and Geosphere - American Chemical Society

Chapter 12

The Fate of Chitin in Quaternary and Tertiary Strata 1,2,5

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B. Artur Stankiewicz , Derek E. G . Briggs , Richard P. Evershed , Randall F . Miller , and Anja Bierstedt 3

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Downloaded by INDIANA UNIV BLOOMINGTON on May 9, 2015 | http://pubs.acs.org Publication Date: June 29, 1998 | doi: 10.1021/bk-1998-0707.ch012

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Biogeochemistry Research Centre, Department of Geology, University of Bristol, Bristol BS8 1RJ, United Kingdom Organic Geochemistry Unit, School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom Steinhammer Palaeontology Laboratory, New Brunswick Museum, Saint John, New Brunswick E2K 1E5, Canada Department of Chemistry, Fritz-Foerster-Bau, Dresden Technische Univeristät, Dresden, D-01069, Germany

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Chitin is one of the most abundant biopolymers on earth. It occurs in a range of organisms but is particularly important as a constituent of arthropod cuticles. Experiments have demonstrated that chitin is more resistant to degradation than protein, but it is rarely preserved in the fossil record. The chitin content of beetle cuticles from 11 Quaternary deposits in Canada, U K and U S A was estimated using pyrolysis-GC/MS and quantitative colorimetric assay. The proportion preserved ranged from 3 to 37 dry weight %. Analyses of insects and fresh-water crustaceans from several European Tertiary biotas revealed that the chitin biopolymer is preserved at levels varying from 2 to 38%. Chitin can survive, even for millions of years, in non-marine clastic sediments that provide favourable environmental conditions, but it is much more susceptible to degradation in marine settings. The 25 Ma lacustrine deposit of Enspel, Germany, preserves the oldest reliable evidence of chitin reported to date. The differences in the proportion of chitin preserved in fossils reflect the environment of deposition more than their age. Chitin is more likely to be found in fossils preserved in terrestrial than in marine strata.

The fate of various macromolecules that make up biological tissues has been the subject of geochemical studies for many years (1-4). This research has focused on biopolymers which contribute to coal and kerogen. Foremost among these are 5

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© 1 9 9 8 American Chemical Society

In Nitrogen-Containing Macromolecules in the Bio- and Geosphere; Stankiewicz, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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212 major components of plants (i.e. cellulose and lignin) and algae (i.e. aliphatic polymers), which are relatively resistant to bio- and chemical degradation during diagenesis (2). Interest in less stable macromolecules (e.g. chitin, proteins) has been limited due to their faster degradation in the biosphere and rare survival into the geosphere. Chitin is one of these less decay-resistant biopolymers (2). Although the annual production i n the biosphere is estimated to be second only to cellulose (5), chitin has received surprisingly little attention among biogeochemists. Chemically it is a polysaccharide, a major component of arthropod exoskeletons where it is cross-linked with proteins (see earlier chapter in this volume). Although chitin is relatively resistant to chemical and physical degradation, it is rapidly broken down in most modern marine and terrestrial ecological systems (6-9, earlier chapter i n this volume) by chitinoclastic microorganisms through enzymatic hydrolysis (10). M a n y studies have investigated the degradation of chitin under both oxic and anoxic conditions (6-9). However, the role of degraders such as fungi, and the influence of physical factors such as p H and temperature of the environment, rate of burial, and organic productivity are still poorly understood. Several recent studies have investigated the preservation of chitin i n fossils ranging i n age from Silurian (ca 420 M a ) to Holocene (11-17). These studies showed that under favourable environmental conditions chitin can be preserved in sedimentary rocks as old as 25 M a (16). Chitin has also been studied using stable isotope ratios as a source of paleoenvironmental information (12,18-20). However, a lack of sufficient material for such analyses is often a limiting factor. The structure of arthropod cuticles appears to be a very important factor in retarding chitin decomposition and promoting its preservation. Thick, heavily sclerotized cuticles, such as those of some beetles, may preserve chitin even in Tertiary strata (16). Beetle elytra are abundant in Quaternary deposits (21-24) where they provide an important indicator of climatic change (25, 26). However, few geochemical studies have investigated these fossils (72, 75). A n important limiting factor in studies of chitin in fossil specimens is the small quantity of cuticle normally available and the recalcitrant nature of the fossil material. Flash pyrolysis, in combination with gas chromatography and mass spectrometry ( p y - G C / M S ) , has proved to be a very efficient tool for elucidating the molecular composition of many modern and fossil materials (3, 4,27). This method was used to demonstrate the preservation of the oldest fossil chitin in beetles from Enspel, Germany (16). Colorimetric assay (28) has allowed the quantitative estimation of chitin in fossils, particularly Quaternary specimens (77). The application of Fourier-transform infrared spectroscopy to the recognition of chitin in fossils is also being investigated (Mastalerz M . , Indiana University, Geological Survey, in preparation). Here we present the results of our analyses of the fate of chitin i n Quaternary sediments, using both p y - G C / M S and colorimetric assay techniques, and briefly review the preservation of chitin in Tertiary sediments. Our goal is to summarize possible factors controlling the preservation of this very important biopolymer i n the geosphere.


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Sample description The Quaternary beetle elytra analyzed (Table 1) represent a range of environmental settings and ages (15, 23, 24, 29-33). The youngest sample is estimated to be 1600 years old (Thornog Bog) and the oldest ca 80,000 years old (Moose Point). The Tertiary fossils came from several European localities (16,17, M c C o b b et al. i n review). The youngest are insects and the crayfish Astacus sp. from the Pliocene lake Willershausen in Germany (ca 3 M a ) . A beetle from the Upper Miocene (8 M a ) of St. Bauzile, France, is preserved i n diatomite. The curculionid beetles from the Oligocène (25 Ma) of Enspel, Germany, are preserved in the volcanoclastic sediments of a maar-lake (16). The insects of the Late Eocene (36 M a ) Bembridge Marls of the Isle of Wight occur in brackish lagoonal and estuarine sediments (McCobb et al. in review). Experimental Quaternary beetles were separated from the sediments by kerosene flotation and sorted using ethanol. The cuticle of the Tertiary specimens was separated mechanically from the rock matrix. Prior to chemical analysis all specimens were extracted ultrasonically with CH2CI2 and methanol to remove extractable constituents and any contaminants introduced by handling. Colorimetric assay was performed on most of the specimens to establish the amount of chitin preserved in the cuticle. The assay is based on K O H hydrolysis, which deacetylates chitin to the glucosamine polymer chitosan, and further conversion of the latter to 2,5-anhydrohexoses using K H S O 4 and N a N 0 (SigmaA l d r i c h Chemical Co). The color reaction was developed using freshly prepared 3methyl-2-benzothiazolone hydrochloride ( M B T H ; Sigma-Aldrich Chemical Co.) and FeCl3 (Sigma-Aldrich Chemical Co.). The absorbance was recorded at 650 nm against reference blank solutions (prepared by following exactly the same protocol but without introducing a sample) using a Cecil C E 2292 Series 2 Digital Ultraviolet Spectrophotometer (see 28,17 for details of the procedure). Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) was performed on 0.05 and 0.5 mg of solvent-extracted cuticle using a C D S (Chemical Data System, Oxford, Pennsylvania) 1000 pyroprobe coupled to a Carlo Erba (Milan, Italy) 4130 G C , interfaced with a Finnigan (San Jose, California, U S A ) 4500 M S . Samples were placed in quartz holders and pyrolyzed in a flow of helium for 10 seconds in a platinum coil at 610°C. Compounds were separated using a Chrompack (Middleburg, The Netherlands) C P Sil-5 C B column (50 m χ 0.32 m m i.d., film thickness 0.4 mm). The G C oven was operated as follows: isothermal for 5 min at 35°C; temperature programmed at 4 ° C min" to 300°C and then isothermal for 10 min. The M S was operated in full scan mode (35-650 daltons, 1 scans/sec, 70 e V electron energy). The identification of pyrolysis products derived from chitin and proteins is described in detail elsewhere (14-16, 27). 2

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Table I. Samples of Quaternary beetles analysed in the course of this study and the results of the colorimetric assay. Sample name

Age*

Lithology

Locality

Chitin

[%] Beetle (Tenebrio molitor) Modern Beetle Ho (ca 1.6 Ka) Beetle (Scarabaeidae) Pie (25-28 Ka) Beetle Pie (25-28 Ka) Beetle (Olophrum boréale) Pie (

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