Tephras in Ecuador

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 (àƒ⣃ ‹’€ 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;àƒ⣃ ‹’€ ”‚¬’„¢160) cal yr B.P. and 2490 (+230;àƒ⣃ ‹’€ ”‚¬’„¢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;àƒ⣃ ‹’€ ”‚¬’„¢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;àƒ⣃ ‹’€ ”‚¬’„¢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;àƒ⣃ ‹’€ ”‚¬’„¢350) and 15,490 (+200;àƒ⣃ ‹’€ ”‚¬’„¢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.

Reference:
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


13 Comments

  1. Steve Sadlov
    Posted Aug 25, 2006 at 10:09 AM | Permalink

    RE: “Could recycling of old carbon have occurred at Quelccaya? ”

    I would imagine yes it could have. Witness the Long Valley finding you alluded to earlier on another thread.

  2. Howard
    Posted Aug 25, 2006 at 10:46 AM | Permalink

    It seems that this could happen anywhere where old carbon is taken up by plants and critters. Found this by quick googling to sciencefrontiers.com website. Sorry not geekified to post links… am old school field hand.

    “For example, M. Grachev et al carbon-dated flatworms and a sponge collected from a bacterial mat near a thermal vent 420-meters deep in Lake Baikal. The apparent ages of these living organisms ranged from 6860 to 10,200 years.

    (Grachev, M., et al; “Extant Fauna of Ancient Carbon,” Nature, 374:123, 1995)”

  3. Armand MacMurray
    Posted Aug 25, 2006 at 11:32 AM | Permalink

    Re:#2
    Just note that, in general, plants do not take up carbon, as they fix atmospheric CO2. Animals, fungi and bacteria generally do take up carbon.

  4. Steve Sadlov
    Posted Aug 25, 2006 at 12:15 PM | Permalink

    RE: #3 – What about hard water being taken up by the roots?

  5. Howard
    Posted Aug 25, 2006 at 12:27 PM | Permalink

    #3 Armand Thanks. You have motivated me to look at the carbon cycle in soil. Still looking. It seems the food web concept is quite complex and very interesting to the subject at hand. I’ll see if I can dig up anything useful either way. Now that global warming threads are running to ground, it is finally becomming interesting.

  6. welikerocks
    Posted Aug 25, 2006 at 12:52 PM | Permalink

    I keep thinking about plants I have that I started from pieces from my grandmother’s garden over 30 yrs ago. Where ever I have moved I’ve brought pieces and restarted new plants. Many times but just sticking a succulant leaf in the ground , or even a stem, and not even a stem with roots. I don’t know whether to say they are 30 yr old plants or not? :)

    So, I wonder does how a plant propagates or if a species has the ability to lie dormant, factor in or not in the carbon dating of plants? (ex: do spoors survive in ice?) I realize the timespans are huge and this might be a ridiculous thought on my part. I would think, no it doesn’t, because the carbon signature is made during it’s growing into a full plant. I just keep thinking about spoors rolling around in a glaciers and moving ice waiting to thaw out.

  7. welikerocks
    Posted Aug 25, 2006 at 12:57 PM | Permalink

    I mean spore not spoor. Sorry!

    Spoor: tracks, droppings from a wild animal.

    Spore: reproductive body produced by mosses

  8. Gary
    Posted Aug 25, 2006 at 8:04 PM | Permalink

    #3 is right that most plants take up atmospheric carbon, but exceptions would be aquatic (particularly submerged) plants whose carbon source would be dissolved in the water and conceivabley could come from atmospheric, biotic (fish respiration), and even geologic (CaCO3) sources. The proportions of each would depend on local conditions. It’s certainly possible there could be significant contamination problems post-mortem as well from the activities of fungi and animals bringing in additional carbon of unknown age. I’d prefer samples from the innermost rings of old stumps over any aquatic plant.

    #6 – the ability to clone plants makes aging individual stems without annual rings somewhat pointless. I once saw the rather rare flowering of a night-blooming Cereus cutting that came from an original plant that was living over 100 years ago. The cutting only had been growing on its own for a decade or so and I don’t know how many “generations” removed from the original. Realize however that the carbon in the young cutting was incorporated recently and very little if any of the original plant’s carbon still resided in it.

  9. Pat Frank
    Posted Aug 26, 2006 at 1:15 AM | Permalink

    #1&2 — Several years ago I found literature warnings of incorrect carbon dating. Three papers,[1-3] for example, showed that living fresh water molluscs could take up 14-C depleted carbon when incorporating ancient humus leached into ground water and then into streams, or when ancient humus was metabolized or oxidized to CO2 and taken up into shells as dissolved bicarbonate. The shells of these snails showed anomalously large ages.

    A really strange report also turned up about mummified seals in Victoria Land, Antarctica, that dated out to between 615 through 4,600 years,[4] even though the frozen tissues could not have survived that long. The author mentioned that, “antarctic sea water has significantly lower carbon-14 activity than accepted as the world standard. Therefore, radiocarbon dating of marine organisms yields apparent dates that are older than true ages, but by an unknown and possibly variable amount.“, and went on to mention that, “A seal freshly killed at McMurdo had an apparent age of 1,300 years.

    [1] M. L. Keith and G. M. Anderson (1963) “Radiocarbon Dating: Fictitious Results with Mollusc Shells” Science 141, 634-636.
    [2] M. Rubin and D. W. Taylor (1963) Radiocarbon Activity of Shells from Living Clams and Snails” Science 141, 637.
    [3] A. C. Riggs (1984) “Major Carbon-14 Deficiency in Modern Snail Shells from Southern Nevada Springs” Science 224, 58-61.
    [4] W. Dort, Jr. (1971) “Mummified seals of southern Victoria Land” Antarctic Journal VI(5) September-October 1971, pp. 210-211.

    One wonders about the generality of regional deficiencies of 14-C, if one such can occur in a large-scale stirred body of water like the antarctic sea; namely, whether adventitious regional deficiencies can occur or have occurred elsewhere for region-specific reasons, which would cause later dating to be experimentally correct but physically wrong. Perhaps one could calibrate a soil column to find out.

    The article about the seals wandered into strange territory, by the way, discussing “numerous seal-bodies” of seal pups that apparently had wandered far inland, into the dry valleys of Victoria Land, where they froze to death and were then slowly ablated away by ice and sand during wind storms. The journal-cover featured a picture of one such seal, half gone on a pedestal of wind-sculpted, body-fluid-containing ice. It was frozen in a rear-body up-turned curve, and was fully 60 km from McMurdo sound. Some seals had managed to climb 1200 meter scarps! The mind boggles at imagining that journey.

  10. Steve McIntyre
    Posted Aug 26, 2006 at 7:39 AM | Permalink

    It’s interesting to see these anomalies develop. It’s remarkable how relatively few samples are dateable to the last millennium and one wonders whether this might be an artifact of problems with a reservoir effect in the radiocarbon. Maybe everything’s OK, but when you see a variety of weird reservoir-affected results, one wonders as to possible over-confidence in the calibrated date error bars.

  11. welikerocks
    Posted Aug 26, 2006 at 8:25 AM | Permalink

    Thank you #8, I spent a good amount of time reading yesterday too.

    I read about Lichenometric-dating curves.
    As in this abstract here:

    “Little Ice Age’ glacier variations in Jotunheimen, southern Norway: a study in regionally controlled lichenometric dating of recessional moraines with implications for climate and lichen growth rates

    http://tinyurl.com/hfwud

    lichen is a really interesting organism/plant whatever it is!

  12. Pat Frank
    Posted Aug 26, 2006 at 3:06 PM | Permalink

    #10 — A thought occurred to me that because a several-thousand year old glacier should contain ice enclosing 14-C depleted CO2, at its bottom, then from the description of ice-melting at Greenland, it seems likely that melt water percolating through the glacier and emerging from the bottom would be enriched in low 14-C bicarbonate.

    If this was taken up by local plants, then they would be endemically and systematically low in 14-C and would give anomalously old ages when tested. One might test for this by looking at 14-C in living plants serially further from the bottom of glaciers.

    It might even be true that the atomsphere immediately around melt streams emerging from glaciers might be low in 14-C CO2 because of the equilibrium between the local atmospheric CO2 and the 14-C depleted dissolved CO2 and bicarbonate in the melt waters. If true, that would also facilitate low 14-C plants around glacier margins.

    Of course, the older the glacier, the more 14-C depleted the emerging melt water, and the more anomalously old-seeming the local plants.

    The same melt-percolation process may explain the low 14-C in the waters around Antractica and would predict 14-C depleted waters around Greenland, too.

  13. Gary
    Posted Aug 26, 2006 at 7:25 PM | Permalink

    The susceptibility of C14-dating to the effects of depletion and enrichment should require extra care in collection and certification of results. The more samples the better. And anomalous results ought to be revealed so that the conclusions can be assigned some level of plausibility. I’d accept one out of 20 or even 10 samples with a weird date, but more than that really begins to erode confidence. Rarely are calculated dates put in context with this information.

Follow

Get every new post delivered to your Inbox.

Join 3,208 other followers

%d bloggers like this: