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Note

n-Alkane distributions in ombrotrophic mires as indicators

of vegetation change related to climatic variation

Chris J. Nott

a

, Shucheng Xie

a

, Luke A. Avsejs

a

, Darrel Maddy

b

,

Frank M. Chambers

c

, Richard P. Evershed

a,

*

aOrganic Geochemistry Unit, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK bDepartment of Geography, Daysh Building, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK

cCentre for Environmental Change and Quaternary Research, GEMRU, C&GCHE, Francis Close Hall,

Swindon Road, Cheltenham GL50 4AZ, UK

Received 7 October 1999; accepted 9 November 1999 (Returned to author for revision 12 October 1999)

Abstract

Lipid analysis of modern bog vegetation revealedn-alkane distributions forSphagnumspecies displaying enhanced abundances of lower chain length homologues (C21±C25). Other plants examined revealed typical higher plant

dis-tributions (C29or C31maxima). Investigation of a 40 cm peat pro®le from Bolton Fell Moss, Cumbria, UK showed a

varying abundance of then-C23homologue down core which appear to be related to vegetation changes, which are

presumed to occur as a result of climate variation.#2000 Published by Elsevier Science Ltd. All rights reserved.

Keywords: n-Alkanes;Sphagnum; Peat; Climate; Holocene

1. Introduction

Peat deposits occur extensively over the Northern Hemisphere and, alone amongst terrestrial ecosystems, their stratigraphy reveals a record of vegetation changes related to climatic variation. The reasonable continuity of this record, in some cases spanning the entire Holo-cene, and the diversity of potentially informative fea-tures have made them the focus of interest for palaeoenvironmental research for many years [Kuder and Kruge (1998) and references therein]. Ombrotrophic mires accumulate peat in a raised mass above the groundwater table and so receive no input of minerogenic water from the surrounding environment. The water bal-ance of these mires becomes dependent upon rainfall, making them particularly sensitive to variations in climate (Aaby, 1976; Barber, 1985, 1995). Thus, raised bogs can provide palaeoenvironmental information through: (i)

di€erences in the relative proportion of taxa determined from the macrofossil record correlating with climatic change (Barber et al., 1994); (ii) humi®cation of the plant matter correlating with changes in temperature or pre-cipitation (Blackford and Chambers, 1993); and (iii) the

d13C values of peat macrofossils being used to reconstruct

atmospheric CO2concentrations (White et al., 1994; Figge

and White, 1995), andd18O anddD values of bulk peat

correlating with temperature changes (Schiegl, 1971). Examples of the use of organic geochemical tools to deduce palaeoenvironmental information from terres-trial environments are limited considering the organic-rich nature of many of these deposits. Studies of organic matter in peat bogs have used pyrolysis GC and GC/MS (Bracewell et al., 1980; Halma et al., 1984; Boon et al., 1986; Heijden, 1994) as well as analysing solvent extractable lipids (Karunen et al., 1983; Quirk et al., 1984; Lehtonen and Ketola, 1993; del Rio et al., 1992; Dehmer, 1993; Farrimond and Flanagan, 1995; Avsejs et al., 1998). In this report we describe the use of n-alkane ®ngerprints to reconstruct plant inputs in response to climatic variation. Molecular stratigraphic

0146-6380/00/$ - see front matter#2000 Published by Elsevier Science Ltd. All rights reserved. P I I : S 0 1 4 6 - 6 3 8 0 ( 9 9 ) 0 0 1 5 3 - 9

www.elsevier.nl/locate/orggeochem

* Corresponding author. Tel.: 117-928-7671; fax: +44-117-925-1295.

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approaches have been employed widely in marine sedi-ments (Brassell et al., 1986; Farrimond et al., 1990; Conte et al., 1992) and lacustrine sediments (Cranwell, 1973; Meyers et al., 1984); however, peat bogs have been largely neglected in this respect (e.g. Farrimond and Flanagan, 1995; Ficken et al., 1998).

Bolton Fell Moss, Cumbria, UK is a large ombro-trophic mire covering an area of around 400 ha. Pre-vious detailed work on the uppermost peat suggested that this mire was particularly responsive to climatic change. Barber (1981) showed that macrofossil assem-blages had responded more or less synchronously to shifts to wetter and/or cooler climates, and that these wet shifts could be correlated with climatic changes known from independent proxy and documentary records. Hence, this site was chosen to develop mole-cular stratigraphic approaches that will complement more conventional palaeoenvironmental techniques.

2. Experimental

2.1. Sample description

A 50 cm peat pro®le was taken, using a monolith tin, from the centre of Bolton Fell Moss, Cumbria, UK. Sam-ples of the modern day covering vegetation were collected and stored, along with the peat monolith, in a freezer at

ÿ20C until required for analysis. Macrofossil stratigraphy

of an adjacent pro®le from the bog was assigned on the basis of microscopic quanti®cation (Barber et al., 1994).

2.2. Lipid extraction and instrumental analysis

The core was sampled by taking contiguous 1 cm sli-ces (discarding the outer layer to eliminate possible contamination) over the top 40 cm, which were then freeze dried. Dried samples were ground to pass through a 0.5 mm sieve before Soxhlet (peat) or ultrasonic (plant) extraction with DCM/acetone (9:1 v/v). The total lipid extracts were separated into acidic and neu-tral fractions by solid phase extraction using aminopro-pyl Bond Elut cartridges (Varian Chromatography). The neutral components were eluted with DCM/isopropanol (2:1 v/v), then further fractionated by ¯ash column chromatography (silica gel 60) using successive elution with hexane, hexane/DCM (9:1 v/v), DCM, DCM/ MeOH (1:1 v/v) and MeOH. Gas chromatography (GC) and GC/mass spectrometry (GC/MS) were performed exactly as described previously (Avsejs et al., 1998).

3. Results and discussion

Whilst conducting GC analyses of the lipid composi-tion of modern day bog vegetacomposi-tion diagnosticn-alkane

Fig. 1. Logarithmic plot of the ratio ofn-C23ton-C31alkane down pro®le (left) compared with the varying proportion ofSphagnum

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distributions were noted. These observations were con-®rmed by GC/MS analyses and ®ndings are summarised in Table 1. It was seen that the six Sphagnum species analysed have quite distinct n-alkane distributions, all containing an increased relative abundance of the C23

homologue (cf. Corrigan et al., 1973). This compound is the predominant alkane homologue in S. recurvum (modal distribution), S. palustre and S. papillosum (bimodal distributions), C31dominatingS. magellanicum,

S. capillifoliumandS. cuspidatum(bimodal distributions). In contrast, the other plants examined revealedn-alkane

distributions characteristic of higher plants. These ranged from C19to C35with an odd-over-even predominance,

maximising at C31, with the exception of Vaccinium

oxycoccusandHypnum cupressiformewhich maximised at C29, and the bog mossAulacomnium palustre which

maximised at C27.

Analysis of the peat horizons showed then-C23alkane

to be increasing and decreasing in relative abundance down the pro®le, shown in Fig. 1 as a ratio of C23/C31.

n-Alkane distribution data for peat horizons are inclu-ded in Table 1. Peat horizons down to 11 cm and from

Table 1

Individualn-alkane homologue data for modern plants and peat horizons expressed as percentage composition

Sample C19 C20 C21 C22 C23 C24 C25 C26 C27 C28 C29 C30 C31 C32 C33 C34 C35

Modern plants

Sphagnum magellanicum 0 0 2.2 0.2 7.9 0.6 18.8 0.6 3.6 0.3 16.7 1.3 33.5 1.7 12.6 0 0

S. recurvum 0.4 0.3 23.3 2.0 38.2 2.7 15.9 1.0 8.1 0.1 4.0 0.3 3.1 0.8 0.5 0 0

S. capillifolium 0.1 0.1 3.6 0.6 11.3 1.1 15.0 0.6 5.1 0.7 13.6 1.2 33.6 1.2 12.6 0 0

S. cuspidatum 0 0 5.4 1.4 26.3 1.6 5.8 0.5 3.4 0.7 8.3 1.5 30.7 0.9 13.3 0 0

S. palustre 0 0 5.4 0.3 31.2 1.3 22.6 0.7 12.4 0.8 7.7 0.7 12.6 0.6 3.7 0 0

S. papillosum 0 0 6.2 1.4 35.6 2.1 20.5 0.8 4.5 0.5 4.3 0.6 16.1 0.8 6.6 0 0

Polytrichumsp. 0 0 0.3 0.3 0.7 0.2 2.5 0.7 11.8 1.3 27.2 1.6 30.9 2.5 18.9 0.4 0.7

Aulacomnium palustre 0 0 0 0 0.9 0.2 4.9 0.8 37.1 0.9 9.4 0.9 28.5 1.9 14.5 0 0

Hypnum cupressiforme 0 0 1.0 0.4 4.0 0.7 8.6 1.6 17.0 1.3 28.2 3.1 22.8 3.3 7.9 0 0

Eriophorum vaginatum 0 0 0.5 0.1 1.2 0.2 1.7 0.3 3.2 0.4 14.8 1.1 54.2 1.4 20.7 0.1 0.2

Eriophorum angustifolium 0 0 0.5 0.1 1.2 0.2 1.7 0.2 3.1 0.1 14.7 1.1 53.8 1.4 20.5 1.3 0.1

Trichophorum cespitosum 0 0 0.1 0.1 0.3 0.1 0.7 0.2 2.0 0.3 11.1 0.8 57.6 1.3 25.4 0.1 0.1

Empetrum nigrum 0 0 0 0 0.1 0.1 0.2 0.1 1.4 0.2 25.5 1.0 49.4 1.2 20.7 0.1 0.1

Vaccinium oxycoccus 0.4 0.2 0.3 0.2 0.6 0.2 1.0 0.9 13.1 2.2 50.4 2.2 26.4 0.6 2.0 0 0

Erica tetralix 0 0 0 0 0.3 0.2 1.1 0.3 4.1 0.3 9.4 1.3 60.4 2.1 20.6 0 0

Calluna vulgaris 0.2 0.1 1.6 0.1 0.7 0.2 2.2 0.5 6.9 0.8 11.3 1.6 43.7 3.1 26.3 0.4 0.8

Andromeda polifolia 0 0 0.1 0.1 0.6 0.4 3.1 0.7 6.4 0.8 38.8 0.9 45.1 0.7 2.4 0 0

Rhynchospora alba 0 0 2.3 0.2 2.3 0.7 3.2 0.5 3.8 0.8 28.2 1.7 44.6 1.0 10.8 0 0

Cladoniasp. 0 0 0.7 0.7 2.3 0.9 4.9 1.2 9.7 2.3 12.4 2.3 38.6 2.7 21.3 0 0

Peat horizons (cm)

0±1 0.1 0.2 2.8 0.5 5.9 0.8 7.7 0.7 5.8 0 18.7 1.0 34.3 1.7 18.9 0.5 0.5

2±3 0.1 0.1 0.9 0.3 1.6 0.4 2.8 0.6 5.6 1.3 15.8 1.5 42.3 1.8 24.0 0.4 0.6

4±5 0.2 0.2 1.4 0.3 2.0 0.4 2.6 0.5 2.9 0.5 9.6 1.3 43.4 2.5 30.9 0.5 0.8

6±7 0.1 0.2 1.2 0.4 2.0 0.4 2.2 0.5 3.2 0.3 8.5 1.7 41.2 3.2 33.2 0.7 1.0

8±9 0.3 0.2 2.6 0.5 2.2 0.5 1.8 0.6 3.8 1.4 15.6 2.8 7.2 5.4 55.8 1.1 1.5

10±11 0.4 0.2 3.2 0.4 2.9 0.4 1.9 0.5 3.4 0 9.9 1.4 42.9 2.6 28.6 0.7 0.7

13±14 1.0 0.5 4.0 1.0 12.2 1.0 3.4 0.7 3.8 1.0 9.2 1.5 31.9 2.6 23.6 0.7 1.0

15±16 1.1 0.4 4.0 1.2 25.3 1.2 5.1 0.8 4.2 0.3 8.6 1.2 26.8 1.7 17.1 0.7 0.4

17±18 1.2 0.3 2.9 1.3 38.8 1.3 4.3 0.7 3.5 0 10.0 1.7 20.9 1.2 11.3 0.3 0.4

18±19 1.0 0.3 2.2 1.6 57.6 1.3 2.8 0.4 2.3 0 6.2 0.2 14.4 0.8 8.4 0.2 0.3

19±20 0.6 0.2 2.3 1.5 45.9 1.1 2.6 0.4 2.5 0 7.6 0.7 20.2 1.1 12.5 0.4 0.4

21±22 0.3 0.2 1.9 1.2 30.5 1.0 2.2 0.5 2.0 0 6.7 1.0 28.8 1.7 20.6 0.6 0.8

23±24 0.2 0.2 1.0 1.1 29.9 1.0 1.9 0.4 2.1 0.2 7.3 1.3 31.5 1.8 19.4 0.3 0.5

25±26 0.3 0.2 1.2 1.0 12.7 1.2 3.0 1.0 3.7 0 10.4 1.3 38.8 1.8 22.3 0.6 0.7

27±28 0.3 0.2 1.2 0.6 7.8 0.8 2.5 0.7 2.9 1.0 8.1 1.6 29.0 2.7 29.1 0.7 0.9

30±31 0.5 0.3 1.8 0.8 4.6 1.0 4.3 0.8 3.4 0 10.1 1.2 36.9 2.4 30.8 1.5 0.9

32±33 0.4 0.2 1.5 0.5 2.8 0.7 3.5 0.6 2.7 0.8 9.3 1.6 38.5 2.6 33.0 0.5 0.9

34±35 0.4 0.2 1.9 0.6 3.1 0.8 4.1 0.6 2.6 0 9.6 1.5 41.5 2.3 29.6 0.5 0.8

36±37 0.3 0.1 1.0 0.4 2.3 0.6 3.1 0.5 2.3 0.9 9.5 1.7 45.2 1.9 29.5 0.3 0.5

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32 to 40 cm showed a familiar higher plant distribution (C31 maximum, odd-over-even predominance), whilst

those from 13 to 31 cm exhibited the C23 component

enhanced in relative abundance. Slight di€erences inn -alkane distributions between peat and reference plants are assumed to be due to environmental conditions a€ecting distributions in source organisms at the time of their growth.

Signi®cantly, the trend of these changes down the peat pro®le appeared to correlate well with the change in vegetation through time, as shown by the macrofossil record. Fig. 1 shows a plot of the variation in then-C23/

n-C31 ratio with depth compared with the variation in

the proportion ofSphagnummacrofossil remains down the pro®le. This parameter illustrates the varying abun-dance of then-C23alkane down core and should re¯ect

the change in relative abundance of Sphagnum and other higher plants, and/or variation inSphagnum spe-cies in Sphagnum dominated areas of the core. The Sphagnummacrofossil record (Fig. 1) showsSphagnum domination between 8±9 and 32±33 cm and the C23/C31

ratio could be recording changes in Sphagnum species over this period. There is a clear increase in the relative abundance of n-C23 present from 33±34 to 19±20 cm

followed by a decrease up to 8-9 cm, with the C23/C31

ratio increasing and decreasing almost linearly. This shift might be related to vegetation changes occuring in response to climate chnage (Barber et al., 1994), hence it is proposed that the linear trends possibly re¯ect the replacement of C31dominatedSphagnumspecies by C23

dominatedSphagnumspecies up to the coldest point of the episode, when the C23 Sphagnum species begin to

retreat and C31 Sphagnum species ¯ourish again. The

low C23/C31ratio either side of this shift also re¯ects the

relatively low Sphagnum abundance, compared to higher plants, as shown in the macrofossil record. The di€erences in trends between the molecular and macro-fossil data highlight the additional potential information available through this type of analysis thereby demon-strating the potential of lipid biomarkers in ombro-trophic mires as proxies for climate change.

Acknowledgements

We thank Mr. J. Carter and Mr. A. Gledhill for their help with the GC/MS analyses. The use of the NERC mass spectrometry facilities (Grants GR3/2951, GR3/ 3758 and FG6/36/01) is gratefully acknowledged. We would also like to thank D. Mauquoy, P. Hughes and N. Cross for their help in obtaining peat samples. Paul Farrimond is thanked for his comments on an earlier version of this paper.

Associate EditorÐA.G.Douglas

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