Note
Palaeoclimate records in compound-speci®c
D
values
of a lipid biomarker in ombrotrophic peat
S. Xie
a, C.J. Nott
a, L.A. Avsejs
a, F. Volders
a, D. Maddy
b,
F.M. Chambers
c, A. Gledhill
a, J.F. Carter
a, R.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, Cheltenham and Gloucester College of Higher Education,
Francis Close Hall, Swindon Road, Cheltenham, GL50 4AZ, UK
Received 23 May 2000; accepted 7 August 2000 (returned to author for revision 11 July 2000)
Abstract
Compound-speci®cD values recorded by means of gas chromatography±thermal conversion±isotope ratio mass spectrometry (GC±TC±IRMS) of the biomarkern-alkane (n-tricosane;n-C23) representative of the dominant
Sphag-numspecies in a 40 cm peat pro®le from Bolton Fell Moss, Cumbria, UK, correlate with vegetation changes in the past >200 years (age depth model based on210Pb dating). The bog vegetation is sensitive to climate change correlating with the global scale cooler period of the later 19th and early 20th centuries. The correlation with meteorological records suggests compound-speci®c D values of lipid biomarkers have potential for use as a new climate proxy.# 2000 Elsevier Science Ltd. All rights reserved.
Keywords:Peat;n-Alkanes; Deuterium;Sphagnum; Climate change; Holocene
1. Introduction
The exploitation of biomarker stratigraphic records as climate proxies has been shown to be eective in the ocean (e.g. Prahl and Wakeham, 1987) and lake sediments (e.g. Street-Perrott et al., 1997) but rarely as yet in peat deposits (Farrimond and Flanagan, 1995; Ficken et al., 1998; Nott et al., 2000). Previous work on the macrofossil record in Bolton Fell moss demonstrated this bog to be climati-cally sensitive (Barber, 1981). While this and other raised bogs (Aaby, 1976) typically provide records for the latter part of the Holocene a coherent record is fre-quently lacking from blanket bogs owing to humi®ca-tion resulting in loss of identi®able macrofossils. One of our goals is to develop biomarker techniques to allow
vegetation records for highly humi®ed peats to be reconstructed. Preliminary work has focused on sections of the peat for which macrofossil records can be corre-lated with biomarkers characteristic of living bog plants, most speci®cally on the peat immediately beneath the modern-day surface (0±40 cm) since detailed meteor-ological records exist for this most recent period pro-viding the basis for validating new climate proxies.
A previous investigation of then-alkane distributions derived from the monolith revealed clear variations in the relative abundance of n-C23 andn-C31 down core (Fig. 1b), corresponding to vegetation changes at the bog surface through time (Nott et al., 2000). In this study we investigate whether or not more detailed cli-matic information could be obtained through stable isotope measurements, anticipating that compound-speci®c stable isotope methods would provide enhanced speci®city and sensitivity compared with bulk techni-ques (May, 1976). The n-C23 component was selected since it was present in all the modern Sphagnum spp.
0146-6380/00/$ - see front matter#2000 Elsevier Science Ltd. All rights reserved. P I I : S 0 1 4 6 - 6 3 8 0 ( 0 0 ) 0 0 1 1 6 - 9
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* Corresponding author. Tel.: 117-928-7671; fax: +44-117-925-1295.
Fig. 1. Pro®les showing temporal variations of the biomarker abundance and compound speci®cDvalues (black squares) correlating
with temperature, rainfall and the macrofossil record [continuous lines: macrofossil record is for 1 cm contiguous samples; the mean temperature is based on the growing season forSphagnum(May±September) in England calculated as a 30-year running average (Manley, 1974; Parker et al., 1992), and the yearly Carlisle rainfall record (Jones, 1983; National Meteorological Library, Bracknell)]: (a)Sphagnummacrofossil distribution (% of the total peat plants); (b) variation in the ratio ofn-C23/n-C31with depth; (c) and (d)
and, most importantly, detectable in all the sections of the peat.
2. Experimental
2.1. Sampling, dating and lipid extraction and separation
A 40 cm monolith was taken from the centre of Bolton Fell Moss, Cumbria, UK, and frozen and stored in the dark atÿ20C until required for analysis. The monolith
was divided into 1 cm slices representing contiguous depth horizons. The outer surface of each of the slices was removed to avoid contamination. Macrofossil analysis was performed according to the method of Barber et al. (1994). Ten samples from the top 30 cm were submitted for 210Pb dating (Flynn, 1968; Robbins, 1978). Lipid extractions and separations were performed as described previously (Avsejs et al., 1998; Nott et al., 2000).
2.2. Instrumental analyses
GC and GC/MS analyses were performed as descri-bed previously (Nott et al., 2000). Measurement of D
values for individual n-alkanes was achieved using a Finnigan MAT DeltaPLUSXL GC±thermal conversion± IRMS (GC±TC±IRMS; Burgoyne and Hayes, 1998; Hilkert et al., 1999). The high temperature conversion system (1450C) quantitatively converts the hydrogen in organic compounds into H2gas prior to analysis in the IRMS. The IRMS has a dispersion of 180 mm, a wide simultaneous detection focal plane for the collection ofm/ z2 and 3, and a gridless retardation lens achieving close to 100% transmission to them/z3 Faraday cup. An addi-tional contribution to them/z3 signal originates from the formation of H3+ ions via ion molecule reactions between H2+and neutral H2in the ion source; this eect was corrected for on a daily basis by determination of small and stable H3-factors (<10). All values for D reported here are relative to VSMOW. A gaseous laboratory H2standard was used to determine the H3 -factor for the day, normalizeDvalues to the VSMOW
scale, and monitor the stability of the system. Another standard n-alkane mixture containing 7 homologues (C20±C30, even Cn), varying by 250% inD, was ana-lysed at least twice a day to assess analytical precision. The external precision () was better than 4.3%, based on the triplicate determinations.
3. Results and discussion
210Pb dating showed that the accumulation rate for the
top 30 cm of the monolith was 1.8 mm aÿ1with
macro-fossil analysis showing this section to be dominated by
Sphagnum spp., although there were ¯uctuations (Fig. 1a) suggesting periods of variation in their relative abundance known to re¯ect changes in wetness and/or temperature (Barber, 1981).Dvalues were recorded for the C23 n-alkane using GC±TC±IRMS (see Experi-mental). Fig. 1c shows the variations in D for n-C23 with depth. Translating depth to calendar age based on 210Pb dating it is clear that the curve forDcorrelates closest with recorded temperature for at least the past 200 years (Manley, 1974; Parker et al., 1992). More negativeDvalues are largely consistent with the global scale cooler climate of the later 19th and early 20th centuries contrasting with the less depleted values recorded for the warming period between 1910 and 1940 (Fig. 1c).D values of bulk peat or peat cellulose have been shown to correlate with climate (Schiegl, 1972; van Geel and Middeldorp, 1988; Dupont and Mook, 1997). However, little is known aboutDvalues of individual lipids (Sessions et al., 1999) and their possible use as proxies for climate, principally because the GC±TC± IRMS method essential for such determinations has only been developed recently (Burgoyne and Hayes, 1998; Hilkert et al., 1999).
The natural deuterium content of organically bound hydrogen in plants is known to correlate with that of local precipitation (Schiegl and Vogel, 1970), and the deuterium content of precipitation depends mainly on climate, decreasing with decreasing temperature (Dans-gaard, 1964). Depletion of deuterium in bulk peat dur-ing colder periods has been shown to correlate with decreases in temperature (Schiegl, 1972). The range of
Dvalues determined forn-C23in the pro®le during the cold episode gives a deuterium temperature coecient of 59%Cÿ1using a 30 year moving average tempera-ture. Statistical comparison of the compound-speci®c dD record with temperature yielded a correlation
coef-®cient (r=0.77), which is unusually high for data of this nature, while the correlation between local rainfall and
Dvalues forn-C23is poor (r=0.14). Interestingly, the observedDtemperature coecients forn-C23recorded
over the maxima and minima of the period is greater than has been observed in precipitation samples (which typically show sensitivities of 5±10%Cÿ1), and is also signi®cantly larger than has been observed in studies of peat components such as cellulose (14±20% Cÿ1; Brennikmeijer et al., 1982; van Geel and Middeldorp, 1988). Signi®cantly, the n-C23 in 6 modern Sphagnum
spp. collected from the bog gave a range of D values narrower than that recorded for the same compound in the peat pro®le (Sphagnum spp. ÿ161 to ÿ142%)
physiologically disparate plant taxa. Hence, any varia-tions in compound-speci®c D values seen at dierent depths will most likely re¯ect temporal variations in cli-mate, e.g. temperature and/or humidity.
Notwithstanding the acknowledged diculty in separating the temperature and moisture signals, it is clear from the data obtained that there is an unusually high statistical correlation of the measured n-C23 D
value with recorded temperature. While it is known that variations inDin precipitation correlate with amount
and humidity (Ehhalt et al., 1963; Dansgaard, 1964) in tropical and subtropical regions, this eect is much less pronounced at northern latitudes (Dansgaard, 1964). This is borne out by the poor correlation obtained between rainfall andD at this location (Fig. 1d). The strong correlation between the measured D values in the peat pro®le and recorded summer temperatures indicates that the variation is in¯uenced either directly or indirectly by temperature. The most likely explana-tion for this is that theDvalues of then-C23is re¯ect-ing the isotopic composition of bog water which will vary between `wet' and `dry' periods due to evaporation, likely being enhanced by increases in temperature, thereby establishing a potential link between the D
value of utilisable water and the isotopic composition of organically bound hydrogen of the bog vegetation. Interestingly, it has been shown thatDand18O in moss cellulose relates to water balance (Aravena and Werner, 1992). Furthermore, evidence for the role of bog hydrol-ogy, and topography, on the isotopic composition of bog vegetation comes from the work of Aravena and Werner (1992) who observed that bog mosses growing on hum-mocks (which were positioned high relative to the water table) were enriched in18O relative to mosses growing in hollows (whose roots were within the saturated zone). Likewise van Geel and Middeldorp (1988) report downcore changes in moss cellulose D that are too
large to be explained by simply temperature eects on
D of precipitation, but that coincide with changes in
Sphagnumspecies distributions in a way that implicates moisture as a dominant control. The results forn-C23of the modern bog vegetation presented above suggest that the observed temporal variation in D for the peat is probably unrelated to species variations, especially in view of the fact that macrofossil analyses show that for the period covered by the monolithS. magellanicumis the sole detectableSphagnumspecies. Thus, temperature related variations in the evaporation of bog water are the most likely cause of the observed changes in theD
value of the synthesisedSphagnum n-C23biomarker. Although further work is required to unambiguously con®rm the origin of the correlation between recorded local temperature and compound-speci®cDvalues the
®ndings reported herein provide a clear indication of the potential that such measurements of individual lipids have for use in the reconstruction of past climate.
Interestingly, the trends observed in the compound-spe-ci®cD record from Bolton Fell peat are analogous to those seen for a similar date range in tree ring cellulose (Schiegl, 1974; Epstein and Yapp, 1976). There is con-siderable appeal in the use of individual lipids, such as
n-alkanes, as carriers of a D signal for palaeoclimate reconstruction due their greater likelihood for survival compared with more labile compounds, such as cellu-lose, which have largely been relied upon until now for establishingDrecords in peat and woody plants.
Acknowledgements
Access to Bolton Fell Moss arranged via Dr Keith Barber from Boothby and Penicuik Peat Company. D. Mauquoy, P. Hughes and N. Cross are thanked for their help in obtaining samples. A. Cundy, South-ampton Oceanography Centre, is thanked for 210Pb dating, and the NERC for mass spectrometry facilities (GR3/E0095). The EPSRC (CJN) and University of Bristol (LAA and FV) are thanked for PhD student-ships. SX was supported by the China Scholarship Council. The authors should like to thank an anony-mous reviewer for most useful comments on an earlier version of this paper.
Associate EditorÐA. G. Douglas
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