A while ago, I summarized an interesting article from the Venezuelan Andes. In that case, they concluded that the glaciers did not exist in the MWP. The post is worth re-reading in the context of Quelccaya. The authors used the continuous deposition of sediments in a proglacial lake to yield evidence of the discontinuous movements of the glacier.
Given that Thompson’s PNAS article is so inadequate as a survey of relevant information, one can only wish that a similar study had been done at one of the Quelccaya proglacial lakes. If one had, you’d think that even Thompson would have thought to mention or that one of the PNAS referees would have asked about it.
But, hey, it’s Thompson so you never know what fundamental information he leaves out. I googled Paco Cocha, the name of one of the proglacial lakes and, needless to say, there was an almost precisely identical study to the one in the Venezuelan Andes, Abbott et al 2003 . It was even done by geologists at Ohio State – Thompson’s own university. You’d think that Thompson would give them a mercy citation – but this is the Team.
Abbott et al concluded that "glaciers were probably absent from the watershed between 10.0 and 4.8 thousand years BP – which raises interesting questions about the 50,000 year old moss. The authors conclude that glaciation has been present in the area since 4800 BP. but with "significant fluctuations."
Multiproxy analyses of Laguna Paco Cocha (13″⟵4S, 71″⟵2W) include studies of sedimentology, geochemistry, physical properties, magnetic susceptibility, and stable isotopes. The age model for the core was produced by linear interpolation between 11 calibrated AMS radiocarbon dates on individual macrofossils (Table 1) (Stuiver et al., 1998). Fig. 4 shows the results from the analysis of organic matter content (LOI at 500″⠃), bulk density, magnetic susceptibility, cellulose-inferred N18Olw, and mass accumulation rates of organic and mineral matter. The abrupt shift to glacial values at 4.8 ka B.P. for all parameters is highlighted by a dashed black line.
This includes decreased organic matter values from ~15 to 65 wt%, higher dry bulk density from 60.5 g/cc to ~0.9 g/cc, higher magnetic susceptibility from 65 to ~10 SI, and an abrupt N18Olw decrease of 3x. We interpret the results of the analyses presented in Fig. 4 to show that glaciers in the Paco Cocha watershed retreated rapidly beginning prior to ~12.7 ka B.P. and were gone from the watershed by 10.0 ka B.P., as indicated by the dashed gray line (Mark et al., 1999). Although glaciers were probably absent from the watershed between 10.0 and 4.8 ka B.P., the lake remained at the overflowing stage during this period, as suggested by analyses of organic matter, sediment density, and magnetic susceptibility. If the lake had desiccated during this period we would expect oxidation of organic matter leading to low values which we do not see. Additionally, cellulose-inferred N18Olw values remained 6310x suggesting that the lake did not become a closed basin during this period. Increased mineral and decreased organic matter accumulation rates after 4.8 ka B.P. also support the return of glacial ice to the watershed at this time and the lack of ice in the drainage basin during the early and middle Holocene. After glaciers returned to the watershed at 4.8 ka B.P. they have been present until today, but analyses of organic matter, sediment density, and magnetic susceptibility in addition to changing accumulation rates suggest significant fluctuations during this period. Cores from Laguna Llacho Kkota, which is located to the south (15″⟰7S, 69″⟰8W), show the onset of wetter conditions at 3.4 ka B.P.
Fig. 4. Sediment-core analyses from Laguna Paco Cocha including organic matter, dry bulk density, magnetic susceptibility, cellulose-inferred N18Olw (thick line is the three-point running average), and mass accumulation rates of organic and mineral matter. The dashed horizontal gray line indicates the retreat of glaciers to at least the neoglacial limit by 10.0 ka B.P., and the horizontal dashed black line shows the return of glaciers to the watershed at 4.8 ka B.P. The vertical dashed lines on the graph of inferred N18Olw provide a framework to interpret the status of the watershed based on the modern calibration samples shown in Fig. 3. The cellulose-inferred N18Olw values were corrected for a systematic o¡set arising from methodological di¡erences between the University of Waterloo and University of Minnesota laboratories (Beuning et al., 2002).
Reference: Abbott et al 2003, Holocene paleohydrology and glacial history of the central Andes using multiproxy lake sediment studies, Palaeogeography, Palaeoclimatology, Palaeoecology 194 (2003) 123-138. url