a palynological study of an extinct arctic ecosystem from the palaeocene of northern alaska

Review of Palaeobotany and Palynology 166 (2011) 107–116

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Review of Palaeobotany and Palynology

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Research paper

A palynological study of an extinct arctic ecosystem from the Palaeocene ofNorthern Alaska

Robert J. Daly a,⁎, David W. Jolley a, Robert A. Spicer b, Anders Ahlberg c

a Department of Geology and Petroleum Geology, University of Aberdeen, King's College, Meston Building, Aberdeen, AB24 3UE, United Kingdomb Department of Earth and Environmental Sciences, Centre for Earth, Planetary and Astronomical Research, The Open University, Milton Keynes, MK7 6AA, United Kingdomc Faculty of Engineering, Lund University, P.O. Box 118, SE-22100 Lund, Sweden

⁎ Corresponding author.E-mail address: [email protected] (R.J. Daly).

0034-6667/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.revpalbo.2011.05.008

a b s t r a c t
a r t i c l e i n f o
Article history:Received 28 January 2010Received in revised form 10 May 2011Accepted 20 May 2011Available online 30 May 2011

Keywords:PalaeoceneArcticPalynologyMetasequoiaVegetationEcology

The Palaeocene arctic supported a vegetation type quite distinct from the tundra and polar desert of today.Here we demonstrate, through the palynological record, the structure of this extinct vegetation and itsdynamics over this period. The Late Palaeocene coal-bearing units of the Sagwon Bluffs on Alaska's NorthSlope (present latitude 69º N) are predominantly fine-grained, non-marine and rich in palynomorph-bearingsediments. From the analysed palynological assemblage we were able to demonstrate, using ‘DetrendedCorrespondence Analysis’ (DCA) and ‘Fuzzy c-Means Cluster Analysis’ (FCM), 1) a complex heterogeneousecosystem, 2) its major successional states, and 3) its development over an extended period. The climax stateof the floodplain was dominated by flood-tolerant, deciduous conifers such as Metasequoia. A moreheterogeneous mid-successional assemblage is represented by angiosperm and gymnosperm co-dominancewith an angiosperm dominance of Corylus, while early-successional ecological groups, dominated by fernsand bryophytes, are considered to represent riparian and post-disturbance niches. The structure of thisvegetation does not remain static over the course of the stratigraphic interval represented. We observe aparticularly dramatic ecological change for instance, following the deposition of a large conglomeratic unit. Itis hypothesized that this corresponds to altered drainage and/or precipitation on the North Slope. Thevegetation examined herein shows marked similarities to that of other palaeobotanical studies from varioussites of similar age at high northern latitudes. It is hence considered to represent an extensive and long-livedcircumpolar arctic biome.

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1. Introduction

Studies of plant fossils from the palaeoarctic and other highlatitude localities are extremely important for our understanding ofecosystem function and adaptation in climatically sensitive areas and,through comparison to modern analogues, can tell us a great dealabout the climate at the time. There have been few palynologicalstudies of the Palaeocene of Northern Alaska, none of which focus onthe plant ecology and vegetation dynamics of the North Slope duringthat interval. Studies by Frederiksen et al. (1988; 1996; 1998) intoCretaceous and Palaeogene successions from the North Slope,including those of the Prince Creek and Sagavanirktok Formations,are the primary reference points for this study, although the ages andboundaries of these formations have since been revised (Mull et al.,2003). Most of our palaeoecological knowledge of this interval inNorthern Alaska is based on megafossil studies from these and othersediments of similar age (e.g. Herman, 2007b; Herman et al., 2004;

Herman andMoiseeva, 2006; Moiseeva et al., 2009; Spicer et al., 1987;Spicer and Parrish, 1990; Wolfe, 1972). Spicer et al. (1987) suggestedthat the arctic represented an ‘evolutionary dead end’ for woodyplants and that diversification in the Arctic during the latestCretaceous and Palaeogene was typically at a low taxonomic level.As expected therefore, both the Sagwon megafloras and palynoflorasare of reasonably low taxonomic diversity. The megafossil assemblageis composed predominantly of broadleaved angiosperms (Corylitesberingianus, Trochodendroides arctica) and coniferous gymnosperms(Metasequoia occidentalis, Fokieniopsis catenulate, Mesocyparis) withrare horsetails (Equisetum arcticum), ferns (Onoclea hesperia) andaquatic angiosperms (Quereuxia angulata, Haemanthophyllum spp.)(Herman, 2007b), corresponding roughly to the palynofloras dis-cussed below. Studies of fossil leaf physiognomy have previously beenused to provide palaeoclimate data for this interval in northernAlaska, a mean annual temperature (MAT) of 6–7 º C at high northernlatitudes during the Palaeocene being proposed by Spicer and Parrish(1990) based on leaf margin analysis. From this, and stable isotopestudies of mollusc shells from the palaeoarctic ocean (Bice et al., 1996;Tripati et al., 2001), it is unlikely that there would have been anysignificant sea ice or continental glaciation at this time (Spicer and

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Sagwon

Brookes Range

Cenozoic Sediments

Sag

avan

irkto

k

Colville

Prudhoe Bay

Mesozoic Sediments

Canada

U.S

.A.

N

100 km

Fig. 1. Location of Sagwon Bluffs on the Sagavanirktok River, North Slope, Alaska, showing extent of Cenozoic sediments.

108 R.J. Daly et al. / Review of Palaeobotany and Palynology 166 (2011) 107–116

Parrish, 1990). As such, temperate broadleaved vegetation was able togrow as far north as the Alaskan north slope, at a palaeolatitude of ~80to 85º N (Glonka and Scotese, 1994; Smith et al., 1981; Ziegler et al.,1983).

The Sagwon Bluffs on the eastern North Slope of Alaska (Fig. 1) yieldplentiful palynomorphs. A thickness of approximately 150 m ofmudstones, sandstones, coals and conglomerates is well exposed onthe banks of the Sagavanirktok River over a distance of approximately 9miles south of pump station 2 of the Trans Alaska Pipeline (69º 24′1″ N148º 35′40.86″ W). These are interpreted as being deposited on awide braided floodplain (Mull et al., 2003). The exposure on the westbankof the river (Fig. 3a) comprises about 100 mof poorly consolidatedto well lithified clay, silt, mudstone and fine to medium-grainedsandstone sandwiched between four laterally extensive coal seams, andculminates in an extensive white-weathering conglomeratic unit(Fig. 3b) of more than 25 m thickness. The exposure on the east bank(Fig. 3c) is approximately 5 miles further down stream and reverts tothe fine-grained clay, silt, mud and fine sand deposits of the west bankexposure, including3 further coal seams. This 20 mor sooffine -grainedsediment is overlain by approximately 30 m of coarse-grained sand-stone and conglomerate. The boundary between the west bankexposure and the ridge-topping conglomerate represents a sequenceboundingunconformity (Figs. 2 and3b). The sediments belowthis are ofthe uppermost Prince Creek Formation, and the conglomerate andsediments above, as exposed downstream on the east bank, are of thebottommost Sagwon Member of the Sagavanirktok Formation (Mullet al., 2003).

These deposits are considered here to be Selandian to Thanetian inage based on the lack of any palynomorphs characteristic of either theMaastrichtian or the Palaeocene Eocene Thermal Maximum (PETM),and by the presence of the fungal spore Pesavis tagluensis (Kalgutkarand Sweet, 1988) and the juglandaceous pollen grain Caryapollenites

40

50

60

PAL

AE

OG

EN

E EOCENE

PALA

EO

CE

NPA

LAE

OC

EN

E

MAASTRICHTIAN Prince

Creek

Fm

Saga

vani

rkto

kFm Franklin Bluffs Mb

White Hills Mb

Sagwon Mb

SW NE150 - 200 Miles

Schrader Bluff Fm

Sagwon Bluffs

Ma

Danian

Selandian

Thanetian55.8

58.7

61.7

65.5

Fig. 2. Eastern North Slope stratigraphy (Modified from Mull et al., 2003).

inelegans, (Nichols and Ott, 1978), both of which are recognisedbiostratigraphic markers for the Upper Palaeocene (Muller, 1981;Norris, 1997). Although Frederiksen et al. (1996) previouslysuggested that the lower section of the uppermost Prince CreekFormation is older (Early Palaeocene) than the upper section of thelowermost Sagavanirktok Formation (Late Palaeocene), we havefound no evidence to support this.

The sedimentary facies from which the palynofloras studied herehave been recovered represent a range of depositional environmentswithin the river floodplain, much of which is composed of fine-grained siliciclastic rock typical of a low energy depositional system.These sediments are ideal for preserving microfossils, and conse-quently yield a high abundance of well-preserved palynomorphs. Themain palynomorph-bearing lithologies in the Sagwon succession arefluvial overbank, lacustrine deposits and coals. Fluvial overbanksediments are generally composed of clay, siltstone and mudstonesinterrupted sporadically by crevasse splay deposits of fine-grainedsandstone. Lacustrine facies are typified by laminated organic shales,and often by the presence of freshwater algae (Schizophacus parvusand/or Botryococcus braunii) representative of floodplain lakes (e.g.Sweet and Braman, 1992; Zippi, 1998). Coal is a common componentof the Sagwon Bluffs, representing a highly organic environmentcharacteristic of peat accumulation in mires and swamps. Coal faciescontain a rough mixture of dull, shaley coal and bright, pure lignite ofboth allochthonous and autochthonous nature and are often lami-nated or interbedded with shales or mudstones. Many coal andsurrounding facies contain drifted logs and/or other plant remains andare often underlain by seat earth palaeosols containing rootlets, plantmegafossils, charcoal and occasionally sphaerosiderite horizons.Palynological analysis of these sediments suggests the Sagwon Bluffsfloodplain environment supported a consistent type of vegetationwhich we attempt here to characterise in terms of ecology.

2. Methods

2.1. Palynology

Samples were collected from 8 sections within the Upper PrinceCreek Formation and SagwonMember as exposed at Sagwon Bluffs. Intotal 230 samples were collected. 158 of these were siliciclastic clays,mudstones, siltstones and fine-grained sandstones, and were pro-cessed using 40% hydrofluoric acid (HF). 72 coal samples wereprocessed using concentrated (70%) and fuming (100%) nitric (HNO3)acids. Where necessary samples were treated with a weak potassiumhydroxide (KOH) solution to remove amorphous organic matterand/or were boiled in 40% hydrochloric acid (HCl) to removeprecipitate. Samples were then mounted on slides using a 2% solutionof Polyvinyl Alcohol (PVA) and counted individually under a light

image of Fig.�1

image of Fig.�2

a

b

c

Fig. 3. Field photographs of the Sagwon Bluffs showing a) a typical section of the west bank exposure (Prince Creek Formation) showing coal III towards the top. b) The ridge-toppingwhite-weathering conglomerate of the lowermost Sagwon Member of the Sagavanirktok Formation unconformably overlying the uppermost coal seam (coal IV) of the Prince CreekFormation. The dashed yellow line indicates the sequence boundary. c) Part of the east bank exposure (SagwonMember) of the Sagwon Bluffs showing the uppermost coal seams (VIand VII) overlain by yellow-weathering coarse sandstone and conglomerate.

109R.J. Daly et al. / Review of Palaeobotany and Palynology 166 (2011) 107–116

microscope for a minimum of 200 palynomorphs per slide, abundancepermitting.

2.2. Statistical analysis

Raw count data was analysed using Detrended CorrespondenceAnalysis (DCA) and Fuzzy c-Means Cluster Analysis (FCM). Using thesetechniques we were able to assess the degree to which sample biascontributed to the palynological assemblage, formulate theoreticalecological groups, and assess how their compositions might overlapand change over time.

2.2.1. Detrended correspondence analysisDetrended Correspondence Analysis (DCA) is a metric ordination

technique based on ‘reciprocal averaging’ (Hill, 1973) used here todemonstrate a three dimensional dataset (represented both spatiallyand temporally) in a two dimensional, graphically simplified form. Byperforming linear transformations of a multidimensional dataset the

most significant trends can be summarized in fewer dimensions,represented by eigenvectors. Eigenvectors are represented in graph-ical form as axes, each of which is assigned an eigenvalue andpercentage variance pertinent to the significance of that particularvector/axis (Kovach, 1993). Detrended Correspondence Analysis(DCA) is used here in order to eliminate the ‘arch’ or ‘horseshoe’effect often created by conventional CA or reciprocal averaging, whichappears when there is a lack of structure in the data (Hill and Gauch,1980). From such plots groups of data points can be identified andtrends hypothesized.

2.2.2. Fuzzy c-means cluster analysisFuzzy c-Means Cluster Analysis (FCM) is a cluster analysis

algorithm developed by Bezdek (1980) which aims to subdivide adataset (X) into subsets of ‘clusters’ (c). In ‘hard’ or ‘non-fuzzy’clustering each data point within X is unequivocally grouped withother members of its cluster and is represented as bearing nosimilarity to other members of X. In ‘fuzzy’ clustering however, each

image of Fig.�3

Table 1Dominant sporomorph taxa as used in statistical analyses making up >0.1% of theassemblage recovered from Sagwon sediments. Recognized botanical affinities areshown where known.

Spores and pollen grains Botanical affinity

Bryophyte taxaStereisporites (Stereisporites) stereioides(Potonié & Venitz)

Sphagnaceae, Sphagnum

Stereisporites (Cingulitriletes) spp. (Pierce) Sphagnaceae, SphagnumStereisporites (Distgranisporis) spp. (Krutzsch) Sphagnaceae, SphagnumStereisporites (Distancoraesporis)germanicus (Krutzsch)

Sphagnaceae, Sphagnum

Pteridophyte taxaLaevigatosporites haardtii (Potonié & Venitz) PolypodiaceaeLaevigatosporites discordatus (Pflug) PolypodiaceaeVerrucatosporites balticus (Krutzsch) PolypodiaceaeBaculatisporites primarius (Wolff) Osmundaceae, OsmundaBaculatisporites nanus (Wolff, Krutzsch) Osmundaceae, OsmundaDeltoidospora adriennis(Potonie & Gelletich, Frederiksen)

Pteridaceae?

Deltoidospora wolfii (Krutzsch) UnknownDeltoidospora maxoides (Krutzsch) Schizaceae?Deltoidospora spp. UnknownLycopodiumsporites reticulatus (Rouse, Dettman) Lycopodiacae, LycopodiumTrilites tuberculifomis (Cookson) Schizaeaceae

Gymnosperm taxaInaperturopollenites hiatus (Potonié) Cupressaceae, MetasequoiaInaperturopollenites distichiforme(Potonié, Jolley & Morton)

Cupressaceae, Taxodium

Inaperturopollenites dubius (Potonié & Venitz) Cupressaceae, CupressusSequoiapollenites polyformosus (Thiergart) Cupressaceae, SequoiaMonocolpopollenites tranquilus (Potonié) Ginkgoaceae, Ginkgo

Angiosperm taxaTriporopollenites coryloides (Pflug) Betulaceae, CorylusTriatriopollenites subtriangulus (Stanley) Myricaeae, MyricaMomipites spp. (Wodehouse, Nichols & Ott) JuglandaceaeAlnipollenites verus (Potonié) Betulaceae, AlnusCupuliferoidaepollenites liblarensis (Thomson, Potonié) Fagaceae, CastaneaCupuliferoipollenites cingulum subsp. fusus (Potonié) Fagaceae, CastaneaTricolpites hians (Stanley) Platanaceae, PlatanusNyssapollenites kruschii subsp. analepticus(Potonié, Thomson & Pflug)

Nyssaceae, Nyssa


point may belong to one or more cluster to varying degrees ratherthan belonging exclusively to just one. This concept was introducedby Zahdeh (1965) and represents the similarity each individual datapoint or sample shares with each other within a function whose valuelies between 1 and 0. Each sample, therefore, will havemembership ofevery cluster to varying degrees. Amembership close to one signifies ahigh degree of similarity between the data point and a cluster, and amembership close to zero implies little similarity, in effect producing‘fuzzy partitions’ of ‘X’ (Bezdek, 1981). Thismethod allows for analysisof ecosystem heterogeneity that is not apparent from DCA.

3. Results

Angiosperm and gymnosperm pollen, fern, lycopod and mossspores as well as fungal and algal material were recovered fromSagwon sediments. Palynomorph taxa which made up less than 0.1%of this assemblage, as well as fungal and algal material and bisaccatepollen taxa, which are adjudged to have been imported from aseparate ‘upland’ ecosystem or hinterland, were omitted from initialstatistical analysis for reasons of clarity, and as such a total of 28 taxaare represented in both the DCA and FCM (Table 1).

3.1. Detrended correspondence analysis (DCA)

A DCA joint plot of samples keyed to lithology (black and greycircles) and palynomorph taxa (inverted triangles) shows no obviousrelationship between coal and siliciclastic facies or between thesefacies and groups of taxa (Fig. 4). Five groups of data pointsrepresenting individual taxa (inverted triangles) are identifiedhowever, each represented by different colours. Based on ourunderstanding of the botanical affinities of defined palynomorphs(Table 1) and their statistical relationships to one another, we wereable to make informed decisions about the nature of each group ofdata points, which, in the absence of any apparent facies bias, areinterpreted to represent ecological communities.

Patterns and trends observable from the plot can be applied to thisecological analysis. The X axis is interpreted to represent ecologicalgradient reflected in the spread of data points representing from leftto right, moss and fern dominated groups (blue and green invertedtriangles), an angiosperm and fern dominated group (red and yellowinverted triangles) and a coniferous gymnosperm dominated group(white inverted triangles). The Y axis is interpreted to representrelative moisture owing to the inferred water tolerances of definedpalynomorph taxa and their positions relative to the vertical plain ofthe plot. The percentage variance (18.011%) and eigenvalue (0.403) ofthe X axis, being higher than those of the Y axis (12.166% and 0.272),indicate that ecological succession is the more significant trenddemonstrated in this dataset. The two axes shown here (X and Y),represent the two largest eigenvectors with the largest cumulativepercentage variance (30.177%) calculated by the DCA. The algorithmhowever gives five axes in total with a cumulative percentagevariance of 45.46% (Appendix C). This was the highest variance wewere able to achieve from the dataset using this method, primarilydue to the removal of pinaceous pollen taxa from the raw dataset.

3.2. Fuzzy c-means cluster analysis (FCM)

FCM identifies four clusters, demonstrated as individual histo-grams (Fig. 5), which are roughly consistent with the interpretationsfrom the DCA. Although fewer clusters were calculated than the fivegroups observable from DCA, the general trend of a fern and moss-dominated early-succesional group (clusters 1 and 2), a moreheterogeneous angiosperm, fern and gymnosperm co-dominantmid-successional group (cluster 3) and a Metasequoia-dominatedclimax community (cluster 4) is apparent. As FCM does not confineeach data point to an individual cluster it is possible to see how

different taxa fit in tomore than one proposed community. Changes inthe dominance of these four fuzzy cluster groups are shown to changeover time as demonstrated in Fig. 6. The occurrence of bisaccatepollen, omitted from the statistical analyses, is shown alongside thefour FCM groups in order to demonstrate input from the hinterland.

4. Discussion

Although DCA and FCM use unrelated algorithms to analysedatasets and are presented graphically in quite different ways (as ascatter plot and histograms respectively), there are notable similar-ities and overlaps observable in their respective results. Here wecollectively interpret them in terms of ecology.

4.1. Palaeoecology

4.1.1. Climax communityThe white inverted triangles of the DCA and Cluster 4 of the FCM

demonstrate marked similarities, most notably in the dominance ofInaperturopollenites hiatus, representative of large Metasequoia typetrees (Cupressaceae). This is the most consistently numerous palyno-morph found throughout the Sagwon section and likely representsthe dominant taxon of the climax state of the ecosystem, particularlyas it is also a ubiquitous component of the megaflora. Similarcupressaceous conifer taxa, Sequoiapollenites polyformosus (Sequoia)and Inaperturopollenites distichiforme (Taxodium), are also consistentlypresent in association with I. hiatus, but occur in much lowernumbers. The main angiosperm association apparent from DCA is

0.403/18.011%

-1.0

-2.0

1.0

2.0

2.9

3.9

4.9

-1.0-2.0 1.0 2.0 2.9 3.9 4.9

0.272/12.166%

D. wolfii

T. subtriangulus

S. (Distgranisporis) spp.

D. maxoides

S. (S.) stereioides

Deltoidospora spp

B. nanus

L. discordatus

V. balticus

S. (Cingulitriletes) spp.

L. reticulatus

D. adriennis

C. liblarensis

B. primariusT. coryloides

S. (D.) germanicusC. cingulum fusus

T. hians

I. hiatus

Post - Fire Disturbance Taxa

Early Successional Taxa

Riparian Taxa

Mid - Successional Taxa

Late Successional/Climax Taxa

Coal Samples

Siliciclastic Samples

L. hardtii

I. dubius

T. tuberculiformis

A. verus

Momipites spp.

M. tranquilus

N. kruschii analepticusS. polyformosus

I. distichiforme

Fig. 4. Detrended Correspondence Analysis (DCA) joint plot for the Sagwon succession showing samples from coal (black circles) and siliciclastic (light grey circles) facies, and five definedgroupsofpalynomorph taxa (inverted triangles).White: Inaperturopollenites hiatus (Cupressaceae,Metasequoia), Sequoiapollenitespolyformosus (Cupressaceae, Sequoia), Inaperturopollenitesdistichiforme (Cupressaceae, Taxodium), Nyssapollenites kruschii subsp. analepticus (Nyssacaea, Nyssa), Cupuliferoipollenites cingulum subsp. fusus (Fagaceae, Castanea), Tricolpites hians(Platanaceae, Platanus), and Stereisporites (Distancoraesporis) germanicus (Sphagnaceae, Sphagnum). Red: Triporopollenites coryloides (Betulaceae, Corylus), Alnipollenites verus (Betulaceae,Alnus), Cupuliferoidaepollenites liblarensis (Fagaceae, Castanea), Laevigatosporites hardtii (Polypodiaceae), Baculatisporites primarius (Osmundaceae, Osmunda), Trilites tuberculiformis(Pteridaceae, Pteris), Inaperturopollenites dubius (Cupressaceae, Cupressus) and Monocolpopollenites tranquilus (Ginkgoaceae). Blue: Deltoidospora maxoides (Schizaceae), D.wolfii(Schizaceae), D. spp. (?), Stereisporites (Stereisoporites) stereioides (Sphagnum), S. (Distgranisporis) spp. (Sphagnum) and Triatriopollenites subtriangulus (Myricaceae, Myrica). Yellow:Deltoidospoa adriennis (Pteridaceae), Lycopodiumsporites reticulates (Lycopodiaceae, Lycopodium) and Stereisporites (Cingulartriletes) spp. (Sphagnum). Green: Laevigatosporites discordatus(Polypodiaceae), Baculatisporites nanus (Osmundaceae, Osmunda) and Verrucatosporites balticus (Polypodiaceae).


with Nyssapollenites kruschii subsp. analepticus (Nyssaceae, Nyssa),representative of flood-tolerant broadleaved trees analogous tomodern Tupelo. Other angiosperm pollen found in association includeCupuliferoipollenites cingulum subsp. fusus (Fagaceae, Castanea) andTricolpites hians (Platanaceae, Platanus), although it is posited here thatthey represent minor mid to late-successional species as they arepresent only in very low numbers and may be considered to overlapwith the mid-successional community. DCA places Stereisporites(Distancoraesporis) germanicus (Spagnaceae, Sphagnum) in the samegroup, although FCM suggests that S. (Stereisporites) stereioides (alsoSphagnum) is the more dominant. The high profusion of moss sporesgenerally and the predominance ofmoisture-loving, flood-tolerant taxasuch asMetasequoia and Nyssa imply that this group occupied a highlysaturated substrate typical of a low-lying floodplain swamp.

In the FCM, as in the DCA, groups interpreted as climax communityand mid seral succession are considered to overlap. Cluster 4 showssimilarities with cluster 3 (Fig. 5). The fern spores Deltoidosporaadriennis and Laevigatosporites haardtii, interpreted asmid-successionalin the DCA are present in both clusters 4 and 3 of the FCM. Similarly, thedominant bryophyte spore in cluster 4, as in cluster 3, is Stereisporites (S)stereioides. This suggests a gymnosperm-dominated climax communitywith a fern understorey, consistent with the climax ‘ecogroup’interpreted from theDCA.Accordingly Inaperturopollenites distichiforme,I. dubius and Sequoiapollenites polyformosus are also present, contribut-ing to the cupressaceous conifer dominance. Triporopollenites coryloidesis also present although in a minimal capacity, consistent with anoverlap of mid and late seral succession.

4.1.2. Mid seral successionAngiosperms are considered co-dominant in mid seral succes-

sion with cupressaceous gymnosperms such as Cupressus types(Inaperturopollenites dubius). The group of taxa interpreted torepresent this ecological stage in the DCA (red inverted triangles) isthe most species rich, containing several angiosperm taxa, principallyof the Betulaceae, the most numerous being Triporopollenitescoryloides (Corylus) and to a lesser extent Alnipollenites verus(Alnus), but also includes examples of other plant families includingCupuliferoidaepollenites liblarensis (Fagaceae, Castanea) andMomipites(Juglandaceae,). This is also reflected in cluster 3 of the FCM, whichindicates amore co-dominant community. Similarities are apparent inthe representation of T. coryloides relative to the other three clusters.Also in common with DCA, fern taxa (Laevigatosporites haardtii,Deltoidospora adriennis, Baculatisporites primarius) and Ginkgophytes(Monocolpopolleniites tranquilus) are present. The species representedhere appear to correspond to a community suited to comparativelydryer, better-drained substrates.

4.1.3. Early seral successionThe blue taxa of the DCA, including fern and bryophyte spores and

Triatriopollenites subtriagulus (Myricaceae,Myrica), interpreted as beingderived from a wetland specialist akin to extant bog myrtle, areconsidered to represent the early stages of ecological development.Stereisporites (Stereisporites) stereioides (Sphagnaceae, Sphagnum) is themost consistently numerous moss spore found throughout the Sagwonassemblage and is considered a major component of early to mid-

image of Fig.�4

Alnipollenites verus

Baculatisporites nanus

Baculatisporites primarius

Cupuliferoidaepollenites liblarensis

Cupuliferoipollenites cingulum subsp fusus

Deltoidospora adriennis

Deltoidospora maxoides

Deltoidospora spp

Deltoidospora wolfii

Inaperturopollenites distichiforme

Inaperturopollenites dubius

Inaperturopollenites hiatus

Laevigatosporites discordatus

Laevigatosporites haardtii

Lycopodiumsporites reticulatus

Monocolpopollenites tranquilus

Nyssapollenites kruschi subsp analepticus

Polypodiaceaesporites spp

Sequoiapollenites polyformosus

Stereisporites (Cingulitriletes) spp

Stereisporites (Distancoraesporis) germanicus

Stereisporites (Distgranisporis) spp

Stereisporites (Stereisporites) stereioides

Triatriopollenites subtriangulus

Tricolpites hians

Triporopollenites coryloides

Verrucatisporites favus

Verrucatosporites balticus

Cluster 1 Cluster 2 Cluster 3 Cluster 4Palynomorph Taxa 20 400 0 20 40 60 0 20 0 20 40 60

Fig. 5. Fuzzy c-Means Cluster (FCM) histograms of the Sagwon assemblage. The axes for each cluster are scaled by the centroid scores for each taxon (Appendix D). Cluster 1 –Mossand fern-dominant: Stereisporites (Distancoraesporis) germanicus dominance with Laevigatosporites hardtii and Deltoidospora adriennis. Cluster 2 – Fern and moss-dominant:Laevigatosporites hardtii dominance with Deltoidospora adriennis and Stereisporites (Distancoraesporis) germanicus. Cluster 3 – Mid Seral succession: Laevigatosporites hardtii,Inaperturopollenites hiatus, Inaperturopollenites dubius, Deltoidospora adriennis, Stereisporites (Stereisporites) stereioides and Triporopollenites coryloides. Cluster 4 – ClimaxCommunity: Inaperturopollenites hiatus dominance.


successional communities.Deltoidosporamaxoides (Schizaceae),D.wolfiiand other unidentifiable species of the genus make up only a smallproportion of the fern spores found, but would undoubtedly have madeup a significant component of the extensive fern-covered floor of thefloodplain. Given the preponderance of bryophytes and ferns, this groupis hypothesized to represent a major early-successional element of theecosystem. We interpret it to represent plants marginal to the riparianswamp-forest ecosystem, possibly as part of an extensive mire.

Clusters 1 and 2 of the FCM are collectively considered to representthis early seral succession. The dominant palynomorphs in both clustersare Stereisporites (Distancoraesporis) germanicus, Laevigatosporiteshaardtii and Deltoidospora adriennis, while Inaperturopollenites hiatus, I.distichiforme and I. dubius are also present in both, likely correspondingto the blown or washed in components of the climax vegetation, whichwould have a significant taphonomic advantage due to its height andwidespread distribution.

4.1.4. Riparian communityThe yellow taxa of the DCA, consisting of Deltoidospora

adriennis, Stereisporites (Cingulitriletes) spp. and Lycopodiumsporitesreticulatus (Lycopodiaceae) are independent of the general left-right downward trend of the plot (Fig. 4), situated low relative tothe Y axis. They may also be considered mid-successional due totheir central position relative to the X axis and perceived overlap

with the red taxa. It is considered therefore that these speciesrepresent a relatively wet environment, constituting either a rivermargin or small intermittent niche. D. adriennis is one of the mostconsistently profuse palynomorphs in the assemblage, which mightbe accounted for by its proximity to the water source. This groupalso includes bryophytes and lycophytes, both plant types depen-dent upon a reliably moist environment for reproduction. Basedon these assessments, and the presence of all three of these taxain multiple fluvial facies, this grouping is considered riparian innature.

4.1.5. Post-fire disturbance vegetationThe green taxa of the DCA include the ferns Baculatisporites nanus

(Osmundaceae, Osmunda), Laevigatosporites discordatus and Verrucatos-porites balticus (both Polypodiaceae). As this cluster is situated towardsthe left end of the plot where there is a distinct overlap with the early-successional group (blue), it is also considered thus. Due to its position atthe very top of the plot relative to the Y axis however, it is interpreted tohave occupied a comparatively dry environment. As charcoal fragmentsare found regularly in sediments above and below coal facies, it issuggestive that wildfires were commonplace (Patterson et al., 1987;Scott, 2000). This groupmay therefore be considered ‘post-disturbance’.Ferns recolonisingareas ravagedbywildfires, as hypothesized above, areto be considered primary colonists (e.g. Spicer et al., 1985).

image of Fig.�5

Fern-Moss Dominance Mid Seral Succession Climax CommunityMoss-Fern Dominance

0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0

Coal I

Coal IV

Coal III

Coal II

M S M C vC GvF F

20

10

30

40

50

5

60

70

80

100

Sequence Boundary

110

90

120

Coal VI

Coal VII

SA

GW

ON

ME

MB

ER

PR

I NC

E C

RE

EK

FO

RM

AT

I ON

Coal V (Not sampled)

0 40 80 120 0 40 80 120 160 200

Piceapollis spp. Pityosporites spp.

Coal

Mudstone/Clay

Channel SandCrevasse Splay

Log Key

Conglomerate

Sequence Boundary

‘Upland’ Floras

Fig. 6. Fuzzy c-Means Cluster (FCM) ecological groupings (L–R: moss and fern-dominated group, fern and moss-dominated group, mid seral succession and climax community) andoccurrence of bisaccate pollen taxa Piceapollis spp. and Pityosporites spp. (interpreted as ‘upland’ vegetation) over time represented by a generalised stratigraphic log of the Sagwonsection. Coal seams I to VII are highlighted in grey. Coal V of the Sagwon Member is shown as unsampled.


4.1.6. SynopsisClearly the designation of species into ecological groups in this way

cannot be considered rigid. Overlaps exist between selected ecologicalgroups in the DCA carried out here and there would certainly haveexisted extensive and varying degrees of heterogeneity. From ataphonomic perspective, it is also important to note that different speciesare likely to be over or under represented as a result of their relativepositions to water sources and sites of deposition, the number of sporesor pollen grains produced by individual species and/or to the size of thesource plants. Accordingly, we have analysed our finds based on what is

knownabout equivalent extant species and ecosystems. It is legitimate todo this because themegafossil remains associatedwith the palynologicalassemblages show high levels of similarity, both taxonomic andphysiognomic, to modern taxa. Within these parameters the analysis ofthis ecosystem based on DCA and FCM appears tomake ecological sense.In the FCMmost taxa occur inmore than one or, inmany cases, in all fourclusters suggesting that successive stages are more of a mosaic of theseproposed plant communities than rigid plant assemblages. The flood-plain environment would indeed have been much more heterogeneousthan DCA would appear to suggest on its own.

image of Fig.�6


4.2. Vegetation dynamics

4.2.1. Base of stratigraphic section – top of coal IIIn the first 25 to 30 m of the studied section the vegetation signal

fluctuates between fern and moss-dominated groups and mid seralsuccession (Fig. 6) with little evidence of late-successional commu-nities becoming established, suggesting that a climax state was rarelyreached. It is likely that this suite of assemblages primarily representriparian vegetation, occupying river banks and levees. Given theregularity of fine-grained sandstone facies during this interval,interpreted here as crevasse splays, it is possible that certain species(e.g. Deltoidospora adriennis) were predominantly associated withsuch deposits. We also see significant numbers of bisaccate pinaceouspollen during this interval, which may be due to a high sediment loadimporting material from the better drained substrates of thehinterland. Such a scenario may also account for the frequency ofcrevasse splays. A relative lack of moss spores suggests that thisinterval might represent a marginally dryer floodplain ecosystemthan further up the section, possibly owing to a lowerwater table and/or to relatively low levels of precipitation. Pinus type pollen(Pityosporites) is replaced temporarily towards the top of coal II bythat of spruce (Piceapollis), possibly indicating a change in ecosystemstructure on the higher ground. This is a short-lived episode, howeverand Piceapollis is rarely found from this point on. The decline in therepresentation of these upland conifers coincideswith the appearanceof bryophyte spores, which are a relatively constant presence in theassemblage until the sequence boundary with the Sagwon Member.

4.2.2. Top of coal II–coal IVThe interval between coals II and IV is dominated by a pattern of

mid seral and climax community dominance (Fig. 6). These mid seral-dominated intervals usually occur in and around coal facies andinclude spikes in fern-dominated floras, however the most notablechange above coal II is a marked decrease in bisaccate pinaceouspollen which does not recover to its former levels. Coal III facies inparticular yield a diverse palynological spectrum representative offern and moss-dominated early to mid seral succession, with aparticularly dramatic increase in fern taxa. Consistent with previouscoal facies, a climax state is rarely reached.

4.2.3. Coal IVLithologically coal IV itself is distinctive, comprising a combination

of bright lignite and dull woody coal containing charcoal fragmentsand sphaerosiderite nodules. Botanically, it constitutes mid seraldominance with a significant fern and moss component (Fig. 6) and,conspicuously, includes the only incidence of the large, highlyornamented monolete spore Reticulosporis foveolatus (Skarby) to-wards the top. The affinity of this is unknown, but it is likely ofpteridophytic origin and as it occurs nowhere else in the Sagwonsuccession before or after coal IV, it is considered significant in thecontext of the physical parameters controlling ecosystem change.Stratigraphically and botanically coal IV demonstrates considerablechange prior to the sequence boundary. Sphaerosiderite concretion,typical of waterlogged, variably anoxic and reducing soils (Retallack,2001), suggests that base level was particularly high at this point.However the presence of distinct layers of siderite nodules followedby charcoal fragments in dull coal layers getting progressivelybrighter would suggest successive rises and falls in base level,allowing siderite concretions to form and wildfires to occur.

4.2.4. Prince creek/sagwon boundary – coal VIIIn the sediments of the Sagwon Member above the basal

conglomerate the palynomorphs are poorly preserved compared withthose below it. They also occur in fewer numbers and constitute fewertaxa. Although roughly the same palynofloras dominate and FCMsuggests analmostblanket coverage of climax communityfloras (Fig. 6),

there is a demonstrably different floral structure. DCA of samples takensolely from the Prince Creek Formation reveals fivewell-defined groupsand a clear general trend almost identical to that described in Fig. 4.Although, as mentioned, Pityosporites and other bisaccate pollen wereoriginally omitted fromDCA, the assemblage from the SagwonMemberon its own produces a more coherent plot when such taxa are included(Fig. 7). The proposed explanation for this is that the environmentbecame drier and/or that the substrate became better drained. Thehitherto more ‘upland’ pinaceous taxa were thus able to colonise thelower ground, invading the immediate floodplain from the hinterland.In support of this hypothesis, there is an almost complete absence ofbryophyte spores and a significant decrease in early and mid-successional vegetation, a possible result of a reduction in moistecosystem niches suitable for colonisation by lower plants. Inaccordance with this, coal seams are considerably thinner and lessextensive indicative of a reduction in peat-forming sphagnum mireenvironments.

5. Conclusions

A rich cool temperate forest ecosystem flourished at Arcticlatitudes during the Palaeocene. A fern-dominated early-successionaland riparian community, angiosperm and gymnosperm co-dominantmid seral stage and gymnosperm-dominated climax state prevailed,with abundant bryophytes occupying multiple niches across thefloodplain. These findings suggest a markedly similar ecosystemstructure for the Palaeocene of northern Alaska to that proposed byother palaeobotanical and palynological studies of the Cretaceous andPalaeogene of various other northern hemisphere localities. Notablythese include the Canadian high Arctic (e.g. Burden and Langille,1991; Francis, 1988, 1990; Jahren, 2007; McIver and Basinger, 1999;Williams et al., 2007), northern Greenland (Boyd, 1990), northeastRussia (Herman, 2007a; Herman et al., 2004; Spicer et al., 2002) andthe North Atlantic Igneous Province (Boulter and Manum, 1989;Jolley, 1998; Jolley and Morton, 2007; Jolley and Whitham, 2004)pointing to a sustained period of dominance of such vegetation overan extended area of the boreal regions of the northern hemisphereand palaeoarctic.

While this ‘palaeoarctic flora’ was clearly part of an extensivenorthern biome during the Palaeocene, it is apparent, at least on theNorth Slope of Alaska, that it was subject to a degree of evolution overan extended period of time. Although taxa vary little throughout thesection there are notable changes in ecosystem structure which aremost likely attributable to a dynamic precipitation and/or drainageregime. This is likely the result of a combination of factors, howeverthe sedimentary succession at Sagwon is likely representative of anautocyclic system driven by eustasy instigating fluctuations in thewater table. Local and/or global environmental change may haveresulted in variable sea level for instance, as well as variable levels ofprecipitation contributing towards this and resulting in an alteredmoisture regime. Further investigation into the palynological recordof rocks of similar age throughout the arctic is required in order toobserve any chronology of these vegetational patterns over anextended area.

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.revpalbo.2011.05.008.

Acknowledgements

The authors would like to thank the Crafoord Foundation (Grant20030705) for sponsoring the two field expeditions to Alaska duringthe summers of 2001 and 2005, and Gil Mull and Mawan Wartes ofthe State of Alaska Division of Geological and Geophysical Surveys fortheir logistical help.

A

B

Baculatisporites primariusCupuliferoipollenites cingulum subsp fusus




Stereisporites (S.) stereioidesStereisporites (Distgranisporis) spp.

S. (Distancoraesporis) germanicus

Inaperturopollenites dubius

Inaperturopollenites hiatus


Sequoiapollenites polyformosusTriporopollenites coryloides

Triporopollenites subtriangularis

Triporopollenites plektosus



Echinatisporites spp.

Echinosporis spp.

Tricolpites spp.

Tricolpites hians

Nyssapollenites kruschii subsp analepticus

-0.8

-1.7

-2.5

-3.4

0.8

1.7

2.5

3.4

4.2

-0.8-1.7-2.5-3.4 0.8 1.7 2.5 3.4 4.20.133/16.548%

0.104/12.883%

Cupuliferoipollenites cingulum subsp pusilus

Baculatisporites nanus

Baculatisporites primarius

C. cingulum subsp fusus



Deltoidospora wolfii Deltoidospora maxoides

Deltoidospora spp.


Laevigatosporites discordatus

Verrucatosporites balticus

Lycopodiumsporites reticulatus

Stereisporites (S) stereioides

Stereisporites (Cingulitriletes) spp.

Stereisporites (Distgranisporis) spp.

Inaperturopollenites dubiusInaperturopollenites hiatus


Sequoiapollenites polyformosus

Triporopollenites coryloidesTriatriopollenites subtriangulus


Momipites spp. Trilites tuberculifomis


Tricolpites hians

N. kruschii subsp analepticus

-0.9

-1.8

0.9

1.8

2.7

3.6

4.5

-0.9-1.8 0.9 1.8 2.7 3.6 4.50.401/18.118%

0.28/12.64%

Group 1

Group 2Group 3

Group 5

Group 4

Group 2Group 4

Group 3

Group 1

Bisaccates

Fig. 7. Comparison of Detrended Correspondence Analyses (DCA) for the Prince Creek Formation (A) and Sagwon Member (B). A: Five observable groups are described. Group 1:Inaperturopollenites hiatus, Sequoiapollenites polyformosus, Inaperturopollenites distichiforme,Nyssapollenites kruschii subsp. analepticus, Cupuliferoipollenites cingulum subsp. fusus andTricolpites hians. Group 2: Triporopollenites coryloides, Alnipollenites verus, Laevigatosporites hardtii, Baculatisporites primarius, Inaperturopollenites dubius, Trilites tuberculiformis,Momipites spp.and Monocolpopollenites tranquilus. Group 3: Deltoidospora maxoides, D.wolfii, D. spp. Stereisporites (Stereisporites) stereioides, S. (Distgranisporis) spp. andTriatriopollenites subtriangulus. Group 4: Deltoidospoa adriennis, Lycopodimusporites reticulatus and Stereisporites (Cingulartriletes) spp. Group 5: Laevigatosporites discordatus,Baculatisporites nanus and Verrucatosporites balticus. This structure is practically identical to the DCA ecosystem analysis outlined in Fig. 4. B: Only 4 groupings are apparent. Group 1:Monocolpopollenites tranquilus, Laevigatosporites haardtii, Tricolpites hians and Echinatisporites spp. Group 2: Inaperturopollenites hiatus, I. distichiforme, Bisaccate pollen,Triporopollenites plektosus (Betulaceae) and Cupuliferoipollenites cingulum subsp. fusus. Group 3: Triporopollenites coryloides, Sequoiapollenites polyformosus, Nyssapollenites kruschiisubsp analepticus, Deltoidospora adriennis, Baculatisporites primarius, Stereisporites (Distgranisporis) spp. and S. (Distancoraesporis) germanicus. Group 4: Cupuliferoidaepollenitesliblarensis, Stereisporites (Stereisporites) stereioides and Bisaccate pollen grains.


image of Fig.�7


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a palynological study of an extinct arctic ecosystem from the palaeocene of northern alaska

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