Geological Society of America | GEOLOGY | Volume XX | Number XX | www.gsapubs.org 1

Manuscript received 23 January 2020 Revised manuscript received 2 April 2020

Manuscript accepted 8 April 2020

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

© 2020 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license.

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

Pleistocene organic matter modified by the Hiawatha impact, northwest GreenlandAdam 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

1 Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen K, Denmark2 Lithospheric Organic Carbon (LOC) Group, Department of Geoscience, Aarhus University, Høegh Guldbergs Gade 2, 8000 Aarhus, Denmark

3 Globe Institute, University of Copenhagen, Øster Voldgade 5–7, 1350 Copenhagen K, Denmark4 Alfred Wegener Institute, Am Handelshafen, 27570 Bremerhaven, Germany

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 micro-brecchia 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 desic-cation 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.

INTRODUCTIONMost terrestrial impact structures contain

only little organic carbon, recycled from target rocks. The ca. 35 Ma Popigai crater, Siberia, contains diamonds transformed from target-rock graphite (Masaitis et al., 1975). Ejecta from the ca. 23 Ma Haughton crater, Canada (Parnell et al., 2007), and the ca. 15 Ma Ries crater, Germany (Osinski 2003), contain organic carbon derived from sedimentary target rocks, and drill core from the 66 Ma Chicxulub crater, Mexico, contains particles of charcoal (Gulick et al., 2019). If the source of the carbon can be identified, it may serve to constrain the maxi-mum age of cratering. For instance, dispersed graphitic carbon in the ca. 500 Ma Gardnos crater in Norway stems from Cambrian Alum Shale (Gilmour et al., 2003; Parnell and Lind-gren 2006). Carbon inclusions in impact glasses linked to the purported ca. 0.8 Ma Darwin crater, Tasmania, preserve biomarkers from contempo-

raneous vegetation (Howard et al., 2013), and the small, twin ca. 3.2 ka Kaali craters, Estonia, contain locally derived charcoal (Losiak et al., 2016, 2019).

With a diameter of 31 km, the newly dis-covered Hiawatha crater under the Greenland Ice Sheet in northwest Greenland is one of the 25 largest impact structures on Earth (Fig. 1; Kjær et al., 2018; Impact Earth, 2020, https://impact.uwo.ca/). A large variety of sand-sized impactite grains originating from the crater floor were found in a glaciofluvial outwash sediment sample (HW21-2016 of Kjær et al.; see their table S1 with sample information) deposited at the front of the Hiawatha Gla-cier just outside the crater rim (Fig. 1). The sand contains shocked quartz, microbreccias, shock-melted mineral glasses, and micromag-matic aggregates; elevated Pt contents and anomalous platinum-group element ratios in this sediment suggest that the impactor was

an iron meteorite (Kjær et al., 2018). The bulk mineral assemblage and chemical composi-tion of the sand indicate sourcing from gran-ulite-grade rocks similar to Paleoproterozoic paragneiss exposed in the ice-free foreland to the crater (locally including sulfidic parag-neiss with graphite flakes; Dawes, 2004). The crater has not been directly dated but is tenta-tively referred to the Pleistocene and possibly as young as the last glacial period, based on indirect evidence such as preservation of the subglacial crater topography, a strongly dis-turbed pre-Holocene ice stratigraphy over the crater identified by airborne radar imaging, and an anomalously high modern-day volume of melt water draining from the impacted area (Kjær et al., 2018).

In this study, 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. We identify the wood fragments to gen-era level and discuss the implications of our findings for the maximum age of the Hiawatha impact.

METHODSSediment samples described by Kjær et al.

(2018), and charcoal and lignite collected by us from in front of the Hiawatha Glacier were inves-tigated by standard optical and backscattered elec-tron scanning electron microscopy (SEM-BSE; Geological Survey of Denmark and Greenland, Copenhagen, Denmark), pyrolysis, and Raman spectra. The Raman spectra were obtained with a WITec alpha300 R system and a 488 nm laser at the Alfred Wegener institute (Bremerhaven, Germany); see the Supplemental Material1 for

Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G47432.1/5051423/g47432.pdfby gueston 06 August 2020

2 www.gsapubs.org | Volume XX | Number XX | GEOLOGY | Geological Society of America

details. Organic carbon geochemistry in bulk sediment sample HW21-2016 and two refer-ence samples was obtained by Hawk pyrolysis at Aarhus University (Aarhus, Denmark) follow-ing Carrie et al. (2012), measuring the quantity of total organic carbon (TOC, in weight percent) and proto-hydrocarbons (S2, in milligrams thermally sensitive, labile bio-macromolecule hydrocarbons per gram sediment) released between 300 and 650 °C from the preserved humic matter. Three grain-size fractions of each sample were mea-sured to check potential variability with grain size. The lignite was dated by the Beta Analytic Carbon Dating Service (https://www.radiocar-bon.com/) with calendar-year calibration using OxCal v. 4.3 with the IntCal13 calibration curve (Reimer et al., 2013).

RESULTSWe found both macro- and microscale

organic matter in the outwash sediments in

front of the Hiawatha Glacier. 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. The microscale, finely dispersed organic car-bon was identified in the impactite grains of sample HW21-2016.

Organic Matter in the Impactite GrainsMicrobreccia grains contain splinters of sili-

cate minerals including shocked quartz as well as partially melted mineral fragments, set in an opaque matrix of amorphous organic matter mixed with altered glass largely derived from shock-melted alkali feldspar (Fig. 2A). Impact glass grains contain appreciable dispersed organic carbon; new magmatic microcrysts within them are commonly surrounded by carbon sheaths (Fig. 2B). A predominantly carbonaceous grain with tiny, angular mineral fragments and clayey material is shown in Figures 2C and 2D. Further examples of the association between shocked quartz, shock melts, and organic matter are pre-sented in the Supplemental Material.

Charcoal and LigniteSand- and gravel-sized pieces of charcoal

from sample HW21-2016 have a largely amor-phous 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 the eastern Hiawatha Glacier front (Fig. 3B) have cellular structures and a low random reflectance (Ro ∼0.2%–0.5%). The ash con-tent is only ∼0.5 wt%. One of these lumps yielded a non-finite 14C age of >43,500 yr B.P. (our sample Beta-471661). Most of the lignite consists of woody material with rows of well-preserved, even-sized cells with central voids, spiraled fibrous cell walls, and bordered pit pairs (Fig. 3C); layers of cork cells have filled inte-riors. Alternating growth zones of spring and summer wood are up to a dozen cells thick. These cell structures are diagnostic of conifer wood, most likely Pinus or Picea, whereas few fragments contain wood cells of uneven size 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 cells with homogeneous (gelatinous) interiors and low reflectance (Ro = 0.1%–0.2%; Fig. 3C), interpreted as remnants of decaying and mildly compacted wood. Conglomerate-like masses of variably decomposed and disrupted wood particles mixed with small vitrinous particles (Fig. 3F) are interpreted as results of normal degradation of lignite and/or impact-related mixing and redeposition. Only very sporadic remnants of plant spores and/or algae have been found, and no leaf cuticula.

Carbon Ordering and Organic Geochemistry

The organic matter was characterized using laser Raman spectrography (Figs. 4A and 4B). The positions and shapes of the carbon peaks in the silicate glasses and microbreccias show that the carbon ordering is mostly very low and vari-able, as is the case for the lignite (Fig. 4A). The highest degree of ordering (narrow Raman peak width) is found in charcoal with Ro = 2%–3.7%, and in small vitrinite particles in the lignite (Ro = 0.5%–0.6%). The carbon peaks in some shock-melted glasses changed shape during analysis. At first, a weak and very broad carbon peak appeared, which then abruptly intensified and narrowed on continued analysis or increased

1Supplemental Material. Microphotographs, Raman methodology, Raman spectra and SEM-EDS spectrum supporting the close association between shocked quartz, impact glass, and organic matter described in the text. Please visit https://doi .org/10.1130/GEOL.S.12298553 to access the supplemental material, and contact editing@geosociety.org with any questions.

Figure 1. (A) Location of the Hiawatha crater and Kap København in Greenland. (B) Elevation map showing circular depression of the crater, covered by the Greenland Ice Sheet (semitranspar-ent), and positions of glaciofluvial sand sample HW21-2016 (78.83305°N, 67.13653°W) and two older reference samples from the outwash plain (HW12-2016, 78.84183°N, 67.29250°W; and HW13-2016 , 78 .88696 °N, 65.99227°W) from Kjær et al. (2018). Modified from Kjær et al. (2018).

A

B

Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G47432.1/5051423/g47432.pdfby gueston 06 August 2020

Geological Society of America | GEOLOGY | Volume XX | Number XX | www.gsapubs.org 3

laser-beam intensity, and finally became perma-nent. This observation shows that the carbon in the glass is principally unordered and finely dis-persed 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 sam-ple 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 the impact area is depleted in thermally sensi-

tive, labile bio-macromolecule hydrocarbons (S2) compared to the two reference samples.

DISCUSSIONThermal 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 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 expan-sion 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 place through the Greenland Ice Sheet, any lig-nite preserved at the margin of the crater would have been further protected by being embed-ded in permafrozen sediments just like the pres-ent Kap København Formation (Funder et al., 2001). Losiak et al. (2016, 2019) reported low-reflecting charcoal at the Kaali craters, without interpreting this observation.

A

B

C

D

Figure 2. Impactite microbreccias with organic carbon in glaciofluvial sand sample HW21-2016 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 feldspar at right margin, and new epitaxial plagioclase microcrysts (Pl). Matrix is glassy with abundant dispersed carbonaceous matter. (B) Feldspar and pyroxene microlites with black carbon sheaths (arrows). (C,D) Microphoto in reflected light (C) and backscattered electron scanning electron microscopy (SEM-BSE) image (D) of predominantly carbonaceous grain 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 image; note, in C, imperfect polishing.

A

B

C

D

E

F

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 parti-cle 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-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.

Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G47432.1/5051423/g47432.pdfby gueston 06 August 2020

4 www.gsapubs.org | Volume XX | Number XX | GEOLOGY | Geological Society of America

The low degree of carbon ordering in Hiawatha lignite, charcoal, and shock-melted glasses compared with graphite in the Hiawatha foreland rocks and Gardnos breccia (Figs. 2 and 4A; Gilmour et al., 2003) and the pyroly-sis data from the three bulk sediment samples suggest that the bulk of the organic carbon at the Hiawatha crater is impact related and not reworked graphite from the crystalline bed-rock. The range of Raman peak positions and widths in the right side of Figure 4A and the variation trend toward initial ordering are the same as observed in Antarctic micrometeorites displaying various degrees of heating on entry into the atmosphere but no further thermal his-

tory (Fig. 4A; Dobrică et al., 2011). With sub-sequent maturation and metamorphism, the Hiawatha carbon peaks would be expected to have obtained still-narrower peak widths and reverted to lower peak positions, as observed in the Gardnos data. There is a pronounced loss of thermally sensitive hydrocarbons (S2) in the bulk sediment sample HW21-2016 relative to the reference sites, located farther away from the impact crater (Fig. 4C). This relative deple-tion of S2 in sample HW21-2016 is attributed to an intense, externally derived temperature excursion at the impact site, which appears to have incinerated virtually all humic substances. Proto-hydrocarbons are typically preserved

through the diagenetic phase under burial tem-peratures of <60 °C (Taylor et al., 1998). There-fore, the distinct loss of labile proto-hydrocar-bons 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 MatterOur 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-tocene 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-tion in North Greenland is the youngest known occurrence of forest at these high latitudes (see Funder et al. [2001] regarding the 2.4 Ma age determination), and the absence of Pinus does not preclude contemporaneous growth in Ingle-field Land some 200 km to the south. In sum-mary, the age of the organic carbon at Hiawatha is probably 3–2.4 Ma, and we favor the younger, 2.4 Ma age as the simplest interpretation and a realistic maximum age of the impact.

CONCLUSIONSCharcoal and abundant dispersed organic

carbon in the impactite grains of glaciofluvial sand draining the Hiawatha crater come from local, thermally degraded conifer trees with a probable late Pliocene to early Pleistocene age of ca. 3–2.4 Ma. The carbon cannot have been derived from the well-crystallized graphite in the crystalline bedrock in the target area. The age of the wood is also the maximum age of the impact, and this study provides the first solid evidence that the Hiawatha crater is the youngest known large impact crater on Earth.

A

B C

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 Fig. 1 for location). (A) Raman positions of carbon peaks plotted against peak widths in range of carbon-bearing impactite grains from sample HW21-2016, showing low and variable order-ing. Raman data from Gardnos crater (Gilmour et al., 2003) and Antarctic micrometeorites (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) Bulk sample pyrolysis of sample HW21-2016 containing impactite grains, and two reference samples without them (see main text). Note relative depletion of thermally sensitive labile proto-hydrocarbons S2 in sample HW21-2016 relative to the two reference samples.

Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G47432.1/5051423/g47432.pdfby gueston 06 August 2020

Geological Society of America | GEOLOGY | Volume XX | Number XX | www.gsapubs.org 5

ACKNOWLEDGMENTSWe sincerely thank Adrian Jones, Timmons Erick-son, two anonymous reviewers, Paula Lindgren, Anna Losiak, and Tod Waight for constructive and precise comments; Nynke Keulen for assistance with SEM imaging; and John Parnell for a Raman spectrograph of Inglefield Land reference graphite. This research was funded by the Carlsberg Foundation (Copenha-gen, Denmark) and the Aarhus University Research Foundation (Denmark).

REFERENCES CITEDBelcher, C.M., New, S.L., Santín, C., Doerr, S.H.,

Dewhirst, R.A., Grosvenor, M.J., and Hudspith, V.A., 2018, What can charcoal reflectance tell us about energy release in wildfires and the prop-erties of pyrogenic carbon?: Frontiers in Earth Science, v. 6, 169, https://doi .org/10.3389/feart.2018.00169.

Bennike, O., 1998, Late Cenozoic wood from Wash-ington Land, North Greenland, in Higgins, A.K., and Watt, W.S., eds., Review of Greenland Activi-ties 1997–1998: Geology of Greenland Survey Bulletin 180, p. 155–158.

Bennike, O., 2000, Notes on the late Cenozoic his-tory of the Washington Land area, western North Greenland, in Dawes, P.R., and Higgins, A.K., eds., Review of Greenland Activities 1999: Geol-ogy of Greenland Survey Bulletin 186, p. 29–34.

Braadbaart, F., and Poole, I., 2008, Morphological, chemical and physical changes during charcoal-ification of wood and its relevance to archaeologi-cal contexts: Journal of Archaeological Science, v. 35, p. 2434–2445, https://doi .org/10.1016/ j.jas.2008.03.016.

Carrie, J., Sanei, H., and Stern, G., 2012, Standardi-sation of Rock-Eval pyrolysis for the analysis of recent sediments and soils: Organic Geochem-istry, v. 46, p. 38–53, https://doi .org/10.1016/ j.orggeochem.2012.01.011.

Dawes, P.R., 2004, Explanatory notes to the Geo-logical map of Greenland, 1:500 000, Humboldt Gletscher, Sheet 6: Geological Survey of Den-mark and Greenland Map Series 1, 48 p.

Dobrică, E., Engrand, C., Quirico, E., Montagnac, G., and Duprat, J., 2011, Raman characteriza-tion of carbonaceous matter in CONCORDIA Antarctic micrometeorites: Meteoritics & Plan-etary Science, v. 46, p. 1363–1375, https://doi .org/10.1111/ j.1945-5100.2011.01235.x.

Funder, S., Bennike, O., Böcher, J., Israelson, C., Petersen, K.S., and Símonarson, L.A., 2001, Late Pliocene Greenland—The Kap Køben-havn Formation in North Greenland: Bulletin of the Geological Society of Denmark, v. 48, p. 117–134.

Fyles, J.G., Marincovich, L., Jr., Matthews, J.V., Jr., and Barendrecht, R., 1991, Unique mollusc find in the Beaufort Formation (Pliocene) on Meighen Island, Arctic Canada, in Current Research, Part B: Interior Plains and Arctic Canada: Geologi-cal Survey of Canada, Paper 91-1B, p. 105–112.

Gilmour, I., French, B.M., Franchi, I.A., Abbott, J.I., Newton, J., and Koeberl, C., 2003, Geochemis-try of carbonaceous impactites from the Gardnos impact structure, Norway: Geochimica et Cos-mochimica Acta, v. 67, p. 3889–3903, https://doi .org/10.1016/S0016-7037(03)00213-8.

Gulick, S.P.S., et al., 2019, The first day of the Ceno-zoic: Proceedings of the National Academy of Sciences of the United States of America, v. 116, p. 19,342–19,351, https://doi .org/10.1073/pnas.1909479116.

Hoadley, R.B., 1990, Identifying Wood: Accurate Results with Simple Tools: Newton, Connecti-cut, Taunton Press, Inc., 223 p.

Howard, K.T., et al., 2013, Biomass preservation in impact melt ejecta: Nature Geoscience, v. 6, p. 1018–1022, https://doi .org/10.1038/ngeo1996.

Kjær, K.H., et al., 2018, A large Pleistocene impact crater beneath Hiawatha Glacier in northwest Greenland: Science Advances, v. 4, eaar8173, https://doi .org/10.1126/sciadv.aar8173.

Losiak, A., et al., 2016, Dating a small impact cra-ter: An age of Kaali crater (Estonia) based on charcoal emplaced within proximal ejecta: Mete-oritics & Planetary Science, v. 51, p. 681–695, https://doi .org/10.1111/maps.12616.

Losiak, A., Belcher, C., Jõeleht, A., Plado, J., and Szyszko, M., 2019, Death from space: Origin of charcoal found in proximal ejecta blanket of Kaali craters (is not what we think): Abstract 2406 presented at the 50th Lunar and Planetary Science Conference, The Woodlands, Texas, 18–22 March.

Masaitis, V.L., Futergendler, S.I., and Gnevushev, M.A., 1975, Diamonds in impactites of the Popi-gai meteoritic crater: Zapiski Vsesoyuzngo Min-eralogicheskogo Obschchestva, v. 101, p. 108–112 (in Russian).

McParland, L.C., Collinson, M.F., Scott, A.C., and Campbell, G., 2009, The use of reflectance val-ues for the interpretation of natural and anthro-pogenic charcoal assemblages: Archaeological and Anthropological Sciences, v. 1, 249, https://doi .org/10.1007/s12520-009-0018-z.

Osinski, G.R., 2003, Impact glasses in fallout suevites from the Ries impact structure, Germany: An analytical SEM study: Meteoritics & Plan-etary Science, v. 38, p. 1641–1667, https://doi .org/10.1111/ j.1945-5100.2003.tb00006.x.

Parnell, J., and Lindgren, P., 2006, Survival of reactive carbon through meteorite impact melting: Geol-ogy, v. 34, p. 1029–1032, https://doi .org/10.1130/G22731A.1.

Parnell, J., Bowden, S.A., Osinski, G.R., Lee, P., Green, P., Taylor, C., and Baron, M., 2007, Organic geochemistry of impactites from the Haughton impact structure, Devon Island, Nuna-vut, Canada: Geochimica et Cosmochimica Acta, v. 71, p. 1800–1819, https://doi .org/10.1016/ j.gca.2007.01.006.

Reimer, P.J., et al., 2013, IntCal13 and Marine13 radiocarbon age calibration curves 0–50,000 years cal BP: Radiocarbon, v. 55, p. 1869–1887, https://doi .org/10.2458/azu_js_rc.55.16947.

Taylor, G.H., Teichmüller, M., Davis, A., Diessel, C.F.K., Littke, R., and Robert, P., eds., 1998, Organic Petrol-ogy: A New Handbook Incorporating Some Revised Parts of Stach’s Textbook of Coal Petrology: Stutt-gart, Gebrüder Bornträger, 704 p.

Printed in USA

Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G47432.1/5051423/g47432.pdfby gueston 06 August 2020