13. Fossilization, Permineralization, Есголƥегцлсрǡ Carbonization
13. Fossilization, permineralization, Ƥ ǡ carbonization and wet wood conservation
13.1 Fossilization
When remains or traces of organisms are preserved in sediments Ń Conclusions about the seasonality of past climates are pos- they are called fossils. The process of fossilization (syn. WL[YPÄJH- sible on the basis of the presence or absence of growth rings. tion or permineralization) begins when organisms, e.g. stems, )HUKLKHNH[LZJHUYLZLTISLNYV^[OYPUNZPUWL[YPÄLK^VVK are buried and remain in anaerobic conditions. Most WL[YPÄLK but they represent periodic mineral deposits, e.g. in stalac- stems are therefore in a horizontal position. Many fossil stems tites. ^LYLWYVIHIS`KYPM[^VVK[OH[^HZI\YPLKI`Å\]PHSZLKPTLU[Z Ń Diagenetic processes (alteration during fossilization) can be Stems in upright position were more likely embedded in vol- reconstructed on the basis of the preservation of tissue and canic ashes. When supersaturated groundwater penetrates the cell-wall structure, and the crystalline composition of miner- wood, the minerals precipitate within cellular spaces and crys- als. Hyphae, barrier zones and coprolites indicate a decay talize. Most frequent are calcium carbonate (CaCO3) or silicate under aerobic conditions, while cell-wall degradation, cell minerals (SiO2). Fossilization occurs mainly in association with deformation and root inclusions indicate decay under anaer- marine or volcanic hydrothermal water. Characteristic for fos- VIPJJVUKP[PVUZILMVYLWL[YPÄJH[PVU+PMMLYLU[JVSVYZVUWVS- sils is the conservation of microscopic structures that can be ished discs are related to different minerals, e.g. red is related observed on polished disks or on micro-sections. Observations to iron, blue and transparent to silicate, green to copper and VUMVZZPSPaLK^VVKSLHKZ[VMV\YWYPUJPWHSÄUKPUNZ! WPUR[VTHUNHULZL4PULYHSJVTWVZP[PVUJHUILX\HU[PÄLK Ń ;H_VUVTPJJSHZZPÄJH[PVUZHYLZ[PSSWVZZPISLVU^LSSWYLZLY]LK on a microscopic section under polarized light. fossils. Ń (ZZLTISHNLZVMMVZZPSZWL[YPÄLK^VVKHUKWVSSLUPUZ[YH[P- See also Taylor et al. 2009 and Selmeier 2015. graphically uniform layers allow a limited reconstruction of past vegetation types.
13.1 Stem disc of a Triassic Araucar- ioxylonZ[LTMYVTHWL[YPÄLKMVYLZ[ in Arizona, USA.
© The Author(s) 2018 183 F. H. Schweingruber, A. Börner, The Plant Stem, https://doi.org/10.1007/978-3-319-73524-5_13 Ƥ
13.2 ,YVKLK WL[YPÄLK Z[LTZ MYVT 13.3 Broken and dislocated petri- 13.4 Reconstruction of the Arau- 13.5 Modern Araucaria araucana Å\]PH[PSL ZLKPTLU[Z VM [OL
Macroscopic aspect of polished disks
13.6 Quercoxylon sp. with distinct 13.7 Conifer with indistinct annual 13.8 Palmoxylon sp., of unknown 13.9 7L[YPÄLKliana of unknown age annual rings. Miocene, approx. rings. Miocene, approx. 20 million age and locality, with distinct vas- and locality. 20 million years ago, Great Basin, years ago, Great Basin, Idaho, USA. cular bundles. Idaho, USA.
Ƥ Ƥ
250 μm 100 μm 50 μm 50 μm
13.10 Rhynia gwynne-vaughanii, 13.11 Cross section of Paradoxylon 13.12 Tangential section of Para- 13.13 Radial section of Paradoxy- a herb with a central stele. Lower leuthardii, one of the Cordaitales doxylon leuthardii with biseriate lon leut hardii with multiseriate Devonian, approx. 400 million years without annual rings. Upper Trias- rays. Upper Triassic, approx. 220 bordered pits on tracheids. Upper ago, Scotland. sic, approx. 220 million years ago, million years ago, Vosges Moun- Triassic, approx. 220 million years Vosges Mountains, France. Slide: R. tains, France. Slide: R. Buxtorf. ago, Vosges Mountains, France. Buxtorf. Slide: R. Buxtorf.
184 Ch 13. Fossilization, permineralization, coalization, carbonization and wet wood conservation Ƥ Ƥ
v
500 μm 250 μm 500 μm 500 μm
13.14 Sequoia sp. with distinct and 13.15 Mimosoideae, Fabaceae, 13.16 Celtixylon cf., Ulmaceae, 13.17 Palmoxylon, Arecaceae, with indistinct rings. Early Oligocene, without annual rings, with para- with distinct annual rings. Oligo- KPZ[PUJ[ SPNUPÄLK WHY[Z VM ]HZJ\SHY approx. 34 million years ago, Floris- tracheal parenchyma. Oligocene, cene, approx. 28 million years ago, bundles. Oligocene, approx. 28 sant, Colorado, USA. approx. 28 million years ago, Vosges Vosges Mountains, France. Slide: R. million years ago, Vosges Moun- Mountains, France. Slide: R. Buxtorf. Buxtorf. tains, France. Slide: R. Buxtorf.
Ƥ
1 mm 1 mm 500 μm
13.18 Cross section of Paradoxylon 13.19 Cross section of Paradoxylon 13.20 Lauraceae cf. with growth 13.21 .YV^[O YPUNZ PU H WL[YPÄLK leuthardii, without growth rings, leuthardii, with growth rings, indi- rings, seasonal climate. Oligo- ring-porous oak stem in Miocene indicating tropical rain forest as hab- cating seasonal climate. Upper Tri- cene, approx. 28 million years ago, deposits, approx. 15 million years itat? Upper Triassic, approx. 220 mil- assic, approx. 220 million years ago, Vosges Mountains, France. Slide: R. ago, Oregon, USA. lion years ago, Vosges Mountains, Vosges Mountains, France. Slide: R. Buxtorf. France. Slide: R. Buxtorf. Buxtorf.
Indicators of Ƥ
500 μm
13.22 )HYYPLY aVULZ PU H WL[YPÄLK 13.23 Barrier zones in a stem of 13.24 Hyphae in a vessel of a Meli- 13.25 Coprolites of a beetle in a stem of a eucalypt tree. Australia, a Lauraceae cf. tree. Oligocene, aceae tree. Reprinted from Selmeier Lauraceae tree. Reprinted from Sel- age unknown. approx. 28 million years ago, Vosges 2015. meier 2015. Mountains, France. Slide: R. Buxtorf.
185 Indicators of Ƥ
50 μm 500 μm 500 μm
13.26 Decayed cell walls in Para- 13.27 Radially compressed dicoty- 13.28 Heavily tangentially com- 13.29 The root of a herb squeezed doxylon leuthardii. Preserved are ledonous wood. Slide: R. Buxtorf. pressed wood of Paradoxylon leu- the soft, anaerobically decayed the thick primary walls and thin ter- thardii. Slide: R. Buxtorf. xylem tissue. Reprinted from Sel- tiary walls. Slide: R. Buxtorf. meier 2015. Permineralization under polarized light
100 μm 100 μm 100 μm 100 μm
13.30 Single crystals in individual 13.31 Many crystals in large early- 13.32 Irregular crystallization of 13.33 Crystallization of different tracheids in Paradoxylon leuthardii. wood vessels in Ulmaceae wood. silicates without any wood ana- minerals relating to groups of tra- Slide: R. Buxtorf. Slide: R. Buxtorf. tomical context in Ulmaceae wood. cheids in Paradoxylon leuthardii. Slide: R. Buxtorf. Slide: R. Buxtorf. 13.2 Permineralization of archaeological artifacts
Permineralization also takes place on metallic archaeological HUH[VTPJHSZ[Y\J[\YL:WLJPLZPKLU[PÄJH[PVUPZ[OLYLMVYLWVZZPISL artifacts. The scabbards and handles of iron swords, bronze dag- If highly concentrated liquids of iron, copper or sulfur soak the gers and axes often contain small mineralized remains of wood. wood, minerals precipitate on cell walls and preserve their struc- Transverse and longitudinal breaks of particles clearly show the ture.
Permineralization of archaeological artifacts
13.34 Wooden scabbard perminer- 13.35 Wooden scabbard perminer- 13.36 Microscopic structure of archae- 13.37 Microscopic structure of archae- alized by dissolved iron. Photo: W. alized by dissolved copper. Photo: ological Quercus sp. wood, perminer- ological Alnus sp. wood, permineral- Tegel. W. Tegel. alized by iron. Photo: W. Tegel. ized by sulfur. Photo: W. Tegel.
186 Ch 13. Fossilization, permineralization, coalization, carbonization and wet wood conservation ͙͛Ǥ͛Ƥ
*VHSPÄJH[PVU PZ H ]LY` ZSV^ WYVJLZZ PU ^OPJO ^VVK PZ [YHUZ- Remnants of wood (xylite, lignite) with well-preserved structures formed into coal at geological time scales. The anaerobic decay are frequently found in brown coal layers (Eocene to Oligocene). VMVYNHUPJZ\IZ[HUJLZPZ[OLÄYZ[Z[HNLVMJVHSPÄJH[PVUZLL*OHW- .LULYHVYL]LUZWLJPLZJHUILPKLU[PÄLKHZSVUNHZ[OLLHYS`- ter 12.2). The ratio of carbon increases during anaerobic decay wood zones are not too much distorted (Dolezych 2005). processes, and the wood loses its stability. The process starts with soft wood, which under high pressure and high temperatures Tertiary wood is often so well preserved that even stages of cell turns into brown coal, IP[\TPUV\Z JVHS HUK ÄUHSS` anthracite. wall decay can be recognized. However, in most cases the early- (UPUJYLHZLPUJHYIVUJVU[LU[KLÄULZ[OLZLNYHKLZ·^VVKJVU- wood zones are compressed, and the corresponding ray features [HPUZ IYV^UJVHS HUKHU[OYHJP[L JHYIVU>OLU HYLKPMÄJ\S[[VVIZLY]L+VSLa`JO softened logs come under mechanical pressure by sediments or ice, the thin-walled earlywood cells collapse, and the stems get KLMVYTLK,HYS`Z[HNLZVMJVHSPÄJH[PVUHYLRUV^UMYVTZ[LTZPU medieval glacier deposits and late glacial clays (subfossil wood).
Fossil tree stump Taxonomically and climatologically relevant features in cross sections of conifers
500 μm 500 μm 100 μm
13.38 Stumps from a brown coal 13.39 Conifer Taxodium taxodii cf. 13.40 Conifer Sciadopityoxylon wett- 13.41 Conifer Doliostroboxylon bed. Photo: Elbwestfale 2003, via without growth zones. Oligocene, steinii without resin ducts and dis- priscum with resin ducts. Eocene, Wikimedia Commons, CC BY-SA approx. 34–23 million years ago. tinct earlywood and latewood zones. approx. 56–38 million years ago. 3.0. Material: M. Dolezych. Miocene, approx. 25–5 million years Material: M. Dolezych. ago. Material: M. Dolezych. Taxonomically relevant features in radial and tangential sections of conifers pit r r r pit r
50 μm 50 μm 50 μm 50 μm 250 μm
13.42 Conifer Larix decidua 13.43 Fenestrate vessel-ray 13.44 Small round pits 13.45 Large round pits in 13.46 Uniseriate rays with with biseriate tracheid pits. pits in the earlywood of in the latewood of Taxo- the early- and latewood of three to ten cells in Taxo- Interglacial period, Italian Sciado pity oxylon wettstei- dium gypsacum. Miocene, Dolio stroboxylon priscum. dium gypsacum. Miocene, Alps, >50,000 years ago. nii. Miocene, approx. 25–5 approx. 25–5 million years Eocene, approx. 56–38 mil- approx. 25–5 million years million years ago. Material: ago. Material: M. Dolezych. lion years ago. Material: M. ago. Material: M. Dolezych. M. Dolezych. Dolezych.
187 Cell wall decay in latewood lw ew ew pit ew wall secondary secondary wall lw secondary walls lw wall secondary secondary wall
25 μm 50 μm 50 μm 50 μm
13.47 Perfectly preserved cell walls 13.48 Anaerobically decayed cell 13.49 Various stages of anaero- 13.50 Primary and tertiary cell walls in latewood tracheids of Glyptosto- walls in latewood tracheids of Larix bic decay in the latewood zone of are preserved, secondary walls are IV_`SVU Y\KVSÄP. Middle Miocene, decidua from the moraine of an Sciadopityoxylon wettsteinii. Mio- completely decayed in latewood approx. 15 million years ago. Mate- alpine glacier. Middle Ages, approx. cene, approx. 15 million years ago. trache ids of Taxodium taxodii cf. rial: M. Dolezych. 1400 AD. Material: M. Dolezych. Material: M. Dolezych. Oligocene, approx. 34–23 million years ago. Material: M. Dolezych. Mechanical deformation of cells resin duct r
r lw ew lw lw ew
ew ew lw lw ew 100 μm 100 μm 250 μm
13.51 Wood of Larix decidua, heavily radially 13.52 Radial compression is indicated by bent 13.53 Compression in a branch of Abies alba cf., compressed by a glacier. Moraine of an alpine rays in wood of Larix decidua. Interglacial period, polarized light. Interglacial period, Switzerland, glacier, Middle Ages, approx. 1400 AD. Italian Alps, >50,000 years ago. Slide stained approx. 45,000 years ago. with Astrablue/Safranin. lw lw deformed ew ew deformed deformed
500 μm 100 μm 100 μm lw
13.54 Compression of two weak zones in ring- 13.55 Intensively compressed earlywood zones 13.56 Intensively compressed earlywood and less wood of the conifer Doliostroboxylon pris- between thick-walled latewood zones in Taxo- latewood zones in Sequoiadendroxylon sp. cum. Eocene, approx. 56–37 million years ago. dium gypsacum. Miocene, approx. 15 million Czech Republic, middle Miocene, approx. 15 Material: M. Dolezych. years ago. Material: M. Dolezych. million years ago.
188 Ch 13. Fossilization, permineralization, coalization, carbonization and wet wood conservation 13.4 Carbonization
Carbonization is the process of transforming wood into char- HZ HUJPLU[ WYLOPZ[VYPJ ÄYLZ·JOHUNPUN O\THU ILOH]PVY HUK coal by pyrolysis. Charcoal used to be one of the most impor- vegetation patterns from the tropics to the arctic—can be docu- tant energy resources in pre-industrial times because it was TLU[LK^P[O[OLPKLU[PÄJH[PVUVMWHY[PJSLZHZZTHSSHZHJ\IPJ easy to transport. Most human civilizations produced charcoal millimeter. PURPSUZ^P[O[OL^VVKSVZPUN ¶ VMP[Z^LPNO[*OHYJVHS burning and grazing were the main reasons for the depletion of During the carbonization process, cell walls shrink by approx. ancient forests all over the world. With the switch to fossil fuels HUKHSZVSVZL[OLPYÄIYPSSVZLZ[Y\J[\YL/V^L]LY[OLZ[Y\J- like coal and coal oil, charcoal lost some of its importance. ture of pits and perforations, and of artifacts caused during It also used to be important for the production of gun pow- decay before the carbonization process, remain. Longitudinal der (black powder). Today, charcoal is still widely used, e.g. for cracks and tangential cell collapses are a sign of vapor pressure SVJHSTL[HSS\YNPJWYVJLZZLZJOLTPJHSÄS[LYPUNIHYILJ\LZHUK during the heating phase. charcoal crayons. Binocular stereomicroscopes and episcopic microscopes facili- ;OLJHYIVUPaH[PVUWYVJLZZKVLZUV[KLZ[YV`[OLZWLJPLZZWLJPÄJ tate the observations. Thin sections, embedded in two compo- anatomical structures of the material. Also, charcoal is very nent epoxy resin, are the basis for photographic presentations stable against biodegradation in both dry and wet environ- (see Chapter 2). ments. Charcoal is therefore of relevance to historical studies,
Charcoal production Carbonization artifacts
1 mm 50 μm
13.57 Establishment of a charcoal kiln in the Black Forest. Photo: T. Lude- 13.58 Radial cracks in this conifer 13.59 Thin carbonized cell walls in mann. are caused by vapor pressure during Alnus sp. the heating phase.
Ƥ
500 μm 500 μm 500 μm 500 μm
13.60 Conifer with an insect gal- 13.61 Charcoal cross section of 13.62 Charcoal cross section of 13.63 Charcoal cross section of SLY`ÄSSLK^P[Ocoprolites. Fraxinus excelsior. Alnus sp. Fagus sylvatica.
189 13.5 Wet wood conservation
Woods in anoxic sediments can keep their form over millennia. and made mostly of wood of deciduous trees. Five-thousand- However, decomposition takes place on a cellular level. Altera- year-old Neolithic stems of deciduous trees with a diameter of tions become obvious when logs or archaeological artifacts dry JT SVZL HWWYV_PTH[LS` VM [OLPY VYPNPUHS ^LPNO[ HUK out. Radial shrinking and weight loss are macroscopic signs of ZOYPURYHKPHSS`^OLUKLO`KYH[LK\W[V ;OLYLMVYLHSHYNL intensive decomposition, cell collapses are visible on a micro- WHY[VMO\THUJ\S[\YHSOLYP[HNL^V\SKILSVZ[^P[OV\[HY[PÄJPHS scopic level. Despite many deformation artifacts, parts such as preservation. Archaeologists preserve the form of large artifacts YVV[ZMYVTWSHU[ZHSVUNSHRLZOVYLZJHUILPKLU[PÄLK mainly with polyethylene glycol (PEG), e.g. the Swedish war- ship Vasa. Smaller wooden instruments are preserved by freeze Natural preservation occurs in the heartwood of logs of pines KY`PUN VY KPTL[O`S L[OLY [YLH[TLU[;OL MVYT HUK Z\WLYÄJPHS soaked in resin, and of oaks with high levels of phenols. The traces of human treatment of ancient wet wood can therefore central parts of stems resist decomposition at the air, but the be kept in museums. The cell wall structures of wet wood do sapwood decays. The situation is much worse in wood of lake survive preservation techniques. dwellings. Most wooden artifacts are of small dimensions,
Macroscopic changes on wet wood after dehydration
decayed
sapwood decayed
heartwood not decayed
resin- soaked
13.64 Late glacial stems of Pinus 13.65 Disc of an air-exposed Pinus 13.66 Disc of a water-saturated 13.67 Disc of the same Quercus sylvestris that have been exposed to sylvestris stem. The large central part archaeological Quercus sp. Its form stem, but dehydrated. Intensively air. The stump surfaces decay and is preserved due to natural impreg- is perfectly preserved. decayed parts shrink dramatically, as ZJPLU[PÄJPUMVYTH[PVUHIV\[[OLSPML nation with resin. The peripheral indicated by the original outline. of those trees is lost. zone without resin decays.
Microscopic changes on wet wood after dehydration collapsed lw ew of last ring
100 μm 100 μm 100 μm 100 μm
13.68 Not dehydrated, heavily 13.69 Dehydrated, heavily degraded 13.70 Dehydrated, heavily degraded 13.71 Dehydrated, heavily degraded degraded wood of late glacial Pinus wood of late glacial Pinus sylvestris. wood of late glacial Pinus sylvestris. wood of Pinus sylvestris. Latewood sylvestris. The cell form and primary Cells lost their original form due to The cell forms remain, but the sec- cells are collapsed, earlywood cells cell walls (red) remain. Secondary contraction, and degraded second- ondary walls are gone. of the outer ring kept their form. The walls lost their original structure ary cell walls appear as unstructured tree died during earlywood forma- HUKHYLKLSPNUPÄLKIS\L bodies. tion in early summer approx. 13,000 years ago.
190 Ch 13. Fossilization, permineralization, coalization, carbonization and wet wood conservation Microscopic changes on wet wood after dehydration Material: Swiss National Museum. roots collapsed vessels
500 μm 500 μm 250 μm
13.72 Modern wood of Fraxinus 13.73 Dehydrated, heavily degraded 13.74 Dehydrated, heavily degraded 13.75 Dehydrated, heavily degraded excelsior, stained with Safranin. All wood of Neolithic Fraxinus excelsior. wood of Neolithic Fraxinus excelsior. wooden artifacts, made from Fraxi- microscopic structures can be rec- Lateral contraction deformed the orig- It maintained its anatomical structure nus excelsior, maintained their origi- ognized. inal structure, but major anatomical despite some lateral deformation by nal form after conservation treat- characteristics for the species remain. roots from plants of the lake shore. ment by freeze drying. collapsed vessels parenchyma cell ÄILY
500 μm 500 μm 25 μm 50 μm
13.76 Modern wood of Quercus 13.77 Dehydrated, heavily degraded 13.78 Dehydrated, heavily degraded 13.79 Well preserved anatomical petraea, stained with Safranin. All wood of Neolithic Quercus sp. Lat- wood of Neolithic Quercus sp. Alter- structure of wood of Neolithic Quer- microscopic structures can be rec- eral contraction deformed the origi- ations due to dehydration are spe- cus sp. after conservation treatment ognized. nal structure, but major anatomical JPÄJ[VJLSS[`WLZ-PILYZHYLOLH]PS` by freeze drying and polyethylene. characteristics for Quercus remain compressed, parenchyma cells with Cell walls are decomposed. e.g. ring porosity, large rays, dark cell contents less so. heartwood. ơ primary and secondary walls secondary wall primary wall
13.80 Neolithic wooden bowl conserved with diethyl ether resin. 13.81 Cell walls without structure 13.82 Partially preserved cell wall after freeze drying. Electron-optical structure after diethyl ether treat- photograph. Reprinted from Bräker ment. Electron-optical photograph. et al. 1979. Reprinted from Bräker et al. 1979.
191 Open Access This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.
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192 Ch 13. Fossilization, permineralization, coalization, carbonization and wet wood conservation