Abstract
Meteorite impacts have caused catastrophic perturbations to the global environment and mass extinctions throughout the Earth’s history. Here, we present petrographic and geochemical evidence of a possible impact ejecta layer, dating from about 11 Ma, in deep-sea clayey sediment in the Northwest Pacific. This clay layer has high platinum group element (PGE) concentrations and features a conspicuous negative Os isotope anomaly (187Os/188Os as low as ~0.2), indicating an influx of extraterrestrial material. It also contains abundant spherules that include pseudomorphs suggestive of porphyritic olivine as well as spinel grains with euhedral, dendritic and spherical forms and NiO contents as great as 23.3 wt%, consistent with impact ejecta. Osmium isotope stratigraphy suggests a most plausible depositional age of ~11 Ma (Miocene) for this layer, as determined by fitting with the seawater evolution curve. No large impact crater of this age is known on land, even within the relatively large uncertainty range of the relative Os age. Thus, we suggest that an unrecognised impact event in the middle or late Miocene produced the impact ejecta layer of the Northwest Pacific.
Highlights
When fitting the secular change curve, we relied first on the seawater Os isotope curve based on pelagic sediment[18,48,49,50,51,52,53,54,55]
Depositional ages of the 30 intervening samples were calculated by linear extrapolation along with their stratigraphic positions (Fig. 1b)
In sample PC11-4_62-64, having a sedimentation rate of 0.04 cm/kyr based on Os isotope dating, a dry density of ~0.5 g/cm[3] and an excess Os concentration of 1,366 ppt, the accumulation rate of extraterrestrial Os was calculated to be 27.3 ng/cm2/Myr, which is much higher than the background cosmic spherule flux of 4 ng/cm2/Myr[58]
Summary
In the interval above the manganese layer, the depositional ages of the uppermost sample (PC11-1_4-6) and the two lowermost samples (PC11-5_4-6 and PC11-5_14-16) were dated at 2.0, 12.0 and 14.8 Ma, respectively, by fitting the seawater Os isotope curve. When fitting the secular change curve, we relied first on the seawater Os isotope curve based on pelagic sediment[18,48,49,50,51,52,53,54,55].
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
