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Supportive comment on: “Morphology and population of binary asteroid
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impact craters”, by K. Miljković, G. S. Collins, S. Mannick and P. A.
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Bland [Earth Planet. Sci. Lett. 363 (2013) 121–132] – An updated
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assessment
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Martin Schmieder1,2, Mario Trieloff3, Winfried H. Schwarz3, Elmar Buchner4 and Fred Jourdan2
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School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
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Western Australian Argon Isotope Facility, Department of Applied Geology and JdL Centre, Curtin University, GPO Box
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U1987, Perth, WA 6845, Australia
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Institut für Geowissenschaften, Universität Heidelberg, Im Neuenheimer Feld 234-236, D-69120 Heidelberg, Germany
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HNU-Neu-Ulm University, Edisonallee 5, D-89231 Neu-Ulm, Germany
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In their recent paper, Miljković et al. (2013) assess the apparent contradiction that the near-Earth asteroid population
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consists of 15% binaries, while the terrestrial (and Martian) impact crater populations have only 2-4% of observable
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doublet craters. The authors suggest that only a small fraction of sufficiently well separated binary asteroids yield
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recognizable doublets. We generally agree with the conclusions by Miljković et al. (2013) and acknowledge the high
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quality and relevance of the study. However, we would like to bring into focus additional geochronologic constraints
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that are critical when evaluating terrestrial impact crater doublets. Miljković et al. (2013) appraised five potential
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terrestrial doublets using the Earth Impact Database (EID; as of 2010). We hereby warn against the indiscriminate
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usage of impact ages compiled in this database without an assessment based on solid isotopic and stratigraphic
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constraints and comment on the geological, geochronological, and geochemical evidence for doublet impact craters
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on Earth.
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Geologic evidence
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Firstly, the confirmation of macroscopic and microscopic shock effects in rocks and minerals recovered from
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candidate terrestrial impact sites is a prerequisite to establish an impact origin of such structures (French and
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Koeberl, 2010). Miljković et al. (2013) included the Crawford and Flaxman structures in South Australia in their
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selection of ‘possible’ terrestrial crater doublets. These structures had been introduced as impact structures in a 1999
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conference abstract. Due to the lack of compelling evidence for impact, both structures were later classified as
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‘possible impact structures’ of uncertain age (Haines, 2005). Only proven impact structures should be part of
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statistical considerations on the terrestrial impact rate.
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Geochronological criteria
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Secondly, we point out that some of the impact ages listed by Miljković et al. are outdated and incorrect. Precise and
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accurate impact ages are crucial when it comes to ‘double impact’. All potential crater pairs of Miljković et al.
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(2013) have apparent crater ages that comply with a theoretical double impact scenario; however, comparatively
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large errors on the ages and, more importantly, a lack of unambiguous accuracy of some of the ages leave evidence
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for double impact doubtful. The dating of terrestrial impact structures has undergone a dynamic evolution in recent
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years, and more and more new and refined isotopic data are now available. Nevertheless, only a few of the ~188
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known terrestrial impact structures have been dated with a satisfying uncertainty of ≤±2% by means of isotopic
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and/or biostratigraphic methods (Jourdan et al., 2012). Isotopic approaches can yield results as precise as ~±0.5% in
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their relative error on the age. Only synchronicity of two neighboring impact events within such narrow
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uncertainties hardens the evidence for double impact.
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Comments on the potential crater doublets listed in Miljković et al. (2013)
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The ≥36 km Clearwater West and ~26 km Clearwater East impact structures in Canada are listed as a ‘very likely’
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doublet in Miljković et al. (2013). The maximum age of both structures is stratigraphically constrained by the
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shocked Ordovician limestones that overlie the Archean basement (e.g., Grieve, 2006). The 290 ± 20 Ma age for
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both impact structures widely cited in the literature and the EID seems to be adopted from early, poorly robust, K-Ar
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ages for Clearwater West published in a 1960s Geologic Survey of Canada report. Our group obtained two 40Ar/39Ar
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plateau ages with a mean of 286 ± 2 Ma for Clearwater West, very similar to the age reported in Bottomley et al.
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(1990). Age data for Clearwater East are still somewhat ambiguous. A mineral isochron Rb-Sr age for melt rocks
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from Clearwater East yielded an age of 287 ± 26 Ma (Reimold et al., 1981), coeval within error with the ages for
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Clearwater West. However, the Rb-Sr technique has frequently failed to provide accurate impact ages (e.g., Mark et
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al., 2013).
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apparent ages around ~460-470 Ma (Bottomley et al., 1990). Similar ages were obtained by our group, suggesting
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that Clearwater West and East are probably not a doublet. Currently, the idea of synchronicity of the two impact
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events entirely relies on a single Rb-Sr age for Clearwater East and, probably, a general ‘agreement’ since the 1960s
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based on the expectation that the two impact structures, closely spaced, must represent a doublet.
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The ~25 km Kamensk and ~3 km Gusev impact structures, also a ‘very likely’ crater doublet in Miljković et al.
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(2013), have been considered a doublet since the 1970s. Both craters are filled with sediments of the Globukaya
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Formation of postulated Cretaceous-Paleogene boundary age, an impact-derived marine resurge breccia that overlies
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the uppermost Cretaceous and is topped by Paleogene sands (Movshovich et al., 1991). Notwithstanding with the
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previous biostratigraphic age estimates,
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yielded a fairly robust Eocene age of 50.36 ± 0.33 Ma (recalculated; Jourdan et al., 2012), contradicting a ~65 Ma
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impact age (as given in Miljković et al. 2013). This also constrains a minimum age for the Globukaya Formation and
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the smaller Gusev crater, and suggests sedimentary reworking processes in the Eocene. Despite some uncertainties
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regarding the exact timing of the Kamensk-Gusev impact, stratigraphic correlation and geographic evidence support
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a likely double impact scenario.
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The ~24 km Ries and ~3.8 km Steinheim impact craters in Southern Germany, associated with the Central European
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tektite strewn field, have been generally regarded as a typical crater doublet since the 1960s (Stöffler et al. 2002).
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Miljković et al. (2013) conservatively labelled them a ‘likely’ crater doublet. Multiple dating campaigns have
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established a robust Miocene
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Jourdan et al., 2012) for the Ries crater. In contrast to a well-established isotopic age data set for the Ries, the nearby
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Steinheim Basin has failed to yield any reliable isotopic dating results so far ( 40Ar/39Ar and (U-Th)/He dating by our
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Ar/39Ar dating of melt rocks from Clearwater East produced a set of significantly older Ordovician
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Ar/39Ar dating of fresh impact glass from Kamensk by Izett et al. (1994)
Ar/39Ar age of 14.83 ± 0.15 Ma (Di Vincenzo and Skála, 2009; recalculated by
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group; Buchner et al., 2011). No coherent Steinheim impact ejecta are preserved outside the crater that could be
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correlated with the Ries ejecta blanket. As a result, the assumed synchronicity of the Ries and Steinheim impacts
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completely relies on the biostratigraphy of the Miocene post-impact crater lake sediments at both sites. However,
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assuming that both crater lakes formed shortly after impact, the oldest known freshwater deposits of the Ries contain
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fossil mammals of the Neogene mammal zone MN6 (Langhian), whereas no evidence for such fossils could be
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observed at Steinheim. Instead, the lowermost Steinheim lake deposits are representative of the younger mammal
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zone MN7 (Serravallian; Heizmann and Hesse, 1995; Heizmann and Reiff, 2002). Therefore, one must consider that
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the Ries crater might be slightly older than the Steinheim Basin. Until such issues are resolved, it is unsafe to treat
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these craters as a ‘proven’ doublet based on their proximity alone.
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Miljković et al. (2013) also listed the ~12 km Serra da Cangalha and ~4.5 km Riachão impact structures as a
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‘possible’ doublet. For both impact structures precise and accurate ages are currently lacking, and only a
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stratigraphic maximum age of ≤250 Ma can be assigned for the Serra da Cangalha impact that affected the ~250-
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260 Ma Permian sandstones of the Pedra de Fogo Formation in the Parnaíba Basin of Brazil (Kenkmann et al.,
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2011). This formation also forms the youngest impact-deformed target rocks at Riachão (Maziviero et al., 2012).
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Due to deep erosion of both impact structures, no post-impact deposits are preserved that could constrain a
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minimum impact age; nor are melt lithologies known from either of these structures to provide material for isotopic
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dating. Based on the poor stratigraphic age constraints, evidence for a crater doublet is far from proven.
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Additional candidate doublets?
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A recent addition to the list of potential impact doublets on Earth are the Paleozoic marine Lockne and Målingen
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impact structures in central Sweden. The ~1 km Målingen structure, ~16 km southwest of the ≥7.5 km Lockne
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impact structure, was confirmed as of impact origin after the release of the paper by Miljković et al. (2013). The
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Målingen impact affected Ordovician orthoceratid limestones and produced a sequence of impactites and a resurge
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breccia overlain by the post-impact Dalby Limestone – a pre- to post-impact sequence essentially identical with that
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at Lockne (Alwmark et al., 2013). No isotopic age data are currently available for these two impact structures, but
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high-resolution biostratigraphic dating plots the Lockne event in the Lagenochitina dalbyensis chitinozoan chron
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(Middle-Late Sandbian; Grahn and Nõlvak, 1993), with a dating precision of ~1%. Despite the high meteorite influx
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during the Middle and Late Ordovician as a consequence of the L-chondrite parent asteroid breakup ~470 Ma ago
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(Schmitz et al., 2007) and the obvious temporal and spatial clustering of impacts in Baltoscandia (also including
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Granby and Tvären in Sweden, as well as Kärdla and the ‘Osmussaar breccia’ in Estonia), most of these impact
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structures and deposits have slightly different biostratigraphic ages (Alwmark et al., 2010). However, neither of the
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post-impact sequences at Lockne and Målingen seems to contain a separate impact ejecta layer that could indicate a
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(slightly) younger impact event close-by, which may be further sedimentologic evidence for two synchronous
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impacts within the same chitinozoan biozone. An Ordovician double impact event in Baltica at ~455 Ma is,
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therefore, reasonably likely, and so far has more solid constraints than the list used by Miljković et al. (2013).
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The ~4 km Suvasvesi North and South impact structures in Paleoproterozoic crystalline rocks of Finland are
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generally perceived as a crater doublet bona fide (Werner et al., 2002). Although the Suvasvesi South impact
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structure is currently not listed in the EID and thus not mentioned by Miljković et al. (2013), there is ample evidence
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for impact (Donadini et al., 2006). The Suvasvesi structures are spaced at ~7 km from center to center and, thus,
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probably overlap. Somewhat surprisingly, 40Ar/39Ar dating of impact melt rock chips recovered from the Suvasvesi
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North drill core yielded a Cretaceous age of ~85 Ma (Schmieder et al., 2012), whereas a Proterozoic
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(minimum) age of ≥700 Ma was obtained for a melt rock sample from Suvasvesi South (Buchner et al., 2009). We
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propose that both craters were produced as a ‘false doublet’ by impacts far apart in time but, by pure coincidence,
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extremely close in space.
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Ar/39Ar
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Geochemical considerations
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Finally, an additional variable not considered by Miljković et al. (2013) is the geochemical signature that, if
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detectable, the asteroid left in impact-produced lithologies. Whereas melt rocks from the Clearwater East impact
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structure carry a distinct chondritic signature, no clear meteoritic signal was detected at Clearwater West (Palme et
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al., 1978). Similarly, no resolvable enrichment of impactor-derived siderophiles was found in the Ries glasses
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(Schmidt and Pernicka, 1994), in contrast to a strong exotic Ni-Co contamination in melt particles from the
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Steinheim Basin (Buchner and Schmieder, 2010). Although seemingly unlikely, these two examples suggest that
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different meteorite types might have been involved in the postulated double impact scenarios. Nevertheless, we
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acknowledge that the theoretical binary impact model still works if one considers a different composition of the
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primary and secondary asteroid; the impact of Asteroid 2008 TC3 – Almahata Sitta in Sudan on October 07, 2008
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revealed that even single meteorite falls can involve different types of impactor material (Bischoff et al., 2010).
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Extraterrestrial chromite grains in the resurge deposits of the Ordovician Lockne impact structure suggested an L-
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chondritic impactor (e.g., Schmitz et al., 2007), whereas a detailed geochemical characterization of the impact
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deposits at its possible twin crater, Målingen, is still outstanding (Alwmark et al., 2013). No detailed information on
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the individual composition of primary and secondary asteroids is currently available for the ~30 known near-Earth
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binaries in space (Delbo et al., 2011).
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Re-assessment and conclusions
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In summary, from a mainly geochronological point of view, several of the suggested terrestrial impact crater
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doublets are associated with ambiguous ages and require further study. This particularly concerns the Clearwater
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and Ries-Steinheim crater pairs traditionally considered as classical doublets by ‘general agreement’, whereas
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current isotopic and stratigraphic evidence suggests they are not. As the exact timing of impact is critical, only a
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combination of accurate and precise impact ages, along with geologic and geochemical evidence, can ultimately
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resolve the existence of terrestrial doublet craters in the geologic record. So far, the number of definitely proven
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doublets on Earth is 0. With somewhat loose criteria, we can consider the Kamensk-Gusev and Lockne-Målingen
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crater pairs as ‘very likely’ doublets. Finally, we wish to comment on a statement by Miljković et al. (2013) on their
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p.124: “[…] even with such a large separation and poorly constrained ages, this crater pair is in such close
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proximity that it is statistically unlikely for them to be formed by two single impacts”. Again, the two closely spaced
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Suvasvesi impact structures demonstrate that ‘false doublets’ can occur by a freak of nature. Considering a growing
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number of impact structures known on our planet, the slightly revised population of possible crater doublets (based
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on analytical evidence) on Earth – currently between 0 and 2 out of ~130 impact structures >5 km in diameter – is
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on the order of ≤1.5%, with major uncertainties. This being a complementary comment, we emphasize and
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independently confirm the very interesting results presented by Miljković et al. (2013) that binary asteroid impacts
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are insufficiently represented by ‘true’ terrestrial crater doublets.
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Acknowledgements: We kindly thank two anonymous reviewers for their constructive comments and the Editor
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Christophe Sotin for careful handling of the manuscript. Lauri Pesonen was involved in previous and ongoing dating
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of the Suvasvesi impacts. M.S., M.T., W.H.S. and E.B. thank the Klaus Tschira Stiftung gGmbH for financial
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support.
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References
161
Alwmark, C., Schmitz, B. and Kirsimäe, K. 2010. The mid-Ordovician Osmussaar breccia in Estonia linked to the
162
163
164
disruption of the L-chondrite parent body in the asteroid belt. GSA Bull. 122, 1039-1046.
Alwmark, C., Holm, S. Ormö, J., Sturkell, E., 2013. Shocked quartz in the Målingen structure – Evidence for a small
twin crater to the Lockne impact structure. In: 44th LPSC, abstract no. 2100.
165
Bischoff, A., Horstmann, M., Pack, A., Laubenstein, M., Haberer, S., 2010. Asteroid 2008 TC3—Almahata Sitta: A
166
spectacular breccia containing many different ureilitic and chondritic lithologies. Meteoritics Planet. Sci. 45,
167
1638-1656.
168
169
170
Bottomley, R.J., York, D., Grieve, R.A.F. 1990. 40Argon-39Argon dating of impact craters. Proc. 20th Lunar Planet.
Sci. Conf., LPI, Houston, pp. 421-431.
Buchner, E., Schmieder, M., Schwarz, W.H., Trieloff, M., Moilanen, J., Öhman, T., Stehlik, H., 2009. A Proterozoic
171
40
172
supplement, no. 5076.
173
174
175
Ar/39Ar age for the Suvasvesi South structure (Finland). In: 72 nd MetSoc Meeting, Meteoritics Planet. Sci.
Buchner, E., Schmieder, M., 2010. Steinheim suevite—A first report of melt-bearing impactites from the Steinheim
Basin (SW Germany). Meteoritics Planet. Sci. 45, 1093-1107.
Buchner, E., Schmieder, M., Cooper, F.J., Hodges, K.V., Jourdan, F., Koeberl, C., Pösges, G., Reimold, W.U.,
176
Schwarz, W.H., Trieloff, M., van Soest M.C., Wartho, J.-A., 2011. Concurrent ages of the Nördlinger Ries
177
and the Steinheim Basin? - An extremely voluminous versus an inexistent isotopic data set for the two impact
178
craters in Southern Germany. Geophys. Res. Abstr.13, EGU2011-1334-8.
179
180
Delbo, M., Walsh, K., Mueller, M., Harris, A.W., Howell, E.S., 2011.The cool surfaces of binary near-Earth
asteroids. Icarus 212, 138-148.
181
Di Vincenzo, G., Skála, R., 2009.
40
Ar–39Ar laser dating of tektites from the Cheb Basin (Czech Republic):
182
Evidence for coevality with moldavites and influence of the dating standard on the age of the Ries impact.
183
Geochim. Cosmochim. Acta 73, 493-513.
184
Donadini, F., Plado, J., Werner, S.C., Salminen, J., Pesonen, L.J., Lehtinen, M., 2006. New evidence for impact
185
from the Suvasvesi South structure, central east Finland. In: Cockell, C., Koeberl, C., Gilmour, I. (eds.)
186
Biological Processes Associated with Impact Events, Impact Studies, pp. 287-307.
187
188
189
190
191
192
Earth Impact Database, 2013.Maintained the University of New Brunswick, Fredericton, New Brunswick, Canada.
Available online at: http://www.passc.net/EarthImpactDatabase/index.html (last accessed 05 February 2013).
French, B.M., Koeberl, C., 2010. The convincing identification of terrestrial meteorite impact structures: What
works, what doesn't, and why. Earth-Sci. Rev. 98, 123-170.
Grahn, Y., Nõlvak, J., 1993. Chitinozoan dating of Ordovician impact events in Sweden and Estonia. A preliminary
note. GFF 115, 263-264.
193
Grieve, R.A.F., 2006. Impact structures in Canada. GEOTEXT 5, Geol. Assoc. Canada, St. John’s, 210 pp.
194
Haines, P.W., 2005. Impact cratering and distal ejecta: the Australian record. J. Australian Earth Sci. 52, 481-507.
195
Heizmann, E.P.J., Hesse, A., 1995. Die mittelmiozänen Vogel- und Säugetierfaunen des Nördlinger Ries (MN 6)
196
und des Steinheimer Beckens (MN 7) – ein Vergleich. Courier Forschungsinstitut Senckenberg 181, 171–185
197
(in German).
198
Heizmann, E.P.J., Reiff, W., 2002. Der Steinheimer Meteorkrater. Pfeil, Munich, 160 pp. (in German).
199
Izett, G.A., Masaitis, V.L., Shoemaker, E.M., Dalrymple, G.B., Steiner, M.B., 1994. Eocene age of the Kamensk
200
buried crater of Russia. LPI Contrib. 825, pp. 55-56.
201
Jourdan, F., Reimold, W.U., Deutsch, A., 2012.Dating terrestrial impact structures. Elements 8, 46-50.
202
Kenkmann, T., Vasconcelos, M.A.R., Crósta, A.P., Reimold, W.U., 2011. The complex impact structure Serra da
203
204
Cangalha, Tocantins State, Brazil. Meteoritics Planet. Sci. 46, 875-889.
Mark, D.F., Lindgren, P., Fallick, A.E., 2013. A high-precision Ar/Ar age for the Dellen impact structure, Sweden.
205
In: Jourdan, F., Mark, D.F., Verati, C. (eds.) Advances in 40Ar/39Ar Dating: from Archaeology to Planetary
206
Sciences. J. Geol. Soc., London, Spec. Pub. 378 (in press).
207
Maziviero, M.V., Vasconcelos, M.A.R., Góes, A.M., Crósta, A.P., Reimold, W.U., 2012. The Riachão ring impact
208
structure, Northeastern Brazil: re-evaluation of its stratigraphy and evidence for impact. In: 43rd LPSC, 1659,
209
p.1511.
210
211
212
213
214
215
216
217
Miljković, K., Collins, G.S., Mannick, S., Bland, P.A., 2013. Morphology and population of binary asteroid impact
craters. Earth Planet. Sci. Lett. 363, 121-132.
Movshovich, Ye.V., Milyavskiy, A.Ye., Titova N., 1991. Geology of the northeastern margin of Donets ridge and
dating of the Kamensk and Gusev impact craters. Int. Geol. Rev. 33, 623-635.
Palme, H., Janssens, M.-J., Takahashi, H., Anders, E., Hertogen, J., 1978.Meteoritic material at five large impact
craters. Geochim. Cosmochim. Acta 42, 313-323.
Reimold, W.U., Grieve, R.A.F., Palme, H., 1981. Rb- Sr dating of the impact melt from East Clearwater, Quebec.
Contrib. Mineral. Petrol. 76, 73-76.
218
Schmidt, G., Pernicka, E., 1994. The determination of platinum group elements (PGE) in target rocks and fall-back
219
material of the Nördlinger Ries impact crater, Germany. Geochim. Cosmochim. Acta 58, 5083-5090.
220
Schmieder, M., Schwarz, W.H., Buchner, E., Pesonen, L.J., Lehtinen, M., Trieloff, M., 2012. Double and multiple
221
impact events on Earth - Hypotheses, tests, and problems. In: 75th MetSoc Meeting, Meteoritics Planet. Sci.
222
supplement, no. 5005.
223
Schmitz, B., Harper, D. A. T., Peucker-Ehrenbrink, B., Stouge, S., Alwmark, C., Cronholm, A., Bergström, S.,
224
Tassinari, M., Xiaofeng, W. 2007. Asteroid breakup linked to the Great Ordovician Biodiversification Event.
225
Nature Geosci. 1, 49-53.
226
227
228
229
Stöffler, D., Artemieva, N.A., Pierazzo, E. 2002. Modeling the Ries-Steinheim impact event and the formation of
the moldavite strewn field. Meteoritics Planet. Sci. 37, 1893-1907.
Werner, S.C., Plado, J., Pesonen, L.J., Kuulusa, M., 2001. The two Suvasvesi lakes in central Finland – A possible
doublet impact structure. Meteoritics Planet Sci. 36, supplement, p. 223-224.