Our PNAS Younger Dryas impact letter is out.

Earlier this year Isabel Israde-Alcántara et al. published a paper detailing the discovery of more putative evidence of an extra-terrestrial impact at the onset of the Younger Dryas interval from a site in Mexico.   I was co-author on one of the responding articles that dealt specifically with the paleoecological interpretations presented by Israde-Alcántara et al.  There were a total of five letters criticizing the original paper, based on a number of points, ecological, chronological and geophysical, and Israde-Alcántara et al. provided responses to each.

There’s an excellent take on the Younger Dryas Impact Hypothesis on Vance Holliday’s website (here), and several papers have been published rebutting the YDIH, for example: Surovell et al. (2009), Pinter et al. (2011) and Pigati et al. (2012).  Israde-Alcántara et al. refute the findings of Surovell et al. and Pinter et al., but not Pigati et al. (the paper wasn’t out in time).

As a co-author on one of the letters I have a vested interest in a non-impact cause for the Younger Dryas, and won’t pretend otherwise.  The most significant limitation we outlined in our letter, is the inconsistent application of paleoecological methods in the Israde-Alcántara et al. study.  It still seems to me that many of the limitations of the YDIH have not been properly addressed, and the fact that physical modeling is capable of explaining the YD without implying a massive asteroid strike (or multiple asteroid strikes, or strikes over glaciers or airbursts) should be cause to consider the YDIH more closely.

Adaptation from the PNAS article described.
Figure 1. This image comes from the Lake Cuitzeo paper, I’ve highlighted the section that is contested, and I continue to fail to see how a linear trend for 14C dates that were rejected can be simply thrown out.

Blaauw et al. (2012) point out that there is no uncertainty estimate in the age-depth model presented in the original paper, that even with the published age-depth model the purported Younger Dryas event is mis-dated, and there is a whole swath of 14C dates that are rejected.  The response is interesting.   Israde-Alcántara et al. imply uniformitarianism to resolve issues of CaCO3 offsets based on modern samples although reservoir effects clearly change through time (see here for example), and are very likely to change given the significant changes in sediment type (and likely in source) shown in Figure 1 (adapted).   Moreover, besides CaCO3 there are many additional ways for 14C dates to be offset, e.g. local volcanic activity or inwash of old soil to the lake. Israde-Alcántara et al. reject uniformitatianism leter in the paper when it comes to the (blue) rejected 14C dates (Figure 1).  The blue dates are rejected because they fail to match a linear depositional trend (I have no idea why anyone would assume that sediment deposition should be constant through time) and the rejected dates come from a region with too much organic matter, so the trend represents a dilution curve that somehow doesn’t affect the dates supporting the YD hypothesis.  Confusing.  It would be really great to see the proper error analysis on this chronology.

Figure two from the Lake Cuitzeo paper
Figure 2. The comparison of incomparables, and evidence of massive overabundance of oak at Lake Cuitzeo (a percentage of nearly 75,000% reported). This alone would make an excellent paper.

The other issue is the paleoecological interpretation.  Figure 2 in the paper (Fig 2) has some major deficiencies.  Not the least of which is the fact that Lake Cuitzeo Quercus percentages are reported to be up to 75,000% in the late-glacial period.  I don’t even know what to say about that.  The La Yeguada concentrations are reported as g/cm2, which, I assume is a typo, and, ultimately comparing concentration to percent is really not useful.  We point this out in the Gill et al. response, but I think that the response misunderstands our point:  Just because percentages and concentrations drop at the same time does not mean the same processes are acting on them.  There is an extra constraint in calculating percentages (the sum to one constraint) that is not present in the concentration data.

I’m not prepared to discuss the other responses since I am not as familiar with the science behind these aspects, but as far as the paleoecology goes, I am happy with our comments to the paper, and feel that in general (and as Figure 2 clearly demonstrates) there are significant deficiencies in the interpretation and presentation of the results.

Songs I’m Listening To:  Record Body Count – The Rheostatics.  This is the first time I’ve seen the video, this is not how I pictured it at all.

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Assistant scientist in the Department of Geography at the University of Wisconsin, Madison. Studying paleoecology and the challenges of large data synthesis.

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