Pleistocene organic matter modified by the Hiawatha impact, northwest Greenland

Adam A Garde,Hamed Sanei,Anne Sofie Søndergaard,Jette Dahl-Møller,Christian Weikusat,Gernot Nehrke,Nicolaj Krog Larsen,Carsten Guvad,Kurt H Kjær,Svend Funder

doi:10.1130/g47432.1

Abstract

AbstractThe 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.

Highlights

Most terrestrial impact structures contain only little organic carbon, recycled from target rocks
The 31-km-wide Hiawatha impact crater was recently discovered under the ice sheet in northwest Greenland, but its age remains uncertain
We investigate solid organic matter found at the tip of the Hiawatha Glacier to determine its thermal degradation, provenance, and age, and a maximum age of the impact

Summary

INTRODUCTION

Most terrestrial impact structures contain only little organic carbon, recycled from target rocks. We investigate organic matter and its thermal degradation in the impactite grains, charcoal, lignite, and bulk sediment in proglacial outwash from the Hiawatha crater floor. The macroscale organic matter comprises angular, sand- to gravel-sized particles of charcoal in sample HW21-2016, as well as pebble-sized lumps of lignite rounded by water transport on the eastern side of the glacier front; small twigs from dwarf bushes unrelated to the impact occur locally on the glacial outwash plain. Charcoal and Lignite Sand- and gravel-sized pieces of charcoal from sample HW21-2016 have a largely amorphous structure (“glassy coal”) and preserve relict cell structures (Fig. 3A) Their reflectance in incident white light is high (Ro ≤ 3.5%), within the range of modern charcoal (Braadbaart and Poole, 2008; McParland et al, 2009). C tive, labile bio-macromolecule hydrocarbons (S2) compared to the two reference samples

DISCUSSION

CONCLUSIONS