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The “Canonical” Varve Thickness Series

Shi et al 2013 use the following five varve thickness series, all of which have become widely used in multiproxy series since their introduction in Kaufman et al 2009: Big Round Lake and Donard Lake, Baffin Island; Lower Murray Lake, Ellesmere Island; and Blue Lake and Iceberg Lake, Alaska. Some of these proxies have been discussed from time to time, with an especially detailed discussion of Iceberg Lake (see tag here.)

The figure below compares a simple scaled average of these five series to the Hvitarvatn varve thickness series (inverted so that the Little Ice Age is shown as “cold” rather than warm. See accompanying discussion of Hvitarvatn here. Whereas Miller et al reported that the 19th century at Hvitarvatn was the period of greatest glacier advance in the entire Holocene, the “Kaufman five” show 19th century levels similar to the 11th century medieval period, with an anomalously ‘warm” 20th century:

shi-2013_compare-to-hvitarvatn
Figure 1. Top – average of five Shi et al varve thickness series; bottom – Hvitarvatn varve thickness (inverted). All in SD Units.

There is no “common signal” in the Kaufman Five according to common methods. The median inter-series correlation is 0.00605, with negative interseries correlation as common as positive interseries correlation. If one examines the eigenvalues of the correlation matrix – a useful precaution in assessing whether the data contains a “common signal” – there are no eigenvalues that are separable from red noise as evident in the barplot below.

shi-2013_varve-eigenvalues
Figure 2. Eigenvalues of (Kaufman Five) Varve Thickness Series

Despite the overwhelming lack of common signal according to these criteria, the average of the Kaufman Five has a distinctly elevated 20th century. Here is a plot of the Kaufman Five. The lack of correlation and lack of significant eigenvalues is evident in the plot, where there is little in common among the series except for one feature: the 20th century in each series is somewhat elevated relative to the 19th century. (As noted above, the average of the five series has a somewhat elevated 20th century, but is relatively featureless in centuries prior to the 20th century, especially in comparison to the well-dated Hvitarvatn series.)

shi-2013_five-sediments
Figure 3. Five Varve thickness series used in Shi et al 2013 (SD Units.)

When parsed in detail, each of the Kaufman Five has troubling defects, some of which I’ll briefly discuss today and which I’ll try to follow up on.

The Iceberg Lake, Alaska series has profound inhomogeneities, especially in its 20th century portion. A major inhomogeneity is that varve thickness is related to distance to the inlet, an observation first made in comments at Climate Audit in comments on Loso 2006. Loso 2009 conceded this point (without mentioning Climate AUdit though it did acknowledge WIllis Eschenbach who corresponded with Loso on a different point) but its remedy (taking logarithms) was hopelessly inadequate to the problem. Dietrich and Loso 2012 acknowledges that inhomogeneities impact their reconstruction, but did not amend or withdraw the earlier series. Interestingly, Dietrich and Loso report glacier advance in Alaska commencing around 1250AD, almost exactly contemporaneous with the well-dated Hvitarvatn advance. The Iceberg Lake series, as used, has a late 20th century uptick coinciding with a major inhomogeneity, the effect of which cannot be separated under any plausible technique known to me.

Major features of the Big Round Lake series (as I’ve observed previously) correspond to major features of the Hvitarvatn series and there is a much higher inter-series correlation between these two series than to other series in the Kaufman Five. The only problem is that this correlation requires inversion of the Big Round series so that thicker varves are generated in the Little Ice Age. There are important geological parallels between the two sites: like Hvitarvatn, Big Round is a proglacial lake, the sediment volume of which is related to proximity of a nearby glacier, which advanced in the Little Ice Age to its Holocene maximum and receded in the 20th century. In order to use the Big Round series in its present orientation, specialists have to explain why its behavior is opposite to Hvitarvatn. And why one should interpret Big Round varve data as showing a Little Warm Period in Baffin Island during Iceland’s Little Ice Age (especially when glacier lines moved 500 m lower in Baffin Island during this period.) The reason why Big Round varves are oriented thick-up by the original authors (Thomas et al 2009) is that there is a positive correlation in the late 20th century between varve thickness and local temperatures. Together with the exclusion of the inhomogeneous Iceberg Lake series, inverting this series (as seems required) would obviously impact the average of the canonical series.

Like Hvitarvatn and Big Round Lake, Donard Lake (Baffin Island) is a proglacial lake whose sediment volume is controlled by proximity of a nearby glacier (Caribou Glacier). Once again, this glacier reached its Holocene maximum in the Little Ice Age, prior to its 20th century retreat. However, the Donard Lake varve thickness series has a slightly negative correlation to the Big Round Lake series. Rather than simply averaging these two incompatible series, specialists need to closely re-examine the data to explain the inconsistency. Donard Lake dating is one thing that needs close examination.

Thomas and associates have recently reported a third proglacial varve thickness from Baffin Island (Ayr Lake), for which they unable to report a significant correlation to instrumental temperature. Thus, they did not report a temperature reconstruction for this site. However, the absence of such correlation surely bleeds back to the other series, inviting a reconsideration of whether their supposed correlations to temperature were spurious – particularly in the context of their inconsistency with the well-dated Hvitarvatn series.

Because varve thickness in these proglacial lakes is profoundly affected by glacier proximity, there is no homogeneous relationship between varve thickness and temperature

Source: 
Climate Audit

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