The tangled web of Silvaplana papers

I’ve not written much about the Lake Silvaplana papers although they are as questionable as the Seebergsee and Zabinskie papers.

I have now digitised the 150-year stratigraphy, my request to the authors for the data having been ignored. Together with the archived 420-year stratigraphy I can now answer questions that I couldn’t earlier.

First question, how large were the count sums? The 150-year paper reports that

The total number of head capsules is highly variable; between 15 and 125 head capsules were counted in samples of ca. 30 mg dry weight, the average being 30 head capsules per sample.

JoPL-06 (copy)

The 150-year stratigraphy.

This is easily demonstrated to be false. The stratigraphic diagram shows the count sums in the third curve from the right. Over 20 samples (of 94) have a count sum to the left of the tick mark at 12.5 head capsules. Some of the samples appear to have counts as low as three (samples 33 & 34 are both ~66% Cricotopus and ~33% one other species).

The 420-year paper makes even stronger claims:

Before sample no. 170 [~1775 CE], the minimum head capsule numbers was 50.

Perhaps the authors counted some more chironomids after publishing the first paper, but the stratigraphic diagram shows that most of the samples above sample 170 have counts smaller than fifty: some appear to be below fifteen.


The 420-year stratigraphy counts


All the papers report that samples with fewer than thirty head capsules were merged before the transfer function was applied. This presumably explains why the reconstruction in the 150-year paper has 69 values rather than one for each of the 94 levels in the stratigraphy. I assume the merged samples form (more or less) the upper 69 samples in the archived assemblage data from the 420-year paper (the oldest assemblage in the 150-year stratigraphy is identical to the 69th in the 420-year paper, which represents 1850).

Even after merging, the samples appear to have some counts below 30. The youngest sample in the 420-year dataset has eight species all with exactly 10% and one with 20% of the assemblage. It is more likely that this assemblage has a count sum of ten than a count sum of at least thirty. The youngest sample in the 150-year stratigraphy has the same composition within the error of digitisation. The reported count sum for this sample is about ten.

Reliable reconstructions cannot be expected from such small samples.

I cannot reproduce the calibration-in-space reconstructions derived from the 150-year or the 420-year stratigraphy as the calibration set has not been archived. Given that the chronological uncertainty is estimated at 15%, it is surprising that the reported correlation between the reconstruction and the instrumental record is so good.

I can, however, attempt to reproduce the calibration-in-time reconstruction shown in a subsequent paper. This reconstruction has 82 levels. I don’t know how the 94 levels were merged to make 82, so I’m going to use the 64 samples from the 420-year stratigraphy which fall into the instrumental period which starts in 1864 (five fall between 1864 and 1850). I also don’t know exactly which years are supposed to be represented by each sample, so I am just going to use the reported year with data from meteoSwiss.

I’m not expecting the result to be exactly the same as the authors, but it should be similar. My 2-component WAPLS model on square-root transformed assemblage data has an apparent R2 between the measured and estimated temperature of 0.45. The leave-one-out cross-validated reconstruction has an R2 of 0.01. This is somewhat short of the reported R2 of 0.51 for the whole record (0.71 for the first 90 years). It would appear, as at Seebergsee, that the authors have not cross-validated their model. In both Silvaplana and Seebergsee, the cross-validated model has no skill, and the published millennium-long reconstruction cannot be relied upon. If my analysis is supported by a re-analysis of the full data used in the calibration-in-time paper, then this paper should be retracted.



About richard telford

Ecologist with interests in quantitative methods and palaeoenvironments
This entry was posted in Peer reviewed literature and tagged , , , , . Bookmark the permalink.

2 Responses to The tangled web of Silvaplana papers

  1. Pingback: The third chironomid calibration-in-time reconstruction | Musings on Quantitative Palaeoecology

  2. Pingback: Reproducibility of high resolution reconstruction – one year on | Musings on Quantitative Palaeoecology

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