Biogeography in the Sub-Arctic. Группа авторов

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СКАЧАТЬ 4 Examples of leaf imprints of vascular plants from interglacial deposits in Iceland. (a) Dryas octopetala from the Víðidalur Formation. (b) Salix herbacea from the Búlandshöfði Formation. (c) Salix arctica type from the Búlandshöfði Formation. (d) Thelypteris limbosperma from the Svínafellsfjall Formation. (e) Alnus cf. viridis from the Svínafellsfjall Formation. (f) Dryas octopetala from the Svínafellsfjall Formation. (g, h) Salix arctica type from the Svínafellsfjall Formation. (h) Salix herbacea from the Svínafellsfjall Formation. Scale bars 5 mm.

      Source: Photographs kindly provided by Friðgeir Grímsson.

      Succession at Elliðaárvogur Near Reykjavík

      About 5 km east of Reykjavík an interglacial succession comprises layers of sand and fine gravel with shells of marine shallow water molluscs. Lake deposits with organic remains are also found (Thorkelsson 1935; Einarsson 1981). Seeds and fruits were identified by K. Jessen who recorded E. nigrum, Menyanthes trifoliata, M. alterniflorum, Potamogeton sp., Carex sp. and Scirpus sp. He also found spicules of Spongilla lacustris. K.L. Henriksen identified insect remains and recorded the beetles N. rufescens, Patrobus septentrionis, Pterostichus diligens, Hydroporus sp., Agabus solieri, Tachinus collaris and Byrrhus fasciatus. Fossil cladocerans were identified by Einarsson (1981) who found seven to eight species that all occur in Iceland at present. In contrast, one of the beetles (Hydroporus sp. non Hydroporus nigrita) and one of the vascular plants (Scirpus sp.) do not occur in Iceland at present. The age of the Elliðaárvogur beds is uncertain, but a last interglacial age is possible (LIGA members 1991; Leifur Símonarson and Jón Eiriksson, personal communication, 2013).

      Late Glacial and Early Holocene

      During the last ice age Iceland was covered by an ice cap, the margin of which extended out on to the edge of the shelf (Andrews et al. 2000). Some parts of Iceland were deglaciated during the late glacial, and late glacial raised marine deposits are known from several sites. Late glacial lake deposits have been recovered from the Skagi peninsula in northern Iceland. Studies of late glacial and Early Holocene deposits were conducted in the 1990s (Rundgren 1995, 1998; Rundgren and Ingólfsson 1999).

      Sediments from the Allerød chronozone contain a fairly diverse pollen flora with woody plants such as B. nana, Salix, Juniperus and Empetrum that formed a significant part of the vegetation, which also included various herbs, grasses and sedges. Prior to the Allerød and during the Younger Dryas, a vegetation with pioneer plants dominated (Rundgren 1995).

      The Early Holocene sediments contain plant macro‐fossils in addition to pollen. Macro‐fossils of B. nana, S. herbacea, Salix cf. phylicifolia, E. nigrum, Vaccinium sp., D. octopetala, C. palustris, Angelica sylvestris and Armeria maritima are recorded. Pollen and plant macrofossil records reflect progressive closing of the vegetation cover, from herb tundra over a dwarf‐shrub phase to a shrub and dwarf‐shrub phase (Rundgren 1998).

      Pollen from tree birch are rare in Early Holocene deposits in northern Iceland and it has been suggested that they represent long‐distance transport, either from outside Iceland or from limited areas in Iceland with local tree birch growth (Caseldine 2001). Birch woodland began to develop in Iceland in lowlands along fjords and in valleys ~2000–4000 years after the beginning of the Holocene, and reached their maximum before 7000 years BP. During the Mid‐ and Late Holocene heaths and mires expanded and woodlands became more open (Hallsdóttir and Caseldine 2005).

Rundgren and Ingólfsson (1999) contended that many plant species survived the Younger Dryas in northern Iceland and that these species also could have survived the last glacial maximum in Iceland. In contrast, Buckland et al. (1986), updated in Buckland and Panagiotakopulu (2010), argued that the virtual absence of endemic species supports a model of late glacial or Early Holocene immigration, and maintained that most species of vascular plants and beetles, including heavy flightless beetles, arrived by ice‐rafting from North‐West Europe at the Pleistocene–Holocene transition, an idea first put forward by Coope (1969).

Greenland

      The Kap København Formation

      The Kap København Formation is a succession of clay, silt and sand in eastern North Greenland. The formation was discovered in 1979 and the sequence has been the focus of several subsequent visits (Funder and Hjort 1980). This short review on the Kap København Formation and its flora and fauna is based on Funder et al. (1984, 1985, 2001), Bennike (1990), Böcher (1995) and Símonarson et al. (1998).

The formation covers an area of ~300 km² and is exposed at numerous places along rivers. It has been divided into members A and B. Member A is at least 50 m thick and is dominated by finely laminated clay and silt with rare stones. This member contains rare shells of bivalves and more frequent tests of foraminifers and valves of ostracods. Member B is 40–50 m thick and is dominated by two sandy units (units B1 and B2), which are separated by a more fine‐grained unit (B2). B1 is dominated by horizontally laminated fine and medium grained sand, but cross bedding and other types of sedimentary structures also occur. The sand is texturally and mineralogically mature quartz sand with heavy mineral concentrations in some places. In the upper part large scour‐and‐fill structures are found, these contain large amounts of organic detritus. The sediments in unit B1 were deposited in coastal environments during rising water depth. Unit B2 is characterized by bioturbated clay and silt that was deposited on the inner shelf. Unit B3 is heterogeneous, but it is dominated by horizontally laminated sand. Logs of small trees (Figure 5a, see Plate section) as well as lenses and layers of organic detritus (Figure 5b) is a characteristic feature for unit B3, which is interpreted as deposited in coastal parts of delta environments. In the western part of the Kap København area unit B2 is missing and unit B3 is devoid of plant and animal remains. Member B in this area is interpreted as fluvial.

      Member A and unit B2 contain relatively rich marine faunas. Member A contains an element of warmth demanding Foraminifera species. Unit B2 has a fairly diverse mollusc fauna that includes the bivalves Cyrtodaria kurriana and Macoma balthica, in addition to rich faunas of foraminifers and ostracods. The highest sea water temperatures probably prevailed during deposition of unit B2, with summer temperatures at the sea bottom 7–8 °C higher than at present. Both members A and B show sign of lowered salinity and influence from freshwater. On the surface of member A exposures strongly abraded shell fragments are rather common. They have been designated the allochthonous fauna; it comprises the bivalve Arctica islandica. The allochthonous fauna is assumed to be older than member A, and A. islandica is one of the most warmth‐demanding mollusc species found in the area.

Unit B1 and especially B3 contain a wealth of well‐preserved remains of non‐marine plants and animals, with many different groups represented. Vascular plants include a mixture of boreal and arctic species (Figure 6). Taxa such as Larix groenlandii, Picea mariana, Thuja occidentalis and Taxus sp., Betula sect. Albae, Myrica arctogale, Cornus stolonifera, Viburnum cf. edule and Scirpus microcarpus belong to the first group, whereas D. octopetala, O. digyna and Papaver sect. Scapiflora belong to the second. All remains of wood come from small trees or shrubs, and the largest log that was found had a diameter of 18 cm. Growth rings are narrow to extremely narrow, which may indicate that the mean temperature for the warmest month of the year was ~10–11 °C. Thuja and Taxus cannot tolerate very cold winters, and the mean СКАЧАТЬ