Rapid deglacial and early Holocene expansion of peatlands in Alaska.
ABSTRACT Northern peatlands represent one of the largest biospheric carbon (C) reservoirs; however, the role of peatlands in the global carbon cycle remains intensely debated, owing in part to the paucity of detailed regional datasets and the complexity of the role of climate, ecosystem processes, and environmental factors in controlling peatland C dynamics. Here we used detailed C accumulation data from four peatlands and a compilation of peatland initiation ages across Alaska to examine Holocene peatland dynamics and climate sensitivity. We find that 75% of dated peatlands in Alaska initiated before 8,600 years ago and that early Holocene C accumulation rates were four times higher than the rest of the Holocene. Similar rapid peatland expansion occurred in West Siberia during the Holocene thermal maximum (HTM). Our results suggest that high summer temperature and strong seasonality during the HTM in Alaska might have played a major role in causing the highest rates of C accumulation and peatland expansion. The rapid peatland expansion and C accumulation in these vast regions contributed significantly to the peak of atmospheric methane concentrations in the early Holocene. Furthermore, we find that Alaskan peatlands began expanding much earlier than peatlands in other regions, indicating an important contribution of these peatlands to the pre-Holocene increase in atmospheric methane concentrations.
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ABSTRACT: Radical restructuring of the terrestrial, large mammal fauna living in arctic Alaska occurred between 14,000 and 10,000 years ago at the end of the last ice age. Steppe bison, horse, and woolly mammoth became extinct, moose and humans invaded, while muskox and caribou persisted. The ice age megafauna was more diverse in species and possibly contained 6× more individual animals than live in the region today. Megafaunal biomass during the last ice age may have been 30× greater than present. Horse was the dominant species in terms of number of individuals. Lions, short-faced bears, wolves, and possibly grizzly bears comprised the predator/scavenger guild. The youngest mammoth so far discovered lived ca 13,800 years ago, while horses and bison persisted on the North Slope until at least 12,500 years ago during the Younger Dryas cold interval. The first people arrived on the North Slope ca 13,500 years ago. Bone-isotope measurements and foot-loading characteristics suggest megafaunal niches were segregated along a moisture gradient, with the surviving species (muskox and caribou) utilizing the warmer and moister portions of the vegetation mosaic. As the ice age ended, the moisture gradient shifted and eliminated habitats utilized by the dryland, grazing species (bison, horse, mammoth). The proximate cause for this change was regional paludification, the spread of organic soil horizons and peat. End-Pleistocene extinctions in arctic Alaska represent local, not global extinctions since the megafaunal species lost there persisted to later times elsewhere. Hunting seems unlikely as the cause of these extinctions, but it cannot be ruled out as the final blow to megafaunal populations that were already functionally extinct by the time humans arrived in the region.Quaternary Science Reviews 06/2013; 70:91–108. · 4.57 Impact Factor
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ABSTRACT: Northern peatlands have accumulated large carbon (C) stocks, acting as a long-term atmospheric C sink since the last deglaciation. How these C-rich ecosystems will respond to future climate change, however, is still poorly understood. Furthermore, many northern peatlands exist in regions underlain by permafrost, adding to the challenge of projecting C balance under changing climate and permafrost dynamics. In this study, we used a paleoecological approach to examine the effect of past climates and local disturbances on vegetation and C accumulation at a peatland complex on the southern Seward Peninsula, Alaska over the past ∼15 ka (1 ka = 1000 cal yr BP). We analyzed two cores about 30 m apart, NL10-1 (from a permafrost peat plateau) and NL10-2 (from an adjacent thermokarst collapse-scar bog), for peat organic matter (OM), C accumulation rates, macrofossil, pollen and grain size analysis.A wet rich fen occurred during the initial stages of peatland development at the thermokarst site (NL10-2). The presence of tree pollen from Picea spp. and Larix laricinia at 13.5–12.1 ka indicates a warm regional climate, corresponding with the well-documented Bølling–Allerød warm period. A cold and dry climate interval at 12.1–11.1 ka is indicated by the disappearance of tree pollen and increase in Poaceae pollen and an increase in woody material, likely representing a local expression of the Younger Dryas (YD) event. Following the YD, the warm Holocene Thermal Maximum (HTM) is characterized by the presence of Populus pollen, while the presence of Sphagnum spp. and increased C accumulation rates suggest high peatland productivity under a warm climate. Toward the end of the HTM and throughout the mid-Holocene a wet climate-induced several major flooding disturbance events at 10 ka, 8.1 ka, 6 ka, 5.4 ka and 4.7 ka, as evidenced by decreases in OM, and increases in coarse sand abundance and aquatic fossils (algae Chara and water fleas Daphnia). The initial peatland at permafrost site (NL10-1) is characterized by rapid C accumulation (66 g C m−2 yr−1), high OM content and a peak in Sphagnum spp. at 5.8–4.6 ka, suggesting the lack of permafrost. A transition to extremely low C accumulation rates of 6.3 g C m−2 yr−1 after 4.5 ka at this site suggests the onset of permafrost aggradation, likely in response to Neoglacial climate cooling as documented across the circum-Arctic region. A similar decrease in C accumulation rates also occurred at non-permafrost site NL10-2. Time-weighted C accumulation rates are 21.8 g C m−2 yr−1 for core NL10-1 during the last ∼6.5 ka and 14.8 g C m−2 yr−1 for core NL10-2 during the last ∼15 ka. Evidence from peat-core analysis and historical aerial photographs shows an abrupt increase in Sphagnum spp. and decrease in area of thermokarst lakes over the last century, suggesting major changes in hydrology and ecosystem structure, likely due to recent climate warming.Our results show that the thermokarst–permafrost complex was much more dynamic with high C accumulation rates under warmer climates in the past, while permafrost was stabilized and C accumulation slowed down following the Neoglacial cooling in the late Holocene. Furthermore, permafrost presence at local scales is controlled by both regional climate and site-specific factors, highlighting the challenge in projecting responses of permafrost peatlands and their C dynamics to future climate change.Quaternary Science Reviews 03/2013; 63:42–58. · 4.57 Impact Factor
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ABSTRACT: The Arctic has experienced much greater warming than the global average in recent decades due to polar amplification. Warming has induced ecological changes that have impacted climate carbon-cycle feedbacks, making it important to understand the climate and vegetation controls on carbon (C) dynamics. Here we used the Holocene Thermal Maximum (HTM, 11–9 ka BP, 1 ka BP = 1000 cal yr before present) in Alaska as a case study to examine how ecosystem Cdynamics responded to the past warming climate using an integrated approach of combining paleoecological reconstructions and ecosystem modeling. Our paleoecological synthesis showed expansion of deciduous broadleaf forest (dominated by Populus) into tundra and the establishment of boreal evergreen needleleaf and mixed forest during the second half of the HTM under a warmer- and wetter-than-before climate, coincident with the occurrence of the highest net primary productivity, cumulative net ecosystem productivity, soil C accumulation and CH4 emissions. These series of ecological and biogeochemical shifts mirrored the solar insolation and subsequent temperature and precipitation patterns during HTM, indicating the importance of climate controls on C dynamics. Our simulated regional estimate of CH4 emission rates from Alaska during the HTM ranged from 3.5 to 6.4 Tg CH4 yr−1 and highest annual NPP of 470 Tg C yr−1, significantly higher than previously reported modern estimates. Our results show that the differences in static vegetation distribution maps used in simulations of different time slices have greater influence on modeled C dynamics than climatic fields within each time slice, highlighting the importance of incorporating vegetation community dynamics and their responses to climatic conditions in long-term biogeochemical modeling.Quaternary Science Reviews 01/2014; 86:63–77. · 4.57 Impact Factor
Rapid deglacial and early Holocene expansion of
peatlands in Alaska
Miriam C. Jones1and Zicheng Yu
Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015
Edited by James P. Kennett, University of California, Santa Barbara, CA, and approved March 11, 2010 (received for review October 2, 2009)
Northern peatlands represent one of the largest biospheric carbon
(C) reservoirs; however, the role of peatlands in the global carbon
cycle remains intensely debated, owing in part to the paucity of
detailed regional datasets and the complexity of the role of
climate, ecosystem processes, and environmental factors in con-
trolling peatland C dynamics. Here we used detailed C accumu-
lation data from four peatlands and a compilation of peatland
initiation ages across Alaska to examine Holocene peatland
dynamics and climate sensitivity. We find that 75% of dated
peatlands in Alaska initiated before 8,600 years ago and that early
Holocene C accumulation rates were four times higher than the
rest of the Holocene. Similar rapid peatland expansion occurred in
West Siberia during the Holocene thermal maximum (HTM). Our
results suggest that high summer temperature and strong season-
ality during the HTM in Alaska might have played a major role in
causing the highest rates of C accumulation and peatland expan-
sion. The rapid peatland expansion and C accumulation in these
vast regions contributed significantly to the peak of atmospheric
methane concentrations in the early Holocene. Furthermore, we
find that Alaskan peatlands began expanding much earlier than
peatlands in other regions, indicating an important contribution of
these peatlands to the pre-Holocene increase in atmospheric
climate seasonality|Holocene thermal maximum|peatland carbon|
climate change (1). Of particular concern and considerable
debate is the long-term effect of climate warming on soil carbon
(C) pools (2–5). Numerous studies have documented that
warming negatively impacts soil C storage by increasing respi-
ration and decomposition (2, 4, 6). However, long-term effects of
warming on C storage remain controversial (2, 3), in part
because these studies only cover relatively short time scales.
Furthermore, most of these studies were performed in mineral
soils, and few studies consider long-term climate sensitivity of C
storage in organic-rich peat soils (5), which represent up to one-
third of the global soil C pool (7). In peatlands, climate warming
has the potential to increase net C accumulation by stimulating
net primary productivity (NPP) but also decrease it through
greater ecosystem respiration (including decomposition of old
peat C) (8). Peatlands accumulate carbon where productivity is
greater than the rate of decay, which occurs when the soil is
waterlogged and water tables are relatively stable (8, 9). Satu-
rated soils are necessary for the existence of peatlands, but the
role of moisture in peatland C accumulation remains unclear. On
relatively short time scales, water table depth manipulations have
not produced consistent results (10, 11), and numerous studies
have shown stronger responses of C dynamics to temperature
than moisture changes (10–13).
Most modern peatlands formed during the Holocene and thus
represent a significant terrestrial carbon sink over this period (14,
15), as well as a methane (CH4) source (16, 17). It is well known
ngoing and future warming at high latitudes has generated
significant interest in terrestrial carbon-cycle feedbacks to
CH4increase (16, 17), but results from previous studies do not
understood. Previous peatland data synthesis studies do not
identify climatic mechanisms of peatland expansion, in part
because of the broad geographic reach and variable regional cli-
climatic controls on peatland C dynamics. Here we place late-
glacial and Holocene peatland C dynamics in Alaska into the
context of the regional climate history through detailed peat-core
analysis, focusing on the early Holocene, a time when summer
temperatures were higher than the 20th century average, winter
temperatures were lower, and conditions were drier overall, as
indicated by low lake levels (19). In addition, we will examine the
connection between deglacial increases in atmospheric CH4and
expansion of Alaskan peatlands.
period of warm climate in the high latitudes (19), attributed to an
orbitally induced increase in summer insolation and a decrease in
winter insolation (20). However, the HTM exhibits a spatio-
temporal asymmetry across the northern hemisphere, owing to
effects of the remnant Laurentide ice sheet and the large thermal
inertia of the ocean (19). Extensive peatlands exist in Alaska and
parts of Siberia (16, 17), each covering almost the same areas of
596,000 km2(21) and 592,440 km2(22), respectively. These two
with maximum seasonality in insolation and presumably temper-
ature (19, 23, 24), making these ideal locations for studying the
effects of climate seasonality and temperature on peatland C
dynamics. We also examine temporal patterns of C accumulation
from four peatlands on the Kenai Peninsula, Alaska, where the
climate at the present is semicontinental due to the rain shadow
precipitation to interior Alaska.
Our analysis of peat basal dates shows a steady increase in the
number of newly formed peatlands across Alaska (Fig. 1) begin-
peatland formation occurred from 12 to 8.6 ka, with a peak ini-
tiation at 10.5 ka, concomitant with the highest insolation sea-
sonality (Fig. 2A). By 8.6 ka, 75% of modern Alaskan peatland
area (63% of total basal dates) formed (Fig. 2C), followed by a
6-fold decrease in the rate of new peatland formation.
the Holocene, we calculated C accumulation rates based on peat-
core data from four peatlands in south-central Alaska (Fig. 2B).
Author contributions: M.C.J. and Z.Y. designed research; M.C.J. performed research; M.C.J.
analyzed data; and M.C.J. and Z.Y. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
1To whom correspondence should be addressed. E-mail: firstname.lastname@example.org.
This article contains supporting information online at www.pnas.org/cgi/content/full/
| April 20, 2010
| vol. 107
| no. 16
(grams of carbon per square meter per year) from 11.5 to 8.6 ka,
four times higher than the average rate of ∼5 g C m−2a−1over the
rest of the Holocene. Visual examination of peat cores and mac-
rofossil analysis of all four cores showed well preserved peat in the
for mid- to late-Holocene peat (Fig. S1) (30).
Rapid Peatland Expansion and Carbon Accumulation During the HTM.
coincides almost exactly with maximum insolation seasonality and
of peatland initiation occurs concomitantly with decreasing inso-
lation and climate seasonality, suggesting that a strong link exists
between temperature seasonality and peatland development.
Comparable explosive peatland expansion in the early Holocene
Holocene climate and seasonality as in Alaska, but with a slightly
later peak in summer temperatures at ∼9 ka (23, 24), roughly
concurrent with themaximumpeatlandexpansionthere(Fig. 2C).
Our data show both the most rapid peatland expansion and C
accumulation during the early Holocene, a period that was pre-
viously deemed unsuitable for peatland development in high
the Kenai Peninsula (Fig. 2B), Holocene C accumulation rates
show a remarkably similar pattern despite differences in vegeta-
tion composition (30) and timing of peatland initiation (Fig. S1).
Peat accumulation rates are controlled by both autogenic and
allogenic factors. Under a steady climate, the rate of peat accu-
mulation is sometimes higher initially and slows down over time,
especially in continental peatlands (32, 33), as the peat surface
also as continued decomposition of the deeper anoxic peat layers
reduces hydraulic conductivity (33). However, our records show
well preserved early Holocene peat, despite botanical differences
of these records initiated during the late-glacial period (∼14 ka)
but did not begin significantly accumulating peat until the early
Holocene, presumably when the climate was more favorable.
These observations rule out autogenic peatland processes as the
dominant factor in controlling C accumulation at these four sites.
groupings, including the deglacial (>15 ka), the Bølling-Allerød (15–13 ka), the Younger Dryas (13–11.5 ka), the early Holocene (11.5–10 ka and 10–8.6 ka), the
mid-Holocene (8–6 ka and 6–4 ka), and the late Holocene (4–2 ka and <2 ka). Paleo-shorelines are depicted with dotted blue lines (adapted from ref. 25). The
location of the Inset is outlined by a dashed-line. (Inset) Four peatland sites on the Kenai Peninsula used for carbon accumulation rate curves (Fig. 2B). 1,
Swanson; 2, No Name Creek; 3, Kenai Gasfield; 4, Horse Trail.
Digital elevation model (DEM) of Alaska. Dots indicate peatland sites with basal dates. Colors and sizes indicate paleoclimatically significant age
| www.pnas.org/cgi/doi/10.1073/pnas.0911387107Jones and Yu
The timing of peat C accumulation rate changes in these four
cores coincides with known changes in Holocene climate in
Alaska. High peat accumulation coincides almost exactly with
the HTM in Alaska. Although low lake levels in Alaska during
the early Holocene suggest overall dry conditions (19, 34), Kenai
Peninsula peatlands, which at the present experience a semi-
continental climate and receive nearly as little summer precip-
itation as interior Alaska because of the rain shadow effect of the
Kenai Mountains, do not appear to have been drought stressed.
They likely received sufficient water input from glacial meltwater
from the Kenai Mountains, whose glaciers receded beyond their
modern limit during the early Holocene (35), or from sufficient
late summer precipitation (36). In other regions of Alaska where
peatlands were expanding, glacial ablation, thermokarst for-
mation, and thawing of ice wedges (19) provided sufficient
moisture for rapid peatland expansion. The end of the HTM,
characterized by a less seasonal cooler, wetter climate, coincides
with a decrease in C accumulation rates in the Kenai lowland
Mean C accumulation rates
(g C m-2 a-1)
C accumulation rates
(g C m-2 a-1)
GISP2 CH4 (ppbv)
Frequency of thermokarst
Percent of cumulative
thermokarst lake formation
Frequency of peatland
Percent of cumulative
Alaska & Siberia
Alaska & Siberia
% Cumulative Alaska
No Name Creek
maximum summer insolation and minimum winter insolation in the early Holocene around 10 ka. We assume that an increase in insolation reflects an
increase in temperature and vice versa (26). (B) Carbon accumulation rates (g C m−2a−1) for four peatland sites on the Kenai Peninsula, Alaska [left axis for
individual sites, right axis for the mean of the four sites (dots ± SEs)]. (C) Frequency of basal dates as expressed in 50-year bins of 2σ calibrated age ranges from
peatlands across Alaska (gray; n = 284), Siberia (light gray; n = 182) (16, 27), and the sum of Alaska and Siberia (black; n = 466). The highest frequency of basal
dates (49% of total) falls within the period of maximum insolation seasonality (∼11.6–8.6 ka), as shown by the vertical shaded bar, which also corresponds
with the HTM in Alaska (19). The axis corresponding with these bars is located to the right. Solid black line, cumulative percentage of Alaska peatland sites;
dashed red line, cumulative percentage of peatland sites in both Alaska and Siberia. The axis for these lines is located on the left. Both regions have similar
peatland areas, so the curves also represent their weights and impacts equally. (D) Frequency of thermokarst lakes from Russia (n = 38), Alaska (n = 20), and
western Canada (n = 11) (28). Red line, cumulative percentage of thermokarst lake formation. (E) Atmospheric methane (CH4) concentrations for GISP2 core in
Greenland (29). The HTM in Alaska is indicated by the red vertical band. Climate intervals in (E) include the Younger Dryas (YD), Bølling-Allerød (B-A),
Termination 1A, and Termination 1B.
Alaskan peatland C dynamics, seasonal climate controls, and the global connection. (A) Summer and winter insolation curves from 60°N (20) showing
Jones and YuPNAS
| April 20, 2010
| vol. 107
| no. 16
cores. Particularly low accumulation rates beginning ∼4 ka cor-
respond with the onset of neoglaciation in Alaska (35), which has
been shown to have decreased and even stopped peat accumu-
lation processes in other boreal regions such as Siberia (37).
Net C accumulation is a function of, and long-term difference
between, NPP and ecosystem respiration (including peat C
decomposition). NPP is controlled primarily by summer temper-
ature and growing season length, whereas respiration is controlled
mostly by soil temperature and waterlogged conditions, with
aerobic respiration occurring at a higher rate than anaerobic res-
piration (38). Although moisture is an important control on eco-
system respiration rates, recent studies have documented that
temperature increases generate a stronger response in CH4and
CO2fluxes than water-table changes in modern peatland manipu-
lation studies (11, 12, 39, 40). Warmer temperatures have been
shown to significantly increase NPP (41), suggesting that longer,
increases in snow depth (42, 43) can increase respiration rates
enough to turn an ecosystem from a C sink to a source (43). A
during the early Holocene would have significantly reduced winter
respiration rates, resulting in greater C sequestration, a climate
winter and a strengthening of the subtropical high in summer (44).
The high temperature seasonality during the early Holocene in
Alaska was likely similar to the continental climate that char-
acterizes several important modern peatland regions, including
western Canada and the West Siberian lowlands. These two
regions experience warm summers and cold winters and mod-
erate rates of precipitation, and have the highest average rates of
Holocene peat accumulation of all northern high latitude peat-
land regions (8). The difference in timing of the HTM across the
northern boreal regions (19, 24) allows for further examination
of the role of the HTM on peat C accumulation rates. The
cooling effect of the Laurentide ice sheet in eastern Canada
delayed the HTM until 5–3 ka, a time that corresponds with high
C accumulation rates there (8). It should be noted that the
timing of the HTM in eastern Canada is out of synch with
maximum insolation seasonality, and true examination of the
role of early Holocene seasonality must take this into account.
This hypothesis can be tested in the southern hemisphere where
the maximum insolation seasonality and HTM timing occur at
5–2 ka (20). Although data are sparse, one peatland record from
Patagonia appears to show higher peat accumulation rates at this
time compared with the rest of the Holocene (45).
Role of Northern Peatlands in Controlling Atmospheric Methane
Concentrations. Although peatlands represent a significant C res-
ervoir, they also are a source of CH4to the atmosphere (16, 17).
Atmospheric CH4concentrations began increasing duringthe last
deglaciation, with two large and abrupt increases, one at the start
of the Bølling-Allerød (Termination 1A) and another at the
remains the subject of much debate (28, 29, 31, 46–49). Several
hypotheses have been proposed, the first of which suggests that
releases from methane hydrates caused the atmospheric CH4
increase (46). This hypothesis is considered by some to be
increasingly unlikely (47, 48). A second hypothesis suggests that
extensive wetland development caused the increase atmospheric
CH4, particularly the abrupt increase at the beginning of the
Holocene (16, 17, 48). Finally, a recent study (28) proposes that
thermokarst lake formation in Siberia, Alaska, and northwestern
Canada during the early Holocene explains most of the atmos-
pheric CH4increase and maintains that the northern peatland
basal date synthesis curve (17) lacks the rapid early Holocene
increase evident in thermokarst lake formation (Fig. 2D). The lag
in peatland initiation dates may partially be explained by the large
ice sheet dynamics and thermal inertia in the North Atlantic
delayed the onset of Holocene warming (19).
Alaskan peatlands began gradually expanding ∼18 ka, almost
5,000 years before Siberian peatlands (Fig. 2C) and >1,000 years
a lack of an ice sheet over most of Alaska. The gradual increase in
Alaskan peat basal dates corresponds with the beginning of the
increase in atmospheric CH4concentrations, which suggests that
these peatlands contributed to the initial deglacial increase in
atmospheric CH4concentrations. The interpolar gradient in CH4
concentrations at this time implies that a northern wetland source
must exist (49), but no sharp increase in peatland area in Alaska is
observed at Termination 1A (∼14.9 ka) to explain the sharp
factors may have contributed to that observed sharp CH4rise.
(12, 39, 40), it is conceivable that the warm Bølling-Allerød tem-
peratures could have increased CH4emissions in existing peat-
lands at that time, even with no additional new peatland
decrease the residence time of CH4in the atmosphere (50). It is
also likely that by that time, peatland expansion had begun to the
south of the Laurentide ice sheet and Europe (17). In addition, it
remains possible that the CH4increase at Termination 1A was
50). It is also possible that thermokarst development increased as
temperatures rose (28), or that subglacial methane was released
from retreating ice sheets (51).
The rate of Alaskan peatland expansion does not decrease
during the Younger Dryas (YD), and therefore it cannot explain
the reduction in atmospheric CH4concentrations. Closer exami-
nation of the spatial expansion pattern (Fig. 1) shows a lower rate
of expansion on the North Slope of Alaska but continued expan-
sion in south-central and eastern Alaska during the YD, a pattern
confirmed by a detailed peatland and paleoclimate analysis from
the Arctic Foothills (52). The decrease in peat expansion on the
North Slope is attributed to colder, drier conditions (52), whereas
YD cooling with greater southerly atmospheric flow, a pattern
simulated by numerous climate models (53–55). The smooth
may have altered sea ice extent and atmospheric circulation pat-
terns (Fig. 1) (55) to allow for continued peatland initiation. If
cooling slowed peat formation and halted thermokarst develop-
ment during the YD because of colder conditions across much of
the ice-free boreal region, then these changes could explain the
decline in atmospheric CH4concentrations. A recent isotopic
analysis of methane from the Greenland GRIP ice core suggests
as a result of a decrease in wetland area (50), but these changes
may have largely occurred outside of Alaska. A portion of the
methane change can also be explained by biomass burning, which
was likely lower during the YD (50).
During the early Holocene, expansion of Alaska peatlands
preceded the expansion of Siberian peatlands by almost 1,000
years and occurred during the period of low thermokarst lake
formation at the beginning of the Holocene, suggesting that
Alaskan peatlands may have contributed most to the initial early
Holocene increase in atmospheric CH4concentrations at Ter-
mination 1B (11.6 ka). The delayed timing of peatland devel-
opment in Siberia can be explained by dry conditions caused by
the diversion or dismantling of westerly air masses by the Eur-
asian ice sheet (56) and may also explain the delayed onset of
thermokarst lake formation, because the majority of sites are
from Russia (28). The slight difference in timing of initiation
| www.pnas.org/cgi/doi/10.1073/pnas.0911387107Jones and Yu
ages in these vast peatland areas, in addition to the increased
rate of thermokarst lake formation (28), may help explain the
broad early Holocene peak in atmospheric CH4concentrations.
By comparing peatlands and thermokarst lakes from the same
region of Siberia and Alaska, where warmer-than-present
summer temperatures (19) correspond to maximum insolation
seasonality (Fig. 2A), we find that 70% of the combined Alaskan
and West Siberian peatlands (Fig. 2C) developed by 8.6 ka,
similar to thermokarst lake pattern (28) (Fig. 2D). We suggest
that extremely rapid expansion of peatlands in Alaska and
Siberia (16) during the early Holocene represents a significant
contribution to the peak CH4concentrations in the early Hol-
ocene. If we conservatively assume that the average early Hol-
ocene rate of peat accumulation was 15 g C m−2a−1and that the
rate of peatland area expansion corresponds with the frequency
of basal dates as a percent of total peatland area (Fig. 2C), we
find that Alaskan peatlands would have sequestered 14.8 Pg of C
between 11.6 and 8.6 ka. This suggests that Alaskan peatlands
contributed significantly to the global soil carbon stock and that
the previous estimates of 29–58 Pg of C uptake from all northern
boreal peatlands in the early Holocene (17) are likely highly
conservative. Although CH4emissions from peatlands are highly
variable, if we assume a conservative rate of 9 g CH4m−2a−1
(11), we estimate that Alaskan peatlands emitted 3 Tg CH4a−1
during the early Holocene. If we assume a percent total of basal
dates by 8 ka represent the percent of peatland area present,
then we estimate that Alaskan peatlands contributed between 3
and 5 Tg CH4per year, based on the estimate of 20–45 Tg CH4
released every year by present boreal peatlands (57). Our esti-
mate is conservative, because many of these peatlands likely
began as minerotrophic fens, which emit more CH4than the
oligotrophic peatlands found more often on the landscape today.
In addition, the effect of warm early Holocene temperatures
likely also contributed to greater CH4 emission (11, 40). By
combining the Siberia and Alaska peatland datasets (Fig. 2C), an
abrupt decline in peatland expansion is observed at 8.6–8.2 ka,
slightly earlier in Alaska than in Siberia, corresponding with a
nearly 100-ppbv decrease in atmospheric CH4 concentrations
(Fig. 2E), which is attributed to the 8.2 ka cooling event (31).
Although the established peatland area did not decrease, the
cooler climate, combined with the drastic decrease in the rate of
new peatland formation, may partially explain the decrease in
The current distribution of peat basal dates is sparse over
much of Siberia (8, 16) and much of the lowland area in Alaska
(Fig. 1), suggesting that sampling of these vast areas may help us
to better understand the impact these peatland regions had on
information about whether peatlands formed by paludification
(peatland initiated or expanded onto uplands) or by terrestrial-
ization (lake-infilling process) will improve our understanding of
climate controls—specifically, increases or decreases in precip-
itation—on peat formation processes (8).
Implications for Carbon-Cycle Feedback to Present Climate Change.
Our data from Alaskan peatlands, along with Siberian peatland
data (16), indicate that peatlands responded strongly to the
heightened seasonality in the early Holocene by sequestering
large amounts of C as well as emitting significant quantities of
CH4. Early deglacial peatland development in Alaska may help
explain the early increase in atmospheric CH4concentrations, but
the gradual increase in peatland initiation cannot explain the
sharp increases and decreases in atmospheric methane concen-
trations over the Bølling-Allerød and Younger Dryas periods,
suggesting that an additional northern hemisphere wetland
source outside of Alaska contributed to the marked CH4changes,
orthat changes in CH4production within existingpeatland during
these known climatic intervals contributed to the CH4concen-
tration changes. Peatland expansion in Alaska is well timed with
increasing insolation and temperature seasonality. Earlier studies
pointed to the role of greater summer insolation and warm cli-
mate on early Holocene peatland expansion (16, 17), but our
present study is, to our knowledge, the first to suggest that winter
processes may also play an important role in carbon accumu-
lation. Specifically, colder winters with low snowfall may have
decreased peat temperatures to significantly reduce winter eco-
system respiration during the early Holocene. Our data suggest
that high early Holocene temperature seasonality played a pri-
mary role in controlling the high rates of peatland C accumu-
lation, and adequate moisture is necessary to maintain the
presence and persistence of peatlands, but it does not determine
the rate of C accumulation in these peatlands. Although we show
that peatlands expandedand accumulated carbon under a climate
warmer than today, we emphasize the importance of strong
temperature seasonality in peat C accumulation at that time. As
opposed to the early Holocene, recent and projected warming in
high-latitude regions is most pronounced in winter and autumn
seasons, owing to strong positive snow and ice feedbacks (1).
Although our study implies that Alaskan peatlands responded
necessarily imply that peatlands will increase long-term C storage
under current climate warming, particularly if warmer winters
increase snow depths in these peatland regions (1), which would
likely increase carbon loss from decomposition.
We compiled 284 basal peat
sources and our own dating results (Table S1) across Alaska (Fig. 1) to assess
the temporal pattern of peatland initiation and expansion in a region where
the well established warm early Holocene climate (19) is concurrent with
maximum seasonality (20). Basal peat14C dates (Table S1) were calibrated to
their 2σ age ranges using the program Calib 5.0 based on the INTCAL04
calibration dataset (58), and the calibrated 2-sigma age ranges were placed
into 50-year bins. This was done to account for older bulk dates with larger
calibrated age ranges and potentially imprecise mean ages. The number of
sites in each bin was tallied to generate Fig. 2C. The percentage of peatland
area was calculated based on cumulative numbers of these 50-year bins.
Carbon accumulation rates (Fig. 2B) are based on 1-cm measurements of C
content and bulk density obtained through combustion (loss-on-ignition)
and dated by 9–13 AMS14C dates for each of the four cores (Table S2). The
mean of the four sites was calculated for each 1,000-year bin using time-
weighted averaged C accumulation rates for each core, and errors are
standard errors of the mean.
14C dates from both previously published
ACKNOWLEDGMENTS. We thank Thomas Ager, Edward Berg, Robert Booth,
Daniel Brosseau, Shanshan Cai, Andrew Gonyo, and Dorothy Peteet for field
and laboratory assistance and for sharing data; and David Beilman and two
anonymous reviewers for providing helpful comments that improved the
manuscript. This work was supported by National Science Foundation Grant
ATM 0628455 (to Z.C.Y.).
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| www.pnas.org/cgi/doi/10.1073/pnas.0911387107Jones and Yu