Читать книгу Biogeography in the Sub-Arctic - Группа авторов - Страница 62

Southern Greenland. Pollen concentrations in samples from MIS 5e from core ODP 646 are five times higher than in Holocene samples, and the concentrations of fern spores are also higher. The assemblages are characterized by dominant Alnus and abundant spores of Osmunda. In samples from sediment core HU‐90‐013‐013 collected near ODP 646, more detailed analyses of MIS 5e document the pollen succession. A rapid increase of Alnus occurred during an early phase of MIS 5e, which suggests rapid development of shrub tundra after ice retreat. A subsequent increase of Osmunda represents a unique event in the last million years. It suggests the development of dense fern vegetation over southern Greenland under climatic conditions not unlike those of the modern boreal forest. Towards the end of MIS 5e, pollen and spore influxes decreased and at the same time herb percentages increased, indicating a change to herb tundra resulting from regional cooling at the onset of ice growth (de Vernal and Hillaire‐Marcel 2008).

Jameson Land. Deposits from the last interglacial are found along the south and west coast of Jameson Land in central East Greenland (Figures 1 and 5c and d). Most widespread are sandy, silty and clayey sediments, which were deposited in shallow marine water along the coast, partly in connection with deltas, in the coastal zone and in rivers. The deposits often contain shells of marine molluscs, and the fauna includes M. edulis, a warmth‐demanding southern extralimital species (Vosgerau et al. 1994). At many sites layers and lenses rich in organic detritus are found. The detritus consists mainly of washed together remains of plants that grew in the catchment area, but it also contains remains of insects and other invertebrates. The plant and animal remains give a regional picture of the non‐marine biotas.

Figure 10 Scanning electron microscope photographs of plant remains from last interglacial deposits in Jameson Land, East Greenland. (a) Catkin scale of Alnus cf. crispa. (b) Leaf of Dryas octopetala. (c) Leaf of Vaccinium uliginosum. (d) Endocarp of Arctostaphylos alpina. (e) Leaf of Phyllodoce coerulea. Scale bars: 1 mm.

Source: From Bennike and Böcher (1994).

The fossil flora includes many species of dwarf shrubs and other woody plants (Figure 10). Most noteworthy are common remains of Betula pubescens, which probably formed shrub or low forest on sheltered sites. At present, B. pubescens is confined to southernmost Greenland, whereas B. nana, today common in Jameson Land, has not been found in interglacial deposits in East Greenland. Samples from many localities contain remains of Alnus cf. crispa, which grows in south‐western Greenland and North America at present. Another woody plant that grows in southernmost Greenland at present is Cornus canadensis, fruits of which are rare in the fossil assemblages. Dwarf shrubs are represented by no less than 11 species, of which the majority is ericaceous. Remains of E. nigrum, V. uliginosum, Arctostaphylos alpina and D. octopetala are particularly common, whereas remains of Cassiope tetragona and Harrimanella hypnoides are rarer. Leaves of S. herbacea are common; this tiny woody plant is characteristic of snow beds. Herbaceous plants are represented by, for example, Melandrium apetalum and O. digyna, which are typical arctic plants that are common in the region today.

The fossil interglacial flora from Jameson Land is also rich in remains of bryophytes. The bryophyte flora is diverse with more than 60 identified taxa and it includes some southern extralimital species such as Climacium dendroides and Sphagnum warnstorfii (Hedenäs 1994). Abundant remains of Polytrichaceae and Racomitrium show that unstable soil with sparse vascular plant cover was important.

The fossil interglacial fauna of Coleoptera comprises almost 25 taxa of beetles, which is highly surprising considering that Jameson Land's present beetle fauna only comprises three species. In contrast to the flora of vascular plants, the beetle fauna comprised several species that are absent from the modern fauna of Greenland. On the other hand, many of Greenland's extant species are not represented as fossils in the interglacial fauna. The fauna comprises A. alpina (see below) and Otiorhynchus nodosus (Figure 11). The latter is a typical example of a Palaearctic species (Figure 12, see Plate section) that lived in Greenland during the last interglacial and the Holocene. The same applies to, for example, B. fasciatus and Simplocaria metallica.

Figure 11 Scanning electron microscope photographs of beetle remains from last interglacial deposits at Narsaarssuk near Thule air base, North‐West Greenland (a, b) and Jameson Land in East Greenland (c). (a) Elytron of Isochnus arcticus. (b) Half pronotum of Amara alpina. (c) Head of Otiorhynchus nodosus. Scale bars: 1 mm.

Source: From Böcher (1989) and Bennike and Böcher (1992).

All beetle species recorded live in the sub‐arctic bioclimatic zone (Bennike and Böcher 1994; Böcher 2012). The difference between the interglacial and the Holocene fauna indicates that chance dispersal plays a large role in determining which species colonized Greenland during the last and the present interglacial period. Half of the species are Palaearctic, and hence they must have immigrated to Greenland from Europe.

At a single site head capsules of midges were found. Some of these head capsules come from species that live in rivers – a biotope that is otherwise unrepresented in the palaeoecological data (Bennike et al. 2000). Furthermore, it may be mentioned that the fauna includes the freshwater bryozoan C. mucedo, which probably lives in south‐west Greenland at present (Fredskild 1983).

Figure 12 Maps of the northern part of the Earth, showing the present‐day range of the ground beetle Amara alpina and the weevil Otiorhynchus nodosus. The dots show fossil finds in Greenland, assumed to come from the last interglacial. Modern distributions according to Böcher (1989) and Bennike and Böcher (1994).

Some of the most warmth‐demanding plant species from the interglacial layers are B. pubescens, Alnus cf. crispa and C. canadensis (Figure 13, see Plate section), and the flora and insect fauna indicate sub‐arctic conditions, implying a mean summer temperature at least 5 °C higher than today and a displacement of the sub‐arctic bioclimatic zone from southernmost Greenland at least 1000 km northwards. The temperature estimate corresponds with results from studies of an ice core from Renland Ice Cap, an isolated ice cap west of Jameson Land (Johnsen et al. 1992; Vinther et al. 2009).

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 and Bennike 2015). In nearby Trifna Sø a similar flora and fauna was found and a radiocarbon age determination of stems of the bryophyte S. scorpioides gave an age > 52 ka (Kusch et al. 2019). We suggest that the remains from Skallingen are of last interglacial age.

Kap København, Peary Land. A fragment of a strongly weathered reindeer Rangifer tarandus antler found at the ground near Kap København gave a non‐finite age, and a last interglacial age has been proposed (Meldgaard and Bennike 1989).

Zackenberg. A fragment of a mesosternum of A. alpina from an Early Holocene delta deposit at Zackenberg must have been reworked from interglacial, possibly Eemian, sediments (Christiansen et al. 2002).