Saturday, 30 March 2019

Discovery of a large impact crater beneath the Hiawatha Glacier in northern Greenland.

Greenland has been an area of interest to explorers for centuries, but due to its remote location and extensive ice cover new features can still be found there with surprising frequency. This applies particularly to areas of the island covered by ice sheets, which were completely hidden until the advent of airborne radar sounding in the 1970s, and which still have not been completely explored.

In a paper published in the journal Science Advances on 14 November 2018, a team of scientists led by Kurt Kjær of the Centre for GeoGenetics at the Natural History Museum of Denmark at the University of Copenhagen, describe the discovery of a large impact crater beneath the Hiawatha Glacier in northwest Greenland, as a result of surveys carried out in the area by the Greenland Ice Mapping Project, combined with data collected between 1997 and 2014 by NASA’s Program for Arctic Regional Climate Assessment and Operation IceBridge.

The crater measures 31.1 km in diameter and has a rim-to-base depth of 320 m, making it one of the largest known impact craters on Earth. The central portion of the crater rises about 50 m above the base (such central peaks are typical of impact craters, helping to distinguish them from similar appearing structures, such as volcanic calderas). The structure is cut by two winding subglacial channels to the southeast, which appear flow into the crater, while to the northwest a third channel cuts through the crater rim, with ice apparently flowing outward to form a distinct tongue on the northern margin of the glacier, about 1 km from the rim. The whole structure is covered by about 930 m of glacial ice.

Geomorphological and glaciological setting of Hiawatha Glacier, northwest Greenland. (A) Regional view of northwest Greenland. Inset map shows location relative to whole of Greenland. Magenta box identifies location of (B) to (D). (B) A 5-m ArcticDEM mosaic over eastern Inglefield Land. Colours are ice surface velocity. Blue line illustrates an active basal drainage path inferred from radargrams. (C) Hillshade surface relief based on the ArcticDEM mosaic, which illustrates characteristics such as surface undulations. Dashed red lines are the outlines of the two subglacial paleochannels. Blue lines are catchment outlines, i.e., solid blue line is subglacial and hatched is supraglacial. (D) Bed topography based on airborne radar sounding from 1997 to 2014 NASA data and 2016 Alfred Wegener Institute (AWI) data. Black triangles represent elevated rim picks from the radargrams, and the dark purple circles represent peaks in the central uplift. Hatched red lines are field measurements of the strike of ice-marginal bedrock structures. Black circles show location of the three glaciofluvial sediment samples described. Kjær et al. (2019).

The Hiawatha Glacier lies atop a terrain of metamorphosed Palaeoproterozoic rock which forms part of the Inglefield Mobile Belt, into which the crater appears to be impacted. Kjær et al. collected sediment samples from the river that runs from the northwest part of the glacier into the Nares Straight; this is fed by the channel that cuts through the northwestern part of the crater rim, and is the most sediment rich river in the region. These sediment samples contained large numbers of shock-deformed quartz grains, which are considered to be typical of impact sites, although they can be formed by other catastrophic events, as well as grains of K-feldspar, mesoperthite, plagioclase, quartz, sillimanite, garnet, orthopyroxene, rutile, ilmenite, apatite, and other accessory minerals from the local bedrock, all of which show intense fracturing, again typical of (but not unique to) impact sites.

Shocked quartz grains from glaciofluvial sediment sample HW21-2016. (A to C) Microphotographs and backscattered electron (BSE) microscope images of planar deformation features. (A) Two sets, symmetrical with respect to the optical and crystallographic c axis. (B) Four sets. (C) Four closely spaced sets throughout a toasted quartz grain. Kjær et al. (2019).

The sediments also produce glassy spherules; these are formed when drops of melted rocks re-solidify in the air, and are commonly associated with impact events, although they can be formed volcanically. These glassy spherules appear to be derived from the local minerals, having chemical compositions similar to biotite, garnet, or feldspar, found in the area, with some notable changes, for example the feldspar-like spherules show reduced magnesium and increased iron compared to the local rock samples, the biotite-like spherules contain more calcium oxide than biotite minerals found in the area, and the garnet-like spherules are enriched in potassium oxides. Some of these glassy spherules also contain tiny fragments of other minerals trapped within them, such as plagioclase, ternary feldspar, orthopyroxene, zoned clinopyroxene, or ilmenite.

Impact-related sediment grains from glaciofluvial sediment sample HW21-2016. (A) Grain 21C-v32: Pale yellow glass grain of biotite–like composition with possibly inherited prismatic sillimanite (Sil) crystals and beginning devitrification in its lower part. (B) 21D-u28: Pale green glass grain of garnet (Grt)–like composition with dark rim and beginning devitrification around small trapped mineral fragments. (C) 21C-t26: Black glass grain of felsic-like composition with new microporphyritic clinopyroxene (Cpx) and ilmenite (Ilm). (D) to (F) 21B-12a: Microperthitic K-feldspar (Kfs) (D) and brown glass of K-feldspar–like composition (E). Inclusions of quartz (Qtz) have acted as nucleation centres for devitrification (F). (G) and (H) 21C-z08: Dark brown, ellipsoid glass particle of garnet-like composition with a central contraction crack and beginning crystallization of slender, prismatic, radial crystallites. (I) and (J) 21C-x20: Pale glass grain of aluminous felsic composition with new microporphyritic orthopyroxene (Opx), zoned cordierite (Crd), and skeletal plagioclase (Pl). (K) 21C-u05: Devitrified glass of felsic-like composition with four quartz fragments with PDFs. Arrows indicate prominent planar deformation features orientations. (L) 21C-w29: Pale brown glass of K-feldspar–like composition; quartz inclusion with planar deformation features (top left) and two round inclusions lined with pale micaceous material, possibly former vesicles in the impact mineral melt. (M) 21C-z22: Lozenge-shaped, toasted quartz fragment with PDFs throughout, rimmed by black amorphous carbonaceous material. (N) and (O) 21D-r06: Quartz fragment with ballen structure (O), set in a matrix of feldspar-like composition with tiny micaceous crystallites. (P) and (Q) 21E-p08: Microbreccia with matrix of minute ternary feldspar grains and numerous tiny voids (Q) and inclusions of quartz, K-feldspar, plagioclase, garnet, and ilmenite, and larger elongate, cuspate voids, and channels in quartz (black arrows) with interior linings of clayey material. White arrow in enlargement pointing at a hole from sample preparation, clearly distinguishable from the neighbouring original void. (R) 21D-u01: Black ellipsoidal grain comprising numerous target mineral fragments and dust in a carbonaceous matrix identified with scanning electron microscopy–energy dispersive spectrometry and indicated by microprobe totals of only 40 to 70 weight %. (S) The entire 21D-u01 grain with hole from polishing. Kjær et al. (2019). 

The sediment samples also show raised levels of nickel, cobalt, chromium, platinum group elements (particularly rhenium and platinum), and gold, which are hard to explain by comparison to other local rocks (in fact the rhenium levels would be surprising in rocks from the Bushveld complex in South Africa, which are noted for their high levels of this element), but which are known to be raised in some iron meteorites.

The ice of the Hiawatha Glacier has been split into three stratigraphic units, based upon drill coring and deep radar surveys of the ice formation. The uppermost unit is interpreted as being of Holocene age (laid down between 11 700 years ago and the present), reaching a maximum thickness of about 700 m to the southeast of the crater and in the southeastern portion of the crater itself. The base of this unit is marked by a dark, debris-rich layer thought to have been laid down during the Younger Dryas cold period (12 800 to 11 700 years ago). Beneath this the second unit present is thought to have been laid down during the Last Glacial Period (between 115 000 and 11 700 years ago). This layer reaches a maximum thickness of about 200 m to the southeast of the crater and in the eastern portion of the crater. Within this layer are a series of four horizons reflective to radar. The youngest is associated with the Bølling-Allerød warm period (roughly 14 700 to 12 800 years ago), while the next of these has been dated to about 38 000 years ago. Unfortunately, none of these reflective layers is present within the crater, nor within about 100 km of it. The lowest unit within the glacier is much darker and contains significant non-ice debris, probably associated with erosion of the underlaying deposits by the action of the glacier (i.e. bits of rock ripped from the underlying surface as the glacier flows over it. This layer reaches a maximum thickness of about 150 m, within the central part of the crater.

Thickness of Holocene, Last Glacial Period (LGP), and basal ice within and near Hiawatha Glacier. Background is a natural-colour composite Landsat-8 scene from 11 August 2015. Black lines are survey tracks. Units are mapped only where identification is unambiguous. Holocene ice thins as ice flows toward the glacier and is extensively exposed at the ice margin. The incomplete Last Glacial Period ice sequence thins significantly downstream of the centre of the Hiawatha impact crater. Conversely, the apparently debris-rich basal ice thickens significantly downstream of the structure’s centre. Inset panels show mean, SD, and distribution of the absolute value of crossover thickness differences. Kjær et al. (2019). 

Kjær et al. estimate that an iron meteorite 1.5 km in diameter impacting at a speed of 20 km per second would produce a crater roughly 20 km in diameter and 7 km deep, which would then collapse to form a structure about 31 km across and 800 m deep, with an uplifted central area, roughly what is seen beneath the Hiawatha Glacier, however such an impact would also produce a layer of ejecta roughly 200 m thick at the rim of the crater, which would thin to about 20 m thick 30 km from the crater, a debris field which is not seen. Should the site have been covered in ice at the time of the impact, then a larger or faster meteorite would be needed to form the crater, but less non-ice ejecta would be formed. In such an eventuality, the majority of the ejecta that was formed would fall onto ice, and be carried away by the motion of the glacier. However, even an impact onto a glacier should produce some debris, and no apparent impact ejecta has been found. Kjær et al. suggest that one way in which this could be explained is by the object coming in at an oblique angle (more than 45° from the vertical), which would lead to a highly asymmetric debris field, which could have been overlooked. They suggest this is not particularly unlikely, as most known Near Earth Asteroids travel on orbits close to that of the plane of the Solar System, so that an impact in the far north is more likely to be oblique than one close to the equator, and that an impact by such an object would be likely to scatter debris preferentially to the north (i.e. into the Nares Straight) where it would not be detected by ice-core drilling.

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