“When I use a proxy,” Humpty Dumpty said, in rather a scornful tone, “it means just what I choose it to mean – neither more nor less.”
“The question is,” said Alice, “whether you can make a proxy mean so many different things.”
With apologies to Lewis Carroll
Just as I was beginning to run out of things to write about in the chironomid reconstruction from Lake Żabińskie (I still want to write about figure 2 from Larocque-Tobler et al and ask why it is missing 18 lakes), along comes another chironomid reconstruction from Lake Żabińskie, this one spanning the last 1000 years (Hernández-Almeida et al 2016).
You might expect that a 1000-year August air temperature reconstruction would be published in its own paper, as it is sure to be included in compilations of palaeo-climate reconstructions, but no, it is tagged onto the end of a multiproxy study where most of the time the chironomid stratigraphy is used as an anoxia indicator.
Chironomids are of course sensitive to anoxia (most oxygen breathers are), but it is not clear why the authors believe that the first principal component of the chironomid stratigraphy is an anoxia indicator. They write
In Lake Żabińskie, between AD 1896 and 2011, the chironomids PC1 had a relationship with changes in anoxic taxa
but don’t inform the reader that the relationship is very weak (Pearson’s correlation coefficient = 0.23) and non-significant. Not much of a relationship. The second PC axis, in so much as you would trust any ordination axis of these data, has a somewhat higher (0.37) and significant correlation.
Since the chironomid assemblage is a mixture of littoral chironomids (73%) that live in the oxygenated water above the thermocline and profundal chironomids that live in deeper, perhaps oxygen-depleted water, the relationship between anoxia and chironomid assemblages over time is likely to be complex and noisy.
Despite the importance of the chironomid record, the stratigraphy is not shown and no reconstruction diagnostics are given. There is no way for the reader to evaluate whether the reconstruction is any good (it is possible that some of this essential information is included in the supplementary online material which does not seem to be available at the moment).
Hernández-Almeida et al also include a chrysophyte-based reconstruction of winter temperature (first presented in Hernández-Almeida et al 2015a), but spend most of the paper using the first principal component of the chrysophyte data as a nutrient indicator. In contrast, Hernández-Almeida et al 2015b, interpret the same chrysophyte stratigraphy as an indicator of calcium concentrations and relate this to May-October zonal wind speed. These two previous papers by Hernández-Almeida et al make no attempt to acknowledge, still less address, these radically different interpretations of the same data. The winter temperature reconstruction follows previous literature and seems to have been planned in advance, whereas the zonal wind reconstruction reads like a fishing expedition.
Of course I understand the motivation for producing more that one reconstruction from a proxy record that took months of microscope time to generate. And of course I fully accept that biotic proxies are influenced by multiple environmental variables and that at different sites different variables might be possible to reconstruct. The problem is in knowing which. This is why well designed palaeoecological experiments are so powerful.
Reconstructing multiple variables from the same data inevitably means that the assumption of transfer functions that
Environmental variables other than the one of interest have negligible influence, or their joint distribution with the environmental variable does not change with time.
must be violated. Juggins (2013) showed that ecologically important secondary environmental variables can severely bias reconstructions. The chironomid anoxia response will bias the temperature reconstruction and vice versa; the chysophyte winter temperature response will bias the May-October zonal wind speed record and vice versa.
Considerable work is needed to demonstrate that a proxy can be used to make meaningful reconstructions of a single variable. Far more is need to make a convincing case for multiple reconstructions: Hernández-Almeida et al don’t even try.
“The question is,” said Humpty Dumpty, “is it publishable – that’s all.”