Palaeogeography, Palaeoclimatology, Palaeoecology 298 (2010) 235–246
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Palaeogeography, Palaeoclimatology, Palaeoecology
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o
Long-term climate variability in continental subarctic Canada: A 6200-year record
derived from stable isotopes in peat
Päivi Kaislahti Tillman a,⁎, Steffen Holzkämper a,b,1, Peter Kuhry a,2, A. Britta K. Sannel a,3,
Neil J. Loader c,4, Iain Robertson c,5
a
b
c
Department of Physical Geography and Quaternary Geology, Stockholm University, 10691 Stockholm, Sweden
Department of Geography, Johannes Gutenberg University Mainz, Becherweg 21, 55099 Mainz, Germany
Department of Geography, Swansea University, Singleton Park, Swansea SA2 8PP, UK
a r t i c l e
i n f o
Article history:
Received 10 May 2010
Received in revised form 9 September 2010
Accepted 29 September 2010
Available online 7 October 2010
Keywords:
Stable isotopes
Subarctic
Sphagnum
Climate reconstruction
a b s t r a c t
The rapid warming of arctic regions during recent decades has been recorded by instrumental monitoring, but
the natural climate variability in the past is still sparsely reconstructed across many areas. We have
reconstructed past climate changes in subarctic west-central Canada. Stable carbon and oxygen isotope ratios
(δ13C, δ18O) were derived from a single Sphagnum fuscum plant component; α-cellulose isolated from stems.
Periods of warmer and cooler conditions identified in this region, described in terms of a “Mediaeval Climatic
Anomaly” and “Little Ice Age” were registered in the temperature reconstruction based on the δ13C record.
Some conclusions could be drawn about wet/dry shifts during the same time interval from the δ18O record,
humification indices and the macrofossil analysis. The results were compared with other proxy data from the
vicinity of the study area. The amplitude of the temperature change was similar to that in chironomid based
reconstructions, showing c. 6.5 ± 2.3 °C variability in July temperatures during the past 6.2 ka.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Holocene palaeoclimate reconstructions of the northern hemisphere and the arctic regions have been developed based upon a range
of proxies including tree-rings, chironomids, diatoms and pollen
(Overpeck et al., 1997; Moberg et al., 2005; Viau et al., 2006;
Barnekow et al., 2007). Many studies were conducted in coastal areas
influenced by maritime air masses but studies from remote areas with
continental climate are rare. Differences in air mass flow from
different sources can impact temporal and spatial climate patterns
(Latif and Barnett, 1994; Edwards et al., 1996; Trouet et al., 2009) and
thus bias large scale reconstructions if data distribution is unevenly
spaced. Climate responses in migrating permafrost zones and
terrestrial ecoregions at high latitudes can act as feedback sources
to climate change by enhancing greenhouse gas and water vapour
release and by seasonal changes of land surface albedo (Christensen et
al., 2004; Chapin et al., 2005; Johansson et al., 2006; MacDonald et al.,
⁎ Corresponding author. Tel.: +46 8 164961; fax: +46 8 164818.
E-mail addresses: [email protected] (P. Kaislahti Tillman),
[email protected] (S. Holzkämper), [email protected] (P. Kuhry),
[email protected] (A.B.K. Sannel), [email protected] (N.J. Loader),
[email protected] (I. Robertson).
1
Tel.: +49 6131 39 26732; fax: +49 6131 39 24735.
2
Tel.: +46 8 164806; fax: +46 8 164818.
3
Tel.: +46 8 164795; fax: +46 8 164818.
4
Tel.: +44 1792 295546; fax: +44 1792 295955.
5
Tel.: +44 1792 295184; fax: +44 1792 295955.
0031-0182/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.palaeo.2010.09.029
2006; McGuire et al., 2006; Schuur et al., 2009). In Canada, the recent
warming has been most pronounced in the north-western parts
(Prowse et al., 2009), but it is expected that the effects will be felt
most extremely in the currently widespread discontinuous and
sporadic permafrost zones, where peatlands cover vast areas in the
boreal and subarctic ecoregions of the continental interior (Kettles
and Tarnocai, 1999; Natural Resources Canada, 2007). An increase in
mean annual temperatures by only 1–2 °C in these regions can lead to
permafrost degradation in peatlands located in discontinuous
permafrost areas with mean annual air temperatures just below
zero, impacting factors such as vegetation, snow cover and local
hydrology (Vitt et al., 1994; Jorgenson et al., 2001). Also transient
warming can significantly impact permafrost environments and
active layer depths (Smith et al., 2001).
Each of the different environmental indicators (proxies) used to
explore climatic changes has particular strengths and weaknesses in
its ability to record changes over different spatio-temporal scales.
Therefore, there is a need to develop multi-proxy studies and new
methods capable of addressing existing research gaps. Several studies
have corroborated the potential of stable isotopes archived in peat
mosses as proxies for climate change (Ménot-Combes et al., 2002;
Sharma et al., 2004; Loader et al., 2007; Lamentowicz et al., 2008; Daley
et al., 2009, 2010; Loisel et al., 2009; Moschen et al., 2009; Kaislahti
Tillman et al., 2010). Plant macrofossil analysis can be used to detect
variations in mire surface wetness as a proxy for changing environmental conditions (Kuhry et al., 1993; Barber et al., 2003; Oksanen,
2006; Väliranta et al., 2007) and is an important tool to identify and
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P. Kaislahti Tillman et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 298 (2010) 235–246
select species for stable isotope studies that otherwise could be
influenced by the varying botanical composition of peat. Climatic
changes have also been reconstructed based on peat humification as
indicator for dry/wet mire surface conditions back in time (Chambers
et al., 1997; Chambers and Blackford, 2001; Roos-Barraclough et al.,
2004; Borgmark, 2005; Payne and Blackford, 2008).
In this study, we reconstruct July temperatures in subarctic westcentral Canada over the last ~6 ka. The study is based on new stable
carbon isotope data from Sphagnum fuscum moss and an improved
age model, in addition to the data presented in a methodological study
by Kaislahti Tillman et al., 2010, where stable isotope and peat
humification palaeodata relationship as well as correlation to modern
(~50 yr) meteorological observations were investigated. Here we also
present new plant macrofossil analyses from two peat sequences that
are used together with normalized and detrended humification
indices and stable oxygen isotope time series to reconstruct past
moisture conditions. Furthermore, we compare our results from
north-eastern Saskatchewan (~ 60°N; 102–104°W) with results from
previous studies in the region, mainly between c. 58–65°N and 94–
110°W in west-central Canada covering the past ~ 6 ka. The
comparison also includes a study from east-central Canada from the
discontinuous permafrost zone with similar climate conditions as our
study region, and reconstructions in continental/hemispheric scale. As
far as we know, no other comparable studies from Sphagnum have
been made in the region. Therefore, other proxies derived from peat
archives, lake sediments and tree-rings are considered. The main
questions addressed in this study are: How did summer temperatures
vary in west-central Canada during the past 6 ka? What conclusions
can be drawn about moisture conditions in this region? Do peat moss
stable isotope records show similar climate changes as other proxy
records?
2. Background
Selwyn Lake and Misaw Lake (Fig. 1) are located in the
discontinuous permafrost zone (DPZ, permafrost covers 50–90% of
the area) of subarctic Canada, where the tree line coincides
approximately with the southern limit of the continuous permafrost
zone (CPZ, permafrost N 90% of the area) and reflects the mean
position of the arctic front in July (Gajewski and Atkinson, 2003).
Peatland formation in this region was generally caused by paludification of upland forests (Zoltai, 1995). Peat plateaux and palsas are
underlain by permafrost in the DPZ, unlike water bodies and many
fens that are free of permafrost.
The peat profiles used in our study, S52 (59°52'N, 104°12'W;
395 m elevation) and S53 (59°55'N, 104°13'W; 394 m elevation),
were collected in 1993 southeast of Selwyn Lake. Profile S52 was cut
from the edge of a forested peat plateau calving into Porcupine Bay
and profile S53 from an unfrozen part of a peat plateau. A hummock
ML3 (59°52'N, 102°34'W; 390 m elevation) representing modern peat
accumulation was sampled in July 2005 at Misaw Lake, c. 90 km east
of Selwyn Lake. The observed vegetation in the peatland (ombrotrophic peat plateau/palsa–minerotrophic fen/collapse scar–complex)
at Selwyn Lake consisted of trees (Picea mariana, and Larix laricina),
shrubs (Chamaedaphne calyculata, Ledum spp, Rubus chamaemorus,
Oxycoccus palustris, Vaccinium uliginosum, and V. vitis-idaea), unidentified lichen species, mosses (Sphagnum fuscum, S. rubellum, S. cf.
magellanicum, and Drepanocladus sp.) and sedges (Carex spp,
Eriophorum sp., and Equisetum sp.). Around the Sphagnum fuscum
hummock at Misaw Lake, vegetation consisted mainly of Picea
mariana, Betula glandulosa, Ledum groenlandicum, and Vaccinium
vitis-idaea. The oldest bottom peat dates back to c. 6600 cal BP at
Selwyn Lake. Permafrost aggradation occurred more than 1 ka later
Fig. 1. Study sites S52 and S53 are located c. 3 km from each other, close to the south-eastern shore of Selwyn Lake in NE Saskatchewan, Canada. Site ML3 is located at Misaw Lake, c.
90 km east of S52. Present permafrost zones: SPZ = sporadic, DPZ = discontinuous and CPZ = continuous permafrost zone (after Zoltai, 1995). Reference studies in the region:
1 Nichols (1967), 2 Kay (1979), 3 Tardif et al. (2008), 4 MacDonald et al. (2009), 5 Moser and MacDonald (1990), 6 Pienitz et al. (1999), 7 D'Arrigo and Jacoby (1993), and 8 Fallu et al.
(2005, in the small scale map), marked by numbers and proxy symbols in the map. The most frequent wind directions in July at stations with continuous instrumental records imply
a more southerly origin of air masses south of the boundary between DPZ and CPZ. The boundary approximately follows the actual tree line reflecting the mean summer position of
the arctic front (Gajewski and Atkinson, 2003).
P. Kaislahti Tillman et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 298 (2010) 235–246
when isolating Sphagnum fuscum peat had become thick enough
(Sannel and Kuhry, 2008).
The present subarctic region in Canada is influenced by cold and
dry arctic air masses most of the seasons. Warm air and (some)
moisture of Pacific origin is transported across the Cordillera by
westerly winds in the summer-time, when the Pacific-North
American pattern characterized by meridional flow is not dominant
(Edwards et al., 1996; Birks and Edwards, 2009). Continuous climate
records were not available from the closest meteorological station at
Stony Rapids (Environment Canada, 2010), so instrumental records
from Ennadai Lake, Cree Lake, Fort Smith, Churchill Airport and Baker
Lake were also used to estimate variations of temperature, precipitation and the most frequent wind direction in the region (Fig. 1). In
Stony Rapids, mean January and July temperatures were -23.7 °C and
+16.0 °C, respectively, mean annual air temperature (MAAT) was
-3.3 °C and mean annual precipitation (MAP) was 430 mm in the
period 1987–2002. In the whole region, MAAT varied between -2.3 °C
(Cree Lake) and -11.8 °C (Baker Lake) and MAP between 270 mm
(Baker Lake) and 446 mm (Cree Lake) in 1971–2000. During 1970–78
when observations were made in all six meteorological stations,
average July temperatures were 15.6 °C in Stony Rapids, 15.3 °C in
Cree Lake, 15.8 °C in Fort Smith, 13.2 °C in Ennadai Lake, 11.5 °C in
Churchill, and 11.3 °C in Baker Lake. Mean temperature anomalies
calculated from a combination of meteorological stations correlated
well with observations from Stony Rapids, whereas precipitation
amounts had a larger deviation. As no observations were reported
from Stony Rapids between 1979 and July 1986, the time period
1987–2004 was chosen to calibrate the relationship between
instrumental climate records and stable isotope ratios (δ13C, δ18O).
When different time intervals between May and September were
used to investigate the relationship between climate and stable
isotope ratios of Sphagnum fuscum plant components, 60% (p b 0.01) of
the variation in δ13C values of α-cellulose in stems were explained by
the variability in July temperatures. No correlation was found
between δ 13 C values and precipitation amounts (R 2 = 0.08;
p = 0.43). δ18O values of α-cellulose from stems had weak positive
correlations to both July temperatures (R2 = 0.38; p = 0.06) and
precipitation amounts (R2 = 0.23; p = 0.16) (Kaislahti Tillman et al.,
2010).
3. Methods
237
3.2. Plant macrofossil analysis
The hummock ML3 was analyzed at 1 cm intervals from the upper
25 cm, only three samples were analyzed below that level. The
uppermost 60 cm of the profile S53 were analyzed, corresponding to a
time period from recent to c. 1200 cal yr BP. Peat samples from ML3
and S53 were washed in NaOH (5%) and sieved with a 125 μm mesh.
Plant macrofossils were identified under a stereo binocular (X25-40
magnification) with reference to literature (Hallingbäck and Holmåsen, 1982; Nilsson, 1986; Mossberg and Stenberg, 2008) and
presented as volume percentages of the total sample. If the occurrence
was less than 1% of the total sum, they were classified as locally “rare”
and identified accordingly. Sphagnum species were identified under
the light microscope mainly by stem leaf morphology (stained with
methyl violet) with X400 magnification (Lange, 1982; Andersson et
al., 1995), and compiled in diagrams as a percentage of species
abundance among other plant remains, based on stem leaf counts
(Johnson et al., 1990; Grimm, 2004–2008). The method description
and results of macrofossil analysis for the profile S52 (SL1) were
published by Sannel and Kuhry (2008).
3.3. Colorimetric analysis
Bulk peat samples (1–2 cm3) were homogenised after oven drying at
~50 °C or freeze-drying. The method described by Blackford and
Chambers (1993) was modified by reducing the sample size to 50 mg,
and by centrifuging samples (10 min, 4000 rpm) after boiling in NaOH
(8%) following the procedure described by Borgmark (2005). Samples
were analyzed with a UNICAM spectrophotometer at 540 nm wavelength, 4 ± 0.5 h after initial mixing. Samples were recorded both as
absorbance in logarithmic scale (high values indicate a high degree of
humification) and as percentage transmission T (%) (low values indicate
a high degree of humification). Absorbance values were normalized
using the equation Ast = (A - μ)/σ, where Ast denotes a standardized
absorbance value, A the original absorbance value, μ the mean value and
σ the standard deviation of the dataset. The detrending was made by
reducing Ast values with a difference to values A*st calculated by a linear
regression between normalized values and the corresponding depths in
the profile. This was made in order to compensate for possible anaerobic
decay in catotelm and to create varying surface moisture conditions at
peat formation and subsequent acrotelm humification (Gunnarson et
al., 2003; Roos-Barraclough et al., 2004).
3.1. Chronology
3.4. Stable isotope analysis
In this study, the chronology of the Selwyn Lake peat profile S53
was improved by an additional AMS radiocarbon date based on
Sphagnum remains, 137Cs analysis of bulk samples and a subsequent
modification of the age model of the top 0–60 cm, compared to the
chronologies previously described by Kaislahti Tillman et al. (2010, in
the Appendix). The modification consisted of extrapolation of
accumulation rates in Sphagnum peat to lower and upper boundaries
of a rootlet layer and a charcoal layer. Also chronological controls
were conducted in order to correlate peat sequences of overlapping
ages. After removal of organic material by ignition at 550 °C,
cryptotephra was searched from residuals of the upper part of the
profile S52 (overlapping S53) by polarizing microscope, but were
absent. From the upper part of the profile S53 (overlapping the Misaw
Lake hummock ML3), 137Cs analysis was made from bulk samples at
the Department of Applied Environmental Science, Stockholm
University. All radiocarbon dates were calibrated using the program
OxCal 4.1.6 (Bronk Ramsey, 2010) and IntCal09 calibration curve
(Reimer et al., 2009) and expressed in calendar years before present
(cal yr BP) ± 2σ. Ages in older reference records, which were reported
in non-calibrated years BP (Nichols, 1967; Kay, 1979), were calibrated
in the same way and rounded off to the nearest multiple of ten as
described by Stuiver and Polach (1977).
In order to increase the temporal resolution at a more even
spacing, additional stable carbon isotope values were incorporated
into the original dataset described by Kaislahti Tillman et al. (2010).
The isolation of α-cellulose followed the original chemical method
(Green, 1963) modified to suit small (moss) samples (Loader et al.,
1997; Rinne et al., 2005; Daley, 2007) and homogenized before
freeze-drying (Loader et al., 2008). Isotope analyses (0.30–0.35 mg of
the Sphagnum α-cellulose, standard Sigma cellulose, C4- and C3sugars and IAEA-standard cellulose) were performed by online
combustion (carbon isotopes) and pyrolysis (oxygen isotopes) in a
PDZ Europa ANCA GSL elemental analyser interfaced with a 20/20
isotope ratio mass spectrometer at Swansea University, UK. Stable
carbon isotope ratios (δ13C) are presented as per mille (‰) deviations
from the VPDB standard and stable oxygen isotope ratios (δ18O) as ‰
deviations from the VSMOW standard. The analytical precision of 13C
for the Sigma cellulose standard was better than 0.1 ‰ (n = 19) and
for the 18O IAEA standard better than 0.4‰ (n = 8). Since carbon
released from anthropogenic sources is depleted in 13C, correction
factors adapted from McCarroll and Loader (2004) were added to δ13C
values after AD 1850. Linear relationships between stable isotope
ratios and climate parameters were evaluated by analysis of variance
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P. Kaislahti Tillman et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 298 (2010) 235–246
(ANOVA), t-test for a slope; the relationship was regarded significant
if the p-value was b0.05.
4. Results
4.1. Chronology
For the temperature reconstruction of the past c. 6.2 ka, only the
upper part of the Selwyn Lake profile S53 (0–58 cm) was used to cover
the hiatus in the upper part of the profile S52 (0–13 cm), enabling high
time resolution with some overlap (Table 1). The age model of the
profile S53 was modified because of the presence of a rootlet layer at 28–
33 cm depth (c. 360–110 cal yr BP) and a charcoal layer at 43–44 cm
depth (c. 770–450 cal yr BP) (Fig. 2a). 137Cs measurements showed high
values in the uppermost 5 cm of this profile collected in 1993 (Fig. 2b),
which was interpreted as possibly fallout from the Chernobyl accident in
1986 and a subsequent uptake by fresh peat, as described by Rosén et al.
(2009). A second peak at c. 13–18 cm was interpreted as 1963 bombtest fallout that had accumulated at 13–14 cm and migrated vertically
into underlying peat. The age–depth model of ML3 was compiled with
1σ error bars (Fig. 2c). The uncertainty increased in samples
accumulated before 1970, and the accumulation rate around 1960
was surprisingly high. Because a higher accumulation rate seems to be
unlikely in peat with a higher decay rate (increase in colorimetric
absorbance values at 27–33 cm depth, Fig. 2d), a plausible explanation is
that the high fallout at the early 1960's has been migrating downward,
as in the profile S53. Sample ages before 1120 cal yr BP were calculated
from the age–depth model of the profile S52, based on the linear
interpolation model in Kaislahti Tillman et al. (2010).
4.2. Stratigraphy and macrofossil analysis
A macrofossil study and subsequent selection of branches or
stems from one moss species is important for stable isotope time
series, as isotope values vary in different species and in different
plant components (Loader et al., 2007). The identification of
Sphagnum species by stem leaf morphology and size was not
always straightforward. In case of uncertainty or when stem leaves
had been separated from stems (as they often are in highly
decomposed peat), species were identified only at section level.
S. fuscum is a species relatively resistant to decay (Johnson et al.,
1990). It belongs to the Sphagnum section Acutifolia (Laine et al.,
2009), and was possibly classified both on the species level and on the
section level in a same sample because of identification problems
(Fig. 3a–b).
The youngest 25 cm of the hummock ML3 were dominated by
Sphagnum fuscum. Low abundances of Dicranum sp. in the upper
part and Polytrichum sp., dark roots, needles, bark, wood and fungal
sclerotia/mycorrhizae cf. Cenococcum were found below 10 cm
depth. Dicranum, Polytrichum and Cenococcum indicate dry permafrost conditions during the hummock growth (Oksanen, 2006). In
the profile S53, S. fuscum was the dominant moss species
throughout the peat sequence. Identification of Sphagnum species
was uncertain in samples at 37–38 cm and 57–58 cm and therefore
partly reported in the section level. Colorimetric values indicated
high peat decay and dry conditions in the rootlet layer (28–33 cm),
where no S. fuscum plants could be identified and picked for
isotope analyses. The humification rate was high also in peat
preserved below the rootlet layer, possibly because of secondary
decomposition, i.e. deep decay during a dry period (Borgmark and
Schoning, 2006; Sannel and Kuhry, 2009). In the topmost sample, a
leafy liverwort Mylia anomala grew among S. fuscum indicating
rather fresh surface conditions.
The stratigraphy and macrofossil analysis of the profile S52 (SL1)
consisting of repeated layers of Sphagnum peat, rootlet peat, and
charcoal layers was slightly modified by adding a 3 cm thick lichen
layer at the surface to all depths in SL1 given by Sannel and Kuhry
(2008). The topmost lichen layer indicates dry surface conditions.
Table 1
137
Cs and AMS-radiocarbon dates used in the age–depth models. Unpublished data is marked with Bold. *Conventional radiocarbon dates, pMC denotes percent Modern Carbon.
Sample depth
(cm)
Lab. number
Age
(14C yr BP)
Profile S53
0
13–14
25–26
35–36
41–42
47–48
57–58
67–68
83–85
129–130
137-Cs
Poz-24369
Poz-32067
Hel-3851*
Poz-26425
Poz-26426
Poz-24370
AECV1967c*
AECV1906c*
70 ± 30
320 ± 30
460 ± 90
960 ± 30
1200 ± 30
1740 ± 30
2490 ± 70
5320 ± 90
Profile S52
0
4–5
13–14
30–31
44–45
48–49
61–62
69–70
86–87
94–95
117–118
127–129
158–159
184.5–185.5
198–199
Poz-19656
Poz-18597
Poz-26424
Poz-16805
Poz-19675
Poz-19658
Poz-19659
Poz-16806
Poz-19660
Poz-19661
AECV1966c*
Poz-19662
Poz-20142
AECV1905c*
115 ± 0.36pMC
1120 ± 30
1335 ± 30
1614 ± 36
1750 ± 30
1770 ± 30
2160 ± 30
2605 ± 32
2915 ± 30
3250 ± 35
3690 ± 90
3890 ± 35
5250 ± 40
5780 ± 90
a
b
c
Agea (cal yr BP) Best fit ±2σ
-43 (AD 1993)
-13 (AD 1963)
83 + 177/-57
387 ± 82
438 + 208/-129
863 + 67/–68
1120 + 119/-107
1639 ± 78
2550 + 186/-187
6105 ± 178
Acc. rateb
(cm/yr)
1 cm agec
(yr)
0.45
0.13
0.03
0.12
0.01
0.04
0.02
0.02
0.01
2.2
8.0
30.4
8.5
70.8
25.7
(51.9)
(55.2)
(78.1)
0.01
0.07
0.06
0.03
0.21
0.02
0.03
0.03
0.06
0.02
0.11
0.02
0.02
(79.0)
14.4
15.8
39.8
4.7
59.1
33.1
39.6
18.1
55.5
8.7
64.0
42.4
-43 (AD 1993)
1023 + 144/-67
1268 + 37/-88
1489 + 109/-81
1648 + 86/-87
1709 ± 104
2182 ± 126
2745 + 34/-125
3062 + 139/-101
3478 ± 84
4061 + 317/-331
4327 + 93/-143
6023 + 156/-101
6595 + 198/-241
Radiocarbon dates are calibrated to calendar years before present (cal yr BP) by OxCal 4.1.6 (Bronk Ramsey, 2010) and IntCal09 (Reimer et al., 2009).
Accumulation rates are calculated from intervals between the overlying and the actual depth.
The age of 1 cm sampling interval is given in years based on accumulation rates. No samples were analyzed for stable isotopes from intervals in ( ).
P. Kaislahti Tillman et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 298 (2010) 235–246
239
Fig. 2. a–d From the left: a) age–depth model of the Selwyn Lake profile S53in cal yr BP, where a black square shows the 137Cs date (13–14 cm depth) and open squares without error
bars show interpolated ages between radiocarbon dates/extrapolated ages to a rootlet peat layer (28–33 cm) and a charcoal layer (43–44 cm), b) 137Cs concentrations in S53 (Bq/g),
c) age–depth model of the Misaw Lake hummock ML3 in 210Pb years AD ± 1σ, and d) normalized (grey) and detrended (black) colorimetric absorbance values from ML3, positive
values indicating increased humification.
4.3. Stable isotopes and climate reconstruction
Because the isotope data in this study was used to reconstruct past
temperatures, an inverse approach to regression (joint variability)
was adopted (cf. Kaislahti Tillman et al., 2010). Additional stable
carbon isotope analyses were performed from the profile S52
(Table 2) in order to improve the uneven temporal resolution caused
by varying peat accumulation in the oldest part of the record. A linear
model equation derived from δ13C values of the uppermost 10 cm of
the Misaw Lake hummock ML3 and July temperatures in Stony Rapids
1987–2004 (Table 2) gives y = 1.86x + 65.1 (R2 = 0.60, p b 0.01). The
estimates made by a regression model that are closer to the mean δ13C
value -26.5‰ have a more narrow confidence interval (±0.68 °C)
than the standard error of the estimate (±0.93 °C at 95% level), while
more extreme temperatures have a wider interval (±1.3 °C for min
value -27.4‰ and max value -25.6‰). Available instrumental data
from Stony Rapids 1960–1978 was not used for the calibration,
because of increasing uncertainty with 210Pb ages (Fig. 2c) in the older
part of ML3. A short overlap of observed temperatures and high
resolution isotope data hampered a proper validation. However, a
calculation based on the linear regression model and an average of
four δ13C values (-26.75‰) from ML3 during the period 1970–78
results in 15.3 °C that is within the standard error of the estimate
compared to average temperature 15.6 °C in Stony Rapids the same
period. Also two modern values reconstructed from the profile S53
match with the instrumental records (Fig. 4). Past July temperatures
calculated by the equation are shown with expected estimate
intervals on the 95% confidence level in Figs. 5 and 6. The sample
ages of the entire time-series were determined by linear interpolation
from age–depth models. The largest confidence interval was ±2.3 °C
calculated from the most extreme δ13C value (-28.3‰) outside the
regression model. As our study sites are located between Stony Rapids
and Ennadai Lake, July temperatures at our study sites should be
approximately 1.2 °C colder than the reconstructed temperatures
using Stony Rapids data.
Our July temperature reconstruction shows increasing, but variable,
temperatures from 6200 ± 180 cal yr BP to 3100 ± 120 cal yr BP (Fig. 5)
with up to 3.0 ± 2.2° higher than modern temperatures at 4300±
120 cal yr BP. As a result of the cooling trend after 3100 ± 120 cal yr BP,
July temperatures fall below the reference value at c. 2500–1300 cal yrs
BP, except a peak at 1700 ± 100 cal yr BP based on a single value
representing only 5 yr (Table 1). A period between 1270 ± 60 and 860 ±
70 cal yr BP was slightly warmer than our reference value interrupted
by a 2.2 °C drop in the middle of the period. After a slight cooling c. 900–
450 cal yr BP, our reconstruction shows a rapid peak comparable to
modern temperatures at 420 ± 150 cal yr BP and then a 4.1 °C drop at
390 ± 80 cal yr BP. No Sphagnum fuscum was identified in the rootlet
layer c. 390–110 cal yr BP, so we lack carbon isotope data from that
period. Around 100 cal yr BP the recovery to warmer temperatures was
shortly interrupted, but the 20th century shows a fluctuating temperature increase (Fig. 6).
Past precipitation amounts were not reconstructed because of
poor correlation. Instead, the local moisture conditions were interpreted from combined macrofossil analysis and the records of
detrended absorbance values and stable oxygen isotope ratios
(δ18O), which are illustrated in Figs. 5 and 6. High absorbance values
show increased humification during dry conditions (if there is no
change in the botanical composition), while high δ18O values may
reflect dry conditions because of high evaporation or higher
temperatures locally or along the path of moisture transport.
Detrending of humification indices had almost no effect on normalized values in profile S53 and only a slight effect in profile S52,
probably because decay in the catotelm was slowed down by
permafrost (Vardy et al., 2000; Sannel and Kuhry, 2009).
Our study shows that climate was relatively dry in the period c.
6000–4400 cal yr BP. A short wet event at 4300 ± 120 cal yr BP and a
longer period of increased precipitation between c. 3500 cal yr BP and
2500 cal yr BP were indicated by low humification indices and low
δ18O values despite no change in the botanical composition according
to the macrofossil analysis. Around 2060 ± 120 cal yr BP, when stable
oxygen isotope values may reflect mainly low evaporation rates due
to cooler temperatures, both the humification index and the botanical
composition indicate locally dry conditions. Short-term fluctuations
to more southerly origin of air masses bringing higher precipitation in
the summer time could explain low humification rates occurring
simultaneously with reconstructed high temperatures at 1700 ± 100
and 1270 ± 60 cal yr BP. Because of the poor overlap in humification
time series from peat profiles S53 and S52, moisture conditions
438
)
1962
25
1961
30
1959
35
1950
40
20
Sphagnum peat
20
40
40
20
Sphagnum peat Rootlet peat
60
40
80
Selwyn Lake S 53
60
80 100
Charcoal layer
20
20
40
s
20
ot
1966
ro
15
k
1977
ar
Misaw Lake ML3
Be
tu
la
Bu lea
f
d
sc
La ale
rix
n
Pi eed
ce
le
a
n
Ba ee
d
rk
/w le
o
C
yp od
er
a
Em ca
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pe
t
Le rum
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m
sp rum
U
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Va ium
cc
m
yr
in
Va ium tillu
cc
ox s
in
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iu
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oc
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en cu
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et
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us
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ae
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21
0Pb
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D
ep rs (
A
th
( c D)
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)
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t
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yp ee
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nk sp
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cc n
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C ini ul ca
en um ig rp
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cu -i m
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S.
5
ra
ph
ic
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M nu
yl m
i
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1995
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2005
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BP
th
ep
yr
b)
D
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C
240
P. Kaislahti Tillman et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 298 (2010) 235–246
45
100
20
Sphagnum-Polytrichum peat
0
10
20
83
30
387
40
863
50
1120
60
60
20
P. Kaislahti Tillman et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 298 (2010) 235–246
Table 2
Additional stable carbon isotope ratios (δ13C) in the Selwyn Lake profile S52 and data
from Misaw Lake hummock and Stony Rapids meteorological station used in the linear
regression model.
S52 depth
(cm)
Age
(cal yr BP)a
δ13C
(‰)
July temp.b
77–8
123–124
162–163
171–172
180–181
184.5–185.5
2450
3810
4580
5160
5740
6020
-26.20
-26.22
-25.53
-25.68
-27.31
-25.78
16.3
16.3
17.6
17.3
14.2
17.1
ML3 Depth (cm)
Age (AD)
δ13C (‰)
July temp.c
0–1
1–2
2–3
3–4
4–5
5–6
6–7
7–8
8–9
9–10
2002–2004 (2005)
2000–2001
1998–1999
1996–1997
1995
1993–1994
1992
1990–1991
1989
1987–1988
-25.96
-26.00
-25.59
-26.21
-27.39
-26.40
-27.13
-26.88
-26.25
-26.81
16.0
17.1
16.3
17.3
13.1
15.6
14.2
16.0
17.5
15.4
a
Ages are derived from the age–depth model and rounded off to the nearest multiple
of ten.
b
July temperatures are calculated from the linear regression model.
c
July temperatures are averaged from observed data. AD 2005 was excluded from
the dataset because samples were collected before the end of July.
around 1000 cal yr BP are unclear. High humification values of c. 500–
100 cal yr BP coinciding with a rootlet layer (c. 360–110 cal yr BP)
indicate dry conditions, possibly reflecting southwards fluctuations of
the arctic front during the so called Little Ice Age. The change in the
botanical composition during the period could be a consequence of
changing climate or a result of a local change in moisture (microtopographical features) that probably caused the discrepancies in the
recent parts of the Selwyn Lake and Misaw Lake records.
5. Discussion
Our temperature reconstruction (Fig. 4) is based on the assumption that the relationship between carbon isotope ratios and July
temperatures has been stable throughout the middle and late
Holocene. It was also assumed that the 13C concentration in the
atmosphere has been constant on pre-industrial levels before AD
1850. The temperature dependence of the fractionation of carbon
isotopes is plausibly because of kinetic effects during diffusion and
photosynthesis. Summer temperature is an important factor for
surface wetness and plant growth in areas where the variation in
the amount of precipitation is low (Scott et al., 1987; Schoning et al.,
2005; Skrzypek et al., 2007), e.g. near the tree-line in west-central
Canada (Tardif et al., 2008), and affect (either directly or indirectly)
the photosynthetic rate of Sphagnum fuscum to a high degree
(Dorrepaal et al., 2003). The effects of uncertainty in the 210Pb age
model can possibly explain a part of the remaining 40% of the
temperature-isotope relation that are not explained by linear
regression (R2 = 0.60), in addition to factors such as other environmental forcing, e.g. pCO2, moisture and light.
Moisture conditions were interpreted from humification indices,
the oxygen isotope record and the macrofossil evidence. Although
wet/dry periods could alter the hydrology at our study site,
macrofossil analyses demonstrate a continuous succession of Sphagnum fuscum interrupted by local dry conditions. An exception was a
very short period of fen succession indicated by Sphagnum cf. jensenii
241
at 4600–4400 cal yr BP after permafrost degradation caused by a local
fire (Sannel and Kuhry, 2008). Since stable isotope analyses were not
conducted from that layer and S. fuscum is a species growing above the
water-table (Lindholm, 1990; Rydin and Jeglum, 2006) in hummocks
on the peat plateau and not in fens and collapse scars, we can regard
the record being mainly influenced by meteoric water. The oxygen
isotope record is influenced by both moisture conditions in the
peatland and temperature variation between the source of precipitation and the rainfall (Zanassi and Mora, 2005). Although there was
more significant correlation between δ18O values and July temperatures than between δ18O values and July precipitation amounts
(Kaislahti Tillman et al., 2010), the discrepancies between the δ18O
record and the temperature reconstruction may reflect changes in
local moisture conditions and/or shifts in the source of precipitation
caused by fluctuations in the position of the arctic front (Edwards et
al., 1996). If the water table had been high for longer periods when
stable oxygen isotopes were derived from the water, it should also be
seen as a change in the botanic composition. Still, there is a possibility
that changes in the plant macrofossil assemblage are not detected
because of the selective decay of peat (Clymo, 1984) or that other
competitive advantages of Sphagnum fuscum inhibit the change. The
weak covariance of humification indices and stable isotope ratios at
some levels has been interpreted to be caused by climatic impact
rather than autocorrelation (Kaislahti Tillman et al., 2010). Colorimetry is a semi-quantitative method depending not only on decomposition rate, but also on the botanical composition (Blackford and
Chambers, 1993; Yeloff and Mauquoy, 2006). Therefore, these
qualitative indicators of climate variation should not be related only
to wet and dry shifts, but also to local conditions. To what extent
changes in relative humidity and evaporation rate control our record
was not evaluated in this study.
The pattern of absorbance and δ18O curves from peat profiles S52
and S53 show a c. 100 yr offset instead of overlap (Fig. 5). Which of the
age models – or both – cause this offset cannot be concluded without
an independent age control, such as the detection of an attributable
tephra horizon or a high resolution pollen record. The error can
contribute to the uncertainty in the temperature reconstruction at c.
1000–1200 cal yr BP although the time series derived from δ13C seem
to coincide at that level. Considering the estimated uncertainties of
calibrated radiocarbon dates, the uncertainty between the profiles is
approximately of the same magnitude as within the profiles. Despite
the uncertainty, the chronology is well constrained by 22 radiocarbon
dates without any reversed ages.
The Holocene Thermal Maximum occurred later in central Canada
than in the north-western parts of the continent because of the
cooling effect of the residual Laurentide Ice Sheet, but the exact timing
varied across central Canada (Kaufman et al., 2004). Nichols (1967)
estimated from the distance of the forest limit from Ennadai (based on
stratigraphical evidence and pollen records, Fig. 1) that mean July
temperatures were about 3 °C warmer than modern values at around
5850–6850 ± 150 cal yr BP and 3950 ± 150–5300 ± 200 cal yr BP. Kay
(1979) derived July temperatures in the Northwest Territories (Fig. 1)
from pollen transfer functions and found 1–3 °C higher July
temperatures than modern values about 6250 ± 250–4070 ±
340 cal yr BP, a 3–5 °C drop until 3590 ± 240 cal yr BP at Long Lake
and an increase in temperatures around 3150 ± 260 cal yr BP. Pollen
and diatom records from Queens Lake and Toronto Lake implied
warmer climate between 5000–3500 cal yr BP and cooling after
3500–3000 cal yr BP (Moser and MacDonald, 1990; Pienitz et al.,
1999; Kaufman et al., 2004). Our results are largely consistent with
the studies mentioned above. Furthermore, our reconstructed warm
event corresponding to a so-called Mediaeval Climatic Anomaly in this
Fig. 3. a–b Stratigraphies and percentage diagrams of plant macrofossils of a) the upper part of the Misaw Lake hummock ML3, and b) the upper part of the Selwyn Lake profile S53.
Rare types (b 1% of total sum) are marked by + (1–10 findings) or ++ (11–20 findings).
242
P. Kaislahti Tillman et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 298 (2010) 235–246
Fig. 4. July temperatures (broken line) are reconstructed from stable carbon isotope ratios (δ13C) in the Selwyn Lake peat profiles S52 (cross) and S53 (open square), and in the
Misaw Lake hummock ML3 (open diamond). Error bars show 95% confident intervals. July average temperatures were recorded in Stony Rapids meteorological station AD 1960–
1978, 1986–2008 (red/solid line). Note different time scales; 0 in cal yr BP is equivalent to AD 1950.
region was divided in two phases in the same manner as e.g. Kay
(1979) described c. 1–2 °C lower July temperatures in 950 ± 220 cal yr
BP than during centuries before and after at Nicol Lake. The trend of
increasing temperatures during the past 450 yr that was temporary
ceased in the middle of the 19th century was rather similar to
reconstructions by Moberg et al. (2005) and Overpeck et al. (1997)
despite that the temperature anomalies were lower in their larger
study area (the northern hemisphere and the arctic region) and multi
proxy studies (Fig. 6). In their tree-ring studies, D'Arrigo and Jacoby
(1993) showed elevated summer temperatures about AD 1770–1830
and after 1930 in Churchill, some decades later than in a region northwest of our study site (Fig. 1). Unfortunately, we have limited data
from the period between c. 390 and 110 cal yr BP (AD 1560–1840)
because of the rootlet layer in our peat record (Fig. 6), but around
100 cal yr BP the recovery to warmer temperatures was shortly
interrupted also in our reconstruction.
Precipitation records from the region are rare and observed
amounts had a large monthly variation between included meteorological stations (Kaislahti Tillman et al., 2010). In our study, a short
wet event around 4300 ± 120 cal yr BP and a drier period around
2000 cal yr BP coincide with annual precipitation anomalies in central
Canada reconstructed by Viau and Gajewski (2009) (Fig. 5). There is
mismatch with the other parts of their record, e.g. when our study
shows that climate was locally dry in the period c. 6000–4400 cal yr
BP and wet after 3500 ± 80 cal yr BP.
The comparison between proxies was limited by the grade of
smoothing of the used reconstruction method, the spatial coverage,
and the temporal resolution. In our study, the reconstruction based on
calculated values show a temperature range of 6.5 ± 2.3 °C (12.5–
19.0 °C) over the past 6.2 ka where 1 cm sample thickness represent c.
2–70 yr. The instrumental record of July average temperatures had an
annual variation of 5 °C (13.1–18.0 °C) during the period 1987–2009
in Stony Rapids (Environment Canada, 2010), i.e. almost the same as
the reconstructed amplitude. The large annual variation is probably
not superimposed on climate reconstructions with averaged values
from several decades, but it shows the possible impact of short
temporal fluctuations, e.g. on the peak around 1700 cal yr BP
representing a c. 5-year period. Although the fluctuations in our
record and the chironomid records by Fallu et al. (2005) and
MacDonald et al. (2009) were approximately of the same magnitudes
(Fig. 5), they were not exactly synchronous, possibly reflecting the
uncertainty range of radiocarbon dates of lake sediments and peat.
However, the comparison indicates that the regions in west-central
and east-central Canada may have been influenced simultaneously by
the arctic front, although several studies have to be compared before
drawing too far reaching conclusions. It also implies that our results
are not only representing a local response to environmental forcing
but can be regarded as reflecting environmental changes at a regional
scale. We were not able to compare our results with tree-ring studies
because of a hiatus during overlapping records. Therefore, we suggest
further studies about contemporary stable isotopes from peat archives
and tree-ring records in order to reveal if they have a potential as
complementary proxies in the future.
6. Conclusions
The reconstruction based on stable carbon and oxygen isotope
ratios, plant macrofossil analysis and colorimetry suggest that main
climate changes are preserved in the Sphagnum fuscum record on
decadal and longer time scales. The record is the first long-term stable
isotope climate reconstruction from peat in the Canadian arctic
region. It implies a potential of the proxy to extend this kind of studies
into larger temporal and spatial scales. A higher resolution is possible
to obtain if samples are analyzed from continuous and thinner
intervals in the future. We draw the following conclusions from our
record:
During the past 6.2 ka, average July temperatures varied by 6.5 ±
2.3 °C. Thermal maxima occurred at c. 4300 ± 120 and 3100 ±
120 cal yr BP with 3.0 ± 2.2 °C and 2.5 ± 1.9 °C higher temperatures,
respectively, than the recent (late 20th century) mean July temperature value. During the period from c. 2500 to 1200 cal yr BP, July
temperatures were as much as 3.5 ± 2.3 °C lower than modern values,
except a short peak event at 1700 ± 100 cal yr BP. Between c. 1300
and 900 cal yr BP, corresponding to a period of Mediaeval warmth,
temperatures of a level similar to that experienced in the late 20th
century appear to have prevailed. The general cooling tendency of the
last millennium experienced a rapid drop by 4.1 °C around 400 cal yr
BP. The latter part of the Little Ice Age was not recorded because of a
(climatically induced?) hiatus in our proxy record. 20th century
temperatures are characterised by an increasing but fluctuating trend.
The early warming period was relatively dry until a short wet
event around c. 4300 ± 120 cal yr BP. From c. 3500 ± 80 cal yr BP, local
conditions were moist until c. 2500 ± 130 cal yr BP. After a dry cooler
period, the (local) wetter periods around c. 1700 ± 100 cal yr BP and
c.1300 ± 60 cal yr BP could be the result of a short-term influence
from more humid southerly air-masses. The Little Ice Age was a dry
period, possibly as a consequence of increased influence and
southwards fluctuations of the arctic front; conclusions here are
further limited by temporal resolution and dating uncertainties in our
archive.
The δ13C temperature record is largely compatible with other
proxy records, despite the difficulties with age controls in peat
P. Kaislahti Tillman et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 298 (2010) 235–246
243
Fig. 5. a–i From the top: a) Colorimetric normalized (grey line) and detrended (black line) absorbance values (this study), b) stable oxygen isotope ratios (δ18O) of Selwyn Lake
profiles S52 and S53 (this study), c) annual precipitation anomalies in central Canada reconstructed by Viau and Gajewski (2009), temperature anomalies in d) the northern
hemisphere by Moberg et al. (2005) and e) central Canada by Viau and Gajewski (2009), summer surface water temperatures (SWT) in f) the Northwest Territories by MacDonald et
al. (2009) and g) east-central Canada by Fallu et al. (2005), h) July temperatures in west-central Canada (this study) reconstructed by stable carbon isotope ratios (corrected for the
decrease of δ13C in CO2atm during the industrial time) with 95% confidence interval (grey solid lines), and i) June insolation 60°N (black line) and July insolation 65°N (grey line) after
Berger and Loutre (1991). Broken grey lines mark reference period means. Samples from the peat profile S52 are marked by crosses and from the profile S53 by open squares.
Comparative datasets c)–f) originate from the NOAA database (NOAA National Climatic Data Center, 2010) and g) is published with personal permission from Marie-Andrée Fallu.
The patterned/yellow bars show warmer periods in our reconstruction and the grey/light blue bars show cold periods discussed in the text.
archives and lake sediments, and differences in temporal resolution.
Our reconstruction shows similar temperature amplitudes compared
with chironomid records, i.e. c. 6 °C range in July temperatures the
past 6.2 ka, nearly simultaneous variations on the centennial scale,
and warm–cold differences of c. 4 °C during the Little Ice Age. The only
deviation in our temperature curve compared with other proxy
reconstructions is a single-value warm period at c. 1700 cal yr BP
supported by a rapid peat accumulation rate at that level. No
comparison with tree-ring data was made in this study because of
lack of temporary overlapping data from our study region. Wet–dry
periods have more mismatches between different records – reflecting
possibly a larger spatial variability and reconstruction uncertainty in
precipitation compared to temperature.
Acknowledgements
From Stockholm University, we thank Karin Helmens for field
assistance, Stefan Wastegård for tephra analysis and Markus Meili for
244
P. Kaislahti Tillman et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 298 (2010) 235–246
Fig. 6. a-i Records covering the last ~ 450 yr from the top: a) colorimetric normalized (grey line) and detrended (black line) absorbance values (this study), b) stable oxygen isotope
ratios (δ18O) of the profile S53 and the hummock ML3 (this study), c) annual precipitation anomalies in central Canada reconstructed by Viau and Gajewski (2009), temperature
anomalies in d) the northern hemisphere by Moberg et al. (2005), e) central Canada by Viau and Gajewski (2009) and f) the arctic by Overpeck et al. (1997), summer surface water
temperatures (SWT) in g) the Northwest Territories by MacDonald et al. (2009) and h) east-central Canada by Fallu et al. (2005), i) July temperatures in west-central Canada (this
study), reconstructed by stable carbon isotope ratios (corrected for the decrease of δ13C in CO2atm during the industrial time) with 95% confidence interval (grey solid lines). Broken
line c. AD 1560–1840 is because of a rootlet layer from which no Sphagnum fuscum samples were picked. Broken grey lines mark reference period means. Samples from the peat
profile S53 are marked by open squares. Comparative datasets c)–g) originate from the NOAA database (NOAA National Climatic Data Center, 2010) and h) is published with personal
permission from Marie-Andrée Fallu.
137
Cs analysis. Päivi Kaislahti Tillman acknowledges financial support
from the Ahlmann Foundation, Bert Bolin Centre for Climate Research
(BBCC), the Lagrelius Foundation and Svenska Sällskapet för Antropologi och Geographi (SSAG). Peter Kuhry received financial support
from Vetenskapsrådet and Britta Sannel from the Ahlmann Foundation and the Royal Swedish Academy of Science. Iain Robertson and
Neil J. Loader thank Tim Daley and Jonathan Woodman-Ralph for
support and technical assistance. NJL and IR acknowledge support
from the EU GOCE “Millennium” 017008, UK Climate Change
Consortium of Wales (C3W), NERC NE/B501504 and NE/G019673.
We also thank Mats Rundgren from Lund University for comments
that improved the early manuscript, and two anonymous reviewers
for their helpful remarks.
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