Donald T. Rodbell et al 2002 (with overlaps to Mark et al and Abbott et al) , entitled " A Late Glacial–Holocene Tephrochronology for Glacial Lakes in Southern Ecuador" , here correlates glacial lakes in southern Ecuador according to widespread tephra (volcanic deposits).
A couple of interesting points – some BIG differences between radiocarbon dates from material adjacent to well-dated Tephra F, dated at 2500 BP in well-dated Pallcacocha and over 4400 BP in another location – the differenc attributed to recycling of old carbon fronm upvalley peat.
Also a curious observation about wind patterns in the last millennium.
Volcanic tephra are widespread in northern Ecuador, but hard to find in southern Ecuador. Rodbell et al report the discovery of 5 widespread tephra in thin layers in proglacial lakes:
In contrast to the widespread cangagua and tephra of the northern two thirds of Ecuador, no such material has been reported from southern Ecuador. Here we report the first such evidence in the form of thin (à⣃ ’ à⺰.1 to 1.0-cm-thick) distal tephras deposited in glacial lakes and peat lands in Las Cajas National Park (Fig. 1). …
Six widespread tephras (~0.1–1.0 cm thick) with rhyolitic to dacitic glass and/or phenocrysts of feldspar or hornblende are preserved in the glacial lakes of Las Cajas National Park, southern Ecuador. In addition to the tephras found in all or most of the sites, there are several tephras that can only be found in one or two of the lake basins studied. The widely recorded tephras were deposited 9900, 8800, 7300, 5300, 2500, and 2200 cal yr B.P.
They observe that these tephra cannot be correlated to any of the observable large tephra in northern Ecuador in the last 2000 years . Their explanation is far from being as satisfactory as one would like:
The lack of tephras in the sediment cores of this study that are correlatable with any of the numerous proximal tephras deposited during the last two millennia (Hall and Mothes, 1994) suggests that the eruptions that produced the distal tephras noted in this study were either especially large or occurred when wind directions were favorable to transport tephras at least 150 km to the south. Prevailing wind directions generally are E–W across the axis of the Andes, but during the austral summer moisture bearing air masses do travel southward along the eastern side of the Andes. In either case, the distal tephras noted in this study are the product of an unusual set of circumstances, which have not been repeated in the past ~2000 yr: unusually voluminous eruptions, strongly prevailing southward winds, or both.
Radiocarbon Dates of Tephra F
Tephra F is the most prominent and best-dated tephra. They report that a radiocarbon date at L Chorreras is nearly 2000 years older than at well-dated L Pallcacocha. (Note that in the actual diagram in Figure 3, Tephra F is VERY close to the 2490 date and if interpolated would be ~2500 BP versus 4430 BP at L Chorrera):
the radiocarbon dates on tephra F also appear anomalously old in the Laguna Chorreras core. In the Pallcacocha core tephra F, which is clearly identified in all lakes by being the thickest and by the presence of horneblende phenocrysts, is found between two intervals that yield radiocarbon dates of 3250 (+100;à⣃ ’ ”¬’¢160) cal yr B.P. and 2490 (+230;à⣃ ’ ”¬’¢130) cal yr B.P. (Fig. 3). In contrast, tephra F in the Laguna Chorreras core (SC = 0.97) is found immediately below an interval that yields a radiocarbon age of 4430 (+90;à⣃ ’ ”¬’¢20) cal yr B.P. (Fig. 3). [SM – comparable to Pallcacocha , would be "bounded" between 4430 and 6820). Tephra F in the Llaviucu core, which yields SC values of 0.94 and 0.95 when compared with the Chorreras and Pallcacocha cores, respectively, is found immediately above an interval dated to 4120 (+110;à⣃ ’ ”¬’¢130) cal yr B.P.
They attribute the discrepancy to recycling of old carbon from upvalley peat:
In the absence of any source of old carbon that could generate a significant hardwater effect in Laguna Chorreras, we suggest that one likely source of old carbon is from the recycling of organic matter from the numerous peatlands that are located upvalley from Laguna Chorreras. Most of these peat deposits are at least 1.5 m thick, and lateral channel migration by the main stream that drains the Chorreras Valley undoubtedly has periodically eroded these deposits and transported aged organic matter into the Lake. A ~10-cm-thick section in the late glacial portion of the Laguna Chorreras core appears to provide a good example of such an event. Upcore from a depth of ~325 cm,the sediment changes abruptly from an organic-rich gyttja (10–15 wt.% organic carbon) to an inorganic silt containing abundant very small fragments of plant macrofossils. Two radiocarbon dates from this material yielded nearly identical ages of ~15,620 (+230;à⣃ ’ ”¬’¢350) and 15,490 (+200;à⣃ ’ ”¬’¢560) cal yr B.P., which are more than 1000 yr older than radiocarbon dates from the gyttja either above or below this interval. Because of the apparent problem arising from the recycling of organic matter on the landscape, we have based much of our chronology on the laminated core from Laguna Pallcacocha (Rodbell et al., 1999). This lake is located in the floor of a cirque that is surrounded by steep slopes with little, if any, peat deposits. We have encountered no radiocarbon reversals such as those noted above or reported from the closely spaced dating of a core from Laguna Llaviucu by Colinvaux et al. (1997)
Detail from Rodbell et al Figure 3. Tephra F is one of the two top tephras.
What does this imply for dating of peat at Quelccaya? Hard to say. At a minimum, we have a radiocarbon sample from a somwhat relevant environment (L Chorreras) which is 2000 years too old. Could recycling of old carbon have occurred at Quelccaya? It’s hard to exclude such a possibility. What would the potential effect be? I guess the range would be between 0 and the amount at Chorreras.
Donald T. Rodbell, Stefan Bagnato, and Jeffrey C. Nebolini, Geoffrey O. Seltzer and Mark B. Abbott, 2002. A Late Glacial–Holocene Tephrochronology for Glacial Lakes in Southern Ecuador , QR 57 http://www.pitt.edu/~mabbott1/climate/mark/Abstracts/Pubs/Rodbelletal02eq.pdf