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b09HW-21GGlaciofluvial grains with organic carbon

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Inglefield Land

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low-Tphyllosililcate

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After prolonged analysis:Large peaks ofmore ordered C

Initial analysis:small peaks ofdisordered C

GraphitesBiri shaleGardnos brecciaSuevite NG-96-133Suevite NG-96-178Suevite (graphite-rich residue)

Compare Gardnos crater:Carbon more ordered

Raman Carbon spectra

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Raman peak width: Generic parameter of order

Peak position: Stages in graphiteformation

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}} Labile proto-hydrocarbons

20 cm

Organic carbon from the Hiawatha impact crater, North-West GreenlandAdam 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, aag@geus.dk, 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.

The Hiawatha impact crater

A likely source of the organic carbon

Objectives and conclusionsGlaciofluvialimpactite grains

• 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 impact structure on Earth, with the fall watched by humans?

• The newly discovered Hiawatha crater has a highly unusual organic carbon content in shock melt glasses, microbreccias, and around shocked quartz grains. Only few other impact structures contain appreciable organic carbon (e.g. Gardnos, Haughton, Darwin, Popigai, see references).• Dispersed organic carbon was incorporated in microbreccias and shock melts.• Solid organic carbon was heated, depleated and homogenised to form particles with high reflectance (R0 = 1.0–3.5). WHAT IS THE SOURCE OF THIS CARBON AND WHAT CAN IT TELL US?• The obvious carbon source is subfossil wood, known from adjacent Washington Land and the 2.5 Ma Kap København Fm in eastern North Greenland. The carbon content is therefore compatible with a very young age of the impact.• In an impacting scenario through ice, the Greenland Ice Sheet would have protected the unconsolidated plant debris from being removed by the airblast.

References

Carbon reflectance (R0)

Detection and characterisationof carbon using Raman spectrography

Evidence of low carbon ordering in shock melt glasses

Subglacial topography and sediment sample site

Subfossil, un-charred, ~2.5 Ma old tree trunks up to 2 m long, washed out from under the ice sheet in adjacent Washington Land

Heated carbonaceous particlefrom impacted subglacial source

Microbreccia with carbon in matrix Impact glass showing expulsionof carbon around micro-orbicules

Sampling glaciofluvial sand recentlywashed out from the Hiawatha crater

Gymnosperm subfossil wood

Location in Greenland

Impact glass withcarbon-coatedmicro-orbicules

Impact glass withcarbon-coated,

immiscibleglobules

Impact glass with shrinkage crack and beginning radial crystallisation.Garnet-like composition.Plume melt droplet?

Shocked quartzfragment withplanar deformationfeatures (PDFs)

Microporphyriticfeldspar- and cpx-

rich melt grain with unmelted quartz

fragments

Shock-meltedmineral glass:perlitic microstructure

Subfossil charcoal collected from the tip of the Hiawatha glacier. It is black, light, and very fragile, and thus qualifies as charcoal as opposed to lignite, in spite of its low reflectivity (R0).

A sudden loss of labile proto-hydrocarbons associated with the proximity to the impact area is difficult to explain by any diagenetic or geological process, except intense external heating which is well constrained by other evidence of this catastrophic impact

Outwash plain in front of the Hiawatha glacier, looking NW from helicopter

HW21: Glaciofluvial sediment draining the crater.

HW12 and HW13: Reference samples with similar bulk composition.

The relationship between sample locality, sediment grain size and labile proto-hydrocarbons for sample HW21 and the two reference samples HW12 and HW13.

The clear ‘bull’s-eye’ spatial distribution of organic carbon can be attributed to an intense external heat source.

A dramatic loss of labile proto-hydrocarbons (’S2’) is observed in the sedimentary organic matter towards the impact site, independently of grain size.

This is associated with increased depletion of total organic carbon (wt.%) and an increased fraction of inert (’dead’) carbon towards the impact site.

• Charred wood: low, bimodal R0. • Carbonaceous grains and carbon in microbreccias: variable, generally higher R0, documenting heating and C-H-O depletion.

Raman peak widths and peak positions document a wide range of carbon ordering and degree of graphitic platyness at Hiawatha. The ordering is lowest in the shock melt mineral glasses. Carbon in mantles around microliths is more ordered.In general, the Hiawatha carbon is much less ordered than the metamorphosed carbonaceous material in Gardnos crater breccia (Gilmour et al. 2003).

• Lumps of charcoal in the Hiawatha glacier, glaciofluvial grains of homogenised carbon, as well as carbon in microbreccias and impact glasses record widely different degrees of impact heating.

• The charcoal found in the Hiawatha glacier stems from conifers such as Pinus and Picea, which are now extinct in North Greenland. Unpublished 14C analysis shows that the Hiawatha wood is older than ~50 ka.

• Trunks of fresh, light brown wood occur in glacial outwash rivers in adjacent Washington Land. Remnants of land plants and trees are common in the 2.5 ma, estuarine Kap København Fm, eastern North Greenland, deposited during a warm period. See maps in box above.

• Such wood from unconsolidated surficial deposits under the Greenland Ice Sheet constitutes the only plausible source of all the carbon found in Hiawatha impactite grains.

The carbon in melt glasses is dispersed.

Laser-induced, in-analysis crystallisation of carbon is observed.

Shocked quartz withPDFs and partially

melted feldspar. Glassy matrix: feldspar

microcrysts and organic carbon

Shock-melted mineral glass: Perlitic microstructure, shocked quartz and carbon

Impact-induced depletion of organic carbon:Results of C pyrolysis of glaciofluvial sediment

Gilmour, I. et al. 2003: Geochemistry of carbonaceous impactites from the Gardnos impact structure, Norway. Geochimica Cosmochimica Acta 67, 3889–3903.Howard, K.T. et al. 2013: Biomass preservation in impact melt ejecta. Nature Geoscience 6, 1018–1022. Kjær, K. H. et al. 2018: A large impact crater beneath Hiawatha Glacier in northwest Greenland. Science Advances 4, eear8173, 11 pp.Lindgren, P. et al. 2007: Preservation of Organic Carbon in Impact Melt Breccia, Haughton Impact Structure. LPS XXXVIII, 1142.Schultz, P. H. et al. 2014: Preserved flora and organics in impact melt breccias. Geology 42, 515–518.

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