Biogeography in the Sub-Arctic. Группа авторов

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СКАЧАТЬ href="#ulink_b74e7e75-8a7e-5c28-8225-72ee21a5da0d">Figure 12 Dark ash layers contrasting with white diatomite sediments on the island of Fur, Limfjord, Jutland. Originally deposited sub‐horizontally, the sequence was later severely deformed by Pleistocene ice sheets.

      Source: Photo by B.G.J. Upton.

The Palaeocene–Eocene Thermal Maximum

      A prolonged period of global warming commencing at 55 Ma (Palaeocene–Eocene thermal maximum) is attributed to the effects of the proto‐Iceland plume. This was ‘a period of climatic turmoil’ that lasted for over 100,000 years during which ocean temperatures increased by 3–10 °C (Nisbet et al. 2009). On land, this period saw the extinction of a large number of mammalian groups that had been dominant in the Palaeocene and the appearance of three modern mammalian orders. These evolutionary changes have been linked to diversification and dispersal in response to rapid environmental changes at this time (Hallam 2004). In the oceans the principal casualties were the benthic foraminifera, the most abundant deep‐water organisms. At ~55 Ma about half of all benthic Foraminifera species were wiped out (a greater loss than had occurred at the Cretaceous–Palaeocene boundary (best known for the extinction of the dinosaurs). This calamity for the foraminiferans has been ascribed to ocean warming and acidification as a result of rising CO₂ content (Hallam 2004; Lovell 2010).

      Methane hydrates (clathrates) are solids resembling ice, composed of water + gas and stable at high pressures and low temperatures that occur beneath the sea floor. Destabilization of these compounds yields free methane, which is a more efficacious ‘greenhouse gas’ for absorption of solar heat than CO₂ (Svenson et al. 2004). There are numerous hypotheses regarding the actual process by which the ‘greenhouse’ gases were emitted. It has been suggested that arrival of the mantle plume resulted in short‐term sea‐floor uplift that caused both a sea‐temperature rise, pressure reduction and consequent dissociation of the hydrates (MacLennan and Jones 2006). Yet another hypothesis is that a great emission of methane came from an enclosed marine basin in which enormous amounts of methane briefly existed. One such basin, specifically suggested, between Norway and Greenland (called ‘the Kilda Basin) and the triggering of gas was related to a rise of the Iceland Plume (Nisbet et al. 2009).

Iceland

The volcanic activity in Iceland over the past 16 Ma is a direct continuation of the magmatism that commenced at 56–55 Ma in East Greenland (Figure 13, see Plate section). The North Atlantic spreading axis can be observed in the Icelandic eastern neo‐volcanic zone at the present day where the plume axis is presently inferred to underlie the Vatnajökull ice‐cap (Wolfe et al. 1997). Rift volcanism re‐commenced in October 2014 in the Bárđarbunga volcanic system and is currently active as I write. Volcanic cycles in Iceland are connected to rift‐jumping, thought to occur in order to bring the spreading centre over the plume axis. These cycles have an approximate life‐span of 8–12 Ma (Brooks 2011 and references therein).

Evidence for Plume Pulsing

That the plume tail arose in pulses of varying temperature was suggested by White et al. (1995) following Vogt (1971), from consideration of the strength of the V‐shaped ridges of the ocean floor south of Iceland. These ridges, signifying thickened oceanic crust, straddle the mid‐ocean ridge on either side of Iceland. The thickenings are held to be due to fluctuating temperatures as unusually hot pulses in the underlying asthenospheric mantle radiate away from Iceland (Poore et al. 2009, 2011). These authors propose that the hot pulses, with a temperature some 25 °C hotter than that of the back‐ground plume temperature, arise intermittently through a plume conduit (with a radius of ~150 km) under Iceland. The upwelling rate in the conduit is estimated at 27 cm/year with the rate of radial spreading beneath the lithosphere being ~40 cm/year (Poore et al. 2011). Since the varying temperature of the out‐flowing mantle causes elevation or subsidence of the overlying crust (White et al. 1995; Wright and Miller 1996; White and Lovell 1997), it has important stratigraphical implications; hot pulses are responsible for sea‐floor elevation, leading in turn to increased erosion. Consequently, the stratigraphic sequences in the sedimentary basins retain records of intermittent uplifts (White and Lovell 1997; Hartley et al. 2011).

Figure 13 Lava fountaining along a fissure (Krafla volcano) northern Iceland 1980, during an episode of extension and rift opening.

      Source: Photo by Halldór Ólafsson.

At times during the Cenozoic parts of the European continental shelf were so markedly elevated as to be raised above sea‐level and river systems were established. The reconstructed history of one of these ancient landscapes, dating from the Palaeocene–Eocene thermal maximum, has shown that it was lifted above sea‐level in three distinct steps, each of 200–400 m. After about one million years of sub‐aerial erosion, it sank again beneath the sea. Thus the asthenospheric mantle radiating from the Iceland plume was inferred not to have had a constant temperature but to have involved a number of pulses that were exceptionally hot, i.e. that the temperature of the plume fluctuates over time intervals of a few million years (White and Lovell 1997). The size and duration of the shelf uplifts were used to constrain the magnitude and velocity of these pulses (Shaw Champion et al. 2008; Hartley et al. 2011). Thus, the stratigraphy of sedimentary rocks on the ocean retains a record of the mantle pulsing (White and Lovell 1997; Hartley et al. 2011).

      Where uplift raises continental margins above sea‐level, their consequent sub‐aerial erosion causes relatively coarse‐grained sediments to be deposited on adjacent sub‐marine shelves. These pulse‐drive uplifts can have significant commercial consequences: for example, the Forties oil‐field is dependent on sandy reservoir rocks that resulted from continental lithosphere uplift at ~55 Ma, marking the arrival of a major thermal input (Lovell 2010). Furthermore, the changes in the elevation of the Greenland–Faeroes–Iceland and Scotland ridge over millions of years have controlled the deep‐water overflow of the Denmark Straits (Wright and Miller 1996; Nisbet et al. 2009; Poore et al. 2009, 2011).

Continental Uplift after Ocean Formation

      In the aftermath of the ocean opening there was notable uplift of the adjacent ‘trailing’ continental margins. The uplift is noteworthy in, for example, western Scotland and Norway, but is most extreme in eastern Greenland where ‘plateau lavas’ erupted close to sea‐level (and which were preceded by marine Mesozoic strata) have been raised, while remaining essentially horizontal. Among the uplifted rocks are those of Gunnbjørns Fjeld, which at 3693 m is the highest mountain in the Arctic. How much strata have been eroded from above it is unknown.

      The pre‐opening loading, by up to 7 km of basaltic lavas, would be expected to have depressed rather than elevated surfaces. However, the uplift is inferred to be the consequence of intrusion of igneous rocks deep in the crust that more than compensate for the surface loading (Larsen et al. 1998). Despite the huge volume of erupted lavas a much larger volume of magma crystallized deep in the crust as ‘underplating’. The east coast of Greenland presents an elongate area of uplift centred on that part (Kangerdlugssuaq) where the plume axis is deduced to have passed from continent to ocean (Lawver and Müller 1994).

      The tilting of the topography in northern Britain from west to east is also attributed to the process of magmatic underplating (Brodie and White 1994). Consequently, the Iceland plume has been instrumental in shaping the landscapes on either side of the ocean (Fitton and Larsen 2001).

Summary

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