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. Geosciences 2018, 8, 223; doi:10.3390/geosciences8060223 www.mdpi.com/journal/geosciences GeosciencesGeosciences2018, 20188, 223, 8, x FOR PEER REVIEW 2 of 322 of 30 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 toabsence 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, burialconditions in impermeable (e.g., high sediments,alkalinity or aridity, acidity, or lowor 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 of their three cellulosic discrete aggregateslayers 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 may resulting result in in relative destruction enrichment of the ofwood lignin sugars [13, 14cellulose]. Deterioration and of thehemicellulose, lignin-rich secondaryresulting in wall relative may enrichment not greatly of disruptlignin [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 oftenlahar locally deposits abundant, provide particularly deep layers in Quaternaryof fine sediment deposits, that where preserve