Pleistocene organic matter modified by the Hiawatha impact, northwest Greenland

Abstract

The 31-km-wide Hiawatha impact crater was recently discovered under the ice sheet in northwest Greenland, but its age remains uncertain. Here we investigate solid organic matter found at the tip of the Hiawatha Glacier to determine its thermal degradation, provenance, and age, and hence a maximum age of the impact. Impactite grains of microbrecchia and shock-melted glass in glaciofluvial sand contain abundant dispersed carbon, and gravel-sized charcoal particles are common on the outwash plain in front of the crater. The organic matter is depleted in the thermally sensitive, labile bio-macromolecule proto-hydrocarbons. Pebble-sized lumps of lignite collected close to the sand sample consist largely of fragments of conifers such as Pinus or Picea, with greatly expanded cork cells and desiccation cracks which suggest rapid, heat-induced expansion and contraction. Pinus and Picea are today extinct from North Greenland but are known from late Pliocene deposits in the Canadian Arctic Archipelago and early Pleistocene deposits at Kap København in eastern North Greenland. The thermally degraded organic material yields a maximum age for the impact, providing the first firm evidence that the Hiawatha crater is the youngest known large impact structure on Earth.

The Younger Dryas impact hypothesis: Review of the impact evidence

“…it is less than a few million years old (see Figure 11). This maximum age is confirmed a year later (Garde et al, 2020). Clearly, this crater is a candidate YD-age impact structure.…”

The Cape York Meteorites, the Younger Dryas, and Their Possible Association with the Hiawatha and Paterson Impact Craters

“…This argument once again underscores the great importance of narrowing down the formation ages for the Hiawatha and Paterson craters. Some progress in this area has recently been achieved by Garde et al who have investigated the degradation of organic matter within the impactite grains sampled from the outwash from the Hiawatha crater floor [51]. Accordingly, they find a maximum crater formation age of between 3 to 2.4 million years before present.…”

Glacial history of Inglefield Land, north Greenland from combined in situ <sup>10</sup>Be and <sup>14</sup>C exposure dating

“…The final deglaciation and opening of Nares Strait have been debated but recent off-shore studies together with a few terrestrial studies place the collapse of the ice saddle in Kane Basin and opening of Nares Strait between ca. 9-8 ka cal BP (Georgiadis et al, 2018;Jennings et al, 2019;Dalton et al, 2020). Recently, Jakobsen et al (2018) and Reusche et al (2018) proposed an overall glacial retreat of the GrIS in north Greenland during the Holocene, but with a possible stillstand of the ice sheet and its outlet glaciers as a response to the 8.2 ka cold events.…”

“…During the LGM, Inglefield Land was completely ice covered, and the ice nourished the Smith Sound Ice Stream primarily through the Humboldt Glacier until the opening of Nares Strait sometime between ca. 9 and 8 ka cal BP (Georgiadis et al, 2018;Jennings et al, 2019;Dalton et al, 2020). The outer coast at Kap Inglefield, Kap Grinnel and Rensselaer Bay in southwest Inglefield Land was deglaciated between ca.…”

“…8). For information about the offshore deglaciation history, the reader is referred to recent reviews by Georgiadis et al (2018 and Dalton et al (2020).…”