Biogeography in the Sub-Arctic. Группа авторов
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Faroe Islands
At Borðoyarvík near Klaksvík in the north‐eastern part of the Faroe Islands, an interglacial deposit is exposed in a coastal cliff section (Rasmussen 1972; Wastegård et al. 2005). The deposit is ~1 m thick and dominated by clay with a content of total organic carbon that decreases from 6% in the lower part to 2% in the upper part. The diatom flora comprises some marine or brackish taxa, but the common presence of statoblasts of the freshwater bryozoan Cristatella mucedo, especially in the lower part of the unit, may contradict a brackish water environment, because the species usually does not occur in brackish water (but see Økland and Økland 2000).
Notes on the pollen flora are found in Rasmussen (1972; sample analysed by Johs. Iversen), in Jóhansen (1985, p. 12) and in Wastegård et al. (2005). Iversen reported a dominance of Empetrum and Poaceae, with some Betula and rare grains of Pinus and Corylus. Jóhansen reported pollen of Buxus, Betula, Lonicera, Plantago lanceolata, Nymphaea, Poaceae, Cyperaceae and Ericales. Wastegård et al. (2005) found Alnus, Betula, Carpinus, Picea, Pinus, Ericaceae, Apiaceae and Asteraceae. A more recent study (Bennike et al. 2018) indicates accumulation in a coastal lagoon in a landscape and climate similar to that of the Mid‐Holocene.
Plant macrofossils are represented by a few pieces of wood. One of them was identified to Pinus and another to Picea or Larix (Rasmussen 1972; Jóhansen 1985). However, it is possible that the wood is driftwood (Wastegård et al. 2005) and if so it may not provide information on the local flora. Betula remains were referred to as Betula sp. by Wastegård et al. (2005), but the remains come from tree birches (Betula sect. Albae, Bennike et al. 2018). Cristatella mucedo, Betula sect. Albae and Ajuga are fairly thermophilous and may indicate summer temperatures slightly higher than at present. The interglacial deposit at Borðoyarvík can be referred to the Eemian, based on tephrochronological studies (Wastegård et al. 2005).
The Faroe Islands were covered by a local ice cap during the last glacial stage (Jørgensen and Rasmussen 1986; Sejrup et al. 2005), and the oldest minimum dates for the deglaciation are ~11.3 ka (Jóhansen 1985; Hannon et al. 2010). The Early Holocene vegetation history of the Faroe Islands is best documented from a high‐resolution study of lake sediments from Lykkjuvøtn on Sandøy (Hannon et al. 2010). Some information is also available from sub‐marine lake deposits from Skálafjørður (Bennike et al. 1998; Tendal 2004; Bennike 2010). Plants documented by macrofossil finds from Early Holocene deposits include the shrubs Salix herbacea, Betula nana, Empetrum nigrum, the herbs Stellaria alsine, Oxyria digyna, Rumex acetosa, Ranunculus flammula, Ranunculus acris, Cardaminopsis petraea, Filipendula ulmaria, Viola palustris, Caltha palustris, the club moss Selaginella selaginoides and the water plants Isoetes lacustris, Potamogeton pusillus, Potamogeton filiformis, Myriophyllum alterniflorum and Sparganium angustifolium (Jóhansen 1985; Bennike et al. 1998; Hannon et al. 2010). The invertebrate fauna from Skálafjørður includes the sponge Racekiela ryderi, an amphi‐Atlantic species that probably arrived by long distance chance dispersal by birds. The same probably applies to the bryozoans Plumatella sp. and C. mucedo. The coleopteran fauna includes the ground beetle Nebria rufescens that is common and widespread in the islands today. The Early Holocene flora and fauna from the Faroe Islands indicate a climate similar to the present.
Iceland
This short review on interglacial biotas in Iceland is mainly based on the excellent and richly illustrated review by Grímsson (2011). Five occurrences with plant remains and one with arthropod remains have been described from Iceland, and exposures of two of them are shown in Figure 3 (see Plate section). The ages of the older formations are relatively well constrained from palaeomagnetic studies and by Ar/Ar age determinations of underlying or overlying volcanic rocks. Alnus fossils, referred to Alnaster viridis fossilis, Alnus aff. viridis and Alnus cf. viridis are found in deposits from the four oldest formations (Figure 4). During the Holocene, Alnus did not reach Iceland.
The Brekkukambur Formation
The Brekkukambur Formation consists of sedimentary rocks that are exposed in south‐west Iceland. The age of the succession is estimated to 2.4–2.1 Ma, and it is dominated by fluvial conglomerate and sandstone with some siltstone and claystone deposited in a lake. Macrofossils of Alnaster viridis fossilis, Betula sp., Vaccinium uliginosum, Dryas octopetala, Salix glauca, Salix lanata and Salix phylicifolia fossilis have been recorded. The pollen flora is dominated by Alnus, Salix and Betula, and the flora can be characterized as boreal with some arctic elements, notably D. octopetala.
Figure 3 (a) Mt. Stöð with an outcrop of the Búlandshöfði Formation showing lagoonal mud overlain by deltaic sandstones. (b) Svínafell with the Svínafellsfjall Formation showing exposed lake sediments.
Source: Photographs kindly provided by Friðgeir Grímsson.
The Víðidalur Formation
The Víðidalur Formation (~ 1.7 Ma) is found in north‐western Iceland and consists of tillite, conglomerate, sandstone, siltstone and claystone. The macrofossil assemblage is dominated by Alnus aff. viridis and Polygonum viviparum but also includes B. nana × pubescens, Vaccinium cf. uliginosum, D. octopetala and Salix sp. The pollen and spore flora are species‐rich and include Pinus, Menyanthes, Myrica, Trollius, Fragaria and Sanguisorba. Overall, the fossil flora indicates forest‐tundra vegetation on uplands in the region.
The Búlandshöfði Formation
This formation is found in western Iceland; it is covered by lavas that are dated to ~1.1 Ma. The lower part of the formation is a glaciomarine deposit with arctic molluscs such as the small bivalve Portlandia arctica. The upper part is dominated by shallow‐water marine deposits with an interglacial fauna that includes the gastropods Littorina littorea and Nucella lapillus, the blue mussel Mytilus edulis and the barnacle Balanus balanoides. The upper part of the formation includes a lacustrine succession with sandstone and siltstone with plant fossils (Figure 4).
Macrofossils of E. nigrum, Vaccinium cf. uliginosum, Potentilla sp., Salix arctica tp., S. herbacea and Valeriana sp. have been recorded. Again, the pollen and spore flora are more diverse than the macroflora and include Pinus, Alnus and Betula, Mercurialis perennis, Plantago coronopus and Polygonum aviculare and overall the fossil flora indicates forest‐tundra vegetation on uplands in the region and an interglacial type of climate similar to that of the Holocene.
The Svínafellsfjall Formation
The Svínafellsfjall Formation in south‐eastern Iceland is approximately 0.8 Ma old and comprises lacustrine siltstone and sandstone with numerous dropstones. Plant macrofossils include Thelypteris limbosperma, Alnus cf. viridis, Vaccinium cf. uliginosum, P. viviparum, Alchemilla sp., D. octopetala, Sorbus aff. aucuparia, S. arctica tp. and S. herbacea (