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| Titel |
Need to re-evaluate the age of Chesapeake Bay and Popigai Craters and their relevance for the Eocene/Oligocene boundary |
| VerfasserIn |
Vera Assis Fernandes, Mario Trieloff, Natalia A. Artemieva, Joerg Fritz, Wolf U. Reimold |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250049553
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| Zusammenfassung |
| Introduction: Large impact craters distribute material globally, and their ejecta layers present important horizon markers allowing inter-correlation of sediments from different sites, e.g. for late Eocene sediments pre-dating the tipping point of Earth’s climate at the Eocene/Oligocene boundary (boundary at33.9±0.1 Ma; [1]). The Global Stratotype Section and Point (GSSP) in Massignano, Italy [2] contains three iridium-rich ejecta layers covering a 2 Ma time interval, with two being attributed to the Chesapeake Bay (85 km Ø) and Popigai (100 km Ø) impact structures, respectively [3]. Coeval with these anomalies the flux of extraterrestrial 3He-rich particles (presumably dust-size) increased [3]. This increased 3He-burial flux into marine sediments may have resulted either from an asteroid shower onto the Earth-Moon system [4&5], or may have been due to a comet shower [3]. This intense shower of dust to km sized extraterrestrial objects onto Earth predates the Eocene/Oligocene boundary by ~1.5 Ma, which marks the major global climatic change from the warm Eocene to the onset of glaciations in the Oligocene, i.e. hot house-ice house transition [6].
Age of the Chesapeake Bay crater: Current radiometric age determinations for the Chesapeake Bay crater are based on total fusion Ar ages of tektites (distal impact glasses; [7-10]). Our preliminary 40Ar-39Ar step heating measurements suggest that precise Ar ages on these tektites cannot be acquired by total fusion. These data, together with optical microscopy suggest a complex trapped component which cannot be deconvolved when performing total fusion Ar extraction experiments.
Age of the Popigai crater: Current radiometric age determination for the Popigai crater is based on 40Ar/39Ar step heating experiments carried out by [11] on several impact melt rocks. The reported age is 35.7± 0.2 Ma based on a single plateau as these authors argued that other Ar-Ar age spectra were likely affected by inherited 40Ar or perturbed by some mechanism which is not fully understood. More recently, [12] recalculated the weighted mean of all four plateaux and two “mini”-plateaux (displaying a disturbed age spectrum and a younger age for Popigai crater. The best age estimate of 36.42±0.81 Ma is based on the four plateaux showing ~70% 39Ar release [12]. These authors concluded that there is a need to further extend the work on age determination on several samples to better evaluate the effects of inherited argon and other disturbances (e.g. weathering alteration) on Popigai impactites.
Conclusion: There is a need to acquire a variety of well preserved impactites that will permit the thorough characterization of the material as well as to enable hand-picking of the best suitable fragments for an extended 40Ar/39Ar study for precise age determinations of the Chesapeake and Popigai craters.
References: [1] Gradstein et al. ( 2004). Internat. Comm. on Stratigr. (ICS) www.stratigraphy.org. [2] Premoli Silva and Jenkins (1993) Episodes 16, 379–382. [3] Farley et al. (1998) Science 280, 1250–1253. [4] Tagle and Claeys (2004) Science 305, 492. [5] Fritz et al. (2007) Icarus 189, 591-594. [6] Zachos et al. (2001) Nature 292, 686-693. [7] Glass et al. (1986) Meteoritics 21, 369-370 (abstr.). [8] Glass et al. (1995) GCA 59, 4071-4082. [9] Albin et al. (1996) LPSC XXVII, 5-6 (abst.). [10] Horton and Izett (2004) USGS Prof. Paper 1688, E1-E30. [11] Bottomley et al (1997) Nature 388, 365–368. [12] Jourdan et al (2009) EPSL 286, 1-13. |
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