Lipid biomarker, δ13C and plant macrofossil stratigraphy of a Scottish montane peat bog over the last two millennia

PII:

Org. Geochem. Vol. 28, No. 3/4, pp. 217±237, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0146-6380/98 $19.00 + 0.00 S0146-6380(97)00126-5

Lipid biomarker, d13C and plant macrofossil stratigraphy of a Scottish montane peat bog over the last two millennia K. J. FICKEN1, K. E. BARBER2 and G. EGLINTON1 1

Biogeochemistry Centre, Department of Geology, Wills Memorial Building, Queens Road, Bristol, BS8 IRJ, U.K. and 2Palaeoecology Laboratory, Department of Geography, University of Southampton, High®eld, Southampton, SO17 1BJ, U.K. (Received 5 February 1997; returned to author for revision 20 May 1997; accepted 19 November 1997)

AbstractÐSeven horizons of a peat core covering the last 2000 years from Moine Mhor, a blanket bog in the Cairngorm Mountains, Scotland have been examined for the carbon number distributions of the long-chain hydrocarbon, alcohol and acid components using gas chromatography (GC) and gas chromatographyÐmass spectrometry (GCÐMS). Compound speci®c d13C values for individual n-alkanes were obtained, using gas chromatographyÐisotope ratio mass spectrometry (GCÐirms). The lipid biomarker distributions and the d13C values have been compared with those of eleven species of living plant dominant at the contemporary surface of the bog. The observed lipid stratigraphy shows only partial agreement with that calculated using macrofossil abundance data and the lipid distributions for the living taxa, a result which re¯ects the inherent uncertainties in both the lipid biomarker and the macrofossil approaches to palaeoenvironmental stratigraphy. The carbon isotope values for the individual n-alkanes of the plants (ÿ27.6 to ÿ36.6-) and of the peat layers (ÿ29 to ÿ31.7-) are as expected for the C3 photosynthetic pathway. However, the n-alkanes from the surface samples were more depleted (by 02-) in 13C than those from the rest of the core, a negative shift which may, in part, re¯ect the shift (ÿ1.2-) in d13C estimated for CO2 in a pre-industrialised to an industrialised atmosphere. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐlipids, d13C, palaeoenvironment, peat, blanket bog, Cairngorms

INTRODUCTION

In this paper we use distributions and individual d13C values of plant lipid biomarkers to document the molecular stratigraphy of Moine Mhor, a well characterized montane blanket peat bog. Where possible, we have made correlations between molecular information and plant macrofossil data acquired from the same samples of peat. The aim of this biogeochemical project is to assess the use of molecular ®ngerprints in the reconstruction of plant inputs at selected time horizons of this peat core and then, when possible, derive new climatic change proxies which can be used to infer palaeoclimatic changes recorded in this and other bogs. This molecular stratigraphic approach has already been employed successfully in marine sediment cores (Brassell et al., 1986; Farrimond et al., 1990; Conte et al., 1992) and to a lesser extent lake (Cranwell, 1973; Meyers et al., 1984) and peat bog cores (Farrimond and Flanagan, 1995). In this paper we combine it with the established macrofossil approach which utilises the changes in relative proportions of taxa recognised as plant fragments in peat horizons (Barber et al., 1994). The extensive blanket bog studied is at Moine Mhor at 950 m altitude in the Cairngorm Mountains, 6 km west of Lochan Uaine, the mon217

tane lake which was also examined as part of the TIGGER IIa project (Battarbee et al., 1996; Barber et al., unpubl. results). This close proximity was deliberate, to minimise di€erences in environmental factors, but Moine Mhor is also one of the least disturbed and ®nest examples of a montane blanket bog in Europe. Analyses for bulk density, humi®cation and macrofossils were carried out (Barber and Maddy, unpubl. results), and 17 radiocarbon assays were performed by the NERC Radiocarbon Laboratory. The pro®le covers the last 2000 yr, with a basal calibrated date of 150 A.D. The bulk density and humi®cation pro®les show substantial and largely coincident changes, the most recent of which is dated to 1715±1850 A.D. (Barber and Maddy, unpubl. results). The peat contained abundant plant macrofossils, unusual for a blanket bog, and the banded upper peats show abrupt shifts between Racomitrium lanuginosum and Sphagnum domination. This lasted until very recently when sedge began to replace moss. Since this part of Moine Mhor is a water-shedding site, these changes are most likely to be climatically-driven, the bog being wetter during the high Sphagnum stages. This paper will discuss the biogeochemical ®ndings of seven peat samples selected from di€erent

218

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depths in the core. In order to calibrate and explain some of the variation in the geochemical analyses from the peat, a collection of living plant material was made in November 1995 from the same area as the peat core and stored at 48C. Although much of the Moine Mhor blanket bog is dominated by Eriophorum vaginatum (cotton sedge) the particular area of bog from which the core was taken has a sedge/moss cover of Carex bigelowii and R. lanuginosum. The other common species collected were the bog-mosses Sphagnum capillifolium and S. fuscum, the lichens Cladonia uncialis, C. arbuscula, Sphaerophorus globosus and Cetraria islandica, the club moss (actually a primitive vascular plant, not a moss) Huperzia selago, and the dwarf shrubs Empetrum nigrum and Vaccinium vitis-idaea. Although there are some 20±30 plant species over the blanket bog, these dominate the core area. However, the sedge component of the macrofossil assemblages below 16 cm was not identi®able to species level and may not be dominated by C. bigelowii. Other species, not collected, or detectable in the macrofossil analyses, could also contribute to the lipidsÐthese would include fungal hyphae (very common in these situations), bacteria and diatoms. The identi®cation and quantitation of biological markers has been widely used to assess the sources of organic matter in ancient sedimentary environments as a means of determining the palaeoenvironment (e.g. Didyk et al., 1978; Meyers and Ishiwatari, 1993). However, relatively little has been recorded for peat bogs, despite their value as tools for palaeoenvironmental reconstruction in other disciplines (Barber et al., 1994; Shaw and Carter, 1994; Gajewski et al., 1995). Most biogeochemical studies of organic matter in peat bogs have used pyrolysis (e.g. Bracewell et al., 1980; Halma et al., 1984; van der Heijden, 1994) but few studies have employed solvent extraction (Lehtonen and Ketola, 1993, 1990; Dehmer, 1993; Farrimond and Flanagan, 1995). In general, previous studies of peat bogs have examined bulk samples of peat and the lipid composition of the various peat forming plants has largely been overlooked (Lehtonen and Ketola, 1993). However, quantitative biomarker analysis provides the opportunity to compare the presumed biological input with those signals actually recorded in the lipids of the peats after the decomposition and mineralisation processes of peati®cation have taken e€ect. To our knowledge, few papers have tested quantitatively the biomarker approach to palaeo assessment of ecosystems. Farrimond and Flanagan (1995) have made one attempt with a Flandrian peat (Northumberland, U.K.) while Lichtfouse et al. (1995) studied soil n-alkanes before and after a change in plant type from C3 to C4. Microbial mats have also been studied, the rationale being the relative simplicity of the ecosystems involved e.g. Zeng

et al. (1992). Usually it has been assumed that the biomarker approach should work at least semiquantitatively, and distributions found in Recent and ancient sediments have frequently been used to infer past inputs without seeking other con®rmatory data. There has been little validation of the approach through quantitative assessment of contemporary inputs and the composition of the sediment formed. The molecular organic approach to stratigraphy employed in this paper is based on leaf wax contributions. It assumes that each species of plant entering a sediment will contribute a distribution of leaf waxes, notably n-alkanes, n-alkanols, n-alkanoic acids and wax esters and that these patterns of long chain homologues will survive diagenesis largely unchanged. For example, the amounts of n-alkanes contributed will vary from species to species and will also depend on other factors such as plant physiology and environmental conditions. However, we can make the working assumption that the resulting distribution in the deposited sediment will be the sum of the inputs from the various plant species. Of course, selective cropping of the herbage by animals will a€ect the input, but n-alkanes are well known to survive largely unchanged passage through the digestive systems of animals (Collister et al., unpubl. results). Metabolic contributions of long-chain n-alkanes to sediments by animals and fungi are generally regarded as negligible. However, several authors e.g. Nichols et al. (1988), Lichtfouse et al. (1994) and Colombo et al. (1996, 1997) have reported the occurrence of long-chain n-alkanes in algae. Also diagenetic processes in sediments, such as microbial oxidation are known to skew distributions of n-alkanes, with longer chains being least a€ected by removal. Overall, the working paradigm holds that n-alkane distributions re¯ect inputs fairly reliably in recent sediments. The approach undertaken in this paper was the fractionation and assessment of the freely extractable n-alkanes, n-alkanols and n-alkanoic acids in the hydrocarbon, alcohol and acid fractions, respectively. However, the combined fractions, including the polar fraction, of the ®ve dominant plant species were also subjected to transesteri®cation so that the changes in the n-alkanol and n-alkanoic acid distributions due to natural diagenesis in the highly acidic (pH 4) peat horizons could also be assessed. In this way it was hoped that a better assessment of the plant species input could be derived. A recent development in this area is the use of compound speci®c carbon isotopic compositions to obtain additional palaeoenvironmental information (e.g. Hayes et al., 1990; Cerling et al., 1991; Ishiwatari et al., 1994; Huang et al., 1995). Carbon isotopic compositions of individual lipids re¯ect both the isotopic composition of the carbon source

Scottish montane peat bog

utilised by the organism and the isotopic fractionations accompanying carbon ®xation and biosynthesis, which are, in turn, dependent on environmental conditions (Hayes, 1993). Although the carbon isotopic compositions of the lipid components of a variety of plants have now been established (e.g. Cranwell, 1973; Rieley et al., 1991, 1993; Collister et al., 1994), most species of peat-forming plants are still, to our knowledge, uncharacterised by these procedures. A combination of biomarker distributions and isotopic measurements can greatly enhance our ability to trace the inputs of organic material and hence obtain information about the biogeochemical processes occurring in the depositional environment. METHODOLOGY

Stratigraphy The macrofossil zones A/B±F (Table 1) were assigned on the basis of microscopic quantitation (Barber and Maddy unpubl. results). Peat components were estimated from 15 averaged quadrat counts under low power magni®cation (10) using a 10  10 square grid graticule in the eyepiece of the microscope. If present, a random selection of moss leaves (>100 per sample interval) were then identi®ed at 400 to the lowest possible taxonomic level and expressed as percentages. 14C ages for the total organic carbon were measured by the NERC Radiocarbon Laboratory. Samples Peat. In the ®eld multiple peat cores were examined to ensure that a representative pro®le was taken. These test cores showed up the banded nature of the top 25 cm of peat over quite a large area of bog. The pro®le for analysis was taken (in July, 1993) using 15  15 cm aluminium monolith tins, 30 cm in length, and a wide diameter Russian corer to a depth of 90 cm. The peat core was kept moist and at 48C until sectioned. For the lipid analysis, a 1 cm slice was removed from the core at the chosen depth (one from approximately the middle of each of the six macrofossil zones identi®ed and the surface sample) and freeze-dried before analysis. Plants. The plants were frozen until analysis and then freeze dried. Any foreign material on the plants was removed before analysis. Both the peat and the plants were ground to a ®ne powder using a pestle and mortar prior to lipid extraction. Bulk d13C Homogenised samples of 19 peat sections were analysed for bulk d13C at the NERC Radiocarbon Laboratory by sealed tube combustion. All bulk d13C values are expressed as - relative to the Pee Dee Belemnite (PDB) standard.

219

Lipid extraction Total lipids were extracted from seven 1 cm sections of powdered, freeze dried peat (0.15±0.4 g) by sonication and centrifugation using a solvent system of sequentially decreasing polarity (3  100% MeOH, 2  1:1 MeOH:DCM and 5  100% DCM). The total extracts were each split into an acid and neutral fraction by solid phase extraction (Aminopropyl Bond Elute; for each sample, a new column prewashed with DCM and DCM:isopropanol, 2:1). Each column quantitatively retains acids when total extracts are ¯ushed through with DCM:isopropanol (2:1). The acid fraction was subsequently recovered with 2% acetic acid in ether. The neutral fraction was fractionated further into a hydrocarbon fraction, alcohol fraction and a polar fraction (not discussed in this paper) by thin layer chromatography (silica gel 60, 0.25 mm thick: solvent: 1% acetic acid in hexane:ethyl acetate 7:2). The acid fraction was methylated with methanolic HCl and the alcohol fraction was derivatised by bis(trimethylsilyl)-tri¯uoroacetamide (BSTFA) prior to analysis by GC. A known amount (15 mg in 100 ml iso-octane) of standard solution (n-C36 alkane) was added to each fraction prior to analysis by GC. The eleven species of powdered freeze dried plant were extracted in exactly the same way as the peat samples. Transesteri®cation All the fractions (including the polars) of the ®ve major plant species (R. lanuginosum, C. bigelowii, S. capillifolium, V. vitis-idaea and C. uncialis) were recombined and dried under a stream of nitrogen. The samples were transesteri®ed under nitrogen using methanolic HCl in toluene (558C for 12 h) and the products extracted with hexane. The hexane was evaporated and the sample was resuspended in DCM. Just prior to gas chromatography the sample was derivatised under nitrogen using BSTFA. The products were identi®ed by GCÐMS. Lipid analysis GC analyses were carried out on a Varian 3400 GC ®tted with a split/splitless injector and FID. A CPsil5CB (Chrompack) fused silica capillary column (50 m  0.32 mm: 0.17 mm ®lm thickness) was used. The oven temperature was held at 608C for 1 min, ramped at 108C minÿ1 to 1808C and then ramped at 48C minÿ1 to 3008C and held there for 25 min. Hydrogen was the carrier gas. GCÐMS analyses of selected samples were performed on a Carlo Erba Mega gas chromatograph (on-column injection, 70 eV EI) interfaced directly with a Finnigan 4500 mass spectrometer. The column and temperature program were the same as for the GC analyses. Helium was used as the carrier gas.

220

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Compound speci®c carbon isotope analyses These were performed using a Varian 3400 GC attached to a Finnigan MAT Delta-S isotope ratio mass spectrometer via a combustion interface consisting of an alumina reactor (0.5 mm i.d.) containing copper and platinum wires (0.1 mm o.d.). The column, carrier gas and the temperature program were the same as for the GC analyses. Each sample was run in duplicate to ensure reproducibility (20.5-). All carbon isotopic ratios are expressed as - relative to the Pee Dee Belemnite (PDB) standard. Samples were concentrated so that a peak height of 0.5±2 V was achieved for the compounds of interest. Peaks below this threshold were generally found not to give reproducible results. In Fig. 1 the hydrocarbon trace is that of the fraction after further puri®cation. The puri®cation step routinely utilised for the hydrocarbon fractions to be analysed by GCÐirms consisted of urea adduction (a saturated solution of urea in methanol was added to the sample and the non adducted hydrocarbons were extracted with hexane. The adducted hydrocarbons (n-alkanes) were extracted from the urea crystals by dissolving the urea crystals in de-ionised water and extracting the solution with n-hexane) followed by elution (n-hexane) from a mini column (5 cm) of freshly activated alumina. Only the n-C29 and n-C31 alkane peaks were abundant enough for d13C measurement in most samples. Contaminants All glassware was washed, ®red at 4008C overnight and rinsed with solvent before use. Extraction blanks were run. The n-C22 alkane is known to be a contaminant, possibly derived from the packing plastics of the TLC plates (Douglas and Grantham, 1973) and so this compound has been removed from the subsequent diagrams and discussion. This problem has been encountered by others analysing peat cores (e.g. Lehtonen and Ketola, 1993). The nC25 alkane coeluted with a plasticiser, identi®ed by GCÐMS (m/z 149) but this was removed upon the puri®cation of the sample.

RESULTS

Lipid geochemistry Typical chromatograms of fractions derived from one of the peat layers (22±23 cm) are shown in Fig. 1. Each fraction contains compounds additional to the straight chain homologues, including steroid ketones, wax esters, higher plant triterpenols, sterols and hopanols, which are not addressed further in detail in this paper. Similar distributions were found for the fractions from all seven horizons.

The sums of the concentrations of all the nalkanes and the n-alkanols (Table 1) show similar patterns down the core. They both decrease in concentration at sample 22±23 cm, whereas the acids increase in this horizon and become the dominant lipid fraction. n-Alkanes The n-alkane distributions for ®ve major plant species are shown in Fig. 2. Only one species of Cladonia, Sphagnum and ericaceae are shown. Data for the other plant species studied are given in Tables 1 and 2 . The n-alkane distributions are di€erent for each of the ®ve species given in Fig. 2. In both species of Sphagnum there is a large contribution from the shorter chain length n-alkanes (nC23 and n-C25 alkane), although S. capillifolium also has a large contribution from the n-C31 alkane, nC27 alkane is dominant in C. bigelowii (sedge) whereas R. lanuginosum has n-C31 alkane as its dominant n-alkane. The Cladonia has an n-alkane distribution pattern that is similar to that of R. lanuginosum (Fig. 2). The V. vitis-idaea has n-C29 as its dominant alkane. The histograms of the n-alkanes in the peat samples show a homologous series of the n-alkanes extending from n-C19 to n-C33 and maximising at nC31 (Fig. 3). In all samples there is a high odd-overeven predominance in the n-C25±35 region (Table 1) which is consistent with an epicuticular leaf wax origin (Eglinton and Hamilton, 1967). Samples 0±1 and 5±6 cm have a similar distribution pattern which is slightly di€erent from the rest, due to a higher proportion of the n-C33 alkane. n-Alkanols The n-alkanols derived from the plants range from n-C20±32 (Tables 1 and 2) and maximise either at n-C24, n-C26, n-C28 or n-C30 (Tables 1 and 2). Like the n-alkanes, the n-alkanol distributions are distinct for each of the plant species (Fig. 2). R. lanuginosum is dominated by n-C28 alkanol, although there is a large contribution of n-C24, nC26 and n-C30 alkanols. The n-alkanol distributions of the two species of Sphagnum are very similar to each other, with n-C24 and n-C26 alkanol being the dominant homologues. C. bigelowii has two dominant n-alkanols, the n-C28 and n-C30. The Cladonia species are dominated by the n-C28 alkanol and also have a large proportion of the n-C30 and n-C32 alkanols. The V. vitis-idaea, like the C. bigelowii, has dominant n-C28 and n-C30 alkanols although it also contains a large proportion of the n-C26 and n-C32 alkanols. The n-alkanols extracted from the peat range from n-C18±32 and show a strong even-over-odd predominance (Fig. 3; Tables 1 and 2). The samples 0± 1 and 5±6 cm are dominated by the n-C28 alkanol whereas in all the other samples the n-C24±32 even

19±33 19±33 19±33 19±33 19±33 19±33 19±33

19±35 19±35 19±35 19±35 19±35 19±35 19±35 19±35 19±35 19±35 19±35

1189 3945 1717 301 398 296 871

45 184 137 53 49 88 853 69 50 97 58

C-No. range

31 31 25 29 27 29 31 31 31 31 31

31 31 31 31 31 31 31

Cmax

n-alkanes

9.0 11.2 10.3 2.9 8.5 8.2 21.8 7.4 9.8 5.7 7.4

16.5 21.1 16.7 23.1 12.9 16.6 13.4

CPI 1

30.2 27.6 25.6 28.9 28.2 28.5 30.3 30.2 30.0 30.3 29.7

30.5 30.4 29.8 30.0 30.1 29.2 29.7

ACL 1

75 214 154 633 19 119 192 38 61 16 60

149 1837 1807 145 213 343 1122

Conc. (m g gÿ1)

20±32 20±32 18±32 20±32 20±32 20±32 20±32 20±32 20±32 20±32 20±32

20±32 18±32 20±32 20±32 20±32 20±32 20±32

C-No. range

28 24 26 28 30 30 30 28 28 28 28

28 28 28 30 30 24 28

Cmax

n-alkanols

9.2 10.7 6.7 9.9 20.3 4.1 6.0 4.5 8.4 5.4 5.1

10.0 18.3 13.3 11.7 15.3 10.0 12.9

CPI 2

27.4 25.4 25.9 27.3 29.1 28.7 28.4 29.1 28.2 27.8 28.3

26.9 28.1 27.8 27.9 27.1 26.6 27.4

ACL 2

227 469 451 778 628 558 652 131 192 48 237

972 1156 369 1870 2197 2209 2347

Conc. (m g gÿ1)

14±32 14±32 14±32 14±32 14±32 14±32 14±32 14±32 14±32 14±32 14±32

16±32 16±34 16±34 16±34 16±34 16±34 16±34

C-No. range

30 24 24 28 24 30 28 30 28 30 30

30 30 24 24 24 24 24

Cmax

n-alkanoic acids

3.3 5.6 3.8 2.8 2.0 2.8 3.0 4.2 1.6 3.7 1.0

4.3 5.1 5.7 7.7 7.9 9.1 9.3

CPI 2

27.2 25.3 25.1 27.0 25.6 28.0 27.6 28.1 27.6 29.2 27.0

28.5 27.9 25.8 26.0 25.6 25.0 25.3

ACL 2

Full plant names: Racomitrium lanuginosum, Sphagnum capillifolium, Sphagnum fuscum, Huperzia selago, Carex bigelowii, Vaccinium vitis-idaea, Empetrum nigrum, Cladonia uncialis, Cladonia arbuscula, Sphaerophorus globosus, Cetraria islandica. Concentrations are in m g gÿ1 dry weight for total fraction (including lower Cnos.). Cno. range represents the maximum extent of distribution observed by GC analysis. Cmax carbon maximum distribution (where bimodal the lower intensity ignored). Carbon preference index CPI1 = 2Sodd C23ÿC31/(Seven C22ÿC30+Seven C24ÿC32) CPI2 = 2Seven C22ÿC30/(Sodd C21ÿC29+Sodd C23ÿC31) Average chain length ACL1 = (S[Ci]  i)/S[Ci], for i = 23±33 where Ci = concentration n-alkane containing i carbon atoms. ACL2 = (S[Ci]  i)/S[Ci], for i = 22±34 where Ci = concentration n-alkanol or n-alkanoic acid containing i carbon atoms.

0±1 F 01994 5±6 F 01920 13±14 E 01790 22±23 D 01380 28±29 C 01080 37±28 A/B 0860 53±54 A/B 0410 Plant R. lanuginosum (moss) S. capillifolium (moss) S. fuscum (moss) H. selago (moss) C. bigelowii (sedge) V. vitis-idaea (ericaceae) E. nigrum (ericaceae) C. uncialis (lichen) C. arbuscula (lichen) S. globosus (lichen) C. islandica (lichen)

Zone

Conc. Age (A.D.) (m g gÿ1)

Table 1. Summary data for concentrations of homologous distributions of the lipid fractions isolated from the horizons of the Moine Mhor peat core and from plant species growing on the present day bog.

Sample depth (cm)

Peat

Scottish montane peat bog 221

222

K. J. Ficken et al.

Fig. 1. Gas chromatograms of lipid fractions extracted from a typical sample (22±23 cm) from the peat core. Dotted peaks represent the homologous series of the n-alkanes, n-alkanols and n-alkanoic acids. (std = the standard, n-C36 alkane).

numbered homologues are prominent (Table 2). The distribution patterns of the n-alkanols change with depth, with no two samples having precisely the same distribution pattern.

n-Alkanoic acids The n-alkanoic (fatty) acids in the plants range from n-C14±32, although those less than n-C20 are not shown in Fig. 2, as they do not derive from epi-

Scottish montane peat bog

223

Fig. 2. Histograms of distributions of homologues (m g gÿ1 dry weight) for lipid classes (n-alkanes, n-alkanols and n-alkanoic acids) extracted from ®ve major plant species prominent in the present vegetation cover of the Moine Mhor peat bog (n-C22 alkane omitted, see text).

cuticular leaf waxes and are also more prone to diagenesis. All samples display an even-over-odd predominance (Fig. 2; Table 1) although this value is less than that for the n-alkanols and the OEP displayed in the n-alkanes. Both species of Sphagnum have similar n-alkanoic acid distributions to each other, maximising at the n-C24 fatty acid (Table 2). All the long-chain fatty acids are present in high proportions in the R. lanuginosum with the n-C30 being the most abundant. C. bigelowii is dominated by the n-C24 and n-C28 fatty acids. The Cladonia species are dominated by the n-C30 and n-C32 alkanoic acids, although the n-C24±28 alkanoic acids are also present in relatively high abundance (Table 2). The V. vitis-idaea is dominated by the n-C30 alkanoic acid. The fatty acids in the peat do not vary greatly throughout the core, except for the samples 0±1 and 5±6 cm which maximise at n-C30, whereas all the other samples maximise at n-C24 (Table 2 and Fig. 3). All the samples have a relatively strong even-over-odd predominance (Table 1). Transesteri®cation and wax esters The percentages of the even carbon number nalkanols and n-alkanoic acids both before and after

transesteri®cation of the ®ve main plant species are given in Table 3 and displayed in Fig. 4. For simplicity, the odd carbon number homologues are not listed, as these are minor components in the plant lipids. The n-alkanol distributions for the Sphagnum and Ericaceae remain essentially unchanged upon transesteri®cation. The Racomitrium changes from a distribution dominated by n-C28 alkanol to one that is dominated by n-C30. Transesteri®cation of C. bigelowii changes the n-alkanol distribution pattern from one dominated by n-C30 to one dominated by n-C28 and the Cladonia changes upon transesteri®cation from n-C28±32 to n-C28 and n-C24 alkanol domination (Fig. 4). Transesteri®cation the acids display a greater proportion of n-C16 fatty acid (not shown in Fig. 4) for all the plants. Like the n-alkanols, the fatty acid distribution for the Sphagnum remains unchanged upon transesteri®cation. The Racomitrium shows an increase in the n-C30 and nC32 fatty acids. C. bigelowii releases a greater proportion of the n-C26 and the Ericaceae species shows an increase in the n-C20 and n-C32. The Cladonia releases an enhanced amount of n-C20 and n-C24 fatty acids. Gas chromatograms of the freely extractable lipids of the plants revealed the presence of wax

Peat 0±1 5±6 13±14 22±23 28±29 37±38 53±54 Plant R. lanuginosum S. capillifolium S. fuscum

Peat 0±1 5±6 13±14 22±23 28±29 37±38 53±54 Plant R. lanuginosum S. capillifolium S. fuscum H. selago C. bigelowii V. vitis-idaea E. nigrum C. uncialis C. arbuscula S. globosus C. islandica

Sample depth (cm)

0.0 0.1 0.3

0.4 0.4 0.4

0.9 11.7 10.9 0.5 3.5 0.9 0.0 0.9 0.7 0.6 0.7

0.5 0.9 0.5 0.7 0.8 0.8 0.7

0.7 0.3 0.4 1.0 1.3 1.2 0.0 0.5 0.7 0.3 0.8

0.5 0.1 0.1 0.2 1.6 0.2 0.0 0.3 0.3 0.1 0.5

0.0 1.3 2.0 1.9 3.4 5.1 4.9

21

1.5 1.0 4.0 5.0 6.2 4.7 5.4

0.1 1.4 1.5 0.8 1.0 1.3 1.4

20

0.0 5.4 2.4 1.5 1.8 2.3 2.2

19

3.1 13.8 8.5

4.7 3.2 5.0 5.6 8.6 12.1 10.3

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0

22

0.8 3.0 3.1

0.2 0.7 1.2 1.1 1.7 2.5 1.7

1.0 17.1 22.3 1.5 5.9 1.6 0.1 1.2 1.0 0.9 1.2

0.3 0.7 3.8 1.2 2.1 9.8 6.0

23

21.1 29.0 28.5

16.4 10.1 14.0 11.0 16.1 18.8 15.2

0.8 0.9 2.2 1.4 1.4 1.7 0.1 0.8 0.7 0.8 1.1

0.3 0.5 0.9 0.5 1.5 1.2 1.0

24

1.1 3.1 4.5

1.4 1.3 2.1 1.3 1.8 2.7 1.9

1.9 18.0 36.2 3.9 3.6 8.2 0.3 2.2 1.9 1.6 2.6

0.5 1.3 5.4 1.2 2.7 5.5 2.5

25

6.0 8.1 11.7 17.8 31.8 16.6 4.9 5.7 6.0 5.1 10.1

2.8 3.0 2.2 4.0 3.2 4.8 3.3

27

1.1 1.7 2.2 4.9 5.1 2.7 1.7 3.7 3.2 3.4 3.4

1.2 0.7 0.7 0.6 0.7 0.4 0.5

28

12.8 27.3 29.8

17.2 12.8 14.4 10.2 12.1 15.5 11.7

1.3 1.4 2.1

2.0 1.7 1.5 1.2 1.2 1.3 1.3

33.9 12.8 14.3

41.6 33.8 20.0 24.2 17.7 18.5 18.5

n-Alkanol homologues

1.2 0.9 2.3 4.4 1.9 2.7 0.1 1.4 1.4 1.4 1.7

0.4 0.4 0.9 0.5 1.6 0.9 0.6

26

n-Alkane homologues

3.0 0.7 2.3

4.4 0.3 0.2 0.3 0.2 0.6 0.2

24.4 9.4 3.9 21.1 14.1 40.1 33.8 19.4 23.6 15.9 25.2

26.8 26.0 29.3 31.4 26.7 25.6 27.4

29

11.4 6.0 4.0

6.6 24.7 17.5 26.2 29.2 12.4 14.8

3.0 1.3 0.6 7.4 1.7 2.5 1.9 2.7 2.1 3.6 2.7

1.8 1.9 2.3 2.2 2.0 1.9 3.7

30

5.5 0.4 0.9

0.9 0.2 0.1 4.8 0.3 0.7 0.2

40.5 20.6 4.4 16.3 17.1 16.5 41.1 44.4 46.2 45.3 36.9

49.0 45.6 47.3 53.0 43.3 34.1 40.3

31

5.6 2.1 1.3

2.7 9.2 19.4 8.6 4.2 9.5 18.2

4.3 3.4 0.8 5.2 2.2 1.1 1.5 2.3 1.5 5.6 2.7

2.4 0.1 0.1 0.3 0.6 1.0 0.4

32

12.4 5.8 1.7 5.0 5.3 3.0 14.2 13.3 9.5 12.9 9.0

14.5 11.6 0.5 0.9 9.5 6.2 5.7

33

0.6 0.2 0.1 8.1 0.5 0.5 0.3 0.5 0.5 0.9 0.6

34

0.8 0.5 0.3 1.4 3.1 0.7 0.1 0.8 1.0 1.6 0.8

35

Table 2. Individual homologue data for composition of the n-alkane, n-alkanol and n-alkanoic acid fractions for peat horizons and the modern plants. Expressed as percentages of each class. n-C22 alkane entered as zero due to contamination (see text)

224 K. J. Ficken et al.

Peat 0±1 5±6 13±14 22±23 28±29 37±38 53±54 Plant R. lanuginosum S. capillifolium S. fuscum H. selago C. bigelowii V. vitis-idaea E. nigrum C. uncialis C. arbuscula S. globosus C. islandica

H. selago C. bigelowii V. vitis-idaea E. nigrum C. uncialis C. arbuscula S. globosus C. islandica

2.7 2.3 4.4 3.4 2.8 1.6 1.9

11.6 2.0 2.0 2.3 5.8 2.9 5.4 1.0 8.6 0.7 39.7

0.3 0.5 0.3 0.3 0.2 0.2 0.2

0.6 0.1 0.3 12.3 0.6 0.3 0.8 1.1 0.5 0.3 1.6

0.1 1.6 0.3 0.1 0.1 0.5 0.2 0.2

0.1 1.1 3.7 0.1 15.2 3.4 1.4 1.8 23.6 0.0 28.2

6.2 0.6 1.0 0.9 1.0 0.6 0.6

0.0 0.8 0.1 0.1 0.0 0.3 1.4 1.1

9.3 11.6 10.8 3.8 11.8 3.7 3.5 5.2 2.4 0.6 3.2

4.4 6.2 10.1 12.6 14.3 16.8 14.3

0.8 0.8 1.3 2.1 0.3 0.8 4.9 0.4

1.9 5.3 6.8 2.7 7.6 1.7 2.6 0.4 8.2 0.2 2.8

2.5 2.1 3.1 3.5 3.9 3.3 3.0

0.3 0.1 0.3 0.3 0.1 0.5 1.7 0.4

12.4 32.3 31.6 8.8 16.5 7.1 7.5 9.8 4.0 5.6 3.4

7.6 11.6 25.4 25.1 26.9 35.2 33.8

8.0 2.1 4.8 12.9 0.8 2.4 6.6 1.8

3.9 5.2 7.0 6.5 4.8 3.4 3.0 4.1 2.3 2.1 1.3

2.3 2.5 6.6 3.2 3.7 3.0 3.3

1.7 0.5 1.1 0.9 0.6 0.9 2.1 0.8

3.9 0.9 2.1 1.4 1.6 2.1 2.9 1.9

34.4 30.6 21.9 17.7 41.5 48.7 40.9 51.5

10.2 21.9 22.4 12.7 6.1 10.9 13.7 13.5 9.3 6.8 3.3

9.3 11.4 20.8 17.7 21.3 22.7 23.0 5.0 2.2 2.3 10.8 6.6 4.7 5.8 7.2 0.2 5.3 1.3

1.5 2.0 1.3 1.0 0.8 0.2 0.8 12.1 7.2 5.3 22.1 16.4 15.6 23.4 13.4 14.7 12.9 4.9

17.0 15.6 12.7 13.2 11.3 6.8 8.2

n-Alkanoic acid homologues

30.5 1.5 12.0 12.7 5.6 14.1 10.3 10.7

5.6 0.6 1.3 1.0 0.3 11.3 6.5 1.0 4.0 8.0 1.1

4.5 4.5 1.3 1.3 0.8 1.4 0.2

3.2 1.3 12.4 1.4 10.3 5.4 5.9 10.0

13.9 6.5 4.1 12.2 5.0 28.4 18.7 20.4 12.5 35.3 5.4

25.9 22.6 7.1 9.7 7.5 3.9 5.1

16.4 57.4 28.3 27.3 17.0 15.4 12.4 10.9

1.5 0.5 0.5 0.5 0.8 1.8 6.6 2.4 1.5 1.8 0.7

2.2 3.5 1.1 1.4 1.2 2.5 2.8

0.0 0.1 1.0 16.4 3.6 1.3 1.3 2.3

11.8 3.5 1.8 4.1 2.5 4.9 1.1 18.8 8.2 20.6 3.3

13.7 13.6 4.4 6.2 3.7 1.4 2.4

0.7 2.2 14.3 6.8 18.6 7.6 9.5 8.1

0.1 0.0 0.1 0.0 0.0 0.0

0.9 0.3 0.5 0.7 0.4 0.3

Scottish montane peat bog 225

226

K. J. Ficken et al.

Fig. 3. Histograms of distributions of homologues of the n-alkanes, n-alkanols and n-alkanoic acids extracted from Moine Mhor peat horizons and the macrofossil estimates of plant contribution (left hand column, Barber and Maddy pers. comm.). The acid distributions are shown only from n-C20 to nC34: major contribution of n-C16 and n-C18 homologues discussed in text. (n-C22 alkane omitted, see text.); R = Racomitrium; C = Carex bigelowii; S = Sphagnum species; E = Ericaceae species; Cl = Cladonia species; U = unidenti®ed organic matter.

10 2 0.5 10 8

6 3 0.5 13 6

8 4 3 16 6 12 6 4 31 16

11 4 2 5 15

6 2 2 17 22 46 5 3 25 5 13 7 60 34 20

Pre Post

3 4 7 6 6

9 11 71 21 56

%30 Pre Post

Principal components analysis (PCA)

9 22 5 12 11

15 30 2 15 7

6 14 9 3 4

11 32 15 8 8

Acids

7 29 11 5 8

18 32 15 9 15 24 32 2 6 1 7 22 4 7 4

8 12 10 4 4 7 7 8 6 9

4 15 1 2 0.5 2 2 2 5 2 0.5 0.5 2 0.5 0

10 2 5 3 1 6 5 8 9 24

6 2 1 5 1

5 2 5 4 14 50 18 47 41 31

0 0 0 0 0 3 1 0.5 2 0.5 0 0 0 0 0

22 13 39 17 21

R. lanuginosum S. capillifolium C. bigelowii V. vitis-idaea C. uncialis

Plant R. lanuginosum S. capillifolium C. bigelowii V. vitis-idaea C. uncialis

Pre refers to the percentage of each homologue before transesteri®cation and post to the percentage of each homologue after transesteri®cation.

3 17 8 5 4

4 24 4 15 11

11 7 15 17 11

39 14 32 27 49

%28 Pre Post %26 Pre Post Pre Post Pre Plant

%16

Post

Pre

%18

Post

%20 Pre

Pre

%22

Post

Transesteri®cation alcohols %24

Table 3. Percentages of the n-C16±32 even carbon numbered homologues of the n-alkanols and n-alkanoic acids before and after transesteri®cation of the major present day plant species

227

esters ranging from C36±42. Upon transesteri®cation the chain lengths of the n-alkanols and n-alkanoic acids suggested chain lengths of C36±48 for the plant wax esters. In R. lanuginosum the main nalkanol moiety of the wax esters was n-C30, in C. bigelowii n-C28, in Sphagnum n-C24 and n-C26, in Cladonia n-C28 and n-C24 and in V. vitis-idaea nC28 and n-C30 were the dominant moieties. However, the gas chromatograms of the peat horizons showed a virtual lack of wax esters and where they were present they were in much lower abundance than those identi®ed in the plants.

%32

Post

Scottish montane peat bog

Principal component analysis (PCA) was performed on a data set comprising 18 objects (11 plants and 7 peat horizons) and 18 variables (normalised lipid abundance data) using SYSTAT for Macintosh, Version 5.2. The lipid abundance data was normalised to reduce the ``size e€ect'', whereby the samples are distinguished by PCA on the basis of total concentration of variables. The lipids used in the PCA were n-C23±33 odd carbon number alkanes and n-C22±34 even carbon number alkanols and alkanoic acids. However, PCA performed with data obtained both before and after transesteri®cation showed little correlation between the major plant inputs with those peat horizons where that plant was dominant. The plant and peat samples plotted separately from each other, although the di€erent plant species grouped together as might be expected e.g. all the lichens plotted closely together. However, although the two Sphagnum species plotted closely together, they plotted completely separately from the other moss species. Those peat samples that were meant, according to the macrofossil data, to contain predominantly sedge plotted well away from the sedge plant sample and likewise for the other plants. We conclude that PCA does not really aid in the deconvolution of the lipid data. Stable carbon isotope biogeochemistry The carbon isotopic compositions of the n-C29 and n-C31 alkanes for the plants vary from ÿ30.9 to ÿ33.6- (mean ÿ32.4-) and ÿ29.9 to ÿ34.4(mean ÿ32.4-), respectively (Table 4). For certain plant species (Sphagnum, Racomitrium and sedge) the n-C29 alkane is more enriched than the n-C31 alkane, while in others (Cladonia and Ericaceae) the n-C31 alkane is more enriched (Table 4). The bulk isotopic values for the peats average around ÿ25-, which is consistent with a C3 source (Deines, 1980; O'Leary, 1981; Fig. 5). The average values for the n-C29 and n-C31 alkanes in the peat range from ÿ29.6 to ÿ31.7- and ÿ29.3 to ÿ31.2-, respectively (Fig. 5, Table 4). The variability down most of the core (from samples 13±14 and 53±54 cm) approximate to 0.5-, which is within the analytical

228

K. J. Ficken et al.

Fig. 4. Histograms of distributions of homologues of the n-alkanols and n-alkanoic acids before (pre) and after (post) transesteri®cation for the major plant species. Only even carbon numbered homologues are shown due to the low proportions of the odd carbon chain length homologues.

error. However, from sample 0±1 cm to 13±14 cm there is a positive shift of approximately 2- for the n-C29 alkane and 1.5- for the n-C31 alkane (Fig. 5). Overall, the n-C29 and n-C31 alkanes in the buried peat horizons are enriched by an average of

approx. 2.5- with respect to the same alkanes in the present day plants. Isotope values for n-alkanols and n-alkanoic acids for two of the peat layers are given in Table 5. Like the n-alkanes the values all re¯ect a C3 source except for the C16 and C18

Scottish montane peat bog

229

Table 4. Compound speci®c d13C (-) data for the individual homologues of selected n-alkanes for the peat horizons and the modern plants. The isotopic values represent the average of two runs for each sample n-Alkane homologue Sample depth (cm)

21

23

25

27

Peat 0±1 5±6 13±14 22±23 28±29 37±38 53±54 Average (13±53) Plant R. lanuginosum S. capillifolium S. fuscum H. selago C. bigelowii V. vitis-idaea E. nigrum C. uncialis C. arbuscula S. globosus C. islandica Average

ÿ36.3 ÿ32.7

ÿ36.6 ÿ34.0

ÿ36.4 ÿ34.6 ÿ27.6 ÿ28.1

ÿ34.5

ÿ35.3

ÿ31.7

fatty acids which show markedly heavier values than do the longer chain length homologues. DISCUSSION

Lipid distributions of the peat core and of contributing modern bog plants The total concentration of the n-alkanoic acids increases and the total concentration of the nalkanes and n-alkanols decrease at and below 22 cm depth in the peat core (Table 1). The shift may be due to changes in input or degree of mineralisation. Thus, below 22 cm depth the peat may have been dominated by plant inputs high in acids and low in n-alkanes and n-alkanols. However, the most likely explanation is that the acids are being diagenetically released from precursor lipids, such as wax esters, as discussed below. The homologous distributions of the lipids from both the plants and the peat show distributions characteristic of epicuticular leaf waxes (Eglinton and Hamilton, 1967; Cranwell, 1976; Tulloch, 1976). The ®ve major plant species all have di€erent lipid distributions (Fig. 2), suggesting that the dominant plant input for a given horizon could be identi®ed. However, the lipid distributions for the peat samples generally show little change with depth, despite major changes in the macrofossil assemblages determined microscopically (Fig. 3). Indeed, this uniformity in the actual lipid stratigraphies contrasts with the varied histograms of the nalkane, n-alkanol and n-alkanoic acid distributions which were calculated from the macrofossil contributions for each peat layer using the distributions observed for selected modern bog plants as the basis. For the predicted peat lipid abundances the

ÿ35.0 ÿ33.9 ÿ34.7 ÿ32.5 ÿ28.8 ÿ31.7 ÿ33.2 ÿ33.0 ÿ32.9

29

31

33

ÿ31.7 ÿ30.6 ÿ29.6 ÿ30.0 ÿ29.7 ÿ30.4 ÿ30.3 ÿ30.0

ÿ31.2 ÿ30.7 ÿ29.9 ÿ29.5 ÿ29.7 ÿ29.3 ÿ29.8 ÿ29.6

ÿ30.4

ÿ33.1 ÿ32.6 ÿ32.6 ÿ31.8 ÿ31.5 ÿ31.2 ÿ30.9 ÿ33.4 ÿ33.4 ÿ32.6 ÿ33.6 ÿ32.4

ÿ33.2 ÿ33.0 ÿ32.6 ÿ30.0 ÿ32.7 ÿ29.9 ÿ30.8 ÿ32.0 ÿ33.2 ÿ34.4 ÿ34.1 ÿ32.4

ÿ32.9 ÿ32.9

Bulk d13C ÿ27.7 ÿ26.5 ÿ25.5 ÿ25.2 ÿ26.2 ÿ25.4 ÿ23.4

ÿ29.6 ÿ30.7 ÿ33.8 ÿ33.0 ÿ33.6 ÿ33.6 ÿ32.5

following plants were used in the calculation: R. lanuginosum, C. bigelowii, both species of Sphagnum, both species of Ericaceae and both species of Cladonia. For example, the predicted lipid abundance of n-C29 alkane the following equation was used: n ÿC29 ˆ ‰aR ‡ bC ‡ c……S1 ‡ S2†=2† ‡ d……E1 ‡ E2†=2†‡e……C1 ‡ C2†=2†Š=100 where R=% n-C29 alkane in R. lanuginosum, C=% n-C29 alkane in C. bigelowii, (S1+S2)=% nC29 alkane in both species of Sphagnum, (E1+E2)=% n-C29 alkane in both species of Ericaceae, (C1+C2)=% n-C29 alkane in both species of Cladonia, a=% of R. lanuginosum, b=% of C. bigelowii, c=% of Sphagnum, d=% of Ericaceae and e=% of Cladonia in that particular peat horizon. The topmost layer (0±1 cm) of the peat might be expected to have the best correlation between its set of lipid histograms and those of the assemblage of modern plants, since the plant debris should be the least decomposed. C. bigelowii is the dominant plant macrofossil in this horizon, followed by R. lanuginosum. However, when the actual peat lipid distributions are compared to those calculated from the macrofossil abundances, the expected similarity is not very apparent, with the n-alkanols being the least similar. Indeed, the actual peat lipid n-alkane distributions more closely resemble those of R. lanuginosum and Cladonia species rather than that of C. bigelowii. Discrepancies between lipid distributions and known biological inputs have been reported before. Farrimond and Flanagan (1995) found near

n.d. ÿ30.5 ÿ31.0 ÿ31.3 ÿ31.4 ÿ31.1 ÿ32.8 ÿ31.4 ÿ32.5 ÿ29.5 ÿ31.2 ÿ30.2 n.d. ÿ29.0 n.d. ÿ19.5 n.d. ÿ23.6 ÿ30.3 ÿ29.7 ÿ31.8 ÿ31.8 ÿ32.9 ÿ31.9 ÿ32.9 ÿ32.0 n.d. ÿ31.4 All values are expressed in - relative to PDB.

ÿ29.9 ÿ29.8 ÿ25.5 ÿ23.4 13±14 53±54

ÿ29.6 ÿ30.3

30 28 26 24

n-Alkanoic acid

22 20 18 16 30 28 26

n-Alkanol

24 22 31

n-Alkane

29 Bulk d13C Sample depth (cm)

identical n-alkane and n-alcohol distributions in Flandrian peat samples with quite di€erent pollen assemblages and also the converse where lipid distributions changed markedly between peat samples having very similar pollen distributions. They account for this inconsistency between the pollen and lipid data with the suggestion that the lipids record the composition of plant debris deposited locally while the pollen assemblage is rather an average signal of the ¯ora over a wider area. Rieley et al. (1991) also found that lipids derived from leaf waxes of common trees surrounding a lake bore little resemblance to the lipid distributions found in the lake sediments. Therefore, it appears that in Recent sediments the lipid signature of the sediment does not always re¯ect the lipid composition of the various input sources and this must lead to questions over the interpretation of ancient sediments with respect to source inputs.

Peat

Fig. 5. Down core plot for Moine Mhor peat bog of the isotopic values of the n-C29 and n-C31 alkanes (®lled square and circle respectively). The ®lled diamonds represent the bulk carbon isotopic composition of the peat. The dates (given to the nearest 10 yr) are corrected 14C dates for the horizons analysed. Macrofossil zones (Barber and Maddy pers. comm.) are indicated by letters A/B±F.

32

K. J. Ficken et al. Table 5. Compound speci®c d13C (-) data for the individual homologues of selected n-alkane, n-alkanol and n-alkanoic acids for two peat horizons. The isotopic values represent the average of two runs for each sample.

230

Scottish montane peat bog

Fig. 6. Down core plot against Calendar age of average d13C (-) for the n-C29 and n-C31 alkanes (squares). Also plotted is d13C for atmospheric CO2 (®lled circles) as estimated by Friedli et al. (1986), Leuenberger et al. (1992), Marino et al. (1992) (shifted by ÿ23- for comparison).

n-Alkanes. The n-alkanes show no major change with depth in the peat core, although there is a slight di€erence between the top two samples and the other ®ve horizons. Despite signi®cant changes in the macrofossil contribution (Fig. 3), only sample 5±6 cm shows a resemblance between the calculated and actual distributions for the peat. The macrofossil assemblage for this layer is composed almost entirely of R. lanuginosum, which has an nalkane distribution pattern similar to that seen in the peat (Fig. 2). The Cladonia species (one is shown in Fig. 2) have similar n-alkane distributions. However, this lichen rapidly degrades and is generally not observed in the macrofossil analysis, making macrofossil based estimates of lipid input problematic. The remaining three dominant plants (S. capillifolium, V. vitis-idaea and the sedge) all appear to have distinctive n-alkane distribution patterns, the Sphagnum species being dominated by the n-C25 and n-C23 alkanes, the sedge by the n-C27 alkane and the ericaceae by n-C29 alkane. The shorter

231

chain length n-alkanes seen in the Sphagnum are consistent with plants growing in a wet environment (Barnes and Barnes, 1978). However, the nC21±27 alkanes are not major components in any of the peat horizons, despite the Sphagnum and the sedge being the dominant macrofossils throughout most of the core. This lack of shorter chain length n-alkanes in the peat could be due in part to selective diagenetic removal. In the Scandinavian Sphagnum peat bogs studied by Lehtonen and Ketola (1993), it was found that, in parallel with increasing humi®cation, the major homologue changed through n-C25 and n-C27 to n-C31 alkane, a trend suggesting selective removal of shorter chain homologues during diagenesis. The two Sphagnum species in the present study have very di€erent n-alkane distribution patterns; S. capillifolium has a large abundance of n-C31 alkane, whereas S. fuscum does not. The n-C31 alkane has been found to be the major n-alkane in Sphagnum by Sever et al. (1972) and in peats by Lehtonen and Ketola (1993). Unfortunately, the macrofossil analysis could not di€erentiate between the two species of Sphagnum found in the present core. Overall, we are left with the major disagreement downcore between relatively invariable n-alkane patterns and highly variable macrofossil distributions. The similarity between the distribution patterns of the n-alkanes of the Cladonia and Racomitrium with those of the peat n-alkane distributions suggests that these two plants were probably much more abundant throughout the peat core than is suggested by the macrofossil analysis. n-Alkanols. Like the distributions of n-alkanes, those of the n-alkanols do not vary greatly downcore, although those of the two uppermost horizons are di€erent from the rest. The situation is similar to that of the n-alkanes in that the distributions calculated from the macrofossil analysis bear little resemblance to those actually found for the peat horizons. Once again, there is the possibility of mineralisation and diagenetic e€ects: these functionalised molecules are expected to be at least as bioavailable as the n-alkanes and subject to preferential attack on the shorter-chain homologues. But this explanation does not appear to be likely, in view of the abundance of short chain homologues in the lower horizons. Also, all of the ®ve major plant species have very di€erent n-alkanol distributions and the peat horizons would be expected to re¯ect this in their distribution patterns (Fig. 2). Some similarities can be identi®ed between the peats and plants, for example, C. bigelowii and V. vitis-idaea have n-alkanol distribution patterns dominated by n-C30 and n-C28 alkanols (Fig. 2) and these do occur as major components in the peats (Fig. 3). V. vitis-idaea also has a large proportion of the n-C32 alkanol (Fig. 2) which is a major homologue in several of the peat horizons (Fig. 3). The

232

K. J. Ficken et al.

distributions in the top two peat samples bear some resemblance to those of R. lanuginosum and the Cladonia, while those of the remaining samples appear to be a combination of the distributions shown by Sphagnum and C. bigelowii. The distribution patterns of the n-alkanols for the Sphagnum species contain higher proportions of the n-C24 and n-C26 alkanols, a distribution pattern seen by Lehtonen and Ketola (1993) in Sphagnum peats as well as in some peat horizons (e.g. 13±14, 37±38 and 53±54 cm) examined in this study. However, the di€erences between the solvent extractable peat nalkanols and those observed in the extracts of the plants may be best explained by the in situ hydrolysis of wax esters releasing n-alkanols. Comparing the even carbon number n-alkanol distributions of the ®ve major plants after transesteri®cation of their solvent extracts (Fig. 4) with those of the extracts of the peat samples (Fig. 3), reveals that the distributions correlate better than they do without transesteri®cation. Indeed, as with the n-alkane distributions the correlation is further improved for all layers if an additional input of Cladonia and Racomitrium in excess of the macrofossil estimates is assumed. n-Alkanoic acids. The homologue distributions of the fatty acids (Fig. 3) are remarkably similar throughout the ®ve lower horizons of the core, maximising at C24 and generally closely resembling those of the Sphagnum species (Fig. 2). Once again, samples 0±1 and 5±6 cm are the only samples with a di€erent distribution pattern, which maximises at C30 and shows some similarity with those of R. lanuginosum and Cladonia (Fig. 3). The lower chain length homologues (
ing free acids and free alcohols which become part of the solvent extractable fraction. It appears from the above that transesteri®cation of the plant extracts does improve the correlation with the peat lipid distributions. Furthermore, the correlation is also improved if a greater input of Racomitrium and Cladonia into the peat than that indicated by the macrofossil analyses, is assumed. Thus, one major di€erence between the plant and peat lipid data is the high proportion of n-C30 and n-C32 alkanols in the peat horizons: upon transesteri®cation, the Racomitrium releases a large amount of the n-C30 alkanol whereas the Ericaceae and Cladonia release n-C32 and n-C28 alkanol, respectively. A similar situation is seen for the n-alkanoic acids. Relatively high proportions of n-C30 and nC32 alkanoic acids are seen in some samples and these homologues are released by Racomitrium, Cladonia and ericaceae upon transesteri®cation. The majority of the peat samples maximise at n-C24 alkanoic acid but also have a high proportion of the n-C26 alkanoic acid, Cladonia releases n-C24 alkanoic acid and C. bigelowii releases n-C26 alkanoic acid during transesteri®cation. Hence, higher inputs than macrofossil estimates of both Cladonia and Racomitrium would result in better correlations of predicted and observed n-alkane, n-alkanol and alkanoic acid distribution patterns. Evaluation of peat lipid pro®le vs. macrofossil composition The major initial conclusion which we draw from the data in Tables 1 and 2 and in Figs 2 and 3 is that the lipid distributions for those plant species dominant in the present bog do not match those seen in the underlying peat samples, even when homologue distributions are calculated using the percentages of each macrofossil observed in each horizon. This discrepancy between the calculated lipid distributions and those actually observed requires detailed consideration. One possibility is that the actual input from a plant species may have been in excess of that indicated by the macrofossil data. Thus, for the nalkanes, the distribution pattern in the peat is similar throughout the core (Fig. 3) and is suggestive of signi®cant input of n-C29 and n-C31 alkanes other than predicted from the macrofossil abundance data. For example, the R. lanuginosum has a similar n-alkane distribution pattern to the peat samples and it is possible that decayed Racomitrium debris, not identi®able under the microscope, is present throughout the core. Similarly, Cladonia is not an abundant macrofossil but is does have an n-alkane distribution similar to that of the peats and could, like the Racomitrium, have been contributed but not be recognised in the macrofossil analysis. Indeed, it was surprising to ®nd Cladonia particles visible to the naked eye in the pro®les to a depth of

Scottish montane peat bog

9 cm, presumably due to the low temperatures and low pH (4.0) inhibiting cellular decomposition. Lichens are a symbiosis of a fungus and an alga and are not normally found sub-fossil in peats. Zygadlo et al. (1993) report that the major nalkanes in 14 species of lichen are the n-C29 and nC31 homologues, which is consistent with our ®ndings for the four lichens species studied (Table 2). Hence, the dominance of these homologues in the peat samples may be due to a greater proportion of lichens contributing to the peat than originally suggested by the macrofossil analysis. Dembitsky et al. (1991, 1992) report fatty acids ranging up to nC28 in eight species of Cladonia from the Volga basin, with the n-C16 and n-C18 fatty acids being the dominant components as in the present study. However, (Table 2, Fig. 2) prominent n-C30 and nC32 saturated fatty acids were not reported by Dembitsky et al. (1991, 1992), although they did cite the unsaturated n-C30 and n-C32 acids as major contributors. Since lichens contain no resistant structural elements such as those found in vascular plants, their physical remains degrade rapidly. Almost certainly, such lichens have contributed substantially throughout the peat pro®le, despite the lack of macrofossil evidence. Hence, lipid distributions have been calculated using di€ering proportions of Cladonia to see if this improved the similarity between the predicted and actual peat lipid distributions. The ®ndings are, that for the nalkane distribution patterns to become similar, there would have to be at least 70% Cladonia in the peat, whereas the n-alkanols need up to 50% Cladonia and the n-alkanoic acid patterns would accommodate between 0 and 70% Cladonia. However, each horizon would need di€erent proportions of each Cladonia lipid fraction (n-alkanes, n-alkanols, n-alkanoic acids) within that horizon. We conclude that, while Cladonia may have almost certainly been more abundant in the bog than is deduced from the macrofossil data, quantitative evaluations of its contributions are not feasible at present. A second consideration is that the input of leaf waxes will be dependent on their concentrations in the actual plant material contributed to the peat. The lipid concentrations in Table 1 are expressed in m g gÿ1 dry weight of total dried plant but they are well known to vary greatly with the plant organ concerned, leaf waxes being high in leaf cuticles. The proportionate inputs of biomass made by ¯ower, leaf, twig and root to the peat cannot be estimated for any of the species, although they must di€er extensively. Both the C. bigelowii and the Sphagnum species have 3±5 times the amount of n-alkanoic acids compared to the other species, relative to the n-alkanes and n-alkanols (Table 1). These two plant species have such high concentrations of the n-alkanoic acids that even where the

233

species are relatively minor contributors, they will greatly in¯uence the lipid composition of the peat. Indeed, it is the Sphagnum species that appear to be the dominant contributors to the peat, if the distributions of the lipid homologues in the peat horizons are considered without reference to the macrofossil estimates. The Sphagnum species have three times the amount of (m g gÿ1 dry weight) of the major n-alkane and twice the amount of the major n-alkanol and n-alkanoic acid, when compared with the other three species. However, the variations in the lipid distributions down the core cannot be explained fully by Sphagnum. Once again, as with the Cladonia, when a model holds for the n-alkane distribution at a particular horizon it may not hold for the n-alkanol or n-alkanoic acid distributions. A third consideration is that the macrofossil estimates (Fig. 3) indicate a value for unidenti®able organic matter (UOM; Barber and Maddy, unpubl. results). For example, this may include decayed Racomitrium and Cladonia, as mentioned above. Contributions of their lipids to the distribution obtained for the peat extracts can therefore be expected without parallel macrofossil data. It is in situations like this where combined microscopic and chemical analyses have proved useful (e.g. Zeng et al., 1992). Another possibility is that algae and small fungi may be signi®cant biomass contributors. Both Nichols et al., (1988) and Colombo et al. (1996, 1997) have reported the occurrence of long-chain n-alkanes in algae. However, the occurrence of these n-alkanes could be reasonably explained by contamination of the diatom communities by minute traces of persistent components of high wax crude petroleums, a possibility not considered by the authors. Lichtfouse et al. (1994), in their study of the Pula oil shale, also report longchain n-alkanes that may be partially derived from algae. However, this shale contains a wealth of higher plant fossil material, so a typical leaf wax source of these n-alkanes is likely to be the major source and not the diagenesis of n-alkadienes produced by B. braunii as proposed by the authors; indeed, the isotopic evidence supports a leaf wax source. Although not discussed in this present paper, functionalised hopanoids were identi®ed in the Moine Mhor peat. These compounds are usually ascribed to bacterial inputs (Ourisson et al., 1979) but they have been previously reported in Sphagnum (Quirk et al., 1984) although none were identi®ed in the Sphagnum species of this study. Wind blown pollen is unlikely to be a major contributor. The fourth consideration, already outlined in the previous section, is that the major di€erences between the calculated distribution patterns and those seen in the peat extracts may be due to the hydrolysis of the contributed wax esters into their

234

K. J. Ficken et al.

component n-alkanols and n-alkanoic acids. Using the data from the transesteri®cation of the plant extracts (Fig. 4) and comparing them with the distributions seen in Fig. 3, we have shown that the peat distributions can be better explained if hydrolysis of the plant wax esters has occurred in this acidic bog. Both samples 0±1 and 5±6 cm would correlate better if there had been some hydrolysis and there was more Cladonia present than is estimated from the macrofossil analysis. Samples 13± 14 to 53±54 cm all correspond quite well if hydrolysis had occurred, together with a higher input from the Cladonia and Racomitrium. Similarly, the bottom peat sample (53±54 cm), would be accommodated by a greater input of Sphagnum, rather than the sedge indicated in macrofossil analysis. The sedge particles may be more resistant to decay, thereby giving a false dominance when examined microscopically. Finally, it is of interest to compare the distributions of the long-chain lipid homologues observed in this northern, sub-arctic/alpine peat bog, which is an acid (pH approx. 4.0), relatively low temperature (mean approx. 58C) site, with those reported for a tropical, raised peat bog (Dehmer, 1993). One obvious feature is the dramatic loss of lower chain length homologues with depth in the tropical core, presumably due to microbial attack, a change which has been seen to parallel increasing peat humi®cation in Scandinavian peat bogs (Lehtonen and Ketola, 1993). However, such changes in chain length distribution are not apparent in our core. The top two samples (0±1 and 5±6 cm) show similar distributions which di€er somewhat from those of the remainder of the core. Sample 13±14 cm occurs in the macrofossil zone E (Barber and Maddy, pers. comm.) which has been interpreted as being a period when the bog was wetter, as indicated by the dominance of Sphagnum in the peat. However, the lipid data has not yielded any signi®cant correlations with water level ¯uctuation. This zone of Sphagnum domination is dated A.D. 1600±1800, during the time of the Little Ice Age. The di€erence seen in the lipid composition between samples 5±6 and 13±14 cm may have resulted from the changes in the vegetation cover brought about as the cool and wet conditions of the Little Ice Age gave way to those of the later climate. Stable carbon isotope biogeochemistry The rationale for studying the lipid molecular stratigraphy of the peat core using compound speci®c isotope analysis is to link the observed values with those of the contributed plant lipids. However, the principal diculty lies in the fact that each individual compound for which a d13C value is obtained will be a mix of the di€erent amounts of that homologue contributed by the whole range

of plants synthesising that homologue in the environment under study. Even a single leaf can have a wide range of d13C values (up to 6-; Collister et al., 1994; Lockhart et al., 1997). Thus, the d13C value for the individual compound is the mean value resulting from the di€erent amounts and d13C values of the variously contributed components. The d13C values for individual homologues extracted from the sediment may hence di€er considerably, depending on the amounts of the contributing species and the load of their waxes which actually survives in the sediment. There is, therefore, the potential for identifying environmental and hence climatic conditions if we can deconvolute the distribution patterns of the homologues and their individual d13C values. However, the number of variables involved renders this a dicult task. In principle, even small shifts of d13C of one nalkane with respect to another homologue could be of value in assessing palaeoclimatic change. The working basis would be that individual homologues could be contributed by di€ering combinations of plant species, each with potentially disparate d13C values. Where the plant species di€er in their response to environmental conditions, such as atmospheric CO2 concentration, temperature, humidity etc., then each homologue contributed would bear the average d13C value for that homologue. The input of C3 plants is re¯ected in the average d13C value (0ÿ 25-) of the bulk peats (Table 4 and Fig. 5) and from the individual long-chain homologues of the peat lipids (Tables 4 and 5). Figure 6 shows that the n-C29 and n-C31 alkanes in the top samples (0±1 and 5±6 cm) of the peat are more depleted (by 02-) in 13C than in the rest of the core. During photosynthesis, plants use CO2 from the atmosphere and so the d13C value of the atmosphere determines that of the plant. In the last 2000 yr. the major change (depletion) in 13C concentration in the atmosphere is believed to be due to anthropogenic causes. Since the industrial revolution, the burning of fossil fuels has caused the depletion of 13C in the atmosphere by approx. 1.3over the past 130 yr., from ÿ6.5- (pre-industrial atmosphere) to the present day atmosphere of ÿ7.8- (Friedli et al., 1986). Therefore, an isotopic shift of up to 1.3- is expected between the plants growing before the mid 1800 s and those growing now. Indeed, Fig. 6 shows that the average isotopic composition of the n-C29 and n-C31 alkanes in the Moine Mhor peat core roughly parallels the atmospheric d13C curve since 400 A.D. We infer that the depletion seen in the top two samples (0±1 and 5±6 cm) is due to the more negative d13C value of atmospheric CO2, following increased industrialisation since 1850. Another possibility for the negative shift in the isotopic values at the top of the peat core is diagenesis. Microbial action and associated diagenetic change is rapid in the top few centi-

Scottish montane peat bog

metres of a sediment but then decreases markedly with depth and thus downcore lipid concentrations tend to show a typical diagenetic curve. During these diagenetic changes it is possible that the isotopic composition could be a€ected, although Hayes et al. (1990) have discounted this possibility. Carbon isotopic fractionation due to diagenetic change has also been discounted by Huang et al. 1997, who found no isotopic change in the longchain n-alkanes derived from Calluna vulgaris during a 23 yr. decomposition experiment, despite a loss of over 90% of the mass of original compounds. However, apart from the top two samples, there is surprisingly little variation in the isotopic composition of the individual n-alkanes extracted from the peat. Little isotopic variation with depth is also seen in the isotopic values for the individual n-alkanols and n-alkanoic acids (Table 5). The changes in the macrofossil composition have been interpreted to be climatically driven, with wetter periods during the high Sphagnum stages (Barber and Maddy, pers. comm.). Changes from wetter to drier periods may also a€ect the isotope composition. C3 plants growing in water stressed conditions have d13C values slightly enriched in 13C relative to the average (e.g. ÿ22 to ÿ26-; Farquhar et al., 1982). Therefore, one would expect those samples with high Sphagnum to be slightly depleted compared to the other samples, though isotopic variations are only approx. 20.5- for most of the peat core and this is within the analytical error. However, care has to be taken when using isotopic data as indicators of climatic variations, as Price et al. (1997) showed a 6- variation in Sphagnum cellulose across a modern bog surface, depending on whether the species was growing in a wet hollow or on a dry hummock. Other factors can also a€ect the isotopic composition of peat, just as they do the lipid distributions. Thus, the carbon isotopic signature of the individual n-alkanes of the peat will not only re¯ect the macrofossil content but also include that of the unidenti®ed organic matter (Barber and Maddy, unpubl. results) and anything else in the peat, such as pollen grains, not recognised or quantitated under the microscope during the macrofossil analysis. The small positive shift in d13C which seems to accompany the conversion from plants to peat, seen from the comparison of the values for the living plants with those of the top surface of the peat (Fig. 5), could be explained in this way. A similar shift of approx. 1- between leaf and soil n-alkanes was noted by Lichtfouse et al. (1995). Diagenetic e€ects may also be contributing, in view of the slightly larger di€erence between the mean value of ÿ32.3- of the n-C29 and n-C31 alkanes for the modern plants and that of ÿ29.7- for the lower ®ve core horizons. However, we are unable to attri-

235

bute speci®c values to the a€ects of industrialisation, diagenesis and changing inputs without further information.

CONCLUSIONS

The limited agreement between the two palaeoenvironmental approachesÐlipid biomarker analysis and macrofossil analysis emerges as the principal ®nding of this study of a peat core. We have taken a critical look at what is frequently assumed i.e. that the biolipid data for a sediment layer can be interpreted in terms of the original biological input to that corresponding palaeoenvironment, together with the operation of the relevant diagenetic processes. Our study emphasises the importance of utilising di€erent approaches to palaeoenvironmental assessment. Both methods (macrofossil and lipid biogeochemistry) are qualitatively useful but neither is truly quantitative. Obvious considerations are the detailed role of early diagenesis in determining the relationship between the original vegetation cover at a site and both the molecular lipid biomarker and macrofossil distributions in the layer of sediment eventually preserved from that time horizon. These and other palaeoenvironmental proxies urgently need further investigation for a variety of depositional environments. Speci®c conclusions follows: 1. The observed distributions of the homologues of n-alkanes, n-alkanols and n-alkanoic acids in the peat extracts generally did not correlate well with the corresponding distributions calculated simply on the basis of the plant macrofossil counts. 2. Because of their high lipid content, Sphagnum species would appear to have been major contributors, even when the macrofossil count in the peat is relatively minor. Lichens and other species may also have made a greater contribution than is indicated by the macrofossil analysis due to rapid breakdown in the peat. 3. A better correlation between the lipid distribution in the peat horizons and those of the plants is obtained when in situ hydrolysis of wax esters (present in the epicuticular leaf waxes of the plants) into their component long chain nalkanols and n-alkanoic acids is taken into account. 4. The more negative value (by 2-) in d13C for the individual n-alkanes in the top of the peat compared with the rest of the core may be explained by the shift (ÿ1.5-) in d13C of atmospheric CO2 which has followed the industrial revolution. The variation (20.5-) in the d13C of the individual n-alkanes below 8 cm downcore is insucient for assessment of changes in palaeoclimate.

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Associate Editor Ð J. W. Collister AcknowledgementsÐMr J. Carter and Mr A. Gledhill are thanked for assistance with the mass spectrometer and the GCÐirms, respectively. We acknowledge ®nancial support provided by the NERC (TIGGER IIa GST/02/701 and GCÐMS AND GCÐirms facilities, GR 3/2951, GR 3/3758 and GR 3/7731). Mrs Jing-hong Yang is thanked for help with the plant extractions. Mr A. McMullen and Mr D. Mauquoy are thanked for the plant collection. Drs John Hayes and Jim Collister are thanked for constructive reviews.

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