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implications for calcium-rich permineralizations. ANDREW C. SCOTT & MARGARET E. COLLINSON. Geology Department, Royal Holloway University of London ...
Journal of the Geological Society, London, Vol. 160, 2003, pp. 857–862. Printed in Great Britain.

Non-destructive multiple approaches to interpret the preservation of plant fossils: implications for calcium-rich permineralizations A N D R E W C . S C OT T & M A R G A R E T E . C O L L I N S O N Geology Department, Royal Holloway University of London, Egham TW20 0EX, UK (e-mail: [email protected]) Abstract: Permineralized fossil coniferous woods from the Pliocene of Dunarobba, Umbria, Italy, and the Jurassic of Swindon, Wiltshire, England, were studied using non-destructive techniques on uncoated polished thin sections to elucidate their preservational history. Specimens were observed using transmitted light, polarized light, reflected light under oil, and cathodoluminescence. Selected areas were studied using a variable pressure SEM in backscattered electron mode. This allowed uncoated specimens to be examined and elemental distributions to be determined using an energy dispersive X-ray microanalyser. The data were used to interpret details of the permineralization history. Results reveal that anatomical interpretations based merely on observations of thin sections in transmitted light can be very misleading and could potentially affect the application of permineralized plant fossils, for example, in evolutionary biology, palaeoclimate analysis and isotope geochemistry. Keywords: fossil plant, preservation, permineralization, microscopy, microanalysis.

fossil record and widely applied in studies ranging from evolutionary biology, through palaeoclimate analysis to isotope geochemistry. The specimens for analysis were selected primarily because they were calcareous permineralizations apparently displaying excellent three-dimensional preservation of organic cells, as is shown by the apparent cell wall structures seen in transverse section in transmitted light (Figs 1a, i and 2a). This kind of preservation is essential for our aims to assess the effects of permineralization on the preservation of organic material. A secondary consideration in sample selection was to include samples from different environments and ages so as to make a preliminary comparative assessment of different preservational contexts.

For key biological innovations in plants to be investigated it is important to find plant fossils that yield anatomical data (Taylor & Taylor 1992). Our extensive understanding of the evolutionary history of many plant groups (e.g. lycopsids, Bateman et al. 1992) relies heavily on anatomical data from permineralized or petrified fossils. For these applications in evolutionary biology an understanding of the permineralization process is important to interpret which original organic material and anatomy may be preserved faithfully, variously modified or totally lost in the fossil. Understanding of permineralization is also necessary to ensure the validity, comparability and repeatability of results from applied studies of permineralized fossils such as in palaeoclimate analysis or isotope geochemistry. In permineralization, minerals (often silica, pyrite or carbonates such as calcite) occupy former cell lumina whereas organic cell walls remain. Subsequently, in petrifaction (sensu stricto) the organic cell walls may decay and be replaced by another phase of minerals, usually with a similar chemical composition to the first phase (Scott 1990). Considerable progress has been made in our understanding of both silicification and pyritization of plant fossils (Kenrick & Edwards 1988; Trewin 1996; Jones & Rowe 1999; Boyce et al. 2001; Dietrich et al. 2001; Grimes et al. 2001, 2002). However, carbonate permineralization (e.g. calcite) is amongst the most common preservation state. Most studies of carbonate permineralized plants have been undertaken on peel sections (Jones & Rowe 1999) with the loss of mineralogical detail because the mineral is dissolved to make the peel. Petrographic and chemical studies have shown that there is a wide range of carbonate mineralization depending on the depositional and diagenetic environment (Scott 1990; Scott et al. 1996, 1997; DeMaris 2000). However, we are not aware of any studies that have used multiple techniques in combination to investigate the details of calcium-rich permineralizations. In this paper we aim to demonstrate the understanding of mineralization that can be gained by applying multiple, nondestructive, analytical approaches to the same group of cells in a polished thin section of a permineralized plant fossil. We have selected wood as our sample material as this is abundant in the

Materials Pliocene, Dunarobba, Italy The Dunarobba Fossil Forest is a late Pliocene fossil forest of taxodiaceous conifers preserved in the Fosso Bianco Formation at Dunarobba, Umbria, Italy (Scott & Freedberg 2000). Scattered woods, both branches and small trunks, as well as leaves occur here, and include numerous specimens that Ravazzi & Van der Burgh (1994) identified to the conifer Glyptostrobus. The specimens used in this study (Dunacs1 and Dunacs2) were collected (by A.C.S.) in situ in clays at the top of the site above the main fossil forest as shown byScott & Freedberg (2000, figs 18 and 20). The enclosing sediments were silty muds with abundant plant debris.

Jurassic, Swindon, UK A number of wood specimens were collected by J. K. Wright from marine sediments (Bed 4) of the Ampthill Clay from South Marston, Swindon, Wiltshire, England, of Mid-Jurassic age (Oxfordian–Kimmeridgian boundary) (Wright 2003). Bed 4 comprised medium grey fossiliferous mudstone with limonitic ooids and phosphatic nodules. Specimen MK1 was used in the present study. 857

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Fig. 1. Transverse sections of permineralized wood from Dunarobba. (a)– (h), (p)– (u) Dunacs1; (i)– (o) Dunacs2. (a) Transmitted light (note smaller early wood cells and larger late wood cells). (b) Reflected light under oil (note complex layering of white and brown minerals). (c) Backscattered electron image. Increasing brightness indicates higher atomic number (black, carbon; orange, calcium; yellow, iron). (d) Carbon map (note little carbon remains in cell walls). (e) Iron map (note concentrations in cell walls). (f) Calcium map (note concentration in cell lumina). (g) Cathodoluminescence micrograph showing dull luminescence and two phases of calcite precipitation. (h) Stained section in transmitted light with blue staining indicating ferroan calcite. (i) Transmitted light. Cells at the bottom right are mineralized and smaller cells at the top right show no mineralization. (j) Transmitted light, crossed polars. Unmineralized cells at the top right show birefringence and the resin groundmass shows patchy extinction. (k) Reflected light under oil showing the mineralized cell walls as white. (l) Backscattered electron image (see (c)). (m) Carbon map (note little carbon in walls). Large area of green indicates mounting resin. (n) Iron map (note lack of iron in unmineralized cells). (o) Calcium map. (p)–(s) Polished slice. (p) Backscattered electron image. (q) Iron map. (r) Carbon map (note remnant carbon in some cell walls). (s) Calcium map. (t) Backscattered electron image showing position of line scan. (u) Line scan showing elemental distribution.

Analytical approaches Polished and petrographic thin sections Polished thin sections were produced using epoxy resin as an embedding and mounting medium with a thickness of 30 ìm,

measured by micrometer and polished using 0.05 ìm aluminium oxide so that no scratches remained. For each specimen studied all observations on polished sections (see below) were made on exactly the same area of cells on the same single section. Separate petrographic thin sections (taken adjacent to polished

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Fig. 2. Transverse sections of permineralized wood from Swindon. (a) Transmitted light. (b) Backscattered electron image. (c) Carbon map. (d) Sulphur map. (e) Calcium map. (f) Phosphorus map. (g) Backscattered electron image showing position of line scan. (h) Line scan showing elemental distribution.

sections) were stained with potassium ferricyanide–Alizarin Red S using the technique of Dixon (1966). Polished slices were used to confirm the presence of carbon (which is present in the mounting medium for the thin sections).

Light microscopy: transmitted, polarized and reflected light The polished thin sections were examined using a Nikon Microphot microscope in transmitted, plane- and cross-polarized and reflected light. Reflectance observations were made using a 320 objective lens under oil (Cargylite immersion oil type B2, with a refractive index of 1.524). Areas selected were photographed using 100ASA colour print film and marked by a small self-adhesive arrow to allow relocation.

Cathodoluminescence The polished thin sections were examined using a Technosyn II cathodoluminescence unit connected to a Nikon Microphot microscope with a 310 objective lens. Images were captured digitally and processed using image capture software Image ProPlus v.4.5 developed by Media Cybernetics and an enhancement program by Porocity Imaging Co. Image capture was for 45 s.

Scanning electron microscopy: elemental distribution, occurrence and quantity The uncoated polished thin sections were studied using a Hitachi S3000N variable pressure SEM with an Oxford/Link energy dispersive X-ray microanalyser. The microscope was operated at a pressure of 70 Pa using a working distance of 15 mm and accelerating voltage of 20 kV. In the variable pressure mode images were obtained using backscattered electrons, which reflect differences in atomic number. Images were captured and processed digitally and the backscattered electron image was thermally coloured so that dark areas represented the element of smallest atomic number and red, orange and yellow represent elements with progressively higher atomic numbers. A qualitative elemental analysis was undertaken across the polished section so that common elements could be identified. Element distributions were then recorded as (1) line scans giving detailed relative variation across individual cell walls and cell lumina and (2) area maps (100 frames) showing the overall distribution across a group of cells. Area maps were coloured to distinguish element concentrations (brighter colours indicate higher concentration; no colour indicates absent) and saved digitally as tif files. Quantitative analysis of elements on selected areas was also made following calibration using a cobalt standard.

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Results In transmitted light the specimens show a pattern in transverse section that could be interpreted readily in terms of expected wood structure as cell walls (brown and almost opaque) and cell lumina (pale and translucent) of the tracheids (and rarer ray cells) of a coniferous wood (Figs 1a and 2a). The weight and hardness of the hand specimens indicates mineralization, leading to an initial interpretation that the specimens are probably simple permineralizations with cell lumina infilled with a clear mineral such as calcite (calcium carbonate) and with organic cell walls retained.

Pliocene Dunarobba; Dunacs1 ‘Apparent cell lumina’. The pale translucent cell infill seen in transmitted light shows high birefringence colours in polarized light, suggesting that it is calcite, and stains a blue colour (Fig. 1h), showing that it is ferroan calcite. The lumen infill is distinct with a moderate atomic number (Fig. 1c, orange) and elemental analyses showing a high concentration of calcium (Fig. 1f and s) and a low concentration of iron (Fig. 1e and q) entirely consistent with ferroan calcite. Quantitative analysis of an uncoated polished slice showed the cell fill having 12% C, 25% Ca, 11% Fe and 52% O. The specimen shows dull luminescence (Fig. 1g) but bright layers suggest that the calcite fill was deposited in more than one phase. ‘Apparent cell walls’. The dark cell outlines seen in transmitted light reveal multiple layering in reflected light (Fig. 1b). Two colours, white and reddish brown, are evident, with white nearer the cell lumen. The cell-to-cell transition can be complex, suggesting separation of wall layers, intercellular spaces or sections through areas of pits on the radial walls. The white layers are distinctive with high atomic number (yellow, Fig. 1c), high concentrations of iron (Fig. 1e) and little or no calcium (Fig. 1f). The reddish brown layers (Fig. 1b) have low atomic number (black, Fig. 1c), lack iron and calcium (Fig. 1e and f) but sometimes show concentrations of carbon in elemental map (Fig. 1d), confirmed very clearly in line scan (Fig. 1t and u). The ‘apparent cell walls’ have therefore been shown to be complexes, with a thick inner layer (nearest the lumen and most of the apparent wall thickness) made of an iron-rich mineral and a thin outer layer. The iron-rich mineral layer itself exhibits some layering variation as a thin redder area within the yellow (Fig. 1t) and as slight calcium peaks within the calcium troughs (Fig. 1u). The thin outer layer (usually shared between two adjacent cells) may include original organic cell wall or may be space into which other carbon-containing substances have penetrated (sedimentary material or mounting–embedding medium). Elemental mapping of an unmounted polished slice (Fig. 1p–s) confirmed the presence of residual carbon in the position of the cell walls (Fig. 1r). A quantitative analysis shows that the iron-rich layer has 11% C, 13% Ca, 24% Fe and 50% O whereas the thin outer layer, which may include part of the original cell wall, has 24% C, 13% Ca, 20% Fe and 42% O.

Pliocene Dunarobba; Dunacs2 There are two main types of preservation in different parts of this specimen, lower left and upper right (Fig. 1i). The lower left part is mineralized and very similar to Dunacs1 fully described above. Slight differences include a more obvious iron content in some cell lumina (Fig. 1n) and possibly higher calcium content in the

iron-rich wall layer (Fig. 1n and o), which also shows a fibrous structure (Fig. 1k). The upper right part of this specimen contains no mineral, as shown by low atomic number (Fig. 1l), absence of iron and calcium (Fig. 1n and o) (and also absence of other elements such as Si, Mg, Mn, P, etc., not shown), and high abundance of carbon (Fig. 1m). The embedding medium shows patchy grey extinction (Fig. 1j). The cell walls in the upper right area are pale yellow– brown in all LM images and show distinct layering but with no suggestion of mineral content in cross-polarized light. These walls are interpreted as original organic cell walls. The wall thickness is very similar to that of the thin outer (furthest from cell lumen) non-mineralized wall layer in mineralized material (see Dunacs1 above). In contrast, the overall cell size differs by a factor of two or more. The elemental map for carbon (Fig. 1m) is not able to distinguish carbon in these original cell walls from surrounding carbon-rich material (mounting–embedding resin). However, elemental mapping of an unmounted polished slice confirmed the presence of residual carbon in the position of the cell walls.

Jurassic Swindon Unexpectedly, elemental analysis of this specimen shows almost identical distributions for calcium (Fig. 2e) and phosphorus (Fig. 2f). We interpret this as mineralization by calcium phosphate, which has infilled most cell lumina, formed a layer round the edge of otherwise empty cell lumina and infilled thin layers (?intercellular spaces) between cells. Carbon (Fig. 2c) occurs in areas lacking calcium and phosphorus. The carbon infilling some cells is the embedding resin and this was confirmed by examining a thin polished slice. The cell pattern is indistinct in elemental maps and the backscattered image (Fig. 2b) because much of the apparent cell wall (brown in transmitted light, Fig. 2a) is also represented by calcium phosphate. At high magnification ‘cell walls’ exhibit layering (lighter vs. darker in Fig. 2a and black vs. orange in Fig. 2h) with varying proportions of carbon and calcium phosphate distinguishing the layers. The rays contain concentrations of iron pyrites (black mineral, Fig. 2a) exhibiting high sulphur (Fig. 2d and g) and iron (Fig. 2g).

Discussion Dunarobba mineralization The Dunarobba specimens show permineralization by a combination of iron-rich minerals (iron carbonates or oxides including siderite and goethite) and ferroan calcite. The deposition of these minerals is largely controlled by Eh and pH (Tucker 2001), which may be affected by a number of factors including the concentration of carbonate ions and the amount of organic matter and its decomposition (Pettijohn 1975; Tucker 2001) (see also calcium carbonate–calcium phosphate switch (Briggs & Wilby 1996;Sagemann et al. 1999) and the pyrite depositional system (Grimes et al. 2001, 2002)). In general, woods are chemically composed of cellulose, hemicelluloses and lignin. It is well established that celluloses break down before lignin (Hatcher & Clifford 1997) and their breakdown yields CO2 and CH4 ; thus organic matter decomposition may provide a reducing environment (oxic to anoxic) and in the absence of sulphur may be post-oxic and methanic (Tucker 2001). In such circumstances, in the presence of iron, siderite (iron carbonate) is commonly precipitated. Chemical analysis of some woods from Dunarobba indicated a significant loss of

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cellulose (Staccioli et al. 1996). Despite this chemical loss, cell walls usually remain coherent (Hatcher & Clifford 1997). It is also possible that, as has been observed with leaves, the precipitation of iron oxides on the surface of organic matter, as biofilms, was initiated by bacterial decay produced CO2, which rapidly adsorbs metal ions (Dunn et al. 1997). Siderite is prone to solution and redeposition (James 1966) so iron oxides can be a recent weathering product of siderite (Tucker 2001). The fibrous structure (Fig. 1k) in Dunacs2 is commonly seen in sphaerosiderite (Tucker 2001). The cell lumina infills are predominantly of ferroan calcite. Calcite may precipitate in preference to siderite where the pH increases from seven to eight (Pettijohn 1975). If, however, there is insufficient Fe2þ relative to Ca2þ then ferroan calcite may form in preference to siderite (Tucker 2001). Ferroan calcite deposition has occurred only in cells where there has also been an iron-rich mineral layer deposited adjacent to the cell lumen infill and this may relate to local pH. The fact that there is variation in permineralization within one specimen (shown here) and that not all Dunarobba woods are permineralized (Scott & Freedberg 2000) suggests that mineral precipitation is strongly influenced by very local changes in Eh and pH. There is major natural variation in lignification between cells and between secondary wall layers of woods (e.g. Donaldson 2002), which, given differential decay of celluloses vs. lignin, provides great potential for subcellular- and cellularscale variations in Eh–Ph conditions during wood decomposition. We conclude that the breakdown of original organic cell walls may have locally changed the Eh–Ph conditions of the pore waters to encourage mineral precipitation.

Swindon mineralization This wood is dominated by calcium phosphate, partially (as a lining) or entirely infilling cell lumina and in thin layers (?intercellular spaces) between cells. It is well known that bacterial decay of organic matter in sediment liberates phosphate and bone is also a potential phosphate source (Tucker 2001). Phosphate may also replace calcite or be precipitated instead of calcite in neutral or slightly acidic pH conditions (Briggs & Wilby 1996; Sagemann et al. 1999). Variation in phosphatization, therefore, is likely to be related to Eh–pH variations discussed above (Briggs & Wilby 1996; Sagemann et al. 1999). Pyrite appears to be mainly restricted to the ray cells. This may reflect locally very distinct Eh–pH conditions (Grimes et al. 2001, 2002) created by decomposition of living ray parenchyma cell contents, as pyrite forms only in partially anoxic environments.

Organic vs. inorganic preservation and significance for cell size and wall thickness Taking account of the crack between mineralized and nonmineralized areas and the way in which cells seem to have originally fitted together across this crack in Dunacs2 (Fig. 1i–o) we suggest that the organic-walled cells have shrunk, possibly through dehydration and cellulose loss. This is supported by the fact that shrinkage of lignified material on fossilization is known elsewhere (e.g. organic seeds within the late Eocene insect limestone, UK, Collinson, pers. observation). Furthermore, if mineral deposition inside the mineralized cells had caused differential expansion one would expect very distorted or ruptured cell outlines and these are not observed. Therefore, original cell size is thought to be reflected in mineralized cells, if measured from lumen centre to lumen centre. The thickness of

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the iron-rich mineral layer is very variable so that ‘apparent’ wall thickness measurements would be very different from those of the original wood. The differences in the thickness of the cell walls of iron- and non-iron-impregnated cells needs further consideration, as this factor will affect, for example, the measurements used for palaeoclimatic interpretations. Equally dramatic is the preservation of the Swindon wood by calcium phosphate. The paucity of carbon in the cell walls was totally unexpected and we have no way of knowing the original dimensions of non-permineralized organic cell walls or cells.

Wider implications Recognition of diagnostic anatomical structure, which requires preservation of original organic tissues, underpins accurate systematic determination of permineralized plant fossils. In woods cell sizes and cell wall thicknesses and their variations can be applied both in systematic studies and in palaeoclimate analyses. Organic components can be extracted from permineralized fossils, carbon-isotope data can be applied for isotope stratigraphy, palaeoatmospheric interpretations or assessment of photosynthetic pathway, and biomarkers may be used in systematic studies. Our observations show that, in striking contrast to our expectations, the majority of the structures that a superficial examination in transmitted light suggested were cell walls in all our permineralizations are not original organic wall layers but are in fact layers of minerals. Although carbon is proven in elemental maps of polished slices (Fig. 1r) it does not produce a very clear map of the cell outlines and is present in very small amounts. Overall it is not clear which, if any, of the original organic cell wall layers remain in the permineralizations. In the Dunarobba specimens carbon is present in both layers of the ‘apparent cell wall’, one of which is dominantly an iron mineral. Therefore at least some of the carbon may have been carried into the specimens during the permineralization process. These observations show that an understanding of the mineralization is a prerequisite for the use of permineralizations for all the applications above, which are underpinned by the potentially false assumption of preservation of original cell walls. Furthermore, the variation in mineralization that we have encountered in just two specimens also indicates that it should never be assumed that any two plant permineralizations are directly comparable in terms of their mineral composition or their potential for preservation of important anatomical or organic geochemical information. In addition to the surprising results from ‘apparent cell walls’ we encountered an unexpected range of mineralization. Instead of the expected calcite, one of our specimens is a calcium phosphate permineralization and the other a combination of ferroan calcite with an iron oxide or carbonate. The abundance of permineralized wood fragments in the fossil record and the variation within and between specimens, combined with the complexity of factors controlling mineral precipitation, provides future potential to apply permineralized wood fossils to help interpret sediment geochemistry.

Conclusions Multiple, non-destructive microscopic and analytical techniques on uncoated polished thin sections have enabled us to elucidate the mineralization of permineralized fossil coniferous woods from the Pliocene of Dunarobba, Umbria, Italy (from continental sediments) and the Jurassic of Swindon, England (from marine sediments). In thin section, using transmitted light, both woods appeared to be simple calcareous permineralizations with organic

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cell walls preserved and cell lumina filled with calcite. In reality, the Italian Pliocene woods are mineralized with ferroan calcite infilling cell lumina and iron-rich minerals (siderite, goethite) forming ‘apparent cell walls’. In some areas where calcite had not filled the cell lumina the organic walls remained. In the Jurassic wood both cell lumina and cell walls are mainly represented by calcium phosphate. Both woods indicate the importance of organic wall breakdown to initiate the precipitation of the mineral phases likely to be controlled by localized (sub-)cellular-scale pH–Eh conditions in the wood. The results illustrate the range and complexity of the permineralization process in calcareous specimens. The combined evidence from multiple non-destructive approaches on the same set of cells has been particularly valuable in revealing the complexity of layers present in the ‘apparent cell walls’ of the Dunarobba specimens and leading us to the interpretation of these as dominantly mineral in composition. Our observations show that an understanding of the mineralization is a prerequisite for the use of permineralizations for any applications that require the (potentially false) assumption of preservation of unaltered organic cell walls. We thank N. McGilp and C. Gummer of Proctor and Gamble (Health and Beauty) for sponsoring this work and for providing a grant for the colour plates. We thank the Royal Collection Trust, Proctor and Gamble (Health and Beauty) and the Italian Government for financial support for field work (to A.C.S.). We thank J. Wright and S. Howard for preliminary observations on the Jurassic Woods and N. Holloway for slide preparation. We thank members of the Electron Microscope Unit of Royal Holloway for their support and the Science Faculty of Royal Holloway University of London for funding the SEM work.

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Received 13 December 2002; revised typescript accepted 28 March 2003. Scientific editing by Jane Francis