Biogeography in the Sub-Arctic. Группа авторов
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Название: Biogeography in the Sub-Arctic
Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Жанр: География
isbn: 9781118561355
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The deposits from Jameson Land have been dated using a number of different methods, the most important being luminescence dating, which shows that the deposits can be referred to the last interglacial stage, or marine isotope stage 5e, which has been dated to the time interval 115 to 130 000 years BP (Funder et al. 1998).
The Thule area. Deposits from the last interglacial period are also fairly widespread in the Thule area, where marine, coastal‐near deposits consisting of gravel, sand, silt and clay are found. The marine fauna includes the barnacle B. balanoides that does not occur so far north today (Kelly et al. 1999). Last interglacial lake sediments with the southern extralimital midge Chaoborus have also been found in the area (McFarlin et al. 2018).
One of the coastal cliff sections contains remains of non‐marine plants and animals (Böcher 1989, 2012; Bennike and Böcher 1992; Brodersen and Bennike 2003; Hedenäs and Bennike 2003). Some of these species, such as the mosses Pleurozium schreberi and Conardia compacta, the shrub Betula sect. Nanae, the clubmoss S. selaginoides and the invertebrates Simocephalus vetulus and C. mucedo, are southern extralimital (Bennike and Böcher 1992; Hedenäs and Bennike 2003). The mean temperature for the warmest month of the year was ~4 °C higher than at present. The layers at Thule are primarily dated by means of luminescence dating.
Remains of the arctic/alpine carabid beetle A. alpina have been reported from last interglacial layers from both the Thule area (Figure 11) and from Jameson Land; in addition, fragments believed to be reworked from last interglacial layers have been recorded from Zackenberg (Christiansen et al. 2002) and Washington Land (Bennike et al. 2000; Bennike 2002). The finds indicate a wide geographical range of A. alpina in Greenland at that time. The species is absent from Greenland today, but has otherwise a wide circumpolar distribution, and it is found as close to North‐West Greenland as Devon Island in high arctic Canada (Figure 12; Lindroth 1957, 1968; Böcher 1989; Allan V. Morgan, personal communication). It is difficult to understand why the species is missing in modern Greenland. One explanation could be that temperatures during the Holocene thermal maximum were not high enough to allow the species to migrate from Canada (Böcher 1989). The occurrence of another Holarctic species, Isochnus arcticus (Curculionidae; Figures 11 and 12) in sediments from both the Thule area and Jameson Land also favours this explanation (Bennike and Böcher 1992). Remains of the seed‐bug Nysius groenlandicus have also been found in last interglacial sediments from North‐West, North and East Greenland (Bennike and Böcher 1992, 1994; Bennike et al. 2000).
Figure 13 Maps of the northern parts of the Earth, showing the present‐day geographical ranges of (a) Betula pubescens, (b) Alnus crispa and (c) Cornus canadensis. Remains of these species have been found in layers from the last interglacial stage in Jameson Land, East Greenland (black dots).
Source: From Bennike and Böcher (1994; 1996).
The most remarkable zoogeographical characteristic of the interglacial beetle fauna is the total absence of exclusively Nearctic taxa of today, whereas Palaearctic and circumpolar species account for 67 and 33%, respectively. This is the same situation as found in modern Greenland, but in an even more extreme version. Accordingly, the fossil beetle fauna demonstrates that even at the beginning of the previous interglacial a similar situation (ice‐rafting?) might have occurred, capable of transporting European taxa over the North Atlantic to Greenland (Bennike and Böcher 1994; Böcher 2012).
The composition of the interglacial fauna deviated strongly from the modern. For instance, two species of Coccinellidae are found both as fossils and living, but the species are different. Four species of Carabidae are found today, but three different species are recorded as interglacial fossils. This striking dissimilarity presumably reflects the small chance of successful long‐distance dispersal from Europe.
Warming Land. Dating of a Salix twig from a sample of organic detritus found in Warming Land, North Greenland yielded a non‐finite radiocarbon age. The fossil flora includes D. octopetala, S. arctica, S. oppositifolia, O. digyna and M. apetalum and is similar to the present flora of the region. In addition to plant remains, the assemblage comprises a mandible of Lepidurus arcticus and five droppings of a small rodent, probably lemming (Meldgaard and Bennike 1989). The plant and animal remains imply interglacial conditions and they may date from the last interglacial period.
Washington Land. Dating of leaves of Dryas integrifolia from a peat deposit in southern Washington Land gave non‐finite ages (Bennike and Jepsen 2000). Radiocarbon dating and optically stimulated luminescence dating of further samples from what is believed to be the same deposits gave Holocene ages, and it is possible that both interglacial and Holocene material is present at the site (Bennike 2002). Further work is needed to clarify this issue. The fossil flora comprises E. nigrum that does not grow this far north at present. Herbaceous plants are represented by, for example, S. oppositifolia, P. viviparum, O. digyna and Ranunculus hyperboreus, which are all found in the region today (Bennike and Jepsen 2000).
From Lafayette Bugt, western Washington Land, dating of a sample of D. integrifolia leaves from thin layers rich in organic detritus gave an age of ~4000 ka BP. In addition to D. integrifolia the sample was dominated by S. arctica, Potentilla sp., S. oppositifolia and Carex stans, all common in the region today. However, the sample also comprised two achenes of the macrolimnophyte P. filiformis, which does not occur so far north today, and a few fragments of the ground beetle A. alpina, which does not occur in Greenland today. The remains of P. filiformis and A. alpina may originate from unknown interglacial deposits in the area and a last interglacial age has been proposed (Bennike et al. 2000; Bennike 2002).
Blåsø, Kronprins Christian Land. Another site with organic detritus has been reported from North‐east Greenland (Bennike and Weidick 2001). A sample of D. integrifolia was dated to the Mid‐Holocene, but the fossil assemblage comprised oospores of the charophyte Nitella sp. and achenes of P. filiformis, which do not occur this far north today. The remains of these taxa may have been reworked from unknown interglacial deposits.
Skallingen, Kronprins Christian Land. In Lille Sneha Sø north of Blåsø interglacial plant and animal remains have been discovered. The interglacial flora and fauna include several species that do not live so far north today, such as Tolypella cf. nidifica, P. filiformis, the ostracod Bradleystrandesia reticulata, the tad‐pole shrimp L. arcticus, an unidentified dytiscid water beetle and the small bivalve Pisidium sp. Several age determinations of Scorpidium scorpioides gave pre‐Holocene ages (Wagner СКАЧАТЬ