Biogeography in the Sub-Arctic. Группа авторов
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basalt lavas, west of the seaward flexing.
Source: Photo by B.G.J. Upton.
Seaward Dipping Reflectors
In the Kangerdlugssuaq region of East Greenland transition from flat‐lying lavas into a zone in which the lavas acquire increasingly steep dips towards the coast was first recorded by Wager (1934, 1947) and described by Larsen and Jakobsdottir (1988) (Figure 10, see Plate section). The zone of down‐warping is accompanied by an increase in the intensity of the coast‐parallel dyke swarm, culminating in an underlying ‘sheeted complex’ comparable to those seen in ophiolite occurrences regarded as uplift portions of oceanic crust (Nielsen and Brooks 1981; Muttervet al. 1982; Brooks 2011). It came to be realized that what marine seismologic surveyors referred to as ‘seaward‐dipping reflectors’, occurring as belts up to 150 km wide, occur on both sides of the Atlantic and also in west Greenland, were the off‐shore equivalents of these down‐warped lavas.
Figure 10 Kap Hammer on the East Greenland coast (67°40′N), within the zone showing maximum seaward flexing. The cliffs here consist almost entirely of dykes, thus composing a ‘sheeted complex’. Since the flexure here is eastwards, the dykes have a corresponding westward dip. The greater the inclination, the older the dyke. A few (young) near‐vertical dykes are seen in the near cliffs.
Source: Photo by B.G.J. Upton.
The lavas acquired their seaward dips long after the time of their eruption, which had been sub‐aerial (Brooks 2011). Since the total thickness of lavas and their associated intrusions on the rifted margins reaches 25–35 km (Reid et al. 1997; Smallwood and White 2002), these great quantities of basaltic materials on the thinned continental lithosphere would have very significantly increased its bulk density causing the down‐warping. Figure 11 (see Plate section) shows subaerial exposures in West Greenland analogous to those of East Greenland where initially sub‐horizontal lavas were down‐warped towards a new ocean floor in Baffin Bay. As demonstrated in East Greenland, the down‐warping was achieved, not by ductile folding but by displacements along a myriad normal faults dipping away from the developing ocean (Nielsen 1975; Nielsen and Brooks 1981).
Ash Beds of Western and Central Europe
Palaeocene–Eocene basaltic ashes occur widely across north‐west and central Europe (Knox and Morton 1988; Egger et al. 2000). It has been concluded that the ashes were the products of extremely energetic fountaining of magma close to the time of ocean‐opening (Fitton and Larsen 2001; Larsen et al. 2003). The continental crust thinned and subsided, eventually sinking beneath sea‐level as it became ever more laden with lavas and mafic intrusions. At this critical stage, close to parting and formation of the embryonic ocean, the rising magmas interacted with shallow sea waters, producing cataclysmic steam‐driven ash eruptions.
Figure 11 Sub‐aerial (picritic) lavas on the Svartenhuk peninsula, West Greenland (71° 30′N). These lie within the flexed zone, dipping westwards towards the Baffin Bay spreading centre. Such ‘seaward‐dipping reflectors’ are generally sunk below sea‐level but in this instance they are well seen sub‐aerially.
Source: Photo by T.C.R. Pulvertaft.
Ashes from these eruptions travelled hundreds of kilometres eastwards and are reported from as far away as Austria (Egger et al. 2000). Those falling into the shallow seas that covered much of western Europe were generally diluted by normal terrestrial sedimentation and are thus hard to recognize. However, in western Denmark in a shallow marine basin in which diatomite sediment was being very slowly accumulated, some two hundred ash layers contrast strikingly with the intervening white sediments (Figure 12, see Plate section). The alternation of the ‘ash’ beds and white clay reflects the repose periods between eruptions (Fitton and Larsen 2001; Larsen et al. 2003).
Whilst the older ash layers are inferred to have come from sub‐aerial volcanoes on the thinning continental lithosphere, the climactic (‘Stage 4’) ashes are attributed to a time when the locus of the proto‐Iceland plume had shifted away from the Greenland continent into the sea‐covered opening rift. Interaction of incandescent magmas and sea water caused the change from relatively quietly effusive to violently explosive eruptions.
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 (