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Auditing the end of the Wisconsin glaciation.

  1. Nov 20, 2007 #1
    If I google 'Wisconsin glaciation' I see periods mentioned of

    Clearly it seems desireable to find the real termination time of the glacial period and see if it matches other ideas on the termination of the ice ages. So let's duck into the formal scientific publications, hunting for dates associated with glacier activity receding.

    However, first we need some background on dating methods, nowadays there is a plethora of them, roughly in three groups, radio-activity series, chemical alterations due to exposure to atmospheric conditions, and the third: counting any kind of annual layers.

    A special method is radiocarbon dating, which is the most widely used due to the abundance of dateable records. There are several old threads on carbon dating, so I only briefly resume the essence of the method. Radioactive 14C with the half time of 5568±30 (Libby) or 5730±40 years (Cambridge), is generated in the atmosphere and taken up in the biologic carbon cycle. Life tissues constantly refresh carbon, keeping their relative rate of radioactive 14C (delta14C) about constant, but after death the interaction stops and the delta14C decreases logaritmically due to the radioactivity. So the delta14C is a measure of time after death.

    It soon showed however that carbon dating did not compare wuth other dating methods because was much more complicated due to many different processes. The most important being variation in the atmospheric delta14C due to variation in pCO2, intensity of the cosmic activity and changing interactions with the oceans. However large (around 1000-3000 years) those differences are constant and robust calibration tables could be made by comparing counted records (annual coral rings, tree rings, lake sediment layers) with the carbon dates. The most current is Intcal04 Reimer et al, Radiocarbon 2004. These tables also circumnavigate the halftime deviation problem as long as the same value is used. Therefore, although wrong, Libby is still in use to avoid recalibrating all the old publications.

    This brings us at the main question: how much older publications, relying on uncalibrated carbon dates, are still used to define borders between periods. Have all the dates been calibrated and updated?

    We'll see.
  2. jcsd
  3. Nov 20, 2007 #2
    The first publication that deserves some interest is:

    Forman, Steven L. , E. Arthur Bettis III, Timothy J. Kemmis, and Barry B. Miller, 1992; Chronologic evidence for multiple periods of loess deposition during the Late Pleistocene in the Missouri and Mississippi River Valley, United States: Implications for the activity of the Laurentide Ice Sheet , Palaeogeography, Palaeoclimatology, Palaeoecology, 93 (1992): 71-83 71 Elsevier Science Publishers B.V., Amsterdam

    The abstract reads:

    (Loess is sort of ultra fine rock particles chafed off from bed rock by glacial activity, it's also known as glacial flour, making glacier lakes milky opaque.)

    So there is "12 ka" for the end of the Loess deposits, hence end of the glacial period. So what about dates in the publication? We find in table 1 and 2 youngest carbon date of 20,540 BP and youngest thermo/(opto)luminescence (TL) date of 16000 BP. They also observe:

    That's a most accurate observation as we know now, but it doesn't answer where that "12 Ka BP" comes from. Seeing the various graphs, nothing is indicating dating confines for the upper boundary of the loess. But then we read:

    So it appears that the "12ka" in the abstract is not the result of own research but merely passing on of older information. Mind that in 1983 the carbon dating problems were not really sorted out yet, so these dates may just be uncalibrated carbon dates as well (haven't located that paper yet) and the researchers knew that there was something fishy about carbon dates. So why not discuss this in relation to that earlier research? Note that 12000 radiocarbon years BP (Before present, present = 1950) is 13,830 calibrated calendar years BP on INTCAL04.

    looking for Ruhe 1983 (Ruhe, R.V., 1983. Depositional environment of late Wisconsin
    loess in the midcontinental United States. In: S.C. Porter (Editor), Late-Quaternary Environments of the United States. 1. The Pleistocene. Univ. Minnesota Press, Minneapolis, pp. 130- I37.)
    Last edited: Nov 20, 2007