Dating the Hiawatha Impact Structure.

The Hiawatha Impact Structure is an approximately 31 km wide geomorphological structure beneath the Hiawatha Glacier in northwestern Greenland, which has been interpreted as an impact structure on the basis of the structure revealed by airborne radar surveys (a relatively flat, circular depression with an elevated rim and a subtle central uplift), structures in the bedrock along the ice margin, which strike tangentially to the subglacial rim, and the presence of shocked quartz and other impact-related grains in glaciofluvial sediments derived from the largest river draining the structure.

Dating this structure has proved to be difficult. It lies on the surface of the highly metamorphosed 1.95– to 1.75–billion-year-old Ellesmere-Inglefield Mobile Belt, and overlain by the Hiawatha Glacier, which is part of the 2.6 million-year-old Greenland Ice Shelf. This gives a maximum possible age of 1.75 billion years, but, due to the constantly moving nature of the glacier, no minimum age. Impact structures have a fairly constant width-to-depth ratio, so it can be predicted that when it formed the 31 km wide Hiawatha Structure would have had a depth of about 800 m when newly formed. Today it has a depth of about 320 m, implying a loss of about 480 m via erosion since the structure first formed. Estimates of the rate of erosion in subglacial environments vary between 10 m and 10 km per million years, giving the Hiawatha Structure an age of somewhere between 50 thousand and 50 million years. It has also been suggested that the anomalous radiostratigraphy of the ice of Hiawatha Glacier compared to the rest of the Greenland Ice Sheet may be a sign that the impact occurred after the formation of the glacier, making it less than 2.6 million years old.

In a paper published in the journal Science Advances on 9 March 2022, a team of scientists led by Gavin Kenny of the Department of Geosciences at the Swedish Museum of Natural History, William Hyde of the Globe Institute at the University of Copenhagen, Michael Storey of the Quadlab at the Natural History Museum of Denmark, and Adam Garde of the Geological Survey of Denmark and Greenland, present the results of a study which aimed to find a date for the Hiawatha Impact using argon-argon dating of impact-related glaciofluvial sands and uranium-lead analysis of shocked zircons from glaciofluvial clasts of impact melt rock.

Argon-argon dating relies on determining the ratio of radioactive argon⁴⁰ to non-radioactive argon³⁹ within minerals from igneous or metamorphic rock (in this case impact melts) to determine how long ago the mineral cooled sufficiently to crystallise. The ratio of argon⁴⁰ to argon³⁹ is constant in the atmosphere, and this ratio will be preserved in a mineral at the time of crystallisation. No further argon³⁹ will enter the mineral from this point, but argon⁴⁰ is produced by the decay of radioactive potassium⁴⁰, and increases in the mineral at a steady rate, providing a clock which can be used to date the mineral.

Zircons are minerals formed by the crystallisation of cooling igneous (or in this case, impact) melts. When they form, they often contain trace amounts of uranium, which decays into (amongst other things) lead at a known rate. Since lead will not have been present in the original crystal, it is possible to calculate the age of a zircon crystal from the ratio between these elements.

Kenny et al. used a sample of well-sorted, fine-grained sand (HW21-2016) collected from a floodplain about 300 m from the terminus of the Hiawatha Glacier, and about 5 km from the Hiawatha Structure. Examination of satellite and aerial images shows that the section of floodplain material from which the sample was collected did not begin to build up until 2010, making Kenny et al. confident that it does contain material which has been washed along current sub-glacial waterways, and therefore does originate from the Hiawatha Structure.

 

Location and geomorphological setting of Hiawatha Glacier, northwest Greenland. (A) Regional view of northwest Greenland. (B) Bedrock topography mapshowing the Hiawatha structure, and sampling locations of glaciofluvial sediment for argon⁴⁰/argon³⁹ analysis (HW21-2016) and clasts of impact melt rock for zircon uranium/lead analysis (HW19-01 and HW19-05). Bed topography based on NASA and Alfred Wegener Institute airborne radar-sounding data. Samples HW19-01 and HW19-05 are from the same location on a wide riverbank 4 km downstream of the terminus of Hiawatha Glacier. White line represents the present-day margin of the Greenland Ice Sheet. Kenny et al. (2022).

In addition to the sand samples, Kenny et al. selected on two pebble-sized clasts (HW19-01 and HW19-05) obtained from a wide riverbank roughly 4 km downstream from the terminus of Hiawatha Glacier and less than 10 km from the edge of the Hiawatha structure. Both are clast-rich impact melt rocks with a dark grey, aphanitic, hemicrystalline melt matrix dominated by lath-like plagioclase feldspar microlites, that are thought likely to have reached the location where they were found via subglacial and glaciofluvial transport. Portions of both these pebbles were crushed an zircons extracted for analysis.

 

Images of impact melt rocks from the Hiawatha structure. (A) Feldspathic microlitic matrix with clasts of toasted quartz (qtz) and checkerboard feldspars (fsp). (B) Lightly toasted quartz fragment with two sets of PDFs that are considered unequivocal evidence of shock metamorphism. (C) Checkerboard feldspar. (D) Petrographic context of a granular and porous zircon (zr) grain in the feldspathic (fsp) matrix of impact melt rock, with accessory biotite (bt), ilmenite (ilm), and altered cordierite (crd). In contrast to zircon grains like this one that were in direct contact with the impact melt, zircon grains within clasts in impact melt rock do not display porous and granular textures. BSE, backscattered electrons; PPL, plane-polarized light; XPL, cross-polarized light. Kenny et al. (2022).

The sand grains extracted from the floodplain close to the glacier edge were examined visually to look for signs of impact melting. Four types of grains were identified within the sample. The first, and most abundant group, making up 40% of the sample, have a greenish gray, yellow, or dark organic-rich matrix with feldspathic microspherulites about 10 to 50 μm across and fragments of quartz and feldspar. The second most abundant grain type, making up 20% of the sample, have a non-crystalline, glassy, or commonly schlieric matrix and mineral fragments. The third most abundant grain type, making up 12% of the sample have a hemicrystalline, presumably feldspathic matrix and numerous mineral fragments. Finally, 6% of the grains have a dark, hemicrystalline, presumably feldspathic matrix and microlites presumably of pyroxene and ilmenite. Another 20% of the sample have overlapping features between these groups or are dark without distinct features. Also included in the study was a grain of pale, ellipsoidal to spherical silica ooids with nuclei of quartz fragments.

Stepwise argon⁴⁰/argon³⁹ analysis of these sand grains produced a range of readings, which is consistent with minerals from older episodes of melting being included within an impact melt, with 29 of the samples producing more than one age (consistent with partial melting and recrystallization of a mineral grain), of which 23 produced a younger age of 58.5 million years. Since no younger age was produced by any grain within the sample, Kenny et al. take this as the most probable age of the impact melt, making the impact a Late (but not Terminal) Palaeocene event. 

Fifteen unshocked zircons were selected from the two pebble-sized clasts, and subjected to uranium/lead analysis, most of which produced ages clustering around 1915 million years old, with the youngest being about 1485 million years old and the oldest about 2300 million years old. This is consistent with the age of intrusive felsic rocks in the area, supporting the hypothesis that the melts are of local origin. The altered zircons within the sample provided a range of ages between 1915 and 57.99 million years old, with the majority clustered at the minimum end of this range.

Unshocked zircons from the two pebble clasts collected about 10 km downstream of the Hiawatha Glacier give uranium/lead ages consistent with those of intrusive felsic rocks which outcrop at a number of sites around the crater, and which are therefore likely also to outcrop beneath it. Shocked zircons from the same material produce uranium/lead ages of about 58 million years. Argon⁴⁰/argon³⁹ analysis of sand particles from closer to the glacier yield a similar age. All of these samples appear to have been washed out from beneath the Hiawatha Glacier by a river which cuts through the rim of the Hiawatha Impact Structure. The simplest explanation for this is that the impact which caused this structure occurred in the Late Palaeocene. 

When the Hiawatha Impact Structure was first discovered it was thought likely to be less than 2.6 million years old; i.e. younger than the ice sheet which covers it. It has even been proposed that it might be as young as 12 900 years old, linking the impact to the onset of the Younger Dryas glacial episode. Kenny et al.'s findings suggest that the impact structure is much older than this, long predating glacier formation in Greenland.

Modelling of the original shape of the Hiawatha Impact Structure suggests that it has suffered about 500 m of vertical erosion since it was formed 58 million years ago, a much lower rate of erosion than has been predicted for subglacial features. This potentially has profound implications for the interpretation of other features beneath the Greenland Ice Sheet, although Kenny et al. are cautious of placing to much emphasis on this result without drill-core data to confirm the current interpretation of the structure of the feature. However, if this is correct then it means that a number of other features beneath the ice sheet are likely to be much older than previously thought, including a substantial river system currently thought to be subglacial in origin, but which might instead represent a long-standing morphological feature.

 

Geological map of Inglefield Land and Prudhoe Land, northwest Greenland. Previously published zircon uranium/lead ages for bedrock samples are shown in black text, and the age of unshocked zircon in clasts of impact melt rock sampled 4 km downstream from the terminus of Hiawatha Glacier (present study) is shown in green text. The dominant age of unshocked zircon in the impact melt rock samples (1915 ± 8 million years) is indistinguishable from the zircon uranium/lead ages of three felsic igneous intrusions in the vicinity of Hiawatha Glacier (bold text), supporting a local origin for the clasts of impact melt rock. Kenny et al. (2022).

Numerous pebble-sized charcoal fragments, many with cellular structures indicative of Conifer wood, have been found in the outwash of the Hiawatha Glacier. These have previously been taken as evidence of an Early Pleistocene forest system in Greenland, but the new date for the Hiawatha Impact Crater suggests that, if these are related to the impact event, then they must also be Palaeocene in origin. This actually fits well with our understanding of the Palaeocene Arctic, with Conifer fossils known from several Arctic sites.

The anomalous radiostratigraphy of the ice of Hiawatha Glacier compared to the rest of the Greenland Ice Sheet has been invoked as evidence for a young age for the impact structure beneath the glacier. If the glacier is in fact much younger than the impact structure, then an alternative explanation for the radiostratigraphy is needed. Kenny et al. suggest that this might have been caused by water flowing into the crater beneath the ice sheet and then building up until it escaped catastrophically. Alternatively, a collapse of the Nares Strait Ice Bridge in the Early Pleistocene could have disrupted ice structures in northwest Greenland.

The boundary between the Palaeocene and the Eocene, 55.93 million years ago, is marked by a global carbon isotope excursion, and the onset of a period of rapid warming that led to the Palaeocene-Eocene Thermal Maximum. This is close to the age of the Hiawatha Impact Structure, but not identical, and is better explained by massive flood basalt volcanism associated with the opening of the northeast Atlantic about 56 million years ago. There was also a significant lava flow outburst in Greenland in the Palaeocene, but this has been dated to 62 million years ago, older than the Hiawatha Impact Structure, and therefore unrelated to it. A number of spherule beds have of Palaeocene age have previously been discovered in western Greenland and on the northeastern coast of the United States, but these are now thought to be of volcanic origin, rather than impact related.

However, the Marquez Impact Structure in Texas has been dated to 58.3 million years ago, which is a very close match with the Hiawatha Impact Structure, suggesting that a link between the two is quite possible. The coincident age of two large impact structures may imply that other impacts happened at the same time, and that evidence of these is either undiscovered or has been lost. The timing of the Hiawatha and Marquez impacts does coincide with the end of the Late Palaeocene Carbon Isotope Maximum, a sudden increase in the proportion of carbon¹³ and a concurrent episode of global cooling, which ended abruptly at about 58 million years ago, when carbon¹³ levels dropped sharply and the Earth began a long-term warming trend. The absence of a distinct ejecta layer associated with the Hiawatha Impact makes it impossible to date this event with sufficient precision to link it to this shift, but Kenny et al. do note that the shift in carbon isotope ratios was far more sudden than is usually observed. The Chicxulub Impact has been linked to a major shift in carbon isotope ratios, but this, much larger, event is also known to have caused major disruption to the biosphere, which is generally assumed to be the cause of the carbon isotope shift. No known shift in the biosphere has been recorded which can be associated with the Hiawatha Impact, and no impact other than the Chicxulub event is known to have had any measurable influence on the Earth's biosphere, but this does not rule out the possibility that an impact such as the Hiawatha event could have caused changes to the biosphere which have not been recorded.

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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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