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Quaternary Science Reviews 26 (2007) 300–311

Temporal and spatial aspects of peatland initiation followingdeglaciation in North America

Eville Gorhama, Clarence Lehmana,�, Arthur Dykeb, Joannes Janssensc, Lawrence Dykeb

aDepartment of Ecology, Evolution and Behavior, University of Minnesota, St. Paul, MN 55108, USAbNorth Canada Division, Geological Survey of Canada, Ottawa, ONT., Canada K1A OE8

cLambda-Max, 1061 25th Avenue S.E., Minneapolis, MN 55414, USA

Received 3 December 2005; received in revised form 13 July 2006; accepted 14 August 2006

Abstract

A set of simple ecological models accounts well for the cumulative initiation of peatlands throughout North America in relation to

glacial retreat. The most parsimonious form incorporates, first, a delay term to account for the lag during which newly deglaciated land

became suitable for peatland initiation and, second, an intrinsic rate of initiation related to the probability of migration and

establishment of plant propagules from elsewhere. The goodness of fit of the models, based on 1680 basal-peat dates throughout the

continent, allows projection of past trends into the future. Factors contributing to the lag of about 4000 years between deglaciation and

peatland initiation are suggested and data on colonization of deglaciated land by beavers (known to initiate peatlands) are presented. The

rate of peatland initiation peaked between 7000 and 8000 years ago, but remains appreciable today. A marked depression of peatland

initiation (8360–8040BP) interrupted the peak rate. The time of the interruption matches the 8200BP cold–dry event recorded in

Greenland ice cores, and suggests that this event caused a substantial, continent-wide depression of an important ecosystem function, i.e.,

carbon sequestration from the atmosphere by peat deposition. Spontaneous initiation of new peatlands is projected to continue for

millennia to come.

r 2006 Elsevier Ltd. All rights reserved.

1. Introduction

Peatlands—bogs and fens—are important features ofnorthern landscapes, making up about 12% of theCanadian land mass (Zoltai, 1998). They are uniqueecosystems that serve as the primary habitats for a varietyof plants and animals, some of them threatened orendangered species and subject to considerable bioticimpoverishment owing to human activities (Gorham,1990). Their role in the global carbon cycle, and theirlikely responses to climatic warming, have recently receivedmuch attention (Gorham, 1991, 1995; Harden et al., 1992;Bridgham et al., 1995; Clymo, 1998; Clymo et al., 1998). Inaddition, some consideration has been given to regionalpatterns of peatland initiation since the retreat of thecontinental ice-sheet from Quebec (Payette, 1984) andcentral Canada (Zoltai and Vitt, 1990; Halsey et al., 2000).

e front matter r 2006 Elsevier Ltd. All rights reserved.

ascirev.2006.08.008

ing author. Tel.: +1612 625 5734; fax: +1 612 624 6777.

ess: lehman@umn.edu (C. Lehman).

The aim of this paper is to examine, on a much broadercontinental scale, the pattern and timing of peatlandinvasion following deglaciation, and in particular the lagbetween the two processes and the rate at which peatlandinitiation has taken place over time. These deduced lagsand rates can be applied to projections of peatlandinitiation in upcoming millennia.

2. Methods

The pattern and timing of peatland initiation wasestablished by compiling from the literature 1680 radio-carbon dates of basal peats (excluding underlying lakesediments when identified as such) throughout Canada andUnited States during the past 21 400 calibrated years. Thesedates were converted to calendar years before present (BPin text) according to Stuiver and Reimer (1993). In thispaper these 1680 sites are referred to as ‘‘sample peat-lands.’’ (Table available as online Supplementary dataat doi:10.1016/j.quascirev.2006.08.008.) The pattern and

ARTICLE IN PRESSE. Gorham et al. / Quaternary Science Reviews 26 (2007) 300–311 301

timing of deglaciation was established from paleogeo-graphic maps of glaciated North America (Dyke, 2004).Modeling of peatland initiation in relation to melting of theice-sheet was accomplished by fitting parameters numeri-cally to a set of abstract ecological models representingvarious levels of simplicity (described in on-line appendix).

Fig. 2. Glacial extent (lightly shaded), freshwater (darkly shaded), and

peatland distribution (open circles) in 16 200BP.

3. Results

No peatlands of the age of the last glacial maximum wereidentified south of the ice sheet or in Beringia. By19 650BP, when less than 1% of the ice-sheet had melted,only two of the sample peatlands had formed, both close tothe ice margin (Fig. 1). One was in New Jersey and theother west of present Lake Michigan. By 16 200BP, whendeglaciation reached 12%, there were 18 dated peatlands(Fig. 2) distributed around the southern and western edgesof the ice-sheets as far north as Alaska and the Yukon,with southern extensions to the northern Coloradomountains and Texas (not shown). By 14 700, deglaciationhad reached 20%, the Cordilleran corridor had opened,and the number of dated sites had reached 52, but peatlanddistribution was much the same, with several more sitessouth of the ice and in Alaska, and an additional southernsite in an alpine region in Nevada (Fig. 3).

By 12 000BP the ice-sheet was just over half gone (51%),there were 235 dated sites, and peatlands had been initiatedfar up the Atlantic coast to New Brunswick and New-foundland, filling in between Vancouver Island and Alaskaon the Pacific coast, to the base of the Aleutian Islandchain in Alaska (not shown), east of the Mackenzie Riverin the Northwest Territories, and to Banks Island in theArctic Archipelago (Fig. 4). Peatlands were absentthroughout most of the vast western area vacated by theCordilleran ice-sheet, although they were present in south-ern British Columbia and Washington. New peatlands had

Fig. 1. Glacial extent (lightly shaded), freshwater (darkly shaded), and

peatland distribution (open circles) in 19 650BP.

Fig. 3. Glacial extent (lightly shaded), freshwater (darkly shaded), and

peatland distribution (open circles) in 14 700BP.

also formed in North Dakota, beyond earlier sites a littlewest of present Lake Superior, and in alpine Wyoming.By 8900BP deglaciation had reached 75%, the number

of dated sites had reached 605, and peatlands had extendedfar out in the Aleutian Island chain (not shown), well intothe Arctic archipelago (Victoria, Prince Patrick, andBathurst Islands), and to an island off the northwest coastof Greenland (Fig. 5). Many sites had been occupied westof Great Bear and Great Slave Lakes, southeastward toLake Athabasca and a little west of Reindeer Lake inSaskatchewan and west of Lake Manitoba. On the Atlanticcoast peatlands had formed in Labrador, opposite thenorthern tip of Newfoundland, and as far south asVirginia.By 7400BP, with 95% of the ice-sheet gone, Hudson Bay

ice-free, and 845 dated sites, peatlands had extended to

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Fig. 4. Glacial extent (lightly shaded), freshwater (darkly shaded), and

peatland distribution (open circles) in 12 000BP.

Fig. 5. Glacial extent (lightly shaded), freshwater (darkly shaded), and

peatland distribution (open circles) in 8900BP.

Fig. 6. Glacial extent (lightly shaded), freshwater (darkly shaded), and

peatland distribution (open circles) in 7400BP.

E. Gorham et al. / Quaternary Science Reviews 26 (2007) 300–311302

Ellesmere Island, Devon Island, and Somerset Island in theArctic, to the north end of Baffin Island, still with an ice-sheet, and down the west coast of Greenland to itssouthern tip (Fig. 6). (We did not include Iceland in ourstudy area.) Several peatlands had formed east of GreatBear and Great Slave Lakes, Lake Athabasca, andReindeer Lake. They had also reached northward fromthe Great Lakes to a little south of James Bay. On theAtlantic coast peatlands had formed north to HamiltonInlet on the coast of Labrador and south to Georgia (notshown). On the Pacific coast a peatland had formed insouthern Oregon.

At present peatlands extend to northern Ellesmere Islandand northeastern Greenland, the south end of BaffinIsland, and the northern tips of Labrador and Quebec on

the Hudson Strait, as shown in Fig. 11, where sites withfossil beaver remains are also shown. In the interior theyare found near Churchill on the west side of Hudson Bay,down the coast around James Bay in what is now theHudson–James Bay Lowland, and throughout northernQuebec on the east side of Hudson Bay. Far to the south,peatlands are found in southern Florida (not shown). Notethat the extensive peatlands of the Hudson Bay Lowlandsin northwestern Ontario are represented by very few datedsites. The most biogeographically significant gaps inpeatland distribution are on the western Great Plains andin the Great Basin. Thus peatland distribution is mainlylimited by aridity and secondarily by temperature alone.Considering only data from north of 39�N latitude, all

sampled peatlands older than 17 900BP formed in tundrabiomes Table 1). Tundra and forest–tundra collectivelyhosted more than 50% of peatland-initiation sites through11 600BP and remained important environments of initia-tion throughout later postglacial time. After 16 800BP, theboreal forest and similar biomes (e.g., subalpine forest)became significant environments of peatland initiation andbecame the dominant environments after 11 600BP.Throughout the period of record, boreal forest and‘‘colder’’ biomes account for more than 70% of peatlandinitiation sites. The rainforest of the Pacific coast (CoastForest) hosted about half of the ‘‘warmer’’ sites, and themixed forest of east-central North America most of theremainder.

4. Models of glacial retreat and peatland invasion

With 1680 dates of basal peats throughout 1373 000 km2

of peatlands (mostly above 40�N latitude) in Canada(Tarnocai et al., 2004) and the United States (Bridghamet al., 2001), it is possible to model the pace of peatlandinitiation over time in relation to the retreat of the glacial

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Table 1

Percentage representation of biomes supporting peatland initiation north of 39� latitude in North America during the postglacial period (Dyke et al., 2004)

Age Tundra Forest

tundra

Boreal/Conifer

forest

Mixed/Deciduous

forest

Coast

forest

Interior

forest

Grass-land/

Steppe

N

Cal BP 14C

19 000 16 000 100 – – – – – – 2

17 900 15 000 100 – – – – – – 1

16 800 14 000 71 – 29 – – – – 7

15 500 13 000 37 16 37 – 11 – – 19

14 000 12 000 42 15 25 3 14 – – 59

13 000 11 000 59 6 28 1 3 – – 68

11 600 10 000 42 10 37 6 2 1 2 123

10 100 9000 33 7 48 4 4 3 1 179

8900 8000 24 3 46 12 12 2 1 172

7800 7000 12 – 56 16 10 3 3 147

6800 6000 16 5 52 20 8 1 – 177

5800 5000 11 10 54 14 7 1 3 166

4500 4000 20 10 44 17 7 – – 189

3200 3000 14 10 47 20 7 1 1 169

2000 2000 18 9 48 18 8 – – 114

900 1000 24 12 46 11 7 – – 94

50 0 24 6 55 4 10 – – 49

N is the number of data points in the sample.

E. Gorham et al. / Quaternary Science Reviews 26 (2007) 300–311 303

ice-sheets. In doing so, we treated each basal date asrepresenting a separate peatland initiation. Although morethan one basal date may represent a single deposit today,particularly in the very extensive, patterned complexes ofcontinental bogs and fens (Glaser, 1989), those complexesprobably coalesced from multiple origins. We also pro-ceeded from the simplest possible model to more complexversions that incorporated additional parameters.

We began by plotting both the cumulative numbers ofpeatland initiations and the cumulative area of thedeglaciated land over time from the glacial maximum21 400 years ago to the present day. The simplest modelassumes that the total number of peatlands at any timedepended solely on the amount of land freed by glacialretreat up to that time. (We included here a small amountof fresh water in glacial lakes, because some of theseeventually drained and permitted peatlands to form, andbecause the pattern of deglaciation is less irregular if suchfreshwater is included. The picture is the same in eithercase). In this simplest equilibrium model the number ofpeatlands, NðtÞ, at time t is directly proportional to thearea of deglaciated land at that time, expressed as

NðtÞ ¼ gAðtÞ. (1)

Here AðtÞ is the area deglaciated at time t and g is thenumber of sample peatlands that can form per unit ofdeglaciated area. The factor g is related in turn to theproportion of the area that is available for peat deposition.No value of g allowed this simplest model to fit the dataadequately ðR2 ¼ 0:71Þ. (Note that the fraction g is reallythe product of two factors, (1) the fraction of deglaciatedlandscape that is ultimately suitable for peatland formationmultiplied by (2) the probability that a peatland is included

in the set of 1680 peatlands sampled here. These twofactors always appear together, so we represent them by thesingle term g in this paper.)We refined the first model by recognizing that not all

land freed by the glaciers was immediately suitable for peatdeposition, so that a delay term must be added to theequation to account for a lag during which newlydeglaciated land became conditioned for peatland initia-tion. (Several reasons for such a delay will be dealt withlater.) The model then becomes

NðtÞ ¼ gAðt� tÞ, (2)

where t is the delay period and Aðt� tÞ is the deglaciatedarea available t years before the time under consideration,a proportion of which would then be suitable for peatdeposition. This model, like the previous one, is anequilibrium model in that the total number of peatlandsis assumed to be in equilibrium with the total deglaciatedarea at a specified time. Fitted curves for this model areshown in Fig. 7A, where both the deglaciated area and thecumulative number of peatland initiations are approxi-mated by sigmoidal curves, along with a rate curve forpeatland initiation that shows a maximum 8000 years ago.Such a simple model fits surprisingly well ðR2 ¼ 0:9962Þ,although there are noticeable deviations of the modelledcurve (dashed line in Fig. 7A) from the actual data. Thedeviations are most apparent before 10 000BP, from 7000to 4000BP, and especially from 2000BP to the present.Two further refinements were explored to improve the fit

and to examine the importance of basic ecologicalprocesses. The first proposed that the rate of peatlandinitiation was an important factor, being associated withpropagule dispersal and related probabilistic events, and

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Fig. 7. Solutions to models of deglaciation, peatland initiation, and rate

of peatland initiation, with sigmoidal area approximations. Open circles

represent the deglaciation data of Dyke (2004), fitted by a solid sigmoidal

curve that is piecewise polynomial; the numerous individual dots forming

a bolder curve represent the actual data, one dot per sample peatland,

fitted by a dashed line representing the solution to the corresponding

model; and the solid line with a mid-postglacial maximum represents the

calculated rate of peatland initiation. (A) Equilibrium model with time lag,

Eq. (2). (B) Non-equilibrium model incorporating a lag and a global rate

of establishment, Eq. (3). Fig. 7B also represents Eq. (4), which includes a

local rate that is indistinguishable from 0.

Table 2

Characteristics of the four models

Eq. R2 Fraction Delay Rate Rate

g t r1 r2

1 0.7135 116.8 0 – –

2 0.9962 162.6 3969 – –

3 0.9998 196.1 1342 0.000229 –

4 0.9998 196.1 1342 0.000229 0.000000

E. Gorham et al. / Quaternary Science Reviews 26 (2007) 300–311304

being dependent partly on the amount of available landthat was not already occupied by peatlands. This non-equilibrium model also incorporates a delay term:

dN

dt¼ r1ðgAðt� tÞ �NðtÞÞ. (3)

The term dN=dt is the rate of peatland formation at time t,in units of sample peatlands initiated per unit time (e.g., peryear). NðtÞ is the cumulative number of sample peatlandsinitiated up to time t. Because gAðt� tÞ is the number ofsites suitable for formation of one of the sample peatlands,the factor gAðt� tÞ �NðtÞ represents the number ofsuitable sites at time t in which peat formation has notyet initiated. This number is multiplied by r1, an intrinsicrate of initiation related to migration to the site andestablishment of peatland plants from elsewhere, expressedas a rate per unit time. The fit of this model to the actualdata is remarkable (Fig. 7B, R2 ¼ 0:9998), with themodelled curve (dashed line in Fig. 7B) being visiblebeneath the actual data only where the data diverge slightlyfrom uniformity (e.g., brief intervals around 8000 and2000BP) or where the data are sparse (before 16 000BP).

This solution fits very well the high end of the peatlandcurve at the present time, making future projections moresecure. This model shows the rate of initiation peakingslightly later, between 7000 and 8000 years ago.The second refinement proposed that peatland plant

propagules became more readily available as peatlandscovered more and more of the deglaciated landscape, i.e.,the process of initiation was to some degree autocatalytic.This formulation incorporates two rates, one the back-ground rate, r1, in the previous model and the other, r2, anadditional rate of propagule migration to the site fromrecently established peatlands throughout the NorthAmerican continent. Rate r1 has the same units as above,but rate r2 is measured as a rate per unit time per samplepeatland initiated. This model is

dN

dt¼ ðr1 þ r2NðtÞÞðgAðt� tÞ �NðtÞÞ. (4)

This model (Eq. (4)) does not noticeably improve the fitover the previous model (Eq. (3)), for the best-fit value ofthe local rate r2 is not significantly different from zero. Inother words, this model, after being fitted to the data,essentially reduces to Eq. (3).Three other treatments of deglaciation data were tried.

One case assumed a piecewise linear fit to the data between17 500 and 6000 years ago, a second applied a periodic areaapproximation to correct a slight cyclical pattern in theresiduals over time, and a third used an exact interpolationof the actual glaciation data. None of these treatmentsimproved on the goodness of fit shown in Fig. 7.Table 2 presents the characteristics of the four models.

Models 2–4 all performed well, but with differentcharacteristics. The R2 values for these three seem onlymarginally different (0.9962 vs. 0.9998), but R2 values arecompressed asymptotically toward 1.0, and the closeness ofthese two values belies a noticeable difference in how thecorresponding curves fit the data (Fig. 7A vs. B). Theresidual variance, which is not compressed toward 1, is905.1 for Model 2 but only 58.3 for Models 3 and 4,indicating a large difference in fit. The Akaike InformationCriterion (AIC), which considers the number of parametersas well as the goodness of fit, is smallest for Model 3,indicating that Model 3 is the most parsimonious.The second equilibrium model (Fig. 7A) shows a much

longer delay than the non-equilibrium Models 3 and 4.That delay (3969 years) matches the gap of approximately

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4000 years that is obvious between the deglaciation andpeatland initiation curves in Fig. 7. The non-equilibriummodels, however, reveal that the 4000-year gap mayactually have two components, (1) a primary lag duringwhich newly deglaciated sites became suitable in one wayor another for peat deposition, and (2) a secondary lagresulting from the finite probability that a suitable sitewould actually be colonized by peat-forming species.Estimates of the two components always summed toapproximately 4000 years, but their individual valuesdepended on the modelled rate of glacial retreat. Variousreasonable estimates of the primary lag ranged from 1000to over 2000 years, with the best-fit value being 1342 years(Table 2). Even though precise values for these lags cannotbe determined from the models at hand, their appearancenonetheless suggests that the establishment of peatlandpropagules at suitable sites includes a very large compo-nent of chance.

Model 3 provides an excellent fit over the entire rangesince deglaciation began, including the most recent 2000years (Eq. (3); Fig. 7B). Projecting forward, that modelindicates that the present rate of peatland initiationremains large, amounting to 35% of the peak rateapproximately 8000 years ago. If these trends continueunchanged, 2000 years hence peatlands will still beinitiating at over 20% of the peak rate. More than 8000years will pass before the rate drops to 5% of its maximumand 15 000 years before it drops to 1%. Global warming is,however, likely to alter the trend, perhaps substantially,with both positive and negative effects in different regions(Gorham, 1991, 1995).

5. Discussion

5.1. Rate of peatland initiation throughout North America

Throughout North America the rate of initiation ofpeatlands peaked between 7000 and 8000 years ago(Fig. 7), when the landscape was 90% deglaciated andice-sheets remaining in Nunavut and northern Quebec wereseparated by the Hudson Strait. The most reasonableexplanation for the subsequent decline is that by this timemany of the topographically suitable sites had beenoccupied and relatively few remained to be exposed byfurther glacial retreat or other geomorphic processes. Thisis analogous to the Type 3 pattern of range extension byindividual species invading new territory (Shigesada andKawasaki, 1997). It is noteworthy that between about15 000 and 12 000BP much of the land opened up was theresult of the melting of the Cordilleran ice-sheet, in an areanot well suited to peatland initiation because of steepslopes and a rather dry climate; this probably contributedto the lag. It may also be significant that the distinctly aridmid-Holocene period in continental North America about9000 to 4000BP (Bradbury et al., 1993) coincided roughlywith a shift to ever-lower rates of peatland initiation,perhaps contributing to their decline.

To some extent the early rate of peatland initiation,before about 12 000BP, may be under-estimated owing toour exclusion of peat layers, often 0.5m thick, buried underthe mineral sediments deposited as a result of glacialreadvances during the Younger Dryas interval, or of slopeinstability. Deposits of this sort are known mainly fromMaritime Canada. Old buried peats have also beenobserved in Virginia (Daniel, 1981). Later rates of peatlandinitiation may also be somewhat underestimated becausethere have been few peat cores taken in the Hudson BayLowland, the second largest peatland in the world, whereinitiation had to wait upon gradual isostatic rebound of itsland mass from Hudson Bay.The peak rate of peatland initiation also must influence

the peak rate of carbon accumulation, because the rate ofpeat accumulation at individual sites, although variable inthe short term, appears to have been more or less constantover the long term (Gorham et al., 2003).

5.2. The lag of peatland initiation in relation to deglaciation

We should consider first of all why there were so fewpeatlands south of the ice-sheet and in Beringia at theglacial maximum (Fig. 1). One very important reason mustbe that unglaciated landscapes and areas of ancient (pre-Wisconsinan) glaciation lack the diversity of topographi-cally suitable sites for peatland initiation that are createdby deglaciation. In Beringia, despite pervasive permafrost,it was apparently too dry for peatlands to form. Eventoday peatlands are very scarce south of 40�N latitude (Fig.7). During the early stages of deglaciation peatlands wereinitiated close to the margins of the extensive ice-sheet(Figs. 1–3), but by 12 000BP, when the Cordilleran ice-sheet had disappeared, relatively few were at all close to theretreating ice (Fig. 4).Table 2 indicates that the best-fit lag of 3969 years in

model 2 (Fig. 7A) is reduced to 1342 years in models 3 and4 (Fig. 7B). This suggests that probabilistic eventsassociated with the background rate, r1, including migra-tion and establishment of propagules of the dominantpeatland plants, required a surprisingly long time, up toalmost 2700 years on average, leaving the remainder of thedelay to result from several other factors discussed below.The fitted local rate, r2, of Eq. (4) is indistinguishable from0 compared with the background rate, r1. Rate r2 ismeasured as a rate per unit time and per sample peatlandalready established. Therefore, the units of r1 are compar-able to units of the product r2NðtÞ. Because NðtÞ representsup to 1680 sample peatlands, the comparable value of r2will be roughly three orders of magnitude smaller than r1.Nonetheless, the fact that the rate r2 vanishes in the fittedequations suggests that colonization from the peatlandsnewly established across the continent did not provide amajor increment of propagules—the background rate r1and the delay factor t being sufficient to explain, bythemselves, the overall accumulation of peatlands inrelation to glacial retreat. Globally dispersing plants such

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Fig. 8. Number of beaver fossils cumulated from 20 000BP. Deglaciation

data are the same as in Fig. 8. The exponentially rising curve is the actual

number of beaver fossil sites (open circles) and the corresponding best-fit

exponential curve (dashed line).

Table 3

Dates by which percentiles of deglaciation, peatland initiation, and beaver

colonization were reached

Percentile Years BP

Deglaciation Peatland initiation Beaver colonization

25 15 500 10 300 5004

50 10 250 7430 2035

75 8900 4630 828

E. Gorham et al. / Quaternary Science Reviews 26 (2007) 300–311306

as Sphagnum could be important components of thebackground rate of peatland initiation included in r1, butlocally dispersing plants such as Carex, colonizing from aline along the fringe of areas becoming suitable forpeatland formation, could also contribute. Thus, thesemodels provide information on possible sources of peat-land propagules that contributed to the lag in peatlandestablishment, but do not distinguish all types of coloniza-tion.

An immediate cause of the lag may well have been thehigh pH, alkalinity, and salinity of the waters that firstpercolated through the ground-up rock flour left behind asthe ice retreated. They could have inhibited establishmentof any peatland plant propagules reaching a site at thattime. The effect, however, is likely to have been short-lived,as soil surfaces were leached by falling rain and meltingsnow. Engstrom et al. (2000) reported that a chronose-quence of lakes on strongly calcareous glacial drift atGlacier Bay, Alaska, left behind after the retreat of the ice,showed very high initial salinity ðmax � 6:2mequivL�1Þ,calcium ðmax � 50 gL�1Þ, and pH ðmax � 8:8Þ, between 10and 200 years after ice-out, but a sharp decline thereafter.

Among other, more important lag factors was thedrainage of large postglacial lakes (Figs. 4 and 5) that leftbehind large, flat sediment-filled basins suitable for theprocess of paludification by which localized peatlands damthe drainage and spread laterally upslope from initial focalpoints to merge into vast fen/bog complexes. These hugelakes formed over about 4000 years, then drained suddenlywith the opening of Hudson Strait to the ocean around8450BP (Barber et al., 1999).

The focal points for peatland expansion elsewhere maywell have been numerous small, ice-block lakes or kettles,which had to fill in with lake sediments before peatlandscould develop. Although our database includes relativelyfew of these, they may have provided multiple foci (nowburied) for many of the vast peatland complexes of fen andbog that are characteristic of the subarctic and especiallythe boreal zone (Zoltai et al., 1988a, b; Glaser, 1989).Because of their small size, they are unlikely to have beenlocated in coring these peatlands, which are relatively flat(Vitt et al., 2000).

A factor likely to have been important in causing thelong lag in initiation was the slow leaching of iron incoarse, sandy podzol (spodosol) soils in humid climates,which precipitates relatively impermeable ‘‘pans’’ at thesurface of the B horizon (Fitzpatrick, 1986), therebyretarding drainage and allowing peatland plants to invadeand swamp out upland heaths and forests (Pearsall, 1950;Ugolini and Mann, 1979). Further contributing to the lagin southeastern coastal areas, rising sea levels graduallyreduced water-table gradients, impeding runoff of streamsowing to sediment buildup. This led to peat deposition inVirginia by about 10 000BP (Whitehead, 1972; Daniel,1981), and in Florida by about 6000BP (Gleason andStone, 1994). In contrast, isostatic rebound of the landmass after ice retreat led to very extensive peatland

formation around the shores of Hudson and James Bayover the past 7000 years (Tarnocai, 1982). Here paludifica-tion was favored by the establishment of permafrost duringemergence, which removed the drying effect of greaterelevation.Another factor, believed by Sjors (1963) to have been

important in providing local foci for wetland formationfollowed by paludification upslope, was the damming ofstreams by beaver (see Davis and Anderson, 2001, for anillustration), very active in the boreal zone. Radiocarbondating of 500 beaver (Castor) fossils from a wide variety ofsites indicates that their numbers were not detectablebefore about 17 000BP, but then grew approximatelyexponentially through the period of deglaciation to thepresent (dashed curve and corresponding data points inFig. 8). Whereas half the ice-sheet was gone by about10 300BP, and half of the peatlands had initiated by about7430BP, fossil-beaver numbers reached the halfway markmuch later, about 2000BP (Table 3), by which time almostall peatlands had been initiated.Although beaver remains may not preserve as well as the

plant remains in peat, it seems likely that beaver were muchslower than peatland plants to colonize the landscape freedup by ice retreat. Figs. 9–11, showing the ice margins andthe distribution of peatlands and beaver fossils at 9500,

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Fig. 9. Glacial extent (lightly shaded), freshwater (darkly shaded), and the

distribution of peatlands (open circles) and beaver fossils (crosses) in

9500BP.

Fig. 10. Glacial extent (lightly shaded), freshwater (darkly shaded), and

the distribution of peatlands (open circles) and beaver fossils (crosses) in

3200BP.

Fig. 11. Glacial extent (lightly shaded), freshwater (darkly shaded), and

the distribution of peatlands (open circles) and beaver fossils (crosses) at

present.

E. Gorham et al. / Quaternary Science Reviews 26 (2007) 300–311 307

3200BP, and the present, indicate that peatlands were to befound much closer to the ice margins than were beaver.

5.3. Peatland response to the 8200 BP event

A distinct cold event, known as the 8200BP or 8 kaevent, is recorded in Greenland ice-sheet cores (Alley et al.,1997). This event spanned the interval from approximately8400 to 8000BP and has now been recorded at numeroussites around the Northern Hemisphere (see review by Alleyand Agustsdottir, 2005). In Greenland ice cores the eventwas characterized by a temperature drop of about 7 �C,similar in magnitude to the maximum global warmingprojected for this century (Cubasch et al., 2001). It was

accompanied by a drop in snow accumulation rateindicating locally drier winter conditions, an increase inwind-blown particles indicating increased wind erosionupwind of Greenland, an increase in micro-charcoalindicating increased forest fires and presumably driersummers upwind of Greenland, and a decrease in methaneindicating a decreased flux of that gas to the atmospherefrom methane-producing wetlands in the Northern Hemi-sphere, if not globally. Barber et al. (1999), by revising themodern radiocarbon reservoir correction for Hudson Baywaters, brought the deglaciation of Hudson Bay and thecatastrophic drainage of giant glacial Lake Agassiz–Ojib-way into reasonable correlation with the onset of the8200BP event by placing it at 8450� 100BP. They thusprovided a plausible mechanism for the cooling; i.e., a shut-down of the North Atlantic oceanic heat pump because ofthermohaline stratification of the ocean due to the suddeninflux of freshwater from Hudson Strait, which was150 000 km3 from the lake drainage alone (Clarke et al.,2003, 2004), enough fresh water to raise ocean levelsapproximately 0.4m globally. Alley and Agustsdottir(2005) emphasized in their review that the 8200BP eventwas a dry anomaly as well as a cold one.Fig. 12 illustrates the number of peatlands initiated per

century. It shows a steep decline in the rate that lasted (Fig.13) from about 8360 to about 8040BP, almost at the heightof peatland initiation rates. The number of peatlandsinitiated fell to about one-third the number initiated in the320-year periods immediately preceding and following thisevent, and to about one-half of the modelled rate.Considering the precisions of the various dating methods,this event in the peatland record is indistinguishable intiming from the event in the Greenland ice cores and itsonset is indistinguishable from the timing of the drainageof Lake Agassiz–Ojibway. Note that this is the largest

ARTICLE IN PRESS

02468101214161820

Cal BP (×103)

0.0

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Pea

tland

s In

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Fig. 12. Number of peatlands initiated per century from 20 000BP to the

present, centered around the century (e.g., 8150–8250BP). Joined arrows

mark the lull in peatland initiation associated with the outburst flood from

Lake Agassiz–Ojibway.

Fig. 13. Number of peatlands initiated from 8600 to 7900BP, showing the

lull in initiation between about 8360 and 8040BP.

E. Gorham et al. / Quaternary Science Reviews 26 (2007) 300–311308

short-term apparent depression of peatland initiation ratesin our record and it has been a persistent feature of therecord as the database has grown from a few hundredsamples to its present size. A simple interpretation is thatatmospheric drying over northern North America wassufficient for effective depression of peatland initiation inmost areas during that interval. Furthermore, lowering ofwater tables in already formed peatlands may havedepressed methane production in the manner seen in theGreenland records. Of the 14 peatlands that formed duringthe ‘‘driest’’ part of this interval (lasting about 200 years),eight formed at coastal sites, one in an alpine setting, one inthe deciduous forest south of Lake Michigan, and three atcontinental interior sites where peat formed above lakesediments, possibly because of lowered water levels.

We also considered whether the depression of wetlandformation could have been a direct hydrological responseto the drainage of Lake Agassiz–Ojibway. We speculatedthat the lake had generated regionally high water tables far

beyond its margins and that the subsequent drop of watertables had inhibited peatland initiation. Preliminarymodeling suggests, however, that water-table changeswould have been strongly controlled by local slope effectsrather than by drainage of the glacial lake and that noregional lowering of water tables would have occurredbecause of glacial lake extinction.Others (Shuman et al., 2002; Seppa et al., 2003) have

observed several century-scale changes in vegetation, asreflected by the pollen record, at this time. Some long-termchanges in vegetation also appear to have been triggered bythis event (see also Dean et al., 2002). The depression ofpeatland initiation observed here reflects a marked changein ecosystem function, i.e., reduced sequestration of carbonfrom the atmosphere into peat deposits. Peatlands con-stitute a very important component of the global carboncycle, sequestering an amount of carbon that is more thanhalf the 720 Pg in the earth’s atmosphere (Cubasch et al.,2001), and accounting for almost one-fifth of all organicbiological sinks: 400 Pg in peat compared to 1100–1200 Pgin non-peaty soils and 600–700 Pg in vegetation (Woodwellet al., 1995; Powlson, 2005). Climate changes of themagnitude occurring today can, therefore, affect thebiogeochemical functioning of earth’s ecosystems.Fig. 12 reveals other declines in peatland initiation that

deserve future investigation, notably around 4000BP, atime of widespread drought in continental North America(Booth et al., 2005).

5.4. The biogeochemical significance of peatland initiation

Increasing coverage of the landscape by peatlandscaused the build-up of large stores of carbon, nitrogen,and sulfur in peat (Gorham, 1995). Some of the carbonwould have been released in dissolved form, acidifyingstreams and lakes draining the peatlands (Gorham et al.,1986, 1998), often rather suddenly once poor fens and bogsbegan to develop above initially circumneutral fens(Gorham and Janssens, 1992). Peat deposits must alsohave had a significant influence upon the atmosphericconcentrations of greenhouse gases, including negativeeffects by sequestering carbon dioxide in the process ofpeat accumulation, and positive effects by releasingmethane produced by anaerobic decomposition in water-logged subsurface peats into the atmosphere. It has,therefore, considerable importance in the study of globalclimate change throughout this interglacial period and intothe future (Gorham, 1991, 1995).

6. Comparison with other studies

Payette (1984), studying 60 basal dates in subarcticQuebec, reported a time lag of 1500 to 2000 years betweendeglaciation and peatland initiation, which he suggestedwas linked to a change from a drier to a more humidclimate there. The most rapid period of initiation wasabout 6300–4200BP, and the rate declined thereafter

ARTICLE IN PRESSE. Gorham et al. / Quaternary Science Reviews 26 (2007) 300–311 309

owing, he suggested, to occupation of the most favorablesites. Harden et al. (1992), in a modeling study of thedynamics of soil carbon during deglaciation in NorthAmerica, indicated a lag of about 4000 years, with the mainperiod of deglaciation at 12 000–8000 BP and the mainperiod of carbon sequestration in peat at 8000–4000BP.

Halsey et al. (1998) did a rather detailed modeling studyof peatland invasion in western Canada, using 90 basaldates. They concluded that the factors favoring invasionwere, in order of importance, low thermal seasonal aridity,minimal topography, and coarse substrate textures. Halseyet al. (2000) provided a generally similar picture of theinvasion of the bog moss Sphagnum following thedeglaciation of North America, based on the presence ofspores in 323 records from lake sediments as well as peatdeposits, inferring the presence of Sphagnum in nearbypeatlands in both cases.

Vitt et al. (2000), in a detailed study in continentalwestern Canada, reported that peatlands there beganaccumulating carbon about 10 200BP (converting fromtheir 14C dates). The rate of storage then was low, whichthey ascribed to maximum insolation during the earlyHolocene, but it rose sharply thereafter, with about half ofthe current carbon accumulated by 4425BP. They indi-cated that the subsequent slowdown was caused byextensive permafrost development in subarctic and borealCanada and peatlands reaching their southern limit in themidcontinent. They also pointed out that carbon storagecontinues in the region.

Turunen (2003) reported that peatland initiation peakedbetween 10 500 and 8000BP in the aapa peatlands ofnorthern Finland under a cool moist climate, whereas inthe raised bogs of southern Finland there were two peaks,an earlier one between 10 500 and 8500BP and a later oneabout 5000–3000 years ago, with an upward excursion alsoat 2000–1500 years ago. The low rate of initiation between8500 and 5000BP was ascribed to drier climatic conditions.

Smith et al. (2004) used 226 basal peat dates in the WestSiberian Lowland, the largest peatland complex in theworld, to examine initiation there. They reported an‘‘explosive, widespread peatland establishment between11 500 and 9000BP,’’ long before our peak rate at about8000BP in North America, which was followed by a sharpdecline to lower rates throughout the following millennia(but with occasional temporary increases). Kremenetski etal. (2003), using a smaller set of 139 basal dates, connectedthe early peak of initiations to the postglacial warming ofnorthern Siberia owing to high summer insolation, the finaldeglaciation of Fennoscandia, and the warming of theNorth Atlantic Ocean. The later decrease coincided withthe onset of warm dry conditions in the mid-Holoceneperiod.

7. Research opportunities

An important gap in the spatial coverage of peatlandinitiation is the Hudson Bay Lowland, the largest peatland

in North America but with very few basal peat dates. Therelative influence of the several factors responsible for thelong lag between peatland initiation and ice retreat alsodeserves investigation.We have available depths for 1336 of the basal peats for

which we have radiocarbon dates, which will allow us toconstruct a relation between depth and age. Using averagedata for bulk density and carbon content, we can thenestimate the accumulation of carbon from the atmosphereby peatlands during the postglacial period, and also projectthe trend of accumulation into the future. That future trendwill, however, be liable to considerable change owing toglobal warming, also a very important topic for furtherresearch on the role of peatlands in the global carbon cycle.With respect to modeling, we have accurate spatial data

for all the sites shown on the maps, which will allowspatially explicit modeling to reveal more of the mechan-isms and timings related to peatland colonization followingdeglaciation. Coupled with data on depths of peat, suchmodeling will allow reasoned regional projections ofcarbon sequestration from the past into the future.

Acknowledgments

We thank Janneke Hille-Ris-Lambers, Adrienne Keen,Ben Kerr, Fred Lehman, Richard McGehee, HeleneMuller–Landau, Andrew Moore, Claudia Neuhauser,Louis Robertson, Shelby Williams, and two anonymousreviewers for helpful advice and assistance. Paul Glaserkindly contributed several basal-peat dates, unpublished atthe time.

Appendix A. Supplementary data

Supplementary data associated with this article can befound in the online version at doi:10.1016/j.quascirev.2006.08.008.

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