small irregularities in the layering start to appear, and below 2800 m the visual stratigraphy becomes more uncertain, perhaps because of penetration into climatically warmer ice.
Comparison of the visual stratigraphy with high-resolution continuous records of chemical impurities and dust reveals a high degree of correlation, which indicates that the visible layers are caused by these impurities. A new approach is used to automatically determine annual layer thicknesses from the visual stratigraphy record by carrying out a frequency analysis of the most prominent visible layers in the profile. The result gives strong support for the NorthGRIP timescale model.
Using the techniques described later, visual layers are clearly discernible.
THey are highly correlated with other data.
THey
strongly support the existing timescale for this area. For the record, the ice sheet here is said to go back 123,000 years.
The ice core is 3085 m long and covers the Holocene, the
entire last glacial period, and part of the previous interglacial
period, the Eemian, back to approximately 123 kyr BP.
The images are obtained digitally using a mechanism which backlights the core indirectly. This is described on page 2.
Nine sample images from various depths are shown on page 3. Certainly the layers show a lot of variation regarding thickness, sharpness, contrast, etc. but are for the most part very clearly discernible.
The article notes that the visual analysis was compared with several types of chemical analyses, including "calcium Ca++, sodium Na+, ammonium NH4
+, sulphate SO4 , nitrate NO3 , the electrolytical conductivity of
the melt water, and the amount of insoluble dust."
The stratigraphy appearance can change significantly with previously determined past climactic changes.
Throughout the glacial period, the ice core stratigraphy
is clearly visible. During the coldest climatic events the
intensity and the frequency of visible layers or cloudy bands
are highest (Figures 2b–2d). During milder interstadials the
layering is also clearly visible after contrast enhancement of
the images, even when the stratigraphy of the core is barely
visible to the naked eye. An example of an abrupt climatic
transition during the glacial period is given in Figure 2e,
which shows the transition into the mild glacial interstadial
19 as a sharp drop in intensity of the VS over some 10 cm of
ice. The glacial profile is very detailed and reveals very
sharp transitions in the occurrence of visible layers, which
show a large variability in intensity and thickness even over
short depth intervals (Figure 3c).
Later on that page the authors describe how the lower layers show bending and are less discernible. Clearly the authors are being cautious not to overstate the case.
Presence of volcanic ash in some layers is clearly discernible and noted. See Figure 3b on page 5.
1506.1 m depth, the visible ‘‘Vedde’’ ash layer in
Younger Dryas (also shown in Figure 2b).
This layers is mentioned in
wikipedias page on Tephochronology, which is the science of using volcanic tephra in dating. The key is:
The premise of the technique is that each volcanic event produces ash with a unique chemical "fingerprint" that allows the deposit to be identified across the area affected by fallout. Thus, once the volcanic event has been independently dated, the tephra horizon will act as time marker.
See also
http://www.tephrabase.org/tephrochron.html.
The Vedde Ash layer has been identified in a number of areas in Iceland and Norway and even Great Britain. It is dated to over 10,000 years before the present.
[url=http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WPN-45N44P5-1X&_user=8505058&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1042009561&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=8505058&md5=a44ab4dd8e109477f7caf5f9421fcb5f]Birks et al[/url] wrote:
The Vedde Ash Bed (mid-Younger Dryas) and the Saksunarvatn Ash (early Holocene) are important regional stratigraphic event markers in the North Atlantic, the Norwegian Sea, and the adjacent land area. It is thus essential to date them as precisely as possible. The occurrence of the Saksunarvatn Ash is reported for the first time from western Norway, and both tephras are dated precisely by AMS analyses of terrestrial plant material and lake sediment at Kråkenes. The Vedde Ash has been previously dated at sites in western Norway to about 10,600 yr B.P. It is obvious in the Younger Dryas sediments at Kråkenes, and its identity is confirmed geochemically. The mean of four AMS dates of samples ofSalix herbacealeaves adjacent to the tephra is 10,310 ± 50 yr B.P. The Saksunarvatn Ash is not visible in the early Holocene lake sediment at Kråkenes. After removal of organic material and diatoms, the identity of the tephra particles was confirmed geochemically, and their stratigraphic concentration was estimated. From curve matching of a series of seven AMS dates of terrestrial plant macrofossils and whole sediment, the radiocarbon age of the ash is 8930–9060 yr B.P., corresponding to an age of 9930–10,010 cal yr B.P. (7980–8060 cal yr B.C.).
THis was confirmed by a number of other abstracts for scientific articles.
Thus, we have additional support for several previous claims.
1) Visual layers, albeit enhanced by various techniques, are visible thousands of meters below the surface.
2) We see not only annual layers, but also larger scale variations in climate based on the brightness or darkness of the bands. I had alluded to this possibility earlier in discussing pictures of snow pits.
3) Visual layers are correlated with many other types of data, including chemical data. FOr example, note the incredible correspondence shown in Figure 4 on page 6 of the Svensson article between the visual stratigraphy and the O18 profile. Sharp rises in the visual intensity correspond exactly with sharp drops in the stable isotope graph.
4) Volcanic events can be discerned within layers and can be used as markers.
Now, to address otseng's suggestion that scientists cannot tell which layers are annual and which are not, note in this (and other articles) scientists are careful to note how confident they can be the layers are annual and when the layers do not have sufficient integrity to be used for dating. There is no indication that scientists are blithely assuming anything they see is an annual layer. Rather, they carefully test whether the visual stratigraphy corresponds with other dating measures.
Note that we have an event, the Vedde Ash layer, dated by carbon dating, fossil analysis and other means not involving ice cores to a little over 10,000 years before the present.
Note that this later, identified by its chemical signature, is found in the Northern Greenland ice core.
Considering the careful and cautious nature of the discussion in this article, if the visual stratigraphy indicated say 20,000 layers prior to the Vedde event instead of about 10,000,
don't you think the scientists writing this article would have noted that. But instead, they note that their visual stratigraphy provides
strong support for the dating conclusions that had already been reached.
How is it at all credible to suggest that scientists are confusing multiple layers created by individual snowfall events with annual layers??
As one final citation, here is an abstract which discusses comparisons of multiple ice cores with several other types of dating evidences.
Abstract
A new Greenland Ice Core Chronology (GICC05) based on
multi-parameter counting of annual layers has been obtained for the last 42 ka. Here we compare the glacial part of the new time scale, which is based entirely on records from the NorthGRIP ice core, to existing time scales and reference horizons covering the same period. These include the GRIP and NorthGRIP modelled time scales, the Meese-Sowers GISP2 counted time scale, the Shackleton–Fairbanks GRIP time scale (SFCP04) based on 14C calibration of a marine core, the Hulu Cave record, three volcanic reference horizons, and the Laschamp geomagnetic excursion event occurring around Greenland Interstadial 10. GICC05 is generally in good long-term agreement with the existing Greenland ice core chronologies and with the Hulu Cave record, but on shorter time scales there are significant discrepancies. Around the Last Glacial Maximum there is a more than 1 ka age difference between GICC05 and SFCP04 and a more than 0.5 ka discrepancy in the same direction between GICC05 and the age of a recently identified tephra layer in the NorthGRIP ice core. Both SFCP04 and the tephra age are based on 14C-dated marine cores and fixed marine reservoir ages. For the Laschamp event, GICC05 agrees with a recent independent dating within the uncertainties.
Note the multi-parameter counting.
Note they freely admit that one chronology might by off by as much as 1000 years from another. However, this is within an overall chronology going back 42000 years. Again, if there was a huge problem discerning between snowfall events which might take place once or twice a year to over a dozen in some areas with annual layers, how would scientists be off by less than 5% between dating techniques that are being used not only in ice but on land and in sea? How would all the different methods and locations still give results that are so close over periods of many tens of thousands of years?
All of this seems to put the Greenland ice sheet at clearly over 40,000 years and with a good deal of confidence over 120,000 years.
" . . . the line separating good and evil passes, not through states, nor between classes, nor between political parties either, but right through every human heart . . . ." Alexander Solzhenitsyn
