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Quaternary Science Reviews
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The importance of northern peatland expansion to the late-Holocene rise of
atmospheric methane
Atte Korhola a, *, Meri Ruppel a, Heikki Seppä b, Minna Väliranta a, Tarmo Virtanen a, Jan Weckström a
a
b
Environmental Change Research Unit, Department of Biological and Environmental Sciences, P.O. Box 65, Viikinkaari 1, 00014 University of Helsinki, Finland
Department of Geology, P.O. Box 64, Gustaf Hällströminkatu 2a, 00014 University of Helsinki, Finland
a r t i c l e i n f o
a b s t a r c t
Article history:
Received 5 October 2009
Received in revised form
15 December 2009
Accepted 15 December 2009
Wetlands have been considered as the most important natural source of the atmospheric methane
concentration (AMC) prior to anthropogenic influences. According to ice cores, AMC varied significantly
during the Holocene, the causes of which are not completely understood. In particular, the reasons for
the increased AMC during the late Holocene (from 5 ka onwards) have been debated widely, including an
anthropogenic explanation. Initially, this increase was associated with increased emissions from
northern wetlands, but estimated peat initiation rates seem not to support the conclusion. Based on
a new data set of 954 basal peat radiocarbon dates that accounts more properly for the horizontal growth
dynamics of northern peatlands (by containing only sites with multiple basal dates per site), we show
here that the most extensive lateral expansion of high-latitude peatlands occurred only after 5 ka,
parallel with the rise of CH4 in the ice cores. Because this explosive increase in the extent of peatlands
resulted in the formation of moist minerotrophic fen ecosystems that emit high amounts of CH4 for
a long time since their formation, and because many Arctic peatlands have remained minerotrophic
throughout their development, northern peatlands cannot be neglected when seeking cause(s) for the
late-Holocene rise in CH4. A similar event in future could enhance climate change by causing a rapid shift
in atmospheric greenhouse gas concentrations.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Ice-core records have demonstrated that the past atmospheric
concentrations of methane, the third most important greenhouse
gas in the atmosphere, fluctuated widely in parallel to rapid glacial
climate changes in the Northern Hemisphere (Chappellaz et al.,
1993; Blunier et al., 1995; Brook et al., 2000). After the termination
of the last glacial epoch, atmospheric methane concentration
(AMC) rose rapidly to levels over 700 parts per billion by volume
(ppbv) in the early Holocene between 11 and 8 ka (kiloyears before
present, with present equal to AD 1950), decreased and reached
a minimum of <600 ppbv at 5.2 ka and then increased again over
the late Holocene to values about 725 ppbv just before the industrial revolution (Chappellaz et al., 1993; Blunier et al., 1995; Brook
et al., 2000). Changes in ice core AMC reflect larger-scale changes in
natural methane sources, which comprise tropical and boreal
wetlands, oceans, thaw lakes (especially in winter), termites,
ruminants and biomass burning (Whiticar, 1993; Quay et al., 1999;
* Corresponding author.
E-mail address: atte.korhola@helsinki.fi (A. Korhola).
Walter et al., 2006). Of these, natural wetlands have been considered as the most important (z75%) source of AMC prior to
anthropogenic influences (Mikaloff Fletcher et al., 2004).
A vigorous debate about the causes of the late-Holocene
increase of the AMC of around 100 ppbv has been underway for
years. Although it is generally agreed that the dominant natural
methane source was wetlands, the relative contributions from the
mid-high northern hemisphere latitudes versus the tropics to the
late-Holocene increase have been open to question (Chappellaz
et al., 1997; Dällenbach et al., 2000). In contrast, it has also been
suggested that anthropogenic activities, namely rice cultivation and
biomass burning, drove much of this increase (Ruddiman, 2003).
This debate has not been settled yet and requires a further analysis
of the potential long-term controls on AMC.
Northern peatlands represent the biggest wetland complex in
the world releasing 20–45 Tg CH4, produced by anaerobic decomposition in waterlogged subsurface peats, into the atmosphere
annually (Gorham, 1991; Mikaloff Fletcher et al., 2004). Recent
research on the relationship between wetlands and AMC over the
Holocene has focused on the aerial extent of peatlands. MacDonald
et al. (2006) collated a comprehensive data set of 1516 radiocarbon
dates of basal peat layers documenting the spread of peatlands
0277-3791/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.quascirev.2009.12.010
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A. Korhola et al. / Quaternary Science Reviews xxx (2010) 1–7
2080
0
20
0
30
0
40
0
00
50 00
4560
5800
0
3420
0
5220
5680
70
00
80
across the circumpolar North. Based on the frequency distribution
of the basal dates they concluded that the rapid early initiation of
northern peatlands very likely contributed to the sustained peak in
CH4 during the early Holocene. However, they argue that the role of
northern peatlands in the more recent rise of AMC after 5 ka was
most likely negligible because new peatland initiation was then
relatively modest, and the peatlands had already transformed from
early minerotrophic fens to drier ombrotrophic bogs, which are
typically weaker sources of CH4 than fens (MacDonald et al., 2006).
Most of the basal peat dates used to document the initiation of
peatlands has been single dates taken from the deepest/oldest part
of a single peatland (Smith et al., 2004; MacDonald et al., 2006;
Gorham et al., 2007). Such dates may reflect well the initial onset of
peatland formation, but neglect the dynamics of the subsequent
lateral spread of the mire, which however accounts for most of the
methane fluxes because the expanding mire margins are typically
minerotrophic fens that are high methane emitters (Rydin and
Jeglum, 2006). The lateral peatland expansion may occur much
later than the initial mire formation (Foster and Wright, 1990;
Korhola, 1994, 1995, 1996; Mäkilä, 1997; Anderson et al., 2003;
Bauer et al., 2003; Belyea and Baird, 2006; Weckström et al., in
press). Most of the northern peatlands have been formed by
primary paludification of mineral ground after emergence from
beneath the ice or sea (Warner et al., 1991). Paludification usually
began in waterlogged topographic depressions from a single central
locus and affected only restricted areas (Foster and Wright, 1990;
Korhola, 1994, 1995, 1996; Mäkilä, 1997; Anderson et al., 2003;
Bauer et al., 2003; Belyea and Baird, 2006). The subsequent lateral
expansion has been driven by water flowing out of the sides of the
peat body and submerging a strip of land just beyond the boundary
of the peat body, initiating peat formation (Frenzel, 1983). This later
lateral expansion of mires has not taken place in an even manner,
however, but has included distinctly faster and slower phases,
governed by local topographic, hydrological and pedological variations as well as climate change (Foster and Wright, 1990; Korhola,
1994, 1995, 1996; Mäkilä, 1997; Anderson et al., 2003; Bauer et al.,
2003; Belyea and Baird, 2006).
For example, the paludification of Reksuo (Fig. 1), a welldeveloped, concentric raised bog in SW Finland with a size of
581 ha and maximum peat depth of 7.3 m, began from a single
central locus around 8.9 ka. Once the shallow depression was filled
with peat, the mire front began to spread into the surrounding
mineral soils until ca. 2 ka, after which there was a clear decline in
the rate of expansion (Korhola, 1992). On the basis of such detailed
accounts on several Fennoscandian peatlands, Korhola (1994) was
able to assess the overall aerial expansion rates of Finnish peatlands. Two distinct phases of more active lateral expansion were
noted: ca. 8.0–7.0 ka and 4.0–3.0 ka (e.g., Korhola, 1994, 1995;
Almquist-Jacobson and Foster, 1995; Charman, 2002; Belyea and
Baird, 2006), both episodes resulting in the formation of vast new
moist minerotrophic fen areas that emit CH4 effectively.
Bog lateral extent is one of the key boundary conditions constraining the dynamics of peatland systems (Belyea and Baird,
2006). The horizontal expansion patterns of peatlands can be
properly studied only by dating multiple basal peat samples along
transects of one and the same peatland (Foster and Wright, 1990;
Warner et al., 1991; Korhola, 1994, 1995, 1996; Mäkilä, 1997;
Anderson et al., 2003; Bauer et al., 2003). Therefore, to provide
a more generalized account of the spatiotemporal dynamics of the
peatland development we compiled a new northern hemispheric
peat data set, including only sites that contain multiple radiocarbon-dated bottom peat samples from different localities of
a single peatland (by ‘bottom peat’ we mean the first peat overlying
mineral soil). By this way we assume to account for the possible
phases in lateral extension of peatlands more adequately than
7120
7420
3940
8750
7370
4830
2930
5450
7450
7200
1000 m
Fig. 1. Plan view of Reksuo bog showing calibrated radiocarbon dates (cal. yr BP) of
basal peats from different locations and approximate isochrones of mire expansion
(redrawn after Korhola, 1994).
using only the single basal date (usually obtained from the deepest
point of the peatland) that may reflect spatially very limited peat
induction.
2. Material and methods
The timing of northern peatland initiation and lateral expansion
was established by compiling a data set of basal peat radiocarbon
dates of circumarctic peats from existing literature and unpublished data sources. All peatlands of the tundra and boreal biome
and closely adjacent peatlands of the temperate forest biome were
recognised as northern peatlands. In North America the southernmost peatlands included were at the 36th latitude and in Europe at
the 50th latitude.
The data set of 1516 circumarctic basal peat dates collected by
MacDonald et al. (2006) was used as a starting point for the data
collation. The baseline was that all determinations were checked
from the original sources and only basal dates with complete
laboratory and geospatial information were included. 1506 determinations fulfilled these criteria.
A data set of 1680 basal peat dates from Canada and the United
States established by Gorham et al. (2007) was examined next. Of
these, 526 determinations were same as in MacDonald et al. (2006).
Gorham et al. (2007) online supplementary data did not contain
source information and from the remaining data only 425 determinations were traceable and were included in our data. The rest
was included without reference and radiocarbon date particulars,
except for 55 dates that could not be confirmed as basal peat in our
literature survey and 17 dates that were from peatlands south of the
36th latitude. The rest of the data used in our analysis were
compiled from other published articles and reports and some from
unpublished sources (for details, see Supplementary Tables 1 and
2). The initial radiocarbon ages were calibrated to calendar years
using the computer program CALIB 5.1 (IntCal04; Stuiver and
Reimer, 1993; Reimer et al., 2004) and rounded to decades. The ages
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in Gorham et al. (2007) were already calibrated ages (initial assays
not given) and used as such. During the literature survey also the
trophic status (ombrotrophy vs. minerotrophy) of the dated peatland was recorded if such data were available. However, in many
cases the division is arbitrary because most peatlands have surfaces
and vegetation cover of both of these categories.
The compilation resulted in a total of 3146 basal peat radiocarbon dates from 2212 sites. Of these, 555 determinations had not
yet been referenced to in the context of aerial spread of peatlands.
From all these data, we compiled a new subset of data, including
only sites that contained a minimum of three radiocarbon-dated
basal peat samples from different sampling points from a single
peatland. We appreciate less than three sample points may only
refer replicate cores taken next to each other. Hence, we consider
three samples to be the lowest number of bottom ages to inform
about the lateral expansion of given mire. Also, site information
included in our database (Supplementary Table 1) show that three
basal dates are always located in different parts of the peatland
under consideration. This new inventory resulted in 138 peatland
sites and 954 radiocarbon dates. Of the new dataset 417 dates were
already reported in MacDonald et al. (2006) and 97 in Gorham et al.
(2007). Accordingly, 440 samples are new in this context. 14
peatland sites are in Russia, 42 in Europe and 82 in North America.
The apparently low number of peatlands in Russia is a consequence
of the difficulty to get access to the studies made in Russian. 51
peatlands contained seven or more dates per site with a total of 615
dates, while 29 peatlands have 10 or more basal dates, of which 15
are from North America, 13 from Europe and one from Russia. This
shows that our data base contains several sites that have been
studied in the level of detail as indicated in Fig. 1.
We analyzed the compiled data set by raw number of initiation/
extension dates, and assigned a value of peatland initiation based
on the basal dates. To illustrate regional differences in peatland
dynamics we also produced spatial representations from the data
set in ArcGIS.
3. Results
Our data source inventory resulted in a total of 2212 sites and
3146 radiocarbon dates, geospatially widely distributed across the
3
northern areas, of which 138 sites and 954 radiocarbon dates fulfilled our criterion of multiple dates per site (Fig. 2; Supplementary
Table 1). The total number of sites is naturally smaller when using
the new-screened data set (Fig. 2A) in comparison to the data set
including peatlands with only single basal dates (Fig. 2B), and some
very large areas such as eastern Siberia and Greenland have almost
no data at all. Also, there are only a very limited amount of dates
older than 12 ka and only a few dates older than 11 ka in our new
data set, which however does not mean the lack of peatlands
during the lateglacial times (see, e.g., Turunen, 2003; Gorham et al.,
2007).
There was a limited amount of information about the trophic
status of the peatlands in the literature. On the basis of 254 sites
that contain such information (Tables S1 and S2), peatlands in
Europe and Russia appear to be predominantly minerotrophic
north of the 65th latitude and more commonly ombrotrophic south
of it. In North America this boundary follows ca. the 55th latitude,
but in reality it varies greatly between the western and eastern part
of the continent and e.g., the marshes on the east coast are abundantly minerotrophic also south of this boundary. In general, our
survey indicates that Arctic peatlands are mostly minerotrophic,
which is in good agreement with Ruuhijärvi (1983).
Our data set on age dating of basal peat layers documents the
spreading of peatlands in the Circum-Arctic domain (Fig. 3), and is
thought to be proportional to methane emissions (Fig. 4B). Fig. 3
illustrates the expansion dynamics of northern peatlands in
1000 yrs time windows, showing that the expansion rates have
varied greatly during Holocene. A particularly pronounced and
spatially evenly distributed acceleration in peatland extent seem to
have taken place around 5 ka. A more thorough examination of the
basal age frequency data based on multiple datings per site (Fig. 4D)
revealed several patterns that differ markedly from a figure
obtained if only the single oldest date of peat initiation was used
(Fig. 4F). First, the rapid establishment of peatlands in the early
Holocene (11–8 ka) following ice retreat and the exposure of land
surfaces was clearly evident in our data. Second, there was a general
declining trend in expansion rates between 8 and 5 ka. Finally,
a substantial increase in the rate of peatland expansion occurred at
around 5 ka (Fig. 4D). This latter pattern becomes even more
evident, the higher the criteria of basal peat assays per site was set,
Fig. 2. Map of geospatial location of peatlands with basal peat radiocarbon datings. A) Sites with one or two basal peat dates per peatland. B) Sites with three or more dates per
a single peatland.
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Fig. 3. Time and location of peatland initiation and lateral expansion of multiple dated peatlands in 1000 years time slices (cal. yr BP). The red dots indicate the number of basal peat
samples from each single peatland in the given time interval. N ¼ 954.
Please cite this article in press as: Korhola, A., et al., The importance of northern peatland expansion to the late-Holocene rise of atmospheric
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Fig. 4. Timing of areal extension of circumarctic peatland complexes compared with June insolation at 60 N, North Greenland Ice Core Project (NGRIP) d18O values, atmospheric CH4
concentrations, and estimate source strength of northern peatlands. A) June insolation at 60 N [yellow (Berger and Loutre, 1991)] and NGRIP d18O values [blue (Vinther et al., 2006;
Rasmussen et al., 2006)]. B) Atmospheric CH4 concentrations as reconstructed from Greenland ice cores [blue (Blunier et al., 1995)]. C) Model-derived estimates of total methane
sources north of 30 N based on interpolar CH4 concentration gradients as obtained from ice core records [blue (Chappellaz et al., 1997; Dällenbach et al., 2000)]. D) The occurrence
frequency of 954 radiocarbon multidates (3 dates per site) of basal peat layers in 1000 years time slices. E) The occurrence frequency of 615 radiocarbon multidates (7 dates per
site) of basal peat layers in 1000 years time slices. F) The occurrence frequency of 2212 oldest radiocarbon assays of basal peat layers in 1000 years time slices (referring to the first
initiation peat formation at each site).
i.e. the more basal dates we added (Fig. 4E, >7 basal peat dates per
a peatland), thus reinforcing our interpretation that most of the
expansion of northern peatlands took place only during the lateHolocene times after 6 ka.
4. Discussion and conclusions
In principle, the overall pattern we observe in peatland expansion follows the records of the variation in atmospheric methane
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concentration that have been obtained from ice cores for the
Holocene (Fig. 4B): AMC rose along general peatland initiation in
northern hemisphere, but this increase was probably also
substantially contributed by emissions from tropical wetlands
(Chappellaz et al., 1993) and methane bubbling from Siberian
thermokarst lakes (Walter et al., 2007). Methane concentration
declined during the mid Holocene mainly due to drying of tropical
wetlands (Street-Perrot, 1993), whereas northern mires remained
as weak methane sources (Blunier et al., 1995 and Fig. 4D).
According to our new multidate peat data base, the horizontal
expansion of northern peatlands during the Holocene was most
intensive between 5 and 3 ka (Figs. 3 and 4D). Where palaeobotanical analyses from the peats are available, they suggest that
this late-Holocene lateral extension took place largely at a time
when the mires were still mostly minerotrophic (e.g., Korhola,
1994, 1995, 1996; Almquist-Jacobson and Foster, 1995). In addition,
the margins of spreading ombrotrophic bogs are always minerotrophic. Since 3 ka the rate of peatland expansion gradually slowed
down, most probably because at this time essentially most areas
that are potentially suitable for the peat accumulation were already
covered by peat (Ruuhijärvi, 1983). However, methane emissions
still continued to rise, because most Arctic wetlands remained
minerotrophic being moist fens even today.
Changes in climate presumably affected the extent of wetlands
causing variations in the methane production from northern
ecosystems. The areal expansion of peatlands within the northern
area was considerably reduced during the Holocene Thermal
Maximum (Fig. 4), when many arctic lakes also dried up (e.g.,
Korhola et al., 2005; Väliranta et al., 2005). The accelerated mire
expansion at around 5 ka is probably a response to increased
moisture availability associated with the orbitally driven Neoglacial
cooling trend (Fig. 4A and B), which is evident in many records from
the northern subpolar regions after the Holocene Thermal
Maximum (see, e.g., Snowball et al., 2001 and references therein).
Moisture and temperature patterns vary regionally, but there are
many lines of proxy evidence that moisture is inversely correlated
to temperature on a large scale. Although cool and wet climate
would decrease peatland emission rates (per unit area), reduced
evaporation would increase peatland extent, resulting in higher net
biogenic emissions (Ferretti et al., 2005).
The frequency distribution of basal peat dates shares many
features with the AMC inferred from the CH4 mixing ratio in gas
bubbles from the Greenland ice cores, in particular with regard to
the late-Holocene rise after 5 ka (Fig. 4C), suggesting a change in
source location from south to north (Chappellaz et al., 1997).
Furthermore, we observe that the peatland expansion intensity
inferred from the frequency distribution of the basal peat dates
shows similar trends as the Holocene methane emission intensity
as inferred from the paleo net primary production (PNPP) for
a Swiss peat bog (Steinmann et al., 2006). Thus, the methane
emissions from the northern peatlands have certainly boosted at
that time and significantly contributed to AMC during the late
Holocene.
The carbon balance of peatlands is determined primarily by
a trade-off between carbon sequestration and the methane fluxes
in the course of peatland succession. In terms of radiative forcing
(RF), atmospheric model simulations on northern peatlands show
that after peat initiation the net RF impact begins as a net warming
that peaks after about 50 years, remains a diminishing net warming
for the next several hundred or several thousand years and thereafter is or will be an ever increasing net cooling impact (Frolking
et al., 2006). A 3D landscape model applied to the well-studied
Reksuo bog (Fig. 1) in southern Finland demonstrated that in this
8900 years old mire a net warming effect prevailed for about 6000
years, during which time the peatland complex exerted a warming
potential of 53 kg carbon dioxide equivalents (CO2-eq) per square
meter (Korhola et al., 1996). About 2.5 ka ago the net RF impact
turned negative (cooling), when the sink of CO2 exceeded the
warming effect of the CH4 emissions. In addition, the vast majority
of Arctic wetlands have remained minerotrophic, high-emitting
fens, throughout their subsequent development. Hence, substantial
horizontal spread of northern peatlands between 5 and 3 ka could
potentially have contributed to the increased CH4 levels in atmosphere for much longer time that suggested by the period of
intensive expansion itself. We postulate that northern mires
contributed to the increasing AMC levels for much of the preindustrial period, although actual net warming impact of the
wetland-derived CH4 emissions was probably not able to counteract the large orbital cooling impact during the late Holocene.
Although we cannot disapprove the anthropogenic explanation
for the late-Holocene methane record on the basis of the current
data, our results strongly suggest that the contribution of northern
peatlands cannot be excluded when seeking causes for the rise in
AMC over the past millennia. It is even likely that the late-Holocene
expansion rate of peatlands inferred here may be badly underestimated due to practises related to peat sampling. Because
younger peatlands tend to be shallower, they are less likely to be
chosen for analysis by peatland paleoecologists, who traditionally
seek long stratigraphic sections for their work. For example, very
few dates are available from the Hudson Bay Lowland (e.g., Gorham
et al., 2007, see Fig. 2), the second largest peatland complex in the
world, which is comprised of younger peats than average because
of the gradual isostatic rebound of the area.
This paper makes a first attempt to assess the dynamics of
lateral expansion of northern peatlands from a global database.
Because the data sets are highly variable in details, and the
northern peatlands were not collectively sampled with such
a numerical analysis in mind, our analysis should be considered
somewhat qualitative. However, our ultimate aim is to refine the
compiled database in future and include quantitative approaches
and ecosystem modelling to achieve more quantitative estimates of
the spatiotemporal contribution of northern peatlands to Holocene
AMC. Finally, our inference about the contribution of northern
wetland ecosystems to AMC has implications also for the future
carbon balance. Annual total precipitation is projected to increase
by roughly 20% over the Arctic by 2100, with most of the additional
precipitation in the form of rain. This may result in moisture excess,
higher water tables, and a new pulse of peatland spreading.
A similar event than the observed late-Holocene peatland initiation
could strengthen climate change in future by causing a rapid shift in
atmospheric greenhouse gas concentrations.
Acknowledgements
We thank the Science Workshop on Past, Present and Future
Climate Dynamics held in Helsinki in November 2008 for fruitful
discussions. We are particularly thankful to G.M. MacDonald and
S.P. Harrison for their insights. We also thank two anonymous
referees for their thoughtful suggestions. Research funding was
provided by the Academy of Finland (122702), the University of
Helsinki (700056) and the Finnish Cultural Fund.
Appendix. Supplementary data
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.quascirev.2009.12.010.
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Please cite this article in press as: Korhola, A., et al., The importance of northern peatland expansion to the late-Holocene rise of atmospheric
methane, Quaternary Science Reviews (2010), doi:10.1016/j.quascirev.2009.12.010