geosciences

Article -Mineralized Wood from , ,

George E. Mustoe 1,* and Graham Beard 2

1 Geology Department, Western Washington University, Bellingham, WA 98225, USA 2 Vancouver Island Museum, , BC V9K 1K7, Canada; [email protected] * Correspondence: [email protected]

Abstract: Calcite-mineralized wood occurs in marine sedimentary rocks on Vancouver Island, British Columbia at sites that range in age from Early to . These fossil woods commonly have excellent anatomical preservation that resulted from a permineralization process where calcite infiltrated buried wood under relatively gentle geochemical conditions. Wood specimens typically occur in in feldspathic clastic . Other concretions in the same outcrops that contain abundant mollusk and crustacea are evidence that plant remains were fluvially transported into a marine basin. Fossiliferous concretions commonly show zoning, comprising an inner region of progressive precipitation where calcite developed as a concentric halo around the organic nucleus. An outer zone was produced by pervasive , which was produced when calcite was simultaneously precipitated in pore spaces over the entire zone.

Keywords: calcite; concretions; ; ; permineralization; silicification; Vancou- ver Island

  1. Introduction Citation: Mustoe, G.E.; Beard, G. Fossil wood occurs at many sites on Vancouver Island, (Figure1), but these fossils Calcite-Mineralized Fossil Wood from have received scant attention from scientists. Early paleobotany work described leaf Vancouver Island, British Columbia, compression fossils preserved in and of the Group (Bell, 1957). Canada. Geosciences 2021, 11, 38. Later, researchers have focused on permineralized plant remains, e.g., flowers, fruits, seeds, https://doi.org/10.3390/geosciences and cones. Much of this work has been done by paleobotanists R.A. Stockey and G.W. 11020038 Rothwell and their students and colleagues e.g., [1–3]. The plethora of fossil plant organ studies is in sharp contrast to the paucity of work on mineralized woods. Taxonomic Received: 1 January 2021 descriptions are limited to the angiosperm Paraphyllanthoxylon vancouverensis preserved Accepted: 16 January 2021 Cunninghamia Published: 20 January 2021 as a 38 cm diameter log at the Eden Main locality [4] and the gymnosperm hornbeyensis from [5]. Fossil palm logs have been reported from Hornby Island beds [6]. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Our research is devoted to the of the wood and the processes of fossiliza- published maps and institutional affil- tion. Regardless of age, fossil wood from Vancouver Island localities is mineralized with iations. calcite. The mineralization of wood with has previously been reported from 13 locations around the world, mostly in terrestrial deposits (Table1). The occurrence of abundant calcite-mineralized wood preserved in shallow marine deposits on Vancouver Island, British Columbia is a notable addition to this list.

Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Geosciences 2021, 11, 38. https://doi.org/10.3390/geosciences11020038 https://www.mdpi.com/journal/geosciences Geosciences 2021, 11, x FOR PEER REVIEW 2 of 16

Table 1. World-wide occurrences of woods mineralized with .

Location Age Reference Lucknow, Poland [7] Isle of Weight, England Cretaceous [8] New Mexico, USA Cretaceous [9] Kansas, USA Cretaceous [10] Vancouver Island, Canada Cretaceous, Paleocene This report Geiseltal, Germany Eocene [11] Ellesmere Island, Arctic Canada Eocene [12] Washington, USA Miocene [13] Florida, USA Neogene [14] Dunrobba, Italy Pliocene [15] California, USA [16,17] Ohio, USA Pleistocene [18,19] Geosciences 2021, 11, 38 Northern Ireland Pleistocene [20] 2 of 16 Weimar, Germany Pleistocene [21]

FigureFigure 1. 1. VancouverVancouver Island Island plant plant fossil fossil locations. locations. ( (11).). Apple Apple Bay; Bay; ( (22).). Shelter Shelter Point; Point; ( (33).). Appian Appian Way; Way;(4). Eden (4). Eden Main; Main; (5). Puntledge(5). Puntledge River; River; (6). ( Trent6). Trent River; River; (7). ( Denman7). Denman Island; Island; (8). (8 Hornby). Hornby Island; Island; (9 ). (9Cranberry). Cranberry Arms. Arms.

2.Table Geologic 1. World-wide Setting occurrences of woods mineralized with calcium carbonate.

Fossil plantLocation localities on Vancouver Island are Age found in the Upper Reference Cretaceous Nanaimo Group and in adjacent Lower Cretaceous and Paleocene strata (Table 2). These Lucknow, Poland Jurassic [7] Isle of Weight, England Cretaceous [8] New Mexico, USA Cretaceous [9] Kansas, USA Cretaceous [10] Vancouver Island, Canada Cretaceous, Paleocene This report Geiseltal, Germany Eocene [11] Ellesmere Island, Arctic Canada Eocene [12] Washington, USA Miocene [13] Florida, USA Neogene [14] Dunrobba, Italy Pliocene [15] California, USA Pleistocene [16,17] Ohio, USA Pleistocene [18,19] Northern Ireland Pleistocene [20] Weimar, Germany Pleistocene [21]

2. Geologic Setting Fossil plant localities on Vancouver Island are found in the Upper Cretaceous Nanaimo Group and in adjacent Lower Cretaceous and Paleocene strata (Table2). These formations comprise conformable beds and intergradational facies that record transitions from alluvial deposits to coastal, deltaic, and shallow marine basin [22–24]. The paleogeographic origin of Vancouver Island sedimentary rocks remains unre- solved. Namaimo Group sediments appear to have been deposited in a forearc basin Geosciences 2021, 11, 38 3 of 16

where the strata were later subjected to thrust faulting and uplift (Figure2). Perhaps these sediments were deposited at or near their present location. An alternate view is that the Nanaimo Group is part of an extensive assemblage of rocks that have been transported by high-angle faulting from a source area ≈3000 km to the south. This “Baja-BC” hypothe- sis, introduced in 1985 [25], is supported by shallow paleomagnetic inclination angles in sedimentary and volcanic rocks [26–28]. An intermediate interpretation is that northern transport was 1000–1600 km [29,30]. These divergent interpretations preclude the use of paleolatitude for paleoenvironmental reconstructions. Molluscan and crustacean fossils are locally abundant and provide evidence for estimating geologic age. These fossils typically occur in transgressive marine environments. Plant fossils in these beds are indicative of a warm temperate climate [31]; this paleoclimate does not solve the paleolatitude conundrum because mild temperatures were globally widespread during the Cretaceous and early Tertiary.

Table 2. Vancouver Island plant fossil localities. Data from [23,32].

North Vancouver Island Depositional Formation Fossil Locality Age Environment Campanian Unnamed strata shallow marine 72–94 Ma Valengian-Barremian Longarm equivalent Apple Bay sandstone Shallow marine 140 Ma South Vancouver Island Paleocene Bay Oyster Bay sandstone Shallow marine 60 Ma unconformity submarine fan, high Gabriola sandstone energy Maastrichtian 66–72 Ma submarine fan, low Spray Shelter Point & mudstone energy conglomerate & submarine fan, high Geoffrey Campanian- sandstone energy Maastrichtian submarine fan, low Northumberland Hornby Island siltstone & mudstone energy conglomerate & submarine fan, High De Courcy sandstone energy Cranberry submarine fan, low Cedar District siltstone & mudstone Arms energy Campanian sandstone, minor Nanaimo Group shallow marine to 72–84 Ma Protection conglomerate and deep submarine fan siltstone marine outer shelf Pender siltstone, mudstone and slope nearshore marine conglomerate, minor and Extension Santonian- sandstone and coal onshore deltaic and Campanian fluvial marine outer shelf Haslam siltstone & mudstone and slope Santonian conglomerate, nearshore marine 84–86 Ma Comox Puntledge River sandstone, mudstone, and fluvial coal Turonian-Coniacian Unnamed strata Eden Main Sandstone, mudstone nearshore marine 90 MA Geosciences 2021, 11, x FOR PEER REVIEW 4 of 16

nearshore marine conglomerate, mi- and Extension nor sandstone and onshore deltaic and coal fluvial

Santonian-Campa- nian siltstone & - marine outer shelf Haslam stone and slope Santonian conglomerate, nearshore marine 84–86 Ma Comox Puntledge River sandstone, mud- and fluvial stone, coal Turonian-Coni- Sandstone, mud- Geosciences 2021Unnamed, 11, 38 strata Eden Main nearshore marine acian 4 of 16 stone 90 MA

Figure 2. Generalized geological structure of the Upper Cretaceous Nanaimo Group.Group. Adapted from [[23].23].

3. Materials and Methods Specimens usedused inin thisthis study study come come from from the the major major Vancouver Vancouver Island Island localities. localities. Acetate Ace- peelstate peels of Apple of Apple Bay specimensBay specimens were were prepared prepared by etching by etching cut surfacescut surfaces with with 5% HCl. 5% HCl. Pet- Geosciences 2021, 11, x FOR PEER REVIEWPetrographicrographic thin thin sections sections were were made made at Western at Western Washington Washington University University and photographed and photo-5 of 16

graphedusing a Zeiss using petrographic a Zeiss petrographic microscope microsco equippedpe equipped with a 5 megapixelwith a 5 megapixel digital camera. digital cam- SEM era.images SEM are images from aare Tescan from Vegaa Tescan scanning Vega electronscanning microscope. electron microscope. EDS spectra EDS were spectra obtained were obtained with an EDAX detector running Genesis software. SEM specimens consisted of 4.with Results an EDAX detector running Genesis software. SEM specimens consisted of rock frag- rockments fragments attached attached to 1 cm diameterto 1 cm diameter aluminum aluminum stubs using stubsepoxy using epoxy adhesive. adhesive. Samples Samples were As mentioned, fossil wood specimens at all localities are mineralized with calcite, weresputter sputter coated coated with Pdwith to Pd provide to provide electrical electr conductivity.ical conductivity. Fossil Fossil specimens, specimens, thin sections,thin sec- preservingtions,and SEM and specimens varyingSEM specimens amounts are archived are of archivedrelict in G.M.’s organic in researchG.M.’s matter. research collection Large collectionpieces at the have Geology at thebeen Geology Department, found, in-De- cludingWesternpartment, driftwood Washington Western logs, Washington University. but small University. fragments are more abundant (Figure 3). Fossil wood is commonly preserved as nuclei in calcareous concretions (Figure 4). 4. ResultsThe preservation of relict tissue is evidenced by the cellular anatomy visible in acetate peels Asmade mentioned, from specimen fossil woodsurfaces specimens that have at been all localities etched with are mineralizedHCl (Figures with 4 and calcite, 5F). Althoughpreserving acetate varying peels amounts have been of relict widely organic used matter.by paleobotanists Large pieces [4], have these been peals found, do not in- providecluding information driftwood logs, about but mineralogy. small fragments Petrographic are more thin abundant sections (Figure viewed3). Fossil under wood polar- is izedcommonly transmitted preserved light reveal as nuclei the inmineral calcareous composition concretions (Figure (Figure 5). 4).

FigureFigure 3. 3. FossilFossil wood wood from from Nanaimo Nanaimo Group Group strata. strata. (A) Puntledge (A) Puntledge River. River. (B) Shelter (B) Shelter Point. Point.(C) Den- (C) manDenman Island. Island.

Figure 4. Acetate peels from Lower Cretaceous calcareous concretions from Apple Bay. (A) Trans- verse view of wood containing teredo borings. (B) Transverse view of limb, showing concentric zonation of concretionary (arrow). (C) containing fragmental plant remains. Geosciences 2021, 11, x FOR PEER REVIEW 5 of 16

4. Results As mentioned, fossil wood specimens at all localities are mineralized with calcite, preserving varying amounts of relict organic matter. Large pieces have been found, in- cluding driftwood logs, but small fragments are more abundant (Figure 3). Fossil wood is commonly preserved as nuclei in calcareous concretions (Figure 4). The preservation of relict tissue is evidenced by the cellular anatomy visible in acetate peels made from specimen surfaces that have been etched with HCl (Figures 4 and 5F). Although acetate peels have been widely used by paleobotanists [4], these peals do not provide information about mineralogy. Petrographic thin sections viewed under polar- ized transmitted light reveal the composition (Figure 5).

Geosciences 2021, 11, 38 5 of 16 Figure 3. Fossil wood from Nanaimo Group strata. (A) Puntledge River. (B) Shelter Point. (C) Den- man Island.

FigureFigure 4. 4. AcetateAcetate peels peels from from Lower CretaceousCretaceous calcareouscalcareous concretions concretions from from Apple Apple Bay. Bay. (A ()A Transverse) Trans- verseview view of wood of wood containing containing teredo teredo borings. borings. (B) Transverse (B) Transverse view view of limb, of li showingmb, showing concentric concentric zonation zonationof concretionary of concretionary matrix (arrow). matrix (arrow). (C) Concretion (C) Concretion containing containing fragmental fragmental plant remains. plant remains.

The preservation of relict tissue is evidenced by the cellular anatomy visible in acetate peels made from specimen surfaces that have been etched with HCl (Figures4 and5F). Although acetate peels have been widely used by paleobotanists [4], these peals do not provide information about mineralogy. Petrographic thin sections viewed under polarized transmitted light reveal the mineral composition (Figure5). Scanning Electron Microscope secondary electron images (Figure6) show anatomical and mineralogic characteristics of fossil woods that have three-dimensional preservation, e.g., intercellular spaces, lumen, or vessels that are not fully mineralized. SEM/EDS spectra show calcium carbonate is the only major mineral constituent (Figure7). For silicified wood, the magnitude of the C peak is an indication of the abundance of relict organic matter. For calcite mineralized wood, the carbonate composition always produces a large C peak in X-ray fluorescence spectra. If C and Ca could both be measured accurately, the C:Ca atomic ratio would be 1:1, with a mass ratio of 0.32. As a result of its very low X-ray energy (0.277 KeV), C abundance cannot be accurately quantified in EDS spectra. However, some clues to the presence of relict organic are offered by the height ratio of C:Ca peaks. In calcite mineralized wood, the height of the C peak is elevated when organic compounds are present, representing C in excess of the amount required to produce CaCO3. Apple Bay and Puntledge River specimens have C:Ca peak height ratios of 0.82 and 0.78, respectively. These ratios are inferred to represent elevated C levels based on the observation that these fossil woods yield anatomically detailed acetate peals. The C:Ca ratio of 3.2 for the Port Hardy wood is evidence of a much higher percentage of relict organic matter than the Apple Bay and Puntledge River fossil woods. Geosciences 2021, 11, 38 6 of 16 Geosciences 2021, 11, x FOR PEER REVIEW 6 of 16

Figure 5. OpticalOptical photomicrographs photomicrographs of of Nanaimo Nanaimo Island Island calcite-mineralized calcite-mineralized wood. wood. (A,B (A) Thin,B) section views views of Upper of Upper Cre- taceousCretaceous wood wood from from Collishaw Collishaw Point, Point, Hornby Hornby Island. Island. (A ()A Transverse) Transverse view view showing showing prominent prominent calcite calcite vein. vein. ( (BB)) Transverse Transverse view showing brown cell walls and lighter colored lumen. (C,D) Thin section views of Upper Cretaceous conifer wood view showing brown cell walls and lighter colored lumen. (C,D) Thin section views of Upper Cretaceous conifer wood from Puntledge River locality. (C) Transverse view showing calcite vein. (D) Transverse view showing thick cell walls and from Puntledge River locality. (C) Transverse view showing calcite vein. (D) Transverse view showing thick cell walls and calcite-filled lumen. (E,F) Acetate peels of Lower Cretaceous conifer wood from Apple Bay. (E) Transverse view showing cellcalcite-filled walls preserving lumen. ( Erelict,F) Acetate organic peels matter. of Lower (F) Oblique Cretaceous transverse conifer view wood of fromconifer Apple leaf, Bay.showing (E) Transverse central vascular view showing bundle surroundedcell walls preserving by mesophyll relict cells. organic matter. (F) Oblique transverse view of conifer leaf, showing central vascular bundle surrounded by mesophyll cells. Scanning Electron Microscope secondary electron images (Figure 6) show anatomical and mineralogic characteristics of fossil woods that have three-dimensional preservation, Geosciences 2021, 11, x FOR PEER REVIEW 7 of 16

e.g., intercellular spaces, lumen, or vessels that are not fully mineralized. SEM/EDS spec- tra show calcium carbonate is the only major mineral constituent (Figure 7). For silicified wood, the magnitude of the C peak is an indication of the abundance of relict organic matter. For calcite mineralized wood, the carbonate composition always produces a large C peak in X-ray spectra. If C and Ca could both be measured accurately, the C:Ca atomic ratio would be 1:1, with a mass ratio of 0.32. As a result of its very low X-ray energy (0.277 KeV), C abundance cannot be accurately quantified in EDS spectra. How- ever, some clues to the presence of relict organic are offered by the height ratio of C:Ca peaks. In calcite mineralized wood, the height of the C peak is elevated when organic compounds are present, representing C in excess of the amount required to produce CaCO3. Apple Bay and Puntledge River specimens have C:Ca peak height ratios of 0.82 and 0.78, respectively. These ratios are inferred to represent elevated C levels based on the observation that these fossil woods yield anatomically detailed acetate peals. The C:Ca Geosciences 2021, 11, 38 7 of 16 ratio of 3.2 for the Port Hardy wood is evidence of a much higher percentage of relict organic matter than the Apple Bay and Puntledge River fossil woods.

FigureFigure 6. SEM 6. imagesSEM images of calcite-mineralized of calcite-mineralized wood. (A) wood. Apple Bay (A) wood Apple (Lower Bay wood Cretaceous) (Lower showing Cretaceous) vertical tracheids and horizontalshowing vertical ray cells. tracheids (B) Apple Bayand wood, horizontal radial viewray cells. showing (B) mineralizedApple Bay castswood, of circularradial view intervessel showing pits. (C) Port Hardymineralized wood (Upper casts Cretaceous), of circular tracheid intervessel cells in radialpits. ( view.C) Port (D )Hardy Port Hardy wood wood, (Upper showing Cretaceous), mineralized tracheid cell walls. (E) Puntledge River wood (Upper Cretaceous), oblique radial view. (F) Puntledge River wood, oblique view of a tracheid with calcite permineralized cell walls, and lumina filled with crystalline calcite. Geosciences 2021, 11, x FOR PEER REVIEW 8 of 16

cells in radial view. (D) Port Hardy wood, showing mineralized cell walls. (E) Puntledge River Geosciences 2021, 11, 38 8 of 16 wood (Upper Cretaceous), oblique radial view. (F) Puntledge River wood, oblique view of a tra- cheid with calcite permineralized cell walls, and lumina filled with crystalline calcite.

FigureFigure 7. 7.EnergyEnergy dispersive dispersive X-ray X-ray fluorescence fluorescence spectra. spectra. Pd Pd peak peak is isfrom from coating coating applied applied to toprovide provide electricalelectrical conductivity. conductivity. C:Ca C:Ca peak peak height height ratios ratios provide provide an an approximate approximate indication indication of of the the abun- abundance danceof relict of relict organic organic matter. matter.

5. Discussion The process of calcite mineralization of ancient wood has received scant attention. Of the occurrences listed in Table2, only three investigators considered fossilization processes. Sweeny et al. [9] described late Cretaceous wood preserved in fine sediments deposited along a low elevation floodplain in New Mexico, USA. Most fossil woods are silicified, but some specimens are mineralized with calcium phosphate, and charcoalified wood is also Geosciences 2021, 11, 38 9 of 16

present. Calcite occurs cell fillings in fragments of charcoal and presumably represents a late stage of mineralization. At Ellesmere Island, Arctic Canada, Eocene calcite-mineralized stumps are associated with coal seams that occur within sandstone and siltstone deposited on a low-elevation delta plain [12]. Higgins [16,17] reported one of the only known marine occurrences of calcite mineral- ized wood, from late Cretaceous deposits near Sacramento, California USA. The paleoen- vironment appears to have been a nearshore basin with free circulation and strong local currents. Fossil wood at Vancouver Island occurs in geologic depositional environments that are quite different from the above examples. These formations are predominately composed of clastic sediments that range in grain size from conglomerate to claystone. Leaf fossils in coal-bearing beds represent coastal lowland forests, but most other fossils were preserved in shallow marine basins. Skeletal remains of mosasaurs, elasmosaurs, icthyosaurs, ple- siosaurs, and sea turtles represent pelagic vertebrates [33], and the Vancouver Island beds are world-famous for their preservation of ammonites and other marine invertebrates. The proximity to land is evidenced by the abundance of transported plant remains in these marine deposits. The accumulation of fine sediments suggests the absence of strong bottom currents. The absence of phosphate mineralization of the wood and the scarcity of inverte- brate fossils were inferred to indicate that the sediments were deposited in a geosynclinal basin that was at least partially blocked off from the open sea. In contrast, wood-bearing beds on Vancouver Island contain abundant molluscan and crustacean fossils, often in the same strata that preserve fossil wood. Framboidal is a common minor constituent of fossil-bearing concretions.

5.1. Fossilization Processes Permineralization occurs when wood tissue is entombed by that precipitated after burial. Although relict organic compounds may preserve the anatomical form of the original tissue, the components of wood are likely to be quite different from the original forms [34]. One of the most important changes is the breakdown of cellulose. Lignin is a complex cross-linked polymer that is relatively resistant to degradation. As a result of its stability, lignin is likely to be a major contributor to relict in fossil wood. For this reason, ancient woods are sometimes described as “lignified”. As described below, mineralization involved localized geochemical gradients that were linked to the degradation of organic matter. For animals, the breakdown of proteins typically results in a strong pH increase because amino and nucleic acids decompose to produce ammonia. For plant tissues, degradation involves different pathways. Although cytoplasm contains proteins and nucleic acids, wood largely consists of empty cells whose walls are composed of cellulose, hemicellulose, and lignin. These polymers are relatively stable, and fungi are the principle agent for their degradation. Cell wall degradation is accomplished by enzymatic reactions, where molecular linkages are cleaved, causing depolymerization [35]. Since cellulose and hemicellulose are polysaccharides, the end products of decomposition are simple sugars (e.g., glucose). Although these reactions do not produce the strong pH shifts that result from the degradation of nitrogenous compounds, the reaction products may serve as nutrients for microbes that may affect geochemical conditions. The common occurrence of framboidal pyrite suggests that bottom conditions were poorly oxygenated; microbial processes were presumably anaerobic. The most visible evidence of wood degradation is the presence of holes carved by pelecypods of the Teredunidae Family, e.g., Teredo (Figure4A). Although anatomical detail is well-preserved in many specimens, some concretions contain fossil wood that shows cellular disorganization caused by decay (Figure8). In subaerial environments, wood decay is primarily caused by fungi, but in anaerobic marine environments, wood degradation is mainly related to [36]. The relatively slow rate of this degradation is consistent Geosciences 2021, 11, x FOR PEER REVIEW 10 of 16

framboidal pyrite suggests that bottom conditions were poorly oxygenated; microbial processes were presumably anaerobic. The most visible evidence of wood degradation is the presence of holes carved by pelecypods of the Teredunidae Family, e.g., Teredo (Figure 4A). Although anatomical de- Geosciences 2021, 11, 38 tail is well-preserved in many specimens, some concretions contain fossil wood10 ofthat 16 shows cellular disorganization caused by decay (Figure 8). In subaerial environments, wood decay is primarily caused by fungi, but in anaerobic marine environments, wood degradation is mainly related to bacteria [36]. The relatively slow rate of this degradation withis consistent observations withof observations recent waterlogged of recent wood. waterlogged Wood-hulled wood. shipwrecks Wood-hulled have shipwrecks remained intacthave remained after several intact centuries after several of sea centuries bottom exposure of sea bottom e.g., [ 37exposure], and wood e.g., [37], buried and in wood Late Pleistoceneburied in Late deposits Pleistocene has ages deposits >100,000 has yearsages >100,000 BP [38,39 years]. Vancouver BP [38,39]. Island Vancouver fossil woods Island commonlyfossil woods show commonly excellent show anatomical excellent preservation, anatomical but preservation, after millions but of after years, millions this relict of tissueyears, isthis no relict longer tissue pristine is no wood. longer pristine wood.

FigureFigure 8.8. PartiallyPartially decayeddecayed LowerLower CretaceousCretaceous coniferconifer woodwood fromfrom Apple Apple Bay. Bay.

5.2.5.2. CalciteCalcite MineralizationMineralization InIn VancouverVancouver Island fossil fossil woods, woods, relict relict cellular cellular tissue tissue is entombed is entombed within within calcite calcite pre- precipitatedcipitated from from porewater. porewater. This This process process of permineralization of permineralization has has long long been been invoked invoked to ex- to explainplain the the preservation preservation of of woods woods preserved preserved with with silica, silica, but recent research showsshows thatthat most most silicifiedsilicified woods woods have have resulted resulted from from replacement, replacement, not not permineralization permineralization [14]. [14]. Therefore, Therefore, the recognitionthe recognition that that Vancouver Vancouver Island Island woods woods are preservedare preserved as true as true permineralizations permineralizations is an is importantan important discovery. discovery. ExperimentalExperimental studiesstudies showshow thatthat thethe precipitationprecipitation of of silicasilica and and calcitecalcite occur occur under under pH pH conditionsconditions thatthat areare largelylargely mutuallymutually exclusive.exclusive. AtAt pHpH >8,>8, calcitecalcite isis favoredfavored eveneven whenwhen dissolveddissolved Si levels are are high. high. Modern Modern sea sea water water pH pH levels levels locally locally range range from from 5.0 to 5.0 9.0, to with 9.0, with an average of ≈8.1 [40]. Oceanic pH values are largely controlled by atmospheric an average of ≈8.1 [40]. Oceanic pH values are largely controlled by atmospheric CO2 lev- CO levels, which have varied over geologic time [41]. The acidity of sea water may els,2 which have varied over geologic time [41]. The acidity of sea water may have been have been very different during the Cretaceous (Figure9). Elevated atmospheric CO 2 very different during the Cretaceous (Figure 9). Elevated atmospheric CO2 results in ele- results in elevated carbonic levels in , which in turn increases the of vated carbonic levels in seawater, which in turn increases the solubility of calcium car- calcium carbonate minerals and calcite. Seawater acidification may have an bonate minerals aragonite and calcite. Seawater acidification may have an adverse effect adverse effect on , including shellfish [42], but the bioavailability of calcium is on marine life, including shellfish [42], but the bioavailability of calcium is evidenced in evidenced in the Vancouver Island formations by the abundance of shelled invertebrate the Vancouver Island formations by the abundance of shelled invertebrate fossils. How- fossils. However, greater Cretaceous seawater acidity may explain why these strata do not ever, greater Cretaceous seawater acidity may explain why these strata do not include include deposits. The abundance of calcareous concretions in lithic sediments is limestone deposits. The abundance of calcareous concretions in lithic sediments is evi- evidence of localized geochemical environments that favored calcite precipitation. These dence of localized geochemical environments that favored calcite precipitation. These zones are probably related to localized geochemical gradients caused by the decomposition of organic matter. Geosciences 2021, 11, x FOR PEER REVIEW 11 of 16

Geosciences 2021, 11, 38 zones are probably related to localized geochemical gradients caused by the decomposi-11 of 16 tion of organic matter.

Figure 9. Atmospheric CO 2 levelslevels through through deep deep time. time. Adapted Adapted from from [41]. [41].

WoodsWoods mineralized with with calcite calcite result from very different fossilization processes comparedcompared to to silicified silicified woods. woods. For For silicified silicified wood, wood, the the process process of of mineralization mineralization begins via “organic templating” when dissolved silicic molecules form hydrogen bonds with organicorganic constituents constituents of of inner inner cell cell walls walls [43]. [43]. The The cell cell anatomy anatomy acts acts as as a template for these silicasilica layers, layers, so so the the incipient incipient mineralization mineralization of ofcell cell walls walls is the is thefirst first step step in preserving in preserving an- atomicalanatomical detail. detail. Later, Later, silica silica may may be deposited be deposited in open in open spaces: spaces: cell celllumen, lumen, vessels vessels and and in- intercellular spaces. Therefore, silicification is a multi-phase process [44]. As water is lost, tercellular spaces. Therefore, silicification is a multi-phase process [44]. As water is lost, hydrous silica is converted to ; subsequent may convert early amorphous hydrous silica is converted to opal; subsequent diagenesis may convert early amorphous opal to opal-CT, chalcedony, or . Silicification occurs concurrently with tissue opal to opal-CT, chalcedony, or quartz. Silicification occurs concurrently with tissue deg- degradation, and the degree of anatomical preservation is related to the respective rates radation, and the degree of anatomical preservation is related to the respective rates of of these processes. In most cases, silicified wood preserves only a small percentage of the these processes. In most cases, silicified wood preserves only a small percentage of the original organic matter [45]. The colors of silicified woods are diverse and include many original organic matter [45]. The colors of silicified woods are diverse and include many bright colors caused by trace metals [46]. bright colors caused by trace metals [46]. The processes of calcium carbonate mineralization of wood are probably quite dif- The processes of calcium carbonate mineralization of wood are probably quite differ- ferent. As with silicification, the absorption of mineral-bearing pore water is likely the ent. As with silicification, the absorption of mineral-bearing pore water is likely the first first step, because the basic cellular structure of wood facilitates fluid transport. Calcite step, because the basic cellular structure of wood facilitates fluid transport. Calcite may may initially precipitate within spaces in multilayered cell walls, but without the hydrogen initiallybonding precipitate “organic templating” within spaces affinity in multilayered that occurs cell with walls, silicic but acid. without With calcitethe hydrogen precipi- bondingtation, much “organic of the templating” organic matter affinity remains, that occurs with with cell walls silicic being acid. entombedWith calcite rather precipita- than tion,replaced. much Similar of the toorganic silicification, matter remains, calcite mineralization with cell walls may being be a entombed multi-stage rather process, than with re- placed.cell lumina, Similar vessels, to silicification, and intercellular calcite spacesmineralization becoming may filled be witha multi-stage crystalline process, calcite with after cellthe lumina, walls have vessels, been and mineralized intercellular (Figure spaces6). Colorsbecoming of calcite-mineralized filled with crystalline wood calcite lack after the thebright walls colors have observed been mineralized in many silicified(Figure 6). woods. Colors Calcite of calcite-mineralized wood is commonly wood a shadelack the of brightbrown, colors the color observed being causedin many by silicified relict organic woods. matter, Calcite not wood metallic is commonly trace elements. a shade of brown,Calcite the color permineralizations being caused by commonlyrelict organic preserve matter, excellent not metallic anatomical trace elements. detail, but in rareCalcite instances, permineralizations growth can commonly disrupt tissues.preserve Figure excellent 10 shows anatomical fossil wooddetail, that but has in rarebeen instances, subject to crystal the radial growth growth can ofdisrupt calcite ti crystalsssues. Figure that have 10 shows torn apartfossil thewood tissue. that Thishas beenphenomenon subject to has the been radial described growth as of “fibrous calcite cr displaciveystals that calcite” have torn [47]. apart The expansive the tissue. effects This phenomenonof calcite crystallization has been described result from as “fibrous the force di ofsplacive crystallinity, calcite” which [47]. The can reachexpansive pressure effects of of10 calcite atmospheres—equivalent crystallization result to from overburden the force pressuresof crystallinity, at a burial which depth can reach of 41 mpressure [47]. The of 10nucleation atmospheres—equivalent of fibrous displacive to overburden calcite may occurpressures at sites at a that burial contain depth abundant of 41 m microbes[47]. The nucleationthat concentrate of fibrous Ca displacive at their cell calcite walls may [48 ].occur Microbial at sites degradationthat contain abundant may be a microbes factor in mineralization of the Port Hardy wood, where the tissue appears to have undergone extensive disintegration. The radiating calcite commonly have a small wood fragment as a nucleus, with a thin zone of cells bordering the outer margin. Geosciences 2021, 11, x FOR PEER REVIEW 12 of 16

that concentrate Ca at their cell walls [48]. Microbial degradation may be a factor in min- Geosciences 2021, 11, 38 eralization of the Port Hardy wood, where the tissue appears to have undergone extensive12 of 16 disintegration. The radiating calcite crystals commonly have a small wood fragment as a nucleus, with a thin zone of cells bordering the outer margin.

FigureFigure 10. 10.Fossil Fossil conifer conifer wood wood from from Port Port Hardy. Hardy. (A ()A Thin) Thin section section shows shows tissues tissues to have to have a popcorn-like a popcorn- texturelike texture known known as fibrous as fibrous displacive displacive calcite, calcite, where wh theere growth the growth of radiating of radiating calcite calcite crystals crystals produced pro- severeduced anatomical severe anatomical distortion. distortion. (B) Acetate (B) peel Acetate illustrates peel illustrates how small how shards small of cellularshards tissueof cellular commonly tissue servedcommonly as nuclei served for as the nuclei growth for ofthe radiating growth crystals.of radiating crystals.

5.3.5.3. FormationFormation ofof CalcareousCalcareous Concretions Concretions Calcite-cementedCalcite-cemented concretionsconcretions havehave worldwideworldwide occurrence,occurrence, whichwhich isis foundfound inin both both terrestrialterrestrial and and marine marine environments. environments. Diameters Diameters range range in in size size from from a a few few cm cm to to several several m m (McBride(McBride et et al., al., 2003). 2003). No No single single model model explains explains all all types types of of concretionary concretionary growth growth [ 49[49],], but but severalseveral generalizations generalizations can can be be made. made. Calcareous Calcareous concretion concretion formation formation has has several several physical physical andand chemical chemical requirements: requirements: a a supply supply of of dissolved dissolved calcium, calcium, permeability permeability in in the the sediment, sediment, a nucleusa nucleus for for calcite calcite crystallization, crystallization, and and environmental environmental conditions conditions that that favor favor this this precipita- precipi- tion.tation. In In sediment sediment where where permeability permeability is is the the same same in in all all directions, directions, concretionary concretionary growth growth producesproduces a a spherical spherical shape. shape. Directional Directional porewater porewater flow flow produces produces elongate elongate concretions. concretions. VancouverVancouver Island Island marine marine sediments sediments contain contain several several different different concretion concretion types. types. These These includeinclude septarian septarian concretions concretions that that contain contain aa central central regionregion ofof mineral-filledmineral-filled fracturesfractures [[50].50]. More common are large ovoid concretions that lack a central nucleus. In other regions, similar concretions have been suggested to have resulted from a chemical self-organization process [47]. A third type has attracted the attention of paleontologists. These concretions, which are abundant, contain plant or animal remains as a nucleus for calcite crystallization. The morphology of fossil has a strong influence on the shape of the concretion. Geosciences 2021, 11, x FOR PEER REVIEW 13 of 16

More common are large ovoid concretions that lack a central nucleus. In other regions, similar concretions have been suggested to have resulted from a chemical self-organiza- Geosciences 2021, 11, 38 tion process [47]. A third type has attracted the attention of paleontologists. These concre-13 of 16 tions, which are abundant, contain plant or animal remains as a nucleus for calcite crys- tallization. The morphology of fossil has a strong influence on the shape of the concretion. Vancouver Island fossiliferous concretions are commonly zoned. They may have Vancouver Island fossiliferous concretions are commonly zoned. They may have smooth exteriors, but weathered, broken, or sawn specimens reveal distinct concentric smooth exteriors, but weathered, broken, or sawn specimens reveal distinct concentric layers (Figure 11). The initial cementation presumably resulted from the precipitation of layers (Figure 11). The initial cementation presumably resulted from the precipitation calcite cement as a halo around the buried wood. This type of concretion growth has been of calcite cement as a halo around the buried wood. This type of concretion growth has describedbeen described as “concentric” as “concentric” [51]. The [51 microenv]. The microenvironmentironment where concentric where concentric growth occurs growth mayoccurs have may been have mediated been mediated by sulfur-reducing by sulfur-reducing bacteria bacteria that were that nourished were nourished by degradation by degra- productsdation products of the original of the tissue, original as evidence tissue, asd evidencedby the common by the presence common of presencepyrite framboids of pyrite [52].framboids [52].

FigureFigure 11. 11. TransverseTransverse view view of ofpermineralized permineralized limb limb in calc in calcareousareous concretion concretion from from intertidal intertidal zone zone at Collishawat Collishaw Point, Point, Hornby Hornby Island. Island. (A) Exterior (A) Exterior view of view weathered of weathered concretion, concretion, showing showing whitish whitish scars leftscars by leftattached by attached barnacles. barnacles. (B) Polished (B) Polished sawn surface. sawn surface. Arrows Arrows mark the mark outer the boundary outer boundary of the con- of the centricconcentric zone zoneof . of calcification. The outer The outersurfaces surfaces of the of concretion the concretion are the are boundary the boundary limit for limit the for the pervasive phase of cementation. Note that the concentric zone is partially bleached near the fossil pervasive phase of cementation. Note that the concentric zone is partially bleached near the fossil wood. The darkest layer is carbon-rich bark. wood. The darkest layer is carbon-rich bark.

ConcretionaryConcretionary zones zones are are inferred inferred to to have have developed developed progressively progressively [47,53]. [47,53]. The The region region closestclosest to to the the fossil fossil nucleus nucleus commonly commonly shows bleachingbleaching (Figures(Figures4 4BB and and 12 12B).B). In In Vancouver Vancou- verIsland Island fossiliferous fossiliferous concretions, concretions, the the concretionary concretionary zone zone is commonlyis commonly surrounded surrounded by by an anouter outer layer layer with with a a sharply sharply defined defined contact.contact. The inner zone zone may may represent represent the the region region wherewhere calcite calcite cement cement precipitated precipitated because because of of a ageochemical geochemical gradient gradient that that was was created created by by degradationdegradation of of the the organic organic nucleus. nucleus. In In this this zone, zone, carbon carbon remobilization remobilization may may be be caused caused by by sulfate-reducingsulfate-reducing bacteria bacteria that that metabolize metabolize the the original original organic organic matter. matter. The The outer outer zone zone is is believedbelieved to to result result from from a aprocess process where where calcite calcite cement cement precipitates precipitates simultaneously simultaneously rather rather thanthan progressively. progressively. This This process process has has been been described described as as “pervasive “pervasive growth”. growth”. This This zone zone has has beenbeen suggested suggested to to result result from from methanogenic methanogenic microbes microbes [54]. [54 Accomplished]. Accomplished by members by members of theof domain the domain Archaea, Archaea, methanogenesis methanogenesis involves involves the anaerobic the anaerobic conversion conversion of decaying of decaying or- ganicorganic matter matter to yield to yield CO2 CO and2 andCH3 CH[54].3 [CO54].2 production CO2 production potentially potentially causes causes the local the pre- local cipitationprecipitation of calcium of calcium carbonate. carbonate. AlthoughAlthough zonation zonation is isa common a common feature feature of fossiliferous of fossiliferous concretions concretions at Vancouver at Vancouver Is- landIsland sites, sites, some some specimens specimens occur occur as asnuclei nuclei in inconcretions concretions that that have have homogeneous homogeneous form. form. TheThe external external shape shape is is commonly commonly determined determined by by the the internal internal fossil fossil (Figure (Figure 12). 12). Geosciences 2021, 11, x FOR PEER REVIEW 14 of 16 Geosciences 2021, 11, 38 14 of 16

FigureFigure 12. 12.LongitudinalLongitudinal view view of of fossil fossil wood inin an an unzoned unzoned concretion, concretion, Shelter Shelter Point. Point. 6. Conclusions 6. Conclusions Vancouver Island sedimentary rocks provide a rare opportunity to study fossil wood thatVancouver was preserved Island in sedimentary shallow marine rocks basin provide environments a rare opportunity that range in to age study from fossil Lower wood thatCretaceous was preserved to Paleocene. in shallow These marine fossil woods basin are environments of particular scientific that range interest in becauseage from they Lower Cretaceousare mineralized to Paleocene. with calcite. These They fossil are woods commonly are foundof particular in strata scientific that contain interest abundant because theyinvertebrate are mineralized fossils thatwith can calcite. be used They to determine are commonly geologic found age basedin strata on . that contain abun- dantPlant invertebrate and animal fossils fossils that are can preserved be used within to determine calcareous geologic concretions age in based detrital on siliceous biostratigra- phy.sediments, Plant and suggesting animal fossils that the are precipitation preserved of within calcite cementcalcareous was triggeredconcretions by substancesin detrital sili- released during the degradation of organic matter. The excellent anatomical detail in most ceousspecimens sediments, is a result suggesting of several that factors: the precipitation the mild geochemical of calcite conditions cement was that triggered favor calcite by sub- stancesprecipitation, released andduring a rate the concretion degradation formation of organic that allowed matter. woodThe excellent that exceeded anatomical the rate detail in mostof tissue specimens degradation. is a result The time of several required factor for growths: the mild of carbonate geochemical concretions conditions is variable, that favor calcitewith precipitation, rates that are determined and a rate byconcretion porewater formation flow rates that and CaCOallowed3 supersaturation. wood that exceeded Math- the rateematical of tissue calculations degradation. show The that time for a required typical slowly for growth flowing of porewater carbonate supersaturated concretions inis vari- 3 able,CaCO with byrates 10 ppm,that are concretions determined of 1 cm by diameterporewater can flow develop rates in and≈500 CaCO years3 [ supersaturation.55]. A much Mathematicallonger time wouldcalculations be required show to that produce for a thetypical final mass,slowly but flowing the early porewater stages of calcitesupersatu- precipitation may have been sufficient to permineralize wood that forms the concretion rated in CaCO3 by 10 ppm, concretions of 1 cm diameter can develop in ≈500 years [55]. A nucleus. much longer time would be required to produce the final mass, but the early stages of calciteAuthor precipitation Contributions: mayG.E.M. have made been petrographic sufficient to thin permineralize sections, obtained wood optical that and forms scanning the con- cretionelectron nucleus. photomicrographs, and wrote manuscript first draft. G.B. was responsible for field work, provided many of the study specimens, and prepared acetate peels. Both authors contributed to Authorwriting Contributions: and editing of G.E.M. the final made manuscript. petrographic All authors thin sectio havens, read obtained and agreed optical to theand published scanning elec- tronversion photomicrographs, of the manuscript. and wrote manuscript first draft. G.B. was responsible for field work, pro- videdFunding: many ofThe the authors study declarespecimens, no specific and prepared funding for acetate this project. peels. Both authors contributed to writing and editing of the final manuscript. All authors have read and agreed to the published version of Data Availability Statement: Specimens used in this study are archived in G.M.’s research collection the manuscript. at the Western Washington University Geology Department; they are available for study by other Funding:researchers The pendingauthors thedeclare end ofno the specific current funding Covid 19 for pandemic this project. restrictions. Data Availability Statement: Specimens used in this study are archived in G.M.’s research collec- tion at the Western Washington University Geology Department; they are available for study by other researchers pending the end of the current Covid 19 pandemic restrictions. Acknowledgments: The authors thank Vancouver Island collectors Gerald Cranham, Joe Morin and Rick Ross for their donation of specimen. Jim Haggart, Canada Geological Survey, provided helpful Geosciences 2021, 11, 38 15 of 16

Acknowledgments: The authors thank Vancouver Island collectors Gerald Cranham, Joe Morin and Rick Ross for their donation of specimen. Jim Haggart, Canada Geological Survey, provided helpful information regarding the biostratigraphy of Cretaceous and Eocene formations. Rick Ross and Mike Viney provided suggestions for the improving the manuscript first draft. Conflicts of Interest: The authors declare no competing interests.

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