otseng wrote:nygreenguy wrote:
I told you HOW layers could become thinner as well. Ice doesnt NEED to change density in order to simply become more compact. Air and other impurities could be pushed out. I also said that the ice may have spread out.
Scotracer wrote:But what you described was a change from liquid water to solid ice with no crystallisation. Snow crystals are something like 92% air and as such can be compressed a huge amount. I mean, just go stand in some freshly fallen snow!
Looking at a concrete example, in
figure a at 1354.65 meters down, it would be ice. Yet, there are no discernable layers in that ice core section. But at 1504.80 m, we see many layers. How can this be explained?
Firstly, while it is apparent that the contrast and sharpness of the layers in "a" is much less than the other figures, I think we can still see that there are changes from light to dark and back that can be characterized as layers.
Secondly, the article explains that the first sample and part of the second have been "relaxed." This means that there has been some change in the core due to the core no longer being under pressure and having been stored for a period of time in less than ideal conditions.
Compared to most of the glacial ice, the Holocene
ice is characterized by being rather transparent (Figure 2a).
In the Holocene ice, bright layers or regions occur occasionally
but not at regular intervals. At high resolution,
millimeter-sized bright patches in the images can be identified
as ice crystal interfaces that have become visible
because of relaxation of the ice. Small submillimeter-sized
spots are air bubbles, whereas clear (dark) layers of ice have
been identified as possible melt layers or ice where bubbles
have converted into clathrate hydrates (Figure 3a)
Compare the first .8 meters of the "c" core with the remainder. The line indicates where the transition from "relaxed" to "unrelaxed occurred. Later the article states:
A very distinct transition between ice drilled during
the 1999 season and ice recovered in the 2000 season is
found at 1751.5 m depth (Figure 2c). The ice that has been
stored for one year at NorthGRIP shows much more
pronounced cloudy bands than the freshly drilled ice. Also
the density of white patches and bubbles are much higher
in the stored ice. This clearly demonstrates that the internal
structure of the ice core relaxes after recovery, even when
the ice is stored under optimal cold conditions. Similar
observations have been made for the GISP2 ice core [Meese
et al., 1997; Ram et al., 2000].
So, the lower resolution of layers in "a" is partly due to this relaxation. Some of it may also be due to simply the nature of the layers.
However, I disagree that it is clear that there are no layers in a.
Secondly, one possible reason for more discernible layers in the lower depths is that compression has enhanced the contrast. It also may be that the climate at those periods created more discernible layers.
Relaxation is alluded several more times in the article, including on
page 9.
The method is applied for depths greater than 1800 m
only. Above 1750 m depth the VS profile has the problem
of relaxation, caused by the storing of the ice, which
introduces noise into the VS intensity profile and dampens
the annual signal (causing our approach to malfunction).
The result of the analysis is shown in Figure 7. The obtained
annual layer thickness profile is seen to vary consistently in
accordance with climate showing thin annual layers during
cold periods and thicker annual layers during periods with
milder climate.
otseng wrote:
I will also point out I alluded to the possibility of what I termed "extrusion" earlier. A section of ice say 10 square cm at the surface could be taking up 20 square cm at depth.
I will also note that some of the articles specifically describe ice flow.
Yes, thin layers would be a result of ice flow, not simply of weight compression.
"Snow falling on the surface does not stay on the surface, but takes a deeper journey through the ice sheet as it is buried by subsequent snow and is compressed into solid ice. Snow falling in the deepest interior parts of Antarctica can take over 100,000 years to reach the ocean."
http://lima.nasa.gov/antarctica/
Ice/snow is not simply moving laterally (as would be the case in extrusion). We also see in the
ice core images that closer to the bottom (particularly g and h), that the layers are irregular. So, ice flow is contributing to this, rather than simply a thinning through extrusion.
So, as you drill vertically, at depth you could be going through layers where the ice was not originally directly under the location at the surface where you are drilling.
I think this is an important point. Because of ice flow, none of the lower layers of ice would have been deposited at the same spot as the ice core sampling. The ice at the bottom of the core would have been deposited a fair distance away from ice at the top of the core. One then cannot say exactly when a lower layer was deposited relative to a higher layer since it was deposited at a different location.
I think it is not fair to say "none of the lower layers would have been deposited at the same spot" as if one drilled the core at a "node" from which the flow was originating, it's possible the vertical integrity of the layers was preserved.
However, even if the flow does create a situation where the lower layers were deposited at the same spot, I fail to see how the lower layers could somehow have come from an earlier period than the higher layers.
Perhaps otseng could explain how this might be possible and if there is any evidence that this could happen.
" . . . 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
