Effect of ice sheet thickness on formation of the Hiawatha impact crater

Highlights

• Hiawatha is a 31 km wide putative impact crater found below the Greenland Ice Sheet.

• The impact should produce a trace of distal rocky ejecta in drill core samples.

• iSALE hydrocode used to study the crater morphology and emplacement of rocky ejecta.

• The presence of an ice sheet inhibits ejection of rocky material.

• The putative Hiawatha crater likely formed in the last 2.6 Myr.

Abstract

The discovery of a large putative impact crater buried beneath Hiawatha Glacier along the margin of the northwestern Greenland Ice Sheet has reinvigorated interest into the nature of large impacts into thick ice masses. This circular structure is relatively shallow and exhibits a small central uplift, whereas a peak-ring morphology is expected. This discrepancy may be due to long-term and ongoing subglacial erosion but may also be explained by a relatively recent impact through the Greenland Ice Sheet, which is expected to alter the final crater morphology. Here we model crater formation using hydrocode simulations, varying pre-impact ice thickness and impactor composition over crystalline target rock. We find that an ice-sheet thickness of 1.5 or 2 km results in a crater morphology that is consistent with the present morphology of this structure. Further, an ice sheet that thick substantially inhibits ejection of rocky material, which might explain the absence of rocky ejecta in most existing Greenland deep ice cores if the impact occurred during the late Pleistocene. From the present morphology of the putative Hiawatha impact crater alone, we cannot distinguish between an older crater formed by a pre-Pleistocene impact into ice-free bedrock or a younger, Pleistocene impact into locally thick ice, but based on our modeling we conclude that latter scenario is possible.

Introduction

Recently, a putative impact crater with the diameter of $31.1\pm 0.3$ km was discovered beneath the Hiawatha Glacier in northwestern Greenland (Fig. 1) (Kjær et al., 2018). The analysis of the glaciofluvial sediment samples collected from the river draining the structure shows the presence of shocked quartz, a marker indicative of meteoritic impact. Further, elevated concentrations of platinum-group elements (PGE) were found in the samples containing shocked quartz, and Kjær et al. (2018) further asserted that the putative impact crater may have been formed by a fairly rare iron asteroid. The size of the crater suggests that its formation likely caused significant regional – and perhaps even global – environmental perturbations (Toon et al., 1997; Erickson et al., 2020). As per scaling laws (Johnson et al., 2016b), to form a 31 km in diameter impact structure, an iron asteroid impacting at 17 km s⁻¹ at an incidence angle of 45° would have to be nearly 2 km wide (Collins et al., 2004). The probability of any composition asteroid of that size hitting Earth is low but non-negligible, occurring once every ∼2 million years (Silber et al., 2018).

One of the major questions concerning the Hiawatha structure is its age. Although material suitable for radiometric dating has not yet been found and analyzed, radiostratigraphic and geomorphologic evidence suggest that the structure most likely was formed after the Pleistocene inception of the Greenland Ice Sheet (Kjær et al., 2018). This tentative conclusion was further supported by identification of impact-heated conifer wood fragments associated with impactite grains from the Hiawatha glaciofluvial outwash that is probably derived from the Early Pleistocene deposits (Garde et al., 2020). So, while the sum of available evidence is suggestive of a geologically young age, no firm evidence of its age yet exists.

Confirmed impact craters generally contain shock-diagnostic materials, such as extensive fracturing and brecciation, high-pressure minerals, and planar deformation features (PDFs) in quartz. Younger craters generally exhibit well-defined morphologic features and are less degraded than older impact structures (French and Koeberl, 2010; Melosh, 1989). For example, fresh craters feature relatively sharp and raised rims with overturned stratigraphy, and the lack of disrupted features (French and Koeberl, 2010; Melosh, 1989). Based on these identifiers, the putative Hiawatha impact structure might be relatively fresh. It has a rim-to-floor depth of $320\pm 70$ m, and a dissected central uplift that is up to 50 m high and whose peaks are up to ∼8 km apart (Kjær et al., 2018). For a subaerial impact (no ice present), simple modeling suggests that a fresh, 31 km-diameter subaerial crater would display a peak ring (Pike, 1985) and have a rim-to-floor depth of ∼830 m (Collins et al., 2005). So, Hiawatha's morphology is muted compared to that expected for a subaerial impact, and yet it retains fundamental elements of a crater morphology (Fig. 1). One would also expect an impact of this size would blanket Greenland in rocky ejecta. If the impact occurred in the late Pleistocene, it is generally assumed that such ejecta should be easily identifiable within the six existing deep ice cores that typically record most of Last Glacial Period (115–11.7 ka) to the present day, including the Bølling-Allerød and the Younger Dryas (YD) transitions (Kjær et al., 2018). The YD is the millennium-long cold period that followed the Bølling-Allerød interstadial near the end of the last ice age at ∼12.8 ka. Although there is a Pt anomaly of possible cosmic origin at the Bølling-Allerød/YD boundary in the Greenland Ice Sheet Project 2 (GISP2) ice core (Petaev et al., 2013), there is no other evidence of rocky ejecta in any ice cores (Seo et al., 2019) and substantial evidence challenging an impact that time (e.g., Sun et al., 2020). The presence of ice, however, would affect the morphology and depth of the final crater, as well as the distribution of rocky ejecta (Senft and Stewart, 2008).

Here we model several possible scenarios for the formation of the putative Hiawatha impact crater using the iSALE-2D shock physics code (Collins et al., 2004; Wünnemann et al., 2006). To understand the effect of a pre-impact ice sheet, we investigate how the presence of thick ice affects the crater morphology and the dynamics and placement of the distal ejecta blanket.