Here is another example of scientists looking at the formation of ice layers.
Surface melting rarely occurs across most of the Antarctic ice sheet, away from the warmer coastal regions. Nonetheless, isolated melt features are preserved in the firn and ice in response to infrequent and short-lived melting events. An understanding of the formation and occurrence of these melt layers will help us to interpret records of past melt occurrences from polar ice cores such as the Siple Dome ice-core record from West Antarctica. A search in the near-surface firn in West Antarctica found that melt features are extremely rare, and consist of horizontal, laterally continuous, one to a few millimeter thick, ice layers with few air bubbles. The melt layers found date from the 1992/93 and 1991/92 summers. Field experiments to investigate changes in stratigraphy taking place during melt events reproduced melt features as seen in the natural stratigraphy. Melting conditions of varying intensity were created by passively heating the near-surface air for varying lengths of time inside a clear plastic hotbox. Melt layers formed due entirely to preferential flow and subsequent refreezing of meltwater from the surface into near-surface, fine-grained, crust layers. Continuous melt layers were formed experimentally when positive-degree-day values exceeded 1C-day, a value corresponding well with air-temperature records from automatic weather station sites where melt layers formed in the recent past.
Note that in this case they are actually running a semi-controlled experiment. They are creating melting events and layers as a result of these events in order to compare them with melting events that occurred in the recent past naturally.
See
http://cat.inist.fr/?aModele=afficheN&cpsidt=17339294
This one is just for future reference.
http://homepages.ulb.ac.be/~fpattyn/ami ... abstr.html
and this.
http://www.igsoc.org/journal/33/. Ignore for now.
The following however is relevant to the current discussion.
From the previously
cited book by Crary.
Note on page 120 that during one of the monitoring trips, snowpits were dug and layers were measured for granularity and hardness. Annual layer interpretation is mentioned. Diagrams of the layers were made. This is in the early 1960's.
Several monitoring trips were made along a trail with bamboo markers used to check for ice motion and accumulation. It looks like an average of around 115 cm of accumulation occurred over a 5-year period from 1958 to 1963. This is on the Ross ice shelf, so evidently the accumulations there are significantly higher than the 5 cm continental average.
Page 123 again refers to stratigraphy analysis. Again, my main point is that scientists have been making observations of layer formation for decades. In this study, they seem to be looking at layers not only from a visual standpoint, but also using density and grain size.
Also, note in the discussion section on page 123 that snowfall accumulations can easily vary from year to year by a factor of 3 to 4. They also note that, especially in areas where the terrain is not flat, that annual layers can vary widely in thickness over small geographic areas.
They also mention, in the right hand column on page 123, the need to consider whether the three year averages of accumulation they calculate will be above or below the climactic average over long periods. This indicates scientists, even in 1963, understood long term climactic changes needed to be taken into account when analyzing layers.
In another article in this same book on
page 223, there is a picture of layers and they discuss a technique using "oil-burning" to enhance contrasts in the layers.
Page 224 on the right discusses some of the difficulties in stratigraphy but also some of the methods to help determine annual layers. Note again the use of density and grain sizes as well as the crustiness of winter versus summer snow. On this page and the next, they discuss the possibility of no accumulation during a year or during one season (eg. summer) of the year and how this can be detected. They even discuss how to detect the phenomenon of summer accumulation being removed during the subsequent winter!
Certainly no one can say scientists have not been careful in considering possible problems with determining annual layers. Certainly it does not seem that scientists are blithely "assuming" layers are annual. They are carefully attempting to determine, but multiple methods, which layers are annual. They are carefully accounting for numerous different seasonal weather patterns, as well as the possibilities for long term climactic changes. They are using observations made at the current time of layers as they form to inform their methods for studying deeper layers.
And again, let me emphasize this is from a book that is 40 years old.
Clearly, if the scientists say a given layer is annual, there is a lot to back up the assertion.
" . . . 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
