Guest “Missed it by that much” by David Middleton
Following this, Kjaer et al. (2018) report the discovery of a large impact crater beneath Hiawatha Glacier in northwest Greenland. From airborne radar surveys, they identify a 31-km-wide, circular bedrock depression beneath up to a kilometre of ice. They further suggest the impactor was over 1 km wide and unlikely to predate the Pleistocene, i.e. it is less than a few million years old (see Fig. 11). This maximum age is confirmed a year later (Garde et al., 2020). Clearly, this crater is a candidate YD-age impact structure.
Sweatman 2021
“Clearly, this crater is a candidate YD-age impact structure.”
Missed it by that much.
Massive asteroid hit Greenland when it was a lush rainforest, under-ice crater shows
By Stephanie Pappas published 3 days agoThe enormous impact crater dates to 58 million years ago.
Scientists now know the age of an enormous impact crater hidden under Greenland’s ice.
The Hiawatha crater, which sits under 0.6 mile (1 kilometer) of ice in northwest Greenland, formed 58 million years ago, according to a study published March 9 in the journal Science Advances. Whereas some initial estimates had gauged the age of the crater at only 13,000 years, the new finding means the impact occurred much earlier, at a time when Greenland was truly green and full of life.
“Greenland was actually covered with a temperate rainforest when the asteroid hit,” said study co-author Michael Storey, a researcher at the Natural History Museum of Denmark who specializes in dating geological materials.
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Live Science
The new publication very robustly dates the impact to the Late Paleocene, approximately 58 million years ago (Ma), approximately the same age as the Marquez impact crater. Many of the coauthors also participated in the first publication on the Hiawatha Crater (Kjaer et al., 2018). The full text is available and it is well-worth reading.
A Late Paleocene age for Greenland’s Hiawatha impact structure
Abstract
The ~31-km-wide Hiawatha structure, located beneath Hiawatha Glacier in northwestern Greenland, has been proposed as an impact structure that may have formed after the Pleistocene inception of the Greenland Ice Sheet. To date the structure, we conducted 40Ar/39Ar analyses on glaciofluvial sand and U-Pb analyses on zircon separated from glaciofluvial pebbles of impact melt rock, all sampled immediately downstream of Hiawatha Glacier. Unshocked zircon in the impact melt rocks dates to ~1915 million years (Ma), consistent with felsic intrusions found in local bedrock. The 40Ar/39Ar data indicate Late Paleocene resetting and shocked zircon dates to 57.99 ± 0.54 Ma, which we interpret as the impact age. Consequently, the Hiawatha impact structure far predates Pleistocene glaciation and is unrelated to either the Paleocene-Eocene Thermal Maximum or flood basalt volcanism in east Greenland. However, it was contemporaneous with the Paleocene Carbon Isotope Maximum, although the impact’s exact paleoenvironmental and climatic significance awaits further investigation.
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DISCUSSION
A new age for the Hiawatha structure
Two impact melt rock samples, collected in 2019 from a riverbank less than 10 km downstream from the edge of the Hiawatha structure, contain unshocked zircon with ages indistinguishable from voluminous local felsic intrusions adjacent to—and presumably also under—Hiawatha Glacier. Shocked zircon in the same samples give a robust ~58-Ma U-Pb age, which agrees with the younger of two 40Ar/39Ar mini-plateau ages from the 2016 sand sample recovered even closer to the structure. In sum, these data indicate sample provenance from a Late Paleocene impact event that occurred somewhere upstream but sufficiently proximal that the target geology is indistinguishable geochronologically from that exposed locally where the samples were recovered. Given existing geomorphic evidence for an eroded complex impact structure beneath Hiawatha Glacier, whose apparent rim is breached by the subglacial channel that ultimately becomes the subaerial river channel from which our detrital samples were recovered (1), the simplest interpretation of our observations—which we explicitly accept for the remainder of the discussion—is that the Hiawatha structure is a relatively large impact structure that formed in the Late Paleocene.
When the Hiawatha structure was first proposed as an impact structure, it was also suggested that it post-dated the Pleistocene inception of the Greenland Ice Sheet at ~2.6 Ma (1). The ~58-Ma age for the structure indicates that it formed long before the inception of the ice sheet, and that it is unrelated to the onset of the Younger Dryas cold period ~12,900 years ago as has been speculated (19).
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Pebble-sized charcoal particles (with cellular structures indicative of conifer wood) and apparently impact-related sand grains rich in organic carbon have been found in Hiawatha Glacier’s glaciofluvial outwash (2, 3). These were assumed to have been derived from organic material in Early Pleistocene deposits and were thus interpreted to support a young impact age (2). However, the new age for the Hiawatha structure indicates that—if these materials are related to the impact—they must instead date to the Paleocene or earlier. Abundant Late Paleocene plant fossils in the Arctic (e.g., on Ellesmere Island, across Nares Strait from Inglefield Land) point to widespread high-altitude coniferous forests at this time (28), providing a plausible source of organic material in impact-related sediments at Hiawatha Glacier.
[…]
Kenny et al., 2022
Chronostratigraphic column and absolute ages after (75). Carbon and oxygen isotope curves after (76) and references therein. Ages for other impact structures with absolute age constraints from compilation of (42) and references therein, except the age for Boltysh, which was published very recently (77). Age of spherules at P-E boundary from (50). Age of peak flood basalt volcanism in east Greenland recalculated from (33) and age of peak eruption of Paleocene lava piles in west Greenland from (34, 35). Approximate age of spherule beds in west Greenland is based on their occurrence at low stratigraphic levels of the Paleocene lava pile of western Greenland (34). Uncertainties shown at 2σ. K., Cretaceous; Maas., Maastrichtian; Dan., Danian; Sel., Selandian; Than., Thanetian; Ypr., Ypresian. (Kenny et al., 2022)
The authors dated unshocked and shocked zircon using the 40Ar/39Ar technique. The unshocked zircon dated to 1.9 Ga, consistent with established dating of the target rock formation. The shocked zircon dated to 58 Ma. The U-Pb method also yielded an age of 58 Ma for the shocked zircon. 40Ar/39Ar (argon-argon) is a very versatile and relatively accurate method for geochronological dating. It can date materials from a few thousand to billions of years old. U-Pb (uranium-lead) is one of the most widely used methods for radiometric dating, particular older rocks.
Based primarily on the work of Garde et al., 2020, Christ et al., 2021 and Silber et al, 2021, I had previously bracketed the age of the impact to have been between 1.4 to 0.9 Ma. Silber demonstrated that the impact probably occurred on an ice-free surface and Garde had placed the maximum age at 2.4 to 3 Ma. Garde made the assumption that the charred organic material was related to Late Pliocene to Early Pleistocene deposits containing fossilized material from trees. They didn’t directly date the material or consider that it might be related to much older periods when Greenland was actually green.
The Hiawatha Crater is now categorically ruled out as a candidate for a Younger Dryas impact structure.
References
Christ, A.J., Bierman, P.R., Schaefer, J.M., Dahl-Jensen, D., Steffensen, J.P., Corbett, L.B., Peteet, D.M., Thomas, E.K., Steig, E.J., Rittenour, T.M., Tison, J-L., Blard, P-H., Perdrial, N., Dethier, D., Lini, A., Hidy, A.J., Caffee, M., Southon, J., 2021, “A multi-million-year-old record of Greenland vegetation and glacial history preserved in sediment beneath 1.4 km of ice at Camp Century”, The Proceedings of the National Academies of Sciences of the United States of America.
Garde, Adam A., Anne Sofie Søndergaard, Carsten Guvad, Jette Dahl-Møller, Gernot Nehrke, Hamed Sanei, Christian Weikusat, Svend Funder, Kurt H. Kjær, Nicolaj Krog Larsen; Pleistocene organic matter modified by the Hiawatha impact, northwest Greenland. Geology 2020;; 48 (9): 867–871. doi: https://doi.org/10.1130/G47432.1
Kenny, Gavin G. et al. 2022. “A Late Paleocene age for Greenland’s Hiawatha impact structure“. Science Advances 8 (10); doi: 10.1126/sciadv.abm2434
Kjær, Kurt, Larsen, Nicolaj, Binder, Tobias, Bjørk, Anders, Eisen, Olaf, Fahnestock, Mark & Funder, Svend & Garde, Adam & Haack, Henning & Helm, Veit, Houmark-Nielsen, Michael, Kjeldsen, Kristian, Khan, Shfaqat, Machguth, Horst, Mcdonald, Iain, Morlighem, Mathieu, Mouginot, Jeremie’ Paden, J., Waight, Tod & MacGregor, Joseph. (2018). “A large impact crater beneath Hiawatha Glacier in northwest Greenland”. Science Advances. 4. eaar8173. 10.1126/sciadv.aar8173.
Silber, E. A., B. C. Johnson, E. Bjonnes, et al. 2021. “Effect of ice sheet thickness on formation of the Hiawatha impact crater.” Earth and Planetary Science Letters, 566: 116972 [10.1016/j.epsl.2021.116972]
Sweatman, Martin B. The Younger Dryas impact hypothesis: Review of the impact evidence, Earth-Science Reviews, Volume 218, 2021, 103677, ISSN 0012-8252, https://doi.org/10.1016/j.earscirev.2021.103677.
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March 18, 2022
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