Backgrounder on Analysis of Freshwater Lakebed Varved Sediment Cores

Deep in the thread following the Climate Audit post The No-Dendro Illusion, "Ianl8888" made a series of remarks that provide valuable context to the 2003 Boreas paper by Mia Tiljander et al.

His (or her) remarks follow, with minor edits for style.

Aug 3, 2010 at 11:59 PM

“Compression” (i.e. lithification) is not a real issue [in the Tiljander03 analysis], as it does not change the parameters of the initial deposition except for water content. In fact, lithification makes the core sample easier to retrieve, and considerably lessens core loss. Such loss makes interpretation of the depositional environment more problematic.

I have analysed the geophysical logs presented in Tiljander’s papers and agree with her estimates of minor core losses.

One of the limiting factors in Tiljander’s thesis is the small area represented by the drillhole locations. This is essentially because the accessible fringe lake area itself is limited. The reason for wishing for wider geographical spread of sample points is that micro-disturbances (e.g. from sudden but small surges of water released by an ice break) can be more easily distinguished from broader conditions around the lake edge.

If the bio-organism layers are identified as [deriving from] from warmer-weather species (as Tiljander et al. do), then bio-organism versus silicate thicknesses are seen as reliable proxies of warmer/colder conditions, in my opinion.

[Mann08 employs four data series from Tiljander et al.'s analysis of the Lake Korttajarvi sediment cores: varve thickness, Darksum, Lightsum, and X-Ray Density. The question of the extent to which these measures are genuinely independent from one another has been raised earlier in the "The No-Dendro Illusion" comment thread.] I cannot see a four-factor situation here, just thicknesses, organic/silicate layer contents, and degree of sedimentation disturbance. But: human activity on the lake edges (especially driven piles for bridges) do most certainly interfere with the slow, delicate process of sedimentation, to the point of local destruction.

Aug 4, 2010 at 3:56 AM

[Six points of clarification:]

1) XRD analysis is to determine both the mineralogical composition and the ratio of inorganic/organic material in a sample. Compression and density have no bearing on this, nor does magnetic susceptibility.

2) Varves are generally regarded as an accumulation of annual “pairs” of strata, crudely summer and winter.

3) In summer [sic -- "spring"? - AMac], when water flows from melting ice are relatively more abundant and turbulent, sand/silt is deposited along with any debris from dying organisms (light-coloured stratum). In winter, clay particles are very, very slowly deposited (because these are so fine), generally with relatively little or no organic debris (dark coloured stratum). This deposition requires very undisturbed water to avoid re-stirring the particles, [which would keep them in suspension,] thus preventing deposition.

4) Because the deposition process is so delicate in both seasons, anything that interrupts the water/sediment flows or equilibrium in any one year will result in a deformation of the annual “pair”. If the interruptions are long-lived (farming, bridge building, ditch-digging), the deformation [will likely result] in damaged strata succession over a considerable period (perhaps many years), making correlation with samples in other local geographical locations highly problematic.

5) Tiljander’s papers may not be absolutely complete, but she knew which way is up (in the physical, strata succession sense), and also that persistent human interference around the lake edges inserted the likelihood of damaged varve succession, [and changes in sediment thickness and composition that are unrelated to the area's climate.]

6) Altogether, [these data series derived from Lake Korttajarvi's varved sediments are] not a good temperature proxy.