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SILICIFICATION! SUSAN H.! BUTTS Yale University, Peabody Museum of Natural History, 170 Whitney Ave., P.O. Box 208118, New Haven, CT 06520-8118 USA

ABSTRACT.—Silicification is the replacement of original skeletal material accomplished through the concurrent dissolution of calcium carbonate and precipitation of silica. The processes is aided by the nucleation of silica to organic matter which surrounds the crystallites within the shell. Factors that control silicification are those that influence the dissolution/precipitation process: shell mineralogy, shell ultrastructure (and, therefore, surface area), the amount and location of organic matter, and the character of the enclosing matrix. Silicification, like all types of fossilization, can produce taphonomic biases: it is far more common in than younger deposits, is more likely to occur in organisms with low- magnesium shells, in carbonate , and in environments with elevated dissolved silica.

INTRODUCTION paleoecological studies because silicification, ! except in rare occurrences, introduces a Silica—dissolved, and as amorphous or taphonomic bias. Silicification is biased by the m i c r o q u a r t z ( o p a l , c h a l c e n d o n y, a n d organism itself (composition, texture, and organic microcrystalline phases), and crystalline content of the shell) and by local depositional megaquartz—is a common component in controls (matrix and pore- ). sediments, and is found in solution and as There is a temporal pattern of silicification—it is nodules, , and clastic material. more common in Paleozoic rocks than younger Fossilization involving silica occurs by three rocks—but it is also strongly linked to local methods: permineralization, entombment, and factors (i.e., high dissolved silica levels associated replacement. Permineralization is the precipitation with volcanic deposits). Linking that pattern to of silica within the natural cavities of porous environmental factors is problematic, but the materials, such as wood and , from silica- decline in post-Paleozoic silicification can be enriched percolating fluids. Silica entombment linked to a shift in the proportion of calcitic to mainly occurs in hydrothermal settings by aragonitic marine organisms. precipitation of silica on the external surfaces of Exceptional preservation with silica organic objects. Silica can precipitate in molds replacement is concurrent with degradation of created by dissolution, but replacement, or organic matter, which creates the active sites silicification sensu strictu, is the concurrent necessary for silica nucleation and provides the dissolution of original material and framework to preserve the ultrastructure of precipitation of silica: this is the primary focus of skeletal material. The presence and location of this . Silica replacement affects shells at the organic matter and the solubility of the mineral exclusion of the matrix, indicating an organismal phase—factors that are controlled taxonomically genesis of the process. It was suggested in Newell —would have the greatest impact on the et al. (1953, p. 173), one of the first discussions of propensity for and fidelity of replacement, skeletal silica replacement in the literature, that assuming a sufficient source of dissolved silica is shells were “for some unknown reason, centers of present. Recent experimental permineralization of acidification, while the surrounding matrix wood (Akahane et al., 2004; Ballhaus et al., 2012) remained at a higher pH level.” gives tremendous insight to the process and Silicified are ideal for taxonomic geochemical controls that can be applied to the studies because they can be extracted without study of silicification in skeletal material. damage using (hydrochloric or acetic). Silicified fossils are fairly common and are However, care must be taken with diversity and found worldwide in rocks of all ages. In: Reading and Writing of the Record: Preservational Pathways to Exceptional Fossilization. The Paleontological Society , Volume 20, Marc Laflamme, James D. Schiffbauer, and Simon A. F. Darroch (eds.). The Paleontological Society Short Course, October 18, 2014. Copyright © 2014 The Paleontological Society. THE PALEONTOLOGICAL SOCIETY PAPERS, V. 20

ultrastructural detail due to the neomorphism of silica to megacrystalline quartz. Many of the well-known silicified faunas were deposited in tectonically active basins associated with ash-producing volcanism, an association that has been discussed peripherally in papers on silicified faunas and . In New York, the upper to Acadian foreland basins (Ver Straeten, 2009) have silicified fossils in rocks containing tephras of clay-rich K-bentonites (Ver Straeten, 2009). The Antler orogeny and associated volcanism correlates to silicified faunas in the of Idaho, Montana, and Wyoming (Schmitt and Boyd, 1981; Daley and Boyd, 1996; Butts, 2007). In Gotland, Sweden, Silurian silicification has been ascribed to volcanism (tuffs, lava, rhyolites) FIGURE 1.—Examples of silicification from the in the Caledonian geosyncline (Laufield and of West Texas. A) Hustedia sp. (YPM. Jeppsson, 1976). Laufield and Jeppsson (1976) 175528), showing silicified spiralium. B) Juvenile discussed the publication history of occurrences Echinaurus sp. (YPM.205721). C, D) Stenocisma sp. of altered layers in association with (YPM.205615); (C) exhibiting partial silicification; silicified beds. K-bentonites are also prevalent in (D) magnification of the umbonal region showing the Permian basins of West Texas (Nicklen, 2003) polygonal structures interpreted to be silicified organic where silicified faunas are common. sheaths surrounding the (dissolved) crystallite. All Silicification has the ability to provide a high- specimens from the Yale Museum, Division of Invertebrate Paleontology. fidelity snapshot of life on Earth when it is pervasive. For example, beds in the Silurian of Silicification controls are more critical on a local Gotland, with complete infiltration of matrix and scale than a regional or temporal scale. taxa with chert, record diversity with exceptional Perhaps the most well-known of all silicified clarity (Laufield and Jeppsson, 1976). Records of fossils (Figure 1) are those of the Permian basins bias are scattered throughout the literature in of West Texas (especially the Midland and notes on preservation in taxonomic treatments of Delaware basins), studied extensively by Newell, faunas, but are often anecdotal, rather than Cooper, and Grant in the Guadalupe Mountains, statistical. The accounts can be parsed into two Mountains, and adjacent ranges (Newell et main categories: taxonomically and lithologically al., 1953; Cooper and Grant, 1972). The induced bias. Sometimes, both factors play a role silicification process was discussed in Newell et in susceptibility to silicification (Newell et al., al. (1953) and Cooper and Grant (1972). Newell 1953; Butts, 2007). Examining the bias recorded et al. (1953) described the variation in in the fossil record allows for interpretation of silicification, stating that most of the silicification how the controls may have operated. is ‘spongy and fragile’ (partial) and ‘hollow’ (non- ! pervasive) (parentheses use terminology of Butts EXTRACTION AND PREPARATION and Briggs, 2011; a scheme for descriptive ! characterization of silicification can be found Extraction of silicified faunas is a simple process therein). Many exhibit beekite rings, concentric that requires few resources and one cubic foot of rings of silica that likely form in times of limited may provide thousands of exceptionally or fluctuating silica supply. Specimens appear to preserved fossils. Large blocks of rock can be be perfectly preserved to the naked eye, but when etched and fossils freed of matrix using examined in petrographic thin sections and under hydrochloric (15% or lower) or acetic acid, SEM, the silicification in the Glass Mountains using safety precautions for strong acids. First, material is of relatively low fidelity. Except in one face of the rock that is to be used as a base is rare occurrences when silicification is partial, painted with latex paint, which is acid-resistant shells of and bivalves lack and allows the block to acidize from the sides and top only to prevent crushing of specimens as

16 BUTTS: SILICIFICATION

purple, while quartz (generally as cryptocrystalline chalcedony that has replaced the skeletal fabric) alternates between first-order red, !yellow, and blue (Daley, 1987). !SILICIFICATION CONTROLS AND BIASES A simplified process for silicification involves the concurrent dissolution of calcium carbonate and precipitation of silica. These processes are controlled intrinsically and extrinsically by taxonomic and lithologic conditions that affect the rate of organism decay and dissolution of skeletal material, and the precipitation of silica (see Butts and Briggs, 2011). Early diagenetic chert deposits of the were dictated by permeability, silica availability, and concentration of organic matter (Knoll, 1985) and these criteria are the main sedimentary controls on silicification through the Phanerozoic. A taphonomic bias can be introduced in the process, but the dearth of studies devoted to silica replacement (as opposed to brief mentions in , sedimentology, or papers) allows for conjectural !interpretation of how the controls lead to biases. Taxonomic controls and biases Organismal control of silicification is evident in the simple fact that silicified fossils can be FIGURE 2.—Processing of silicified fossils using hydrochloric acid. A) Immersion of rock into acid bath. found in otherwise unaltered rocks. Silicification B) Fossil residue on screen in colander after acid can vary by taxon, presenting a significant bias in etching is complete. interpretation of fossils. Marine organisms have a range of skeletal mineralogies, bound together by matrix is removed. The rock is placed painted- varying amounts of organic matrix, in a range of side down on a nylon window screen and inserted ultrastructural configurations that create the in a bucket with drainage holes (or into a sieve or skeletal component of the organism. These three colander), then immersed in an acid bath (Fig. factors (mineralogy, distribution of organic 2A). Occasionally, a rock requires very gentle matter, and ultrastructure) are the taxonomic shaking if the rock contains fine-grained controls on the dissolution of calcium carbonate siliciclastic material or dolomitized matrix. After and precipitation in silica. the rock is digested, the acid bath is replaced with Shell mineralogy.—The rate and conditions gently flowing water and rinsed for several hours. for dissolution of carbonate are well known The bucket, sieve, or colander is extracted (Fig. (Walter, 1983, 1985; Walter and Morse, 1984, 2B), and the screen is very gently lifted and 1985). There are three common biogenic spread out on a table by two people holding the carbonate phases: high-magnesium calcite (HMC corners. Live-insect forceps are used to pick >8.5 mol% MgCO3), low-magnesium calcite specimens. More elaborate contraptions for acid (LMC <4 mol% MgCO3), and aragonite (Chave, etching and storage of individual specimens are 1954a). Skeletal mineralogy is taxonomically described in Cooper and Grant (1972). controlled, and organisms may be constructed of In thin section, silicified skeletal material is more than one type of mineral. There are best observed under crossed nicols with insertion additional variations in the content based on of the first-order red (gypsum) plate (Scholle and latitude (Chave, 1954a), position within the shell Ulmer-Scholle, 2003). As the stage is rotated, (Buening and Carlson, 1992), and the ambient sea carbonate changes between first-order white and water (Stanley and Hardie, 1998). Dissolution is

17 THE PALEONTOLOGICAL SOCIETY PAPERS, V. 20 affected by the stability of the calcite phase, the material and crystalline and/or calcite saturation state of the pore water solution, and the (Carter, 1990). reactive surface area of the shell (review in Shell ultrastructure dictates surface area Morse, 1983; Walter, 1985). Solubility is available for dissolution of skeletal material. influenced by variations in seawater Mg/Ca, and Early experiments in controls on dissolution were the relative solubility of these phases has varied conducted with carbonate material free of organic through time (Ries, 2010; references therein). matter, but recent studies have also used complete HMC is more stable than LMC, which is more skeletal material or bioclasts with the organic stable than aragonite in seawater with a Recent component intact, which show that increased Mg/Ca ratio (Chave et al. 1962; Berner, 1975), surface area of the crystallites results in an although in some studies, HMC and aragonite increased dissolution rate of skeletal material dissolution rates are equivalent (Berner et al., (Chave 1954b; Henrich and Wefer, 1986; Glover 1976; Walter and Morse, 1985). The and Kidwell, 1993; Harper, 2000). crystallography of aragonite, a calcite polymorph, Location and abundance of organic matter.— is influenced by the minor presence of large Abundance and location of organic matter in cations, notably Mg (Folk, 1974). It is stable at extinct organisms is not always obtainable, but it higher pressures and lower temperatures than influences the geochemical microenvironment in calcite, but is less stable at atmospheric ways that influence silicification. There are conditions, and neomorphoses to more stable significant variations in mineralogy, calcite (Klein et al., 1993), making it increasingly microstructure, and the location and content of less common in rocks of increasing age (Land, organic matter between different taxa (e.g., 1967). However, original aragonite is known in brachiopods and mollusks) and within taxonomic shells as old as Devonian, and aragonite groups (e.g., orders) and between preservation is ubiquitous in some Paleozoic units shell layers in a single taxon, especially with (e.g., the Upper Carboniferous Breathitt multiple ultrastructures (Fig. 3). Skeletal organic Formation of Kentucky; Brand, 1983). Due to the matter varies by organism, and occurs in very high solubility of aragonite, dissolution of shell minute amounts within the crystallites material frequently occurs prior to burial, a point (intracrystalline) and surrounding the crystallites that will be discussed later in terms of secular (intercrystalline) (Crenshaw, 1982). variation in ocean geochemistry and dissolution Organic matrix is secreted by the organism of carbonate bioclasts. along the growing edge of the shell. The Shell ultrastructure.— is composition of the matrix serves as a nucleating mediated by the organic matrix which, by agent and controls for the calcium carbonate controlling parameters such as saturation, ionic polymorph by regulating the shape and strength, pH, surface energies, and character and orientation of the crystals. Since organic matter quantity of active sites, results in the precipitation and biomineralization of marine organisms is a of a mineralized skeleton or shell of an organism study in and of itself, two morphologically similar (Crenshaw, 1990). Skeletal biomineralization and and commonly fossilized organisms are ultrastructure varies taxonomically, and is compared: brachiopods and bivalves. ‘Articulate’ thoroughly characterized in Carter (1990) for both brachiopods (including rhynchonellates and invertebrates and vertebrates. Brachiopods have a strophomenates, as discussed in the ultrastructure primary layer or granular calcite in a matrix of section) have less than 5% of total weight glucosaminoglycans, an organo-crystalline (periostracum, soft-tissue, and shell) in organic secondary layer, and less commonly, a tertiary matter (Jope, 1965), but may have 40–50% of prismatic layer (Williams, 1997). In some their organic material contained within the shell organisms, ultrastructure is taxonomically (Curry and Ansell, 1986; Curry et al., 1989). uniform. For example, the two most abundant Features of brachiopod shells, such as spines and classes, and , have punctae, contain additional organic material fibrous and cross-bladed laminar ultrastructures (Williams, 1997), and one modern punctate (Fig. 3) in the secondary layer, respectively. brachiopod (Liothyrella uva) has up to 75% of the Mollusks have at least nine different organic material in the shell (Peck et al., 1987). ultrastructures (i.e., nacreous, foliated, prismatic; Linguliform brachiopods have very high organic Fig. 3) that vary taxonomically, but all have one shell content (Cusack, 2001). Bivalve mollusks or more ultrastructure of multilayered organic have 15–30% of total weight as organic matter

18 BUTTS: SILICIFICATION

Strophomenate and rhynchonellate brachiopod and bivalve shell microstructures Brachiopod - S trophomenata Brachiopod - Rhynchonellata Bivalve cross-bladed laminar fibrous periostracum calcitic regular simple prismatic primary layer calcitic homogeneous

secondary layer calcitic fibrous prismatic calcitic laminated

F F foliated PUNCTAE PUNCTAE F nacreous absent present absent present semi-nacreous Billingsellida Protorthida Orthida: (laminar/fibrous) Dalminellidina Productida Orthida: aragonitic fibrous prismatic Orthidina Orthotetida : crossed lamellar or Pentamer ida* Retziidina complex crossed lamellar Rhynchonellida Spiriferinida Microstructures vary taxonomically, but are all multilayered organic material and crystalline Atrypida Thecideidina* aragonit e and/or calcite, with one or more KEY common textures described above Athyridida: Terebratulida B rachiopod orders with extant Athyrididina representatives are in bold text Bivalve Koninckinidina variations: organic periostracum periostracum F acicular or granular F outer shell layer Rhynchonellate secondary variations: la yer reduced F F crystallite in F organic sheath or absent Living theceidines F F middle shell layer (arag) prismatic F F F Living terebratulids prismatic inner shell layer (arag) cross-bladed laminar Some fossil spiriferides tertiary layer F F F and pentamerides Argopecten irradians Streblochondria stantonensis * with some exceptions Rec. (right valve) U. Carb. (left valve)

FIGURE 3.—Shell structures of the two most abundant classes of brachiopods, Rhynchonellata and Strophemata, and generalized molluscan ultrastructure. Compiled from Carlson and Leighton (2001) and Carter (1990). (periostracum, soft-tissue, and shell) (Peck et al., would be considered intercrystalline, as defined in 1987; Peck, 1993). Mollusk shells are 95% to Crenshaw, 1982) can serve as a scabbard 99% CaCO3 and 1% to 5% organic matrix, which protecting the crystallite from dissolution (noted is a mix of proteins, glycoproteins, proteoglycans, by slow dissolution in sterile environments). chitin, polysaccharides (Marin and Luquet, 2004), However, at a certain point, the decay of organic and 16 known molluscan shell proteins (Keith et material initiates conditions that actually al., 1993; Hattan et al., 2001). accelerate crystallite dissolution (Glover and Organic matter can either accelerate or Kidwell, 1993) both by lowering the pH and by decelerate the process of decay (Emig, 1990; mechanical degradation as the crystallites become Glover and Kidwell, 1993; Tomasovych and unbound from the ultrastructure. Rapid alteration Rothfus, 2005) and it is nearly insoluble in dilute of organic matter within the secondary shell layer acid solutions (Hudson, 1967). Differential of terebratulid brachiopods caused weakening and dissolution of organic-poor and organic-rich fragmentation of the shell structure and reduction aragonitic molluscan shell material was observed of the shell to microfibers within six to seven by Glover and Kidwell (1993) in sterile and non- months. In marine settings, punctate (terebratulid) sterile controlled seawater composition brachiopods have a higher rate of shell environments. In the initial stages of degradation, degradation than impunctate (rhynchonellid) the presence of intercrystalline matter brachiopods due to a greater abundance of (terminology note: Glover and Kidwell, 1993 skeletal organic matter (Tomasovych and Rothfus, refer to material surrounding the crystallite as 2005). Well-preserved nacreous microstructures ‘intracrystalline,’ but in the definitions above, it found in ammonoids are considered to

19 THE PALEONTOLOGICAL SOCIETY PAPERS, V. 20 result from protection by organic matter (Clark, and microstructure (the perimeter of each 1999). crystallite is silicified) was established by Daley Silicification is often discussed tangentially in and Boyd (1996) and Sun and Balinski (2008). a systematic treatment of a faunal assemblage. Both papers included SEM pictures showing fine- Synthesis of these discussions gives insight into scale textural replacement preserving susceptibility and the taxonomic hierarchy of ultrastructure in Mississippian brachiopods. silicification. There is overlap of causes, and However, these authors attributed the some contradiction, but silicification is biased by microstructural control to thickness of the shell the original shell mineralogy, microstructure (e.g., and infiltration of fluids preferentially along the surface area of crystallites), and organic matter shell layers. Location and preservation of organic location and abundance. It is often difficult to tell matrix in shell and matrix and the permeability of mold-filling silicification from replacement, so the matrix are listed as possible contributors, but only articles in which the textural replacement is not discussed (Daley, 1987; Daley and Boyd, indicated (or interpreted from other information) 1996; Sun and Balinski, 2008). are referenced. Differential silicification of brachiopods is A literature study of 291 papers with commonly noted as well. Brown et al. (1969) occurrences of silicified faunas found brachiopods found that in partially silicified fossils from the and mollusks have the highest tendency to silicify, Cretaceous of England, rhynchonellids followed by corals, , and echinoderms silicified preferentially (more completely) than (Schubert et al, 1997). In Permian rocks of West terebratulids. Spirifers are known to preferentially Texas, Newell et al. (1953) observed taxonomic silicify (Loope and Watkins, 1989; Tucker and susceptibility to silicification, listed from the most Wright, 1990). Taxonomic bias was attributed to to least susceptible: 1) bryozoans, tabulate corals, ultrastructural controls in the Mississippian and punctate brachiopods; 2) impunctate Lodgepole Limestone in Wyoming. In Lodgepole brachiopods and mollusks; 3) echinoderms (as Limestone brachiopods, fibrous (rhynchonellate) crust on an ossicle, not replacement); 4) ultrastructure was more susceptible to ; and 5) calcareous and silicification than laminar (strophomenate) dasyclads. When impunctate brachiopods are ultrastructure and prismatic ultrastructure (a silicified, then punctate brachiopods (which have tertiary layer in spiriferides and athyridides; decreased susceptibility relative to impunctate Daley and Boyd, 1996). Holdaway and Clayton brachiopod), are silicified as well. If all taxa are (1982) attributed selective silicification directly to silicified, then the matrix can also be silicified the size of crystallites, emphasizing a taxonomic (Newell et al., 1953). Erwin and Kidder (2000) control. reported the following hierarchy: echinoid spines, In some cases, differential silicification can be brachiopods, and scaphopods; bellerophonts; and observed in a single taxon. The Permian Park City straparolids and pleurotomarid gastropods (in a Formation (Wyoming) contains mixed aragonite/ gastropod-dominated fauna), with smaller calcite bivalves, where calcite is silicified, but specimens having more complete silicification. aragonite is not: presumably, dissolution preceded In fossils from the Carboniferous of Idaho, silicification (Schmitt and Boyd, 1981). The same there is excellent silica replacement in pattern is seen in pseudomonotids from West brachiopods, rare corals, and a single vertebra, but Texas (Newell and Boyd, 1970). Silicification of pelmatozoans, bryozoans, inarticulate outer lamellar layers in brachiopods and exclusion brachiopods, rare bivalves and gastropods, algal of inner prismatic layers has been noted by mats, and trilobites were not silicified (Butts, several authors (Daley and Boyd, 1996; Butts, 2003). In the Silurian of Gotland, silicification is 2007), and is likely due to the outer silicified most prevalent in LMC taxa (brachiopods and layers precluding interaction with porewaters bivalves), followed by corals, ostracods, rather than being a strictly taxonomic control. gastropods, , bivalves, trilobites, Aragonitic faunas, with high relative solubility, spicules, and Tentaculites (Laufield and Jeppsson, often dissolve prior to silicification of other 1976). As with the Permian material studied by organisms (Holdaway and Clayton, 1982; Cherns Newell et al. (1953), silicification operated and Wright, 2000; Wright et al., 2003). hierarchically, and in the most drastic cases, Many have argued about the influential role affected even the matrix. of organic matter in the silicification of marine The link between selectivity of silicification organisms (Holdaway and Clayton, 1982; Knoll,

20 BUTTS: SILICIFICATION

1985; Klein and Walter, 1995; Erwin and Kidder, Lithological controls and biases 2000). One of the first papers to propose the Silicification occurs in deposits ranging from influence of organic matter in the replacement of supratidal to basinal (Knoll, 1985). The fossils with silica was by Holdaway and Clayton geochemical environment of the sediments, (1982), who found structures resembling the including matrix composition, permeability and configuration of organic material in brachiopods porosity, and organic matter content, has a direct from the Upper Cretaceous of the UK. In these effect on the dissolution of carbonate and brachiopods, silicification preserved the precipitation of silica. As with the taxonomic microstructural fabric and appeared to be both controls, the propensity for silicification with encasing the crystallite and replacing the variations in these conditions is poorly studied. crystallite (Holdaway and Clayton, 1982). The The composition of the sediment—carbonate association is well established in microbial and versus siliciclastic—mediates chemical reactions silicification (Barghoorn and Tyler, 1965; by controlling solubility of bioclasts and Leo and Barghoorn, 1976; Francis et al., 1978a; b; interaction with clays. Solubility and precipitation Walters et al., 1977; Knoll, 1985; Akahane et al., of silica are reduced in the presence of fine- 2004), and silicified illustrate the affinity of grained siliciclastics. Organic matter is silica to organic matter even at the micrometer incorporated into the matrix and serves a source scale (Boyce et al., 2001). for silica nucleation. Organic matter (Hinman, Despite these commonalities in the hierarchy, 1990) and certain derived from carbonate and silicification does not always work on a similar clay (Lancelot, 1973; Hinman, 1998) hierarchy, or the details of possible controls are also influence the rate of silica phase changes. not well established in the literature. In the Diagenetically, -A to opal-CT conversion is Silurian of Utah, silicification was most common optimal in carbonates and retarded in clays in trilobites; brachiopods, bryozoans, and sediments (Kastner et al., 1977). ostracods were partially silicified; and gastropods Matrix composition of the sediment and were not replaced. In correlative influences susceptibility and fidelity of rocks in central Nevada, trilobites were not silicification. Silicification is enhanced and of replaced, but brachiopods and gastropods were higher fidelity in open-marine, siliciclastic-poor silicified, with the disparity being attributed to sediments (Erwin and Kidder, 2000; Butts, 2007). depositional environment of the basin (Hintze, Silicification was more common and of higher 1953). In the Mesozoic of Slovakia, Mišík (1995) fidelity in open-marine environments of the found bivalves silicified more readily over Permian basins of southwestern United States brachiopods, echinoderms, corals, and other taxa. (Arizona, New Mexico, and West Texas)—a link From these occurrences, it can be determined that attributed to greater silica availability and more the pattern of silicification is biased by original rapid burial (Erwin and Kidder, 2000). Newell et mineralogy, with calcitic organisms (i.e., al. (1953) also noted that silicification is more brachiopods) being more susceptible to common in open-marine (basinal) carbonate silicification than aragonitic faunas, and LMC settings, and also noted that silicification is favoring silicification over HMC, probably due to associated with depositional structures (toe of relative rates of dissolution for those carbonate talus slope). In the Carboniferous Antler Foreland phases (i.e., aragonitic faunas dissolve at a rate Basin, silicification is more common in that surpasses the rate of silica precipitation). carbonates than in fine-grained siliciclastics Presence of organic matter plays a role based on (Butts, 2007). In the Devonian Helderberg Group evidence that punctate brachiopods, with a greater of New York, there is a positive correlation proportion of organic matter in their shells, between lithology and silicification. Heterolithic silicify more readily than impunctate brachiopods. carbonate and siliciclastic (silt- and clay-stone) Note that punctate representatives are found in outcrops have uniform faunas from shallow- both of the two most abundant clades of marine environments (probably lagoonal), but brachiopods, Strophomenata and Rhynchonellata, silicification is more common in carbonate than in which are characterized by different skeletal fine-grained siliciclastics (Butts, 2004; pers. obs.). ultrastructures of the same mineralogy. However, Incomplete or poor silicification can occur, these interpretations are not a substitute for a particularly in mollusks, in shallow-water statistical analysis of silicified taxa. calcareous fine-grained siliciclastic-rich ! sediments (Newell et al., 1953; Kidder and Erwin,

21 THE PALEONTOLOGICAL SOCIETY PAPERS, V. 20

2001; Butts, 2007). shell. However, the role of organic matter is Stratigraphic position (related genetic factors significant in both the dissolution of carbonate of deposition, such as increasing or decreasing and precipitation of silica, as demonstrated in w a t e r d e p t h , e m e r g e n c e , e t c . ) a n d experiments and fossil analysis. sedimentological controls can also play a role (see Degradation of organic matter by microbial Butts and Briggs, 2011). For example, lithological activity within the skeletal material lowers the compartmentalization occurring in icehouse pH, increases CO2, and promotes dissolution of climates (Read, 1998) may create porosity calcium carbonate (Jacka, 1974; Schmitt and barriers that locally are favorable to silicification Boyd, 1981; Holdaway and Clayton; 1982; on flanks of reefs (Newell et al., 1953) or Cherns and Wright, 2000). Organic matter Waulsortian mounds (Meyers, 1977).The nucleates silica, a process that has been influence of cements can preclude silicification by demonstrated in the lab (Amores and Warren, decreasing the permeability of the matrix. 2007) and in natural settings, such as sinters ossicles, which are HMC and therefore could be (Guidry and Chafetz, 2003; Konhauser et al., expected to silicify readily, often have early 2004 [although they argue microbes do not syntaxial cements when found in crinoid mounds contribute to the process, they acknowledge the and banks, and can resist silicification due to the passive role of microbial organic matter in large crystal size of the . Silicified nucleating silica precipitation]; Lynne et al., brachiopods in crinoid grainstone suggest that 2007) and desert varnishes (mineralized coatings while syntaxial cements prevented the on rock surfaces; Perry, 2003). In laboratory replacement of echinoderm bioclasts with silica, studies, silicification of microbes (Amores and they did not prevent the silicification of Warren, 2007) and skeletal material (Paraguassu, brachiopod bioclasts contained within the 1976; Butts et al., 2011) occurred in silica- grainstone matrix (Butts, 2007). saturated conditions in acidic, low-temperature Silicified fossils and matrix from the Silurian conditions. This indicates that a taxonomic bias of Gotland conform to bedding and directly may be introduced by the presence and underlie bentonite deposits, indicating an obvious distribution of organic matter in skeletal material. connection between the silica source and the In experiments using ground rhyolitic obsidian as occurrence of silicification (Laufeld and a silica source in aqueous solution at at 100°C, Jeppsson, 1976). Silicification in the Upper plant tissues attracted silica complexes from Proterozoic of Greenland is more prevalent in solution that subsequently precipitated onto sediments high in organic matter than in organic- organic matter (Ballhaus et al., 2012). Knoll poor sediments (Knoll, 1985), a correspondence (1985) also suggested that a modest amount of first noted in Siever (1962). In some areas, such degradation of organic matter would enhance as sponge bioherms, all taxa have a greater silicification by increasing the reactivity (number likelihood of silicification (Cooper and Grant, of sites available for hydrogen bonding) and 1972). Silicification is more common and higher introduction of structural gaps (i.e., microcracks fidelity in carbonate sediments versus fine- or holes) for silica infiltration. The generalized grained siliciclastics, likely due to the reactivity processes and controls for silicification are shown of clays and the increased solubility of calcite in Figure 4. bioclasts in an undersaturated environment. Silica is adsorbed to organic matter and ! undergoes further polymerization to more stable PROCESS silica phases by hydrogen bonding to hydroxyl ! groups (Iler, 1979); the rate is slower at lower pH Knauth’s (1979) classic model of silicification (Hinman, 1987). Organic matter has weak acidic proposed that the mixing of meteoric and marine functional groups that form surface complexes ground promotes a geochemical shift, with relatively basic surface hydroxyls. Relatively which then promotes silica precipitation. In many acidic surface hydroxyls form weaker complexes, examples of silicification, however, there is no and under natural (freshwater) conditions, may link to terrestrial emergence. Maliva and Siever not be covered with organic material (Davis, (1988) discussed force of crystallization as the 1982). Silicic acid can form a variety of primary driver of the replacement of calcite with complexes with ions and organic molecules silica, and felt the organic matter within a shell including mucopolysaccharides and hydroxyl was insufficient to promote dissolution of the amino acid-enriched glycoproteins. Organic

22 BUTTS: SILICIFICATION

LMC, HMC, aragonite carbonate surface area phase (taxa, ultrastruture) Mg/Ca ambient

decay templating silica organic volcanism source siliceous organisms content

FIGURE 4.—Process and controls of silicification. Arrows directed away from the silicified richthofenid represent processes or controls that affect dissolution of carbonate, arrows directed away affect the precipitation of silica, and coupled controls are bonded. substances can contribute to the polymerizing, experiments of Butts et al. (2011) at Yale cross linking, and hardening (by elimination of University showed that silica infiltrates the water) of silicic acid, and can incorporate metals intercrystalline organic matter within the shell, as well (Perry, 2003). Polymerization occurs even as carbonate persists, which could indicate between pH 3–10, but the optimal rate is at pH 7– isolated microenvironments. A shift in 9 (Iler, 1979). geochemical conditions, such as rapid increase in Based on this process, silicification acidity, could also trigger silica precipitation. technically could happen at any point in the pre- Basinal fluctuations in temperature or pH could lithification diagenetic history of an organism, promote wide-scale silica precipitation. even high-fidelity silicification that retains the Silicified fossils grow increasing susceptible character of shell ultrastructure. Intercrystalline to loss of fidelity as silica neomorphism occurs. organic matter is taphonomically robust, and has After precipitation, silica goes through a series of been found in fossils as old as the Pennsylvanian. diagenetic transformations: opal-A (amorphous) > The organic matter retains the texture of the opal A’ (secondary) > opal CT > opal CT crystallites (polygonal in prismatic ultrastructures, (reordered phase) > cryptocrystalline quartz or tabular in nacreous), viewable by performing chalcedony > microcrystalline quartz (Iler, 1979; critical point-drying on the specimens after Williams and Crerar 1985; Hesse 1989). The etching. Protection by organic matrix, and from increasingly coarse nature of the silica may conditions that also precluded degradation or obliterate preserved ultrastructure, which may be dissolution of the crystallites themselves (with the case for the Permian Basin faunas. low permeability rock matrix), is attributed to the ! protection of aragonite in shells from the EXPERIMENTAL SILICIFICATION Cretaceous (Clark, 1999). ! The conditions conducive to silicification are The earliest accounts of experimental not exactly consistent with general marine silicification are examples of of wood geochemical conditions either in carbonate or for experimental and utilitarian purposes that are siliciclastic environments, so the establishment of referred to in Leo and Barghoorn (1976). One microenvironments is likely essential to this example from the 1500s states, “When one brews process. Schmitt and Boyd (1981) felt that an beer, alderwood is simmered in the pot until the early diagenetic micritic envelope was enough to hops are done. Afterwards, one buries the wood create an acidic microenvironment. The structural for three years in fresh sand or gravel in a cellar, framework of the shell ultrastructure and the after which it turns hard, making the best whet- or resistance of skeletal organic matter to decay flintstone.” (from Meyer’s 1791 account, could also provide isolation of geochemical translated in Leo and Barghoorn, 1967, p. 10). In conditions. Nascent experimental silicification that case, barley grains contain a significant

23 THE PALEONTOLOGICAL SOCIETY PAPERS, V. 20 supply of amorphous silica as influenced by temperature (55°C was optimal) (abundant in Poaceae), which go into solution and and water content (neither wet nor dry materials precipitate in the cavities of the wood, and silicified because ethyl is immiscible in dehydration completes the petrifaction. An water). These experiments concluded that excellent historical overview is provided in Leo silicification can create a high-fidelity and Barghoorn (1976). Contrary to conventional reproduction of an organism, although internal wisdom of the slow nature of fossilization, the structures may not preserve and shrinkage can process of silicification is proving to be a rapid alter the final product, and that silica precipitation process in both laboratory and natural was favored by the complexing of silica by experimental procedures (Leo and Barghoorn, organic matter (Francis et al., 1978a). 1976; Karawoe and Jefferson, 1987; Akahane et Field examples indicate a geologically rapid al., 2004; Ballhaus et al., 2012). This section rate of permineralization. Wood fragments placed discusses experimental silicification, both in the in silica-saturated hot springs (70°C, pH=3, laboratory and in natural settings, which have 323.3–378 mg/l monomeric silica) were silicified helped to elucidate the processes and controls on 40% (by weight) after just seven years of silicification. immersion. The authors concluded that silicified ! wood can form in silica-rich hot waters associated Experiments with microbes and with volcanic or pyroclastic rocks in the span of Research on experimental fossilization with tens to hundreds of years (Akahane et al. 2004). silica is most robust with respect to Leo and Barghoorn (1976) found permineralized permineralization (Leo and Barghoorn, 1976) and specimens of wood retrieved from a hot spring entombment (of microbes) (Barghoorn and Tyler, that had formed in the last 13 years. 1965; Walters et al., 1977; Francis et al., 1978a, Industrially, wood permineralization b). Permineralization of wood was achieved in a experiments using various carriers for silica have laboratory setting at near-neutral pH conditions by aimed to produce a more durable wood product. soaking wood in water to remove gas, then Tetraethoxysilane (TEOS) was determined to be immersing it in ethyl silicate solutions for months the most effective vehicle for penetration of silica at 70°C with intermittent vacuum impregnation and preservation of wood structures. TEOS Without vacuum impregnation, permineralization hydrolyzes during the transition from gel to sol was incomplete, and the structure crumbled when and forms monosilicic acid, which then the organic matter was removed. The siliceous polymerizes to opal-A (chemical reactions in lithomorphs were constructed of highly Götze et al., 2008). Other solutions, such as silica disordered α-cristobalite, similar to Recent sols and , were less effective fossilized wood specimens in volcanic ash from or damaged the wood (sodium metasilicate is the 1886 eruption of Mt. Tarawera, New Zealand highly alkaline). (Leo and Barghoorn, 1976). Leo and Barghoorn One laboratory experiment utilized a silica (1976) proposed that silicification occurred as source that would be found in nature—ground monomeric (or possibly low molecular weight rhyolitic obsidian— which, at 100°C, went into polymeric) silicic acid infiltrated void spaces in solution to produce clay minerals, ionic K+ and the wood structure and walls, which, by Na+, and silicic acid. Addition of NaOH was hydrogen bonding between soluble silica and necessary to promote dissociation of neutral polysaccharides in contact with organic matrix, silicic acid species. At pH of 9.5 and below, the preserved the histological characters of the wood. obsidian solution contained around 6.24 mmol/L Based on the experiments by Leo and SiO2. Despite the initial spiking with NaOH, the Barghoorn (1976), Francis et al. (1978a, b) addition of wood caused acidification, decreased undertook experimental silicification of and silica solubility, and precipitation of silica, with microbes (entombment) using tetraethyl the introduced pH gradient being a primary orthosilicate (TES) at 55–60°C for weeks to control on the process (Ballhaus et al., 2012). months in an attempt to correlate with silicified ! structures in . Silicification Experiments with replacement correlated with organic content. It did not occur Despite the abundance of studies on the on freshwater microbes that lacked an organic experimental permineralization of wood, little sheath, and was greatest in organisms that had a experimental work has been performed on shelly robust extracellular sheaths. Silicification was material to emulate replacement. The first

24 BUTTS: SILICIFICATION published experiment to replace skeletal material Ocean geochemistry and global climate of an organism was performed in the 1970s by Ocean geochemistry has fluctuated with Paraguassu (1976). Paraguassu suspended live- respect to Mg/Ca over the course of geologic collected mollusk shells from threads in a solution time, driven mostly tectonically by the production of sodium metasilicate and adjusted the pH to of ocean crust at spreading centers (Stanley and promote dissolution of skeletal material (2g/liter Hardie, 1998, 1999) and ρCO2 (Zhuravlev and SiO2, pH=2 was ideal for a 0.5g shell). Silica Wood, 2009), as demonstrated by sampling flocculated on the exterior of the shell within evaporate deposits (Hardie, 1996), abiogenic three weeks. After three weeks, the shells were carbonates (Sandberg, 1986), and the Mg/Ca ratio removed from the solution and were found to be of organisms that secreted their tests in flexible. After eight months, shells were removed equilibrium with the Mg/Ca of seawater (Stanley and dehydrated in ambient air conditions. The and Hardie, 1998). Silicification necessarily is shells contracted slightly due to loss of water, and preceded by the dissolution of skeletal material, were found to be 75% silica in composition; the and differential dissolution rates of carbonate remainder was organic matter and unmobilized phases are based on local and global microelements. He performed additional geochemistry. Because of the importance of experiments removing Ca2+ from solution, which carbonate solubility to silicification, the effect is can decrease the solubility of silica (Paraguassu, seen in the abundance and selectivity of silicified 1976). faunas through time. The link between relative Experimental silicification by Butts et al., solubility and silicification is not direct, but rather (2011) linked the process of silicification to the a contributing factor because it influences the adsorption of silica to intercrystalline organic relative dissolution of organisms at a given time, a matter of modern bivalves. Mytilus were live- key part of the silicification process. In global collected, stripped of flesh, and placed in a greenhouse climates, deposition is characterized solution of sodium metasilicate. Sodium by thick sequences of carbonate lithologies with metasilicate is a strong base, so an initial addition small spatial variations in depositional of HCl was necessary. Afterward, the experiment environments laterally and vertically (Read, was kept at constant low pH (1, 2, and 4) in 1998). Since silicification is favored in carbonate replicate sets. Mytilus contains two skeletal lithologies, silicification may be overrepresented ultrastrutures, prismatic and nacreous (both in greenhouse climates due to the higher relative aragonitic in composition), with two proteins that proportion of carbonate rocks. In terms of global are common only in their respective shell layer. ocean geochemistry, calcite-saturated greenhouse These two proteins are thought to be the controls seas would favor dissolution of aragonite (see on the construction of the shell ultrastructure Palmer et al., 1988). As suggested by Kidder and (Weiner, 1983). The adsorption of silica to organic Erwin (2001), greenhouse and icehouse matter was observed in a span of tens of days. In conditions also influence burial. In these experiments, the silica extended beyond icehouse climates, increased siliciclastic dissolution of the crystallite to within the deposition may reduce the abundance of silicified ultrastructure where carbonate material was still faunas, bearing in mind that no connection has intact as seen under electron microprobe. There been drawn between coarse- and medium-grained was no significant difference between siliciclastic sediment and a silicification bias; a silicification in the two layers, so it was assumed negative bias is attributed only to fine-grained that the particulars of the shell organic matrix (silt- and clay-sized) siliciclastic sediments. The composition, and perhaps among all taxa, are of high-amplitude, high-frequency sea-level changes minor importance (Butts et al., 2011). in icehouse climates repeatedly expose sediment ! to meteoric diagenesis, which would result in TEMPORAL BIASES IN SILICIFICATION either rapid dissolution preceding silicification ! (see Palmer et al., 1988) or conversely, A temporal bias in silicification has been geochemical conditions favoring silicification connected to secular variations in sea water (see Knauth, 1979). Seas saturated with aragonite geochemistry, abundance of siliceous and calcium favor the dissolution of both HMC and LMC taxa, carbonate , abundance of chert deposits, with preference to LMC. sea-level changes, and rock volume, but patterns In the absence of a strong greenhouse/ of silicification are still poorly constrained. icehouse signal (Kidder and Erwin, 2001) or with

25 THE PALEONTOLOGICAL SOCIETY PAPERS, V. 20 carbonate rock volume (Schubert et al., 1997), amorphous silica (Iler, 1979; Fleming and Crerar, patterns seen from surveys of silicified faunas 1982; Ballhaus, 2012). Monomeric silicic acid is may relate to sampling of exceptionally the most common dissolved silica source in preserved, compartmentalized, silicified faunas marine and fresh waters (Tréguer et al., 1995). that override the general signal of silicification Silicic acid (H2SiO4) is a weak acid and has four preference during greenhouse times. Also hydroxyl groups per molecule, making it highly contributing to the lack of a pattern is the reactive (Iler, 1979; Leo and Barghoorn, 1976). It superposition of the marine diversity curve over is the raw material for the tests of siliceous geologic time onto the global ocean climate organisms, including , , and curve, specifically the abundance and diversity of silicoflagellates (algae), and so is an important potentially silicified organisms of specific transfer in silica cycling (Goering et al., 1973). mineralogy (HMC, LMC, or aragonite), and the The primary source of removal of silica at present abundance and diversity of silicified faunas as a is diatoms (Tréguer et al., 1995), but Paleozoic silica source. removal was by radiolaria and sponges (Siever, The relative solubility of marine organisms of 1992; Kidder and Erwin, 2001). Dissolved silica a given mineralogy is susceptible to shifts in is a minor constituent in natural waters with an ocean Mg/Ca, making it the greatest variable in average concentration of 120 µg/l in natural skeletal dissolution, and therefore, tendency to waters (Milliman and Boyle, 1975). Siever (1957) silicify. It can also result in large-scale dissolution offered the following assessment of dissolved as seen in Upper and marine silica in natural systems: rivers have from a few to deposits. These calcitic greenhouse seas promote 35 ppt silica, some lakes and rivers are higher (up dissolution of aragonitic faunas (Palmer et al., to 75 ppm), alkaline lakes can be 300 ppm, 1988). In one example from the Silurian of ground waters are 50–60 ppm (but can be higher Gotland, early diagenetic silicification preceded in areas of meteoric mixing), oceans have less aragonite dissolution, and resulted in loss of than one up to 12 ppm, ocean sediments can have molluscan diversity in silicified versus non- up to 68 ppm, and hydrothermal deposits up to silicified deposits (70% versus 3%; Cherns and 400 ppm. Silicic acid occurs abiologically through Wright, 2000). In the Jurassic of South Wales, the breakdown of siliceous microorgansims there is a 65% decrease in the diversity of bivalve (Calvert, 1974; Tréguer et al., 1995) and through genera where dissolution of aragonite preceded terrestrial and submarine (Tréguer et silicification (Wright et al., 2003). Seas saturated al., 1995), notably devitrification of volcanic ash with aragonite favor the dissolution of HMC and and diagenesis (Siever, 1957; Leo LMC taxa, with preference to LMC. and Barghoorn, 1976), so local concentrations can The dissolution of skeletal matter may be be much higher (Calvert, 1974) in the vicinity of significantly altered by shifting geochemical these factors. conditions that can produce large-scale Silicified flora and fauna are commonly dissolution events (Henrich, 1985; Palmer et al., associated volcanic ashes, including deposits of 1988; Cherns and Wright, 2000; Wright et al., the Permian Basin of Texas (e.g., bentonites, 2003; James et al., 2005). Aragonitic faunas Nicklen and Bell, 2007; Nestell et al., 2012), become increasing dominant through the Gotland (Laufeld and Jeppsson, 1976), and New Phanerozoic because of declining pCO2 leading to York (Bald Hills volcanics with K-bentonite ash decreasing total and dissolved inorganic layers, Dennison and Textoris, 1979; Smith and and increasing pH, assisted by mass- Way, 1988; Butts, 2004), petrified woods extinction events that enhanced the survival of (Scurfield and Segnit, 1984; Senkayi et al., 1985), aragonitic faunas (Zhuravlev and Wood, 2009). and Recent hot-spring deposits (Leo and ! Barghoorn, 1976; Akahane et al., 2004). Dissolved silica supply in the oceans Permineralization of wood is associated with The limiting factor in silicification is silica volcanic ashes, afforded by both the ample supply availability. Silica in the oceans is input by rivers, of silica and by creating anoxic conditions which hydrothermal systems, and from weathering of preclude the degradation of lignin, the primary sediments. Uptake is mediated by the production component of wood (Leo and Barghoorn, 1976). of siliceous tests by marine organisms (Goering et ! al., 1973). Crystalline silica is not considered a Proterozoic silicification silica source because it is much less soluble than Silica concentrations are low today due to the

26 BUTTS: SILICIFICATION efficacy of organisms in removing silica from shifts were found in the facies distribution of ocean water, and there is little fossil evidence for chert in the Paleozoic: Cambro–Ordovician silica-secreting organisms prior to the (increasingly peritidal to subtidal chert related to Phanerozoic (Siever, 1992). The in the the radiation of sponges and radiolarians in the Precambrian has typically been interpreted as Ordovician), and Late Cretaceous to Paleogene abiologically mediated by weathering, tectonism, (chert deposition moves to deep shelf and ocean and hydrothermal activity (Maliva et al., 2005), basin environments, corresponding to the and lacking a strong biological output, interpreted radiation of radiolarians) (Maliva et al., 2005). An by the lack of fossil record of siliceous organisms upper Carboniferous peak in silicification at this time (Siever, 1992); however, this coincides with a rebound in shelfal cherts (Maliva interpretation was recently challenged by Sperling et al. 2005). Kidder and Erwin (2001) found that et al. (2010). drops in abundance in chert and silicified fossils Silica-replaced remains of microbes have and bedded cherts coincided with four of the five been found in cherts of the 2000 My Gunflint Iron mass-extinction events at the end of the Formation (Barghoorn and Tyler, 1965). Ordovician, late Devonian, end-Permian and the iron formations, including some Cretaceous–Paleogene. Increases were linked to containing microbial remains, were deposited by radiations in the Ordovician and Siluro–Devonian widespread primary silica precipitation directly (Kidder and Erwin, 2001). from silica-supersaturated seawater (Maliva et al., A literature review by Schubert et al. (1997) 2005). Proterozoic examples of silicification are looked specifically at silicified faunas based on limited to cherts containing that are photographic documentation and observances in considered early diagenetic peritidal deposits manuscripts, and found that they are more (Maliva et al., 2005; references therein). common in Paleozoic (21% of publications) Abundant organic matter in peritidal sediments than post-Paleozoic sediments (4% of environments may have served as a nucleation publications). In decreasing order, compared to site for silica precipitation (Maliva et al., 2005). unsilicified occurrences and normalized for Reactions with organic matter to precipitate silica duration of time period, silicification was most would have been common in Proterozoic prevalent during the Silurian, Ordovician, sediments, and organic compounds can be Carboniferous, and (Shubert et al., 1997). extracted from Proterozoic sediments (Summons This was correlated to a post-Paleozoic shift from and Walter, 1990). Experiments suggest some calcitic to aragonitic faunas (Wilkinson, 1979; promote silicification (Birnbaum and Railsback and Anderson, 1987), decrease in Wireman, 1984, 1985), as do the products of sponge spicule abundance, and possibly to a shift organic degradation (Siever, 1992). Organic from nearshore to offshore cherts related to the matter, abundant in Proterozoic carbonate radiation of diatoms in the Cretaceous (Schubert sediments (Knoll, 1985), would degrade, creating et al., 1997). However, they found the decline in complex compounds capable of polymerizing silicified fossils in the Triassic predates the rise of silica (Siever, 1992). diatoms, indicating they do not appear to be ! driving the silica cycle (Shubert et al., 1997). Phanerozoic silicification Ultimately, local patterns, such as silica-rich The evolution of silica biomineralizing environments associated with volcanism and organisms over the course of the Phanerozoic hydrothermal activity or geochemical shifts, may introduced a major biological influence into the make broad patterns difficult to establish. silica cycle (Maliva et al., 1989). Secular trends in ! silicification should be tied to the relative CONCLUSIONS abundance of organisms of the different carbonate ! phases, their relative solubility based on the Mg/ Permineralization, entombment, and replacement Ca ratio of the ocean at the time, and the with silica occur by similar processes because abundance of siliceous organisms (as a source of silica adsorbs to either the organic matter next to a silica). void space in wood, the organic sheath of a Maliva et al. (2005) attributed the temporal microbe, or the organic matter that permeates the and spatial distribution of chert to the interior of a marine organism’s shell. Silicification evolutionary history of siliceous organisms, is the concurrent dissolution of carbonate skeletal particularly sponges, radiolaria, and diatoms. Two material and precipitation of silica, aided by the

27 THE PALEONTOLOGICAL SOCIETY PAPERS, V. 20 nucleation of silica by organic matter. As such, the diversification, and extinction of major siliceous process is mediated by the original mineralogy, biota, such as sponges and radiolarians. On a ultrastructure, and location and abundance of smaller scale, silicification is clearly biased organic matter within the shell (taxonomic towards preservation of lower solubility LMC controls), and geochemical environmental at organisms, as aragonite and, to a lesser degree, deposition and during the early diagenetic history HMC faunas, fall victim to dissolution prior to of the fossil (lithological controls). silicification unless in exceptional situations. Dissolved siliceous skeletal material has long ! been considered the source for silica in ACKNOWLEDGMENTS replacement, but considering the frequent co- ! occurrence of volcanic ash sediments and The author acknowledges the contribution of her silicified faunas, a non-biogenic source is evident colleagues, Derek E. G. Briggs, and Richard A. as well. Modern examples of silicification occur Krause, Jr., for their collaboration on in sinters, geothermal waters enriched in silica, experimental silicification and analysis of and with very a different set of geochemical taphonomic bias, which was funded by the Yale constraints than those of normal marine carbonate Peabody Museum. Eric Peavey assisted with environments, the depositional environments in photography of fossils in Figure 1. The author which most silicified fossils are found, but the thanks the Volume Editors for the invitation and creation of geochemical microenvironments, such the Special Publications Editor for their roles in as a shell enclosed within a micritic envelope, or producing this Paleontological Society Short crystallites within an organic sheath, may allow Course. Thank you to Matthew Clapham and for such conditions to exist. Fine-scale textural Douglas Erwin for their constructive reviews of evidence of silica-replaced flora and fauna and this manuscript. laboratory experiments suggest that silicification ! of any type can be geologically instantaneous. REFERENCES The demonstrated proclivity for organic matter to ! nucleate silica, and the persistence of organic AKAHANE, H., T. FURUNO, H. MIYAJIMA, T. matter in skeletal material, suggests that the pre- YOSHIKAWA, AND S. YAMAMOTO. 2004. Rapid lithification window for silicification could be wood silicification in hot spring water: an sizable. Temporal patterns of silicification show explanation of silicification of wood during the that silicification was more prominent in the Earth's history. Sedimentary , 169:219– Paleozoic, and has decreased over time, a pattern 228. AMORES, D. R. AND L. A. WARREN. 2007. Identifying ascribed to the evolution of more aragonitic when microbes biosilicify: The interconnected faunas through the Phanerozoic. requirements of acidic pH, colloidal SiO2 and Understanding the process of silicification, exposed microbial surface. Chemical Geology, and particularly the association with organic 240:298–312. matter, may be useful for recognizing chert and BALLHAUS, C., C. T. GEE, C. BOCKRATH, K. GREEF, T. silicified deposits as petroleum source rocks MANSFELDT, AND D. RHEDE. 2012. The because it points to the location of organic matter. silicification of trees in volcanic ash—an More importantly, understanding the processes experimental study. Geochimica et Cosmochimica and controls on silicification of fossils allows an Acta, 84:62–74. understanding of a taphonomic filter that biases BARGHOORN, E. S. AND S. A. TYLER. 1965. the interpretation of the record of biodiversity from the Gunflint Chert—These structurally preserved Precambrian fossils from through time. The exact nature of this bias is not Ontario are the most ancient organisms known. clearly ascertained with our present knowledge of Science, 147:563–575. silicification patterns and the process and controls BERNER, R. A. 1975. The role of magnesium in the on silicification. Broad patterns of silicification crystal growth of calcite and aragonite from sea could certainly be driven by global ocean water, Geochimica et Cosmochimica Acta, geochemistry and climate, and by the control of 39:489–504. the and variation of organisms by BERNER, R. A., E. K. BERNER, AND R. S. KEIR. 1976. mineralogy, and presence and abundance of silica. Aragonite dissolution on the Bermuda Pedestal: Its The most notable effect is the transition from depth and geochemical significance. Earth and Paleozoic calcite-dominated taxa to post- Planetary Science Letters, 30:169–178. Paleozoic aragonitic faunas and the origin, BIRNBAUM, S. J., AND J. W. WIREMAN. 1984. Bacterial

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