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The role of angiosperms in Palaeocene arctic ecosystems: A palynological study from the Alaskan North Slope
Palaeogeography, Palaeoclimatology, Palaeoecology 309 (2011) 374–382
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Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o
The role of angiosperms in Palaeocene arctic ecosystems: A palynological study from the Alaskan North Slope R.J. Daly a,⁎, D.W. Jolley a, R.A. Spicer b a b
Department of Geology and Petroleum Geology, University of Aberdeen, Meston Building, King's College, Aberdeen, AB24 3UE, UK Department of Earth and Environmental Sciences, The Open University, Milton Keynes, MK7 6AA, UK
a r t i c l e
i n f o
Article history: Received 7 March 2011 Received in revised form 5 July 2011 Accepted 6 July 2011 Available online 14 July 2011 Keywords: Angiosperm Pollen Arctic Alaska Ecology Palaeocene
a b s t r a c t The North Slope of Alaska preserves a rich fossil record of the vegetation history of the palaeoarctic, facilitating an in depth palaeoecological assessment of plant ecosystems at high latitude within the framework of a greenhouse climate. Here we examine the palynological record of sediments of Upper Palaeocene age (~ 60 Ma) from Sagwon on the eastern North Slope. The assemblage is dominated by coniferous gymnosperm pollen belonging to the Cupressaceae and Pinaceae as well as numerous fern and bryophyte spores, together with several different angiosperm pollen taxa. Although constituting only 5.5% of the entire palynomorph assemblage, nearly 60% of which is of a single taxon (Triporopollenites coryloides), it is apparent that angiosperms were ecologically significant. The leaf megafossil assemblage from the same succession, for instance, contains a far higher proportion of broadleaved angiosperm to gymnosperm foliage, and little evidence of ferns, bryophytes or pinaceous conifers, which are so abundant in the palynofloras. We consider flowering plants to have comprised two main associations: 1) a riparian swamp-forest and 2) a raised mire complex distal to the water course. The first of these associations was dominated by cupressaceous gymnosperms such as Metasequoia, with an angiosperm component limited primarily to nyssaceous plants analogous to modern Tupelo, but which also included platanaceous and possibly fagaceous taxa. The second association, dominated by ferns and bryophytes, included scrubby betulaceous and myricaceous taxa such as alder and myrtle-types, and T. coryloides, which is analogous to modern hazel. Understanding the role of these flowering plants at or near the North Pole ~ 60 Ma, derivatives of which are common at mid latitudes today, can give us an idea as to the nature of high latitude floras in warmer climates. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Sagwon is the name given to the site of an abandoned pipeline construction camp on the North Slope of Alaska, situated at a latitude of 69° N, well within the Arctic Circle. The vegetation consists of scrubby arctic tundra, grazed by musk ox and caribou during the summer and blanketed by ice and snow in the winter. During the Upper Palaeocene, some 60 Ma, Alaska lay considerably further north sitting on or near to the pole at approximately 85° N (Smith et al., 1981; Ziegler et al., 1983; Glonka and Scotese, 1994). Within the framework of a greenhouse climate quantitative studies of fossil leaf physiognomy estimate the mean annual temperature to have been approximately 6 to 7 °C (Spicer and Parish, 1990), and consequently large, warm-temperate plants of an arboreal habit were able to colonise the most northerly latitudes. It is possible to reconstruct this plant ecosystem in some detail using the abundant spores and pollen recovered from the coal-bearing rocks at Sagwon (69° 24′1″ N
⁎ Corresponding author. Tel.: + 44 1224 273430. E-mail address: [email protected] (R.J. Daly). 0031-0182/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2011.07.007
148° 35′40.86″ W). Angiosperm pollen, constituting a small but significant proportion of this palynological assemblage, is the focus of this study and here we assess the role flowering plants had in a primarily gymnosperm-dominated palaeoenvironment. The Sagwon Bluffs expose a succession of Upper Palaeocene (Selandian–Thanetian), non-marine sediments of the eastern Colville Basin (Gryc et al., 1951; Detterman et al., 1975; Mull et al., 2003). The deposits crop out on the banks of the Sagavanirktok River on the eastern North Slope of Alaska approximately 100 km from Prudhoe Bay to the north and 80 km from the foothills of the Brooks Range to the south (Fig. 1). The exposure consists of vertical river bluffs, the majority of which are situated between 3 and 14 km south–southwest of Pump Station 2 of the Trans Alaska pipeline. The section is centred at VABM Gard, located on a ridge-topping, white-weathering conglomerate on the west bank of the river (Detterman et al., 1975). The section in its entirety encompasses sediments exposed for approximately 10 km south–south-west upstream and 2.5 km east– north-east downstream along both banks of the river. Several plant megafossil studies have been undertaken from localities in the Sagwon area, and from the eastern North Slope from rocks of similar age (e.g. Spicer et al., 1987; Spicer and Parish,
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Fig. 1. Location map showing the extent of Mesozoic and Cenozoic sedimentary deposits and the position of the Sagwon site on the Sagavanirktok River.
1990; Herman et al., 2004; Herman and Moiseeva, 2006; Herman, 2007; Herman et al., 2009; Moiseeva et al., 2009). Comparatively little work has been carried out on the palynology of the Sagwon section or eastern North Slope however. Frederiksen et al. (1988, 1996, 1998) focussed on the palynostratigraphy of the formations of the eastern North Slope, but here we analyse the ecological roles played by the plants represented in the palynoflora, concentrating on the importance of the angiosperms. 2. Geological setting Most of the bluffs are composed of brown-weathering, finegrained, coal-bearing strata. Rocks of this lithology on the west bank, across the river from the abandoned Sagwon airstrip, are overlain by a large white-weathering coarse sandstone and conglomeratic unit that forms the ridge top of the west bank exposure. The east bank exposure, 5 km downstream, is a continuation of this succession, representing the same coal-bearing lithology as the west bank, albeit weathering a lighter grey colour. This is topped by large units of pale yellow-weathering gravel and conglomerate grading down to coarse sandstone (Mull et al., 2003). The section in its entirety represents approximately 200 m of the uppermost Prince Creek Formation and the Sagwon Member of the lowermost Sagavanirktok Formation (Fig. 2). Just under 90 m of coal-bearing rocks on the west bank of the river comprises the Prince Creek Fm., while roughly 25 m of the ridgetopping conglomeratic unit unconformably overlies this, forming the base of the Sagwon Mb. A continuation of the Sagwon Mb. is exposed on the east bank for approximately 30 m in the form of coal-bearing beds similar to those of the Prince Creek Fm. and is overlain by approximately 30 m of sandstone, gravel and conglomerate in one metre thick beds representing the northernmost extent of the Sagwon Bluffs (Fig. 2). The age of the Sagwon exposure is considered here to be Late Palaeocene (~60 Ma). This assessment is based on the lack of any palynomorphs typical of either the Maastrichtian or the Palaeocene Eocene Thermal Maximum (PETM), and particularly by the presence of the fungal spore Pesavis tagluensis (Kalgutkar and Sweet, 1988) and the juglandaceous pollen taxon Caryapolenites inelegans (Nichols and Ott, 1978). Both of these palynomorphs have stratigraphic ranges between the Upper Palaeocene and Eocene (Muller, 1981; Norris, 1997), suggesting an age no older than Selandian and no younger than Thanetian.
3. Methods Samples were collected at approximately 50 cm intervals, where possible, from fine-grained siliciclastic and coal facies, but not from lithologies coarser than fine-grained sandstone due to lack of preservational potential. A total of 230 samples were analysed for palynology, of which 158 were from siliciclastic muds, silts, clays and fine sandstones, and 72 were coal samples. 10 g samples of rock were crushed and dissolved in either 50 ml of 40% hydrofluoric acid (HF) for siliciclastics or 45 ml of 100% (fuming) nitric acid (HNO3) for coals. Samples which contained an abundance of charcoal or other organic matter sufficient to obscure palynomorphs and prevent suitable analysis were treated with 40 ml of concentrated (70%) HNO3 for several minutes or until suitably oxidised. Potassium hydroxide (KOH) was used as an oxidising agent to remove finer amorphous organic matter, and, where precipitate formed (usually during dissolution in HF) samples were boiled in 200 ml of 37% hydrochloric acid (HCl) for 20 min. Each sample was neutralised in warm water and sieved through a 180 μm sieve to remove coarse material, and the collected residue through a fine 7 μm mesh before being mounted on a glass microscope slide in a 2% solution of polyvinyl alcohol (PVA). Up to 200 palynomorphs were counted under a light microscope where numbers allowed, following which the remainder of the slide was scanned for any additional significant components and to ensure minor taxa were represented. 4. Results and discussion The palynological assemblage from Sagwon is dominated by coniferous gymnosperm pollen, particularly Inaperturopollenites hiatus (Cupessaceae, Metasequoia) and Pityosporites spp. (Pinaceae, Pinus), which collectively make up nearly 50% of the N26,000 terrestrial sporomorphs counted during this study (Table 1). These two taxa, along with other similar, but less numerous, types, are interpreted to represent 1) a riparian swamp-forest ecosystem occupying the lowlying floodplain, and 2) the washed in components of a better-drained hinterland respectively. Within this assemblage there is also a wealth of bryophyte (Stereisporites spp.) and pteridophyte (e.g. Deltoidospora adriennis and Laevigatosporites hartdtii) spores, which together account for over 30% of the total abundance, and which were likely representative of a number of different niches within the floodplain ecosystem. Lycophyte spores (Lycopodiumsporites reticulates) and
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Fig. 2. Generalised stratigraphic log of the Sagwon Bluffs section.
Table 1 Principal pollen and spore taxa from the Sagwon palynological assemblage representing over 95% of the total assemblage. Shown are the average percentage occurrences and botanical affinities for each taxon. Sagwon pollen and spore taxa
%
Affinity
Inaperturopollenites hiatus (Potonié, 1931) Pityosporites spp. (Seward, 1914) Laevigatosporites hardtii (Potonié & Venitz, 1934) Deltoidospora adriennis (Potonié & Gelletich, 1933) Stereisporites (Stereisporites) stereioides (Potonié & Venitz, 1934) Inaperturopollenites dubius (Potonié & Venitz, 1934) Triporopollenites coryloides (Pflug, 1953) Sequoiapollenites polyformosus (Thiergart, 1953) Piceapollis spp. (Krutzsch, 1971) Baculatisporites primarius (Wolff, 1934) Lycopodiumsporites reticulates (Rouse ex Dettman) Nyssapollenites kruschii subsp. analepticus (Thomson & Pflug, 1953) Cupuliferoipollenites cingulum subsp. fusus (Potonié) Cupuliferoidaepollenites liblarensis (Thomson; Potonié, 1960) Monocolpopollenites tranquillus (Potonié) Momipites spp. (Wodehouse, 1933; Nichols & Ott, 1978) Alnipollenites verus (Potonié, 1931) Triatropollenites subtriangulus (Stanley, 1965) Tricolpites hians (Stanley, 1965)
28.8 20.82 15.34 7.85 5.53 5.51 3.33 2.4 1.87 1.53 0.98 0.68 0.51 0.38 0.33 0.23 0.18 0.17 0.12
Cupressaceae, Metasequoia Pinaceae, Pinus Polypodiaceae Pteridaceae Sphagnaceae, Sphagnum Cupressaceae, Cupressus Betulaceae, Corylus Cupessaceae, Sequoia Pinaceae, Picea Osmundaceae, Osmunda Lycopodiaceae Nyssaceae, Nyssa Fagaceae, Castanea Fagaceae, Castanea Ginkgoaceae, Ginkgo Juglandaceae Betulaceae, Alnus Myricaceae, Myrica Platanaceae, Platanus
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ginkgophyte pollen (Monocolpopollenites tranquillus) are also present, albeit as minor components (just over 1% and 0.3% respectively) of the palynological spectrum. Angiosperm pollen represents approximately 5.5% of the assemblage, with a predominance of Triporopollenites coryloides (Betulaceae, Corylus), which, although constituting only 3.3% of the total assemblage, is by far the most numerous angiosperm taxon (Table 1). Leaf megafossils from Sagwon (e.g. Herman, 2007; Moiseeva et al., 2009) roughly mirror this palynological spectrum. Leafy shoots of Metasequoia occidentalis (Newberry) and fruits of Nyssidium arcticum (Heer) for example are probably represented by Inaperturopollenites hiatus and Nyssapollenites kruschii subsp. analepticus respectively. Similarly Corylites beringianus (Gardner), and the morphologically similar Tilaephyllum tsagajanicum (Newberry), both analogous to the Betulaceae, are likely represented by Triporopollenites coryloides or pollen grains of a similar affinity (Plate I). The relative lack of Pinus type foliage, fern fronds or lycophyte megafossils for example, is testament to the taphonomic disparity between the palynological and megafossil assemblages. The high representation of Pinus and Metasequoia in the palynological record for instance, is likely due to high pollen production in these two taxa. 4.1. Botanical relationships In Fig. 3 the pollen and spore data are presented grouped within broad botanical groups. These include pteridophytes, lycophytes, bryophytes, cupressaceous and pinaceous gymnosperms, which dominate the assemblage, ginkgophytes and angiosperms, the component taxa of which are shown as a collective. When individual angiosperm taxa are examined however, it is clear that different species occur in discreet groups rather than as one assemblage (Fig. 4). Particularly notable is the distribution of Triporopollenites coryloides, the dominant angiosperm pollen type, which makes up over 59% of the angiosperm pollen counted. It shows two intervals of particularly high abundance in the Prince Creek Fm. separated by intervals of relative scarcity and, despite a small peak prior to the sequence boundary of the Prince Creek Fm. and Sagavanirktok Fm., becomes a comparatively minor component of the assemblage thereafter. Both T. coryloides and angiosperms in general become increasingly uncommon, particularly in the Sagwon Mb. Being so numerous compared with other angiosperm taxa, the pattern of occurrence of Triporopollenites coryloides roughly mirrors the overall angiosperm contingent as demonstrated in Fig. 2. However, several less numerous species of angiosperm pollen are present in the Sagwon assemblage. Most notable is Nyssapollenites kruschii subsp. analepticus (Nyssaceae, Nyssa), analogous to modern tupelos, which makes up just over 12%, and the comparatively minor Alnipollenites verus (Betulaceae, Alnus), Triatriopollenites subtriangulus (Myricaceae, Myrica), Tricolpites hians (Platanaceae, Platanus), Momipites spp. (Juglandaceae), Cupuliferoidaepollenites liblarensis and Cupuliferoipollenites cingulum subsp. fusus (both Fagaceae). These angiosperm taxa appear to occur in a much more sporadic manner throughout the section. It is apparent however that certain taxa occur roughly concurrent with others. Triporopollenites subtriangulus, for example, vaguely mirrors the peaks of T. coryloides, which would suggest that its parent plant, assumed to be akin to modern myrtle (Myrica), was associated with stands of hazel (Corylus), the proposed affiliate of T. coryloides. This observation invites the hypothesis that these two plant types occupied similar ecological niches and/or spatial distribution patterns. Both of these, along with A. verus, are likely representative of small scrubby plants, occupying mires comparatively distal to the water course (Fig. 4). Pollen taxa analogous to alder (A. verus) appear to be part of the same loose grouping as Corylus and Myrica, occurring roughly parallel with T. coryloides and T. subtriangulus. Both alder and hazel belong to the Betulaceae and their respective ecologies are not dissimilar, alder and hazel stands
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tending to occur on base-rich soils with impeded drainage (e.g. McVean, 1956). This may well have been the case on low-lying floodplains in the Palaeocene Arctic. Nyssapollenites kruschii subsp. anelepticus, however, more readily mirrors the distribution of the cupressaceous pollen and is likely representative of the same riparian swamp ecosystem. Its botanical affiliate of Nyssa is considered analogous to modern Water Tupelo, Nyssa aquatica, which commonly occurs in lowland swamps together with bald cypress, Taxodium distichum, in south-central North America (e.g. Mitsch et al., 1979; Schneider and Sharitz, 1986, 1988; Megonigal et al., 1997). Given that Metasequoia, another cupressaceous genus, is so similar to Taxodium, such swamps are considered here a useful analogue for the riparian margins of Palaeocene arctic floodplains. Platanus-type pollen (Tricolpites hians) displays a similar pattern of occurrence, and we conclude that these plants would also have had an association with this swamp ecosystem. Tricolpites hians however occurs in very low numbers, representing just over 0.1% of the total assemblage, and only 2.2% of the angiosperm taxa, suggesting that platanoids constituted only a very minor component of the vegetation. The two pollen taxa here associated with the Fagaceae (Cupuliferoipollenites cingulum subsp. fusus and Cupuliferoidaepollenites liblarensis), despite their similar affinities, to modern chestnut (Castanea), do not occur in the same pattern throughout the succession, suggesting that the ecologies of their respective parent plants, although overlapping to some extent, were not the same. The occurrence of the slightly less numerous C. liblarensis roughly mirrors that of Triporopollenites coryloides, and can therefore be assumed to represent a similar stage of ecological succession, a similar environment or both. Judging by the distribution of chestnut and hazel in modern broad-leaved woodlands (e.g. Castanea sativa and Corylus avellana in British riparian woodland and the Caucasus of southern Russia), we suggest they could have occupied similar substrates under similar hydrological conditions in the Palaeocene (e.g. Mason et al., 1984; Pridnya et al., 1996). The only apparently anomalous pollen grains found throughout the succession are those of the genus Momipites, which are relatively abundant only at the bottom of the Prince Creek section and rarely found thereafter. The recorded affiliation of Momipites species is to the Juglandaceae (Nichols and Ott, 1978). An increase in the occurrence of ferns and Sphagnum moss type spores (Stereisporites spp.) following this interval might suggest that the floodplain was becoming too wet for juglandaceous plants. It is worth noting that towards the bottom of the Prince Creek Fm. Alnipollenites verus occurs in a similar pattern to Momipites spp., perhaps indicating that Juglandaceous plants gave way to alder types. 4.2. Lithological relationships It is necessary to consider lithological relationships in relation to the assessments made above. The principal associations appear to be between particular plant types and coal facies hypothesised to represent peat deposits and between particular plant types and siliciclastic facies hypothesised to represent overbank deposits of near riparian association. The most obvious distributional pattern is that of cupressaceous gymnosperm pollen, which generally occurs in greater numbers within siliciclastic rocks, and comparatively low numbers within coal facies (Fig. 3). A similar relationship within the angiosperms is apparent in Nyssapollenites kruschii subsp. analepticus, which is roughly mirrored by Tricolpites hians and to a slightly lesser extent by Cupuliferoipollenites cingulum subsp. fusus (Fig. 4). As discussed above these taxa are considered to represent the vegetation of a riparian swamp forest-type environment. 4.2.1. Coal facies Peat would have accumulated on the Sagwon floodplain from mires rich in Sphagnum type mosses, ferns and scrubby angiosperms.
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Fig. 3. Pollen and spores of the Sagwon palynomorph assemblage grouped into broad botanical types against the Prince Creek Fm. and Sagwon Mb. of the Sagavanirktok Fm. Left to right: pteridophyte, lycophyte and bryophyte spore taxa, cupressaceous and pinaceous gymnosperm, ginkgophyte and angiosperm pollen taxa and the total sporomorph count from each sample. The X-axis units are raw palynological counts. Grey and white horizontal stripes denote coal and siliciclastic facies respectively.
Although each of the six coal seams sampled from the Sagwon section displays different lithological characteristics, at least two (I and IV) are dominantly, and one (III) partially, autochthonous and will have developed, at least in part, from such mires. These in situ coals are generally lacking in gymnosperm pollen, and are often comparatively rich in fern and moss spores, but the distribution of angiosperm pollen does not follow a specific pattern. The occurrence of Triporopollenites coryloides for example clearly does not adhere to a strict facies relationship as it is present in significant quantities in siliciclastic rocks as well as coals. This is most notable between coals II and III, however the largest peak of T. coryloides observed in this interval is from a small coal horizon which is not part of any of the main seven coal facies. Angiosperm pollen taxa which follow a similar pattern include Triatriopollenites subtriangulus, Cupuliferoidaepollenites liblarensis and Alnipollenites verus, although their distribution is not uniform within all six sampled coal facies as discussed below. Coal I is the bottommost coal facies situated at the very base of the studied section. It is composed of bright and dull coal layers and,
although varying laterally in thickness, is generally quite narrow. Layers of coal are interbedded with mud and clay palaeosols containing abundant rootlets and plant material, suggesting an autochthonous nature. Coal I is generally very poor in angiosperm pollen, although Cupuliferoidaepollenites liblarensis is present at the very bottom of the facies but disappears and is replaced by Momipites spp. towards the top. Nyssapollenites kruschii subsp. analepticus, Tricolpites hians and Alnipollenites verus are also present in very low numbers along with cupressaceous pollen and fern spores. In contrast coal II, further up the stratigraphy, is predominantly allochthonous, consisting of drifted material within lacustrine shale, suggestive of anoxic lake conditions. Although it is marginally less diverse than coal I, it contains a greater abundance of angiosperm pollen, particularly N. kruschii subsp. analepticus and Triporopollenites coryloides in the lower half of the facies. Coal III is considerably more heterogeneous than any of the other six. It is appreciably thicker in places, although this varies laterally, and comprises a mixture of drifted and in situ coaliferous material
Plate I. Spores, pollen grains and leaf megafossils from Sagwon Bluffs samples. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
Pityosporites spp. (Seward) Inaperturopollenites hiatus (Potonié) Deltoidospora adriennis (Potonié & Gelletich) Lycopodiumsporites reticulates (Rouse ex Dettman) Laevigatosporites hardtii (Potonié & Venitz) Stereisporites spp. (Potonié & Venitz) Triporopollenites coryloides (Pflug) Triatriopollenites subtriangulus (Stanley) Alnipollenites verus (Potonié) Momipites spp. (Wodehouse, Nichols & Ott) Cupuliferoidaepollenites liblarensis (Thomson, Potonié) Cupuliferoipollenites cingulum subsp. fusus (Potonié) Nyssapollenites kruschii subsp. analepticus (Thomson & Pflug) Tricolpites hians (Stanley) Corylites beringianus (Gardner) Tiliaephyllum tsagajanicum (Newberry) Nyssidium arcticum (Heer) fruit. Scale bars are 10 μm for palynomorphs and 1 cm for leaf megafossils.
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Fig. 4. The eight dominant angiosperm pollen representing 5.5% of the Sagwon palynomorph assemblage against the Prince Creek Fm. and Sagwon Mb. of the Sagavanirktok Fm. Shown, from left to right, are Cupuliferoipollenites cingulum subsp. fusus (Fagaceae), Cupuliferoidaepollenites liblarensis (Fagaceae), Triporopollenites coryloides (Betulaceae, Corylus), Triatriopollenites subtriangulus (Myricaceae, Myrica), Momipites spp. (Juglandaceae), Alnipollenites verus (Betulaceae, Alnus), Tricolpites hians (Platanaceae, Platanus) and Nyssapollenites kruschii subsp. analepticus (Nyssaceae, Nyssa). As with Fig. 2 the X-axis units are raw palynological counts. Dark grey stripes denote coal facies and light grey stripes denote siliciclastic facies.
interbedded with organic-rich clastic shale, mud and clay. This part of the succession likely represents a mire-lake continuum fluctuating between angiosperm rich and poor lithologies and containing abundant ferns and bryophytes. Gymnosperms remain a conspicuously minor component however. Small peaks of Triatriopollenites subtriangulus occur where there is a decline in the dominant Triporopollenites coryloides, which also corresponds with small numbers of Cupuliferoipollenites cingulum subsp. fusus. Coal IV, which unconformably underlies the ridge-topping conglomerate of the bottommost Sagwon Mb. contains sphaerosiderite nodules in layers of autochthonous dull and bright coal, suggesting an emergent mire depositional environment. Layers of charcoal above these siderite horizons indicate wildfires when the water table was sufficiently low. A peak of Triporopollenites coryloides just below coal IV is followed by that of Triatriopollenites subtriangulus before it is replaced by Alnipollenites verus (Alnus) towards the top of the facies, suggesting a Corylus–Myrica–Alnus succession pattern which likely, given the nature of the coal, was associated with the relative moisture of the substrate. Prior to coal IV a number of Nyssapollenites kruschii subsp. analepticus and Tricolpites hians along with a high abundance of cupressaceous angiosperms suggest a transition from riparian swamp to emergent mire. The bottommost coal seam of the Sagwon Mb. (coal V) was inaccessible due to adverse river conditions, and was not sampled. Coals VI and VII above this, however, are both very narrow compared
with the coal facies of the Prince Creek Fm. and are composed predominantly of bright coal, although coal VI is dull at the base overlying a layer of woody clay. They are considered to be predominantly allochthonous, composed of drifted material, possibly in a lacustrine setting. As would appear to be the case from the minimal diversity of angiosperm pollen taxa and relative dominance of cupressaceous gymnosperm pollen, these coals were likely formed in a riparian swamp-type depositional environment. Of the angiosperm pollen there are small peaks of Triporopollenites coryloides and Nyssapollenites kruschii subsp. analepticus.
5. Conclusions A temperate plant community of dominantly arborescent taxa existed as far as the highest northern latitudes during the Palaeocene. Palynological and macropalaeobotanical fossil assemblages of the same age from Sagwon collectively reveal an ecosystem dominated by cupressaceous gymnosperms, ferns, bryophytes and angiosperms. The preponderance of broad-leaved megafossils suggests that angiosperms were at least locally abundant relative to gymnosperms, particularly Metasequoia, the remains of which are ubiquitous compared with those of angiosperms which are more heterogeneous in their distribution. The palynofloras, on the other hand, would appear to indicate that they were a more marginal component of the
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Fig. 5. Schematic diagram representing the floodplain ecosystem of the Palaeocene North Slope. Shown are the two hypothesised botanical associations of 1) swamp forest dominated by Cupessaceae-Nyssa around the riparian margins and 2) raised mires dominated by bryophytes, ferns and Corylus and Myrica-type angiosperms distal to the water course.
plant ecosystem. The reality is probably somewhere in between (Fig. 5). Two loose associations of angiosperm pollen appear throughout the Sagwon succession. The first, dominated by Triporopollenites coryloides, is interpreted here to represent a mire complex and was likely to have been composed of small scrubby trees distributed throughout the floodplain distal to the main water courses. The second, with a prevalence of Nyssapollenites kruschii subsp. analepticus, is representative of riparian swamp forests dominated by Metasequoia and other cupressaceous gymnosperms. Along with these higher plants, numerous pteridophyte spores, representing a number of different fern types, are present throughout the succession. Due to their occurrence in multiple coal and siliciclastic facies types, however, they likely represented niches in both the near riparian back swamp and the distal mires. Given this, and the differences in the palynological content of coal facies, it is likely that the different lithologies here do not represent precise botanical divisions and are more ambiguous. From this assemblage of terrestrial palynomorphs we can conclude that angiosperms played a significant, if minor role in a gymnosperm-dominated wetland ecosystem, characterising different ecological niches along the floodplain. Acknowledgements The authors would like to thank the Crafoord Foundation (Grant 20030705) for sponsoring the 2001 and 2005 field expeditions to Alaska, and Gil Mull and Mawan Wartes of the State of Alaska Division of Geological and Geophysical Surveys for their logistical help. References Detterman, R.L., Reisner, H.N., Brosgé, W.P., Dutro Jr., J.T., 1975. Post Carboniferous Stratigraphy, Northeastern Alaska: USGS Professional Paper 886. Frederiksen, N.O., Ager, T.A., Edwards, L.E., 1988. Palynology of Maastrichtian and Paleocene rocks, lower Colville River region, North Slope of Alaska. Canadian Journal of Earth Science 25, 512–527. Frederiksen, N.O., Sheehan, T.P., Ager, T.A., Collet, T.S., Fouch, T.D., Franczyk, K.J., Johnsson, M., 1996. Palynomorph Biostratigraphy of Upper Cretaceous to Eocene Samples from the Sagavanirktok Formation in its Type Region, North Slope of Alaska: USGS Open File Report 96-84. Frederiksen, N.O., Andrle, V.A.S., Sheehan, T.P., Ager, T.A., Collet, T.S., Fouch, T.D., Francczyk, K.J., Johnsson, M., 1998. Palynological Dating of Upper Cretaceous to
Middle Eocene Strata in the Sagavanirktok and Canning Formations, North Slope of Alaska: USGS Open File Report 98-471. Glonka, J., Scotese, C.R., 1994. Phanerozoic palaeogeographic maps of Arctic margins. ICAM-94 Proceedings: Stratigraphy and Palaeogeography. Gryc, G., Patton Jr., W.W., Payne, T.G., 1951. Present cretaceous stratigraphic nomenclature of Northern Alaska. Journal of the Washington Academy of Sciences 41 (5), 159–167. Herman, A.B., 2007. Comparative paleofloristics of the Albian–Early Paleocene in the Anadyr-Koryak and North Alaska Subregions, part 2: the North Alaska subregion. Stratigraphy and Geological Correlation 15 (4), 373–384. Herman, A.B., Moiseeva, M.G., 2006. Floristic changes at the Cretaceous–Palaeogene boundary in the Northern Pacific (based on the Paleocene flora of Northern Alaska). In: The Cause–Effect Relationships and Factors of the Global Turnovers in the Biosphere During the Phanerozoic: Proceedings of the Geological Institute of the Russian Academy of Sciences, 580, pp. 83–89 (in Russian). Herman, A.B., Moiseeva, M.G., Spicer, R.A., Ahlberg, A., 2004. Maastrichtian–Paleocene floras of Northeastern Russia and North Alaska and floral changes at the Cretaceous–Paleogene boundary. Stratigraphy and Geological Korrelyatsiya 12 (5), 55–64 (Stratigr. Geol. Korrelyatsiya 12 (4), 485–494). Herman, A.B., Akhmetiev, M.A., Kodrul, T.M., Moiseeva, M.G., Iakovleva, A.I., 2009. Flora development in Northeastern Asia and Northern Alaska during the Cretaceous– Paleogene transitional epoch. Stratigraphy and Geological Correlation 17 (1), 79–97. Kalgutkar, R.M., Sweet, A.R., 1988. Morphology, taxonomy and phylogeny of the fossil genus Pesavis from northwestern Canada. Geological Survey Canadian Bulletin 379, 117–133. Mason, C.F., Macdonald, S.M., Hussey, A., 1984. Structure, management and conservation value of the riparian woody plant community. Biological Conservation 29, 201–216. McVean, D.N., 1956. Ecology of Alnus glutinosa (L.) Gaertn: V. Notes on some British alder populations. Journal of Ecology 44 (2), 321–330. Megonigal, J.P., Conner, W.H., Kroeger, S., Sharitz, R.R., 1997. Aboveground production in southeastern floodplain forests: a test of the subsidy–stress hypothesis. Ecology 78 (2), 370–384. Mitsch, W.J., Dorge, C.L., Wiemhoff, J.R., 1979. Ecosystem dynamics and a phosphorus budget of an alluvial cypress swamp in Southern Illinois. Ecology 60 (6), 1116–1124. Moiseeva, M.G., Herman, A.B., Spicer, R.A., 2009. Late Paleocene flora of the Northern Alaska Peninsula: the role of Transberingian plant migrations and climate change. Paleontological Journal 43 (10), 1298–1308. Mull, C.G., Houseknecht, D.W., Bird, K.J., 2003. Revised Cretaceous and Tertiary Stratigraphic Nomenclature in the Colville Basin, Northern Alaska: USGS Professional Paper 1673. Muller, J., 1981. Fossil pollen records of extant angiosperms. The Botanical Review 47 (1), 1–123. Nichols, D.J., Ott, H.L., 1978. Biostratigraphy and evolution of the momipites–caryapollenites lineage in the Early Tertiary in the Wind River Basin, Wyoming. Palynology 2, 93–112. Norris, G., 1997. Paleocene–Pliocene deltaic to inner shelf palynostratigraphic zonation, depositional environments and paleoclimates in the Imperial ADGO F-28 Well, Beaufort-Mackenzie Basin. Geological Survey of Canada Bulletin 523, 1–71. Pridnya, M.V., Cherpakov, V.V., Pailet, F.L., 1996. Ecology and pathology of the European chestnut (Castanea sativa) in the deciduous forests of the Caucasus Mountains in Southern Russia. Bulletin of the Torrey Botanical Club 123 (3), 213–222. Schneider, R.L., Sharitz, R.R., 1986. Seed bank dynamics in a southeastern riverine swamp. American Journal of Botany 73 (7), 1022–1030.
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Schneider, R.L., Sharitz, R.R., 1988. Hydrochory and regeneration in a bald cypresswater tupelo swamp forest. Ecology 69 (4), 1055–1063. Smith, A.C., Hurley, A.M., Briden, J.C., 1981. Phanerozoic Palaeocontinental World Maps. Cambridge University Press. Spicer, R.A., Parish, J.T., 1990. Late Cretaceous–Early Tertiary Palaeoclimates of northern high latitudes: a quantitative view. Journal of the Geological Society of London 147, 329–341.
Spicer, R.A., Wolfe, J.A., Nichols, D.J., 1987. Alaskan Cretaceous–Tertiary floras and arctic origins. Palaeobiology 13 (1), 73–83. Ziegler, A.M., Scotese, C.R., Barrett, S.F., 1983. Mesozoic and Cenozoic palaeogeographic maps. In: Brosche, P., Sündermann, J. (Eds.), Tidal Friction and the Earth's Rotation, II. Springer-Verlag, Berlin, pp. 240–252.
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Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o
The role of angiosperms in Palaeocene arctic ecosystems: A palynological study from the Alaskan North Slope R.J. Daly a,⁎, D.W. Jolley a, R.A. Spicer b a b
Department of Geology and Petroleum Geology, University of Aberdeen, Meston Building, King's College, Aberdeen, AB24 3UE, UK Department of Earth and Environmental Sciences, The Open University, Milton Keynes, MK7 6AA, UK
a r t i c l e
i n f o
Article history: Received 7 March 2011 Received in revised form 5 July 2011 Accepted 6 July 2011 Available online 14 July 2011 Keywords: Angiosperm Pollen Arctic Alaska Ecology Palaeocene
a b s t r a c t The North Slope of Alaska preserves a rich fossil record of the vegetation history of the palaeoarctic, facilitating an in depth palaeoecological assessment of plant ecosystems at high latitude within the framework of a greenhouse climate. Here we examine the palynological record of sediments of Upper Palaeocene age (~ 60 Ma) from Sagwon on the eastern North Slope. The assemblage is dominated by coniferous gymnosperm pollen belonging to the Cupressaceae and Pinaceae as well as numerous fern and bryophyte spores, together with several different angiosperm pollen taxa. Although constituting only 5.5% of the entire palynomorph assemblage, nearly 60% of which is of a single taxon (Triporopollenites coryloides), it is apparent that angiosperms were ecologically significant. The leaf megafossil assemblage from the same succession, for instance, contains a far higher proportion of broadleaved angiosperm to gymnosperm foliage, and little evidence of ferns, bryophytes or pinaceous conifers, which are so abundant in the palynofloras. We consider flowering plants to have comprised two main associations: 1) a riparian swamp-forest and 2) a raised mire complex distal to the water course. The first of these associations was dominated by cupressaceous gymnosperms such as Metasequoia, with an angiosperm component limited primarily to nyssaceous plants analogous to modern Tupelo, but which also included platanaceous and possibly fagaceous taxa. The second association, dominated by ferns and bryophytes, included scrubby betulaceous and myricaceous taxa such as alder and myrtle-types, and T. coryloides, which is analogous to modern hazel. Understanding the role of these flowering plants at or near the North Pole ~ 60 Ma, derivatives of which are common at mid latitudes today, can give us an idea as to the nature of high latitude floras in warmer climates. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Sagwon is the name given to the site of an abandoned pipeline construction camp on the North Slope of Alaska, situated at a latitude of 69° N, well within the Arctic Circle. The vegetation consists of scrubby arctic tundra, grazed by musk ox and caribou during the summer and blanketed by ice and snow in the winter. During the Upper Palaeocene, some 60 Ma, Alaska lay considerably further north sitting on or near to the pole at approximately 85° N (Smith et al., 1981; Ziegler et al., 1983; Glonka and Scotese, 1994). Within the framework of a greenhouse climate quantitative studies of fossil leaf physiognomy estimate the mean annual temperature to have been approximately 6 to 7 °C (Spicer and Parish, 1990), and consequently large, warm-temperate plants of an arboreal habit were able to colonise the most northerly latitudes. It is possible to reconstruct this plant ecosystem in some detail using the abundant spores and pollen recovered from the coal-bearing rocks at Sagwon (69° 24′1″ N
⁎ Corresponding author. Tel.: + 44 1224 273430. E-mail address: [email protected] (R.J. Daly). 0031-0182/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2011.07.007
148° 35′40.86″ W). Angiosperm pollen, constituting a small but significant proportion of this palynological assemblage, is the focus of this study and here we assess the role flowering plants had in a primarily gymnosperm-dominated palaeoenvironment. The Sagwon Bluffs expose a succession of Upper Palaeocene (Selandian–Thanetian), non-marine sediments of the eastern Colville Basin (Gryc et al., 1951; Detterman et al., 1975; Mull et al., 2003). The deposits crop out on the banks of the Sagavanirktok River on the eastern North Slope of Alaska approximately 100 km from Prudhoe Bay to the north and 80 km from the foothills of the Brooks Range to the south (Fig. 1). The exposure consists of vertical river bluffs, the majority of which are situated between 3 and 14 km south–southwest of Pump Station 2 of the Trans Alaska pipeline. The section is centred at VABM Gard, located on a ridge-topping, white-weathering conglomerate on the west bank of the river (Detterman et al., 1975). The section in its entirety encompasses sediments exposed for approximately 10 km south–south-west upstream and 2.5 km east– north-east downstream along both banks of the river. Several plant megafossil studies have been undertaken from localities in the Sagwon area, and from the eastern North Slope from rocks of similar age (e.g. Spicer et al., 1987; Spicer and Parish,
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Fig. 1. Location map showing the extent of Mesozoic and Cenozoic sedimentary deposits and the position of the Sagwon site on the Sagavanirktok River.
1990; Herman et al., 2004; Herman and Moiseeva, 2006; Herman, 2007; Herman et al., 2009; Moiseeva et al., 2009). Comparatively little work has been carried out on the palynology of the Sagwon section or eastern North Slope however. Frederiksen et al. (1988, 1996, 1998) focussed on the palynostratigraphy of the formations of the eastern North Slope, but here we analyse the ecological roles played by the plants represented in the palynoflora, concentrating on the importance of the angiosperms. 2. Geological setting Most of the bluffs are composed of brown-weathering, finegrained, coal-bearing strata. Rocks of this lithology on the west bank, across the river from the abandoned Sagwon airstrip, are overlain by a large white-weathering coarse sandstone and conglomeratic unit that forms the ridge top of the west bank exposure. The east bank exposure, 5 km downstream, is a continuation of this succession, representing the same coal-bearing lithology as the west bank, albeit weathering a lighter grey colour. This is topped by large units of pale yellow-weathering gravel and conglomerate grading down to coarse sandstone (Mull et al., 2003). The section in its entirety represents approximately 200 m of the uppermost Prince Creek Formation and the Sagwon Member of the lowermost Sagavanirktok Formation (Fig. 2). Just under 90 m of coal-bearing rocks on the west bank of the river comprises the Prince Creek Fm., while roughly 25 m of the ridgetopping conglomeratic unit unconformably overlies this, forming the base of the Sagwon Mb. A continuation of the Sagwon Mb. is exposed on the east bank for approximately 30 m in the form of coal-bearing beds similar to those of the Prince Creek Fm. and is overlain by approximately 30 m of sandstone, gravel and conglomerate in one metre thick beds representing the northernmost extent of the Sagwon Bluffs (Fig. 2). The age of the Sagwon exposure is considered here to be Late Palaeocene (~60 Ma). This assessment is based on the lack of any palynomorphs typical of either the Maastrichtian or the Palaeocene Eocene Thermal Maximum (PETM), and particularly by the presence of the fungal spore Pesavis tagluensis (Kalgutkar and Sweet, 1988) and the juglandaceous pollen taxon Caryapolenites inelegans (Nichols and Ott, 1978). Both of these palynomorphs have stratigraphic ranges between the Upper Palaeocene and Eocene (Muller, 1981; Norris, 1997), suggesting an age no older than Selandian and no younger than Thanetian.
3. Methods Samples were collected at approximately 50 cm intervals, where possible, from fine-grained siliciclastic and coal facies, but not from lithologies coarser than fine-grained sandstone due to lack of preservational potential. A total of 230 samples were analysed for palynology, of which 158 were from siliciclastic muds, silts, clays and fine sandstones, and 72 were coal samples. 10 g samples of rock were crushed and dissolved in either 50 ml of 40% hydrofluoric acid (HF) for siliciclastics or 45 ml of 100% (fuming) nitric acid (HNO3) for coals. Samples which contained an abundance of charcoal or other organic matter sufficient to obscure palynomorphs and prevent suitable analysis were treated with 40 ml of concentrated (70%) HNO3 for several minutes or until suitably oxidised. Potassium hydroxide (KOH) was used as an oxidising agent to remove finer amorphous organic matter, and, where precipitate formed (usually during dissolution in HF) samples were boiled in 200 ml of 37% hydrochloric acid (HCl) for 20 min. Each sample was neutralised in warm water and sieved through a 180 μm sieve to remove coarse material, and the collected residue through a fine 7 μm mesh before being mounted on a glass microscope slide in a 2% solution of polyvinyl alcohol (PVA). Up to 200 palynomorphs were counted under a light microscope where numbers allowed, following which the remainder of the slide was scanned for any additional significant components and to ensure minor taxa were represented. 4. Results and discussion The palynological assemblage from Sagwon is dominated by coniferous gymnosperm pollen, particularly Inaperturopollenites hiatus (Cupessaceae, Metasequoia) and Pityosporites spp. (Pinaceae, Pinus), which collectively make up nearly 50% of the N26,000 terrestrial sporomorphs counted during this study (Table 1). These two taxa, along with other similar, but less numerous, types, are interpreted to represent 1) a riparian swamp-forest ecosystem occupying the lowlying floodplain, and 2) the washed in components of a better-drained hinterland respectively. Within this assemblage there is also a wealth of bryophyte (Stereisporites spp.) and pteridophyte (e.g. Deltoidospora adriennis and Laevigatosporites hartdtii) spores, which together account for over 30% of the total abundance, and which were likely representative of a number of different niches within the floodplain ecosystem. Lycophyte spores (Lycopodiumsporites reticulates) and
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Fig. 2. Generalised stratigraphic log of the Sagwon Bluffs section.
Table 1 Principal pollen and spore taxa from the Sagwon palynological assemblage representing over 95% of the total assemblage. Shown are the average percentage occurrences and botanical affinities for each taxon. Sagwon pollen and spore taxa
%
Affinity
Inaperturopollenites hiatus (Potonié, 1931) Pityosporites spp. (Seward, 1914) Laevigatosporites hardtii (Potonié & Venitz, 1934) Deltoidospora adriennis (Potonié & Gelletich, 1933) Stereisporites (Stereisporites) stereioides (Potonié & Venitz, 1934) Inaperturopollenites dubius (Potonié & Venitz, 1934) Triporopollenites coryloides (Pflug, 1953) Sequoiapollenites polyformosus (Thiergart, 1953) Piceapollis spp. (Krutzsch, 1971) Baculatisporites primarius (Wolff, 1934) Lycopodiumsporites reticulates (Rouse ex Dettman) Nyssapollenites kruschii subsp. analepticus (Thomson & Pflug, 1953) Cupuliferoipollenites cingulum subsp. fusus (Potonié) Cupuliferoidaepollenites liblarensis (Thomson; Potonié, 1960) Monocolpopollenites tranquillus (Potonié) Momipites spp. (Wodehouse, 1933; Nichols & Ott, 1978) Alnipollenites verus (Potonié, 1931) Triatropollenites subtriangulus (Stanley, 1965) Tricolpites hians (Stanley, 1965)
28.8 20.82 15.34 7.85 5.53 5.51 3.33 2.4 1.87 1.53 0.98 0.68 0.51 0.38 0.33 0.23 0.18 0.17 0.12
Cupressaceae, Metasequoia Pinaceae, Pinus Polypodiaceae Pteridaceae Sphagnaceae, Sphagnum Cupressaceae, Cupressus Betulaceae, Corylus Cupessaceae, Sequoia Pinaceae, Picea Osmundaceae, Osmunda Lycopodiaceae Nyssaceae, Nyssa Fagaceae, Castanea Fagaceae, Castanea Ginkgoaceae, Ginkgo Juglandaceae Betulaceae, Alnus Myricaceae, Myrica Platanaceae, Platanus
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ginkgophyte pollen (Monocolpopollenites tranquillus) are also present, albeit as minor components (just over 1% and 0.3% respectively) of the palynological spectrum. Angiosperm pollen represents approximately 5.5% of the assemblage, with a predominance of Triporopollenites coryloides (Betulaceae, Corylus), which, although constituting only 3.3% of the total assemblage, is by far the most numerous angiosperm taxon (Table 1). Leaf megafossils from Sagwon (e.g. Herman, 2007; Moiseeva et al., 2009) roughly mirror this palynological spectrum. Leafy shoots of Metasequoia occidentalis (Newberry) and fruits of Nyssidium arcticum (Heer) for example are probably represented by Inaperturopollenites hiatus and Nyssapollenites kruschii subsp. analepticus respectively. Similarly Corylites beringianus (Gardner), and the morphologically similar Tilaephyllum tsagajanicum (Newberry), both analogous to the Betulaceae, are likely represented by Triporopollenites coryloides or pollen grains of a similar affinity (Plate I). The relative lack of Pinus type foliage, fern fronds or lycophyte megafossils for example, is testament to the taphonomic disparity between the palynological and megafossil assemblages. The high representation of Pinus and Metasequoia in the palynological record for instance, is likely due to high pollen production in these two taxa. 4.1. Botanical relationships In Fig. 3 the pollen and spore data are presented grouped within broad botanical groups. These include pteridophytes, lycophytes, bryophytes, cupressaceous and pinaceous gymnosperms, which dominate the assemblage, ginkgophytes and angiosperms, the component taxa of which are shown as a collective. When individual angiosperm taxa are examined however, it is clear that different species occur in discreet groups rather than as one assemblage (Fig. 4). Particularly notable is the distribution of Triporopollenites coryloides, the dominant angiosperm pollen type, which makes up over 59% of the angiosperm pollen counted. It shows two intervals of particularly high abundance in the Prince Creek Fm. separated by intervals of relative scarcity and, despite a small peak prior to the sequence boundary of the Prince Creek Fm. and Sagavanirktok Fm., becomes a comparatively minor component of the assemblage thereafter. Both T. coryloides and angiosperms in general become increasingly uncommon, particularly in the Sagwon Mb. Being so numerous compared with other angiosperm taxa, the pattern of occurrence of Triporopollenites coryloides roughly mirrors the overall angiosperm contingent as demonstrated in Fig. 2. However, several less numerous species of angiosperm pollen are present in the Sagwon assemblage. Most notable is Nyssapollenites kruschii subsp. analepticus (Nyssaceae, Nyssa), analogous to modern tupelos, which makes up just over 12%, and the comparatively minor Alnipollenites verus (Betulaceae, Alnus), Triatriopollenites subtriangulus (Myricaceae, Myrica), Tricolpites hians (Platanaceae, Platanus), Momipites spp. (Juglandaceae), Cupuliferoidaepollenites liblarensis and Cupuliferoipollenites cingulum subsp. fusus (both Fagaceae). These angiosperm taxa appear to occur in a much more sporadic manner throughout the section. It is apparent however that certain taxa occur roughly concurrent with others. Triporopollenites subtriangulus, for example, vaguely mirrors the peaks of T. coryloides, which would suggest that its parent plant, assumed to be akin to modern myrtle (Myrica), was associated with stands of hazel (Corylus), the proposed affiliate of T. coryloides. This observation invites the hypothesis that these two plant types occupied similar ecological niches and/or spatial distribution patterns. Both of these, along with A. verus, are likely representative of small scrubby plants, occupying mires comparatively distal to the water course (Fig. 4). Pollen taxa analogous to alder (A. verus) appear to be part of the same loose grouping as Corylus and Myrica, occurring roughly parallel with T. coryloides and T. subtriangulus. Both alder and hazel belong to the Betulaceae and their respective ecologies are not dissimilar, alder and hazel stands
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tending to occur on base-rich soils with impeded drainage (e.g. McVean, 1956). This may well have been the case on low-lying floodplains in the Palaeocene Arctic. Nyssapollenites kruschii subsp. anelepticus, however, more readily mirrors the distribution of the cupressaceous pollen and is likely representative of the same riparian swamp ecosystem. Its botanical affiliate of Nyssa is considered analogous to modern Water Tupelo, Nyssa aquatica, which commonly occurs in lowland swamps together with bald cypress, Taxodium distichum, in south-central North America (e.g. Mitsch et al., 1979; Schneider and Sharitz, 1986, 1988; Megonigal et al., 1997). Given that Metasequoia, another cupressaceous genus, is so similar to Taxodium, such swamps are considered here a useful analogue for the riparian margins of Palaeocene arctic floodplains. Platanus-type pollen (Tricolpites hians) displays a similar pattern of occurrence, and we conclude that these plants would also have had an association with this swamp ecosystem. Tricolpites hians however occurs in very low numbers, representing just over 0.1% of the total assemblage, and only 2.2% of the angiosperm taxa, suggesting that platanoids constituted only a very minor component of the vegetation. The two pollen taxa here associated with the Fagaceae (Cupuliferoipollenites cingulum subsp. fusus and Cupuliferoidaepollenites liblarensis), despite their similar affinities, to modern chestnut (Castanea), do not occur in the same pattern throughout the succession, suggesting that the ecologies of their respective parent plants, although overlapping to some extent, were not the same. The occurrence of the slightly less numerous C. liblarensis roughly mirrors that of Triporopollenites coryloides, and can therefore be assumed to represent a similar stage of ecological succession, a similar environment or both. Judging by the distribution of chestnut and hazel in modern broad-leaved woodlands (e.g. Castanea sativa and Corylus avellana in British riparian woodland and the Caucasus of southern Russia), we suggest they could have occupied similar substrates under similar hydrological conditions in the Palaeocene (e.g. Mason et al., 1984; Pridnya et al., 1996). The only apparently anomalous pollen grains found throughout the succession are those of the genus Momipites, which are relatively abundant only at the bottom of the Prince Creek section and rarely found thereafter. The recorded affiliation of Momipites species is to the Juglandaceae (Nichols and Ott, 1978). An increase in the occurrence of ferns and Sphagnum moss type spores (Stereisporites spp.) following this interval might suggest that the floodplain was becoming too wet for juglandaceous plants. It is worth noting that towards the bottom of the Prince Creek Fm. Alnipollenites verus occurs in a similar pattern to Momipites spp., perhaps indicating that Juglandaceous plants gave way to alder types. 4.2. Lithological relationships It is necessary to consider lithological relationships in relation to the assessments made above. The principal associations appear to be between particular plant types and coal facies hypothesised to represent peat deposits and between particular plant types and siliciclastic facies hypothesised to represent overbank deposits of near riparian association. The most obvious distributional pattern is that of cupressaceous gymnosperm pollen, which generally occurs in greater numbers within siliciclastic rocks, and comparatively low numbers within coal facies (Fig. 3). A similar relationship within the angiosperms is apparent in Nyssapollenites kruschii subsp. analepticus, which is roughly mirrored by Tricolpites hians and to a slightly lesser extent by Cupuliferoipollenites cingulum subsp. fusus (Fig. 4). As discussed above these taxa are considered to represent the vegetation of a riparian swamp forest-type environment. 4.2.1. Coal facies Peat would have accumulated on the Sagwon floodplain from mires rich in Sphagnum type mosses, ferns and scrubby angiosperms.
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Fig. 3. Pollen and spores of the Sagwon palynomorph assemblage grouped into broad botanical types against the Prince Creek Fm. and Sagwon Mb. of the Sagavanirktok Fm. Left to right: pteridophyte, lycophyte and bryophyte spore taxa, cupressaceous and pinaceous gymnosperm, ginkgophyte and angiosperm pollen taxa and the total sporomorph count from each sample. The X-axis units are raw palynological counts. Grey and white horizontal stripes denote coal and siliciclastic facies respectively.
Although each of the six coal seams sampled from the Sagwon section displays different lithological characteristics, at least two (I and IV) are dominantly, and one (III) partially, autochthonous and will have developed, at least in part, from such mires. These in situ coals are generally lacking in gymnosperm pollen, and are often comparatively rich in fern and moss spores, but the distribution of angiosperm pollen does not follow a specific pattern. The occurrence of Triporopollenites coryloides for example clearly does not adhere to a strict facies relationship as it is present in significant quantities in siliciclastic rocks as well as coals. This is most notable between coals II and III, however the largest peak of T. coryloides observed in this interval is from a small coal horizon which is not part of any of the main seven coal facies. Angiosperm pollen taxa which follow a similar pattern include Triatriopollenites subtriangulus, Cupuliferoidaepollenites liblarensis and Alnipollenites verus, although their distribution is not uniform within all six sampled coal facies as discussed below. Coal I is the bottommost coal facies situated at the very base of the studied section. It is composed of bright and dull coal layers and,
although varying laterally in thickness, is generally quite narrow. Layers of coal are interbedded with mud and clay palaeosols containing abundant rootlets and plant material, suggesting an autochthonous nature. Coal I is generally very poor in angiosperm pollen, although Cupuliferoidaepollenites liblarensis is present at the very bottom of the facies but disappears and is replaced by Momipites spp. towards the top. Nyssapollenites kruschii subsp. analepticus, Tricolpites hians and Alnipollenites verus are also present in very low numbers along with cupressaceous pollen and fern spores. In contrast coal II, further up the stratigraphy, is predominantly allochthonous, consisting of drifted material within lacustrine shale, suggestive of anoxic lake conditions. Although it is marginally less diverse than coal I, it contains a greater abundance of angiosperm pollen, particularly N. kruschii subsp. analepticus and Triporopollenites coryloides in the lower half of the facies. Coal III is considerably more heterogeneous than any of the other six. It is appreciably thicker in places, although this varies laterally, and comprises a mixture of drifted and in situ coaliferous material
Plate I. Spores, pollen grains and leaf megafossils from Sagwon Bluffs samples. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
Pityosporites spp. (Seward) Inaperturopollenites hiatus (Potonié) Deltoidospora adriennis (Potonié & Gelletich) Lycopodiumsporites reticulates (Rouse ex Dettman) Laevigatosporites hardtii (Potonié & Venitz) Stereisporites spp. (Potonié & Venitz) Triporopollenites coryloides (Pflug) Triatriopollenites subtriangulus (Stanley) Alnipollenites verus (Potonié) Momipites spp. (Wodehouse, Nichols & Ott) Cupuliferoidaepollenites liblarensis (Thomson, Potonié) Cupuliferoipollenites cingulum subsp. fusus (Potonié) Nyssapollenites kruschii subsp. analepticus (Thomson & Pflug) Tricolpites hians (Stanley) Corylites beringianus (Gardner) Tiliaephyllum tsagajanicum (Newberry) Nyssidium arcticum (Heer) fruit. Scale bars are 10 μm for palynomorphs and 1 cm for leaf megafossils.
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Fig. 4. The eight dominant angiosperm pollen representing 5.5% of the Sagwon palynomorph assemblage against the Prince Creek Fm. and Sagwon Mb. of the Sagavanirktok Fm. Shown, from left to right, are Cupuliferoipollenites cingulum subsp. fusus (Fagaceae), Cupuliferoidaepollenites liblarensis (Fagaceae), Triporopollenites coryloides (Betulaceae, Corylus), Triatriopollenites subtriangulus (Myricaceae, Myrica), Momipites spp. (Juglandaceae), Alnipollenites verus (Betulaceae, Alnus), Tricolpites hians (Platanaceae, Platanus) and Nyssapollenites kruschii subsp. analepticus (Nyssaceae, Nyssa). As with Fig. 2 the X-axis units are raw palynological counts. Dark grey stripes denote coal facies and light grey stripes denote siliciclastic facies.
interbedded with organic-rich clastic shale, mud and clay. This part of the succession likely represents a mire-lake continuum fluctuating between angiosperm rich and poor lithologies and containing abundant ferns and bryophytes. Gymnosperms remain a conspicuously minor component however. Small peaks of Triatriopollenites subtriangulus occur where there is a decline in the dominant Triporopollenites coryloides, which also corresponds with small numbers of Cupuliferoipollenites cingulum subsp. fusus. Coal IV, which unconformably underlies the ridge-topping conglomerate of the bottommost Sagwon Mb. contains sphaerosiderite nodules in layers of autochthonous dull and bright coal, suggesting an emergent mire depositional environment. Layers of charcoal above these siderite horizons indicate wildfires when the water table was sufficiently low. A peak of Triporopollenites coryloides just below coal IV is followed by that of Triatriopollenites subtriangulus before it is replaced by Alnipollenites verus (Alnus) towards the top of the facies, suggesting a Corylus–Myrica–Alnus succession pattern which likely, given the nature of the coal, was associated with the relative moisture of the substrate. Prior to coal IV a number of Nyssapollenites kruschii subsp. analepticus and Tricolpites hians along with a high abundance of cupressaceous angiosperms suggest a transition from riparian swamp to emergent mire. The bottommost coal seam of the Sagwon Mb. (coal V) was inaccessible due to adverse river conditions, and was not sampled. Coals VI and VII above this, however, are both very narrow compared
with the coal facies of the Prince Creek Fm. and are composed predominantly of bright coal, although coal VI is dull at the base overlying a layer of woody clay. They are considered to be predominantly allochthonous, composed of drifted material, possibly in a lacustrine setting. As would appear to be the case from the minimal diversity of angiosperm pollen taxa and relative dominance of cupressaceous gymnosperm pollen, these coals were likely formed in a riparian swamp-type depositional environment. Of the angiosperm pollen there are small peaks of Triporopollenites coryloides and Nyssapollenites kruschii subsp. analepticus.
5. Conclusions A temperate plant community of dominantly arborescent taxa existed as far as the highest northern latitudes during the Palaeocene. Palynological and macropalaeobotanical fossil assemblages of the same age from Sagwon collectively reveal an ecosystem dominated by cupressaceous gymnosperms, ferns, bryophytes and angiosperms. The preponderance of broad-leaved megafossils suggests that angiosperms were at least locally abundant relative to gymnosperms, particularly Metasequoia, the remains of which are ubiquitous compared with those of angiosperms which are more heterogeneous in their distribution. The palynofloras, on the other hand, would appear to indicate that they were a more marginal component of the
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Fig. 5. Schematic diagram representing the floodplain ecosystem of the Palaeocene North Slope. Shown are the two hypothesised botanical associations of 1) swamp forest dominated by Cupessaceae-Nyssa around the riparian margins and 2) raised mires dominated by bryophytes, ferns and Corylus and Myrica-type angiosperms distal to the water course.
plant ecosystem. The reality is probably somewhere in between (Fig. 5). Two loose associations of angiosperm pollen appear throughout the Sagwon succession. The first, dominated by Triporopollenites coryloides, is interpreted here to represent a mire complex and was likely to have been composed of small scrubby trees distributed throughout the floodplain distal to the main water courses. The second, with a prevalence of Nyssapollenites kruschii subsp. analepticus, is representative of riparian swamp forests dominated by Metasequoia and other cupressaceous gymnosperms. Along with these higher plants, numerous pteridophyte spores, representing a number of different fern types, are present throughout the succession. Due to their occurrence in multiple coal and siliciclastic facies types, however, they likely represented niches in both the near riparian back swamp and the distal mires. Given this, and the differences in the palynological content of coal facies, it is likely that the different lithologies here do not represent precise botanical divisions and are more ambiguous. From this assemblage of terrestrial palynomorphs we can conclude that angiosperms played a significant, if minor role in a gymnosperm-dominated wetland ecosystem, characterising different ecological niches along the floodplain. Acknowledgements The authors would like to thank the Crafoord Foundation (Grant 20030705) for sponsoring the 2001 and 2005 field expeditions to Alaska, and Gil Mull and Mawan Wartes of the State of Alaska Division of Geological and Geophysical Surveys for their logistical help. References Detterman, R.L., Reisner, H.N., Brosgé, W.P., Dutro Jr., J.T., 1975. Post Carboniferous Stratigraphy, Northeastern Alaska: USGS Professional Paper 886. Frederiksen, N.O., Ager, T.A., Edwards, L.E., 1988. Palynology of Maastrichtian and Paleocene rocks, lower Colville River region, North Slope of Alaska. Canadian Journal of Earth Science 25, 512–527. Frederiksen, N.O., Sheehan, T.P., Ager, T.A., Collet, T.S., Fouch, T.D., Franczyk, K.J., Johnsson, M., 1996. Palynomorph Biostratigraphy of Upper Cretaceous to Eocene Samples from the Sagavanirktok Formation in its Type Region, North Slope of Alaska: USGS Open File Report 96-84. Frederiksen, N.O., Andrle, V.A.S., Sheehan, T.P., Ager, T.A., Collet, T.S., Fouch, T.D., Francczyk, K.J., Johnsson, M., 1998. Palynological Dating of Upper Cretaceous to
Middle Eocene Strata in the Sagavanirktok and Canning Formations, North Slope of Alaska: USGS Open File Report 98-471. Glonka, J., Scotese, C.R., 1994. Phanerozoic palaeogeographic maps of Arctic margins. ICAM-94 Proceedings: Stratigraphy and Palaeogeography. Gryc, G., Patton Jr., W.W., Payne, T.G., 1951. Present cretaceous stratigraphic nomenclature of Northern Alaska. Journal of the Washington Academy of Sciences 41 (5), 159–167. Herman, A.B., 2007. Comparative paleofloristics of the Albian–Early Paleocene in the Anadyr-Koryak and North Alaska Subregions, part 2: the North Alaska subregion. Stratigraphy and Geological Correlation 15 (4), 373–384. Herman, A.B., Moiseeva, M.G., 2006. Floristic changes at the Cretaceous–Palaeogene boundary in the Northern Pacific (based on the Paleocene flora of Northern Alaska). In: The Cause–Effect Relationships and Factors of the Global Turnovers in the Biosphere During the Phanerozoic: Proceedings of the Geological Institute of the Russian Academy of Sciences, 580, pp. 83–89 (in Russian). Herman, A.B., Moiseeva, M.G., Spicer, R.A., Ahlberg, A., 2004. Maastrichtian–Paleocene floras of Northeastern Russia and North Alaska and floral changes at the Cretaceous–Paleogene boundary. Stratigraphy and Geological Korrelyatsiya 12 (5), 55–64 (Stratigr. Geol. Korrelyatsiya 12 (4), 485–494). Herman, A.B., Akhmetiev, M.A., Kodrul, T.M., Moiseeva, M.G., Iakovleva, A.I., 2009. Flora development in Northeastern Asia and Northern Alaska during the Cretaceous– Paleogene transitional epoch. Stratigraphy and Geological Correlation 17 (1), 79–97. Kalgutkar, R.M., Sweet, A.R., 1988. Morphology, taxonomy and phylogeny of the fossil genus Pesavis from northwestern Canada. Geological Survey Canadian Bulletin 379, 117–133. Mason, C.F., Macdonald, S.M., Hussey, A., 1984. Structure, management and conservation value of the riparian woody plant community. Biological Conservation 29, 201–216. McVean, D.N., 1956. Ecology of Alnus glutinosa (L.) Gaertn: V. Notes on some British alder populations. Journal of Ecology 44 (2), 321–330. Megonigal, J.P., Conner, W.H., Kroeger, S., Sharitz, R.R., 1997. Aboveground production in southeastern floodplain forests: a test of the subsidy–stress hypothesis. Ecology 78 (2), 370–384. Mitsch, W.J., Dorge, C.L., Wiemhoff, J.R., 1979. Ecosystem dynamics and a phosphorus budget of an alluvial cypress swamp in Southern Illinois. Ecology 60 (6), 1116–1124. Moiseeva, M.G., Herman, A.B., Spicer, R.A., 2009. Late Paleocene flora of the Northern Alaska Peninsula: the role of Transberingian plant migrations and climate change. Paleontological Journal 43 (10), 1298–1308. Mull, C.G., Houseknecht, D.W., Bird, K.J., 2003. Revised Cretaceous and Tertiary Stratigraphic Nomenclature in the Colville Basin, Northern Alaska: USGS Professional Paper 1673. Muller, J., 1981. Fossil pollen records of extant angiosperms. The Botanical Review 47 (1), 1–123. Nichols, D.J., Ott, H.L., 1978. Biostratigraphy and evolution of the momipites–caryapollenites lineage in the Early Tertiary in the Wind River Basin, Wyoming. Palynology 2, 93–112. Norris, G., 1997. Paleocene–Pliocene deltaic to inner shelf palynostratigraphic zonation, depositional environments and paleoclimates in the Imperial ADGO F-28 Well, Beaufort-Mackenzie Basin. Geological Survey of Canada Bulletin 523, 1–71. Pridnya, M.V., Cherpakov, V.V., Pailet, F.L., 1996. Ecology and pathology of the European chestnut (Castanea sativa) in the deciduous forests of the Caucasus Mountains in Southern Russia. Bulletin of the Torrey Botanical Club 123 (3), 213–222. Schneider, R.L., Sharitz, R.R., 1986. Seed bank dynamics in a southeastern riverine swamp. American Journal of Botany 73 (7), 1022–1030.
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R.J. Daly et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 309 (2011) 374–382
Schneider, R.L., Sharitz, R.R., 1988. Hydrochory and regeneration in a bald cypresswater tupelo swamp forest. Ecology 69 (4), 1055–1063. Smith, A.C., Hurley, A.M., Briden, J.C., 1981. Phanerozoic Palaeocontinental World Maps. Cambridge University Press. Spicer, R.A., Parish, J.T., 1990. Late Cretaceous–Early Tertiary Palaeoclimates of northern high latitudes: a quantitative view. Journal of the Geological Society of London 147, 329–341.
Spicer, R.A., Wolfe, J.A., Nichols, D.J., 1987. Alaskan Cretaceous–Tertiary floras and arctic origins. Palaeobiology 13 (1), 73–83. Ziegler, A.M., Scotese, C.R., Barrett, S.F., 1983. Mesozoic and Cenozoic palaeogeographic maps. In: Brosche, P., Sündermann, J. (Eds.), Tidal Friction and the Earth's Rotation, II. Springer-Verlag, Berlin, pp. 240–252.