Support for two subglacial impact craters in northwest Greenland from Earth gravity model EIGEN 6C4 and other data

doi:10.1016/j.tecto.2020.228396

Tectonophysics

Volume 780, 5 April 2020, 228396

https://doi.org/10.1016/j.tecto.2020.228396 Get rights and content

Highlights

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Independent support of discovery of two impact craters under the ice of Greenland
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Gravity aspects derived from EIGEN 6C4 with a ground resolution of 10 km applied
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Magnetic anomaly from EMAG-2 interpreted in favour of existence of the impact events

Abstract

We support the very recent discovery of two impact craters under the ice of northwest Greenland (Hiawatha Glacier and Paterson). These discoveries are based mainly on geology and bedrock topography. We added an analysis of gravity field aspects (descriptors) in addition to the traditional gravity and magnetic anomalies. The gravity aspects (the Marussi tensor of the second derivatives, the gravity invariants and their special ratio, strike angles and virtual deformations) provide more complex and comprehensive information about the underground density variations due to a causative body than ordinary gravity anomalies. They show signals typical for the individual geological features like a mountain/volcano, fault, (river)valley, (paleo)lake, (ground)water, hydrocarbon/mineral deposits, etc., as well as for the targets known as impact craters. Our method has been tested on various geological features on the Earth and the Moon. The gravity aspects are, in our case, derived from the recent global static combined Earth gravity field model EIGEN 6C4 with a ground resolution ~9 km and a precision ~10 mGal. A further data come from the digital magnetic field database EMAG 2 with resolution ~5 km. Our method is novel and independent of anything which led to discoveries of these craters in Greenland.

Introduction

There are now nearly 200 proven impact craters known on the Earth and many other features not yet confirmed (but suspected) as impact craters (Rajmon, 2017). Two new possible impact craters buried under the ice of Greenland have been discovered very recently.

Kjær et al. (2018) reported the discovery of an impact crater in NW Greenland under Hiawatha Glacier (in the depth 0–2 km beneath the surface); here we call this crater “Hiawatha”. The bedrock topography from a special scanning radar carried aboard airplanes (known as the radio echo sounding) shows (i) a flat depression with a diameter 31.1 ± 0.3 km, (ii) a rim-to-floor depth 320 ± 70 m and (iii) a central uplift. We reproduce their result in our Fig. 1 with coordinate system of geodetic latitudes and longitudes added. The crater is slightly asymmetric, with a gentler slope towards the southwest and maximum depth in the southeast of the structure (Kjær et al., 2018). From their geological and related findings, we quote that investigations of the deglaciated foreland identified overprinted structures within Precambrian bedrock along the ice margin, that glaciofluvial sediment from the largest river draining the crater contains shocked quartz and other impact related grains or that geochemical analysis indicates that the impactor was a fractionated iron asteroid.

The age of the object is not clear but probably from Pleistocene allowing it to be at the onset of Younger Dryas (Kletetschka et al., 2018; Wittke et al., 2013; Wolbach et al., 2018a, Wolbach et al., 2018b). It looks like that Kjær et al. (2018) did not use gravity data.

MacGregor et al. (2019) reported the discovery of a second impact crater in NW Greenland near Hiawatha Glacier, SE of the Hiawatha crater, in Humboldt Glacier. They call this crater Paterson (so here we label it also “Paterson”). The bedrock topography shows a similar depression for both craters, “flat depressions with a diameter of 36.5±0.2 km for Paterson and … a rim-to-floor depth 160±70 m. It also displays a central uplift.” While maximum central peak is 8 km wide and 50 m high for Hiawatha, flat Paterson has a 20 km central peak width and 20 m high central uplift. The thickness of ice is ~2000 m over Paterson compared to 0–950 m at Hiawatha. Paterson is more eroded than Hiawatha. Minimum age of overlying ice is 79 ka for Paterson while 12.8 ka for Hiawatha, claim the authors. The key properties of both subglacial features are compared in Table 2, p.4 of (MacGregor et al., 2019) and show specific craters' locations, dimensions, ice thicknesses, and minimum ages.

For Paterson, they primarily evaluated various Greenland-specific remote sensing data sets. There are no direct geological data, in a contrast to Hiawatha, no direct sampling. From their Table 1 we can see that they used local gravimetry data and caesium magnetometer. They did not use the gravity aspects.

MacGregor et al. (2019) also speculate: “…statistical analysis of the frequency of two unrelated but nearby large impacts indicates that it is improbable but not impossible that this pair is unrelated”.

The background of our approach is described in detail in Klokočník et al. (2017a) and in references therein. We outline the method in the next section, where we note on the physical meaning of the gravity aspects and inform readers about the data used.

Section snippets

Method

Classical gravity anomalies Δg provide only a limited information about the stress state of the rocks causing them. In order to broaden the potential information about the state of rocks we compute specific novel gravity functions of the disturbing gravitational potential called the gravity aspects or descriptors (Klokočník et al., 2017a).

The gravity aspects (gravity anomalies/disturbances Δg, second radial derivatives T_zz as a part of the Marussi tensor, the gravity invariants I_i and their

Results and discussion

First let us recall how the typical gravity aspects for impact craters look like; this has many times been verified for proven impact craters on the Earth and the Moon (see some examples in the references to our works mentioned above).

For an impact crater, it is expected that Δg and T_zz will be negative inside the crater (missing masses), changing to positive and to negative values for the respective rims and space between them. For a very large crater, or crater basin or mare, inside the

Conclusions

We independently support discovery by Kjær et al. (2018) and MacGregor et al. (2019) of two possible subglacial impact craters in NW Greenland. We used the gravity aspects computed from the harmonic geopotential coefficients of the global static gravity field model EIGEN 6C4 and our previous experience from analyses of other similar phenomena like volcanoes, lakes, paleolakes and impact craters (e.g., Klokočník et al., 2017a, Klokočník et al., 2018b, Klokočník et al., 2017c, Klokočník et al.,

CRediT authorship contribution statement

Jaroslav Klokočník: Conceptualization, Methodology, Writing - original draft, Writing - review & editing. Jan Kostelecký: Software, Data curation, Visualization. Aleš Bezděk: Writing - original draft, Writing - review & editing, Software, Data curation, Visualization, Validation. Václav Cílek: Formal analysis, Investigation. Gunther Kletetschka: Investigation, Writing - original draft, Writing - review & editing, Validation. Hana Staňková: Software, Project administration.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work has been prepared in the frame of the project RVO #679 858 15 (Czech Academy of Sciences, Czech Republic, CR), projects LTT18011 and LO1506 (PUNTIS) from the Ministry of Education of the CR. Support for GK came from the MEYS LTAUSA 19141, CSF 20-08294S, and RVO #679 858 31. The input data, the harmonic geopotential coefficients of EIGEN 6C4 and the magnetic anomalies from EMAG 2, are publicly available; data to our figures (in surfer, png files) and our high resolution figures can be

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View full text

Support for two subglacial impact craters in northwest Greenland from Earth gravity model EIGEN 6C4 and other data

Highlights

Abstract

Introduction

Section snippets

Method

Results and discussion

Conclusions

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgments

Impact-pressure controlled orientation of shatter cone magnetizations in Sierra Madera, Texas, USA

Stud. Geophys. Geod.

The Canyon Diablo impact event: projectile motion through the atmosphere

Meteorit. Planet. Sci.

Digital Data Grids for the Magnetic Anomaly Map of North America Rep

The second derivative method of gravity interpretation

Geophysics

The latest combined global gravity field model including GOCE data up to degree and order 2190 of GFZ Potsdam and GRGS Toulouse (EIGEN 6C4)

5th GOCE User Workshop, Paris 25–28, Nov

Antarctic Mapping Tools for Matlab

Comput. Geosci.

A large impact crater beneath Hiawatha Glacier in northwest Greenland

Sci. Adv.

The origin of magnetic anomalies in lower crustal rocks, Labrador

Geophys. Res. Lett.

Stability analysis of invariant points using Euler spheres, with an application to FMAS granulites

J. Metamorph. Geol.

Pressure effects on martian crustal magnetization near large impact basins

Meteorit. Planet. Sci.

Cosmic impact event in lake sediments from central Europe postdates the Laacher See eruption and marks onset of the Younger Dryas

J. Geol.

Gravity Disturbances, Marussi Tensor, Invariants and Other Functions of the Geopotential Represented by EGM 2008

J. Earth Sci. Res.

Gravitational atlas of Antarctica

A support for the existence of paleolakes and paleorivers buried under Saharan sand by means of “gravitational signal” from EIGEN 6C4

Arab. J. Geosci.