Abstract
Stratigraphic records of impact ejecta preserved in a pelagic deep-sea setting occur within Upper Triassic successions of the subduction-generated accretionary complexes of central Japan. A significant biotic turnover in radiolarians occurred during the ~ 300 kyr time interval after the impact event, which is characterized by a remarkable reduction in the burial flux of radiolarian silica. However, the nature of the environmental conditions at this time remains unclear. To investigate the environmental changes that triggered a decline in radiolarian burial flux after the impact event, geochemical proxies (major, trace, and rare earth elements) were applied to the middle–upper Norian (Upper Triassic) bedded chert succession of the Mino Belt, central Japan. A progressive environmental deterioration is evident from (1) a post-impact shutdown of burial flux of primary and silica- and apatite-secreting organisms; and (2) a subsequent abrupt increase in chemical weathering associated with a sustained reduction in the burial flux of radiolarian silica. No significant redox changes were observed across the impact event. The continental weathering proxies suggest a transient increase in weathering intensity occurred during the decline of radiolarian burial flux, likely in response to a short-term warm and humid period. Our results delineate a remarkable record of progressive environmental changes in the Panthalassa Ocean after this large impact event.
Highlights
An Upper Triassic (Norian) impact event has been inferred from anomalous concentrations of platinumgroup elements (PGEs) and a negative osmium (Os) isotope excursion, in addition to occurrences of microspherules and Ni-rich magnetite, in a claystone layer in an Upper Triassic bedded chert succession in the Sakahogi section, central Japan (Onoue et al 2012; Sato et al 2013; Sato et al 2016) (Fig. 1)
We examined variations in the compositions of major, trace, and rare earth elements (REE) in bedded cherts in the Sakahogi section to identify the environmental changes responsible for the decline in radiolarian burial flux after the middle Norian impact event
5 Discussion On the basis of high-resolution profiles of biogeochemical signatures (Si, Ba, Ca, and P), redox-sensitive elements, REE, and Chemical Index of Alteration (CIA; Nesbitt and Young 1982) values obtained from the Triassic bedded cherts, we assessed the environmental changes that triggered a decline in radiolarian burial flux after the Norian impact event
Summary
An Upper Triassic (Norian) impact event has been inferred from anomalous concentrations of platinumgroup elements (PGEs) and a negative osmium (Os) isotope excursion, in addition to occurrences of microspherules and Ni-rich magnetite, in a claystone layer in an Upper Triassic bedded chert succession in the Sakahogi section, central Japan (Onoue et al 2012; Sato et al 2013; Sato et al 2016) (Fig. 1). An Upper Triassic (Norian) impact event has been inferred from anomalous concentrations of platinumgroup elements (PGEs) and a negative osmium (Os) isotope excursion, in addition to occurrences of microspherules and Ni-rich magnetite, in a claystone layer in an Upper Triassic bedded chert succession, central Japan (Onoue et al 2012; Sato et al 2013; Sato et al 2016) (Fig. 1). Onoue et al (2016a) showed that extinctions of middle Norian radiolarian species occurred in a stepwise fashion in the ~ 1 Myr interval above the ejecta horizon, which was associated with the radiation of late Norian radiolarians (Fig. 3a). High-resolution paleontological and geochemical data revealed that two paleoenvironmental events occurred during the initial phase of the radiolarian turnover interval. Norian paleolatitude of deposition range: 0.3 14.4 N (Ando et al, 2001, Uno et al, 2015)
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