Specialist literature on varves e.g. Besonen et al 2008 – coauthor Raymond Bradley -(which is cited by Tingley and Huybers) make the obvious observation that varves are compacted within a core. Besonen et al 2008 allow for compaction by estimating annual mass accumulation as a more appropriate measurement of varve “thickness”, rather than uncompacted varve thickness. In their abstract, Besonen et al stated:
In many studies of lakes from the High Arctic, varve thickness is a good proxy for summer temperature and we interpret the Lower Murray Lake varves in this way. OnOn that basis, the Lower Murray Lake varve thickness record suggests that summer temperatures in recent decades were among the warmest of the last millennium, comparable with conditions that last occurred in the early twelfth and late thirteenth centuries, but estimates based on the sediment accumulation rate do not show such a recent increase
They report later in the article:
On the other hand, because of compaction, the thickness of recent varves is not directly comparable with those varves that are buried deeper in the sediment pile. This problem can be addressed by calculating a packing index (a simple ratio of the area occupied by sediment grains versus the area occupied by matrix in the varve BSE images) and then calculating the sediment accumulation rate on an annual basis (assuming a constant sediment density of 2.65 g/cm3, for quartz). This procedure compensates for compression of the sediment with depth, and results in a suppression of the trend over the last century seen in the varve thickness record (Figure 7).
An excerpt from Besonen’s Figure 7 is shown below. The top panel shows varve thickness unadjusted for compaction, while the bottom panel shows mass accumulation. The top panel (with no allowance for compaction) shows somewhat elevated 20th century levels, while the bottom panel (after allowing for compaction) does not – the phenomenon noted in their abstract.
A related article (Cook et al 2009 – also with Bradley as coauthor) the following year on a different Murray lake core made similar observations about compaction. Cook et al provided a temperature reconstruction using mass accumulation rate, showing a rather elevated MWP as shown below. I’m not inclined to put much weight on simplistic reconstructions from varve thickness (see also my discussion of Gifford Miller’s observations on this topic), but show this series as evidence that mass accumulation was used by this group as the relevant index.)
Tingley and Huybers 2013 have three classes of proxy data: MXD data, which has the familiar divergence problem; ice core O18 which doesn’t have a Hockey Stick shape and varves. The Murray Lake varve series is used. Tingley and Huybers provide an excellent SI, including exact URLs for data sets as used – a simple enough protocol that unfortunately is seldom observed. (They forgot to archive their actual reconstruction, though I presume that this is a mere oversight since their intent is clearly to provide a comprehensive archive).
In their SI, they state:
Details and references for the lake varve records used in the analysis are available in Table S.1. Unless the description of the data indicates otherwise, we use the total varve thickness.
Of the two Murray Lake versions (different cores taken by the same group), they cite the Besonen et al version.
For the Murray lake record, we use the unfiltered version of the shorter (1000 year) record posted at the NOAA Paleolimnology site .
However, when one compares their archive of data as used to the NOAA archive, one can immediately determine that they used varve thickness without compensating for varve compaction (the series match), rather than mass accumulation as used by Cook et al 2009 in their temperature reconstruction. This decision gives a pronounced upward bias, as shown by the difference between the two series (after taking a log and then scaling as in Tingley and Huybers.)
I haven’t yet looked at how the other varve series handled compaction, but it seems like an important issue in any attempt to deduce temperatures from this sort of data. In the particular case of the Murray Lake series, it seems to me that the original data clearly “indicates” that mass accumulation be used as an index, rather than varve thickness unadjusted for compaction and that this should have been used according to the stated methodology of Tingley and Huybers.
As previously noted, Tingley and Huybers also used the contaminated portion of the Korttajarvi sediment data, so there are multiple problems with their varve reconstruction. These are not complicated issues, but ones that ought to be within the scope of even Nature peer reviewers.