Permian wood misinterpreted as fossil charcoal
deutsche Version
What looks like fossil charcoal at first sight may be no such. More often than not, palaeobotanists have formed their opinion without looking closely enough, as with the conspicuous black wood fragments in white or red chalcedony pictured here.
Permian wood fragments (no charcoal) in chalcedony
Fig.1: Lower Permian wood fragments in white chalcedony, Döhlen basin, Germany.
cell files rolled up under shear deformation
Fig.2: Same sample, shear cracks with cell files partially rolled up (see idealized drawing) and squeezed while soft.
shear crack with cell files coiled

The aspect had suggested the idea of brittle charcoal broken into angular fragments and dust-like debris. 
(M. Barthel, private communication 1993). Doubts arose from the observation that some cracks in the angular fragments look as if the wood was soft when they were formed. The suspicion was confirmed by the complex deformation phenomenon seen in the upper half of Fig.2: Two cracks (above and left), whose planes are separated by 10 files of wood cells, had been propagating between files, driven by overall shear stress (Mode II cracks in terms of fracture mechanics) until their overlapping stress fields offered another option: connecting their crack tips by a strip of sheared but not broken wood. As a peculiar fact, the overall shear displacement in the strip is realized by a combination of local slip and winding of cell files into coils.
Permian wood crack with cell files bridging the gap
Fig.3: Same sample, gap bridged with bent but not broken cell files.

The tiny remains of pith ray cell walls protruding into the cavities in Fig.3 indicate that fracture started in the pith rays while the main part of the wood was still coherent. A succession of broken pith rays can appear and mechanically behave as a big crack bridged by bent cell files, as also seen here.
It would be hard to imagine that delicate structures like the sheared strip connecting two cracks (Fig.2) or the bent cell files bridging the gap (Fig.3) could have persisted through a forest fire without breaking. As the coils and bent files are virtually never found in a broken state, one can safely assume that what is seen here is no charcoal but simply wood slowly deformed and torn asunder after it had lost most of its strength by lying in water and soft silica gel for some time. As a result of such evidence, other images of this specimen have not been offered as charcoal in [1], Fig.9, and in [7], Fig.199.

Fossil charcoal has become a favourite interpretation always offered by some palaeobotanists when fossil wood looks black and fragmented (Fig.4), even if it comes in apparently enigmatic arrays as in Fig.5.
Permian tree trunk cross-section allegedly consisting of charcoalPermian tree trunk section allegedly consisting of charcoal
Fig.4: Cross-section of Permian tree trunk allegedly turned into charcoal without disintegrating into a pile of fragments before silicification. Winnweiler, Rhine-Palatinate, Germany. Detail from [2], Bild 449, width of the picture 14cm.

Fig.5 (right): Permian tree trunk lengthwise section, Winnweiler, Rhine-Palatinate, Germany. Detail from [3], Abb.6, there interpreted as fossil charcoal blocks but recognized as problematic. Height of the picture 1.3cm.

Apparently the authors [2,4] thought it possible that a tree trunk turns into charcoal throughout
although every charburner knows that without intricate process control it would end up in ashes.
The authors [3] saw the problems arising with Fig.5 and tried to overcome them with the assumption that the silica gel split the charcoal into blocks and subsequently separated the blocks from each other. However, there are virtually no other cases known where silica gel effected disintegration.
The authors [3] saw more problems with turning massive trunks into charcoal. So they invoked the idea of a catastrophic event which piled the trunks into big heaps so as to resemble a charcoal kiln if incinerated. However, this would not work for lack of process control.
They did not see the simple solution: Their problems vanish with the assumption that there is no charcoal involved. The observations suggest a quite different explanation:
Apparently the wood substance underwent some slow process of degradation while submerged in water containing silica, possibly mediated by microbes, which brought about a black aspect, reduced strength, and shrinkage with fragmentation. According to the anisotropy of the wood strength, the pattern formation in Fig.5 began with vertical through-cracks. The gaps between the vertical wooden strips or rods became filled with silica gel which stuck to the wood and whose strength was sufficiently high to prevent lengthwise contraction of the rods as a whole. Hence, shrinkage stress built up until the rod broke somewhere. As shrinkage continued, the broken parts broke again until the fragments were so short that the shrinkage stress did no more exceed the fracture strength. With simple reasoning, such scenario leads to the conclusion that wider rods should produce longer fragments, which is indeed realized in Fig.5. All cracks and the wood fragments became filled with silica gel and finally turned into chalzedony by uptake of more SiO2 and expulsion of water.
Similar wood decay into rectangular pieces, less regular in the absence of silica gel, is known as brown cubicle rot.
The interpretation as fossil charcoal had been refuted by means of the above arguments [5]. Nevertheless it is stubbornly being upheld at the Museum für Naturkunde Chemnitz, despite of plain contrary evidence.
Figs. 1-3: sample W/55, own collection, found in 1992 at Wilmsdorf golf course, Possendorf near Dresden, Photographs:
 M. Barthel.

Addendum 2015:
  They have got it at long last !
It took the authors [1-4] about ten years to absorb the idea that they were wrong with their charcoal interpretation, and they offer this as a new insight now [6]. They could have arrived at this change of mind earlier if they had heeded my repeated advice [5] from 2005 on. This gives reason for hope that relentless criticism over a long time will produce favourable results also in other cases where authors, particularly
 R. Rössler and R. Noll, shy away from discussions on their publications.
Addendum 2017:
Palaeobotanists, of course, would like to find fossil charcoal as probable evidence of past wildfires whose frequency and extension would allow conclusions to be drawn concerning the climate of bygone times.
The notorious difficulty to distinguish between charcoal, which often comes in small crumbs only, and black fossil wood requires particularly careful examination of anatomical details of the cell structure, preferably with the Scanning Electron Microscope [8].

H.-J. Weiss
    2011, 2015, 2017

[1]  M. Barthel : Pflanzenfossilen im rechten Licht. Veröff. Mus. Naturkunde Chemnitz 19(1996), 49-62.
[2]  R. Rössler: Der versteinerte Wald von Chemnitz. Museum f. Naturkunde Chemnitz, 2001, 179.
[3]  R. Noll, D. Uhl, S. Lausberg : Brandstrukturen an Kieselhölzern der Donnersberg Formation.
      Veröff. Mus. Naturkunde Chemnitz 26 (2003), 63-72.
[4]  R. Noll, V. Wilde :  Conifers from the „Uplands“ – Petrified wood from Central Germany, 
       in: U. Dernbach, W.D. Tidwell : Secrets of Petrified Plants, D'ORO Publ., 2002, 88-103
[5]  H.-J. Weiss : Seltsame Strukturen in Siliziten (Strange structures in silicites), 
oral presentation, 4th  Chert Workshop (2005),
       Museum f. Naturkunde Chemnitz.

[6]  R. Rössler, R. Noll, D. Dietrich, V. Annacker, M. Merbitz: Taphonomic features of fossilised wood ...
     23rd Int. Workshop on Plant Taphonomy,  Museum f. Naturkunde Berlin, 11/2014.
[7] M. Barthel : Die Rotliegendflora der Döhlen-Formation. Geologica Saxonica 61(2015), 105-238.  (Voricht, falsche Größen !)
[8] A.Jasper, D. Uhl et al.: Evidence of wildfires in the Late Permian …,  Current Science 110 No3 Feb.2016, 419-423.
quartz crystal with wood inside
Fossil Wood News  9

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