Biogeography in the Sub-Arctic. Группа авторов

Чтение книги онлайн.

СКАЧАТЬ København Formation is that of Amara alpina, so common both in last interglacial sites from Greenland and in the Beaufort Formation (Matthews and Fyles 2000; Elias and Matthews 2002).

      The dating of the Kap København Formation is based on a number of different methods, of which the most important are biostratigraphy, palaeomagnetic analyses and amino acid analyses. The biostratigraphically most important groups are foraminifers, ostracods, molluscs and mammals. The occurrence of the extinct rabbit Hypolagus sp. and the extant hare Lepus sp. in member B3 is particularly important. These genera co‐occurred in North America during the time period from ~2.3 to 2.0 million years BP (Repenning et al. 1987). This is in good agreement with the latest age estimate based on benthic foraminifera, which indicate an age for member B of ~2 Ma (Bennike et al. 2010), perhaps corresponding to marine isotope stage 77 and one of the so‐called super interglacials that have been documented in Arctic Russia (Melles et al. 2012).

      The Île de France Formation

At the type locality the lower part of the Île de France Formation in North‐East Greenland is heterogeneous, consisting of diamicton, fine‐grained sand, homogenous silt and rhythmically layered silt and sand. This part contains no macro‐fossils and was presumably laid down in a glaciomarine environment. The upper part of the sequence is more homogenous and consists of bioturbated silt and fine sand with scattered shell fragments of marine molluscs. At a few sites on northern Île de France sand with organic detritus is found (Bennike et al. 2002a).

      Twenty‐four species of marine molluscs have been found and the fauna includes Trichotropis bicarinata, which formerly as a fossil in Greenland has only been reported from the Kap København Formation (Bennike 1989), and Astarte alaskensis, which is not known from other sites in Greenland. Diatoms are represented by 99 species, of which the vast majority are marine species, but the diatom flora also includes some freshwater taxa. The latter as well as colonies of the fresh water algae Pediastrum sp. and Botryococcus sp. indicate some outflow of freshwater. The diatom flora comprises Fossula arctica, which is associated with sea ice and is only known from the Arctic.

      The marine fauna show that the sediments were deposited at sub‐littoral water depths on the inner shelf. The molluscs comprise warmth demanding species such as A. islandica, which shows that the water temperature was considerably higher than at present. The dinoflagellate cysts indicate polar to sub‐polar environments.

      In addition to remains of marine organisms and microscopic freshwater algae scattered fragments of wood and bark are found, as well as rare seeds and fruits. This material comprises Picea, probably P. mariana, Thuja sp. and M. trifoliata. These sparse remains may imply that the adjoining land areas were covered by coniferous forest or forest tundra.

      The Île de France Formation may have been deposited during the Olduvai normal polarity subchron at 1.95–1.78 Ma, based on palaeomagnetic studies and data from benthic foraminifera (Bennike et al. 2010). This means that it is slightly younger than member B of the Kap København Formation.

      The Store Koldewey Formation

      The Store Koldewey Formation is found at ~120 m above sea level and consists of bioturbated marine silt and fine sand (Bennike et al. 2010). The marine mollusc fauna comprises 13 species of bivalves and 5–6 species of gastropods. The presence of M. balthica and C. kurriana indicates lowered salinity, perhaps near a river outlet.

      At two localities thin layers with species‐rich, well‐preserved remains of non‐marine macrofossils were found. The fossil flora comprises remains of trees (Larix sp., Betula sect. Albae sp. and Alnus sp.), shrubs and dwarf‐shrubs (Arctostaphylos uva‐ursi, Andromeda polifolia and E. nigrum) and herbs (Viola sp., Saxifraga oppositifolia and S. microcarpus), which indicate the presence of forest‐tundra vegetation in the region. The presence of several species of macro‐limnophytes and statoblasts of the freshwater bryozoan C. mucedo testifies to the presence of various freshwater biotopes. Coleoptera remains were scarce and represented by five taxa that are also found in the Kap København Formation. Only a single taxon, the circumpolar, low arctic‐temperate rove beetle Tachinus elongatus was identified to species level.

      The moss remains from Store Koldewey were discussed by Hedenäs and Bennike (2009). A total of ~50 species were found, of which several are southern extralimital species. Two of them, Hamatocaulis vernicosus and Hygrohypnum montanum, do not occur in Greenland at present. They are the only examples of non‐Greenland moss species found as fossils in Greenland.

      It was suggested that the age of the Store Koldewey Formation is similar to the age of the Île de France Formation by Bennike et al. (2010), mainly based on palaeo‐magnetic studies and data from benthic foraminifera.

      Other Possible Early Quaternary Occurrences

      At several sites in northern Greenland occurrences of reworked wood that have yielded non‐finite radiocarbon ages have been located (Bennike 1990, 1998, 2000b). These occurrences may represent remains of Early Quaternary interglacial forests. The largest concentrations of wood are found in Washington Land, where wood is found at elevations up to 600 m above sea level (Bennike 2000b). At none of these sites have sedimentary deposits been found in connection with the wood.

      Marine Oxygen Isotope Stage 13

      Analyses of pollen and spores from a marine sediment core (ODP 646) indicate that southern Greenland was characterized by shrub‐tundra vegetation (de Vernal and Hillaire‐Marcel 2008) during marine oxygen isotope stage 13, which is dated to ~500 ka.

      Marine Oxygen Isotope Stage 11

      Analyses of samples from core ODP 646 indicate that marine oxygen isotope stage 11 was characterized by a much reduced, nearly vanished Greenland ice sheet, with boreal coniferous forest with abundant Picea and some Abies in southern Greenland (de Vernal and Hillaire‐Marcel 2008). The MIS 11 interglacial is different from other Middle Quaternary interglacials because of its near 50 000‐year duration. It is dated to ~400 ka. The dominance of Picea from the beginning to the end of the interglacial period suggests the presence of forest vegetation throughout the entire interval, at least over southern Greenland. The base of MIS 11 is marked by higher proportions of shrub and herb pollen, indicating more open vegetation and a cooler climate, but Picea was probably already present regionally. Detailed examination of the Picea pollen indicates the occurrence of several species, among which Picea abies dominated. Picea abies is presently common in northern Europe.

      Studies of the basal silty‐rich ice from the 2 km long DYE‐3 ice core from south‐central Greenland showed that the ice contains remains of DNA molecules that could be assigned to Picea, Pinus, Alnus, Taxaceae, Poaceae, Asteraceae and Fabaceae. The presence of these plant taxa indicates open northern boreal forests, very different from today's Arctic environment (Willerslev et al. 2007). These taxa were identified by independent laboratories; in addition, Achillea, Betula, Cerastium, Festuca, Luzula, Plantago, Poa, Saxifraga, Symphoricarpos and Populus were recorded but lacked identity between independent laboratories. Most of these genera are common in northern boreal forests. The ice also contained arthropod DNA from Lepidoptera and probably Coleoptera, Diptera, Arachnida and Nymphalidae. The age of the DNA molecules from DYE‐3 is uncertain, but it is possible that they correlate with marine oxygen isotope stage 11.

      Marine СКАЧАТЬ