Australia: The Land Where Time Began
Hiawatha Glacier in Northwest Greenland – A Large Impact Crater
In this paper Kjær et al. report the discovery of a large impact crater beneath the Hiawatha Glacier in northwest Greenland. They identified a crater that is a 31 km wide circular bedrock depression beneath up to 1 km of ice. There is an elevated rim around this depression that cross-cuts tributary subglacial channels and a subdued central uplift that appears to be eroding actively. They identified structures that were overprinted within the bedrock that dates to the Precambrian along the margin of the ice that strike tangent to the subglacial rim, by the use ground investigations of deglaciated foreland. Shocked quartz, and other grains that were compact-related, are contained in glaciofluvial sediments from the largest river that drains the crater. It is indicated by geochemical analysis of this sediment that the impactor was a fractionated iron asteroid, which must have been more than a kilometre wide in order to produce the crater that was identified. It is shown by the radiostratigraphy of the ice that the ice from the Holocene is continuous and conformable, though all deeper and older ice appears to be rich in debris or is disturbed heavily. At the time of writing the age of this crater was not known, though from the geological and geophysical evidence that has been collected, Kjær et al. concluded that it is not likely to predate the Pleistocene inception of the Greenland ice sheet.
As a result of the remote location of Greenland and its cover of ice the scientific exploration has been extended for centuries. A new development that is relatively recent is exploration of features beneath the ice, because of the advent of borehole drilling through ice and century radar sounding in the mid-20th century:
1) While airborne radar sounding of the Greenland Ice Sheet began in the 1970s,
2) Surveying of the ice sheet that increasingly comprehensive has only been possible over the last 2 decades. Extensive airborne radar sounding that began in the mid-1990s has revealed a landscape that was hitherto hidden, as well as elucidating and processes and events that have led to the topography of the present,
3) The Late Pleistocene and Holocene history of the ice sheet itself has been revealed through internal stratigraphy that was detected by this radar sounding,
4) In this paper Kjær et al. have described a new feature of the landscape in remote northwest Greenland, which was initially identified by airborne radar sounding, and ground-based field studies of the ice sheet and the deglaciated foreland.
Identification of the Hiawatha impact crater
It was concluded by Kjær et al. based on the characteristic crater morphology beneath the ice, including a subdued central uplift, the rim-tangent structures that were imposed in bedrock foliations next to the margin of the ice, and the fresh glaciofluvial sediments that contain shocked quartz, other grains that are impact-related, and elevated concentrations of siderophile element concentrations that were strongly suggested by observations to have originated from beneath the Hiawatha Glacier, that beneath the Hiawatha Glacier there is an impact crater. There are other impact features that are diagnostic of impact features, which are expected to be subglacial in this case, also, they have not yet performed a gravity survey across the Hiawatha Glacier. There is no ejecta layer associated with this structure that has yet been identified, beyond the grains in the sediment sample that was interpreted by Kjær et al. as possibly being ejecta. An impact origin for the structure beneath the Hiawatha Glacier is the simplest explanation for their observations, which they accept for the remainder of this discussion, in spite of the lack of such additional evidence.
Potentially, this crater is 1 of the 25 largest impact structures on Earth, and it is the only one that still has a significant portion of its original surface topographic expression.
Preliminary estimates of impactor and ejector properties
The kinetic energy of the impactor is constrained by the diameter of the impact crater. About 3 x 1021 J of energy is required for the formation of an impact crater that is 31 km wide (Collins et al., 2005). The dimeter of the impactor was about 1.5 km and its velocity at impact was 20 km/s, if it is assumed that the Hiawatha impactor was iron with a density of 8000 kg/m3. According to Kjær et al. it is expected that the impact would initially produce a cavity that was bowl-shaped that was about 20 km in diameter and about 800 m deep with a central uplift (Collins et al., 2005). Up to ~20 km3 of crystalline target rock, and it is estimated that approximately half of which would have remained within the crater, to form a melt sheet that was up to ~50 m deep.
An ejector layer that could be associated with the Hiawatha impact crater has not yet been identified in either the rock of Greenland or its ice records. The symmetric ejecta layer would be ~200 m thick at the rim, and thinning to <20 m at a distance of 30 km from the rim (Collins et al., 2005), if there was no ice present at the time of an impact at a high angle of >45o. During most of the Pleistocene, however, the impact area was covered by an ice sheet (Bierman et al., 2016). A more energetic projectile is required to produce a crater of the observed size, and the fraction of the non-ice debris in the ejecta would be smaller than if the impactor hit land that was free of ice (Senft &Stewart, 2008), if ice was present and its thickness was comparable to the dimeter of the impact. Also, the ice cover that was regionally extensive at the time of the impact could have resulted in a significant fraction of the ejecta landing on the surface of the Greenland or the Innuitian ice sheets, and not on bare ground. The site has almost certainly been ice free during 1 or several short, about 15 ka, interglacial periods during the Pleistocene, such as had been predicted for the Eemian about 125 ka (Born & Nisancioglu, 2012). Most of any ejecta that was deposited on the ice sheet would have been transported to the margin of the ice within about 10 ka, on the basis of ice flow speeds of the present. Similarly, any such ejecta would have been less than half of its original thickness within 10 ka, based on vertical strain rates in the Holocene (MacGregor, 2016).
At the time of the impact if the Greenland Ice Sheet was present and a high-angle impact occurred during the Late Pleistocene (LGP), then the ejecta should be present in 4 deep ice cores that had been recovered from central and northern Greenland that span the majority of the (LGP), though none has been identified so far. The expected initial thickness of a symmetric ejecta layer for an impact of the size of a Hiawatha sized impact on rock is about 0.7 mm thickness with an average particle diameter of about 0.4 mm (Collins et al., 2005), at 2 of the ice cores (GISP2 and GRIP) which are located at the greatest distance from the crater, >1,000 km. This thickness increases roughly 2-fold in the closer ice cores. A significant fraction of the ejecta would also be ice if ice was present at the site of impact (Senft & Stewart, 2008), though the presence of any rock ejecta should be unambiguous in an ice core. The angle of the impact is presently not known, which is a possible complicating factor in interpreting the lack of ejecta in ice cores to the south of the structure. It is indicted by modelling that oblique impact of less than 45o produce ejecta that is asymmetric predominantly downrange of the crater with a shadow zone that is ejecta-free uprange and that this effect becomes more pronounced with a decrease of the angle of impact (Shuvalov, 2011). At 78.72oN, the Hiawatha impact crater is located further north than any other known impact crater, which increases the probability of an oblique that is northwards directed given the majority of asteroids that cross the path of the Earth move in or near the ecliptic plane. Such a scenario might be analogous to the Mjølnir Crater from the Late Jurassic, which is also a large crater with a diameter of 40 km at the high latitude of 73.8oN, which also produced an asymmetric ejecta layer that was northwards focused (Shuvalov & Dypvik, 2004).
The estimates by Kjær et al. of the size of the impactor, initial size of the crater, melt volume of the impactor, and thickness and extent of the ejecta should be considered preliminary, as it is yet to be determined whether the Greenland Ice Sheet covered this region at the time of the impact, or its thickness at that time, or the angle of impact.
Hiawatha impact crater – age
For impact craters on Earth they are often dated by the use of radiometric decay systems, though to date there are no samples recovered from the Hiawatha impact crater that are suitable for an absolute age determination. According to Kjær et al. they can assume confidently that the structure is younger than the age of 1.985 to 1.740 Ga of the Palaeoproterozoic bedrock that outcrops in the immediately adjacent foreland. Also, independent, though tentative, constraints on the age of the crater are provided by multiple lines of evidence that are mostly derived from their radar-sounding survey.
The depth of the crater, 329 ± 70 m, is muted compared to that predicted for a fresh subaerial crater of the same diameter, about 800 m (Collins et al., 2005; Melosh, 1989), which could have resulted from either rapid erosion over a short time period or slower erosion over a longer time span. Fluvial and subglacial rates of erosion that have been reported span a range of about 10-5 to 10-2 m/year (Koppes & Montgomery, 2009; Cowton et al., 2012; Strunk et al., 2017; Young et al., 2016). An erosion rate that is at the upper end of that range implies a minimum period of about 5 Ka to erode the rim and central uplift, as well as partially fill the floor of the crater, to form the morphology of the present, if it is assumed that ice has covered the crater for almost all of its existence. A maximum erosion period that is constrained loosely of 50 Myr is yielded by a lower end erosion rate. It appears a subglacial erosion rate, and therefore a younger age, is favoured by the radar evidence obtained by Kjær et al. of subglacial erosion at present (movie S1) and active deposition of sediment at the front of the glacier.
The rim of the crater cuts across the northern channel and effectively terminates it east of the crater. Part of the southern channel is redirected by the rim to its southeast, therefore it is inferred by Kjær et al. that both channels predate the formation of this structure. According to Kjær et al. these 2 channels are comparable to the Palaeofluvial channel network of the neighbouring Humboldt Glacier (Livingston et al., 2017) and the megacanyon (Bamber et al., 2013) of the central Greenland Ice Sheet (about 2.6 Ma) (Bierman et al., 2016). It was noted by Kjær et al. that it is required by this interpretation is that the channels that subsequently merged later breached the rim itself.
It appears that radar evidence of active basal melting, the full column stratigraphic synclines, and subglacial storage of water (groundwater table) within and beneath the Hiawatha Glacier, respectively, appear to be anomalous as compared to grounded ice-marginal settings across northern Greenland. An anomalous subglacial heat source could be the origin of basal melting, and it is consistent with, though not conclusive of residual heat from the impact itself. It was suggested by of hydrothermal systems within subaerial impact craters on Mars, that such systems have a lifespan of about 100 kyr for a crater that is 30 km in diameter (Abramov & Kring, 2005). In the case of the terrestrial Hiawatha glacier an ample supply of water for such a hydrothermal system during the Pliocene and Pleistocene would have been provided by the overlying ice sheet, though it would have also exported heat from the system more efficiently than for a subaerial crater, which there would have been a shorter lifespan for any possible post-impact hydrothermal system than would be the case on Mars.
Finally, the radiostratigraphy of the Hiawatha Glacier is highly anomalous when compared to the remainder of the Greenland Ice Sheet (movies S1 to S3). The ice from the Last Glacial Period is neither complete nor conformable across the entire crater. It would have taken only a few millennia for deeper ice to flow across the crater, given the surface velocities of the present, therefore, the age of the glacier’s structure is still not explained clearly by steady, uninterrupted ice flow to the crater from the ice sheet. The deformed radiostratigraphy of this deeper, older ice was interpreted by Kjær et al. as indicating that the ice flow there, that was strongly affected by a transient there following the deposition of most of ice during the LGP. The retreat of the Humboldt Glacier that occurred about 9 to 8 ka, which unblocked the Nares Strait (Senft & Stewart, 2008; Jennings et al., 2011; Knudsen et al., 2008), is a candidate regional perturbation. Surface mapping and dating of moraines, as well as coring in the strait, have yet to show that the ice flow at the retreating margin (Jennings et al., 2011; Funder et al., 2011) was affected significantly by that perturbation, therefore there is no clear reason why the effect on the ice flow of that event appears to be focused within and to the south of the Hiawatha impact crater. The anomalous radiostratigraphy could be explained by the subglacial pooling of water within the topographic depression that was formed by the pre-existing crater, which then outburst catastrophically, and possibly repeatedly, through the breach in the rim (i.e., jökulhaup), which ultimately affected the local ice flow. A significant local or upstream source of water is required by such a scenario, from either basal melting beneath thick ice or from melting at the surface. Alternatively, the apparent change in ice flow could be a reflection of the response of the ice sheet to the impact that formed the crater – if it occurred when ice was present there. Such an impact would have melted, vaporised and excavated ice locally, and would have provided a local source of heat that would have continued melting ice that was flowing into the crater for a period post-impact that is yet to be determined. Between the crater and the local ice divide, which was about 100 km upstream, the ice sheet would have accelerated and thinned in response to the impact, transporting the ejecta of ice and rock ejecta to the margin of the ice. There is not enough evidence at present to favour 1 of these hypotheses on the origin of the anomalous radiostratigraphy of the Last Glacial Period over the other.
It was suggested by the sum of these tentative constraints on age that it was during the Pleistocene that the Hiawatha impact crater was formed, as this age is most consistent with inferences from data that is available at present. If the impact occurred prior to the Pleistocene it cannot explain clearly the combination of the relative freshness of the morphology of the crater and the apparent ongoing equilibrium of the ice sheet with the presence of the crater. Kjær et al. emphasised that even this broad estimation of age remains uncertain and that further investigation of the age of the Hiawatha impact crater is necessary. Whatever the exact age of the Hiawatha crater, it is very likely that there were significant environmental consequences for the Northern Hemisphere, possibly globally as well (Chapman & Morrison, 1994).
Hiawatha impact crater – significance
This is the only well-preserved impact crater that has been found in Greenland, which is partially due to the ice sheet that covers 80% of the island. In this study, multiple lines of evidence, that included high-resolution radar sounding data and geological evidence on the microscale, of a large crater that is buried beneath the ice sheet. An iron asteroid on a Kilometre scale would be required to generate the energy necessary to produce a crater that is 31 km wide. Morphological deviations of the Hiawatha impact crater from a typical complex crater are likely to be due to glaciofluvial and subglacial erosion of the rim and central uplift, deposition of sediment within the crater, and post impact collapse of the rim, though the overall appearance of the crater is relatively fresh. The Hiawatha crater is the only known terrestrial crater of this size that has retained aspects of its original surface topographic expression. It is not known at present what the age of the crater is, though an impact at some time during the Pleistocene is consistent with geological and geophysical data that is presently available.
It is suggested by this study that there are several avenues for further research into the nature and the age of the Hiawatha impact crater as well as other possible impact craters. An improved geochronology for this impact event is in need of the discovery and analysis of more samples, which form within the crater itself or the surrounding area. The southwest of the crater is one of the most promising regions, which appears to be rich in debris both englacially and subaerially (Risbo & Pedersen, 1994). At least part of the age range of the Pleistocene and the impact scenario inferred by Kjær et al., could be tested by evidence of ejecta, or lack of ejecta, north of the structure and its chronostratigraphy. At times the consequences of impacts into ice masses have been considered for extraterrestrial bodies, though only rarely has this been done for ice masses on Earth. An understanding of the evolution of the Hiawatha impact crater would be helped by modelling of both the dynamics of large impacts into an ice sheet, the post-impact modification of the morphology of the crater by flowing ice masses, and the internal structure of those ice masses.
Kjær, K. H., et al. (2018). "A large impact crater beneath Hiawatha Glacier in northwest Greenland." Science Advances 4(11).
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|