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 DOI: 10.1177/0959683609350389

2010 20: 285 originally published online 3 November 2009The HoloceneJulie Loisel, Michelle Garneau and Jean-François Hélie

13C values as indicators of palaeohydrological changes in a peat bogδSphagnum   

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(Rice, 2000). As a result, environmental variables such as temper-ature (Ménot and Burns, 2001; Jedrysek and Skrypek, 2005), lightintensity (Farquhar et al., 1989; Ménot and Burns, 2001) andmoisture (Price et al., 1997; Ménot-Combes et al., 2004) influenceSphagnum carbon isotopic composition (δ13C; Eq. (1)). Since itcan be assumed that temperature and light variations are negligi-ble in a peatland at a local scale and at a given time, it has beenhypothesised that Sphagnum δ13C values reflect surface-moistureconditions (Rice and Giles, 1994, 1996; Price et al., 1997; Ménotand Burns, 2001; Lamentowicz et al., 2008; Loisel et al., 2009).Under wet conditions, discrimination against 13C seems to declineas water increases the resistance to carbon dioxide (CO2) uptake,by creating a barrier to CO2 diffusion (Farquhar et al., 1989).Conversely, discrimination against 13C tends to increase underdrier conditions, leading to more negative δ13C values (Price et al.,1997; Rice, 2000). Such results suggest that Sphagnum δ13C val-ues can potentially yield additional information to peat-basedhydroclimatic reconstructions, assuming that δ13C values remainunchanged over time. Previous studies have demonstrated thatpeat isotopic signature is either preserved or only slightly alteredby diagenetic processes (Aucour et al., 1999; Asada et al., 2005;Jedrysek and Skrzypek, 2005).

δ13C (‰) = ((13C/12C)sample / (13C/12C)standard – 1)1000where the standard is V-PDB (Coplen, 1995)

(1)

Several methods have been developed over the last 20 years toapply isotopic tools (13C, 18O, 2H) to peat-based palaeoclimaticstudies. While carbon isotopic measurements of bulk peat has

Introduction

Peat bog stratigraphy constitutes a valuable record of Holoceneenvironmental changes, which has been extensively used over thelast decades (Aaby, 1976; Mauquoy et al., 2002). Palaeohydrologicalreconstructions derived from these records are primarily based onsurface-moisture proxy indicators such as plant macrofossils(Barber et al., 1994), testate amoebae (Charman, 2001; Booth,2002) and humification analysis (Blackford and Chambers, 1993).Although these moisture proxies generally show a fairly goodagreement within organic sequences (Blundell and Barber, 2005;Hugues et al., 2006), limitations have also been highlighted inprevious studies. For example, Caseldine et al. (2000) demon-strated that peat humification data are not well understood andpartly linked to plant macrofossil composition. Moreover,Hugues et al. (2006) showed that some plant macrofossil typesmay only be sensitive to major surface-moisture shifts because oftheir resistance and resilience to some water table fluctuations.Such limitations may partly explain why peat-based records arestill underestimated when compared to other archives such as treerings and lake sediments (Chambers and Charman, 2004).Development of new tools should hence be promoted to verify,refine and complement palaeohydrological reconstructions derivedfrom peatland stratigraphy.Sphagnum mosses constitute ~50% of northern peatlands bio-

mass (Rydin and Jeglum, 2006). Because they lack stomata, thesenon-vascular plants are unable to regulate their carbon uptake

Abstract: Late-Holocene peat stratigraphy of a Canadian boreal peat bog was examined to evaluate the potentialof Sphagnum δ13C values as surface moisture proxy indicators. Isotopic measurements were compared totestate amoebae-inferred water-table depth reconstructions, plant macrofossil assemblages and past decompo-sition levels inferred by colorimetric humification. AMS radiocarbon dates were coupled with 210Pb dates toestablish peat chronology. Although the mechanisms underlying carbon fractionation processes in peatlands arenot well understood, we demonstrate that Sphagnum δ13C values have potential for palaeohydrological recon-structions, since variations presented by the isotopic values can generally be correlated with the other proxyrecords, especially during drier episodes.

Key words: Carbon isotopes, peat bogs, Canada, late-Holocene, testate amoebae, Sphagnum.

The Holocene 20,2 (2010) pp. 285–291

© The Author(s), 2010. Reprints and permissions: http://www.sagepub.co.uk/journalsPermissions.nav

10.1177/0959683609350389

*Author for correspondence (e-mail: jul208@lehigh.edu)

Sphagnum δδ13C values as indicators ofpalaeohydrological changes in a peat bogJulie Loisel,1,2* Michelle Garneau1 and Jean-François Hélie3

( 1Université du Québec à Montréal, GEOTOP Research Center and Departément de Géographie,

Montréal H3C 3PA, Canada; 2Lehigh University, Department of Earth and Environmental Sciences,

Bethlehem PA 18015, USA; 3Université du Québec à Montréal, GEOTOP Research Center and

Département des Sciences de la Terre, Montréal H3C 3PA, Canada)

Received 10 April 2009; revised manuscript accepted 6 August 2009

been reported as problematic because several components (sedges,mosses, fungi, etc.) contribute to the carbon pool (Pancost et al.,2003), cellulose extraction of well-preserved plant macrofossilshave provided more reliable records (White et al., 1994; Figge andWhite, 1995; Hong et al., 2000; Pendall et al., 2001; Hong et al.,2001). However, Ménot-Combes et al. (2004) showed that bulkSphagnum macrofossils and cellulose from the same samplesexhibited similar trends in their δ13C isotopic values, probablybecause cellulose and hemicellulose are abundant in Sphagnumtissues (Rydin and Jeglum, 2006). Species-specific morphologicalfeatures might also influence carbon assimilation and its associ-ated isotopic composition, although these effects seem small whencompared to that of moisture (Rice, 2000; Loisel et al., 2009).Loader et al. (2007) and Moschen et al. (2009) further demon-strated that Sphagnum stems and branches from a single specieshave statistically different isotopic signatures, which ledLamentowicz et al. (2008) to measure Sphagnum δ13C values onstems only for their palaeohydrological reconstruction.

This paper aims at evaluating the potential of Sphagnum δ13Cvalues as palaeohydrological proxies. Carbon isotope analyseshave been performed on Sphagnum stems from a peat section thatspans clear hydrological shifts that have already been recon-structed by changes in testate amoebae-inferred water table depth,plant macrofossil assemblages and peat humification data (Loiseland Garneau, submitted). We hypothesise that Sphagnum isotopicsignatures will indicate similar hydrological changes than theseother proxies.

Study site

Lac Le Caron peatland (52°17’N, 75°50’W, 248 m asl) is locatedin the James Bay lowlands region, Northern Québec, Canada. Dueto its flat topography and cool and relatively humid climate,30–60% of this region is covered by peatlands (Tarnocai et al.,2000). Mean annual temperature is –3.1°C, ranging from –23.2°Cin January to 13.7°C in July. Mean annual precipitation is 684mm, from which 267 mm falls as snow (Environment Canada,2007). The studied region was free of ice by 8400 cal BP (Dykeet al., 2003) and peat started to accumulate as a fen around 7510cal BP (Beta-223743) on the site.

The studied ombrotrophic peatland covers ~2.5 km2 and has awell-developed hummock-hollow patterned surface. Ericaceousshrubs such as Chamaedaphne calyculata, Kalmia angustifolia,Rhododendron (Ledum) groenlandicum and Andromeda glauco-phylla are distributed following the surface-moisture gradient.Eriophorum vaginatum spp. spissum, Trichophorum cespitosum,Carex spp., Scheuchzeria palustris, Rubus chamaemorus andSarracenia purpurea characterise the herbaceous layer. Theground layer is largely dominated by the following Sphagnaceae:Sphagnum fuscum and S. capillifolium on hummocks, S. magel-lanicum, S. rubellum and S. pulchrum in lawns, and S. majus andS. lindbergii in hollows.

Methodology

A peat core was collected in July 2006 from a lawn section with a105 × 8 × 8 cm Box sampler (Jeglum et al., 1992) and waswrapped in a plastic gutter and stored at 4°C until required foranalyses. At the laboratory, stratigraphy was described followingTroëls-Smith (1955).

Every 2 cm throughout the peat core, fossilised Sphagnum wereremoved from 5–10 cm3 peat sub-samples and identified to the

section or the species level. Taxonomy is from Crum andAnderson (1981). At least 50 Sphagnum stems (Loader et al.,2007; Lamentowicz et al., 2008; Moschen et al., 2009) were thenextracted from each sub-sample, washed with de-ionized water,dried at 45°C and ground. δ13C measurements were performed onthose sub-samples (0.035–0.045 mg) with a Micromass IsoprimeTM

IRMS continuous flow coupled with a Carlo Erba NC 1500TM ele-mental analyzer at GEOTOP Research Center (Université duQuébec à Montréal). The resulting carbon isotope ratios werecalibrated against V-PDB and duplicates were processed at 10sample increments. The analytical error was ±0.1‰ and repro-ducibility was better than ±0.07‰.

Each sample also had a corresponding testate amoebae-inferredwater table depth, a plant macrofossil assemblage and a humifica-tion measurement. A detailed presentation of the methods andresults can be found in Loisel and Garneau (submitted), along withprecisions on the age-depth model that was used. Peat sub-samples(2 cm3) for testate amoebae analysis were prepared following thestandard procedures (Hendon and Charman, 1997) and at least 150specimens were counted per sample. A weighted average partialleast square (WA-PLS) transfer function developed by Booth (2008)provided water table depth (WTD) reconstructions with a verticalmean error range of 8.2 cm. The software package C2 was used todevelop this transfer function and a bootstrapping method wasapplied to evaluate the model performance (Booth, 2008 for moredetails). For plant macrofossils, the abundance of Sphagnum remnantsand of unidentifiable organic matter (UOM) were determined aspercentage volumes of each sample (5 cm3) and indicate the state ofpreservation of the peat (Yu et al., 2003). For humification data, drypeat samples (0.2 g) were analysed following Blackford andChambers (1993). Values are presented as a percentage of lighttransmission (T) where low transmission values are related to highlyhumified peat, and high transmission values, to less decomposedpeat (Blackford and Chambers, 1993). Detrended transmittance val-ues (Td) were obtained through a quadratic model. The age-depthmodel was constrained by five radiocarbon and ten lead-210 dates.To facilitate comparison between Sphagnum 13C values and theother proxy signals, a zonation scheme was plotted on the diagramsbased on reconstructed WTD values. Wet (dry) shifts were identi-fied when WTD was below (above) the mean value for the core(–12.6 cm). A covariance analysis was also performed to determinethe relationship between WTD (x) and δ13C (y) values (Eq. (2)).

Cov(x,y) = (Σ(x−x–)(y−y–))/n (2)

Results and discussion

Comparison of isotopic data withreconstructed WTD valuesSignificant changes in the Sphagnum C isotopic signature arefound along the peat sequence (Figure 1). The averaged δ13C valuerecorded is –26.5 ‰ and the distribution ranges from –29.3 to–23.5‰. Results are conforming to the C3 fractionation pathway.As previously mentioned, Sphagnum δ13C values are expected toprovide information on peatlands surface-moisture by recordingvariations of the water resistance to carbon diffusion during plantgrowth (Rice and Giles, 1996; Price et al., 1997; Ménot andBurns, 2001; Lamentowicz et al., 2008; Loisel et al., 2009).Depleted (more negative) δ13C values are expected to be recordedwhen the water resistance is small (i.e. when the WTD is low),while enriched (less negative) δ13C values are expected to berecorded as the water resistance gets greater (i.e. as the WTD getscloser to the peat surface).

286 The Holocene 20,2 (2010)

Eleven hydrological zones are reconstructed, based on testateamoebae-inferred WTD values (Figure 1). Most variations presentedby Sphagnum δ13C and WTD values can be correlated visually(Figures 1 a–b), with drier (wetter) episodes generally charac-terised by more (less) negative isotopic values. However, resultsfound within zones 1, 10 and 11 do not match the WTD recon-struction. Furthermore, no clear statistical relation was foundbetween those two proxies (Figures 1 c–d). This poor relationshipmay be due to: (1) the error range associated with the recon-structed WTD values; (2) the limited use of isotopic data as quan-titative hydrological proxies (qualitative information only); (3) theinfluence of other environmental conditions (moisture level, car-bon source, etc.) on Sphagnum δ13C values; and/or (4) the influ-ence of diagenetic processes on Sphagnum δ13C values.

Environmental controls on the isotopic dataWhen coupled with testate amoebae, plant macrofossils and humifi-cation data provide complementary information on peatland dynam-ics, hence strengthening palaeoecological reconstructions (Blundelland Barber, 2005; Hugues et al., 2006). In the present study, the sen-sitivity level of each of these proxies might help interpreteSphagnum δ13C values. Overall, there is a good agreement betweenplant macrofossils, testate amoebae assemblages and humificationdata (Figure 2). Wetter episodes (Zones 1, 3, 5, 7 and 9) are mostlycharacterised by near-surface WTD values, high Sphagnum peat type

percentages, with S. sect. Acutifolia, S. magellanicum and S. sect.Cuspidata, and positive detrended light transmittance values. Drierepisodes (Zones 2, 4, 6, 8 and 10) are characterised by low WTD val-ues, less Sphagnum remnants and negative humification data. Zone11 corresponds to the acrotelm layer, where a fluctuating WTD influ-ences Sphagnum pulchrum growth.

Hollow mosses, such as Sphagnum sect. Cuspidata and S. pul-chrum, are physiologically more sensitive to water table changesthan hummock and lawn mosses since they are directly sur-rounded by water, possess less well-developed capillary watertransport systems, reside in a more open carpet and have a smallerwater-holding capacity (Titus and Wagner, 1984; Rice, 2000;Gauthier, 2001). Studies also pinpointed that desiccation, which ismore likely to occur in hollows (Rydin, 1985), leads to increaseddiscrimination against the heavier isotope (Williams andFlanagan, 1996; Ménot and Burns, 2001), implying that 13C-depleted values recorded by hollow mosses are influenced by theindirect effect of moisture and temperature on their metabolicactivity. Therefore, hollow species may be more easily influencedby water table changes, and their C isotopic signature is likely tobe wider than for other species. In the present study, hollowspecies are identified within a few wet episodes (Zones 3, 5, 10and 11). Their associated isotopic data range between –27.8 and–24.4‰ (mean = –25.9‰ and SD = 1), with more negative valuesfound within Zone 3 and less negative values, in the acrotelm.

Julie Loisel et al.: Sphagnum δ13C values as a palaeohydrological indicator 287

Figure 1 Sphagnum δ13C values (a) are compared to testate amoebae-inferred WTD reconstruction (b). Zonation is based on WTD values. (c)Sphagnum δ13C values: mean, range and standard deviation (SD) for WTD-inferred wet and dry episodes. (d) Correlation between Sphagnum δ13Cvalues and reconstructed WTD.

Sphagnum species growing in hollows might also take up dis-solved inorganic carbon (DIC) from the surrounding water (Ménotand Burns, 2001). The isotopic signature of this dissolved C canimpact on partially submerged mosses δ13C values. Raghoebarsinget al. (2005) similarly showed that submerged Sphagnum speciesmay also take up CH4 through symbiosis with methanotrophicbacteria. However, it has not been possible to evaluate the role ofthose processes from our data. Using a mass balance equation, thedeuterium isotopic composition (δD) of Sphagnum may poten-tially become a mean for evaluating the amount of CH4 taken upby those plants. Both mechanisms presented above can complicatethe interpretation of the isotopic signature recorded by Sphagnumspecies growing in hollows.

The global atmospheric δ13C value, which has been decreasingfrom –6.5 to –8.0‰ since the Industrial Revolution (Francey et al.,1999), might also have influenced the isotopic signature of plantsfound within the acrotelm (zone 11; Farquhar et al., 1989). In thepresent study, the top two samples (–26.2 and –26.4‰) exhibitedmore negative values than that of other acrotelm samples (mean =–24.9‰). The effect of the atmospheric δ13C signature onSphagnum might be a response to this process, but that cannot beassessed from our dataset as a lowering of the water table was alsoreconstructed for those levels.

Early-stage and long-term diageneticcontrols on the isotopic dataSome studies (Melillo et al., 1989; Pancost et al., 2002; Asada et al.,2005) suggested that early-stage diagenetic processes might alterSphagnum δ13C values by preferentially removing more labileorganic compounds such as carbohydrates (e.g. sugars, cellulose),which are mostly 13C-enriched (Wilson and Grinsted, 1977), whileother studies did not find such a trend (Wedin et al., 1995).Although a preferential loss of 13C-enriched compounds duringearly-stage peat decay may explain the values for the uppermosttwo samples in the present study, the lower WTD and the post-1850 atmospheric carbon isotopic composition also probably con-tributed to the recorded Sphagnum isotopic signatures, making itdifficult to evaluate the role of early-stage decay alone.

Similarly, long-term anaerobic decomposition can potentiallymodify Sphagnum δ13C values through time. In the present study,however, zones that present indications of intensified decay (i.e.high unidentifiable organic matter percentages) also exhibit 13C-depleted values, implying that a preferential loss of 13C-enrichedcompounds through intensive aerobic decomposition is not likelyto have happened along the peat sequence. However, fossilisedSphagnum stems seem to become slightly depleted in 13C withdepth (Figure 3), which may be due to the preferential preserva-tion of refractory C that is 13C-depleted (Melillo et al., 1989;Wedin et al., 1995). Lipids, which are found in great amounts inSphagnum tissues, might be responsible for such a trend, sincethey are poorer in the heavier isotope than other organic com-pounds (Rundel et al., 1979; Hillaire-Marcel, 1986; Farquhar et al.,1989; Ménot and Burns, 2001). Another factor to consider isSphagnum species-specific resistance to decay, with hollow (hum-mock) species being generally more (less) easily decomposed(Johnson and Damman, 1991). In the case where hollow (wet)species are enriched in the heavier isotope (as suggested in ourhypothesis), but are prone to preferentially lose 13C-enriched com-pounds through rapid decay, their signature might overlap withthat of hummock (drier) species. In the present study, isotopic val-ues obtained for the “wet” zones get progressively more negativeas a function of time (Figure 3). To a lesser extent, the same trendis observed for the “dry” horizons. These results suggest thatanoxic diagenesis might modify Sphagnum δ13C values with time,but measurements of recalcitrant compounds (e.g. waxes) and/or

biomarkers would be necessary to validate this interpretation(Pancost et al., 2002; Nichols et al., 2006).

Palaeohydrologic reconstructionIn NE North America, the ‘Medieval Warm Period’ (MWP) andthe ‘Little Ice Age’ (LIA) have been identified in many records (Viauet al., 2002; Hugues et al., 2006). In Northern Québec however,these events have rarely been reconstructed, due to the low reso-lution and the paucity of available data. In fact, the present studyis the first one to detail late-Holocene peatland dynamics in themid-boreal region. Filion (1984) reconstructed a warm and humidperiod (960–700 cal BP) based on sand dune stabilisation that sheattributed to the MWP, while low lake levels (Payette and Filion,1993) were reconstructed during the LIA and associated with drywinters. Both studies were conducted in the Hudson Bay low-lands. Those periods were potentially identified from our multi-proxy reconstruction from ca. 1200–850 cal BP (Zone 7) and fromca. 400 cal BP to the present (Zones 10 and 11), respectively.Assuming that the MWP was warm and humid and the LIA wascool and dry in this region, contrasting δ13C values should havebeen recorded in Sphagnum tissues during those two periods.Results presented in Figure 2 do not support this hypothesis, as theisotopic data from Zones 7 and 10 completely overlap and are bothrelated to wet conditions. The presence of wet Sphagnum taxa(S. pulchrum and S. rubellum-capillifolium) within Zones 10 and11 (LIA), however, is in agreement with the isotopic data. It ispossible that the WTD reconstruction was imperfect within thiszone and/or that other environmental controls affected Sphagnumδ13C values. In contrast with the reconstructions from the HudsonBay region, Hugues et al. (2006) reconstructed warmer and drierconditions during the MWP, and cooler and wetter conditions dur-ing the LIA from a blanket bog located in Newfoundland. Theirresults imply that regional effects complicate the direct compari-son of both the Newfoundland and the Hudson Bay reconstruc-tions with the ones from our study region.

Conclusion

Measurements of Sphagnum δ13C values showed a variation of5.8‰ along a 1-m-long peat core, with a mean carbon isotopevalue of –26.5‰. Surface-moisture conditions seem to dominateSphagnum mosses fractionation processes, since comparison oftheir isotopic signature were closely related with other palaeo-hydrological proxies. However, Sphagnum species-specificphysiognomy, the isotopic signature of each carbon source usedfor photosynthesis and the position along the hummock-hollowgradient all constitute environmental controls affecting Sphagnumδ13C values. In addition, although aerobic decay did not appearto alter Sphagnum mosses isotopic signature as the peat decom-posed, anoxic decay seems to have a slight 13C depleting effect.Overall, Sphagnum δ13C values show potential for palaeohy -drological reconstructions, but further analyses are needed tobetter understand underlying fractionation processes withinmoss tissues.

Acknowledgments

Financial contributions from Hydro-Québec Production (Dr A.Tremblay), NSERC, FQRNT and NSTP-Canada are acknowl-edged. Special thanks to L. Pelletier, H. Asnong, É. Rosa and A.A.Ali for helpful information and discussions. The authors alsoacknowledge the anonymous reviewers for their critical commentson an earlier version of the manuscript.

288 The Holocene 20,2 (2010)

Julie Loisel et al.: Sphagnum δ13C values as a palaeohydrological indicator 289

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290 The Holocene 20,2 (2010)

Figure 3 Sphagnum δ13C values (a) are detrended through a regression model (b). Zonation is based on reconstructed WTD values (see Figure1).

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