Stratigraphy and floristics of Eocene swamp forests from Axel

1914
Stratigraphy and floristics of Eocene swamp forests from Axel Heiberg Island,
Canadian Arctic Archipelago
Dnvrn R. GnseNwoool
lNo huBs
F. BnsrNcnn
Department of Geological Sciences, University of Saslcatchewan,Sasl<atoon,SK S7N 0W0, Canada
Received March 22, 1993
Revision acceptedAugust 3I, 1993
A record of polar Eocene forests is preserved as in situ tree-stump fields and leaf-litter mats in Buchanan Lake Formation
sediments on Axel Heiberg Island, in the Canadian Arctic Archipelago. Stratigraphic examination at the centimetre to metre
scale of peat-coal lithology and macrofossil floristics in two levels of thesefossil forests reflects small-scale changesin forest
composition and swamp hydrology horizontally and temporal variation vertically. Root system structure and tree base stratigraphy suggestthat exposedtree stumps may not include only coeval individuals of a single forest stand, but rather also individuals representing different phasesof the forest through one cycle of the hydrological development of this Eocene polar forest
community. Earlier calculations of stand density and biomass, based upon the assumption that all stumps represent coeval
trees, may therefore be greatly overestimated. A mosaic of Alnus - fern bog, mixed coniferous community, and taxodiaceous
(Metasequoia-Glyptostrobus) swamp appears to have produced both the leaf mats and the in situ stumps, with the taxodiaceous swamp the dominant peat-accumulating phase.
Les s6dimentsde la Formation de Buchanan sur I'ile Axel Heiberg, dans I'archipel de I'Arctique canadien, renferment
des champs de souches d'arbres et des tapis de litiEres de feuilles in situ qui document I'existence d'une for0t polaire Dr
I'EocEne. L'6tude stratigraphique, i I'echelle du centimbtre au mbtre, des unitds lithologiques de tourbe-houille et de la flore
macrofossilifdre repr6sentantdeux niveaux de ces for0ts fossilis6es reflEte i petite 6chelle des changementshorizontaux dans
la composition des espCcesforestidres et de I'hydrologie des marais, et verticalement des variations temporelles. La structure
du systdme de racines, jointe i la stratigraphie fond6e sur les espbcesd'arbres, suggdre que les souchesd'arbres expos6es
ne repr6sentent pas uniquement des sp6cimens contemporains d'un peuplement forestier unique, mais plut6t des sp6cimens
qui appartiennent i diff6rentes phases d'une for6t soumise i un cycle de d6veloppement hydrologique qui a affect6 cette
communaut6 forestidre polaire de I'EocEne. Les dvaluations faites auparavant de la densit6 et la biomasse du peuplement
6taient fond6es sur I'hypothtse que toutes les souchesreprdsententuniquement des arbres contemporains, par cons6quentelles
sont fortement surestim6es. Une mosarque formde d'Alnus - fougbres de tourbibre, d'une communaut6 mixte de conifdres
et d'un marais i Taxodiacdes (Metasequoia-Glyptostob,ns) a vraisemblablement produit les tapis de feuilles et les souches
in situ, et c'est dans ce marais i Taxodiac6esqu'est apparue la plus importante phase d'accumulation de la tourbe.
[Traduit par la r6daction]
Can.J. EarthSci.30, l9l4-1923 (1993)
Introduction
Fossil forests are assemblagesof tree stumps preserved in
their growth positions at a single stratigraphic level (Jefferson
1982; Francis l99l). Such autochthonousplant fossil assemblages provide information on the stand structure and synecology of ancient forest communities (e.g., Fowler et al.
1973; Jefferson 1982; Collinson and Scoff 1987; Francis
l99l; Taylor et al. 1991). In contrast with allochthonousfluviolacustrineplant fossil deposits, fossil forests preserve patterns of abundance and biomass of forest ecosystems
approximately intact, as well as spatial relationships of some
of the forest constituents. Recent studies on high-latitude
fossil-forest standsindicate stem densities of between 325 and
484 trees/h for EoceneArctic forests (Francis l99l) and 258
trees/h for Triassic Antarctic forests (Taylor et al. l99l). A
key problem in determining stem densities, spatial relationships,and biomass,however, is time-averaging;that is, determining whether the in situ stumps were all alive at the same
time and, in turn, killed and preserved at the same time.
If treesfrom within a single continuouslygrowing forest die
at different times and are preserved over a geologically short
time interval (500-1000 years), later compression of the
swamp-peat may bring tree stumps from different time intervals (i.e., lived and died at different times) into stratigraphic
juxtaposition. Subsequenterosion may exposeall of thesestumps
rPresentaddress: Department of Paleobiology, NHB MRC 121,
SmithsonianInstitution, Washington, DC 20560, U.S.A.
Printed in Canada / Imprimd au Canada
and give the impression that all of the stumps represent coeval
members of a single forest community which were killed and
preseryedin a single catastrophicevent. Evaluation of standing biomass (as stems per hectare) and of stand structure and
dynamics, based upon assumptionsthat stumps are coeval,
may therefore be grossly overestimatedand may misrepresent
species composition and relative abundance.
Leaf litter of modern forests reflects the spatial arrangement, canopy composition, and relative biomass (stem basal
area) of the standing forest (Chaney 1924; Ferguson 1985;
Greenwood 1991, 1992; Burnham et al. 1992). Therefore,
autochthonous fossil leaf litter layers will reflect the spatial
arrangementand biomass of the palaeovegetation.Such reconstruction of forest structure is also dependenton whether a litter layer representsdepositionwithin the life of a single forest,
or an accumulation of successivestands. Floristic variation
from point to point must be determined as being caused by
either spatial (lateral facies changes) or temporal (vertical
stratigraphic changes)variation (e.9., Taggart 1988; Barrett
and Christophel 1990; Greenwood l99l).
The standstructure and dynamics of someof the fossil forests
of Axel Heiberg have been briefly examined (Francis l99l;
Basingerl99l). The original studyby Francis(1991)was based
on the unstated assumption that all of the stumps within a
single layer were alive at the same time. Stem densitiesand
biomassestimateswere derived by measuringthe size and position of all exposedstumps. Preliminary examination of the leaf
layers had suggestedthat lateral variation in speciescomposition reflected local community structure but did not address
l9t5
GREENWOOD AND BASINCER
variation with depth within the leaf litter layer (Basinger
1991).
Lithology of peats and low-grade coals can be assessed
basedon colour and the preservationor absenceof recognizable plant macrofossils (Moore 1989; Moore and Hilbert
1992). Differences in coal lithology within a single seam can
be attributed to the former existence of a mosaic of plant communities, controlled in part by local hydrological conditions,
and the plant types present, as well as subsurfacediagenetic
processes(McCabe 1984; Moore 1986, 1987; Collinson and
Scott 1987; Cameron et al. 1989; Moore and Hilbert 1992).
(Modern taxodiaceousswamps in Florida contain mosaics of
different plant subcommunities,reflecting differences in local
hydrology, substrate, and disturbance history (Davis 1946:'
Spackman et al. 1969; Collinson and Scott 1987).) Stratigraphic examination of floristic and lithological character
within seams will therefore demonstrate differences in the
hydrological regime and plant community type within a coeval
surfaceand successionalchangesover time in vertical section
(McCabe 1984; Moore 1986; Cameron et al. 1989).
If the assumptionof coeval stumps is valid, then each stump
should be rooted at the same stratigraphic level, with roots
similarly penetratingthe peat substrate.Variation in floristics
laterally is likely to have been controlled by local hydrology
and therefore may be reflected in local lithological differences
within the peat leaf-litter layer. In a similar manner, temporal
changesin floristics, controlled by either local hydrological
changesor disturbance events, are likely to be reflected by
both changesin peat lithology and floristics with depth through
the peat layer.
Recent fieldwork, reported here, questions some of the
assumptionsupon which earlier analysesof the Axel Heiberg
Eocenefossil forests were based. A detailed lithostratigraphy
(centimetreto metre scale) and spatial changesin floristics of
two laterally extensive peat-coal layers are presented. To
assesswhether thesefossil-forest standsrepresentcoeval trees
or a successionof stands,root systemspatialand stratigraphic
organizationhas been examined. Also consideredis the relationship between individual leaf layers and local spatial and
temporal mosaicsof hydrologic conditions and plant community structure.
Materials and methods
Macrofloras in BuchananLake Formation (upper coal member) sedimentspreservea record of near-polar vegetationand
environmentsnear the Geodetic Hills on Axel Heiberg Island
(Fig. l) in the CanadianArctic Archipelago (Basinger1991;
Mclntyre 1991). Ricketts(1991) interpretsthis sequenceas a
meanderplain fed by alluvial fans. The upper coal member of
the BuchananLake Formation has been dated palynologically
as Eocene (Ricketts and Mclntyre 1986; Mclntyre l99l).
Well-preserved taxodiaceousfossil forests are preserved in
carbonaceouspaludal facies of these sediments (Basinger
l99l; Francis l99l). Rickett's (1991) upper coal member of
the Buchanan Lake Formation is representedat the fossil
forest site by a repeatingsequenceof lacustrine shales,fluvial
and fluviolacustrine sandstonesand sandyclaystones,paleosols,
and peaty coal seams(Fig. 2a) exhibiting varying degreesof
humifaction(Ricketts l99l; Goodarziet al. l99l). Peatycoal
layers at the site were originally labelled (e.9., levels M, N,
and O) in descendingalphabetical series (i.e., M is stratigraphicallyhigher than levels N or O) by Francis (1991), and
this convention is continued here. Each major coal layer is
Heiberg
lsland
F o s s iI
Q Forest
Locality
Frc. l. Locationmap of the Eocenefossil-forestsite, Buchanan
LakeFormation,GeodeticHills area,Axel HeibergIsland,Canadian
Arctic Archipelago.
about 0.5-2 m thick and may be laterally continuousfor more
than2 km (seealso Ricketts l99l). In situ stumpscomprising
"fossil forests" commonly occur within or overlying coal
seams. One of the most extensive fossil forests, level N of
Francis(1991), is embeddedin a coal layer in which mummified leaf litter is clearly recognizable(Figs. 2b and 2c). The
fossil forest of level N and the peat layer of level M are considered here (Figs. 2a and 2b).
To test the assumptionof coeval fossil-forest stump and litter layers of earlier studies(Basingerl99l; Francis l99l), a
seriesof trencheswere dug through vertical edge exposuresof
the in situ stump-litter layer (level N of Francis' nomenclature) (Fig.2b) anda trench dug horizontally within the surface
exposureof this layer (Figs. 2c and 2d). Samplesof the leaf
litter - coal (10 x l0 x 5 cm) were collected in vertical
sequencewithin the edge sectionsto reveal the stratigraphyof
the peat leaf-litter layer. The horizontal trench was dug to
exposeroot systemsof sometree stumpsto reveal their spatial
arrangement, laterally and vertically, within the litter layer
and any lateral and vertical variation in the microtopography
of the peat surface.
The horizontal trench in level N (Figs. 2c and 2d) passed
near several small ( < l0 cm diam. at peat surface) in situ
stumps, a medium-sizedstump (>10-30 cm diam.) and a
large stump (l m diameter). An accesstrench was first dug
40 cm from the stumpsand the peat stockpiledat the site. Slabs
CAN. J. EARTH SCI. VOL. 30. 1993
\i*
-
.!r"
Y' r;
-.|:t-nft
tr'
.,
a
..
\
-
{' -- +'-.'
.-'EiK.
--tr7-'
Frc. 2. Stratigraphyofthe uppercoal memberandthe fossil forest. (4) Photographshowingaltematingsequenceof swamp-forest peat-coal
fossil-forest
site(eastridge,Basingerl99l). Scalebar = l0 m. (r) Photolacustrineshales,andpaleosols;
layers(K-R), fluvial sandstones,
graphshowingexposedhorizontally lying peat(leaf litter) layer with in situ tree stumpsfrom levels N andO (arrowheadsapprox. I m diam.).
The positionsof the vertical stratigraphicsectionsthrough level N are indicatedwith arrowheadsand the numbersI -8; tr, position of the
horizontaltrench. (c) Photographshowinghorizontaltrenchthroughleaflitter layer of level N. Note in situ stumpsand root systems.kn, Position of putativepneumatophoreor taxodiaceous"knee." Personfor scale.(d) Photograph(detail) of trench showing root systemsof in situ
stumps(trenchwidth 0.6 m).
of peat, approximately20 x 20 x l0 cm, were then sequentially removed along a transectand each sample labelled with
its vertical orientation recorded on the sample. After removal
of the peat samples, small spatulasand brusheswere used to
exposethe root systemsof the in situ stumps.
Detailed examinationof the stratigraphicsectionsexposed
by the vertical edge trenchesthrough level N (Fig. 2b, l-8)
were used to assessstratigraphic variation within a 30 m
lateral transect. Floristic variation with depth was assessed
from samplescollected from one of these sections, section 7
(labelled trench G elsewhere),using the procedureoutlined
below.
A lessdetailedtransectfollowed the stratigraphicallyhigher
level M laterally for about 1.5 km to assesslarge-scalevariation in floristics. A traverse followed level M horizontally
"east ridge" to a point
from the fossil forest site proper, along
where level M becametotally obscuredby cover (Fig. 3).
Samplesof coal, 10- 15 x 10 X 5 cm, were collectedfrom
the basal clay-coal contact at about 100-125 m intervals.
The coal sampleswere composedlargely of mummified coniferous material (foliage, cones, and other organs), and so
macrofossilswere extractedby simply dispersingthe dry peat
manually. Material was sorted according to organ type (cone
or other reproductivestructure,foliage, etc.). A semiquantitative analysisof the level M transectis presentedhere using a
five point abundancescale (whole or equivalent recognizable
t9t7
GREENWOOD AND BASINGER
east ridge
Frc. 3. Sketch of the east ridge and the fossil-forest site (/) showing the position of level M and the transect sample points (l - l0).
Fig. 2a was photographedat s and the campsitewas at c. The distancebetweeneach samplepoint from points 3 to l0 is approximately 100 m
and the total distance betweenf and s is 2 km.
organs,see below) used in Quaternarymacrofossilstudies
(Grosse-Brauckmann
1986,p. 608).
i.F
Results
Root systems
The excavationof the horizontal trench revealed intact root
systems for the two small stumps and for the medium-sized
stump (Fig. 2d); however, no evidencefor the root systemof
the large stump was found in the excavatedpeat, with only a
minimal amountof root material remaining at the peat surface.
It would appear that the root system of this individual either
rotted after burial, or the individual was dead prior to burial
and the root systemwas already in an advancedstateof decay.
Clay was found to be domed beneaththis large stump as well
as other large stumps of level N. The root systemsexamined
were consistentwith conifer root systems.The flattenedplanklike appearanceof the major roots (e.g., Fig. 2d) is probably
due to compression,but may also reflect a degreeof planarity
in the living roots that may be indicative of shallow-rooted
trees in swamp conditions (Munaut 1986). A root structure
exposed in excavation strongly resembled a pneumatophore
(or "knee"; Fig. 4) such as those produced by some Taxodiaceae.This structure was geniculate, resembling the form
produced by the modern Glyptostrobus pensilis Koch (Fowler
et al. 1973\, and not conical as seen in Taxodium distichum
(L.) Rich. Similar structureswere found projecting from the
unexcavatedpeat surface.
The positions of the stump baseswere variable within the
peat, but the basal level of the peat (possibly representingthe
establishmentmicrotopographyof the precedingpaleosol)was
also variable. Compactionof the sedimentsmay have contributed to variations in apparentthickness and microtopography
of the peat and peat-clay contact. However, it is likely, given
the very low compaction ratios of these sediments, that the
microtopography of the peat and peat-clay contact reflects
original highs and lows in the swamp forest. Similar microtopographic patterns seen in modern peat swamps correlate
with small-scalefloristic changes,due to differences in water
level.
The roots that radiate from the small to medium stumps
occur at the mid-level of the peat layer; however, a significant
numberof large roots and branch roots extend to lower levels.
Theseroots did not appearto penetratethe basal clay. Significantly, the remnants of the large stump root system encountered in the trench, and additional large stumps nearby, stand
in relief and appearto sit atop the peat layer. However, excavation beneath these stumps demonstrated that they were
underlain by clay, suggestingthat they were rooted directly in
the precedingclay paleosol. The implication is that the largest
"knee" notedin Fig. 2c, exposed
FIc. 4. Closeupof taxodiaceous
in the trenchthroughlevel N. Scalebar : 2.0 cm.
size stumpsmay representtrees that existed throughout much
of the period of peat (leaf litter) accumulation preserved in
level N.
Vertical peat stratigraphy (Ievel N)
Sections through level N revealed interbedding of peatycoal, clayey-coal, coaly-clay, and clay layers (Fig. 5). The
thicknessand lateral continuity of distinct layers were variable,
with individual layers thinning and thickening, anastomosing,
and occasionallydisappearingand reappearing(Fig. 6). Individual peat-coal and clay layers within level N were generally between 6 and 30 cm thick.
Net peat accumulation (as thickness) reflects the result of
gross productivity (as leaf-fall and root systems,in this case),
loss from decay and herbivory, and compaction(Clymo 1983;
Moore 1989; Moore and Hilberl 1992).In thick sequencesof
peat ( > 150 cm), total thicknessof peat may be a poor reflection of original biomassand the rate of accumulation,as decay
and compactionvary with depth (Clymo 1983). Very shallow
peatswill better reflect accumulation, as compaction is more
even and loss through decay is similar at depth. The thickness
of eachpeat layer provides an estimateof accumulationrates,
and thereforeof the period of peat accumulation.This estimate
provides a rough time frame for the fossil forests.
McCabe (1984, 1987) estimatesthat rates of accumulation
r9l8
CAN. J. EARTHSCI. VOL. 30. 1993
Frc. 5. Photographsof vertical sectionsthrough level N. (c) Section 5. (b) Section 7. Peat (leaf mat) layers appear darkest, with coaly-clay
layers gray and mineral-clay layers pale. The pen in the photographs is l0 cm in length. See Fig. 6 for an explanation of lithology.
ffi peat
2
5
r
tr
l---=
coaly clay
I
lctl
0
clay
1
2
","
3
m
4
5
[:"
x 10.
betweenmeasuredsections(l -8), Venical exaggeration
Frc. 6. Compositestratigraphyof level N. Stratigraphyis diagrammatic
Prominentin situ roots (r) were observedat section3 and an in situ stump (s) at the baseof section l. The coal layer found betweenl0 and
20 cm abovethe bas€is stratigraphicallycontinuousover the whole lateral transectand is markedly fossiliferous, b€ing dominatedby angiospermleaves(a) at section2, and 'ich it Chamaecyryis (c) at s€ction8. At section7 (seeTable 2) this layer is dominatedby Metasequoia,
with significant amountsof angiospermleaves,the undescribedTaxodiaceaetaxon, ^trd Glvtostrobut-
TnsLn l. Accumulation rates of peat in Tertiary and modern peat-forming vegetation
Accumulation rate
Type of vegetation
Mixed podocarp and angiosperm swamp
(Miocene)
Mixed podocarp and angiosperm swamp
(Miocene)
Tropical angiosperm swamp forest (modern)
Mixed angiosperm and Taxodiutn swamp
(modern)
Taxodium swamp (modern)
Quoted
60 m / 4.8xIff a
( 0 . 1 3m m ' a - ' )
60 m / lff-lOs a
(0.6-6.0 mm'a-')
2.2-2.8 mm' a-r
I -2 mm'a-l
5.9m l7O00a
( 0 . 8 4m m ' a - r )
Years/IO cm
770
Source
Luly et al. 1980
t7 -170
George 1975
36-46
Anderson 1983
50-100
l2O
Gower et al. 1985
Cohen 1985: Retallack 1990
GREENWOOD AND BASINGER
1919
Tnslr 2. Generalized floristics of level N coal through vertical
for Recentpeats range from 0.1 to 2.3 mm'a-r. Table I
section 7 (trench G)
Tertiary
summarizes peat accumulation for some modern and
forested swamp communities. Accumulation rates for these
examplesrange from 0.13 to 6 mm'a-r. The estimateof
0.13 mm'"-l provided for Miocene podocarp-angiosperm
swampsby Luly et al. (1980) is basedon pollen accumulation
Distance
ratesand pollen density in lignites and may be conservativefor
from base
this swamp community. Similarly, the upper limit of 6 mm'
(cm)
a-l for these Miocene swamps estimated by George (1975)
(1990,
p.
that
272)
suggests
Retallack
as
may be excessive,
2-3
2-3
43
5
peat cannot accumulateunder forest at rates much greater than
2-3
2-3
42
5
I mm'a-I. Accumulation rates for the Axel Heiberg fossil
2- 3
2- 3
5
4l
forest peats of around l-2 mm'a-l seem likely, based on
2-3
2-3
5
40
2-3
2-3
5
accumulation rates in modern taxodiaceousswamps (Gower
39
Clay parting
38
et al. 1985).On this basis, l0 cm of peat in the Arctic Eocene
Clay paning
37
Buchanan Lake Formation fossil-forest sequencesrepresents
Clay parting
36
is
a
between 50 and 100 years of forest development. This
5
35
of
implicit
assumption
is
an
as
there
estimate,
conservative
5
34
productivity and loss through decay or compaction equivalent
5
33
to the Recentexamples.It is possiblethat the time represented 32
+
+
5
by this thicknessof peat is much longer (200-400 years). The
I
+
5
3l
layers of clay and coaly clay may representsingle events, such
I
+
5
30
as a minor flooding, but may also reflect temporary changes
t
5
I
29
Clay parting
28
in the hydrological regime lasting decades or longer. The
I
27
5
whole sequenceof level N deposition, therefore, most prob|
2-3
26
5
ably represents 500- 1000 years of forest development,
+
+
5
25
although much longer time frames (2000 years or more) could
2
5
24
be representedif loss has been excessive.
Clay parting
23
A period of 500- 1000 years for the deposition of a single
Clay parting
22
peat, or leaf litter layer (such as level N), is within the lifeClay parting
2l
span of modern taxodiaceous trees (such as Taxodium and
2
3
5
20
Sequoia). However, the small size of the largest stumps
2
2
5
+
t9
(approx. 2 m diam.) and typical ring widths of 3 mm (Francis
+
4
L-2
4
l
18
+
4
t-2
4
l
t7
l99l) suggestthat the fossil-forest stumps of level N may be
2
5
+
l6
300-400 years old, and therefore potentially younger than the
2- 3
2-3
l5
5
l
lowest horizons of level N. However, these larger stumps in
3
t4
5
level N appearto be rooted directly in the basal clay, whereas
3
l3
5
+
the smaller stumpsappearto be rooted in the peat (seeabove).
3-5
2-3
5
r
2
t
2
At least some of the largest stumpswere deadbefore the fossil
3-5
2-3
ll
5 r-2
forest was buried by the succeedingclay, becausetheir hearts
2-3
3-5
5 r-2
l0
show evidenceof having rotted and in some casesfilled with
3
5 3-5
9
debris (Francis 1991).
5
4
l
8
5
The degree of humification of the coal layers was variable,
2
7
5
2
6
with individual layers composed of essentially leaf litter to
Unidentifiable plant fragments
5-0
predominantly twigs and wood to unrecognizable organic
material. The leaf litters of level N were composedmostly of
Norps: Samplesare I cm apart. +, only a few pieces (l-2 specimens)of
Metasequoia leafy twigs, rarely of broad-leaved angiosperm
tissueremains (e.g., angiospermleaf pieces), representing llVo of material;
with fruits, cones, or seedsof the taxon generally not present; l, l-3Vo of
leaves (mostly Betulaceae,especiallyAlnus), fern fronds and
material, or 3-5 fruits, cones, or seedsof the same taxon; 2,4-10% of
rhizomes, or other (often diverse) assemblagesof conifers,
material. or 5- 14 fruits, cones, or seeds; 3, 10-25% of material, or > 14
such as Pinaceae(e.g., Pseudolarix) and Cupressaceae(Chafruits, cones, or seeds;4,25-50% of material; 5, >50% of material.
l99l)
maecyparis) (LePage and Basinger l99l; Basinger
(Table 2; Fie. 7).
The peat of level N was generally richer in angiosperm
suggests that other conifers (especially Glyptostrobus, an
leaves (principally Alnus) towards the base (7-18 cm above
taxodiaceous conifer, and Chamaecyparis\ may
undescribed
5
on
the
abundance
basal clay) of section 7, ranging from 3 to
scale (10-25% to >50% of material). Other angiosperms dominate in place of either Metasequoia or Alnus (t other
angiosperms)over small areas (2-5 m2) and short stratiwere rarely present,then in small amounts(2-3 on the abungraphic intervals (2-4 cm).
dancescale, or approx. l0% of material) higher in the section
Smaller stumps encountered in the lateral section transects
7
section
was
throughout
(39-43 cm). Metasequoia
dominant
(levels N and M) were most commonly rooted in the top
(Table 2; Fig.7), with the exceptionof three intervals (7, 8,
l0-20 cm of peat, the roots extendingonly as far down as the
and 17- l8 cm) where it was either codominantwith Alnus (at
preceding
(7
clay or peaty-clay layer within the level (e.9.,
cm).
17-18 cm), or replacedas dominantby Alnus and 8
were typically not rooted within the same
Fig.
8).
Stumps
freld
season
1992
N
in
the
of
level
examination
Subsequent
$ € s $* *q gF
. *rE
s s Es 3.{$
cAN. J. EARTH SCr. vOL. 30, 1993
1920
abundance score
0
43
1
2
3
4
5
0
1
2
3
4
5
0
1
2
3
4
5
0
1
2
3
4
5
0
1
41
39
37
35
33
31
29
E
()
s
c
o
!
27
25
7t
21
19
17
15
13
11
I
7
5
Osmunda
other angiosperm
Alnus
undescribed
Taxodiaceae
Pseudolarix
Glyptostobus
Metasequoia
Frc. 7. Floristic composition at centimetre intervals within level N peat through section at section 7. Abundance scale as in Table 2.
Tlnln 3. Plant associations(common macrofossils) along level M
transect(1.5 km), baseof section(<25 cm from basalclay), based
on field observations
Sample points
4
Metasequoia
Foliage
Cones
Glyptostrobus
Other conifer
Alnus
Other angiosperm
Onoclea (fern)
Osmunda (fern)
No macrofossils
Frc. 8. Photograph (detail) of level M showing in situ stump in
upper part of the level and root system. The pen lying against the
stump is l0 cm in length.
horizon in this topmost layer, suggestingestablishment(?and
death) of trees at either different times or at different microtopographic positions.
Horizontal transect (level M)
The macrofossil taxon most commonly found in abundance
in level M (Fig. 3) was Metasequoia, representedby both foliage and cones (Tables 3 and 4). Glyptostrobuswas rare, and
remains of other coniferous taxa virtually absent. Very few
specieswere found in individual samplesand throughout the
transect,indicating a conservativecommunity of low diversity
X
X
X
X
5
6
l0
7
X
X
Tnnr-B 4. Plant associations(common macrofossils) along level M
transect(1.5 km), middle of section (25-50 cm from basal clay),
based on quantitative examination of samples
Sample points
Metasequoia
Foliage
Cones
Glyptostrobus
Other conifer
Alnus
Other angiosperm
Onoclea (fern)
Osmunda (fern)
Wood pieces
5
+
2
3
5
l
3
4
5
5
6
7
l
5
+
10
5
2-3
I
3
+
I
f
+
+
2
+
+
5
I
3
4
f
3
2
5
3
Nore: Abundance scale as in Table 2.
+
2
3
2
1
4
3
2
t92l
GREENWOOD AND BASINGER
over a large area. At sample points 5, 6, and 10, thick leaf
mats dominatedby Alnus (Betulaceae)were present(Table 4).
The fern remains in level M include Osmunda and Onoclea.
At one samplepoint, level M was dominated by Alnus leaves
in association with abundant fern fronds and mummified
whole fern plants of Onoclea. The amount of woody matter
(small branches,bark, etc.) varied between samples. Angiosperm leaves were most commonly preserved as either rare
fragmentswithin minor clay partings within otherwiseconiferous leaf mats (e.g., sample9) or as densemats (e.g., sample6).
Alnus was present in all but two samples,but was most common where Metasequoia was rare or absent.
Although some samples include an abundance of angiosperm leaves, as is apparent from Tables 3 and 4, higher in
level M (i.e., )50 cm from the basal clay) peat-coals were
composedof either mixed conifer (including Metasequoia)or
pure Metasequoialeaf mats. Mixed conifer layers were commonly succeededby Metasequoia-dominated leaf mats higher
in the layer. In situ stumps in level M were uncommon, typically associated with Metasequoia leaf mats, and generally
occurred from the middle to near the top of level M (Figs. 3
and 8). Cones of Metasequoia were present in four samples,
being common in only one sample.Alnus was presentin all but
two samples, but was most common where Metasequoia was
less common, or where Metasequoia was absent.
Discussion
Litter accumulation in modern taxodiaceous swamps is
high, producing peat dominated by Taxodium foliage (Spackman et al. 1969; Conner and Day 1976; Wing 1984).
Nevertheless,modern taxodiaceousswamps in the southern
United Statescontain a mosaic of plant communitiesreflecting
the interactionof hydrology (variations in the water table) and
ecology (Davis 1946; Spackmanet al. 1969; Conner and Day
1976; Collinson and Scott 1987). Different subcommunitiesin
such swamp mosaics reflect differences in substrate, water
level, and successionalresponsesto disturbance. Spackman
et al. (1969) have suggestedthat successionfrom Taxodium
forest islands ("Cypress heads") in the Florida swamp-marsh
communitiesto hardwood-dominatedforest may be causedby
the accumulation of peat and the resulting lowering of the
water table. Recordsof thesesubcommunitiesare preservedin
the peats. Open-water plant communities commonly produce
fine-grained peat lacking obvious plant remains, whereas
forested swamps produce coarse-grainedpeat with obvious
leaf and twig remains (Spackmanet al. 1969).
The polar Eocene taxodiaceousswamps of Axel Heiberg
Island would seem to have had a similar ecological dynamic
to modern taxodiaceousswamps, although open-water plant
communitiesappearto have been a more minor componentof
the ancient swamps. The associationof Onoclea with Alnus
leaves in the peats is consistent with the preference of the
modern speciesOnoclea sensibilis andAlnus spp. for wet soil
or marshy forests and is similar to the associationof remains
of O. sensibilis with betulaceous remains and Metasequoia
foliage in the Paleoceneof Alberta (Rothwell and Stockey
l99l). It is plausible that the Alnus-Onoclea association
representshardwood stands or thickets within the taxodiaceousswamp, on local slightly raisedor exposedareasof peat.
It seemslikely, therefore, that hydrological differencespromoted developmentof a vegetationalmosaic of taxodiaceous
and betulaceousswamp and bog forests, and that hydrological
evolution of the swamp was complex. The stratigraphic
arrangement of root systemsin level N may reflect redirection
of the roots during the life of thesetrees, indicating fluctuating
water-table levels (Munaut 1986). Repeatedpatterns of succession of plant communities have not been recognized for the
Eoceneswamp forestsof Axel Heiberg Island. The occasional
presence of layers rich in nontaxodiaceousconifers (e.9.,
Chamaecypari s, I-arix, andPseudolarix ; LePage and Basinger
l99l1' Basinger 1991) indicates the existence of relatively
diverse mixed-conifer plant communities within the mosaic.
Thesecommunitieswere encounteredin level N, but were not
seen in the analysisof the level M transect. Nevertheless,the
majority of the peat is composed of taxodiaceouslitter (principally Metasequoia).
The predominanceof taxodiaceouspeat in the paludal facies
of the upper coal member of the Buchanan Lake Formation
does not imply that theseplant communities were the dominant
vegetation of the meander plains. However, Metasequoiadominated forests were by far the widespread vegetation in
areas of high net accumulationof peat.
Conclusions
A successionof coaly layers, representingtree-dominated
swamps, is found interbeddedwith lacustrine shales, fluvial
sands, and paleosols within the Buchanan Lake Formation of
Axel Heiberg Island. These coals may include well-preserved
leaf matter and in situ stumps. Lateral variation in the stratigraphy of these coaly layers indicatesthat hydrological conditions, and as a consequencethe floristic and ecology of the
plant communities, may have been highly variable laterally
within these swamps. Vertical changesin sedimentcharacter
within coal layers indicate that hydrological conditions also
varied over time, apparently influencing local community composition and structure. Semiquantitativechangesin macrofossil
taxon abundance(Tables 2-4; Fig. 7) within levels M and N
indicate the former existence of a mosaic of taxodiaceous
swamp (Metasequoia- Glytostrobus), angiosperm- fern (Onoclea-Osmunda), and mixed coniferous communities. Taxodiaceous swamp was the dominant peat-forming community
and must have been intimately associated with areas of soil
saturation. Such peat accumulation reflects higher water-table
levels and hencesuppressionofdecay and doesnot necessarily
imply higher productivity (Moore 1987).
The thicknessof individual coal layers within levels M and
N suggeststhat each represents500- 1000 (or more) years of
swamp-forest growth. Individual stumpsmay representcoeval
members of a forest killed and buried contemporaneously,but
may also have been shown to be the remains of trees from
previous stands. Laterally exposed horizontal layers with in
situ stumps may therefore contain superimposed individuals
representingdifferent components of the temporal and spatial
vegetational mosaic. Inclusion of all stumps in stand-density
calculations, based on the assumption that all stumps in such
beds are coeval, may substantially overestimate biomass,
stand density (stems per hectare), and productivity.
Acknowledgments
We thank C. Nelson, E. Mclver, and B. LePage(Saskatchewan), L. Savard (Laval), T. Sweda and S. Kojima (Nagoya),
and K. Hayashi (Ehime) for assistanceduring the 1991 field
season.This researchwas made possible through field support
from the Polar Continental Shelf Project (No. 169-85) of
1922
CAN. J. EARTH SCI. VOL. 30. 1993
Energy, Mines and Resources, Canada, and the Natural
Sciences and Engineering Research Council of Canada
(NSERC) operating grant OGP0001334 to J.F.B., and was
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