Degradation and preservation of palaeozoic wood

Petrified wood is known in various states of preservation. It may reveal tiny details of the tissue or consecutive stages of decay before and during silicification, also damage to the living tree. A few related observations are reported in the following. As a remarkable fact, severely decayed wood is often seen immediately beside well preserved tissue on cut faces of silicified wood (Fig.1). This suggests the idea that highly selective processes had been at work which affected individual cells, often spreading from one cell to the next, leaving some cells well preserved but others completely decayed.
The pictured sample is problematic as there is more than one thinkable explanation of Fig.1. The distorted pith rays above seem to indicate that first came a process spreading preferably along the radial cell files and pith rays, filling the cells with transparent chalcedony with a brown hue, which makes a dark aspect if one looks into the tracheid tubes. The cells preserved in this way were not accessible to the subsequently encroaching rot which caused disorder by weakening or consuming the cell walls of larger parts of the wood, giving rise to the deposition of whitish chalcedony and to the growth of crystalline quartz so that a few distorted pith rays is all that is left of the former structure.
dark cell files

Fig.1: Well preserved radial cell files adjacent to decaying coniferous-type wood indicated by a few distorted pith rays above. Cell diameter about 50µm.

Another sign of selectivity is seen in Fig.2, where individual cells are (partially) filled with one dark clot each, which resembles the clots produced by fungi in extant wood [1].

Fig.2, right: Dark clots, often smaller than the cells, scattered throughout the tissue. Same sample as Fig.1.

Figs.3,4: Clots within cells in radial files.

A combination of the two features making rounded dark clots within cells in radial files has been observed in Permian wood from Germany (Fig.3) and China (Fig.4). The latter clots have been questionably interpreted as oribatid mite coprolites [2].

Fig.5 (right): Intact wood structure on the left bordering on disintegrated tissue on the right. Note the dark pith rays.

Degradation or loss of strength may reveal itself by large irreversible deformation under load as seen in Fig.5, where there is a rather sharp boundary between the regular wood structure on the left and the distorted tissue on the right. Although the contributions of the various processes possibly involved in degradation are not known, it can be stated that large shear deformation or kinking is usually not the cause of decay, as it is also seen in this sample where one can find small internal kinks without disintegration (Fig.6). Hence, Fig.5 suggests that first came local degradation, then local shear and decay.

Fig.6 (left): Kink in wood without disintegration of tissue.

Obviouly the cells had not been empty during deformation, otherwise they would have been squeezed into rhomboid or oblique shapes as often seen in petrified wood, also in Fig.7, below left.

wood decayed into white clots

Fig.7: Light-coloured clots representing cell lumina of disintegrated wood next to dark areas of coherent tissue stained red with hematite. Width of the picture 1.2mm.

Loss of mechanical strength of the wood, revealing itself as large deformations, must be due to weakening of the cell walls. This can be inferred from Fig.5 but is not immediately obvious. It is obvious from a peculiar variety of local disintegration as in Fig.7, where the cell walls were essentially consumed, and if the cell lumina had not been preserved somehow as white clots, one would not have a strong argument for the assumption that there had been wood with cells in radial files where chaos reigns now. Incidentally, this image is different since there is a horizontal dark line which mediates between the random clot distribution and the regular tissue.
Larger random assemblages of whitish clots had been mentioned earlier [3] and will be the subject of a forthcoming contribution. If not adjoining to well-preserved tissue as in Fig.7, the clots may easily be mistaken for remains of the central pith. Clots of this kind, like the more frequently seen dark ones, have repeatedly been mistaken for mite coprolites.
The samples in Figs.1,2,5,6,7 were found at the Borxleben gravel pit near the Kyffhäuser Mountains, Germany, uppermost Carboniferous [4]. The sample in Fig.7 has been provided by W.+G. Etzrodt, Borxleben, who also supported the collection of other samples at the location. The sample in Fig.3 (Lower Permian) has been provided by Ch. Krüger , Schallodenbach, .
Sample labels: Figs.1,2,5: KyB/112.1, Fig.6: KyB/112.2, Fig.7: KyB/12.1,

The observations are compatible with the following interpretation:
- There had been interrelations between processes promoting decay or preservation.
- The processes did not work simultaneously throughout but locally by spreading of affected areas, resembling the spread of rot in live or dead wood.
- The loss of strength and decay of cell walls had probably been brought about by fungus activity.
- Metabolic waste released by some fungus attacking the wood and /or substances from partially decaying wood triggered the precipitation of SiO₂ , with a tendency to preserve what had been left of the tissue.
- The processes of both decay and preservation could have begun with individual affected cells and subsequently spread across the tissue.
- The rich non-biological structure of fossil wood from several locations, notably of the type found at the Kyffhäuser mountains, is the result of competing processes whose unravelling has just begun and will reqire much more effort.

Discussing the formation of non-biological structures mediated by biological agents is not idle talk since the subject has been the source of misinterpretations by professionals from about 200 years ago until present.

H.-J. Weiss 2013

[1] F. Schwarze: Fungal strategies of wood decay in trees. Springer, Berlin 2004.
[2] Zhuo Feng, Jun Wang, Lu-Yun Liu:
      First report of oribatid mite (arthropod) borings and coprolites in Permian woods from the Helan Mountains of northern China.
      Palaeogeography, Palaeoclimatology, Palaeoecology 288(2010), 54-61.
[3] H.-J. Weiss: Beobachtungen an Kieselhölzern des Kyffhäuser-Gebirges.
      Veröff. Mus. Naturkunde Chemnitz 21(1998), 37-48.
[4] J. Schneider, R. Rößler, B. Gaitsch: Stratigraphy and facies of the Middle European continental Carboniferous and Permian excursion guide A5, 1995.