The Quelccaya glacier is at a similar latitude to Kilimanjaro and is also receding. It’s a logical point of comparison. Core 1 is 163.6 m deep (Summit Core- 154.8 m) and is attributed a start date of 470 AD (Summit Core: 744 AD). Annual dust layers are a guide to dating in the upper portions. In Core 1, the layer dated to 1800 AD is at 106 m in depth, the layer dated to 1590 AD at 130 m in depth (Summit – 120 m). It is both much younger and much thicker than Kilimanjaro. If you calculate accumulation rates at both glaciers according to a thickening model, it turns out that the assumed accumulation at Kilimanjaro is about 100 times lower than at Quelccaya, which is a young glacier. Precipitation levels appear to be comparable.
Glaciers become increasingly compressed with age. The usual equation linking the total thickness of the glacier H and the annual accumulation a to the age at a given depth z is as follows:
The main sensitivity in this equation is to a. For Quelccaya, the average accumulation is said to be 1.368 m. See footnote at ftp://ftp.ncdc.noaa.gov/pub/data/paleo/icecore/trop/quelccaya/q83cor1.txt. There is an interesting aliasing artifact discussed by Hans Erren here.
Assuming that I’ve done this right – I’ve cross-checked it, but this is a new calculaiton for me – putting H at 165 m, the value of a needed to yield the 1800 AD layer at 106 m is 0.85 m; to yield AD1590 at 130 m is 0.65 cumulatively (i.e. a much lower rate from AD1800 to AD1590) and to yield AD500 at 160 m is cumulative 0.38 m. I’m unaware of any detailed discussion of the reasoning for this at Quelccaya. But, aside from this, the key point here is that the glacier is both much younger and much thicker than Kilimanjaro.
Here are the ages at 50 m under various accumulation rates a for H=55 m and H=80 m. Thompson does H=50 m, which leads to singularity in the equation, for which he does an odd coercion. Thompson concludes that the accumulation rate is 0.0128 m and that the basal layer is 11700 BP through this odd coercion. For the argument here, it doesn’t matter whether a=0.0128 m or a= 0.0067 m. The point here is just how sensitive this age calculation is to the accumulation rate a and just how low the Kilimanjaro accumulation rate is compared with Quelccaya where, in the well-measured period since AD1800, a=0.85.
|Accumulation Rate a (m)||Age (H=55 m)||Age (H=80 m)|
Thompson is saying that Kilimanjaro a is about 1% of the value at Quelccaya. Thompson says that precipitation occurs in all months at Kilimanjaro with monthly totals "typically" less than 100 mm. I’m looking for annual precipitation totals at Quelccaya; if the average accumulation is said to be about 1.268 m, it would appear that annual precipitation at both Quelccaya and Kilimanjaro are pretty similar. Ergo, the average annual ablation rate at Kilimanjaro must be pretty nearly equal to the average annual precipitation. Add in some autocorrelation and I cannot imagine how you can get a plausible age model resulting in Kilimanjaro being 11700 years old. I’m not even sure how you can prove that it’s as old as Quelccaya on the information proffered to date.
It would have been nice if Thompson had commented in the original publication on these astonishingly low accumulation rates and how they reconcile with (say) Quelccaya, where he had previously worked.