Organic carbon from the Hiawatha , North-West

Adam A. Garde1, Svend Funder2, Carsten Guvad1, Kurt H. Kjær2, Nicolaj Krog Larsen3, Jette Dahl-Møller4, Gernot Nehrke5, Hamed Sanei3, Anne Sofie Søndergaard3 and Christian Weikusat5.

1Geological Survey of Denmark and Greenland (GEUS), Denmark, [email protected], 2Centre for GeoGenetics, Natural History Museum, University of Copenhagen, Denmark, 3Department of Geosience, Aarhus University, Denmark, 4Geobiology and Minerals Section, Natural History Museum, University of Copenhagen, Denmark, 5Alfred Wegener Institute, Bremerhaven, Germany.

20 cm

Subfossil, un-charred, ~2.5 Ma old tree trunks up to 2 m long, Gymnosperm subfossil wood Heated carbonaceous particle Microbreccia with carbon in matrix Impact glass showing expulsion Sampling glaciofluvial sand recently washed out from under the ice sheet in adjacent Washington Land from impacted subglacial source of carbon around micro-orbicules washed out from the Hiawatha crater

Glaciofluvial The Hiawatha impact crater Objectives and conclusions

grains A 77°30’N 80°W 70°W 60°W B • The newly discovered Hiawatha crater has a highly unusual organic carbon content in shock melt glasses, microbreccias, and around grains. Only few Shocked quartz fragment with other impact structures contain appreciable organic carbon (e.g. Gardnos, Haughton, planar deformation Darwin, Popigai, see references). features (PDFs) Washington Land HW21-2016 • Dispersed organic carbon was incorporated in microbreccias and shock melts. Kane Basin Active drainage path 79°N Nares Strait 77°30’N B Humboldt • Solid organic carbon was heated, depleated and homogenised to form particles Inglefield Land glacier Smith Sound with high reflectance (R0 = 1.0–3.5). Ice margin Hiawatha Kap glacier København Qtz PDFs WHAT IS THE SOURCE OF THIS CARBON AND WHAT CAN IT TELL US? Shocked quartz with Greenland 750 m Ice Sheet Height PDFs and partially Thule • The obvious carbon source is subfossil wood, known from adjacent above Air Ba se melted feldspar. crater Washington Land and the 2.5 Ma Kap København Fm in eastern North Greenland. floor Glassy matrix: feldspar Greenland Ice Sheet 30 Km 78°30’N The carbon content is therefore compatible with a very young age of the impact. microcrysts and Ca pe York 0 m Island 0 10050 0 500 organic carbon 75°N Ba ffin Ba y Km Km 78°N 66°W 64°W • In an impacting scenario through ice, the Greenland Ice Sheet would have protected Location in Greenland Subglacial topography and sediment sample site the unconsolidated plant debris from being removed by the airblast.

• Discovered in 2015, first publication by Kurt H. Kjær and others in 2018 (Science Advances). • Hidden under the Greenland Ice Sheet, d = 31 km, one of the 25 largest terrestrial impact structures • Possibly the youngest large on Earth, with the fall watched by humans? Impact-induced depletion of organic carbon: Results of C pyrolysis of glaciofluvial sediment Impact glass with carbon-coated Detection and characterisation A dramatic loss of labile proto-hydrocarbons (’S2’) micro-orbicules is observed in the sedimentary organic matter Sediment sample sites of carbon using Raman spectrography towards the impact site, independently of grain size. Impact glass with Carbon HW12 HW21 carbon-coated, coats Raman peak widths and peak positions document This is associated with increased depletion of total 79°N Peak position ∆/cm-1 Outwash grains HW13 immiscible 1620 Raman Carbon spectra e03 carbonaceous a wide range of carbon ordering and degree of Peak position: d04 carbonaceous organic carbon (wt.%) and an increased fraction of globules Stages in b09 carbon-matrix silt 30 Km graphitic platyness at Hiawatha. graphite e06 ‘glassy coal’ inert (’dead’) carbon towards the impact site. Greenland Ice Sheet 1610 formation Graphitic d08 black glass g01 glass d07 glass The ordering is lowest in the shock melt mineral g10 perlitic glass f07 perlitic glass 1600 HW21: Glaciofluvial sediment glasses. e11 microcrystalline Grain size vs. C depletion d05 microcrystalline draining the crater. g12 microbreccia Reference } samples Carbon in mantles around microliths is more 1590 Charcoal HW12 and HW13: Outwash plain in front Impact glass with shrinkage Graphite <63 μ m } HW-21 standard of the Hiawatha glacier, ordered. Raman peak width: Reference samples with similar crack and beginning radial 1580 Compare Gardnos crater: Generic parameter Reference HW-12 Bulk looking NW from helicopter Carbon more ordered of order } HW-13 Bulk bulk composition. crystallisation. Amorphous samples In general, the Hiawatha carbon is much less Graphites HW-21 Bulk b Biri shale } HW-21 HW-21 Bulk a Garnet-like composition. Gardnos 125–63 μ m 1570 ordered than the metamorphosed carbonaceous NG-96-133 Plume melt droplet? Suevite NG-96-178 Reference Suevite (graphite-rich residue) } material in Gardnos crater breccia (Gilmour et al. Peak width ∆/cm-1 samples 1560 } HW-21 Labile proto-hydrocarbons Microporphyritic 2003). 0 20 40 60 80 100 120 140 160 250–125 μ m A sudden loss of labile proto-hydrocarbons feldspar- and cpx- 0 0.2 0.4 0.6 0.8 1 rich melt grain with associated with the proximity to the impact unmelted quartz Evidence of low carbon ordering in shock melt glasses The relationship between sample locality, sediment grain area is difficult to explain by any diagenetic fragments size and labile proto-hydrocarbons for sample HW21 and or geological process, except intense The carbon in melt glasses C OH C the two reference samples HW12 and HW13. is dispersed. external heating which is well constrained by other evidence of this catastrophic Quartz Laser-induced, in-analysis low-T Initial analysis: After prolonged analysis: The clear ‘bull’s-eye’ spatial distribution of impact phyllosililcate small peaks of Large peaks of crystallisation of carbon is peaks disordered C more ordered C organic carbon can be attributed to an Shock-melted observed. mineral glass: intense external heat source. perlitic microstructure

A likely source of the organic carbon Carbon reflectance (R0) • Charred wood: low, bimodal R . • Lumps of charcoal in the Hiawatha glacier, Subfossil charcoal collected 0 n 20 glaciofluvial grains of homogenised carbon, as well as from the tip of the • Carbonaceous grains and carbon in Glaciofluvial grains with organic carbon HW-21G b09 Hiawatha glacier. It is black, microbreccias: variable, generally higher R , HW-21 carbon in microbreccias and impact glasses record light, and very fragile, and 0 e12 widely different degrees of impact heating. thus qualifies as charcoal as documenting heating and C-H-O depletion. 15 d04 opposed to lignite, in spite e06 • The charcoal found in the Hiawatha glacier stems of its low reflectivity (R ). 2 cm Raman n e03 from conifers such as Pinus and Picea, which are now 0 Charcoal 10 spectra Grain 1 40 extinct in North Greenland. Unpublished 14C analysis shows that the Hiawatha wood is older than ~50 ka. Grain 3 5 Intensity / arbitrary unit 20 Grain 2 Shock-melted mineral glass: • Trunks of fresh, light brown wood occur in glacial Perlitic microstructure, Grain 3, R0 outwash rivers in adjacent Washington Land. Remnants high-R0 0 0 shocked quartz and carbon Raman shift, cm-1 particle of land plants and trees are common in the 2.5 ma, 1000 2000 3000 0 10.5 0 0.5 1 1.5 2 2.5 3 3.5 R0 estuarine Kap København Fm, eastern North Greenland, deposited during a warm period. See maps in box above. b09 d04 e06 e03 References 2 Gilmour, I. et al. 2003: Geochemistry of carbonaceous from the Gardnos impact structure, • Such wood from unconsolidated surficial deposits 1 Norway. Geochimica Cosmochimica Acta 67, 3889–3903. Howard, K.T. et al. 2013: Biomass preservation in impact melt ejecta. Nature Geoscience 6, 1018–1022. under the Greenland Ice Sheet constitutes the only Kjær, K. H. et al. 2018: A large impact crater beneath Hiawatha Glacier in northwest Greenland. 3 e12 Science Advances 4, eear8173, 11 pp. plausible source of all the carbon found in Hiawatha Lindgren, P. et al. 2007: Preservation of Organic Carbon in Impact Melt Breccia, Haughton Impact impactite grains. Structure. LPS XXXVIII, 1142. 0.1 mm 0.2 mm 0.5 mm 0.5 mm Schultz, P. H. et al. 2014: Preserved flora and organics in impact melt . Geology 42, 515–518.