Читать книгу Biogeography in the Sub-Arctic - Группа авторов - Страница 34

Palaeobotanical investigations in the sub‐arctic regions of the North Atlantic date back to the middle nineteenth century (Heer 1859 et seq., Iceland, Greenland, Svalbard; Gardner 1887, Island of Mull; Hartz 1903, Faroe Islands). The first mention of fossil plants in this region dates back to 1772 (Ólafsson 1772). Heer believed all the Arctic floras to be of Miocene age (e.g. Heer 1868), based on comparison with the (truly) Miocene flora of Öhningen. This age estimate was questioned by Gardner (1884), who suggested an Eocene age for the floras of Mull, Spitsbergen and Greenland based on the presence of Macclintockia in these floras. Gardner's assessment was later corroborated by evidence from molluscs (Ravn 1922). With the general acceptance of plate tectonics in the early 1960s and subsequent first radiometric dating of basalts bordering the North Atlantic basin (e.g. Moorbath and Bell 1965), the modern picture of the evolution of the North Atlantic region emerged. The Cenozoic floras of the northern North Atlantic region belong to the Brito‐Arctic Igneous (floral) Province (BIP; Figure 1) according to Boulter and Manum (1989). The BIP floras are the result of the initial break‐up of the northern North Atlantic during the early Cenozoic and the widening of the North Atlantic during the Neogene (cf. Denk et al. 2011). Whereas the floras at the western (Greenland) and eastern (Scotland, Faroe Islands) margins of the North Atlantic are of Paleogene age, the intra‐basaltic plant‐bearing sedimentary rocks of Iceland are of Neogene age (Figure 2). As such, all these palaeofloral assemblages provide a unique and almost continuous archive of Cenozoic vegetation development in the sub‐arctic North Atlantic region.

Figure 1 The northern North Atlantic part of the Brito‐Arctic Igneous (floral) Province (BIP). Volcanics (dark grey) and sedimentary rocks on West and East Greenland, the Faroe Islands and the British Isles are of Paleogene age, while the rocks on Iceland are of Neogene age. Paleogene BIP‐floras discussed in the text are indicated. 1 – Agatdalen flora; 2 – Upper Atanikerluk A flora; 3 – Upper Atanikerluk B flora; 4 – Hareø flora; 5 – Mykines flora; 6 – Prestfjall flora; 7 – Mull flora.

In early accounts of Paleogene plant fossils from high latitudes angiosperm plant remains were mainly ascribed to modern genera (e.g. Alnus, Castanea, Fagus, Laurus, Populus, etc.; Heer 1868 et seq.). This assessment of high‐latitude fossil plants, and their alleged close relationship to the modern northern temperate woody flora, brought Engler (1879) to establish the term ‘Arcto‐Tertiary element’, used for plant groups that today dominate temperate forest regions and have prominent fossil representatives in Paleogene floras at high latitudes. Later authors considered many of the fossils to represent extinct taxa, commonly with unknown botanical affinities (Koch 1963; Boulter and Kvaček 1989; Kvaček et al. 1994; Mai 1995; Manchester 1999). In contrast, Budantsev and Golovneva (2009) and Grímsson et al. (2015, 2016a) suggested that a substantial number of alleged extinct taxa actually belong to modern genera (Aesculus, Alnus, Betula, Carpinus, Fagus, Quercus and Ulmus), thus supporting Engler's hypothesis about the ‘Arcto‐Tertiary element’.

Figure 2 Relative age of Cenozoic (pre‐Holocene) volcanics and sedimentary rocks of the Brito‐Arctic Igneous (floral) Province (BIP). Stratigraphic occurrences of Paleogene floras of West Greenland, the Faroe Islands and the British Isles (Scotland) discussed in the text are indicated. 1 – Agatdalen flora; 2 – Upper Atanikerluk A flora; 3 – Upper Atanikerluk B flora; 4 – Hareø flora; 5 – Mykines flora; 6 – Prestfjall flora; 7 – Mull flora. Geological time scale based on ICS (2020).

Neogene plant fossils from the sub‐arctic North Atlantic have recently been shown to be important for understanding modern biogeographic patterns in northern temperate plant groups and assessing the subsidence history of the Greenland‐Scotland Transverse Ridge (Tiffney and Manchester 2001; Grímsson and Denk 2007; Tiffney 2008; Denk et al. 2010a, 2011). Biogeographic inferences from plant fossils have also important implications for understanding molecular differentiation in plants and animals (e.g. Denk and Grimm 2010; Kornobis et al. 2011).

In this chapter, we review previous work on the plant fossil record from Palaeocene to Pleistocene sedimentary formations of the sub‐arctic North Atlantic region. This includes Paleogene plant assemblages from Greenland, the Faroe Islands and Scotland, as well as Neogene floras from Iceland (Figures 1 and 2). In addition, we present revised macro‐fossil and new palynological data from Palaeocene to Eocene sediments of Greenland and the Faroe Islands, which considerably change our understanding of the biogeographic history of several northern temperate woody plant taxa that are among the most ecologically important tree species in the mid‐latitudes of North America, Europe and East Asia.