geosciences

Article Non-Mineralized Fossil Wood

George E. Mustoe ID

Geology Department, Western Washington University, Bellingham WA 98225, USA; [email protected]; Tel.: +1-360-650-3582


Received: 25 May 2018; Accepted: 11 June 2018; Published: 18 June 2018


Abstract: Under conditions where buried wood is protected from microbial degradation and exposure to oxygen or harsh chemical environments, the tissues may remain unmineralized. If the original organic matter is present in relatively unaltered form, wood is considered to be mummified. Exposure to high temperatures, whether from wild fires or pyroclastic flows, may cause wood to be converted to charcoal. Coalification occurs when plant matter undergoes gradual metamorphosis, producing bituminous alteration products. Examples of all three types of non-mineralized wood are common in the geologic record. This report describes some of the most notable occurrences, reviews past research and introduces data from several localities in North America.

Keywords: fossil wood; carbonized; charcoal; coalified; mummified; paleobotany

1. Introduction From a deep time perspective, forests are a transient phenomenon, their existence depending on environmental factors that include elevation change, atmospheric physics, ocean currents patterns, and tectonic plate motion. These factors determine whether a region is an arid desert or a lush rainforest. Foresters describe forests that contain trees with ages of only a few hundred years as “old growth”. The oldest known age record for an individual tree is a Bristlecone Pine (Pinus longaeva) growing in White Mountains of California USA; a tree core taken in 2012 was observed to have 5067 annual rings [1]. The ages of clonal tree clusters, where individual trunks share a common root network, may be much greater, e.g. the 80,000 year age estimated for ~47,000 component trees of a Quaking Aspen (Populus tremuloides) grove in central Utah, USA [2]. The ages of these living trees are brief in comparison to fossil evidence of trees that comprised ancient forests. The infiltration and replacement of inorganic minerals produces petrified wood that has sufficient durability to be preserved for many millions of years. Recent reports [3,4] describe these mineralization processes in detail, and list earlier literature. Less well-known are fossil forests where ancient trees have been preserved as original tissue, or as organic material that has undergone structural alteration. This paper provides a detailed review of fossil wood occurrences where tissues have not been mineralized. These fossils are sometimes described as “carbonized wood”, but unmineralized ancient wood can be divided into three categories. Mummified wood consists of original tissues that may have undergone anatomical distortion or desiccation, but which are otherwise free of alteration. Charcoalified wood originates when wood is combusted in an anaerobic environment, causing much of the organic materials to be reduced to pure carbon. Coalified wood consists of tissues that have been altered by heat and pressure during deep burial, converting the original organic constituents to a mixture of pure carbon and various hydrocarbons.

2. Analytical Methods Scanning electron microscope (SEM) images were taken by the author using a Tescan Vega SEM at Western Washington University, Bellingham, WA USA.

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Scanning electron microscope (SEM) images were taken by the author using a Tescan Vega SEM 3. Mummifiedat Western WoodWashington University, Bellingham, WA USA. A common question asked of fossil wood researchers is “how long does it take for wood to become 3. Mummified Wood petrified?” There is no uniform answer, because fossilization processes are highly variable. In siliceous hot springs,A common wood may question become asked rapidly of fossil impregnated wood researchers with silicais “how [3 ,long5–7], does but init take other for environments, wood to become petrified?” There is no uniform answer, because fossilization processes are highly variable. buried wood may be preserved for tens of millions of years without experiencing mineralization. In siliceous hot springs, wood may become rapidly impregnated with silica [3,5–7], but in other The most important conditions required for mummification are an absence of microbial and chemical environments, buried wood may be preserved for tens of millions of years without experiencing degradation,mineralization. and burial The most conditions important where conditions mineral-bearing required for groundwater mummification is unable are an to absence penetrate of the tissues.microbial Mummified and chemical wood degradation, consists of originaland burial tissues conditions that havewhere undergone mineral-bearing minimal groundwater degradation is of cellularunable constituents. to penetrate This the preservationtissues. Mummified is a result wood ofconsists several of factors:original inhibitiontissues that ofhave wood-destroying undergone microbes,minimal decreased degradation oxygen of availability, cellular constituents. and the absence This preservation of harsh chemical is a result or physical of several conditions factors: (e.g., highinhibition alkalinity of or wood-destroying acidity, or elevated microbes, temperatures). decreased Environmentsoxygen availability, that favorand the mummification absence of harsh include deeplychemical submerged or physical wood, burial conditions in impermeable (e.g., high sediments, alkalinity or aridity, acidity, or low or temperatureselevated temperatures). [8–12]. Lignin and cellulose,Environments the primarythat favor constituents mummification of wood, include may deeply undergo submerged alteration wood, on burial a microscopic in impermeable level, even sediments, aridity, or low temperatures [8–12]. Lignin and cellulose, the primary constituents of though the overall appearance of the ancient wood may appear unchanged. Wood cell walls consist of wood, may undergo alteration on a microscopic level, even though the overall appearance of the an outer primary wall that encloses a multilayered secondary wall. The primary wall, which may be ancient wood may appear unchanged. Wood cell walls consist of an outer primary wall that encloses relativelya multilayered thick, consists secondary mostly wall. of The cellulose primary and wall, hemicellulose, which may be inrelatively contrast thick, to the consists thinner mostly secondary of wall thatcellulose is largely and hemicellulose, composed ofin acontrast mixture to the of cellulosethinner secondary and lignin. wall Thisthat is secondary largely composed wall commonly of a consistsmixture of three of cellulose discrete and layers lignin. (lamellae) This secondary that differ wall in commonly the orientation consists of oftheir three cellulosic discrete aggregates layers and the(lamellae) degree that of lignification. differ in the orientation Microbial of degradation their cellulosic of woodaggregates may and result the in degree destruction of lignification. of the wood sugarsMicrobial cellulose degradation and hemicellulose, of wood mayresulting result in in relative destruction enrichment of the ofwood lignin sugars [13, 14 cellulose]. Deterioration and of thehemicellulose, lignin-rich secondary resulting in wall relative may enrichment not greatly of disrupt lignin [13,14]. the overall Deterioration cellular of architecture the lignin-rich as long as thesecondary cellulosic wall outer may walls not greatly remain disrupt relatively the overall intact cellular (Figure architecture1). When as it long is preserved, as the cellulosic cellulose outer has walls remain relatively intact (Figure 1). When it is preserved, cellulose has scientific value because scientific value because carbon and oxygen isotope ratios in this substance can be used to estimate carbon and oxygen isotope ratios in this substance can be used to estimate paleoprecipitation and paleoprecipitationpaleotemperature and [15–18]. paleotemperature [15–18].

Figure 1. Anatomical preservation in mummified food from western Washington, USA. (A,B) Figure 1. Anatomical preservation in mummified food from western Washington, USA. Transverse views, late Pleistocene wood, Whidbey Formation, Swantown, Whidbey Island, Island ( A,B) Transverse views, late Pleistocene wood, Whidbey Formation, Swantown, Whidbey Island, Island County, Washington State, USA. (C) Transverse view, Wilkes Formation compressed wood. (D) Late Pleistocene wood showing extensive decay. Radial view. Net-like structures are relict ray cell walls, Whidbey Formation, Cama Beach, Camano Island, Island County, Washington, USA. Geosciences 2018, 8, x FOR PEER REVIEW 3 of 32

County, Washington State, USA. (C) Transverse view, Wilkes Formation compressed wood. (D) Late Pleistocene wood showing extensive decay. Radial view. Net-like structures are relict ray cell walls, Whidbey Formation, Cama Beach, Camano Island, Island County, Washington, USA. Geosciences 2018, 8, 223 3 of 30

Reported occurrences of mummified wood range in age from Paleocene to Holocene. This subfossilReported wood occurrencesis often locally of abundant, mummified particularly wood range in Quaternary in age from deposits, Paleocene where to Holocene. glacial, fluvial, This andsubfossil volcaniclastic wood is often lahar locally deposits abundant, provide particularly deep layers in Quaternary of fine sediment deposits, that where preserve glacial, organic fluvial, materials.and volcaniclastic The following lahar deposits section provide describes deep some layers of of finethe sediment best-known that localities, preserve organic but it materials. is not a comprehensiveThe following section list. describes some of the best-known localities, but it is not a comprehensive list.

3.1.3.1. Canadian Canadian Arctic Arctic Localities Localities SpectacularSpectacular examples examples of of mummified mummified wood wood occur occur in in the the Canadian Canadian Arctic, Arctic, ranging ranging in in age age from from PaleocenePaleocene to to Pliocene Pliocene (Figure (Figure 2).2). Although Although the the modern modern High High Arctic Arctic is a ispolar a polar desert desert typified typified by long by coldlong winters cold winters and short and cool short summers, cool summers, early Tertiary early Tertiary conditions conditions were very were different, very different, with a withwarm a ice-freewarm ice-free environment environment where whereplants plantsbenefited benefited from high from humidity high humidity and a andgrowing a growing season season that had that threehadthree months months of continuous of continuous light. Three light. months Three months of continuous of continuous darkness darkness during Arctic during Circle Arctic winters Circle favoredwinters favored conifers conifers rather rather than angiosperms.than angiosperms. These These fossil fossil forests forests are are typically typically dominated dominated by by MetasequoiaMetasequoia,, an an unusual unusual deciduous deciduous conifer conifer particularly particularly suited suited for for long long dark dark winters. winters. Many Many other other taxa taxa areare represented, represented, particularly in the Miocene and Pliocene occurrences [[19,20].19,20].

FigureFigure 2. 2. FossilFossil forests forests in inthe the Canadian Canadian Arctic. Arctic. (A) Axel (A) AxelHeiberg Heiberg Island Island (Eocene); (Eocene); (B) Banks (B) Island Banks (Miocene/Pliocene);Island (Miocene/Pliocene); (C) Cornwallis (C) Cornwallis Island Island (Miocene); (Miocene); (E) (E Ellesmere) Ellesmere Island Island (Paleocene/Eocene, Pliocene);Pliocene); ( (MM)) Meighen Meighen Island Island (Miocene). (Miocene).

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3.2.3.2. EllesmereEllesmere Island—LateIsland—Late Paleocene/earlyPaleocene/early Eocene,Eocene, PliocenePliocene FossilFossil woodwood waswas discovereddiscovered atat northernnorthern EllesmereEllesmere IslandIsland inin 18831883 atat aa sitesite laterlater describeddescribed asas “Brainard’s“Brainard’s PetrifiedPetrified Forest”,Forest”, namednamed afterafter aa SargentSargent D.L.D.L. Brainard,Brainard, aa survivorsurvivor ofof thethe ill-fatedill-fated GreelyGreely ExpeditionExpedition ofof 1881–18831881–1883 [[21].21]. InterestInterest waswas revivedrevived inin thethe 1980s1980s whenwhen GeologicalGeological SurveySurvey ofof CanadaCanada geologistgeologist NeilNeil McMillanMcMillan observedobserved fossilfossil woodwood atat EllesmereEllesmere Island.Island. TheThe mainmain fossilfossil forestforest areasareas atat StrathconaStrathcona andand StenkulStenkul FjordsFjords containcontain siderite-mineralizedsiderite-mineralized woodwood preservedpreserved inin LateLate Paleocene-EarlyPaleocene-Early EoceneEocene coal-bearingcoal-bearing sedimentssediments ofof thethe Iceberg Iceberg BayBay FormationFormation [22[22–24].–24]. PliocenePliocene mummifiedmummified woodwood occursoccurs atat thethe BeaverBeaver PondPond fossilfossil sitesite nearnear thethe headhead ofof StrathconaStrathcona Fjord,Fjord, wherewhere braidedbraided riverriver depositsdeposits ofof thethe BeaufortBeaufort FormationFormation preservepreserve woodywoody debrisdebris [[25].25]. ManyMany ofof thethe twigstwigs havehave teethteeth marksmarks leftleft byby ancestralancestral beaverbeaver (Figure(Figure3 ).3). PliocenePliocene woodwood alsoalso occurs occurs at at nearby nearby Meighen Meighen Island, Island, including including both both individualindividual logslogs andand finefine woodywoody debrisdebris [ [26].26].

Figure 3. Pliocene wood from Canadian Arctic sites. (A) Natalia Rybcynski with log, Fyles Leaf Bed, Figure 3. Pliocene wood from Canadian Arctic sites. (A) Natalia Rybcynski with log, Fyles Leaf Bed, ~3 km south of Beaver Pond locality. Ellesmere Island. (B) Beaver-gnawed twigs preserved in ~3 km south of Beaver Pond locality. Ellesmere Island. (B) Beaver-gnawed twigs preserved in Pliocene Pliocene sediments at Beaver Pond site. (C) Tree trunk in Beaufort Fm. Strata at Meighen Island. (D) sediments at Beaver Pond site. (C) Tree trunk in Beaufort Fm. Strata at Meighen Island. (D) Fine woody Fine woody debris. Meighen Island. Photos courtesy of Natalia Rybcynski. (E) William Hagopian debris. Meighen Island. Photos courtesy of Natalia Rybcynski. (E) William Hagopian with freshly excavatedwith freshly trunk excavated fragment, trunk Stenkul fragment, Fjord, Stenkul Ellesmere Fjord, Island Ellesmere 2005. PhotoIsland by 2005. Ann Photo Jefferson. by Ann Jefferson.

3.3. Axel Heiberg Island—Middle Eocene 3.3. Axel Heiberg Island—Middle Eocene The fossil forest on this island was discovered in 1985 by helicopter pilot Paul Tudge, who had The fossil forest on this island was discovered in 1985 by helicopter pilot Paul Tudge, who had been transporting Canadian Geological Survey scientists to Arctic locations for years. Tudge notified been transporting Canadian Geological Survey scientists to Arctic locations for years. Tudge notified University of Saskatchewan paleobotanist James Basinger, who organized the first expeditions to University of Saskatchewan paleobotanist James Basinger, who organized the first expeditions to visit visit the site. the site. Located in the Geodetic Hills, the fossil forest is preserved in sediments of the middle Eocene Located in the Geodetic Hills, the fossil forest is preserved in sediments of the middle Eocene Buchanan Lake Formation. The fossils are preserved in a basin where fluvial sedimentation included Buchanan Lake Formation. The fossils are preserved in a basin where fluvial sedimentation included episodic floods that killed trees, burying stumps in situ together with fallen logs and abundant forest episodic floods that killed trees, burying stumps in situ together with fallen logs and abundant floor leaf litter. The total depth of sediment exceeds 150 m, and preserves at least 33 separate forest floor leaf litter. The total depth of sediment exceeds 150 m, and preserves at least 33 separate fossil-bearing layers [27]. As noted later in this report, although mummified wood is a dominant fossil-bearing layers [27]. As noted later in this report, although mummified wood is a dominant component or the Axel Heiberg Island fossil forest, some specimens have been coalified. component or the Axel Heiberg Island fossil forest, some specimens have been coalified. Metasequoia (Dawn Redwood) wood is by far the most abundant fossil (Figure 4). Other conifers Metasequoia (Dawn Redwood) wood is by far the most abundant fossil (Figure4). Other conifers include fir, cypress, ginkgo, larch, redwood, spruce, pine and hemlock; Angiosperm trees include include fir, cypress, ginkgo, larch, redwood, spruce, pine and hemlock; Angiosperm trees include maple, alder, birch, hickory, chestnut, beech, ash, holly, walnut, sweetgum, sycamore, oak, willow, maple, alder, birch, hickory, chestnut, beech, ash, holly, walnut, sweetgum, sycamore, oak, willow, and elm. Herbaceous plants are represented by honeysuckle and sumac; other taxa include several and elm. Herbaceous plants are represented by honeysuckle and sumac; other taxa include several species of ferns and mosses [27]. This botanical diversity is evidence of a taxonomically diverse species of ferns and mosses [27]. This botanical diversity is evidence of a taxonomically diverse ecosystem, where plant communities flourished in a continually-changing floodplain environment. Despite the rigors of conducting research in a remote location with a harsh modern polar climate, the Axel Heiberg fossil forest has been the subject of enthusiastic scientific inquiry. Published research

Geosciences 2018, 8, 223 5 of 30 ecosystem, where plant communities flourished in a continually-changing floodplain environment. Despite the rigors of conducting research in a remote location with a harsh modern polar climate, Geosciences 2018, 8, x FOR PEER REVIEW 5 of 32 the Axel Heiberg fossil forest has been the subject of enthusiastic scientific inquiry. Published research includes general descriptions [[19,22,28–32],19,22,28–32], as well as more specialized reports. These include analyses of paleoclimate and and paleoecology paleoecology [15,18,33–40], [15,18,33–40], and and plant plant anatomy anatomy and and taxonomy taxonomy [14,41–43]. [14,41–43 ].

Figure 4. Axel Heiberg Island, Canadian Arctic. (A) Geodetic Hills site, 1999. (B) Metasequoia stumps Figure 4. Axel Heiberg Island, Canadian Arctic. (A) Geodetic Hills site, 1999. (B) Metasequoia stumps exposed by weathering. Photos courtesy of David Greenwood. (C) Metasequoia stump, 2000, photo by exposed by weathering. Photos courtesy of David Greenwood. (C) Metasequoia stump, 2000, photo William Hagopian. (D) Close view of Metasequoia wood, showing annual rings and desiccation by William Hagopian. (D) Close view of Metasequoia wood, showing annual rings and desiccation fractures. Photo from Kaelin et al. [44]. fractures. Photo from Kaelin et al. [44].

3.4. Banks Island—Middle Miocene-Pliocene 3.4. Banks Island—Middle Miocene-Pliocene At Banks Island, the Miocene Ballast Brook Formation consists of sediments deposited on an At Banks Island, the Miocene Ballast Brook Formation consists of sediments deposited on an ancient floodplain, where an upper peat layer preserves cones, foliage, twigs, and logs (Figure 5) ancient floodplain, where an upper peat layer preserves cones, foliage, twigs, and logs (Figure5)[ 45]. [45]. In situ stumps are exposed at the top of the peat player; logs buried in the peat lie adjacent to In situ stumps are exposed at the top of the peat player; logs buried in the peat lie adjacent to their their corresponding stumps [46]. Separated by an unconformity, the overlying Beaufort Formation is corresponding stumps [46]. Separated by an unconformity, the overlying Beaufort Formation is a a Pliocene braided river deposit where logs and woody debris are preserved as transported material Pliocene braided river deposit where logs and woody debris are preserved as transported material (Williams 2006. Williams et al. 2008). Paleobotanical studies date to the 1960s, when University of (Williams 2006. Williams et al. 2008). Paleobotanical studies date to the 1960s, when University of Alberta professor Len Hills began studying Banks Island plant fossils [47–50]. Alberta professor Len Hills began studying Banks Island plant fossils [47–50].

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Figure 5.5. Miocene Beaufort Formation at Ballast Brook, Banks Island,Island, CanadianCanadian Arctic.Arctic. (A) GeneralGeneral site view. view. (B (B) )Tree Tree trunk trunk with with intact intact root root crown. crown. (C) Excavated (C) Excavated trunk trunk showing showing branching. branching. 2005 photos 2005 photoscourtesy courtesy of Christopher of Christopher J. Williams. J. Williams. (D,E) Partially (D,E) Partially exposed exposedlogs, 2010, logs, photos 2010, courtesy photos courtesyof William of WilliamHagopian. Hagopian.

3.5. Cornwallis Island—Miocene 3.5. Cornwallis Island—Miocene A single sample of mummified collected from a roadcut at Resolute has been described as A single sample of mummified collected from a roadcut at Resolute has been described as having having elevated levels of Ca, Fe, and S relative to modern wood, but scanning and transmission elevated levels of Ca, Fe, and S relative to modern wood, but scanning and transmission electron electron microscopy show no evidence of mineralization [14]. microscopy show no evidence of mineralization [14].

3.6. Northwestern Canada Late Paleocene/early Eocene Eocene woods woods have have been been preserved preserved in volcaniclastic material filling filling the upper zoneszones ofof kimberlitekimberlite pipes pipes in in the the Lac Lac de de Gras Gras region region of Slave of Slave Province, Province, Canada, Canada, the site the of site many of openmany pitopen diamond pit diamond mines mines (Figure (Figure6). The 6). wood The iswood inferred is inferred to represent to represent a tree froma tree the from boreal the forestboreal surroundingforest surrounding the eruption the eruption zone, where zone, a trunk where collapsed a trunk into collapsed the diatreme, into becoming the diatreme, preserved becoming in the sterilepreserved environment in the sterile of theenvironment volcaniclastic of the kimberlite. volcaniclastic Other kimberlite. mummified Other woods mummified have been woods recovered have 13 18 2 frombeen recovered another diamond from another mine indiamond the same mine region in the [51 ].same Cellulose regionδ [51].13C, δCellulose18O and δδ2 HC, values δ O and indicate δ H thevalues Paleogene indicate climatethe Paleogene of the Canadianclimate of subarcticthe Canadian was subarctic 12–17 ◦C was warmer 12–17 and °C fourwarmer times and wetter four times than presentwetter than [52,53 present]. [52,53].

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Figure 6. Paleocene/Eocene mummified wood from kimberklite pipes, Subarctic Canada. (A,B) Figure 6. Paleocene/Eocene mummified wood from kimberklite pipes, Subarctic Canada. Metasequoia heartwood showing tyloses (arrow), Panda Pipe Ekati Diamond Mine. Specimen (A,B) Metasequoiaprovided by heartwoodAlex Wolfe. (C showing) Piceaoxylon tyloses from Diavik (arrow), Diamond Panda Mine. Pipe Photo Ekati by Diamond Benjamin Hook Mine. [52]. Specimen provided byFigure Alex 6. Wolfe. Paleocene/Eocene (C) Piceaoxylon mummifiedfrom wood Diavik from Diamond kimberklite Mine. pipes, Photo Subarctic by Canada. Benjamin (A,B Hook) [52]. 3.7. UnitedMetasequoia States ofheartwood America showing tyloses (arrow), Panda Pipe Ekati Diamond Mine. Specimen 3.7. United StatesMummifiedprovided of America by Alex wood Wolfe. occurs (C) Piceaoxylon in many from locations, Diavik Diamond particularly Mine. Photo in by Pleistocene Benjamin Hook and [52]. Holocene sediments. Examples where intact logs are preserved include the Farmdale Geosol in Illinois [54] Mummified3.7. United wood States of occurs America in many locations, particularly in Pleistocene and Holocene sediments. and Mt. Pleasant Bluff, Louisiana [55]. This report focuses on three locations that offer notable Examplesexamples whereMummified of intact wood logs mummification wood are occurs preserved in that many includerange locations, in age the from Farmdale particularly late Miocene Geosol in Pleistoceneto inlate Illinois Pleistocene and [54 Holocene] and(Figure Mt. 7). Pleasant Bluff, Louisianasediments. [55 Examples]. This where report intact focuses logs are on preserved three locations include the that Farmdale offer Geosol notable in Illinois examples [54] of wood mummificationand Mt. that Pleasant range Bluff, in age Louisiana from [55].late MioceneThis reportto focuses late Pleistocene on three locations (Figure that7 ). offer notable examples of wood mummification that range in age from late Miocene to late Pleistocene (Figure 7).

Figure 7. Mummified wood localities in USA. (1) Wilkes Formation, Washington, late Miocene.

(2) Northern Puget Sound, Washington, late Pleistocene. (3) Two Creeks Forest, Wisconsin, late Pleistocene. (4) Farmdale Geosol, Illinois, late Pleistocene. (5) Mt. Pleasant Bluff, Louisiana, late Pleistocene. Geosciences 2018, 8, x FOR PEER REVIEW 8 of 32

Figure 7. Mummified wood localities in USA. (1) Wilkes Formation, Washington, late Miocene. (2) Northern Puget Sound, Washington, late Pleistocene. (3) Two Creeks Forest, Wisconsin, late Pleistocene. (4) Farmdale Geosol, Illinois, late Pleistocene. (5) Mt. Pleasant Bluff, Louisiana, late Pleistocene.

3.8. Wilkes Formation, Washington, USA—Late Miocene The Wilkes formation consists of a stacked series of volcanic mudflow (lahar) sediments that episodically inundated a lowland forest, alternating with lacustrine strata that were deposited Geosciencesduring quiescent2018, 8, 223 intervals. Abundant mummified wood is evidence of paleoenvironment: upright8 of 30 stems where mudflows buried living vegetation (Figure 8A), and layers of woody peat that 3.8.represents Wilkes Formation,organic debris Washington, transported USA—Late by the Miocene mudflow (Figure 8B). Radiometric dating of a tephra interbed gave a Late Miocene age of 6.13 ± 0.08 Ma [56]. Taxonomy of the wood has not been attempted,The Wilkes but fossil formation pollen consists provides of evidence a stacked of series a swamp of volcanic forest dominated mudflow (lahar) by Taxodium sediments (Swamp that episodicallyCypress) and inundated Nyssa (Tupelo), a lowland with forest, diverse alternating angiosperms with as lacustrine minor constituents strata that were [57]. deposited during quiescentPreservation intervals. quality Abundant of the mummified wood is variable,wood is evidence typically of consisting paleoenvironment: of wood thatupright has stems been wherecompressed mudflows from buried burial living pressure, vegetation and (Figuredeformed8A), by and desiccation. layers of woody Exposure peat that to weathering represents organic causes debrismany transportedmummified by specimens the mudflow to be (Figure shattered8B). Radiometric (Figure 9A,B), dating but of intact a tephra specimens interbed can gave be a found Late ± Miocene(Figure 9C). age of Brown 6.13 mummified0.08 Ma [56 ]. wood Taxonomy is abundant of the wood (Figure has 9D), not been as well attempted, as wood but that fossil has pollen been providespartially evidencecoalified (Figure of a swamp 9E,F). forest Even dominated in the freshest-looking by Taxodium (Swamp specimens, Cypress) tissues and commonlyNyssa (Tupelo), show withstructural diverse damage angiosperms (Figure as1C). minor constituents [57].

Figure 8. Miocene mummified wood in Wilkes Fm. Strata at Salmon Creek, Lewis County, Figure 8. Miocene mummified wood in Wilkes Fm. Strata at Salmon Creek, Lewis County, Washington, Washington, USA. (A) Upright stems buried by volcanic mudflow. (B) Transverse view of in situ USA. (A) Upright stems buried by volcanic mudflow. (B) Transverse view of in situ stump, showing stump, showing compression of original circular shape and deformation of annual rings. (C) Three compression of original circular shape and deformation of annual rings. (C) Three thin wood mat layers thin wood mat layers separated by thin mudflow sediments. (D) Close view of the lowest wood mat, separated by thin mudflow sediments. (D) Close view of the lowest wood mat, showing flattening of individualshowing flattening stems. of individual stems.

Preservation quality of the wood is variable, typically consisting of wood that has been compressed from burial pressure, and deformed by desiccation. Exposure to weathering causes many mummified specimens to be shattered (Figure9A,B), but intact specimens can be found (Figure9C). Brown mummified wood is abundant (Figure9D), as well as wood that has been partially coalified (Figure9E,F). Even in the freshest-looking specimens, tissues commonly show structural damage (Figure1C). Geosciences 2018, 8, 223 9 of 30

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Figure 9. Late Miocene wood from Wilkes Formation at Salmon Creek, Washington USA. (A,B) Figure 9. A B LateHighly Miocene fractured wood wood from in clay Wilkes bed hat Formation has been exposed at Salmon to surface Creek, weathering. Washington (C,D) Mummified USA. ( , ) Highly fractured woodwood in typically clay bedshows hat compaction has been and exposeddistortion from to surfaceburial pressures. weathering. (E,F) Some( smallC,D )specimens Mummified wood typically showsshow compaction incipient coalification. and distortion from burial pressures. (E,F) Some small specimens show

incipient3.9. coalification. Two Creek Forest, Wisconsin USA—Late Pleistocene The Two Creeks forest bed is perhaps the most famous Quaternary deposit in the Great Lakes 3.9. Two Creekregion Forest, in north-central Wisconsin USA USA—Late [58]. The widespread Pleistocene distribution of subfossil wood was first mapped in 1970 [59], though the Two Creeks fossil forest had been described a century earlier [60], with later The Twostudies Creeks published forest in bedthe 1930s is perhaps [61,62]. Radiometric the most ages famous for the Two Quaternary Creeks forestdeposit sediments inaverage the Great Lakes region in north-central13,500 14C years USA BP (Before [58]. Present), The widespread equivalent to distributiona calendar age of of 11,850 subfossil years BP wood [57,63], was a time first mapped in 1970 [59], thoughwhen spruce the forest Two covered Creeks much fossil of the forest Lake Michigan had been region described following athe century recession earlier of the great [60 ], with later continental ice sheet. Tree-ring analyses show a duration age of the forest to have been 252 years [64] studies publishedto 310 years in the [65]. 1930s The wood [61 ,is62 extremely]. Radiometric well preserved, ages typically for the showing Two Creeks shrinkage forest cracks, sediments but few average 13,500 14C yearsother BP structural (Before defects Present), (Figure equivalent 10). Detailed to a descriptions calendar age of theof 11,850 geologic years setting BP and [57 ,63], a time when sprucepaleoenvironment forest covered can much be found of in the references Lake Michigan[59,66–69]. region following the recession of the great continental ice sheet. Tree-ring analyses show a duration age of the forest to have been 252 years [64] to 310 years [65]. The wood is extremely well preserved, typically showing shrinkage cracks, but few other structural defects (Figure 10). Detailed descriptions of the geologic setting and paleoenvironment can be foundGeosciences in references 2018, 8, x FOR [59 PEER,66 REVIEW–69]. 10 of 32

Figure 10. Late Pleistocene Spruce log from Two Creeks Forest, east-central Wisconsin, USA. Figure 10. Late Pleistocene Spruce log from Two Creeks Forest, east-central Wisconsin, USA. 3.10. Puget Sound, Washington—Late Pleistocene Mummified wood is abundant in Late Pleistocene interglacial deposits in the northern Puget Sound lowlands, northwest Washington, USA, where tree trunks, limbs, and small wood fragments occur in the Whidbey Formation. The region experienced multiple advances and retreats of the cordilleran ice sheet, but evidence of earlier events has largely been obscured by erosion and deposition related to the last glacial episode, the Fraser Glaciation that occurred between ~25,000–10,500 years ago. An exception occurs at Whidbey Island, where Fraser glacial deposits are underlain by prominent exposures of the much older Whidbey Formation. These sediments have ages that exceed the maximum age that can be determined by 14C (>40,000 years); thermoluminescence dates suggest an age of 150,000–100,000 years BP [70]. The Whidbey Formation has an exposed thickness of ~60 m at the type section; other localities expose up to 90 m of strata. In general, the strata originated as fluvial and lacustrine sediments deposited by meandering streams flowing along a coastal floodplain. Wood samples used in this study were collected from coastal exposures at Swantown, Lagoon Point, and Double Bluffs on Whidbey Island, and Cama Beach on nearby Camano Island (Figure 11).

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3.10. Puget Sound, Washington—Late Pleistocene Mummified wood is abundant in Late Pleistocene interglacial deposits in the northern Puget Sound lowlands, northwest Washington, USA, where tree trunks, limbs, and small wood fragments occur in the Whidbey Formation. The region experienced multiple advances and retreats of the cordilleran ice sheet, but evidence of earlier events has largely been obscured by erosion and deposition related to the last glacial episode, the Fraser Glaciation that occurred between ~25,000–10,500 years ago. An exception occurs at Whidbey Island, where Fraser glacial deposits are underlain by prominent exposures of the much older Whidbey Formation. These sediments have ages that exceed the maximum age that can be determined by 14C (>40,000 years); thermoluminescence dates suggest an age of 150,000–100,000 years BP [70]. The Whidbey Formation has an exposed thickness of ~60 m at the type section; other localities expose up to 90 m of strata. In general, the strata originated as fluvial and lacustrine sediments deposited by meandering streams flowing along a coastal floodplain. Wood samples used in this study were collected from coastal exposures at Swantown, Lagoon Point, and Double Bluffs on Whidbey Island, and Cama Beach on nearby Camano Island (Figure 11). Geosciences 2018, 8, x FOR PEER REVIEW 11 of 32

Figure 11. Late Pleistocene Whidbey Formation mummified wood localities, Whidbey and Camano Figure 11.Islands,Late Washington Pleistocene USA. Whidbey (1) Swantown, Formation (2) Lagoon mummified Point, (3) Double wood localities,Bluffs, (4) Cama Whidbey Beach. and Camano Islands, Washington USA. (1) Swantown, (2) Lagoon Point, (3) Double Bluffs, (4) Cama Beach. Wood occurs in two sedimentary environments. Individual logs are preserved in thick sandy beds, where they have been transported by fluvial processes. (Figure 12) Wood also occurs in beds of Wood occurs in two sedimentary environments. Individual logs are preserved in thick sandy woody peat, ranging in form from abundant flattened stems and small twigs to large branches and beds, whererare tree they trunks have that been accumulated transported in local by fluvialbogs (Figure processes. 13). Peat (Figure beds occur 12) at Wood multiple also levels occurs in the in beds of woodyWhidbey peat, ranging Formation, in form interspersed from abundant with sandy flattened fluvial deposits stems (Figure and small 14). twigs to large branches and rare tree trunks that accumulated in local bogs (Figure 13). Peat beds occur at multiple levels in the Whidbey Formation, interspersed with sandy fluvial deposits (Figure 14).

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Figure 12. Logs in Whidbey Formation sand beds, Swantown beach, Whidbey Island, Washington Figure 12.USA.Logs (A) View in Whidbey of Whidbey Formation Formation sandstrata. beds,(B–D) Logs Swantown exposed beach,by weathering. Whidbey Island, Washington USA. (A) View of Whidbey Formation strata. (B–D) Logs exposed by weathering. Geosciences 2018, 8, x FOR PEER REVIEW 13 of 32 The Whidbey Formation wood is well-known to local geoscientists, but this material has received little scientific study. Poor anatomical preservation hinders taxonomic identification of the wood, but the forest that produced it is known from pollen obtained from Whidbey Formation peat beds at the Double Bluffs type locality (Figure 14) [71,72]. Pinus contorta (Lodgepole Pine), Tsuga heterophylla (Western Hemlock), and Pseuodtsuga menziesii (Douglas Fir) are abundant elements in the palynoflora. Pollen analysis from the main wood-bearing peat bed in the Whidbey Formation type section contains the above-mentioned conifers as the most abundant taxa, but small amounts of Juniperus (Juniper), Abies (Balsam Fir), and Betula (Birch) are present. A thin peat bed ~20 m higher in the stratigraphic section is dominated by Alnus (Alder) [72]. The pollen flora indicates that during the Whidbey Formation interglacial period the region was inhabited by boreal conifer forest that closely resembles the modern flora. Preservation quality is variable, largely affected by compression from the weight of overlying sediments. Small twigs and stems in woody peat layers are commonly highly flattened, but larger limbs and trunks are more likely to retain three-dimensional shapes. Regardless of shape, it is rare for specimens to preserve uncompressed cells (Figures 1A and 15A-D).

Figure 13. Logs in Whidbey Formation peat, Double Bluff, Whidbey Island, Washington USA. (A) Figure 13.PleistoceneLogs in strata Whidbey at Double Formation Bluffs. Large peat,wood fragments Double occur Bluff, in Whidbey2 m-thick peat Island, beds (arrow). Washington (B) USA. (A) PleistoceneBlocks of strata peat that at Doublehave fallen Bluffs. to lie at Large the base wood of the fragmentsbluff. (C,D) Wood occur in infallen 2 m-thick peat blocks. peat beds (arrow). (B) Blocks of peat that have fallen to lie at the base of the bluff. (C,D) Wood in fallen peat blocks.

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The Whidbey Formation wood is well-known to local geoscientists, but this material has received little scientific study. Poor anatomical preservation hinders taxonomic identification of the wood, but the forest that produced it is known from pollen obtained from Whidbey Formation peat beds at the Double Bluffs type locality (Figure 14)[71,72]. Pinus contorta (Lodgepole Pine), Tsuga heterophylla (Western Hemlock), and Pseuodtsuga menziesii (Douglas Fir) are abundant elements in the palynoflora. Pollen analysis from the main wood-bearing peat bed in the Whidbey Formation type section contains the above-mentioned conifers as the most abundant taxa, but small amounts of Juniperus (Juniper), Abies (Balsam Fir), and Betula (Birch) are present. A thin peat bed ~20 m higher in the stratigraphic section is dominated by Alnus (Alder) [72]. The pollen flora indicates that during the Whidbey Formation interglacial period the region was inhabited by boreal conifer forest that closely resembles theGeosciences modern 2018 flora., 8, x FOR PEER REVIEW 14 of 32

FigureFigure 14. 14.Stratigraphic Stratigraphic sectionsection ofof late late Pleistocene Pleistocene Whidbey Whidbey Formation Formation of of interglacial interglacial sediment sediment at at DoubleDouble Bluff, Bluff, Whidbey Whidbey Island, Island, Washington Washington USA. USA. Adapted Adapted from from [73 [73].].

Preservation quality is variable, largely affected by compression from the weight of overlying sediments. Small twigs and stems in woody peat layers are commonly highly flattened, but larger limbs and trunks are more likely to retain three-dimensional shapes. Regardless of shape, it is rare for specimens to preserve uncompressed cells (Figures1A and 15A–D).

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Whidbey Formation wood is not visibly mineralized, the only exception being a few specimens 2+ 2+ · Geosciencesthat have 2018 thin, 8 coatings, x FOR PEER of REVIEW vivianite (Fe Fe 2(PO4)2 8H2O), (Figure 16). 15 of 32

Figure 15. Scanning electron electron microscopy microscopy (SEM) (SEM) photos photos of of mummified mummified conifer wood from the late Pleistocene Whidbey Whidbey Formation, Formation, Whidbey Whidbey Island, Island, Washington Washington USA. USA. (A) Wood (A) Wood from peat from bed peat at Double bed at DoubleBluff. Radial Bluff. Radial view of view tracheids, of tracheids, showing showing circular circular pits (arrow).pits (arrow). (B) Wood(B) Wood from from log log in sandin sand bed bed at atSwantown Swantown beach. beach. Cell Cell walls walls are composed are composed of relict of relict organic organic matter, matter, but anatomical but anatomical details details are mostly are mostlyobliterated. obliterated. (C) Pits ( inC) ray Pits cells in of ray wood cells from of wood Double from Bluff Double peat, showing Bluff peat, excellent showing preservation. excellent preservation.(D) Radial view (D of) woodRadial from view Swantown of wood log,from showing Swantown two pitslog, on showing a cell that two has pits altered on a surface cell that texture. has altered(E) Transverse surface viewtexture. of ( woodE) Transverse from a limb view buried of wood in from peat beda limb at Doubleburied in Bluff. peat Cellsbed at walls Double preserve Bluff. Cellsoriginal walls multilayered preserve architecture, original multilayered but cells are architecture, highly compressed. but cells (F ) are Transverse highly view compressed. of the same (F) Transversewood at higher view magnification, of the same wood showing at higher the sigmoidal magnification, shape showing of a single the compressed sigmoidal tracheid.shape of a single compressed tracheid.

Whidbey Formation wood is not visibly mineralized, the only exception being a few specimens that have thin coatings of vivianite (Fe2+Fe2+2(PO4)2·8H2O), (Figure 16).

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Figure 16. LateLate Pleistocene Pleistocene wood wood from from northwest northwest Washington, Washington, USA, USA, containing coatings of blue vivianite (arrows).

3.11. European European Locations Locations MummifiedMummified wood occursoccurs atat manymany locationslocations in in throughout throughout the the world, world, including including numerous numerous sites sites in inEurope. Europe. Examples Examples that that have have been been reported reported in in scientific scientific literatureliterature includeinclude localities in Belgium Belgium [74], [74], England [75], [75], Austria Austria [76], [76], and and Siberia Siberia [77,78]. [77,78]. Other Other mummified mummified wood wood sites sites are are located located in in Germany, Germany, Poland, Czech Republic, and The Netherlands. The The following following section section describes describes some some of of the most spectacularspectacular occurrences occurrences at at localities localities where where efforts efforts have have been been made made to display to display evidence evidence of fossil of forests fossil forestsfor public for viewing.public viewing.

3.12. Ipolytarnóc Ipolytarnóc Fossil Fossil Forest, Forest, Hungary—Late Miocene InIn July 2007, miners working working at at an an open-pit coal coal mine mine at at Bükkábrány, Bükkábrány, Hungary discovered 16 upright tree tree trunks trunks in in the the sand sand layer layer just justabove above the lignite the lignite coal seam coal (Figure seam 17 (FigureA). A team 17A). of A geologists team of geologistsand paleontologists and paleontologists was quickly was organized quickly organized to study thisto study spectacular this spectacular occurrence, occurrence, the only the location only locationworldwide worldwide where large where trees large are preservedtrees are preserved intending intending in their original in their forest original setting forest [79 setting,80]. The [79,80]. fossil Theforest fossil is believed forest is tobelieved have originated to have originated when rising when lake rising waters lake drowned waters drowned the forest the during forest theduring late theMiocene, late Miocene, with the with trunks the protected trunks protected from decay from by decay their deep by their burial deep in water-saturated burial in water-saturated sediment. sediment.The trees have The been trees identified have been as Glyptostroboxylon identified as Glyptostroboxylon rudolphii (Figure 17 rudolphiiB), a taxon (Figure previously 17B), described a taxon previouslyfrom a site described in Germany from [81 a,82 site]. Inin 2010,Germany a new [81,82]. visitor In center 2010, a was new opened visitor atcenter the Ipolytarnwas openedóc Fossils at the IpolytarnócNature Reserve, Fossils where Nature five Reserve, large stumps where from five Bükklargeá stumpsbrány are from displayed Bükkábrány (Figure are 17 displayedC,D). (Figure 17C,D).The fossil trees have commonly been described as being mummified, with the wood soft enough to cutThe with fossil a razor trees blade have after commonly specimens been were described soaked in as boiling being water mummified, [80]. Microscopic with the examination wood soft enoughand chemical to cut analyses with a confirmrazor blade that theafter wood specimens of the buriedwere soaked trees is relativelyin boiling pristine; water [80]. levels Microscopic of cellulose examinationand phenolic and compounds chemical inanalyses wood within confirm the that underlying the wood lignite of the are buried significantly trees is relatively lower, evidence pristine; of levelsincipient of cellulose gelification and [83 phenolic]. compounds in wood within the underlying lignite are significantly lower, evidence of incipient gelification [83].

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Figure 17. Fossil forest at Bükkábrány, northern Hungary. (A) Miocene stumps excavated in 2007 Figure 17. Fossil forest at Bükkábrány, northern Hungary. (A) Miocene stumps excavated in 2007 open open pit lignite mine. (B) Glyptostroboxylon rudolphii wood. Photo by Zsigmund Atilla. (C) Large pit lignite mine. (B) Glyptostroboxylon rudolphii wood. Photo by Zsigmund Atilla. (C) Large stumps that stumps that have been relocated for outdoors display at Ipolytarnóc Fossils Nature Center, opened in have been relocated for outdoors display at Ipolytarnóc Fossils Nature Center, opened in 2010. Photo 2010. Photo courtesy of Tamás Elter. (D). Interior of Nature Center. Photo (D) courtesy of Sándor courtesy of Tamás Elter. (D). Interior of Nature Center. Photo (D) courtesy of Sándor Kovács. Kovács. 3.13. Dunarobba Fossil Forest, Italy—Pliocene 3.13. Dunarobba Fossil Forest, Italy—Pliocene Discovered during clay mining in 1980, Dunarobba Fossil Forest is preserved in lacustrine Discovered during clay mining in 1980, Dunarobba Fossil Forest is preserved in lacustrine sediments of the Tiberino Basin, a Plio-Plesitocene intermontane basin along the western margin of sediments of the Tiberino Basin, a Plio-Plesitocene intermontane basin along the western margin of the Apenine Mountains in central Italy [84]. More than 50 upright trunks up to 2.5 m in diameter and the Apenine Mountains in central Italy [84]. More than 50 upright trunks up to 2.5 m in diameter and 9 m in height are preserved in clay and fine sand sediment, representing a Pliocene conifer forest that 9 m in height are preserved in clay and fine sand sediment, representing a Pliocene conifer forest flourished along a marshy lake shore ~3–2 Ma (Figure 18). Originally named as a member of the form that flourished along a marshy lake shore ~3–2 Ma (Figure 18). Originally named as a member of the genus Taxodioxylon [83], the dominant species is now considered to be Glyptostrobus europaeus based on form genus Taxodioxylon [83], the dominant species is now considered to be Glyptostrobus europaeus the “whole plant” association of foliage, cones, and wood [85]. The wood is commonly described as based on the “whole plant” association of foliage, cones, and wood [85]. The wood is commonly mummified, resulting from the protection against degradation provided by the impermeable clay-rich described as mummified, resulting from the protection against degradation provided by the matrix, which restricted influx of oxygenated groundwater. However, the composition of Dunarobba impermeable clay-rich matrix, which restricted influx of oxygenated groundwater. However, the wood has complexities. Chemical analyses reveal that leaching has removed most polyoses and composition of Dunarobba wood has complexities. Chemical analyses reveal that leaching has cellulose, leaving lignin as the major structural component [86,87]. Some specimens are partially removed most polyoses and cellulose, leaving lignin as the major structural component [86,87]. mineralized with calcite, chlorapatite, chloromagnesite, and clays [88]. Permineralized wood collected Some specimens are partially mineralized with calcite, chlorapatite, chloromagnesite, and clays [88]. from clays above the main fossil forest layer are mineralized by a combination of siderite, goethite, and Permineralized wood collected from clays above the main fossil forest layer are mineralized by a ferroan calcite [89,90]. Mineral composition may vary within a single specimen, evidence that mineral combination of siderite, goethite, and ferroan calcite [89,90]. Mineral composition may vary within a precipitation was strongly influenced by localized changes in Eh and pH. single specimen, evidence that mineral precipitation was strongly influenced by localized changes in Eh and pH. Conservation efforts in the 1990s led to the establishment of Dunrobba Botanic Palaeontology Centre, which includes both a small exhibition hall and a path that directs visitors to outdoor displays where upright stumps are protected by individual A-frame shelters. Preservation efforts continue in an ongoing attempt to minimize degradation caused by microbial decay and atmospheric exposure.

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Figure 18. Dunarobba Fossil Forest, Umbria region of central Italy. (A) Fossil forest, circa 1995, prior Figure 18. Dunarobba Fossil Forest, Umbria region of central Italy. (A) Fossil forest, circa 1995, prior to to conservation. Wikipedia photo. (B) One of many outdoor stumps protected by shelters at conservation. Wikipedia photo. (B) One of many outdoor stumps protected by shelters at Dunarobba Dunarobba Fossil Forest. Copyrighted photo from www.alamy.com, used with permission. (C) View Fossil Forest. Copyrighted photo from www.alamy.com, used with permission. (C) View of stump withinof stump its shelter. within ( itsD) Close-upshelter. (D view) Close-up of mummified view of wood mummified showing wood shrinkage showing cracks. shrinkage Photos (C,D cracks.) by FrancoPhotos Bianchi,(C,D) by courtesyFranco Bianchi, of Marta courtesy Pinzaglia. of Marta Pinzaglia.

3.14. Fossano Fossil Forest, Northern Italy—Pliocene Conservation efforts in the 1990s led to the establishment of Dunrobba Botanic Palaeontology Centre,A major which Pliocene includes fossil both aforest small occurs exhibition in northwestern hall and a path Italy, that where directs upright visitors stumps to outdoor are displaysexposed wherealong the upright banks stumps of the areStura protected di Dimonte by individual River near A-frame the small shelters. town of Preservation Fossano. The efforts site is continue known as in antheongoing Fossano attempt Fossil Forest.to minimize The sitedegradation is in proximity caused by to microbial the alps; decay the river and has atmospheric incised Quaternary exposure. sediments to expose Pliocene continental sediments (Figure 19). Two successive forests are 3.14.represented Fossano inFossil separate Forest, stratigraphic Northern Italy—Pliocene layers [91–93]. As in other Pliocene swamp forests in Italy, GlyptostrobusA major Plioceneeuropaeus fossilis the forest dominant occurs tree in northwesterntype. The scientific Italy, wheresignificance upright of stumpsthis newly-exposed are exposed alongfossil forest the banks is sure ofthe to result Stura diin Dimonteadditional River descriptions near the smalland interpretations town of Fossano. in future The site years. is known as the Fossano Fossil Forest. The site is in proximity to the alps; the river has incised Quaternary sediments to expose Pliocene continental sediments (Figure 19). Two successive forests are represented in separate stratigraphic layers [91–93]. As in other Pliocene swamp forests in Italy, Glyptostrobus europaeus is the dominant tree type. The scientific significance of this newly-exposed fossil forest is sure to result in additional descriptions and interpretations in future years.

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Figure 19. Fossano Fossil Forest, Italy. (A,B) Upright stumps of Glyptostrobus europaeus exposed in Figure 19. Fossano Fossil Forest, Italy. (A,B) Upright stumps of Glyptostrobus europaeus exposed in Pliocene sediment that forms the bed of the Stura di Dimonte River, NW Italy. (C) Wood exposed to Pliocene sediment that forms the bed of the Stura di Dimonte River, NW Italy. (C) Wood exposed to surface conditions. Photos courtesy of Edoardo Martinetto. surface conditions. Photos courtesy of Edoardo Martinetto. 3.15. Stura di Lanzo Fossil Forest, Northwest Italy—Pliocene 3.15. Stura di Lanzo Fossil Forest, Northwest Italy—Pliocene Like the Fossano Fossil Forest, the Stura de Lanzo locality is a site where mummified Pliocene Liketree trunks the Fossano have been Fossil exposed Forest, by the river Stura erosion de Lanzo[94–96]. locality The muddy is a site sediments where mummifiedthat enclose large Pliocene tree trunksGlyptostrobus have stumps been exposed also preserve by river seeds erosion and cones, [94 –allowing96]. The “whole muddy plant” sediments identification that enclose[85]. Thelarge Stura di Lanzo Fossil Forest has been described in detail in a 2005 booklet [97] published in Italian by Glyptostrobus stumps also preserve seeds and cones, allowing “whole plant” identification [85]. Parco la Mandria (Mandria Provincial Park). Conservation efforts at the site have included The Stura di Lanzo Fossil Forest has been described in detail in a 2005 booklet [97] published in participation of teachers and young students [98,99]. Italian by Parco la Mandria (Mandria Provincial Park). Conservation efforts at the site have included participation3.16. Other of Locations teachers and young students [98,99].

3.16. OtherIn LocationsEast Asia, Oligocene mummified wood occurs in the Nanning Basin of southern China. The deposit includes fossilized mollusks, fish, reptiles, and mammals as well as plants. Wood occurs as Instumps East and Asia, trunks Oligocene in a lagerstätte mummified that also wood preserves occurs foliage in theand Nanningseeds [100]. Basin Pliocene of southernmummified China. The depositwood has includes been described fossilized from mollusks, central Japan fish, [101]. reptiles, and mammals as well as plants. Wood occurs as stumps and trunks in a lagerstätte that also preserves foliage and seeds [100]. Pliocene mummified wood4. has Charcoalified been described Woodfrom central Japan [101].

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Fire has been present throughout the Earth’s history, so the abundance of charred wood in the geologic record is not surprising. Production of charcoal has two primary causes: natural wildfires (Figure 20), and ignition of wood by volcanic flows or hot volcanic ash (ignimbrite) (Figure 21). Charcoal in the fossil record ranges from intact logs to tiny fragments dispersed in clastic sediments and coal. The latter form is commonly called fusain. Regardless of size, ancient charcoal may yield important information about Earth history. Much of our knowledge of ancient charcoal comes from the work of University of London professor Andrew C. Scott. His research is described in a recent book [96], and in many journal publications [102–106]. Several papers provide general overviews of fossil charcoal research [104-106]. Because charred wood typically has excellent anatomical preservation, intact charcoal specimens are well suited for taxonomic identifications. For relatively young deposits, charcoal is in an ideal material for 14C radiometric dating. Like other plant fossils, charcoal can be used to Geosciences 2018, 8, 223 18 of 30 reconstruct paleoenvironmental conditions. On a global scale, ignition temperature of plant material is related to atmospheric oxygen levels, so the presence of charcoal can be used to trace atmospheric evolution [107]. 4. Charcoalified Wood Charring of wood by volcanic processes has been described for the Soufrière Hills Volcano, FireMonserrat has been [103], present the Miocene throughout volcanic the field Earth’s at Cormandel history, Peninsula, so the abundance New Zealand of charred[108] and wood Recent in the geologicignimbrite record in is the not Taupo surprising. volcano, Production New Zealand of charcoal[109]. In the has latter two example, primary charring causes: temperatures natural wildfires (Figureestimated 20), and from ignition optical ofreflectance wood byranged volcanic from flows267 °C orto 414 hot °C, volcanic generally ash decreasing (ignimbrite) at distances (Figure 21). Charcoalaway in from the the fossil volcanic record vent. ranges At Montseratt, from intact charring logs to temperatures tiny fragments are estimated dispersed to inmainly clastic be sedimentsin the range of 200°C–340 °C, with a possible maximum of ~450 °C [104]. In volcanic environments, it is and coal. The latter form is commonly called fusain. Regardless of size, ancient charcoal may yield common for tree trunks to be completely charred, though some specimens are less charred in importantinterior information regions. about Earth history.

Figure 20. Charcoalification of wood in modern wild fires. (A) California, USA, 2015. Photo: National Figure 20. Charcoalification of wood in modern wild fires. (A) California, USA, 2015. Photo: National Atmospheric & Space Administration. (B) Charred trees after 2007 fire, Minnesota, USA. Photo Atmosphericcourtesy &of SpaceEli Sagor. Administration. (B) Charred trees after 2007 fire, Minnesota, USA. Photo courtesy of Eli Sagor. Geosciences 2018, 8, x FOR PEER REVIEW 21 of 32

Figure 21. Charred wood resulting from volcanic processes. (A) Tree ignited by basalt flow from Figure 21. Charred wood resulting from volcanic processes. (A) Tree ignited by basalt flow from Kilauea Volcano, Hawaii, USA. Photo by Brian W. Schaller (Wikipedia). (B) Charred trees after 1959 Kilauea Volcano, Hawaii, USA. Photo by Brian W. Schaller (Wikipedia). (B) Charred trees after 1959 Kilauea eruption. USGS Hawaii Volcano Observatory photo. (C) Tephra cloud engulfing palm grove, Kilauea1991 eruption. Mt. Pinatubo USGS eruption, Hawaii Philippine Volcano Observatory Islands. Photo photo. by Alberto (C) Tephra Garcia. cloud (D) Recent engulfing charcoal palm in grove, 1991ignimbrite, Mt. Pinatubo Taupo eruption, volcanic field, Philippine New Zealand. Islands. Photo Photo by Brent by Alberto Alloway. Garcia. (D) Recent charcoal in ignimbrite, Taupo volcanic field, New Zealand. Photo by Brent Alloway. 4.1. Evidence of Ancient Wildfires Fossil evidence of wildfires primarily comes from charcoal. The oldest known charcoal comes from the Silurian [110]. However, wildfires did not become prominent until the appearance of forests in the Middle Devonian [111,112]. By the Carboniferous, high-biomass forests were episodically swept by fires, ultimately producing coal that contains as much as 20% charcoal by volume [99]. The occurrence of fire is related to atmospheric oxygen levels, which determine the ignition temperature of organic materials. Atmospheric oxygen levels during Phanerozoic time was estimated from the organic carbon and sulfur content of clastic sediments [113–115], but the usefulness of fossil charcoal for making more precise evaluations was quickly recognized. A minimum oxygen concentration of 13% is required for wildfires; oxygen levels above 35% would reduce ignition temperatures to a point where sustained forest growth would be prohibited [114]. Atmospheric oxygen concentrations are estimated to have been above 26% throughout the Carboniferous and Permian periods, declining abruptly near the time of the Permian-Triassic mass extinction, fluctuating during the Triassic and Jurassic, and declining beginning in the mid Cretaceous to reach the present-day value of ~21% [116-118]. Dispersed charcoal is commonly preserved in clastic sediments, but also occurs in lignite coal [119].

4.2. Anatomical Evidence Dispersed charcoal fragments are useful for tracing wildfire history, but macroscopic specimens are valuable for taxonomic purposes. Light microscopy can be used, but SEM images are richer in detail. Although most research has been done on charred wood, vascular plants (e.g., ferns) may also be represented as fossil charcoal [120]. In higher plants, charred tissues become black and brittle, with a silky sheen. Because charcoal results from heating, compression forces are limited to minor shrinkage effects, so wood retains its 3-dimensional structure The reflectance of the charcoal can be

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Much of our knowledge of ancient charcoal comes from the work of University of London professor Andrew C. Scott. His research is described in a recent book [96], and in many journal publications [102–106]. Several papers provide general overviews of fossil charcoal research [104-106]. Because charred wood typically has excellent anatomical preservation, intact charcoal specimens are well suited for taxonomic identifications. For relatively young deposits, charcoal is in an ideal material for 14C radiometric dating. Like other plant fossils, charcoal can be used to reconstruct paleoenvironmental conditions. On a global scale, ignition temperature of plant material is related to atmospheric oxygen levels, so the presence of charcoal can be used to trace atmospheric evolution [107]. Charring of wood by volcanic processes has been described for the Soufrière Hills Volcano, Monserrat [103], the Miocene volcanic field at Cormandel Peninsula, New Zealand [108] and Recent ignimbrite in the Taupo volcano, New Zealand [109]. In the latter example, charring temperatures estimated from optical reflectance ranged from 267 ◦C to 414 ◦C, generally decreasing at distances away from the volcanic vent. At Montseratt, charring temperatures are estimated to mainly be in the range of 200◦C–340 ◦C, with a possible maximum of ~450 ◦C[104]. In volcanic environments, it is common for tree trunks to be completely charred, though some specimens are less charred in interior regions.

4.1. Evidence of Ancient Wildfires Fossil evidence of wildfires primarily comes from charcoal. The oldest known charcoal comes from the Silurian [110]. However, wildfires did not become prominent until the appearance of forests in the Middle Devonian [111,112]. By the Carboniferous, high-biomass forests were episodically swept by fires, ultimately producing coal that contains as much as 20% charcoal by volume [99]. The occurrence of fire is related to atmospheric oxygen levels, which determine the ignition temperature of organic materials. Atmospheric oxygen levels during Phanerozoic time was estimated from the organic carbon and sulfur content of clastic sediments [113–115], but the usefulness of fossil charcoal for making more precise evaluations was quickly recognized. A minimum oxygen concentration of 13% is required for wildfires; oxygen levels above 35% would reduce ignition temperatures to a point where sustained forest growth would be prohibited [114]. Atmospheric oxygen concentrations are estimated to have been above 26% throughout the Carboniferous and Permian periods, declining abruptly near the time of the Permian-Triassic mass extinction, fluctuating during the Triassic and Jurassic, and declining beginning in the mid Cretaceous to reach the present-day value of ~21% [116–118]. Dispersed charcoal is commonly preserved in clastic sediments, but also occurs in lignite coal [119].

4.2. Anatomical Evidence Dispersed charcoal fragments are useful for tracing wildfire history, but macroscopic specimens are valuable for taxonomic purposes. Light microscopy can be used, but SEM images are richer in detail. Although most research has been done on charred wood, vascular plants (e.g., ferns) may also be represented as fossil charcoal [120]. In higher plants, charred tissues become black and brittle, with a silky sheen. Because charcoal results from heating, compression forces are limited to minor shrinkage effects, so wood retains its 3-dimensional structure The reflectance of the charcoal can be used to estimate charring temperature [105, 107] The most significant structural change is the fusing of the cell walls into a single homogeneous layer, obliterating the original multilayered architecture. Experimental studies show that cell wall homogenization may occur when modern conifer wood is heated for one hour at 350 ◦C[105,116]. In other aspects, anatomical preservation may be excellent (Figure 22). Geosciences 2018, 8, x FOR PEER REVIEW 22 of 32 used to estimate charring temperature [105, 107] The most significant structural change is the fusing of the cell walls into a single homogeneous layer, obliterating the original multilayered architecture. Experimental studies show that cell wall homogenization may occur when modern conifer wood is Geosciencesheated for2018 one, 8, 223hour at 350 °C [105,116]. In other aspects, anatomical preservation may be excellent20 of 30 (Figure 22).

Figure 22. Modern charred wood from Whatcom County, Washington USA. (A) Thuja plicata (Red Cedar), wildfirewildfire wood; wood; ( B (B–D–D) Acer) Acer macrophyllum macrophyllum(Bigleaf (Bigleaf Maple), Maple), camp camp fire wood.fire wood. (B) Tangential (B) Tangential view, view,showing showing cross-section cross-section of a ray of with a ray a conspicuous with a conspicuous resin canal. resin Adjacent canal. tracheid Adjacent cell tracheid walls show cell brittle walls showfracture. brittle (C), Transverse fracture. (C view), Transverse showing earlywood view showing (large earlywood cells) and latewood (large cells) (small and cells). latewood (E,F) Charred (small cells).conifer ( woodE,F) Charred from an excavated conifer wood landfill. from (E ) an Tangential excavated view, landfill. showing (E cross-section) Tangential view view, of showing ray cells. (cross-sectionF) Radial view view showing of ray well-preservedcells. (F) Radial borderedview showing pits. well-preserved bordered pits.

A few occurrences have been reported where partially-charred wood has become silicified A few occurrences have been reported where partially-charred wood has become [108,121]. A previously unreported occurrence is represented by a single specimen from the Miocene silicified [108,121]. A previously unreported occurrence is represented by a single specimen from Columbia Plateau basalts of central Washington, USA (Figure 23A). Although the opal mines at the Miocene Columbia Plateau basalts of central Washington, USA (Figure 23A). Although the opal Virgin Valley are famous for silicified wood, a few samples show charring, presumably from mines at Virgin Valley are famous for silicified wood, a few samples show charring, presumably from pyroclastic flows that affected forests bordering the ancient caldera. These examples provide pyroclastic flows that affected forests bordering the ancient caldera. These examples provide evidence evidence that charred wood is not susceptible to silicification, probably because the pyrolized tissues that charred wood is not susceptible to silicification, probably because the pyrolized tissues are not capable of acting as organic templating matrices for the precipitation of silica. However, uncharred interior regions and empty cell lumen may be sites for silica precipitation (Figure 23B,C). Geosciences 2018, 8, x FOR PEER REVIEW 23 of 32

Geosciences 2018are ,not8, 223capable of acting as organic templating matrices for the precipitation of silica. However, 21 of 30 uncharred interior regions and empty cell lumen may be sites for silica precipitation (Figure 23B,C).

Figure 23. Silicification of charred wood. (A) Miocene wood from Columbia River Basalt Group, Figure 23. Silicification of charred wood. (A) Miocene wood from Columbia River Basalt Group, central central Washington, USA. Exterior charred layer surrounds silicified interior wood. (B,C) Miocene Washington,wood USA. from Exterior Royal Peacock charred Opal layer Mine, surrounds Virgin Valley, silicified Nevada interior USA, showing wood. homogenization (B,C) Miocene of wood from Royal Peacockcharred Opal cell walls. Mine, Cell Virgin lumen Valley,contain lepispheres Nevada USA, of opal-CT. showing homogenization of charred cell walls. Cell lumen contain lepispheres of opal-CT. 5. Coalified Wood 5. Coalified WoodMummified wood retains a high proportion of the original cellulosic constituents, in contrast to coalified wood, where loss of cellulose and hemicellulose results in relative enrichment of lignin [73]. MummifiedDuring coalification, wood retains wood aand high other proportion plat remains of are the transformed original to cellulosic peat as a result constituents, of humification in contrast to coalified wood,and gelification, where loss which of celluloseare biochemical and hemicellulose processes. Humification results involves in relative the enrichmentdecomposition of of lignin [73]. original organic constituents to produce dark brown organic polymers. Because these polymers are During coalification,resistant to microbial wood and attack, other peat plat is a relativelyremains stable are transformed material. The to amorphous peat as a nature result of of these humification and gelification,organic which alteration are products biochemical causes processes. plant tissues Humification to lose their original involves architecture, the decomposition producing a of original organic constituentsgelatinous texture. to produce The conversion dark brown of peat organic to coal polymers. is a metamorphic Because process, these where polymers organic are resistant to microbialcompounds attack, are peat altered is as a a relatively result of heat stable and pressure, material. leading The to an amorphous increase in carbon nature content of theseand organic decreases in oxygen and hydrogen. These reactions involve dehydration, bituminization, and alterationgraphitization, products causes and they plant can be tissues divided to into lose four their stages: original peat is successively architecture, changed producing into lignite, a gelatinous texture. Thesubbituminous conversion ofcoal, peat bituminous, to coal is a and metamorphic anthracite process, [122]. Coal where is organic composed compounds of three are altered as a resultmicroscopically-recognizable of heat and pressure, leading groups to of an organic increase constituents in carbon that arecontent known and as macerals decreases. Derived in oxygen and hydrogen.from These cellular reactions tissues, involve vitrinite isdehydration, the most abundant bituminization, and most homogeneous and graphitization, maceral, the and main they can be contributor to the shiny black appearance of coal. U.S. Coals typically contain as much as 80 wt % divided intovitrinite four [123]. stages: Liptinite peat is (also successively called exinite) changed originates into from lignite, spores, subbituminous pollen, resins, lipids, coal, bituminous, waxes, and anthracitealgae, [122 ].and Coal bacterial is composed proteins. Inertinite of three is microscopically-recognizable derived from oxidation. Coalified wood groups is most of organiccommonly constituents that are knownfound in as themacerals lowest-rank. Derived coals, lignite from and cellular subbituminous, tissues, where vitrinite the macroscopic is the most organic abundant remains and most homogeneousmay be maceral, preserved. the Preservation main contributor of wood in to lignite the shiny (brown black coal) appearance is very common of coal.(e.g., [124–126]). U.S. Coals typically contain as much as 80 wt % vitrinite [123]. Liptinite (also called exinite) originates from spores, pollen, resins, lipids, waxes, algae, and bacterial proteins. Inertinite is derived from oxidation. Coalified wood is most commonly found in the lowest-rank coals, lignite and subbituminous, where the macroscopic organic remains may be preserved. Preservation of wood in lignite (brown coal) is very common (e.g., [124–126]). However, cellular tissues in a mineralized Permian tree fern have been observed to consist of anthracite [127]. Conversion of original tissue to vitrinite involves the diagenetic alteration of lignocelluloses. In general, both hemi-cellulose and cellulose are greatly reduced or destroyed, and lignin-based components are selectively enriched. [127,128]. Key drivers of vitrinite formation are removal of cellulosic materials followed by physical compression. The result is that cellular anatomy is likely to be destroyed, producing logs and limbs that retain their original form, but often with significant flattening. Coalified wood typically has a vitreous or semi-vitreous luster, brittle with Geosciences 2018, 8, x FOR PEER REVIEW 24 of 32

However, cellular tissues in a mineralized Permian tree fern have been observed to consist of anthracite [127]. Conversion of original tissue to vitrinite involves the diagenetic alteration of lignocelluloses. In general, both hemi-cellulose and cellulose are greatly reduced or destroyed, and lignin-based components are selectively enriched. [127,128]. Key drivers of vitrinite formation are Geosciences 2018, 8, 223 22 of 30 removal of cellulosic materials followed by physical compression. The result is that cellular anatomy is likely to be destroyed, producing logs and limbs that retain their original form, but often with significant flattening. Coalified wood typically has a vitreous or semi-vitreous luster, brittle with conchoidal fracture (Figure 24). Microscopically, coalified wood cells are typically comminuted and conchoidal fracture (Figure 24). Microscopically, coalified wood cells are typically comminuted and homogenizedhomogenized (Figure (Figure 25). 25).

Figure 24. Coalified wood. (A) Coalified wood fragment from Eocene Colestin Formation, Siskiyou Figure 24.Pass,Coalified Oregon USA wood. [129]. (A ) (B Coalified–E) Eocene wood Chuckanut fragment Formation, from Eocene Whatcom Colestin County, Formation, WA USA. (B Siskiyou) Pass, Oregoncoalified USA log, [Bellingham129]. (B–E Coal) Eocene Mine. Chuckanut (C–E) Driftwood Formation, logs in fluvial Whatcom sandstone County, at Lookout WA USA. Mountain. (B) coalified log, BellinghamOuter bark Coallayer Mine.has been (C coalified,–E) Driftwood inner wood logs has in decayed fluvial and sandstone been replaced at Lookout by sandstone. Mountain. Outer bark layer has been coalified, inner wood has decayed and been replaced by sandstone. Geosciences 2018, 8, x FOR PEER REVIEW 25 of 32

Figure 25. SEM images of coalified wood. (A,B) Late Pleistocene Whidbey Formation, Swantown Figure 25.beach,SEM Washington, images ofUSA. coalified (A) longitudinal wood. views (A,B) showing Late Pleistocene brittle fracture Whidbey of tracheids, Formation, (B) transverse Swantown beach, Washington,view showing USA.homogenized (A) longitudinal cell walls, (C views,D) Oligocene showing Colestin brittle Formation, fracture ofSiskiyou tracheids, Pass, Oregon (B) transverse view showingUSA. Arrows homogenized show relict pits. cell walls, (C,D) Oligocene Colestin Formation, Siskiyou Pass, Oregon USA. Arrows show relict pits. 5.1. Coexistence of Mummifoed and Coalified Wood Fossil forests in the Whidbey Formation and Wilkes Formation in Washington, USA and Axel Heiberg and Ellesmere Islands in the Canadian Arctic primarily contain both mummified wood, but coalified wood is also present. The occurrence of both mummified and coalified wood in a single formation, sometimes within a single stratum, is evidence that conditions of preservation are subject to localized variation. Because the alteration of original organic matter to bituminous products involves a reduction in volume, coalified specimens are typically compressed. At the Eocene fossil forest at Axel Heiberg Island, Canadian Arctic, coalified wood is estimated to have been compressed at a ratio of ~6:1 relative to the original wood [44].

5.2. Accessory Minerals in Mummified and Coalified Wood The original preface of this paper is a description of fossil woods that are composed of original organic matter or its alteration products. However, nature seldom obeys simple rules, and it is not unusual for non-silicified wood to contain small amounts of inorganic minerals that were introduced during diagenesis. This phenomenon usually involves the precipitation of microcrystalline minerals within empty cell lumen. Figure 26 shows Eocene wood from Yuba River, California. Cell walls have been fused to produce a homogenous carbon layer, suggestive of charring. Individual cell lumen contain a variety of different diagenetic minerals, including apatite, pyrite, and gypsum. Siderite (FeCO3) has previously been reported as a major component of Neogene coalified wood in Alaska, USA [130]. Cretaceous wood from New Mexico, USA, has been reported to contain quartz, apatite, and calcite [131]; small amounts of pyrite, siderite, and clay minerals occur in coalified Miocene tree stumps at the Bilna Mine, Czech Republic [132].

Geosciences 2018, 8, 223 23 of 30

5.1. Coexistence of Mummifoed and Coalified Wood Fossil forests in the Whidbey Formation and Wilkes Formation in Washington, USA and Axel Heiberg and Ellesmere Islands in the Canadian Arctic primarily contain both mummified wood, but coalified wood is also present. The occurrence of both mummified and coalified wood in a single formation, sometimes within a single stratum, is evidence that conditions of preservation are subject to localized variation. Because the alteration of original organic matter to bituminous products involves a reduction in volume, coalified specimens are typically compressed. At the Eocene fossil forest at Axel Heiberg Island, Canadian Arctic, coalified wood is estimated to have been compressed at a ratio of ~6:1 relative to the original wood [44].

5.2. Accessory Minerals in Mummified and Coalified Wood The original preface of this paper is a description of fossil woods that are composed of original organic matter or its alteration products. However, nature seldom obeys simple rules, and it is not unusual for non-silicified wood to contain small amounts of inorganic minerals that were introduced during diagenesis. This phenomenon usually involves the precipitation of microcrystalline minerals within empty cell lumen. Figure 26 shows Eocene wood from Yuba River, California. Cell walls have been fused to produce a homogenous carbon layer, suggestive of charring. Individual cell lumen contain a variety of different diagenetic minerals, including apatite, pyrite, and gypsum. Siderite (FeCO3) has previously been reported as a major component of Neogene coalified wood in Alaska, USA [130]. Cretaceous wood from New Mexico, USA, has been reported to contain quartz, apatite, and calcite [131]; small amounts of pyrite, siderite, and clay minerals occur in coalified Miocene tree stumpsGeosciences at 2018 the, 8 Bilna, x FOR Mine, PEER REVIEW Czech Republic [132]. 26 of 32

Figure 26. Cont.

Figure 26. SEM images of accessory minerals in Eocene charred wood from Yuba River, California, USA. (A) Transverse view shows homogenized cell walls, with diagenetic apatite crystals dispersed over inner surfaces of some cells. (B) Longitudinal view of two tracheids, showing abundant apatite. Apatite does not occur in intracellular spaces. (C) Apatite crystals are attached to surfaces of cell wall separating two adjacent tracheids. (D) Tabular gypsum crystals occur on outer surfaces of the wood, apparently representing a very late stage precipitation event. (E,F) Pyrite crystals are present in some cell lumen. The presence of gypsum (CaSO4·2H2O) and pyrite (FeS2) are evidence of the availability of dissolved sulfur in pore fluids during diagenesis.

Funding: This research received no external funding.

Acknowledgments: The author thanks the following people for generously providing photographs: Brent Alloway, Franco Bianchi, Tamás Elter, Alberto Garcia, David Greenwood, Hope Jahren, William Hagopian, Benjamin Hook, Sándor Kovács, Edoardo Martinetto, Mark Orsen, Marta Pinzagli, Natalia Rybcynski, Eli Sagor, Brian Schaller, Chris Williams. Fossil wood specimens were contributed by David Lawler, Jim Meissner, and Alex Wolfe. Charles Wampler and Dan Carnevale provided technical support for the SEM used in this research.

Conflicts of interest: The author declares no conflicts of interest.

References

Geosciences 2018, 8, x FOR PEER REVIEW 26 of 32

Geosciences 2018, 8, 223 24 of 30

Figure 26. SEM images of accessory minerals in in Eocene Eocene charred charred wood wood from from Yuba Yuba River, California, USA. ( A) Transverse viewview showsshows homogenizedhomogenized cell walls,walls, withwith diageneticdiagenetic apatite crystals dispersed over inner surfaces of some cells. ( B)) Longitudinal Longitudinal view view of of two two tracheids, tracheids, showing showing abundant abundant apatite. apatite. Apatite does not occur in intracellular spaces. (C) Apatite crystals are attached to surfaces of cell wall separating two adjacent tracheids. ( (DD)) Tabular Tabular gypsum crystals occur on on outer surfaces surfaces of of the the wood, wood, apparently representing a very late stage precipitation event. ( E,F) Pyrite crystals are present in some

cell lumen. The presence ofof gypsum (CaSO(CaSO44··2H2H2O)O) and and pyrite pyrite (FeS 2)) are are evidence evidence of of the the availability availability of dissolved sulfur in pore fluidsfluids during diagenesis.

Funding: This research received no external funding.

Acknowledgments:Funding: This research The received author no thanks external the funding. following people for generously providing photographs: Brent Acknowledgments:Alloway, Franco Bianchi,The Tamás author Elter, thanks Alberto the Garcia, following David people Greenwood, for generously Hope Jahren, providing William photographs: Hagopian, BenjaminBrent Alloway, Hook, Franco Sándor Bianchi, Kovács, Tam Edoardoás Elter, Martinetto, Alberto Garcia, Mark Orsen, David Greenwood,Marta Pinzagli, Hope Natalia Jahren, Rybcynski, William Hagopian,Eli Sagor, BrianBenjamin Schaller, Hook, Chris Sándor Williams. Kovács, Fossil Edoardo wood Martinetto, specimens Mark were Orsen, contributed Marta Pinzagli,by David Natalia Lawler, Rybcynski, Jim Meissner, Eli Sagor, and Brian Schaller, Chris Williams. Fossil wood specimens were contributed by David Lawler, Jim Meissner, and Alex Wolfe. Charles Wampler and Dan Carnevale provided technical support for the SEM used in this research. Alex Wolfe. Charles Wampler and Dan Carnevale provided technical support for the SEM used in this research. ConflictsConflicts ofof Interest:interest: The author declares no conflictsconflicts ofof interest.interest.

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