Exploring abrupt climate change induced by comets and asteroids during human history

Love Notes: Wittke and van Hoesel in PNAS


Cosmic impact or natural fires at the Allerød–Younger Dryas boundary: A matter of dating and calibration

Wittke et al. (1) present evidence of a major cosmic impact at the onset of the Younger Dryas (YD) episode, including some markers found in the top of the well-known Usselo marker horizon (UH). This finding is contrary to our extensive radiocarbon dating effort from this horizon (2), which shows that the UH at Aalsterhut postdates the onset of the YD. Furthermore, Wittke et al. (1) misinterpret the origin of the UH: this horizon is a well-defined paleosoil that formed during the Allerød and the early YD in the top part of coversand. This coversand was deposited before the Allerød, during cold and dry conditions, and is part of the European Sandbelt. Wildfires were common and occurred throughout this period, rather than synchronously with the onset of the YD (3).

Wittke et al. (1) cite our paper on the UH at the Aalsterhut site (2) and state that we neglected to address calibration issues when interpreting our radiocarbon dates. This comment is simply not true. Moreover, the statement that we used a value of 12,900 ± 30 cal y B.P. for the onset of the YD is incorrect: this age is not in our report. We report an age range of 11,000–10,900 14C yr B.P. (2). Using the older, discontinued IntCal04 calibration, this radiocarbon age range gives a calibrated age for the onset of the YD of 12,900 ± 30 cal y B.P.. Wittke et al. (1) compare some of our radiocarbon dates from the UH, calibrated using the presently recommended IntCal09 curve (4), to a value for the onset of the YD calibrated using a different (unspecified) calibration curve, thus creating a problem with calibration issues. Had Wittke et al. used the current IntCal09 calibration curve for the YD dating, which yields 12,815 ± 90 cal y B.P., there would not be a calibration issue.

Calibration increases the uncertainty of the timing of the YD because of a long-lasting “plateau” in the calibration curve for this time period (4). This aspect is why we directly compared the radiocarbon ages, showing that the charcoal in the UH at Aalsterhut is somewhat younger than the Allerød–YD boundary (2). When using the calibrated ages, as Wittke et al. (1) did, it can indeed be argued that our data are consistent with the Allerød–YD boundary. However, this does not indicate that our site is consistent with the diamond-rich layer reported by Kennett et al. (5), as suggested by Wittke et al. (1). The UH, and associated wildfire, at Aalsterhut (10,870 ± 15 14C yr B.P. or 12,785–12,650 cal y B.P.; n = 12) is clearly younger than the Black Mat where Kennett et al. (5) found nanodiamonds (11,070 ± 10 14C yr B.P. or 13,080–12,915 cal y B.P.; n = 7).

Accurate dating is imperative for establishing causal relationships between deposits and events and, to compare calibrated radiocarbon dates, these must be obtained using the same calibration curve. It is thus essential to report both the original radiocarbon ages and the calibration curve used.


The authors declare no conflict of interest.


1. Wittke JH, et al. Evidence for deposition of 10 million tonnes of impact spherules across four continents 12,800 y ago. Proc Natl Acad Sci USA. 2013;110(23):E2088–E2097. [PMC free article][PubMed]
2. van Hoesel A, et al. Nanodiamonds and wildfire evidence in the Usselo horizon postdate the Allerød–Younger Dryas boundary.Proc Natl Acad Sci USA. 2012;109(20):7648–7653.[PMC free article] [PubMed]
3. Kaiser K, et al. Palaeopedological marker horizons in northern central europe: Characteristics of lateglacial Usselo and Finow soils. Boreas. 2009;38(3):591–609.
4. Reimer PJ, et al. IntCal09 and Marine09 radiocarbon age calibration curves, 0–50,000 years CAL BP. Radiocarbon.2009;51(4):1111–1150.
5. Kennett DJ, et al. Shock-synthesized hexagonal diamonds in Younger Dryas boundary sediments. Proc Natl Acad Sci USA.2009;106(31):12623–12628. [PMC free article] [PubMed]

Articles from Proceedings of the National Academy of Sciences of the United States of America are provided here courtesy of National Academy of Sciences

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6 Responses

  1. van Hoesel has the YDN shown per IntCal04 in her Fig 2, but she pretends like she never used IntCal 04 in her 2012 paper. By displaying IntCal04 instead of IntCal09, she “seems” to show that the fires are 100 years too later for the YDB. When the YDB is shifted to match the same IntCal09 as the rest of her data, the mismatch vanishes – the YDB lines up perfectly with her wildfires.

    She also harps on Wittke as using IntCal04 in his paper.

    Wrong. Everywhere he SHOWS IntCal04 (YDB at 12.9 kya) Wittke clearly points out that by IntCal09 the proper date is 12.8 kya. He is VERY clear about this, and BOTH dates are shown, with the proper IntCal referenced.

    She makes a mountain out of a mole hill of her own making – and has NOTHING to back her up.

  2. Wittke repeats the IntCal04 vs IntCaal09 labels for every site in his paper. It is repetitive (and boring), but it is necessary. He covered all the bases. van Hoesel couldn’t handle the truth. Even when it is in front of her a dozen times or more.

  3. Hi Steve,

    As I stated earlier, we always used the IntCal09 calibration curve when calibrating radiocarbon ages. We never used the IntCal04 calibration curve (as implied in the wittke et al. 2013 paper). The figure you are referring to is based on UNCALIBRATED radiocarbon ages. I am intrigued to find out how I used the wrong calibration curve when not calibrating these data.

    Also, I should have bothered to bring up the use of the old calibration curve in other publications and take the cuetzeo site as an example (and it is not related to your erroneous statement about my paper), but since I did, here are two quotes from the literature:
    From Wittke et al (2013): “We adopted the chronology of Israde et al. (2012),”
    From Israde Alcantara et al (2012): “To test and refine that model, we acquired six more AMS 14C dates on bulk sediment, for a total of 22 dates, and calibrated them using the IntCal04 calibration curves”

    Maybe I somehow misread and misinterpreted those quotes as usage of the old IntCal04 calibration curve. If that is the case, I would like to apolagize for my assumptions in the matter.

    I would like to leave it at that.

  4. Thanks for your contribution here at the Cosmic Tusk, Annelies. You are welcome any time.

    I will try to hold the dogs back.



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