Pleistocene Organic Matter Modified by the Hiawatha Impact, Northwest Greenland Adam A

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https://doi.org/10.1130/G47432.1

Manuscript received 23 January 2020 Revised manuscript received 2 April 2020 Manuscript accepted 8 April 2020

Pleistocene organic matter modified by the Hiawatha impact, northwest Greenland Adam A. Garde1, Anne Sofie Søndergaard2, Carsten Guvad1, Jette Dahl-Møller3, Gernot Nehrke4, Hamed Sanei2, Christian Weikusat4, Svend Funder3, Kurt H. Kjær3 and Nicolaj Krog Larsen3 1Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen K, Denmark 2Lithospheric Organic Carbon (LOC) Group, Department of Geoscience, Aarhus University, Høegh Guldbergs Gade 2, 8000 Aarhus, Denmark 3Globe Institute, University of Copenhagen, Øster Voldgade 5–7, 1350 Copenhagen K, Denmark 4Alfred Wegener Institute, Am Handelshafen, 27570 Bremerhaven, Germany

ABSTRACT an iron meteorite (Kjær et al., 2018). The bulk The 31-km-wide Hiawatha impact crater was recently discovered under the ice sheet mineral assemblage and chemical composi- in northwest Greenland, but its age remains uncertain. Here we investigate solid organic tion of the sand indicate sourcing from gran- matter found at the tip of the Hiawatha Glacier to determine its thermal degradation, ulite-grade rocks similar to Paleoproterozoic provenance, and age, and hence a maximum age of the impact. Impactite grains of micro- paragneiss exposed in the ice-free foreland brecchia and shock-melted glass in glaciofluvial sand contain abundant dispersed carbon, to the crater (locally including sulfidic parag- and gravel-sized charcoal particles are common on the outwash plain in front of the crater. neiss with graphite flakes; Dawes, 2004). The The organic matter is depleted in the thermally sensitive, labile bio-macromolecule proto- crater has not been directly dated but is tenta- hydrocarbons. Pebble-sized lumps of lignite collected close to the sand sample consist largely tively referred to the Pleistocene and possibly of fragments of conifers such as Pinus or Picea, with greatly expanded cork cells and desic- as young as the last glacial period, based on cation cracks which suggest rapid, heat-induced expansion and contraction. Pinus and Picea indirect evidence such as preservation of the are today extinct from North Greenland but are known from late Pliocene deposits in the subglacial crater topography, a strongly dis- Canadian Arctic Archipelago and early Pleistocene deposits at Kap København in eastern turbed pre-Holocene ice stratigraphy over the North Greenland. The thermally degraded organic material yields a maximum age for the crater identified by airborne radar imaging, impact, providing the first firm evidence that the Hiawatha crater is the youngest known and an anomalously high modern-day volume large impact structure on Earth. of melt water draining from the impacted area (Kjær et al., 2018). INTRODUCTION raneous vegetation (Howard et al., 2013), and In this study, we investigate organic matter Most terrestrial impact structures contain the small, twin ca. 3.2 ka Kaali craters, Estonia, and its thermal degradation in the impactite only little organic carbon, recycled from target contain locally derived charcoal (Losiak et al., grains, charcoal, lignite, and bulk sediment in rocks. The ca. 35 Ma Popigai crater, Siberia, 2016, 2019). proglacial outwash from the Hiawatha crater contains diamonds transformed from target- With a diameter of 31 km, the newly dis- floor. We identify the wood fragments to gen- rock graphite (Masaitis et al., 1975). Ejecta covered Hiawatha crater under the Greenland era level and discuss the implications of our from the ca. 23 Ma Haughton crater, Canada Ice Sheet in northwest Greenland is one of the findings for the maximum age of the Hiawatha (Parnell et al., 2007), and the ca. 15 Ma Ries 25 largest impact structures on Earth (Fig. 1; impact. crater, Germany (Osinski 2003), contain organic Kjær et al., 2018; Impact Earth, 2020, https:// carbon derived from sedimentary target rocks, impact.uwo.ca/). A large variety of sand-sized METHODS and drill core from the 66 Ma Chicxulub crater, impactite grains originating from the crater Sediment samples described by Kjær et al. Mexico, contains particles of charcoal (Gulick floor were found in a glaciofluvial outwash (2018), and charcoal and lignite collected by us et al., 2019). If the source of the carbon can be sediment sample (HW21-2016 of Kjær et al.; from in front of the Hiawatha Glacier were inves- identified, it may serve to constrain the maxi- see their table S1 with sample information) tigated by standard optical and backscattered elec- mum age of cratering. For instance, dispersed deposited at the front of the Hiawatha Gla- tron scanning electron microscopy (SEM-BSE; graphitic carbon in the ca. 500 Ma Gardnos cier just outside the crater rim (Fig. 1). The Geological Survey of Denmark and Greenland, crater in Norway stems from Cambrian Alum sand contains shocked quartz, microbreccias, Copenhagen, Denmark), pyrolysis, and Raman Shale (Gilmour et al., 2003; Parnell and Lind- shock-melted mineral glasses, and micromag- spectra. The Raman spectra were obtained with gren 2006). Carbon inclusions in impact glasses matic aggregates; elevated Pt contents and a WITec alpha300 R system and a 488 nm laser linked to the purported ca. 0.8 Ma Darwin crater, anomalous platinum-group element ratios in at the Alfred Wegener institute (Bremerhaven, Tasmania, preserve biomarkers from contempo- this sediment suggest that the impactor was Germany); see the Supplemental Material1 for

CITATION: Garde, A.A., et al., 2020, Pleistocene organic matter modified by the Hiawatha impact, northwest Greenland: Geology, v. 48, p. 867–871, https://doi.org/10.1130/G47432.1

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/9/867/5135263/867.pdf by guest on 26 September 2021 A Charcoal and Lignite Sand- and gravel-sized pieces of charcoal from sample HW21-2016 have a largely amorphous structure (“glassy coal”) but also preserve relict cell structures (Fig. 3A). Their reflectance

in incident white light is high (Ro ≤ 3.5%), within the range of modern charcoal (Braad- baart and Poole, 2008; McParland et al., 2009). Pebble-sized lumps of lignite collected at Figure 1. (A) Location of the Hiawatha crater the eastern Hiawatha Glacier front (Fig. 3B) and Kap København in have cellular structures and a low random Greenland. (B) Elevation reflectance (Ro ∼0.2%–0.5%). The ash con- map showing circular tent is only ∼0.5 wt%. One of these lumps depression of the crater, yielded a non-finite14 C age of >43,500 yr B.P. covered by the Greenland Ice Sheet (semitranspar- (our sample Beta-471661). Most of the lignite ent), and positions of consists of woody material with rows of well- glaciofluvial sand sample preserved, even-sized cells with central voids, HW21-2016 (78.83305°N, spiraled fibrous cell walls, and bordered pit pairs 67.13653 W) and two ° (Fig. 3C); layers of cork cells have filled inte- B older reference samples from the outwash plain riors. Alternating growth zones of spring and (HW12-2016, 78.84183°N, summer wood are up to a dozen cells thick. 67.29250°W; and HW13- These cell structures are diagnostic of conifer 2016, 78.88696°N , wood, most likely Pinus or Picea, whereas few 65.99227°W) from Kjær et al. (2018). Modified from fragments contain wood cells of uneven size Kjær et al. (2018). and distinct perforation plates, which is the characteristic of the angiosperm genus Betula (Hoadley, 1990). Some conifer cork layers have greatly inflated cells with numerous spherical voids (Fig. 3D), which we interpret as results of expansion by heating with sudden release of volatile components from the lipid-rich cork cells. Common branching and tapering systems of shrinkage cracks are likewise interpreted as evidence of rapid loss of volatile components by heating (Fig. 3E). Other lumps display flattened details. Organic carbon geochemistry in bulk front of the Hiawatha Glacier. The macroscale cells with homogeneous (gelatinous) interiors

sediment sample HW21-2016 and two refer- organic matter comprises angular, sand- to and low reflectance (Ro = 0.1%–0.2%; Fig. 3C), ence samples was obtained by Hawk pyrolysis gravel-sized particles of charcoal in sample interpreted as remnants of decaying and mildly at Aarhus University (Aarhus, Denmark) follow- HW21-2016, as well as pebble-sized lumps compacted wood. Conglomerate-like masses ing Carrie et al. (2012), measuring the quantity of of lignite rounded by water transport on the of variably decomposed and disrupted wood total organic carbon (TOC, in weight percent) and eastern side of the glacier front; small twigs particles mixed with small vitrinous particles proto-hydrocarbons (S2, in milligrams thermally from dwarf bushes unrelated to the impact (Fig. 3F) are interpreted as results of normal sensitive, labile bio-macromolecule hydrocarbons occur locally on the glacial outwash plain. degradation of lignite and/or impact-related per gram sediment) released between 300 and The microscale, finely dispersed organic car- mixing and redeposition. Only very sporadic 650 °C from the preserved humic matter. Three bon was identified in the impactite grains of remnants of plant spores and/or algae have been grain-size fractions of each sample were mea- sample HW21-2016. found, and no leaf cuticula. sured to check potential variability with grain size. The lignite was dated by the Beta Analytic Organic Matter in the Impactite Grains Carbon Ordering and Organic Carbon Dating Service (https://www.radiocar- Microbreccia grains contain splinters of sili- Geochemistry bon.com/) with calendar-year calibration using cate minerals including shocked quartz as well The organic matter was characterized using OxCal v. 4.3 with the IntCal13 calibration curve as partially melted mineral fragments, set in an laser Raman spectrography (Figs. 4A and 4B). (Reimer et al., 2013). opaque matrix of amorphous organic matter The positions and shapes of the carbon peaks in mixed with altered glass largely derived from the silicate glasses and microbreccias show that RESULTS shock-melted alkali feldspar (Fig. 2A). Impact the carbon ordering is mostly very low and vari- We found both macro- and microscale glass grains contain appreciable dispersed organic able, as is the case for the lignite (Fig. 4A). The organic matter in the outwash sediments in carbon; new magmatic microcrysts within them highest degree of ordering (narrow Raman peak

are commonly surrounded by carbon sheaths width) is found in charcoal with Ro = 2%–3.7%, 1Supplemental Material. Microphotographs, Raman (Fig. 2B). A predominantly carbonaceous grain and in small vitrinite particles in the lignite

methodology, Raman spectra and SEM-EDS spectrum with tiny, angular mineral fragments and clayey (Ro = 0.5%–0.6%). The carbon peaks in some supporting the close association between shocked material is shown in Figures 2C and 2D. Further shock-melted glasses changed shape during quartz, impact glass, and organic matter described in the text. Please visit https://doi.org/10.1130/ examples of the association between shocked analysis. At first, a weak and very broad carbon GEOL.S.12298553 to access the supplemental material, quartz, shock melts, and organic matter are pre- peak appeared, which then abruptly intensified and contact [email protected] with any questions. sented in the Supplemental Material. and narrowed on continued analysis or increased

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DISCUSSION Thermal Degradation of the Organic Matter The organic matter is directly linked to the Hiawatha impact by its close association with impactite grains of microbreccias with shocked quartz and mineral melt glass. The charcoal itself documents a high-temperature B D event, presumably impact-induced incineration of surficial and/or originally shallowly buried plant material (Fig. 3A); we have not discovered formerly airborne organic particles originating from wildfires. The textural evidence of expansion and contraction in lignite (Figs. 3D and 3E) also suggests that some of it was subjected to a distinct heating episode, although its low reflectance shows that it was not affected by direct incineration (e.g., Belcher et al., 2018). In the possible scenario where the impact took Figure 2. Impactite microbreccias with organic carbon in glaciofluvial sand sample HW21-2016 place through the Greenland Ice Sheet, any lig- from the Hiawatha crater, northwest Greenland (Kjær et al., 2018; see Fig. 1 for location). (A) Microbreccia with shocked quartz (Qtz) particle (arrows along shock lamellae), partially melted nite preserved at the margin of the crater would feldspar at right margin, and new epitaxial plagioclase microcrysts (Pl). Matrix is glassy with have been further protected by being embed- abundant dispersed carbonaceous matter. (B) Feldspar and pyroxene microlites with black ded in permafrozen sediments just like the pres- carbon sheaths (arrows). (C,D) Microphoto in reflected light (C) and backscattered electron ent Kap København Formation (Funder et al., scanning electron microscopy (SEM-BSE) image (D) of predominantly carbonaceous grain 2001). Losiak et al. (2016, 2019) reported low- with very fine-grained matrix containing tiny, angular mineral fragments, clayey material, and carbonaceous particles with high reflectivity (arrows), bright in reflected light and dark in BSE reflecting charcoal at the Kaali craters, without image; note, in C, imperfect polishing. interpreting this observation.

laser-beam intensity, and finally became perma- nent. This observation shows that the carbon in A D the glass is principally unordered and finely dispersed and suggests that accumulation of laser beam energy led to localized ordering and crys- tallization of platy, more ordered carbon. The quantity and geochemical composition of the organic carbon in the bulk sediment sample HW21-2016 and reference samples HW12- 2016 and HW13-2016 (Kjær et al., 2018) were analyzed by a Hawk carbon analyzer (Figs. 1 and 4C). Sample HW21-2016 directly draining B E the impact area is depleted in thermally sensi-

Figure 3. Charcoal and lignite from the front of Hiawatha Glacier, northwest Greenland, next to the sampling site of glaciofluvial sand sample HW21-2016 (Kjær et al., 2018; see Fig. 1 for location). (A) Gravel-sized charcoal particle with vacuolated, former cell wall structure

and high reflectance (Ro ≤ 3.5, “glassy coal”). (A, C–F) Optical microscopy, reflected light. (B) Lumps of lignite. (C) Partly degraded coni- C F fer wood with remnants of spiraled cell walls and annual spring and summer growth rings; summer wood is compact. (D) Swollen area of conifer cork, where cells contain numerous rounded voids of different sizes interpreted as former bubbles. Other, greatly expanded cell walls confine large single voids. (E) Branch- ing and tapering shrinkage cracks in lignite. (F) Radial longitudinal section of gymno- sperm wood and wood debris with vitrinite fragments.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/9/867/5135263/867.pdf by guest on 26 September 2021 A through the diagenetic phase under burial tem- peratures of <60 °C (Taylor et al., 1998). There- fore, the distinct loss of labile proto-hydrocarbons in sample HW21-2016 cannot be explained by normal terrestrial diagenetic processes, but rather by an intense, localized external heating event that is well constrained with the previously reported compelling evidence of the Hiawatha impact from shocked quartz and mineral melt glasses.

Origin and Age of the Organic Matter Our observations and analyses of the organic carbon in front of the Hiawatha Glacier show that it stems from organic-rich beds formed at a time when tree growth at this high northern latitude was possible. Pliocene to early Pleis- tocene deposits at ∼80°N are known, e.g., from the 2.4 Ma Kap København Formation in North Greenland (Funder et al., 2001) and the adjacent ca. 3 Ma Beaufort Formation at Meighen Island, Canada (Fyles et al., 1991). The likely genera recorded in the Hiawatha lignite, Picea-Pinus and Betula, are common in the Pliocene-Pleis- BCtocene as well as present northern boreal forests, and therefore do not allow correlation with spe- cific deposits. However, a correlation seems obvi- ous with wood commonly found along rivers on the adjacent Washington Land, 150 km northeast of the Hiawatha Glacier (Fig. 1; Bennike, 1998, 2000). This wood stems from small trees, trans- ported by melt water from an unknown source under the Greenland Ice Sheet, and comprises both Picea and Pinus, but not Betula, which is less resistant to wear. Similar sediments might well exist under the Greenland Ice Sheet where it borders Inglefield Land, although no timber has been found along the rivers here. The early Pleistocene, 2.4 Ma, Kap København Forma- Figure 4. Analytical properties of organic matter and dispersed carbon in Hiawatha (northwest Greenland) impactite grains from glaciofluvial sand sample HW21-2016 (Kjær et al., 2018; see tion in North Greenland is the youngest known Fig. 1 for location). (A) Raman positions of carbon peaks plotted against peak widths in range occurrence of forest at these high latitudes (see of carbon-bearing impactite grains from sample HW21-2016, showing low and variable order- Funder et al. [2001] regarding the 2.4 Ma age ing. Raman data from Gardnos crater (Gilmour et al., 2003) and Antarctic micrometeorites determination), and the absence of Pinus does (Dobrică et al., 2011) are shown for comparison. Δ—Raman shift. (B) Example of carbon peak with position and determination of width, measured at half peak height above background. (C) not preclude contemporaneous growth in Ingle- Bulk sample pyrolysis of sample HW21-2016 containing impactite grains, and two reference field Land some 200 km to the south. In sum- samples without them (see main text). Note relative depletion of thermally sensitive labile mary, the age of the organic carbon at Hiawatha proto-hydrocarbons S2 in sample HW21-2016 relative to the two reference samples. is probably 3–2.4 Ma, and we favor the younger, 2.4 Ma age as the simplest interpretation and a The low degree of carbon ordering in tory (Fig. 4A; Dobrică et al., 2011). With sub- realistic maximum age of the impact. Hiawatha lignite, charcoal, and shock-melted sequent maturation and metamorphism, the glasses compared with graphite in the Hiawatha Hiawatha carbon peaks would be expected to CONCLUSIONS foreland rocks and Gardnos breccia (Figs. 2 have obtained still-narrower peak widths and Charcoal and abundant dispersed organic and 4A; Gilmour et al., 2003) and the pyroly- reverted to lower peak positions, as observed carbon in the impactite grains of glaciofluvial sis data from the three bulk sediment samples in the Gardnos data. There is a pronounced loss sand draining the Hiawatha crater come from suggest that the bulk of the organic carbon at of thermally sensitive hydrocarbons (S2) in the local, thermally degraded conifer trees with a the Hiawatha crater is impact related and not bulk sediment sample HW21-2016 relative to probable late Pliocene to early Pleistocene age reworked graphite from the crystalline bed- the reference sites, located farther away from of ca. 3–2.4 Ma. The carbon cannot have been rock. The range of Raman peak positions and the impact crater (Fig. 4C). This relative deple- derived from the well-crystallized graphite in the widths in the right side of Figure 4A and the tion of S2 in sample HW21-2016 is attributed crystalline bedrock in the target area. The age of variation trend toward initial ordering are the to an intense, externally derived temperature the wood is also the maximum age of the impact, same as observed in Antarctic micrometeorites excursion at the impact site, which appears to and this study provides the first solid evidence displaying various degrees of heating on entry have incinerated virtually all humic substances. that the Hiawatha crater is the youngest known into the atmosphere but no further thermal his- Proto-hydrocarbons are typically preserved large impact crater on Earth.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/9/867/5135263/867.pdf by guest on 26 September 2021 ACKNOWLEDGMENTS Dobrică, E., Engrand, C., Quirico, E., Montagnac, Losiak, A., Belcher, C., Jõeleht, A., Plado, J., and We sincerely thank Adrian Jones, Timmons Erick- G., and Duprat, J., 2011, Raman characteriza- Szyszko, M., 2019, Death from space: Origin son, two anonymous reviewers, Paula Lindgren, Anna tion of carbonaceous matter in CONCORDIA of charcoal found in proximal ejecta blanket of Losiak, and Tod Waight for constructive and precise Antarctic micrometeorites: Meteoritics & Plan- Kaali craters (is not what we think): Abstract 2406 comments; Nynke Keulen for assistance with SEM etary Science, v. 46, p. 1363–1375, https://doi presented at the 50th Lunar and Planetary Science imaging; and John Parnell for a Raman spectrograph .org/10.1111/j.1945-5100.2011.01235.x. Conference, The Woodlands, Texas, 18–22 March. of Inglefield Land reference graphite. 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