Just when I’d despaired of ever seeing anything as mundane as a site map of the Quelccaya glacier on a scale that did not also show all of South America, I stumbled (due to the wonders of google) on an interesting article by Mark et al (2002), a geologist from Ohio State – the same university as Lonnie Thompson – on glacial moraines in the Quelccaya area. Of course neither the NAS panel nor Thompson deign to cite an article as mundane as one by a geologist. It was nice to read an article that focussed on the presentation of facts without moralizing all the time.
I’ve mentioned that I’d like to see other stratigraphic and contextual information relative to the report of the 5000-year old Distachia reported in a recent pamphlet by Thompson – that it was published in PNAS does not elevate Thompson et al 2006 from being IMHO a "pamphlet". In any event, I’ve teased out some information including this from Mark eet al 2002:
Other peat exposed at the modern ice margin [from their Table 1, this appears to have been collected in 1977] dates to 2760 cal yr B.P., implying that the Quelccaya Ice Cap at that time was smaller than present and may have disappeared completely during the middle Holocene. Two ice cores drilled 154.8 and 163.6 m to bedrock through the ice cap contain records that are only 1350 and 1500 years old (Thompson et al., 1985).
Mark et al describe the Quelccaya ice cap as follows together with the Location Map shown below:
The summit of the highest of four domes on the ice cap reaches 5645 m, and short, steep outlet glaciers descend the escarpment of the plateau to elevations as low as 4950 m (Mercer and Palacios, 1977). To the west of Quelccaya is a broad till and outwash plain with an abundance of peat bogs, lakes, and moraines.Two valleys in particular provide the opportunity to date multiple series of moraines: the Upismayo Valley on the northwestern side of the Cordillera Vilcanota and the Huancane Valley on the western side of the Quelccaya Ice Cap. These valleys were visited by Mercer and colleagues, who provided the first radiometric dates for some of the moraines (Mercer and Palacios, 1977; Mercer, 1983).
FIG. 2. Map of the study area….. (b) the Huancane Valley, Quelccaya Ice Cap. Moraines and the numbered sample sites for radiometric dates (coinciding with Table 1) are shown. Also presented are the modeled paleoglaciers, overlaid on the topography of the digital elevation model (shown by contour lines).
Mark et al visited the site in 1997, describing their work program as follows:
During June and July 1997, extensive field reconnaissance was carried out in the Cordillera Vilcanota–Quelccaya Ice Cap region to map and date moraines (Fig. 2). Five lakes and three bogs were cored using a square-rod piston corer (Wright, 1991), samples for cosmogenic isotopic analysis were taken from erratics on moraine crests, and soils were described and sampled (Goodman et al., 2001). Organic material recovered in the field from lake cores, bogs, and buried peat was dated by accelerator mass spectrometry. Dates are presented as calibrated years before present (cal yr B.P.), based on CALIB version 4.0 (Stuvier and Reimer, 1993; Stuvier et al., 1998) (Table 1).
From moraine and other information, they estimated the size of the paleoglaciers at different periods as follows (H refer to the ones at Quelccaya that we’re interested in at present), with H1 being the LIA moraine:
Note. Preserved moraines define the extent of three different paleoglaciers in each valley, labeled in descending order from oldest to youngest as follows: Ice3, Ice2, and Ice1 for the Upismayo Valley; H3, H2, and H1 for the Huancane Valley. Volume (km3) of each reconstructed paleoglacier is calculated using gridded-model surfaces and the digital elevation model. Modern glacier volume was estimated from surface area by the formula V=28.5 S1.36 (after Chen and Ohmura, 1990). Deglacial interval (yr) represents the conceivable time range over which the paleoglacier deglaciated from successively less extensive end moraine positions. The interval is presented as a mean surrounded by the one-sigma range in calibrated radiocarbon ages. Where available radiocarbon dates include more than one constraining age for a moraine, the maximum and minimum possible intervals are provided as large and small intervals, respectively. Deglacial Volume (km3) represents the volume lost from the paleoglacier in two possible deglacial scenarios: a large volume from complete deglaciation and a small volume considering only the volume lost between successive moraine positions. Deglacial Rate (10à⣃ ’ ”¬’¢5 km3 yrà⣃ ’ ”¬’¢1) is calculated by dividing the deglacial volume by the deglacial interval, such that the small rate equals small volume divided by large interval, and large rate equals large volume divided by small interval.
They discuss how they dated the various moraines, relying primarily on radiocarbon dating of peat deposits, often buried in moraines in disturbed contexts.
Peat buried in sandy outwash at 4745m altitude upvalley from the Huancane III terminus provides a minimum age of 14,290 cal yr B.P. for H3… Disturbed peat found under till associated with a Huancane II moraine provides a maximum limiting age for the H2 glacier advance of 12,830 cal yr B.P. The difference between this date and the minimum age of H3 cited above provides a minimum estimate for the deglacial interval of H3 as 1460 yr…. at no time since ~12,800 cal yr B.P. has the Quelccaya Ice Cap margin been more than 1 km beyond that of today (Rodbell and Seltzer, 2000)…. Huancane I moraines are unvegetated and, although present in all valleys surrounding the Quelccaya Ice Cap, are not dated directly in the main Huancane Valley. Peat entrained in a Huancane I moraine from a neighboring Valley provides a maximum limiting age of 300 (+202/à⣃ ’ ”¬’¢300) cal yr B.P. for the H1 paleoglacier, noted by Mercer to be in the range of the Little Ice Age. Thus, the deglacial interval for H1 is 290 yr, the difference between the maximum H1 Peat buried beneath a Huancane I moraine dates to 11,190 cal yr B.P., a minimum age for the H2 paleoglacier, which indicates that deglaciation of H2 was rapid and extensive. By 11,190 cal yr B.P. the glacier was reduced greatly in size and within its Little Ice Age extent. This results in a 1640 yr deglacial interval for H2. ….Other peat exposed at the modern ice margin dates to 2760 cal yr B.P., implying that the Quelccaya Ice Cap at that time was smaller than present and may have disappeared completely during the middle Holocene. Two ice cores drilled 154.8 and 163.6 m to bedrock through the ice cap contain records that are only 1350 and 1500 years old (Thompson et al., 1985).
What information from this is relevant to the 5000-year old Distachia? Well, if the 5000-year old Distachia had been under a glacier uninterruptedly for 5000 years, then how did the peat form in 2760 BP at the "modern ice margin"? This is the same kind of problem that we observed in the Green Alps and in Alberta – there seem to be glacier advances and retreats. How do we know that the Quelccaya Distachia wasn’t exposed at surface at some point in its history? The only argument on this seems to be that, if it had been at surface, it would have decayed. How solid is this argument? I’ve seen references in passing to how long things can survive at surface in cold dry climates. How do we know that this didn’t survive? I’ll have more to say on this in another post.
Bryan G. Mark and Geoffrey O. Seltzer, Donald T. Rodbell and Adam Y. Goodman, 2002. Rates of Deglaciation during the Last Glaciation and Holocene in the Cordillera Vilcanota-Quelccaya Ice Cap Region, Southeastern Peru, Quaternary Research 57, 287–298 (2002) http://geog-www.sbs.ohio-state.edu/faculty/bmark/QRMarketal.pdf
Mercer, J. H. (1983). Cenozoic glaciation in the southern hemisphere. Annual Reviews in Earth and Planetary Science 11, 99–132.
Mercer, J. H. (1984). Late Cainozoic paleoclimates of the southern Hemisphere south of the equator. In “Late Cenozoic Paleoclimates of the Southern Hemisphere,” (J. C. Vogel, Ed.), pp. 45–58. Balkema, Rotterdam.
Mercer, J. H., and Palacios, O. M. (1977). Radiocarbon dating of the last glaciation in Per´u. Geology 5, 600–604.