[Palaeontology, Vol. 52, Part 1, 2009, pp. 187–192]
A NEW METHOD FOR THE EXTRACTION OF MACROFOSSILS FROM CALCAREOUS ROCKS USING SULPHURIC ACID by RADEK VODRA´ Zˇ KA Czech Geological Survey, Kla´rov 3, 118 21 Prague 1, Czech Republic; e-mail:
[email protected] Institute of Geology and Palaeontology, Faculty of Science, Charles University Prague, Albertov 6, 128 43 Prague 2, Czech Republic; e-mail:
[email protected] Typescript received 7 February 2008; accepted in revised form 19 May 2008
Abstract: A new method for the extraction of calcified and ⁄ or partly pyritized macrofossils has been developed. This method is based upon the differential speeds for the dissolving of microcrystalline and macrocrystalline calcite in 38% sulphuric acid. The effectiveness of the sulphuric acid treatment is also influenced by the volume of clay minerals in the host rock. Therefore, this method is highly applicable for the extraction of macrofossils from marlstones, marly limestones, and other lithified calcareous sediments. The main advantages of this method, when compared with other chemical methods, are (1)
Palaeontologists dealing with calcitic or calcified macrofossils within solid calcareous rocks, apply different mechanical and chemical methods for their extraction. Whereas mechanical preparation is usually time-consuming and with a high risk of damaging surface structures, different types of chemical preparation have been employed for various types of calcareous rocks. During palaeontological work on the Upper Cretaceous sediments of the Bohemian Cretaceous Basin, a rich abundance of macrofossils have been collected; however, some of these could not be treated by any known chemical method. Therefore, a new chemical method has been developed for the extraction of calcitic, calcified and ⁄ or partly pyritized macrofossils from calcareous rocks (marly limestones, lithified marlstones, and other compact calcareous rocks). The methodology was developed experimentally in 2003, when sulphuric acid was employed for macrofossil extraction, for the first time. This proved an excellent and effective method for cleaning of the macrofossils from different Upper Cretaceous calcareous rocks (e.g. Upper Cenomanian – Middle Coniacian rocks of the Bohemian Cretaceous Basin and Campanian rocks of the Lower Saxony region of Germany). Published and figured materials
ª The Palaeontological Association
the short treatment time, (2) the capability of dissolving the sediment on the fossil’s surface, and (3) its efficiency in dissolving calcareous rocks with low porosity. This method has been successfully applied to Upper Cretaceous macrofossils from the Bohemian Cretaceous Basin. The surface of extracted macrofossils remained undamaged, exhibiting minute skeletal details; perhaps even encrusters and bioerosions. Key words: preparation, acid treatment, sulphuric acid, cal-
careous macrofossils, carbonates.
extracted by the sulphuric acid method are represented by Upper Cretaceous sponges (Vodra´zˇka 2005), bryozoans (Za´gorsˇek and Vodra´zˇka 2006), sponges, corals, serpulids, oysters, brachiopods, echinoderms and bryozoans (Zˇı´tt et al. 2006), as well as various phosphatized macrofossil moulds (Vodra´zˇka et al. in press).
PROCEDURE Mechanical preparation If the surface of macrofossil is covered by a thick crust of obscuring matrix, initial mechanical preparation is recommended. Thick matrix crusts of compact and highly calcareous rocks are slowly dissolved by sulphuric acid, and to minimize the time of acid-treatment they should be removed. Fast and effective mechanical surface cleaning can be done with vibrating tools, hammer, chisels, needles, and scribers. Mechanical preparation need not be time consuming, because only thick crusts of obscuring matrix are necessary to be removed. Care must be taken not scratch the surface of the fossil; therefore it is not recommended to try to remove any thin crusts of sediment
doi: 10.1111/j.1475-4983.2008.00829.x
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on the shell (skeleton) surface during mechanical preparation. Waterblasting (the use of high-pressurized water; Jakobsen 2003; Nielsen and Jakobsen 2004), as well as sandblasting (e.g. Stucker et al. 1965; Aichinger 1969; Hannibal et al. 1988; Hannibal 1989) may be employed during mechanical preparation. With waterblasting, the water creates an effective abrasive spray; one not as damaging as sandblasting. Therefore, it is particularly recommended for softer calcareous rocks. The character of a skeleton’s preservation must be carefully inspected and noted upon a sediment-free portions of the fossil, in order to observe its pristine condition. The quality of preservation must not be degraded by the chemical treatment to follow.
Drying the sample It is necessary that the fossil and matrix are completely dry, because the concentration of the acid must not be reduced. To avoid this, the rock samples were dried at about 100C, for one day. However, it does not seem necessary to dry samples that have been kept in dry conditions for several weeks.
well as an increase of the thickness of the residue on the surface of the fossil.
Neutralization After samples are removed from the sulphuric acid bath, they need to be washed (within either a plastic or glass container) with abundant and constantly flowing tap water, and then consequently neutralized in a solution of water and sodium carbonate. The washing procedure must be rapid, because when the acid on the surface of the fossil is diluted in water, the concentration of sulphuric acid is reduced and becomes more reactive with calcite fossils. Washing should last 1–5 minutes depending on the volume of the adhering residue (chemically-degraded sediment, calcium sulphate, a.o.). After washing, the remainder of the sulphuric acid has to be neutralized. For this purpose, a weak solution of water and sodium carbonate (one full tablespoon per liter of water) worked well. Samples have to be immersed in this solution, within a vessel for 2–6 hours, immediately after the washing.
Final cleaning Bath in sulphuric acid Safety notice. When working with sulphuric acid, always wear safety glasses. Although short contact times of 38% sulphuric acid with the skin usually does not cause burns, rubber gloves should be used. The procedure should be carried out under a fume hood. The dry samples were submerged in a vessel of 38% sulphuric acid. The vessel (plastic or glass) should then be covered, because of the need to exclude moisture, and as protection against any foam arising from the acid. During the chemical reaction, small bubbles of carbon dioxide produce foam above the air-acid surface. Small sediment particles are exfoliating from the surface of the sample and consequently turn into a soupy mud. Dissolving of the obscuring matrix is accompanied by the formation of a very thin insoluble residue on the surface of the fossil and the matrix (for removing this coating, see Final cleaning step below). The residence time of the samples in the acid is variable, and depends on the mineralization of the shell (skeleton) and surrounding matrix. Fossils treated in an acid bath for less that 2 hours show no (or very weak) corrosion. Some more compact clayey limestones with calcified fossils may be treated for up to 5 hours in the acid bath, without obvious signs of destruction of the fossil surface. However, prolonged bath times of 3– 10 hours increase the risk of damage to fossil surfaces, as
Residues have to be removed carefully after the neutralization, using one of three processes, differing in their time-consumption and effectiveness: (1) waterblasting, (2) ultrasonic cleaning, and (3) cleaning with a soft paintbrush. The waterblasting technique (Jakobsen 2003; Nielsen and Jakobsen 2004) is suitable for the final treatment of all types of macrofossils. In this study a hand-held watergun was used (Wagner Model W 400 SE), with adjustable water pressure up to 180 bars, at 10 mm from its orifice. The intensity of the pressure washer can be varied by either raising or lowering the velocity of the water stream by adjusting the water pressure, or by changing the distance between the orifice and the fossil surface. Ultrasonic cleaning (e.g. Wetzel 1950) also shows quite good results, although it is not recommended for brittle shells and skeletons that can be destroyed if the power level of the ultrasonic cleaner is set too high. Cleaning by use of a soft paintbrush, under a water stream, is also efficient; and if done carefully, it does not damage the fossil surface. However, some deep fissures, filled with residual coatings and degraded sediment are difficult or impossible to clean this way. The same is true for fossils with a complex and rugged surface (e.g. skeletons of sponges). In some cases it is useful to combine the methods described above, to obtain the best results. After the final cleaning, the fossils have to be dried, before study.
V O D R A´ Zˇ K A : A N E W M E T H O D F O R T H E E X T R A C T I O N O F M A C R O F O S S I L S
COMPARISON WITH THE OTHER METHODS Calcareous microfossil extraction from chalk rocks was one of the earliest micropalaeontological procedures to be documented (Heron-Allen 1894), although the procedure was not formally described until later (Williams-Mitchell 1948). Extraction techniques for calcareous microfossils from carbonate sediments include: heating in sodium sulphate (Kirchner 1958), use of petroleum spirit and sodium carbonate (Bolli 1952), use of sodium hydroxide (Williams-Mitchell 1948), and use of concentrated acetic acid (see below). Nevertheless, except for the method employing highly concentrated acetic acid, these methods are not suitable for extraction of calcareous macrofossils (cf. Green 2001). The new method described herein, and the method using highly concentrated acetic acid, are both based upon differential speeds for dissolution of microcrystalline and macrocrystalline calcite, and employ some similar procedural steps. A weak solution of acetic acid dissolves the matrix of chalk and limestones, and is only efficient for extraction of non-calcareous (especially phosphatized and silicifed) fossils (Reid 1958; Mu¨ller 1962; Zankl 1965; Rudner 1972; Jeppsson et al. 1985, 1999). Bourdon (1956, 1962), followed by Lethiers and Crasquin-Soleau (1988), employed concentrated acetic acid (95% or more) for the extraction of calcareous microfossils from calcareous sediments. Their method of ‘hot acetolysis’ was successfully applied to the extracting of calcareous microfossils e.g. by Bubı´k and Kosma´k (1991). Za´gorsˇek and Va´vra (2000) extracted bryozoans, combining acetolysis in acetic acid with artificial weathering, and then cleaning with Quaternary ‘O’TM (Geigy Industrial Chemicals, New York, NY, USA). Nielsen and Jakobsen (2004) described an acid-hot water method, which is efficient for large bulk samples of limestones that have both high permeability and porosity. Their method, using highly concentrated acetic acid, resembles a modified and much improved version of the method given by No¨tzold (1965). Methods employing concentrated acetic acid, and the new method described herein, were simultaneously tested upon identical samples from the Bohemian Cretaceous Basin. There are three main advantages of the sulphuric acid treatment, when compared with hot acetolysis (using concentrated acetic acid; for methodology see Green 2001). Sulphuric acid treatment is: (1) much faster; (2) dissolving just the sediment on the surface of the fossils, instead of disaggregation of brittle macrofossils and macrofossil clusters; and (3) dissolving some crusts of calcareous sediment, which were unsuccessfully treated with concentrated acetic acid. The main advantage, when compared with Nielsen’s and Jacobsen’s acid-hot water method (2004), is greater efficiency in dissolving calcareous rocks without high permeability and porosity.
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Until now, sulphuric acid has been employed as a catalyst (in solution with acetic anhydride) by palynologists in order to remove unwanted cellulose, in a process called acetylation (for details see Green 2001).
RESULTS The use of sulphuric acid is herein demonstrated on Upper Turonian macrofossils from the U´pohlavy Quarry (Bohemian Cretaceous Basin). Well-preserved, calcitic, calcified or partly pyritized, macrofossils (sponges, echinoderms, serpulids, thick-shelled bivalves, and brachiopods) were gathered from marly limestones with volume of calcium carbonate ranging between 70 and 85% (Cˇech et al. 1996). It should be noted that the majority of ´ pohlavy Quarry are sponges, originally sponges from the U with siliceous skeletons (hexactinellids and demosponges); nevertheless, their skeletons having become calcified and ⁄ or partly pyritized due to diagenetic processes. Thick crusts of sediment were removed from the surfaces of the specimens, using a vibrating engraver (Record Power, model 7417070). Having been housed in dry conditions for several months, no additional drying was necessary. Specimens were subsequently immersed in 38% sulphuric acid in lidded plastic (polyethylene) and glass vessels. After 2 hours, they were removed from the acid, and carefully washed under a stream of water for 4 minutes. Afterwards, the fossils were covered with a neutralizing solution of water and sodium carbonate for 3 hours. Final cleaning was carried out by both ultrasonic cleaning and waterblasting (hand-held watergun). Treatment of calcified sponges, thick-shelled bivalves, echinoids, and other echinoderms all showed very good results, in the sulphuric acid. Before the treatment, the surfaces of the sponges were obscured by a thin crust of marly limestone, obstructing identification of any spicules, as well as any description of skeletal characteristics. Application of the procedures, described above, led to clean skeleton surfaces (compare Text-figs 1C to 1D, 1G to 1H, and 1I to 1J); allowing exact determinations, based on spicular characteristics. Moreover, the clean surface of the sponges revealed numerous well-preserved encrusters (Text-fig. 1D), providing palaeoecological and taphonomic information. The treatment of echinoderms was also successful (compare Text-fig. 1A, B, E, and F). Moreover, the clean surfaces of echinoids bear different kinds of bitemarks (Text-fig. 1B) and other bioerosional structures, as well as numerous epizoans, mainly represented by oysters, serpulids, and bryozoans. The surfaces of the macrofossils, cleaned by sulphuric acid, were examined at high magnifications, using both optical and electron microscopes. Applying a 50x
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A
B C
E
D
D
F
H
I
J
T E X T - F I G . 1 . A comparison of the macrofossils, before and after sulphuric acid treatment. A–B, Micraster cf. leskei (Desmoulins, 1837), RV92, 0.7x. A, before the treatment. B, after the treatment. C–D, Ventriculites chonoides (Mantell, 1822), RV93, 0.8x. C, before the treatment. D, after the treatment, arrows indicate encrusting oysters Pycnodonte vesicularis (Lamarck, 1809). E–F, Gauthieria radiata (Sorignet, 1850), RV94, 1.5x. E, before the treatment. F, after the treatment. G–H, Ventriculites alcyonoides Mantell, 1822, RV95, 0.8x. G, before the treatment. H, after the treatment. I–J, Ventriculites alcyonoides, RV96, 1.1x. I, before the treatment. J, after ´ pohlavy quarry (Upper Turonian, Bohemian Cretaceous Basin). Specimens coated with the treatment. All specimens from the U ammonium chloride before photographing. All specimens are housed in the Collections of the Czech Geological Survey, Prague.
magnification documented none or only slight damage of the sponge spicules (Text-fig. 2A, B). Very good results were also obtained by treating ichnofossils filled with fine- to coarse-grained bioclastic debris [e.g. Pseudobillobites rugosum (Reuss) and Thallasinoides burrows]. However, the treatment of some brachiopods (e.g. Woodwardirhynchia cuneiformis (Pettitt)
and Cretirhynchia minor Pettitt) led to the exfoliation of thin laminae, forming the shell. Destruction of thinshelled bivalves and other thin-shelled fossils is caused by the infiltration of the acid through shell fissures to the rock underneath the shell. Subsequent disintegration of the rock is followed by exfoliation of the shell fragments.
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A 500 µm µm
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B 500 µ m
Skeletal structure of sponges generated after sulphuric acid treatment. SEM micrographs. Note, that before the treatment, the surfaces of the sponge skeletons were completely obscured by a 0.2–1 mm thick crust of marly limestone. A, calcified desmas forming choanosomal skeleton of Phymatella intumescens (Roemer, 1864), RV97. B, calcified and partly pyritized lychniscs ´ pohlavy quarry, (Upper Turonian, forming choanosomal skeleton of Pyrospongia vrbaei Zaha´lka, 1900 (RV98). Specimens from U Bohemian Cretaceous Basin). Specimens are housed in the Collections of the Czech Geological Survey, Prague. TEXT-FIG. 2.
(Note: The character of surface preservation, before chemical treatment, was examined on sediment-free portions of the fossils).
CONCLUSIONS A new chemical preparation method, based on a sulphuric acid treatment, is highly applicable for the cleaning of calcitic, calcified and ⁄ or partly pyritized macrofossils from marlstones, marly limestones, and other lithified calcareous sediments. Although there are other methods for releasing calcareous fossils from calcareous rocks, this method fills the gap for fast, non-destructive, yet intense, treatment of macrofossils. The method has been successfully applied to different macrofossils from the Upper Cretaceous calcareous sediments of the Bohemian Cretaceous Basin and the Lower Saxony region of Germany. Macrofossils such as calcified sponges, corals, serpulids, thick-shelled bivalves, some brachiopods, echinoderms, bryozoans, various phosphatized moulds, and ichnofossils with coarse-grained infill were successfully treated. The surfaces of these extracted macrofossils remained undamaged, exhibiting minute skeletal details. Nevertheless, some brachiopods and thinshelled bivalves were damaged during this preparation process. Further experiments are needed to test this methodology on supplementary samples from different stratigraphic levels. Acknowledgements. This research was funded by the Grant Agency of the Czech Republic (No. 205 ⁄ 06 ⁄ 0842). I am
indebted to A. Pisera, M. Bubı´k and J. Zˇı´tt and for invaluable discussions and comments. A. Pisera is also acknowledged for taking SEM-photos in Polish Academy of Sciences (Phillips XL-20). I also wish to thank Palaeontology editor Philip C. J. Donoghue and two anonymous reviewers, whose suggestions and critical remarks helped to improve the manuscript. My thanks go also to J. Dasˇek (Czech Geological Survey) for technical support.
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