Organic Geochemistry 39 (2008) 1790–1799

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Organic Geochemistry

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Neutral monosaccharides as biomarker proxies for bog-forming plantsfor application to palaeovegetation reconstruction in ombrotrophicpeat deposits

Guodong Jia a,b, Jennifer A.J. Dungait a, Elizabeth M. Bingham a,Minna Valiranta c, Atte Korhola c, Richard P. Evershed a,*

a Organic Geochemistry Unit, Bristol Biogeochemistry Research Centre, School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UKb State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Chinac Environmental Change Research Unit (ECRU), Department of Biological and Environmental Sciences, University of Helsinki, P.O. Box 65, Viikinkaari 1, 00014, Finland

a r t i c l e i n f o a b s t r a c t

Article history:Received 12 November 2007Received in revised form 13 June 2008Accepted 1 July 2008Available online 9 July 2008

0146-6380/$ - see front matter � 2008 Elsevier Ltddoi:10.1016/j.orggeochem.2008.07.002

* Corresponding author. Tel.: +44 (0)117 928 7671129.

E-mail address: R.P.Evershed@bristol.ac.uk (R.P.

The application of molecular approaches to palaeovegetation reconstruction in peat is stillrelatively rare, with molecular level studies of carbohydrate organic geochemistry beinggenerally uncommon. In this report, neutral monosaccharides derived via acid hydrolysiswere investigated in modern bog-forming plants in order to assess their potential applica-tion as biomarkers in peat palaeovegetation reconstruction. The concentrations of majorneutral monosaccharides, i.e. glucose (Glu), xylose (Xyl), arabinose (Ara), galactose (Gal),mannose (Man), rhamnose (Rha) and fucose (Fuc), were determined using gas chromatog-raphy (GC) for 7 lichens, 10 Sphagnum species and 7 vascular plants collected from threeombrotrophic mires across northern Europe. Based on factor analysis of the modern plantmonosaccharide compositions, two carbohydrate proxies: [(Man + Gal):(Ara + Xyl)] and %contributions of Rha and Fuc in total Glu-free monosaccharides [%(Rha + Fuc)], wereselected as biomarkers for bog-forming plants. The three plant groups could be separatedby [(Man + Gal):(Ara + Xyl)], which showed decreasing values following the order:lichens > Sphagna > vascular plants. The high [%(Rha + Fuc)] in Sphagna allowed their sep-aration from lichens and vascular plants. These two factors were applied as plant group-specific indices to investigate vegetation change in a peat core from Kontolanrahka Bog,Finland. Our findings show strong correspondence with fossil plant abundances from thesame core, thereby confirming the potential of carbohydrate compositional parametersas proxies for palaeovegetation reconstruction in peat bogs.

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1. Introduction

Local water balance is the crucial climatic parameter forombrotrophic mires and its reconstruction from peat sedi-ment records is valuable for palaeoclimate research. Bog-forming vegetation is sensitive to precipitation and thusto the water table of the bog. Plant macrofossils, pollenanalysis (Barber et al., 1998, 1999; Mauquoy et al., 2002),

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Evershed).

humification indices (Blackford and Chambers, 1993) andtestate amoebae (Charman, 2001) have been used to inferpast hydrological variations and, by extension, the regionalpalaeoclimate. In addition, several lipid biomarkers havebeen shown to be characteristic of the major peat-formingplants, such as C23 n-alkane (Baas et al., 2000; Nott et al.,2000), n-alkane ratios (Nott et al., 2000; Pancost et al.,2002) and long chain ketones (Nichols and Huang, 2007)for Sphagnum species, 5-n-alkylresorcinols for sedges (Av-sejs et al., 2002) and triterpenoids for ericaceous shrubs(Pancost et al., 2003). Lipid biomarkers thus offer a poten-tially important tool for palaeoclimate reconstruction

G. Jia et al. / Organic Geochemistry 39 (2008) 1790–1799 1791

based on peat records, particularly where humification hasresulted in the loss of identifiable macrofossils (Fickenet al., 1998; Nott et al., 2000).

Polysaccharides are the major biochemical compo-nents of terrestrial vegetation and represent the mainform of photosynthetically assimilated carbon in thebiosphere. Given their ubiquity and abundance, carbohy-drates represent a potentially powerful tool for elucidat-ing the sources, processes and diagenetic pathways ofbiologically important organic materials in sedimentaryenvironments. However, they have been applied rela-tively infrequently for biological source discrimination(Cowie and Hedges, 1984; Hamilton and Hedges, 1988;Moers et al., 1990; Hicks et al., 1994; Boschker et al.,1995; D’souza et al., 2003). Given their importance insediments, it is surprising they have been so little ex-plored as palaeoenvironmental reconstruction tools(Swain and Rogers, 1966; Bourdon et al., 2000). Althoughin most settings carbohydrate compositions have beenshown to be highly sensitive to diagenetic alteration,the organic matter-rich nature of peat deposits and thepotential for preservation below the acrotelm makesombrotrophic mires eminently suitable for exploringthe potential of carbohydrates as palaeovegetation prox-ies. Despite this, few studies have investigated the carbo-hydrate compositions of peat and peat-forming plants(Morita and Montgomery, 1980; Morita, 1983; vanSmeerdijk and Boon, 1987; Moers et al., 1989; Francez,1991; Wicks et al., 1991; Bourdon et al., 2000; Comontet al., 2006). However, Bourdon et al. (2000) utilisedcarbohydrates, amongst other proxies based on a varietyof macromolecular components, to infer past inputs to atropical peat deposit. More recently, Comont et al. (2006)provided a tantalising glimpse of the potential carbohy-drates hold as source indicators for moss and sedgeinputs to peat deposits, reporting the following sourceindicators: xylose and arabinose for Cyperaceae; rham-nose, galactose and mannose for mosses, and ribose(and to a lesser extent, hemicellulose glucose) for micro-bial synthesis.

Described herein are the results of investigations aimedat exploring the potential application of carbohydratecompositional parameters as proxies for palaeovegetationchange in ombrotrophic peat deposits. This was achievedby: (i) characterising the carbohydrate composition of awide range of representative contemporary bog-forming li-chens, Sphagnum species and vascular plants in order to as-sess the potential of carbohydrates as source-specificproxies; (ii) exploring downcore variations in the derivedcarbohydrate-based proxies in a well characterised peatcore; and (iii) comparison of the variation in the carbohy-drate proxies with the fossil plant records for the samecore.

2. Materials and methods

2.1. Bog-forming plants

A total of 24 species common in European ombrotroph-ic mires were selected (Table 1). They included seven

lichens, ten Sphagna and seven vascular plants, includingrepresentatives of the Ericaceae (heath), Cyperaceae(sedge) and Betulaceae (birch) families. Most Sphagnaand lichens were collected from Kontolanrahka Bog,Finland, with the exception of Sphagnum magellanicum,which was collected from Butterburn Flow, UK. All vascu-lar plants were collected from Bissendorfer Moor,Germany. Whole plants were analysed unless otherwisestated (Table 1).

2.2. Peat core analysis

In addition to those of the bog-forming plants, mono-saccharide distributions were investigated from depths of(a) 0–148 cm with 4 cm resolution and (b) 260–318 cmwith 2 cm resolution, from a peat core from KontolanrahkaBog, Finland (60�470N, 22�470E). The core had been 14C-da-ted and analysed for fossil plant remains and pollen (seeVäliranta et al., 2007, for further details). Additional workon the humification and testate amoebae was also con-ducted as part of the ACCROTELM project.

2.3. Carbohydrate analysis

2.3.1. Neutral monosaccharide preparation and analysisThe method of Blakeney et al. (1983) as modified by

Docherty et al. (2001) was employed to prepare alditol ace-tate derivatives of total monosaccharides extracted fromplants and peat sediments. Pentaerythritol was used asinternal standard.

2.3.2. Gas chromatography (GC) and GC/mass spectrometry(GC/MS)

Identification of individual monosaccharide derivativeswas performed by comparison of retention times definedby GC analysis of a range of authentic monosaccharidestandards, and their identity as pentose or hexose sugarsconfirmed using GC/mass spectrometry (GC/MS). GC wasperformed using a Hewlett–Packard 5890 series II chro-matograph. Monosaccharide derivatives were separatedwith a Varian VF23ms 50% cyanopropyl fused-silica col-umn (60 m � 0.25 mm i.d. � 0.25 lm thickness) using Heas carrier gas (10 psi head pressure). The oven temperaturewas programmed from 50 �C (1 min) to 200 �C at20 �C min�1, then to 230 �C (held 22 min) at 4 �C min�1.Individual monosaccharide concentrations (C) were calcu-lated using the following expression: C = Ps/PIS �MIS/W,where Ps is the peak area of the monosaccharide alditolacetate; PIS is the peak area of pentaerythritol acetate(internal standard); MIS is the mass of pentaerythritol(nonacetylated) added to a sample before treatment; andW is the sample dry weight.

GC/MS analysis was performed using a ThermofinniganTrace GC–MS system. The column and temperature pro-gramme details were as described above. The interfacewas set to the maximum oven temperature, the ion sourcewas set to 30 �C below this and the mass spectrometerscanned a range from m/z 50 to 650 at 1.7 scan s�1. Datawere acquired and processed using Xcalibur Data SystemVersion 1.4.

Table 1Concentrations of individual monosaccharides (mg g�1 dry weight) and % contributions to dry weight in lipid-extracted plant samplesa

Bog-forming plants Rhamnose Fucose Arabinose Xylose Mannose Galactose Glucose Total

mg g�1 % mg g�1 % mg g�1 % mg g�1 % mg g�1 % mg g�1 % mg g�1 % mg g�1

LichensLichen 1 3.1 0.5 0.3 0.1 8.6 1.5 5.3 0.9 168.6 29.3 238.3 41.4 151.6 26.3 575.7Lichen 2 4.4 0.6 0.8 0.1 7.6 1.0 3.6 0.5 299.6 38.1 315.2 40.1 154.6 19.7 785.8Lichen 9 5.5 1.0 0.9 0.2 15.6 2.9 22.1 4.1 200.4 37.2 179.5 33.3 114.4 21.3 538.4Lichen 12 6.3 0.7 1.6 0.2 17.8 2.0 16.0 1.8 328.5 37.4 299.1 34.1 208.3 23.7 877.6Lichen 13 4.6 0.7 1.8 0.3 12.4 1.9 3.6 0.5 154.3 23.6 171.9 26.2 306.1 46.7 654.7Cladonia stellaris 3.1 0.5 0.2 0.0 9.1 1.4 1.4 0.2 303.7 46.4 189.0 28.9 147.4 22.5 653.9Cladonia stygia 3.0 0.4 0.8 0.1 8.4 1.2 3.5 0.5 218.5 31.9 179.7 26.2 270.8 39.5 684.7

Mean 4.3 0.6 0.9 0.1 11.4 1.7 7.9 1.2 239.1 34.8 224.7 32.9 193.3 28.5 681.5Stdev (±) 1.3 0.2 0.6 0.1 4.0 0.6 7.9 1.4 70.6 7.3 60.6 6.2 71.3 10.4 117.3

SphagnaS. magellanicum 4.2 1.8 2.3 1.0 11.7 5.1 31.2 13.6 24.9 10.8 40.5 17.6 115.2 50.1 230.1S. balticum 10.1 2.0 3.9 0.7 13.3 2.6 65.5 12.7 63.1 12.2 109.1 21.1 251.4 48.7 516.4S. cuspidatum 12.0 1.7 5.6 0.8 28.1 3.9 84.7 11.8 79.8 11.2 150.6 21.1 353.8 49.5 714.6S. rubellum 9.6 2.2 3.0 0.7 11.6 2.6 53.4 12.1 42.9 9.7 100.4 22.7 222.0 50.1 442.9S. capillifolium 3.4 1.4 1.8 0.7 8.2 3.4 22.3 9.2 29.1 12.0 50.0 20.7 127.3 52.6 242.1S. tenellum 4.6 1.6 2.5 0.9 11.1 3.8 39.5 13.6 31.2 10.7 58.9 20.2 143.3 49.2 291.2S. lindbergii 5.4 1.2 2.9 0.6 13.7 3.0 64.1 14.0 47.4 10.4 111.0 24.3 212.0 46.4 456.5S. majus 7.6 2.5 2.9 1.0 49.6 16.4 31.1 10.3 39.6 13.1 58.7 19.4 113.1 37.4 302.6S. fuscum 4.9 1.6 2.4 0.8 13.2 4.3 37.5 12.2 35.1 11.4 67.8 22.0 147.5 47.8 308.4S. angustifolium 5.0 1.2 3.6 0.9 20.8 5.1 49.9 12.2 48.2 11.8 91.3 22.3 190.2 46.5 408.9

Mean 6.7 1.7 3.1 0.8 18.1 5.0 47.9 12.2 44.1 11.3 83.8 21.1 187.6 47.8 391.4Stdev (±) 3.0 0.4 1.1 0.1 12.5 4.1 19.3 1.5 16.7 1.0 34.5 1.9 75.6 4.1 149.2

Vascular plantsEriophorum angustifolium(roots)

1.3 0.3 2.0 0.5 86.9 21.3 107.3 26.3 4.9 1.2 43.4 10.6 163.0 39.9 408.8

Eriophorum vaginatum 1.7 0.7 0.7 0.3 69.1 29.3 77.1 32.7 3.1 1.3 10.5 4.5 73.4 31.2 235.6Calluna vulgaris (leaves) 5.9 1.6 0.9 0.2 70.0 18.9 69.1 18.6 9.1 2.5 65.3 17.6 150.3 40.6 370.6C. vulgaris (stems) 1.4 0.5 0.4 0.1 17.4 6.2 107.7 38.3 8.7 3.1 26.5 9.4 118.9 42.3 281.0Empetrum nigrum 5.4 2.0 1.8 0.7 53.4 20.0 53.0 19.8 12.4 4.6 55.7 20.8 85.8 32.1 267.6Erica tetralix 1.8 0.9 0.4 0.2 35.0 17.8 58.2 29.6 6.0 3.0 30.3 15.4 64.9 33.0 196.5Vaccinium oxycoccos(leaves)

0.5 0.6 0.2 0.2 23.4 28.7 19.5 23.9 2.0 2.4 7.4 9.0 28.7 35.2 81.6

V. oxycoccos (stems) 2.0 0.6 1.1 0.3 29.6 8.4 109.1 31.1 13.1 3.7 39.0 11.1 157.4 44.8 351.2Betulasp. 0.8 0.5 0.7 0.4 22.4 13.4 50.7 30.3 4.4 2.6 10.9 6.5 77.2 46.2 167.1

Mean 2.3 0.9 0.9 0.3 45.3 18.2 72.4 27.8 7.1 2.7 32.1 11.7 102.2 38.4 262.2Stdev (±) 1.9 0.6 0.7 0.2 25.3 8.0 31.0 6.3 4.0 1.1 20.6 5.3 47.2 5.7 105.0

a Analysis conducted on whole plants unless otherwise stated.

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2.3.3. Statistical analysisFactor analysis of monosaccharide compositions of the

plants was performed on the basis of the percent contribu-tions of individual monosaccharides to total dry weight.The factor extraction method was principle componentanalysis (PCA) and the factor rotation method was Varimaxwith Kaiser normalization, using SPSS 13.0 for Windows.

3. Results and discussion

A literature method for carbohydrate analysis, i.e. aldi-tol acetates of total monosaccharides, was employed andwhich had been tested rigorously in our laboratory (Doch-erty et al., 2001) and employed by Macko et al. (1998) inthe analysis of sphagnum peat and, more recently by Teeceand Fogel (2007) to analyse carbohydrates in higher plants.The alditol acetate method allows analysis of a wider rangeof monosaccharides than the methylboronic (MBD) meth-od used by van Dongen et al. (2001) and produces a singlepeak for each sugar, unlike trimethylsilylation which pro-

duces a complex mixture of anomers for each monosaccha-ride; van Dongen et al. (2001) point out that glucitol mayderive from either glucose or fructose; however, fructoseis not reported as a significant component of either Sphag-num cuspidatum or Erica tetralix or other plants consideredherein.

3.1. Monosaccharide compositions of bog-forming plants

Seven major neutral monosaccharides, i.e. Glu, Xyl, Ara,Gal, Man, Rha and Fuc were found in all the species studied.Whole plants were analysed, apart from the vascular plantsCalluna vulgaris and Vaccinium oxycoccus (leaves and stem),and Eriophorum angustifolium (roots). Gas chromatogramsof typical monosaccharide distributions in vascular plants,lichen and Sphagna are shown in Fig. 1. Monosaccharideswere quantified (mg g�1 dry weight) for all plant speciesand used to calculate the % contributions of individualmonosaccharides (Table 1). A range of values for eachplant group (lichens, Sphagna and vascular plants) weredetermined. For example, mean Man concentration was

Fig. 1. Typical monosaccharide distributions for vascular plants repre-sented by Eriophorum vaginatum, lichens represented by Lichen 1 andSphagna represented by Sphagnum majus. Peak numbers refer to alditolacetate derivatives of individual monosaccharides: 1, rhamnose; 2,fucose; 3, pentaerythritol (internal standard); 4, arabinose; 5, xylose; 6,mannose; 7, galactose; and 8, glucose.

G. Jia et al. / Organic Geochemistry 39 (2008) 1790–1799 1793

239.1 ± 70.6 mg g�1 in lichens; 83.8 ± 34.5 mg g�1 Gal inSphagna; and, 72.4 ± 31.0 mg g�1 Xyl for vascular plants.Mean total monosaccharide concentration for each plantgroup was 681.5 ± 117.3 mg g�1 for lichens, 391.4 ±149.2 mg g�1 for Sphagna and 262.2 ± 105.0 mg g�1 for vas-cular plants. Glu was the most abundant monosaccharide inSphagna and higher plants, accounting for 47.8 ± 4.1% and38.4 ± 5.7% of total concentration, respectively, and alsocontributed 28.5 ± 10.4% to lichen carbohydrate concentra-tion (Fig. 2). In contrast, Rha and Fuc were the minor com-ponents constituting <2.5% of the total concentration ofmonosaccharides for all plants. Significantly, more thantwice the contribution of Rha, and three and eight timesmore Fuc, was found in Sphagna compared to lichen andvascular plants, respectively. These minor monosaccharidecomponents have been suggested to be derived frommicrobial or algal inputs to aquatic organic matter or sedi-

ment by several authors (Klok et al., 1984; Mopper et al.,1995; Zhou et al., 1998; Ogier et al., 2001; Engbrodt andKattner, 2005). Pertinently, Raghoebarsing et al. (2005) ob-served bacteria in the hyaline cells of submerged Sphagnumspecies. In fact, the carbohydrate compositions of sub-merged or wet lawn Sphagna, Sphagnum cuspidatum,Sphagnum balticum and Sphagnum rubellum (Välirantaet al., 2007), displayed significantly higher concentrationsof Rha and Fuc than other species (Table 1). As bacteria,and their necromass, are ubiquitous in sediments, thissource must also be considered when assessing the validityof proxies for palaeovegetation reconstruction. However,the significantly higher abundances of Rha and Fuc in Spha-gna suggest they hold potential for use in differentiatinginput of this plant group vs. other bog-forming plants.

Xyl and Ara were the second and third most abundantmonosaccharides, respectively, in the vascular plant group.This was expected as they are the major components ofhemicellulose polysaccharides in angiosperms, whilstgymnosperms have a high proportion of Man (Cowie andHedges, 1984; Hoffman et al., 2005). Gymnosperms werenot included in the selection of modern plants due to theirabsence from the fossil plant assemblages comprisingombrotrophic peat deposits (Väliranta et al., 2007). Galwas the second most abundant monosaccharide in Spha-gna, consistent with the recent findings for Sphagnum fallax(Comont et al., 2006). Lichens exhibited a distinctive com-position, in which the sum of Gal, Man and Glu accountedfor >95% of total concentration; moreover, both Gal andMan were 4% and 6% more abundant, respectively, thanGlu. However, the finding that non-vascular land plants,especially lichens, are rich in Man and Gal has only becomeapparent recently (Svihus and Holand, 2000; Olafsdottirand Ingolfsdottir, 2001; Popper and Fry, 2003; Omarsdottiret al., 2005) and has yet to be applied in palaeoenviron-mental reconstruction.

3.2. Development of carbohydrate proxies forpalaeovegetation reconstruction

Factor analysis of plant monosaccharide compositionwas conducted to identify possible underlying factors thatcould explain the observed variance. The technique per-mits the grouping of components that are covariant. Theanalysis revealed two major factors, the first (F1) describ-ing 53%, and the second (F2) 31%, of the total variance.The loadings of the individual sugars and the scores ofthe plant samples on the two factors are shown in Fig. 3.Two groups, (Xyl and Ara) and (Man and Gal), with highloadings, respectively, on the positive and negative sidesof F1 are clearly observed. Meanwhile, Fig. 3 shows thatlichens, Sphagna and vascular plants are clearly separatedalong the F1 axis. Therefore, the ratio of [(Man + Gal):(Ara+Xyl)], can be used to differentiate the three plant groups.Interestingly, similar parameters have been proposed byother researchers for other settings to reflect environ-ment-specific inputs. For example, ratios of [(Man + Ga-l):(Ara + Xyl)] or [Man:Xyl] have been used in marine,coastal and lacustrine environments as indicators of rela-tive inputs of phytoplankton and higher plants (Cowieand Hedges, 1984; Boschker et al., 1995; D’souza et al.,

Fig. 2. Mean contribution (%) of individual monosaccharides to total monosaccharide content in lichens, Sphagna and vascular plants.

Fig. 3. Factor analysis of monosaccharide compositions from bog-formingplants, based on % contribution of individual monosaccharides to totalmonosaccharide. Key: (q) lichen, (�) vascular plant, (M) Sphagna and (+)monosaccharide.

1794 G. Jia et al. / Organic Geochemistry 39 (2008) 1790–1799

2003), and in soil as indicators of relative inputs of bacteriaand higher plants (Oades, 1984).

The third group, Rha, Fuc and Glu, displayed high load-ings on the positive side of F2 that allowed separation ofSphagna from other plants (Fig. 3). However, other studiesof carbohydrate inputs (Cowie and Hedges, 1984; Boschkeret al., 1995; D’souza et al., 2003) did not use Glu in proxydevelopment because of its ubiquitously high abundancein biological systems that tends to obscure differencesreflected in the less abundant carbohydrates (Cowie andHedges, 1984; Boschker et al., 1995). In order to includeboth the major component Glu and minor componentsRha and Fuc, the sum of the normalised abundances ofthe three monosaccharides, [Rhan + Fucn + Glun], was usedas a proxy for separating Sphagna from other plants.Normalization involved a simple division of each monosac-charide’s % contribution by its mean % contribution in allthe plants investigated, being 1.1% for Rha, 0.4% for Fucand 39.4% for Glu. Nevertheless, based on the previousapplications, the relative concentration of (Rha + Fuc) as a% of total Glu-free monosaccharide [%(Rha + Fuc)], was alsoused for comparison.

The selected parameters: (i) [(Man + Gal):(Ara + Xyl)]and (ii) [Rhan + Fucn + Glun] and (iii) [(%(Rha + Fuc)], wereapplied to probe the monosaccharide compositions of themodern plant samples. The ratio [(Man + Gal):(Ara + Xyl)]gave values for lichens of 30.5 ± 15.9, Sphagna 2.0 ± 0.4and vascular plants 0.3 ± 0.2. The Sphagna specific proxyof [Rhan + Fucn + Glun] gave values for Sphagna of 4.5 ±0.5, while for lichens and vascular plants the values were2.5 ± 0.7 and 1.5 ± 0.4, respectively. The %[(Rha + Fuc)],another Sphagna-specific proxy, gave values for Sphagnaof 4.8 ± 0.8, while for lichens and vascular plants the valueswere 1.1 ± 0.4 and 1.9 ± 1.0, respectively. Thus, it appearsthat %[(Rha + Fuc)] is more effective for separating Sphagna

from other plants; hence, it was applied to investigatemonosaccharide compositions in a peat core in order to as-sess Sphagna contributions. The findings give the first indi-cation that the selected parameters are robust proxies forthe carbohydrates of different bog-forming plants. On thisbasis, we applied the above parameters to a peat core to as-sess their applicability to reconstruct palaeovegetationchanges in peat archives.

G. Jia et al. / Organic Geochemistry 39 (2008) 1790–1799 1795

3.3. Downcore variation in monosaccharide parameters

3.3.1. Differential degradation among monosaccharidesIn order to assess whether the carbohydrate proxies (i)

differentiate between plant groups using the proxy[(Man + Gal):(Ara + Xyl)] and (ii) distinguish Sphagna fromlichens and vascular plants using the proxy [%(Rha + Fuc)],a study of total monosaccharide distributions was con-ducted for a peat core extracted from Kontolanrahka Bog,Finland. Fig. 4 shows that, overall, concentrations of mono-saccharides from 0 to 148 cm were higher than in thedeeper 260–318 cm section, indicating degradation of car-bohydrates downcore. More detailed investigation revealedtwo main patterns of variation in concentration of individ-ual monosaccharides. The first showed a rapid decrease inAra and Gal from the surface to 25 cm depth, and thereaf-ter their concentrations were remarkably constant to318 cm. The second pattern showed a comparatively slowdecline downcore for Rha, Fuc, Xyl, Man and Glu, indicatingdifferential degradation between monosaccharides (ormore precisely the polysaccharides from which theyderive). These trends suggest that Ara and Gal are compo-nents of polysaccharides that are more degradable thanother carbohydrates. Differential degradation amongcarbohydrates has been observed in several investigationsof aquatic sediments. Glu, the main component of storagepolysaccharides in algae, has been found to be more labilethan structural polysaccharide components (Klok et al.,1984; Bhosle and Wagh, 1989; Hernes et al., 1996; Ogieret al., 2001). Our results do not show a clear degradationtrend downcore for Glu, perhaps reflecting its high concen-tration and role as a component of structural polysaccha-

Fig. 4. Variation in concentrations (mg g�1) of individual monosa

rides, e.g. cellulose, in bog-forming plants. Thus, thepreferential degradation of Gal and Ara suggests that thesemonosaccharides were in a less refractory form. Because ofthe diverse physiological roles and structures of plant car-bohydrates, one might expect those of different plantgroups to be more prone to degradation than others, i.e.through physiochemical protection as a result of lignifica-tion in vascular plants. However, exploring the extent ofdegradation of intact plant polysaccharides from sedimen-tary environments presents an analytical challenge.

Diagenetic alteration of neutral sugars has been investi-gated in a variety of environments (Cowie and Hedges,1984; Ittekkot and Arian, 1986; Hedges et al., 1994; Cowieet al., 1995; Hernes et al., 1996; Opsahl and Benner, 1995;Benner and Opsahl, 2001). The studies consistently foundincreases in % deoxysugars (fucose and rhamnose) and de-creases in % glucose as diagenesis progressed, with theexception of the work by Amon and Benner (2003), whichshowed an enrichment in Glu, Fuc and Rha, and a depletionin Ara, Gal and Man as diagenesis progressed. The increasein % deoxysugars has been attributed to contributions fromexternal sources, such as microbial biomass (Cowie andHedges, 1984; Hamilton and Hedges, 1988; Ogier et al.,2001; Panagiotopoulos and Sempéré, 2007) or to selectivepreservation of deoxysugars (Hedges et al., 1994; Amonand Benner, 2003). However, both attributions were morespeculative than demonstrative, because distinctions be-tween the selective preservation and contributions frommicrobes have never been satisfactorily resolved. In ourwork, no increasing trend existed downcore, so selectivepreservation of Fuc and Rha does not appear to apply inthis context.

ccharides in a peat core from Kontolanrahka Bog, Finland.

0 50

150

120

90

60

30

0

300

270

0 80 0 50 100 0 1 2 0 0.1 0.20 1 2 3 2 4 6 8

Dep

th (c

m)

B C

D

E

F

G

40

Vascular plants

A

Lichens Sphagna [Man:Xyl] [(Man+Gal):(Ara+Xyl)] [(Rha+Fuc):(Man+Xyl)] [%(Rha+Fuc)]

Fig. 5. Comparison of monosaccharide parameters with macrofossil records. Key: (A) % lichen, (B) % vascular plants, (C) % Sphagna (NB. Vertical lines denoteM. anomala), (D) [Man:Xyl]; (E) [(Man+Gal):(Ara+Xyl)], (F) [(Rha + Fuc):(Man + Xyl)], (G) [%(Rha + Fuc)]. Shaded grey areas denote proxy values for modernSphagna.

1796 G. Jia et al. / Organic Geochemistry 39 (2008) 1790–1799

As to the selective compositional alteration among Ara,Gal, Man and Xyl, it is impossible to give a distinct degra-dation order because of the variable behaviour evidencedby the current literature. For example, Hedges et al.(1985) proposed the following order of decreasing stabilityfor angiosperm wood monosaccharides: Ara > Gal > Fuc >Rha > Glu > Man > Xyl. However, the authors admitted thattheir results were different from some other reports. Pana-giotopoulos and Sempéré (2007) found that pentoses (Araand Xyl) degraded faster than hexoses (Man and Gal),while Opsahl and Benner (1993, 1999) found that Gluand Xyl degraded more rapidly than other neutral sugars.Hernes et al. (1996) proposed that there are differing reac-tivities among structural carbohydrates, and that the lessrefractory structural component was enriched in Gal andAra, while the more refractory structural component wasenriched in Rha, Fuc, Xyl and Man. Our work shows themore labile nature of Ara and Gal, consistent with Herneset al. (1996). Thus, the impact of preferential degradationof Ara and Gal on the [(Man + Gal):(Ara + Xyl)] and[%(Rha + Fuc)] proxies had to be considered.

The effect of omitting Ara and Gal from [(Man + Gal):(Ara + Xyl)] was tested using [Man:Xyl] and the [%(Rha +Fuc)] parameter was tested by using [(Rha + Fuc):(Man +Xyl)]. Fig. 5 shows the results of the application of all fourpossible carbohydrate proxies downcore; it is clear thatany bias due to preferential degradation is minimal. Thegrey vertical bands (Fig. 5) show the range of each proxyfor modern Sphagna. It can be seen that [Man:Xyl] and[(Rha + Fuc):(Man + Xyl)] ratios recorded in the peat coreare closer to modern Sphagna carbohydrate distributions;

hence, these proxies were applied to investigate carbohy-drate contribution to the core in relation to macrofossilrecords.

3.3.2. Comparison of monosaccharide parameters withmacrofossil records

In general, the carbohydrate proxies tracked the patternof fossil plant remains (Fig. 5). The changes in the latterhave been discussed in detail by Väliranta et al. (2007).Plant fossil remains in the upper part of the core from 0to 148 cm revealed that vegetation input was >80% Spha-gna, with relative abundances varying among Sphagnumfuscum, S. rubellum and S. balticum. An exception to the pat-tern was the liverwort Mylia anomala, which accounted forca. 50% of the non-vascular plants at the top of the core (0–12 cm); unfortunately, no modern M. anomala specimenswere available for comparison in this study. Vascular plantcontributions increased between 50 and 80 cm depth, butconstituted <20% total fossil plants. Lichens increased to10% at a depth of 48 cm and 30% at 148 cm. The deeperpart of the core, from 260 to 318 cm, showed relatively lar-ger shifts in vegetation type. Maxima in lichen abundancepreceded those in vascular plant abundance in the lowerpart of the core from 288 to 318 cm (Fig. 5). There weretwo major declines in Sphagna abundance centred at 288and 306 cm which correspond to increased abundancesof vascular plants. From 260 to 288 cm, Sphagna domi-nated, with a slight decrease at ca. 270 cm.

Sphagna-dominated intervals displayed [Man:Xyl] and[(Rha + Fuc):(Man + Xyl)] ratios in the range for modernSphagna. Most of the variability beyond the ranges for

Fig. 6. Monosaccharide distributions from 278, 300 and 306 cm depths ofa peat core collected from Kontolanrahka Bog, Finland. Peak numbersrefer to alditol acetate derivatives of individual monosaccharides as forFig. 1.

G. Jia et al. / Organic Geochemistry 39 (2008) 1790–1799 1797

modern Sphagna results from contributions from vascularplants or lichens, as reflected in the fluctuations in theplant macrofossil records (Fig. 5). The coincident low val-ues of the two proxies at 60–84, 270, 292 and 304–318 cmare consistent with an increase in the dominance ofvascular plants. Increases in relative lichen abundance cor-responded to relatively high [Man:Xyl] values. The promi-nent [Man:Xyl] peak at ca. 300 cm depth is coincident witha relatively low [(Rha + Fuc):(Man + Xyl)] value (Fig. 5),reinforcing the evidence of lichen input. Fig. 6 showsmonosaccharide distributions from peat sampled fromhorizons which are Sphagna (278 cm), lichen (300 cm)and vascular plant (306 cm) dominated. Comparison ofthe contributions clearly illustrates individual carbohy-drates varying systematically with depth, reflecting palae-ovegetation changes consistent with the plant macrofossilrecord.

However, an increase in [(Rha + Fuc):(Man + Xyl)] at288 cm depth was not consistent with a peak in lichenfossil remains, although [Man:Xyl] remained relativelyhigh. Moreover, there was another significant increase in

[(Rha + Fuc):(Man + Xyl)] values at 44 cm that was notconsistent with the lichen macrofossil record. Therefore,the [Man:Xyl] ratio seems to be recording vegetationchange more faithfully than the [(Rha + Fuc):(Man + Xyl)]proxy. This is perhaps due to the contribution of Rha andFuc from microbes, which would have been more activein the deeper aerobic acrotelm during drier periods. Hopa-noids, which have a bacterial provenance (Quirk et al.,1984; Pancost et al., 2000), were also present in higherconcentration around this depth. Since Rha and Fuc wereminor components of the total monosaccharides, their rel-ative contributions would have been prone to significantfluctuations resulting from varying microbial input.

4. Conclusions

Seven major neutral monosaccharides i.e. glucose (Glu),xylose (Xyl), arabinose (Ara), galactose (Gal), mannose(Man), rhamnose (Rha) and fucose (Fuc), were identifiedand quantified for 24 modern plant species and used to cal-culate the % contributions of individual monosaccharidesto total monosaccharides. The major monosaccharide invascular plants and Sphagna was Glu, while lichen carbo-hydrates were dominated by Man and Gal. Vascular plantsalso contained significant abundances of Xyl and Ara, andSphagna displayed significantly higher abundances of Fucand Rha. Therefore, the three modern plant groups couldbe separated by their relative abundances of monosaccha-rides using the proxy [(Man + Gal):(Ara + Xyl)] in the orderof decreasing values: lichens > Sphagna > vascular plants.This proxy was modified to [Man:Xyl] to accommodate po-tential faster degradation of Ara and Gal, and was found torecord vegetation change more faithfully based on macro-fossil evidence when applied to the analysis of carbohy-drates in a peat core from Kontolanrahka Bog, Finland.Similarly, [(Rha + Fuc):(Man + Xyl)] was found to be a morereliable proxy for Sphagna than [%(Rha+Fuc)] in the core,perhaps due to climate-driven fluctuations of microbe-de-rived Rha and Fuc. This supposition needs to be confirmedby additional microbial biomarker analysis. However,incompatibilities between the lichen macrofossil recordand [(Rha + Fuc):(Man + Xyl)] showed that [Man:Xyl] wasthe most reliable proxy overall, as it showed a high degreeof correspondence with the abundances of macrofossilsfrom the same core, especially lichen input. The findingssuggest that carbohydrate proxies have the potential tobe applied to reconstruct palaeovegetation change in peatbogs, and could become a valuable addition to the reper-toire of organic geochemical proxies available for palaeo-environmental reconstruction based on these relativelyunexploited molecular archives.

Acknowledgments

We thank D. Mauquoy for the identification of plantspecies and E. McClymont for preparation and lipidextraction of peat. We also thank I. Bull and R. Berstanfor technical support in the Organic Geochemistry Unit,University of Bristol, and two anonymous reviewers forconstructive comments. The work was funded in part by

1798 G. Jia et al. / Organic Geochemistry 39 (2008) 1790–1799

a China Scholarship Council awarded to G.J. The NERC arethanked for mass spectrometry facilities.

Associate Editor — G.D. Abbott

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