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n-Alkane distributions in ombrotrophic mires as indicators of vegetation change related to climatic variation
Organic Geochemistry 31 (2000) 231±235
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n-Alkane distributions in ombrotrophic mires as indicators of vegetation change related to climatic variation Chris J. Notta, Shucheng Xiea, Luke A. Avsejsa, Darrel Maddyb, Frank M. Chambersc, Richard P. Eversheda,* a Organic Geochemistry Unit, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK Department of Geography, Daysh Building, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK c Centre for Environmental Change and Quaternary Research, GEMRU, C&GCHE, Francis Close Hall, Swindon Road, Cheltenham GL50 4AZ, UK
b
Received 7 October 1999; accepted 9 November 1999 (Returned to author for revision 12 October 1999)
Abstract Lipid analysis of modern bog vegetation revealed n-alkane distributions for Sphagnum species displaying enhanced abundances of lower chain length homologues (C21±C25). Other plants examined revealed typical higher plant distributions (C29 or C31 maxima). Investigation of a 40 cm peat pro®le from Bolton Fell Moss, Cumbria, UK showed a varying abundance of the n-C23 homologue 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 Holocene, and the diversity of potentially informative features 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 balance 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) * Corresponding author. Tel.: +44-117-928-7671; fax: +44117-925-1295. E-mail address: [email protected] (R.P. Evershed).
dierences 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 precipitation (Blackford and Chambers, 1993); and (iii) the d13C values of peat macrofossils being used to reconstruct atmospheric CO2 concentrations (White et al., 1994; Figge and White, 1995), and d18O and dD values of bulk peat correlating with temperature changes (Schiegl, 1971). Examples of the use of organic geochemical tools to deduce palaeoenvironmental information from terrestrial environments are limited considering the organicrich 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 nalkane ®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. PII: S0146-6380(99)00153-9
232
C.J. Nott et al. / Organic Geochemistry 31 (2000) 231±235
approaches have been employed widely in marine sediments (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 ombrotrophic mire covering an area of around 400 ha. Previous detailed work on the uppermost peat suggested that this mire was particularly responsive to climatic change. Barber (1981) showed that macrofossil assemblages 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 molecular 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. Samples of the modern day covering vegetation were collected and stored, along with the peat monolith, in a freezer at
ÿ20 C 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 slices (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 neutral fractions by solid phase extraction using aminopropyl 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 composition of modern day bog vegetation diagnostic n-alkane
Fig. 1. Logarithmic plot of the ratio of n-C23 to n-C31 alkane down pro®le (left) compared with the varying proportion of Sphagnum macrofossils in an immediately adjacent pro®le (right).
C.J. Nott et al. / Organic Geochemistry 31 (2000) 231±235
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), C31 dominating S. magellanicum, S. capillifolium and S. cuspidatum (bimodal distributions). In contrast, the other plants examined revealed n-alkane
233
distributions characteristic of higher plants. These ranged from C19 to C35 with an odd-over-even predominance, maximising at C31, with the exception of Vaccinium oxycoccus and Hypnum cupressiforme which maximised at C29, and the bog moss Aulacomnium palustre which maximised at C27. Analysis of the peat horizons showed the n-C23 alkane 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 included in Table 1. Peat horizons down to 11 cm and from
Table 1 Individual n-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 S. recurvum S. capillifolium S. cuspidatum S. palustre S. papillosum Polytrichum sp. Aulacomnium palustre Hypnum cupressiforme Eriophorum vaginatum Eriophorum angustifolium Trichophorum cespitosum Empetrum nigrum Vaccinium oxycoccus Erica tetralix Calluna vulgaris Andromeda polifolia Rhynchospora alba Cladonia sp.
0 0.4 0.1 0 0 0 0 0 0 0 0 0 0 0.4 0 0.2 0 0 0
0 0.3 0.1 0 0 0 0 0 0 0 0 0 0 0.2 0 0.1 0 0 0
2.2 23.3 3.6 5.4 5.4 6.2 0.3 0 1.0 0.5 0.5 0.1 0 0.3 0 1.6 0.1 2.3 0.7
0.2 2.0 0.6 1.4 0.3 1.4 0.3 0 0.4 0.1 0.1 0.1 0 0.2 0 0.1 0.1 0.2 0.7
7.9 38.2 11.3 26.3 31.2 35.6 0.7 0.9 4.0 1.2 1.2 0.3 0.1 0.6 0.3 0.7 0.6 2.3 2.3
0.6 2.7 1.1 1.6 1.3 2.1 0.2 0.2 0.7 0.2 0.2 0.1 0.1 0.2 0.2 0.2 0.4 0.7 0.9
18.8 15.9 15.0 5.8 22.6 20.5 2.5 4.9 8.6 1.7 1.7 0.7 0.2 1.0 1.1 2.2 3.1 3.2 4.9
0.6 1.0 0.6 0.5 0.7 0.8 0.7 0.8 1.6 0.3 0.2 0.2 0.1 0.9 0.3 0.5 0.7 0.5 1.2
3.6 8.1 5.1 3.4 12.4 4.5 11.8 37.1 17.0 3.2 3.1 2.0 1.4 13.1 4.1 6.9 6.4 3.8 9.7
0.3 0.1 0.7 0.7 0.8 0.5 1.3 0.9 1.3 0.4 0.1 0.3 0.2 2.2 0.3 0.8 0.8 0.8 2.3
16.7 4.0 13.6 8.3 7.7 4.3 27.2 9.4 28.2 14.8 14.7 11.1 25.5 50.4 9.4 11.3 38.8 28.2 12.4
1.3 0.3 1.2 1.5 0.7 0.6 1.6 0.9 3.1 1.1 1.1 0.8 1.0 2.2 1.3 1.6 0.9 1.7 2.3
33.5 3.1 33.6 30.7 12.6 16.1 30.9 28.5 22.8 54.2 53.8 57.6 49.4 26.4 60.4 43.7 45.1 44.6 38.6
1.7 0.8 1.2 0.9 0.6 0.8 2.5 1.9 3.3 1.4 1.4 1.3 1.2 0.6 2.1 3.1 0.7 1.0 2.7
12.6 0.5 12.6 13.3 3.7 6.6 18.9 14.5 7.9 20.7 20.5 25.4 20.7 2.0 20.6 26.3 2.4 10.8 21.3
0 0 0 0 0 0 0.4 0 0 0.1 1.3 0.1 0.1 0 0 0.4 0 0 0
0 0 0 0 0 0 0.7 0 0 0.2 0.1 0.1 0.1 0 0 0.8 0 0 0
Peat horizons (cm) 0±1 2±3 4±5 6±7 8±9 10±11 13±14 15±16 17±18 18±19 19±20 21±22 23±24 25±26 27±28 30±31 32±33 34±35 36±37 39±40
0.1 0.1 0.2 0.1 0.3 0.4 1.0 1.1 1.2 1.0 0.6 0.3 0.2 0.3 0.3 0.5 0.4 0.4 0.3 0.4
0.2 0.1 0.2 0.2 0.2 0.2 0.5 0.4 0.3 0.3 0.2 0.2 0.2 0.2 0.2 0.3 0.2 0.2 0.1 0.2
2.8 0.9 1.4 1.2 2.6 3.2 4.0 4.0 2.9 2.2 2.3 1.9 1.0 1.2 1.2 1.8 1.5 1.9 1.0 1.4
0.5 0.3 0.3 0.4 0.5 0.4 1.0 1.2 1.3 1.6 1.5 1.2 1.1 1.0 0.6 0.8 0.5 0.6 0.4 0.5
5.9 1.6 2.0 2.0 2.2 2.9 12.2 25.3 38.8 57.6 45.9 30.5 29.9 12.7 7.8 4.6 2.8 3.1 2.3 3.4
0.8 0.4 0.4 0.4 0.5 0.4 1.0 1.2 1.3 1.3 1.1 1.0 1.0 1.2 0.8 1.0 0.7 0.8 0.6 0.8
7.7 2.8 2.6 2.2 1.8 1.9 3.4 5.1 4.3 2.8 2.6 2.2 1.9 3.0 2.5 4.3 3.5 4.1 3.1 4.2
0.7 0.6 0.5 0.5 0.6 0.5 0.7 0.8 0.7 0.4 0.4 0.5 0.4 1.0 0.7 0.8 0.6 0.6 0.5 0.6
5.8 5.6 2.9 3.2 3.8 3.4 3.8 4.2 3.5 2.3 2.5 2.0 2.1 3.7 2.9 3.4 2.7 2.6 2.3 3.0
0 1.3 0.5 0.3 1.4 0 1.0 0.3 0 0 0 0 0.2 0 1.0 0 0.8 0 0.9 0
18.7 15.8 9.6 8.5 15.6 9.9 9.2 8.6 10.0 6.2 7.6 6.7 7.3 10.4 8.1 10.1 9.3 9.6 9.5 11.0
1.0 1.5 1.3 1.7 2.8 1.4 1.5 1.2 1.7 0.2 0.7 1.0 1.3 1.3 1.6 1.2 1.6 1.5 1.7 1.5
34.3 42.3 43.4 41.2 7.2 42.9 31.9 26.8 20.9 14.4 20.2 28.8 31.5 38.8 29.0 36.9 38.5 41.5 45.2 44.5
1.7 1.8 2.5 3.2 5.4 2.6 2.6 1.7 1.2 0.8 1.1 1.7 1.8 1.8 2.7 2.4 2.6 2.3 1.9 1.8
18.9 24.0 30.9 33.2 55.8 28.6 23.6 17.1 11.3 8.4 12.5 20.6 19.4 22.3 29.1 30.8 33.0 29.6 29.5 25.8
0.5 0.4 0.5 0.7 1.1 0.7 0.7 0.7 0.3 0.2 0.4 0.6 0.3 0.6 0.7 1.5 0.5 0.5 0.3 0.4
0.5 0.6 0.8 1.0 1.5 0.7 1.0 0.4 0.4 0.3 0.4 0.8 0.5 0.7 0.9 0.9 0.9 0.8 0.5 0.7
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C.J. Nott et al. / Organic Geochemistry 31 (2000) 231±235
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 dierences in nalkane distributions between peat and reference plants are assumed to be due to environmental conditions aecting 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 the n-C23/ n-C31 ratio with depth compared with the variation in the proportion of Sphagnum macrofossil remains down the pro®le. This parameter illustrates the varying abundance of the n-C23 alkane down core and should re¯ect the change in relative abundance of Sphagnum and other higher plants, and/or variation in Sphagnum species in Sphagnum dominated areas of the core. The Sphagnum macrofossil record (Fig. 1) shows Sphagnum 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 C31 dominated Sphagnum species by C23 dominated Sphagnum species 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/C31 ratio either side of this shift also re¯ects the relatively low Sphagnum abundance, compared to higher plants, as shown in the macrofossil record. The dierences in trends between the molecular and macrofossil data highlight the additional potential information available through this type of analysis thereby demonstrating the potential of lipid biomarkers in ombrotrophic 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
References Aaby, B., 1976. Cyclic climatic variations in climate over the past 5,500 yr re¯ected in raised bogs. Nature 263, 281±284. Avsejs, L.A., Nott, C.J., Maxwell, J.R., Evershed, R.P., 1998. Hydroxy and ketonic androstanes: a new class of sterol diagenetic product in peat. Organic Geochemistry 28, 749± 753. Barber, K.E., 1981. Peat Stratigraphy and Climate Change: a Palaeoecological Test of the Theory of Cyclic Peat Bog Regeneration. Balkema, Rotterdam. Barber, K.E., 1985. Peat stratigraphy and climatic change: some speculations. In: Tooley, M.J., Sheail, G.M.. The Climatic Scene. George Allen and Unwin, London, pp. 175± 185. Barber, K.E., 1995. Peat bog archives of past climates. NERC News, January 1995. Barber, K.E., Chambers, F.M., Maddy, D., Stoneman, R., Brew, J.S., 1994. A sensitive high-resolution record of late Holocene climatic change from a raised bog in northern England. The Holocene 4, 198±205. Blackford, J.J., Chambers, F.M., 1993. Determining the degree of peat decomposition for peat-based palaeoclimatic studies. International Peat Journal 5, 7±24. Boon, J.J., Dupont, L., de Leeuw, J.W., 1986. Characterization of a peat bog pro®le by Curie point pyrolysis-mass spectrometry combined with multivariant analysis and by pyrolysis gas chromatography-mass spectrometry. In: Fuchsman, C.H. (Ed.), Peat and Water. Elsevier, London, pp. 215±239. Bracewell, J.M., Robertson, G.W., Williams, B.L., 1980. Pyrolysis±mass spectrometry studies of humi®cation in a peat and peaty podzol. Journal of Analytical and Applied Pyrolysis 2, 53±62. Brassell, S.C., Eglinton, G., Marlowe, I.T., P¯aumann, U., Sarnthein, M., 1986. Molecular stratigraphy: a new tool for climatic assessment. Nature 320, 129±133. Corrigan, D., Kloos, C., O'Connor, C.S., Timoney, R.F., 1973. Alkanes from four types of Sphagnum moss. Phytochemistry 12, 213±214. Conte, M.H., Eglinton, G., Madureira, L.A., 1992. Long-chain alkenones and alkyl alkenoates as palaeotemperatures indicators: their production, ¯ux and early sedimentary diagenesis in the Eastern North Atlantic. Organic Geochemistry 19, 287±298. Cranwell, P.A., 1973. Chain-length distribution of n-alkanes from lake sediments in relation to post-glacial environmental change. Chemical Geology 11, 307±313. Dehmer, J., 1993. Petrology and organic geochemistry of peat samples from a raised bog in Kalimantan (Borneo). Organic Geochemistry 20, 349±362. Farrimond, P., Flanagan, R.L., 1995. Lipid stratigraphy of a Flandrian peat bed (Northumberland, UK): comparison with the pollen record. The Holocene 6, 69±74. Farrimond, P., Poynter, J.G., Eglinton, G., 1990. A molecular stratigraphic study of Peru Margin sediments, Hole 686B, Leg 112. In Suess, E. (Ed.), Proceedings of the Ocean Drilling Program, Scienti®c Results 112. pp. 547±553. Ficken, K.J., Barber, K.E., Eglinton, G., 1998. Lipid biomarker, d and plant macrofossil stratigraphy of a Scottish montane peat bog over the last two millennia. Organic Geochemistry 28, 217±237.
C.J. Nott et al. / Organic Geochemistry 31 (2000) 231±235 Figge, R.A., White, J.W.C., 1995. High-resolution Holocene and late glacial atmospheric CO2 record: variability tied to changes in thermohaline circulation. Global Biogeochemical Cycle 9, 391±403. Halma, G., Dam, D., van Haverkamp, J., Windig, W., Meuzelaar, H.L.C., 1984. Characterisation of an oligotrophiceutrophic peat sequence by pyrolysis±mass spectrometry and conventional analysis methods. Journal of Analytical and Applied Pyrolysis 7, 167±183. Heijden, van der E.A., 1994. A combined anatomical and pyrolysis mass spectrometric study of peati®ed plant tissues. Ph.D. thesis, University of Amsterdam. Karunen, P., Ekman, R., Salin, M., 1983. Sphagnum mosses as sources of sterols in peat. In: Fuchsman, C.H., Spigarelli, S.A. (Eds.), Proceedings of the International Symposium on Peat Utilization, Bemidji State University, Bemidji, MN, pp. 487± 493. Kuder, T., Kruge, M.A., 1998. Preservation of biomolecules in sub-fossil plants from raised peat bogs Ð a potential paleoenvironmental proxy. Organic Geochemistry 29, 1355±1368.
235
Lehtonen, K., Ketola, M., 1993. Solvent-extractable lipids of Sphagnum, Carex, Bryales and Carex-Bryales peats: content and compositional features vs peat humi®cation. Organic Geochemistry 20, 363±380. Meyers, P.A., Kawka, O.E., Whitehead, D.R., 1984. Geolipid, pollen and diatom stratigraphy in post-glacial lacustrine sediments. Organic Geochemistry 6, 727±732. Quirk, M.M., Wardroper, A.M.K., Wheatley, R.E., Maxwell, J.R., 1984. Extended hopanoids in peat environments. Chemical Geology 42, 25±43. Rio del, J.C., Gonzalez-Vila, F.J., Martin, F., 1992. Variation in the content and distribution of biomarkers in two closely situated peat and lignite deposits. Organic Geochemistry 18, 67±78. Schiegl, W.E., 1971. Deuterium content of peat as a paleoclimate recorder. Science 175, 512±513. White, J.W.C., Ciais, P., Figge, R.A., Kenny, R., Markgraf, V., 1994. A high-resolution record of atmospheric CO2 content from carbon isotopes in peat. Nature 367, 153±156.
Note
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n-Alkane distributions in ombrotrophic mires as indicators of vegetation change related to climatic variation Chris J. Notta, Shucheng Xiea, Luke A. Avsejsa, Darrel Maddyb, Frank M. Chambersc, Richard P. Eversheda,* a Organic Geochemistry Unit, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK Department of Geography, Daysh Building, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK c Centre for Environmental Change and Quaternary Research, GEMRU, C&GCHE, Francis Close Hall, Swindon Road, Cheltenham GL50 4AZ, UK
b
Received 7 October 1999; accepted 9 November 1999 (Returned to author for revision 12 October 1999)
Abstract Lipid analysis of modern bog vegetation revealed n-alkane distributions for Sphagnum species displaying enhanced abundances of lower chain length homologues (C21±C25). Other plants examined revealed typical higher plant distributions (C29 or C31 maxima). Investigation of a 40 cm peat pro®le from Bolton Fell Moss, Cumbria, UK showed a varying abundance of the n-C23 homologue 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 Holocene, and the diversity of potentially informative features 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 balance 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) * Corresponding author. Tel.: +44-117-928-7671; fax: +44117-925-1295. E-mail address: [email protected] (R.P. Evershed).
dierences 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 precipitation (Blackford and Chambers, 1993); and (iii) the d13C values of peat macrofossils being used to reconstruct atmospheric CO2 concentrations (White et al., 1994; Figge and White, 1995), and d18O and dD values of bulk peat correlating with temperature changes (Schiegl, 1971). Examples of the use of organic geochemical tools to deduce palaeoenvironmental information from terrestrial environments are limited considering the organicrich 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 nalkane ®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. PII: S0146-6380(99)00153-9
232
C.J. Nott et al. / Organic Geochemistry 31 (2000) 231±235
approaches have been employed widely in marine sediments (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 ombrotrophic mire covering an area of around 400 ha. Previous detailed work on the uppermost peat suggested that this mire was particularly responsive to climatic change. Barber (1981) showed that macrofossil assemblages 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 molecular 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. Samples of the modern day covering vegetation were collected and stored, along with the peat monolith, in a freezer at
ÿ20 C 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 slices (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 neutral fractions by solid phase extraction using aminopropyl 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 composition of modern day bog vegetation diagnostic n-alkane
Fig. 1. Logarithmic plot of the ratio of n-C23 to n-C31 alkane down pro®le (left) compared with the varying proportion of Sphagnum macrofossils in an immediately adjacent pro®le (right).
C.J. Nott et al. / Organic Geochemistry 31 (2000) 231±235
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), C31 dominating S. magellanicum, S. capillifolium and S. cuspidatum (bimodal distributions). In contrast, the other plants examined revealed n-alkane
233
distributions characteristic of higher plants. These ranged from C19 to C35 with an odd-over-even predominance, maximising at C31, with the exception of Vaccinium oxycoccus and Hypnum cupressiforme which maximised at C29, and the bog moss Aulacomnium palustre which maximised at C27. Analysis of the peat horizons showed the n-C23 alkane 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 included in Table 1. Peat horizons down to 11 cm and from
Table 1 Individual n-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 S. recurvum S. capillifolium S. cuspidatum S. palustre S. papillosum Polytrichum sp. Aulacomnium palustre Hypnum cupressiforme Eriophorum vaginatum Eriophorum angustifolium Trichophorum cespitosum Empetrum nigrum Vaccinium oxycoccus Erica tetralix Calluna vulgaris Andromeda polifolia Rhynchospora alba Cladonia sp.
0 0.4 0.1 0 0 0 0 0 0 0 0 0 0 0.4 0 0.2 0 0 0
0 0.3 0.1 0 0 0 0 0 0 0 0 0 0 0.2 0 0.1 0 0 0
2.2 23.3 3.6 5.4 5.4 6.2 0.3 0 1.0 0.5 0.5 0.1 0 0.3 0 1.6 0.1 2.3 0.7
0.2 2.0 0.6 1.4 0.3 1.4 0.3 0 0.4 0.1 0.1 0.1 0 0.2 0 0.1 0.1 0.2 0.7
7.9 38.2 11.3 26.3 31.2 35.6 0.7 0.9 4.0 1.2 1.2 0.3 0.1 0.6 0.3 0.7 0.6 2.3 2.3
0.6 2.7 1.1 1.6 1.3 2.1 0.2 0.2 0.7 0.2 0.2 0.1 0.1 0.2 0.2 0.2 0.4 0.7 0.9
18.8 15.9 15.0 5.8 22.6 20.5 2.5 4.9 8.6 1.7 1.7 0.7 0.2 1.0 1.1 2.2 3.1 3.2 4.9
0.6 1.0 0.6 0.5 0.7 0.8 0.7 0.8 1.6 0.3 0.2 0.2 0.1 0.9 0.3 0.5 0.7 0.5 1.2
3.6 8.1 5.1 3.4 12.4 4.5 11.8 37.1 17.0 3.2 3.1 2.0 1.4 13.1 4.1 6.9 6.4 3.8 9.7
0.3 0.1 0.7 0.7 0.8 0.5 1.3 0.9 1.3 0.4 0.1 0.3 0.2 2.2 0.3 0.8 0.8 0.8 2.3
16.7 4.0 13.6 8.3 7.7 4.3 27.2 9.4 28.2 14.8 14.7 11.1 25.5 50.4 9.4 11.3 38.8 28.2 12.4
1.3 0.3 1.2 1.5 0.7 0.6 1.6 0.9 3.1 1.1 1.1 0.8 1.0 2.2 1.3 1.6 0.9 1.7 2.3
33.5 3.1 33.6 30.7 12.6 16.1 30.9 28.5 22.8 54.2 53.8 57.6 49.4 26.4 60.4 43.7 45.1 44.6 38.6
1.7 0.8 1.2 0.9 0.6 0.8 2.5 1.9 3.3 1.4 1.4 1.3 1.2 0.6 2.1 3.1 0.7 1.0 2.7
12.6 0.5 12.6 13.3 3.7 6.6 18.9 14.5 7.9 20.7 20.5 25.4 20.7 2.0 20.6 26.3 2.4 10.8 21.3
0 0 0 0 0 0 0.4 0 0 0.1 1.3 0.1 0.1 0 0 0.4 0 0 0
0 0 0 0 0 0 0.7 0 0 0.2 0.1 0.1 0.1 0 0 0.8 0 0 0
Peat horizons (cm) 0±1 2±3 4±5 6±7 8±9 10±11 13±14 15±16 17±18 18±19 19±20 21±22 23±24 25±26 27±28 30±31 32±33 34±35 36±37 39±40
0.1 0.1 0.2 0.1 0.3 0.4 1.0 1.1 1.2 1.0 0.6 0.3 0.2 0.3 0.3 0.5 0.4 0.4 0.3 0.4
0.2 0.1 0.2 0.2 0.2 0.2 0.5 0.4 0.3 0.3 0.2 0.2 0.2 0.2 0.2 0.3 0.2 0.2 0.1 0.2
2.8 0.9 1.4 1.2 2.6 3.2 4.0 4.0 2.9 2.2 2.3 1.9 1.0 1.2 1.2 1.8 1.5 1.9 1.0 1.4
0.5 0.3 0.3 0.4 0.5 0.4 1.0 1.2 1.3 1.6 1.5 1.2 1.1 1.0 0.6 0.8 0.5 0.6 0.4 0.5
5.9 1.6 2.0 2.0 2.2 2.9 12.2 25.3 38.8 57.6 45.9 30.5 29.9 12.7 7.8 4.6 2.8 3.1 2.3 3.4
0.8 0.4 0.4 0.4 0.5 0.4 1.0 1.2 1.3 1.3 1.1 1.0 1.0 1.2 0.8 1.0 0.7 0.8 0.6 0.8
7.7 2.8 2.6 2.2 1.8 1.9 3.4 5.1 4.3 2.8 2.6 2.2 1.9 3.0 2.5 4.3 3.5 4.1 3.1 4.2
0.7 0.6 0.5 0.5 0.6 0.5 0.7 0.8 0.7 0.4 0.4 0.5 0.4 1.0 0.7 0.8 0.6 0.6 0.5 0.6
5.8 5.6 2.9 3.2 3.8 3.4 3.8 4.2 3.5 2.3 2.5 2.0 2.1 3.7 2.9 3.4 2.7 2.6 2.3 3.0
0 1.3 0.5 0.3 1.4 0 1.0 0.3 0 0 0 0 0.2 0 1.0 0 0.8 0 0.9 0
18.7 15.8 9.6 8.5 15.6 9.9 9.2 8.6 10.0 6.2 7.6 6.7 7.3 10.4 8.1 10.1 9.3 9.6 9.5 11.0
1.0 1.5 1.3 1.7 2.8 1.4 1.5 1.2 1.7 0.2 0.7 1.0 1.3 1.3 1.6 1.2 1.6 1.5 1.7 1.5
34.3 42.3 43.4 41.2 7.2 42.9 31.9 26.8 20.9 14.4 20.2 28.8 31.5 38.8 29.0 36.9 38.5 41.5 45.2 44.5
1.7 1.8 2.5 3.2 5.4 2.6 2.6 1.7 1.2 0.8 1.1 1.7 1.8 1.8 2.7 2.4 2.6 2.3 1.9 1.8
18.9 24.0 30.9 33.2 55.8 28.6 23.6 17.1 11.3 8.4 12.5 20.6 19.4 22.3 29.1 30.8 33.0 29.6 29.5 25.8
0.5 0.4 0.5 0.7 1.1 0.7 0.7 0.7 0.3 0.2 0.4 0.6 0.3 0.6 0.7 1.5 0.5 0.5 0.3 0.4
0.5 0.6 0.8 1.0 1.5 0.7 1.0 0.4 0.4 0.3 0.4 0.8 0.5 0.7 0.9 0.9 0.9 0.8 0.5 0.7
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C.J. Nott et al. / Organic Geochemistry 31 (2000) 231±235
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 dierences in nalkane distributions between peat and reference plants are assumed to be due to environmental conditions aecting 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 the n-C23/ n-C31 ratio with depth compared with the variation in the proportion of Sphagnum macrofossil remains down the pro®le. This parameter illustrates the varying abundance of the n-C23 alkane down core and should re¯ect the change in relative abundance of Sphagnum and other higher plants, and/or variation in Sphagnum species in Sphagnum dominated areas of the core. The Sphagnum macrofossil record (Fig. 1) shows Sphagnum 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 C31 dominated Sphagnum species by C23 dominated Sphagnum species 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/C31 ratio either side of this shift also re¯ects the relatively low Sphagnum abundance, compared to higher plants, as shown in the macrofossil record. The dierences in trends between the molecular and macrofossil data highlight the additional potential information available through this type of analysis thereby demonstrating the potential of lipid biomarkers in ombrotrophic 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
References Aaby, B., 1976. Cyclic climatic variations in climate over the past 5,500 yr re¯ected in raised bogs. Nature 263, 281±284. Avsejs, L.A., Nott, C.J., Maxwell, J.R., Evershed, R.P., 1998. Hydroxy and ketonic androstanes: a new class of sterol diagenetic product in peat. Organic Geochemistry 28, 749± 753. Barber, K.E., 1981. Peat Stratigraphy and Climate Change: a Palaeoecological Test of the Theory of Cyclic Peat Bog Regeneration. Balkema, Rotterdam. Barber, K.E., 1985. Peat stratigraphy and climatic change: some speculations. In: Tooley, M.J., Sheail, G.M.. The Climatic Scene. George Allen and Unwin, London, pp. 175± 185. Barber, K.E., 1995. Peat bog archives of past climates. NERC News, January 1995. Barber, K.E., Chambers, F.M., Maddy, D., Stoneman, R., Brew, J.S., 1994. A sensitive high-resolution record of late Holocene climatic change from a raised bog in northern England. The Holocene 4, 198±205. Blackford, J.J., Chambers, F.M., 1993. Determining the degree of peat decomposition for peat-based palaeoclimatic studies. International Peat Journal 5, 7±24. Boon, J.J., Dupont, L., de Leeuw, J.W., 1986. Characterization of a peat bog pro®le by Curie point pyrolysis-mass spectrometry combined with multivariant analysis and by pyrolysis gas chromatography-mass spectrometry. In: Fuchsman, C.H. (Ed.), Peat and Water. Elsevier, London, pp. 215±239. Bracewell, J.M., Robertson, G.W., Williams, B.L., 1980. Pyrolysis±mass spectrometry studies of humi®cation in a peat and peaty podzol. Journal of Analytical and Applied Pyrolysis 2, 53±62. Brassell, S.C., Eglinton, G., Marlowe, I.T., P¯aumann, U., Sarnthein, M., 1986. Molecular stratigraphy: a new tool for climatic assessment. Nature 320, 129±133. Corrigan, D., Kloos, C., O'Connor, C.S., Timoney, R.F., 1973. Alkanes from four types of Sphagnum moss. Phytochemistry 12, 213±214. Conte, M.H., Eglinton, G., Madureira, L.A., 1992. Long-chain alkenones and alkyl alkenoates as palaeotemperatures indicators: their production, ¯ux and early sedimentary diagenesis in the Eastern North Atlantic. Organic Geochemistry 19, 287±298. Cranwell, P.A., 1973. Chain-length distribution of n-alkanes from lake sediments in relation to post-glacial environmental change. Chemical Geology 11, 307±313. Dehmer, J., 1993. Petrology and organic geochemistry of peat samples from a raised bog in Kalimantan (Borneo). Organic Geochemistry 20, 349±362. Farrimond, P., Flanagan, R.L., 1995. Lipid stratigraphy of a Flandrian peat bed (Northumberland, UK): comparison with the pollen record. The Holocene 6, 69±74. Farrimond, P., Poynter, J.G., Eglinton, G., 1990. A molecular stratigraphic study of Peru Margin sediments, Hole 686B, Leg 112. In Suess, E. (Ed.), Proceedings of the Ocean Drilling Program, Scienti®c Results 112. pp. 547±553. Ficken, K.J., Barber, K.E., Eglinton, G., 1998. Lipid biomarker, d and plant macrofossil stratigraphy of a Scottish montane peat bog over the last two millennia. Organic Geochemistry 28, 217±237.
C.J. Nott et al. / Organic Geochemistry 31 (2000) 231±235 Figge, R.A., White, J.W.C., 1995. High-resolution Holocene and late glacial atmospheric CO2 record: variability tied to changes in thermohaline circulation. Global Biogeochemical Cycle 9, 391±403. Halma, G., Dam, D., van Haverkamp, J., Windig, W., Meuzelaar, H.L.C., 1984. Characterisation of an oligotrophiceutrophic peat sequence by pyrolysis±mass spectrometry and conventional analysis methods. Journal of Analytical and Applied Pyrolysis 7, 167±183. Heijden, van der E.A., 1994. A combined anatomical and pyrolysis mass spectrometric study of peati®ed plant tissues. Ph.D. thesis, University of Amsterdam. Karunen, P., Ekman, R., Salin, M., 1983. Sphagnum mosses as sources of sterols in peat. In: Fuchsman, C.H., Spigarelli, S.A. (Eds.), Proceedings of the International Symposium on Peat Utilization, Bemidji State University, Bemidji, MN, pp. 487± 493. Kuder, T., Kruge, M.A., 1998. Preservation of biomolecules in sub-fossil plants from raised peat bogs Ð a potential paleoenvironmental proxy. Organic Geochemistry 29, 1355±1368.
235
Lehtonen, K., Ketola, M., 1993. Solvent-extractable lipids of Sphagnum, Carex, Bryales and Carex-Bryales peats: content and compositional features vs peat humi®cation. Organic Geochemistry 20, 363±380. Meyers, P.A., Kawka, O.E., Whitehead, D.R., 1984. Geolipid, pollen and diatom stratigraphy in post-glacial lacustrine sediments. Organic Geochemistry 6, 727±732. Quirk, M.M., Wardroper, A.M.K., Wheatley, R.E., Maxwell, J.R., 1984. Extended hopanoids in peat environments. Chemical Geology 42, 25±43. Rio del, J.C., Gonzalez-Vila, F.J., Martin, F., 1992. Variation in the content and distribution of biomarkers in two closely situated peat and lignite deposits. Organic Geochemistry 18, 67±78. Schiegl, W.E., 1971. Deuterium content of peat as a paleoclimate recorder. Science 175, 512±513. White, J.W.C., Ciais, P., Figge, R.A., Kenny, R., Markgraf, V., 1994. A high-resolution record of atmospheric CO2 content from carbon isotopes in peat. Nature 367, 153±156.