Also for the record, the 123 kyr BP date they gave was not a result from their findings, but a reiteration from earlier works.
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.
This date poses no problem for the FM. But, where it does pose an issue is how can all the layers below it be explained? I've got my own theory on this and still pondering over it.
(Just for my reference:)
It is thought that Katla is the source of the Vedde Ash (tephra dated to 10,600 years BP[1]) found at a number of sites including Norway, Scotland and North Atlantic cores.
http://en.wikipedia.org/wiki/Katla
2) We see not only annual layers, but also larger scale variations in climate based on the brightness or darkness of the bands.
I'm not so sure they can "see" accurately annual layers. They mention several problems regarding this including subannual layers and variability in layer counts due to different contrast settings.
because some years may experience more
depositional events than others, some annual layers will
appear as multiple visible layers in the VS profile, while
others may only be weakly represented in the stratigraphy.
Inspection of the VS profile, at depths where the annual
layers can be identified from the CFA profiles, shows that
multiple-layer years appear frequently. Another difficulty
is caused by the great variability in intensity of the visible
layers, which complicates the counting. Depending on the
contrast enhancement of the images and on the selection
criteria used for identifying the layers, one can end up
counting a wide range of layers within the same ice core
section, e.g., as the contrast of an image is increased, more
and more layers tend to appear.
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.
There does seem to be an inverse correlation between the two. However, there is also a direct relationship between the dust concentration and stratigraphy intensity. So, it could simply be dust that is causing the O18 profile, rather than attributing it to yearly changes. Also, looking at the charts do not reveal any yearly patterns.
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.
After reading the article, I'm now under the impression that there is not a standard method to determine annual layers.
The articles cites Shimohara et al as one approach to determine annual layers:
One way to overcome the multiple-layer problem for
annual layer counting is to smooth the intensity profile such
as proposed by Shimohara et al. [2003] in a case study of
10 sections of GRIP and NorthGRIP glacial ice. Although
this approach is promising, the smoothing method assumes
preknowledge of the mean annual layer thickness from
modeling, which may not always be justified. Furthermore,
the number of annual layers that is identified within a
section of ice is likely to vary with the width of the filter
applied for smoothing the profile.
The paper presents a different method to determine annual layers by "applying a frequency analysis to
sections of the VS intensity profile."
Both of these methods do not simply "count" the layers, but require mathematical modeling and for the underlying assumptions to be true. In the Shimohara approach, it "assumes preknowledge of the mean annual layer thickness." In the Svensson approach, a "successful outcome of such a method requires a certain regularity in the annual layer thicknesses within each considered section." It also relies on the ss09sea timescale model.
