Thompson et al (PNAS 2006) stated that the discovery of a plant deposit with radiocarbon age of around 4000 BP (calibrated to 5138 BP) near the receding margin of the Quelccaya glacier provided “strong evidence” that the “current retreat of Quelccaya is unprecedented for the last 5 millennia”. The plant deposit is Distichia muscoides, which is a component of Andean peat deposits whose present limit is 400-500 m lower than the discovery.
In order to better understand the significance of this discovery, it would be nice to have information of the stratigraphy of the discovery as glacial deposits in other localities which have been mapped in detail, often have extremely complicated histories with re-working of deposits.
Of course, it’s Thompson, so we get a pamphlet, rather than a geologic report. However, with a bit of googling, I identified the following other recent discoveries of plant deposits from receding margins of Quelccaya glacier: The plant deposit reported in the PNAS article appears to date from Thompson’s 2002 expedition (url), but other plant deposits cited in news releases were not reported in the PNAS article.
- Mark et al 2002 reported the discovery of peat dated to 2700 BP at the receding margin of the
glacierice cap. Their Table 1 indicates that the discovery was in a 1977 expedition.
- in the 2003 campaign, a plant deposit was discovered dated to 2200 years BP
- also in the 2004 campaign, a deposit of moss was identified at the maximum radiocarbon date of 50,000 years – from which Thompson speculates that the moss may date to the Eemian period 110,000 years ago
- in the 2005 campaign, more plant deposits were discovered with dates varying from 4500 to 6500 BP.
With respect to the 5000 year old plant, Thompson said:
We know the first plant could not have been exposed at any time during in that 5,200-year history or it would have decayed
From the discovery of the 50,000 year old plant, Thompson said in a news release:
The plant had to have remained covered and protected for most of that time, which means that the ice cap most likely has not deteriorated to its current size for any length of time in more than 50,000 years.
But don’ t these discoveries prove too much? If the 5000-year old plant has been covered by glacier continuously, then how did the 2200-year old plants or the 2700-year old plants grow? Surely the problem here is similar to that faced in the Green Alps, where stumps have been dated to different periods and that evidence has been interpreted as showing the retreat and advance of the glacier. Similarly if the 50,000-year old glacier – if it’s been covered, how did any of the 5000-year old 2700-year old or 2200-year old plants get formed?
Why didn’t Thompson report these various plant deposits in his PNAS pamphlet? Why didn’t the NAS panel ask for an explanation? I’d certainly be interested in a considered explanation from thoughtful people. But Thompson failed to report all the plant deposits and the NAS panel failed to search for other plant deposits.
I’ve collated information from Thompson’s various news releases and pictures here.
BTW Antonio Regalado of the Wall Street Journal had an interesting contemporary article in Nov 2004 from the glacier.