A preliminary biostratigraphic study of the section at Sakahogi shows that most of the middle Norian radiolarian taxa are present in the 30 cm interval above the ejecta layer and apparently survived well into the late Norian7 (Supplementary Fig. S4). However, our new biostratigraphic data collected from above the previously sampled horizon indicate that extinctions of middle Norian species occurred in a stepwise fashion in the ~1 m interval above the ejecta horizon. Furthermore, our high-resolution palaeontological and geochemical data reveal that two palaeoenvironmental events occurred during the initial phase of the radiolarian extinction interval (Fig. 3). The first event (E1) consisted of the post-impact shutdown of primary productivity and a remarkable decline in the amount of biogenic silica preserved before the first phase of diversification (D1). The second event (E2) consisted of a large and sustained reduction in the sinking flux of radiolarian silica and the proliferation of siliceous sponges, occurring before the second phase of diversification (D2) and lasting for ~0.3 Myr after the impact.

During the initial E1 event, the post-impact reduction in primary productivity occurred during deposition of the clay layer, as suggested by the negative δ13C org excursion observed in the claystone. Concentrations of TOC with low δ13C org values could be interpreted to represent a change in the proportion of organic matter derived from land plants versus oceans25,26. However, low and relatively stable C/N ratios (mean, 1.8; standard deviation, 0.6) throughout the study section indicate that contributions of land-derived organic matter are minor to absent. The rapid decrease in the biogenic silica content in the claystone (Supplementary Table S3) further indicates that the productivity of silica-secreting radiolarians was sharply reduced and that marine primary productivity declined during the initial E1 event. Laboratory studies show that radiolarians prey on phytoplankton, such as dinoflagellates, haptophytes and thecate nonmotile algae27,28; however, we know little about the ecological factors that influence radiolarian productivity. A decline in radiolarian production in the near-surface zone may imply major changes at the base of the marine food chain, such as a substantial reduction in primary productivity or a shift toward primary producers not favored by pre-impact taxa.

The negative δ13C org excursion and the rapid decrease in biogenic SiO 2 content observed in the upper part of the claystone occurs within a 3.2-cm-thick section, suggesting a very short time scale for the E1 event. We used previously published 187Os/188Os ratio results9 to estimate the duration of low-productivity conditions after the impact. Previous studies have revealed that 187Os/188Os ratios declined abruptly, from 0.477 to 0.126, in the lower sublayer of the claystone. This excursion can be interpreted as a result of the mixing of ambient seawater Os (characterized by relatively high 187Os/188Os ratios) with meteoritic Os (characterized by low 187Os/188Os ratios); the meteoric Os was vaporized at the time of the impact and was subsequently dissolved into seawater. The 187Os/188Os ratio, which is lowest in the lower sublayer, gradually increases towards the upper sublayer and reaches pre-impact levels in chert samples overlying the claystone layer. No discrete extraterrestrial particles were observed in the upper sublayer claystone, which indicates that the 187Os/188Os ratio of this sublayer records the Os isotope composition of ancient seawater9. Therefore, the recovery of 187Os/188Os values in the upper sublayer claystone after the impact event may reflect post-impact removal of excess dissolved meteoritic Os from seawater, which occurred over a period of 104–105 yr, as Os residence times range from 10 to 60 kyr (refs 29 and 30). Although the marine residence time of Os in the Late Triassic is not precisely known, deposition of the upper sublayer clay may have occurred over the period 104–105 yr after the impact event. This interval is hypothesized to represent the duration required for the restoration of productivity by primary and silica-secreting organisms after the middle Norian impact. However, simulations using an ocean–atmosphere/carbon-cycle model31, which suggest a global collapse of primary productivity (in the Strangelove Ocean) resulting in the delivery and cycling of carbon in the oceans and on land, cannot explain such short-term (104–105 yr) shifts in δ13C org values across the ejecta layer. The global implications of the magnitude and short duration of the negative δ13C org excursion reported here remain to be verified at other middle–late Norian boundary intervals worldwide.

Following the resurgence in primary productivity after the E1 event, the biogenic silica content had recovered to pre-impact values by the first chert bed overlying the claystone layer. However, our analysis reveals that major biotic components of the bedded chert changed temporarily from radiolarians to siliceous sponges, for ~0.3 Myr after the impact. Assuming a constant sedimentation rate in the middle–upper Norian chert succession of 1.1–1.6 mm kyr−1 and a constant dissolution rate of biogenic silica during that time, the mass accumulation rates (MAR) of radiolarian silica in the pre-impact chert beds is estimated at ~0.1 g cm−2 kyr−1, which is the same as the biogenic silica flux near the equator in the modern Pacific Ocean (0.1–0.3 g cm−2 kyr−1)32. After the impact event, the MAR decreased to 0.02 ± 0.01 g cm−2 kyr−1 in the upper sublayer claystone and then remained low during the deposition of sponge spicule-rich cherts. On the other hand, the MAR of the siliceous sponge silica increased markedly across the claystone layer of the ejecta deposit, from ~0.01 to ~0.08 g cm−2 kyr−1 and subsequently decreased to ~0.01 g cm−2 kyr−1 after the E2 event. Our data and previous data9 on terrigenous elements (e.g., Ti, Al and K) indicate that the flux of terrestrial components (aeolian dust) derived from continental crust22,33 did not change substantially through the study interval; thus, the significant and sustained reduction in the flux of radiolarian silica appears to have coincided with an increasing volume of siliceous sponge spicules. During the Mesozoic, the concentration gradient caused by the export of silica from surface to deep waters by sinking of marine plankton may not have been as intense as in modern diatom-dominated oceans34 and such concentration gradients in any case would have been briefly interrupted by fluctuations in radiolarian productivity. Even lacking a direct record of Triassic silica concentrations, it is likely that a reduction in radiolarian productivity in the Panthalassa Ocean during the E2 event, which probably increased the amount of dissolved silicic acid in seawater, favored the proliferation of siliceous sponges after the impact event. Laboratory experiments on silicon uptake by siliceous sponges reveal that an increased concentration of silicic acid in water has a striking positive effect on both the size and robustness of siliceous sponge spicules35. Our hypothesis that dissolved silicic acid increased in seawater at the time of the E2 event is supported by the observation that longer and more robust skeletons of siliceous sponge spicules were dominant only in the spicules of the spicule-rich chert (Supplementary Fig. S10).

Decreases in the sinking flux of radiolarian silica during the E1 and E2 events may reflect a decline in radiolarian production in middle Norian taxa, including in Capnodoce and Capnuchosphaera species. These middle Norian radiolarians are very rare above the E1 interval, whereas a small spumellarian species is abundant within the E2 interval; this spumellarian species is reported as Spumellaria gen. et sp. indet. A and its occurrence can be used to identify the stratigraphic position of the ejecta layer in other Triassic chert sections within the Jurassic accretionary complexes in Japan9. These taxa can be considered as short-lived opportunistic species, as they disappeared at the end of the radiolarian faunal turnover interval. The present biostratigraphic analysis also reveals that radiation of late Norian taxa was contemporaneous with a temporal bloom in the numbers of opportunistic spumellarian species in the E2 interval. The timing of these radiation events suggests that the decrease in radiolarian biomass in the middle Norian taxa enhanced the bloom of opportunistic radiolarian species and the evolutionary radiation of late Norian taxa in the E1 and E2 intervals. Hence, the gradual extinction of middle Norian radiolarian taxa during the ~1 Myr period could be explained by ecological pressures imposed by late Norian taxa, provided that the late Norian taxa were more rapidly growing and more efficient phytoplankton feeders than the middle Norian taxa. These unusual radiation patterns are similar to those observed in the Panthalassic TJB sections in Japan and Canada21,22. As with the TJB event, changes in seawater acidity, temperature, and/or a reduced nutrient levels in ocean surface waters are possible drivers for the decline in the production of middle Norian radiolarian taxa. The primary cause of this decline is difficult to identify, but the relatively long period of the E2 interval (~0.3 Myr after the impact) largely excludes the possibility that the decline was triggered by instantaneous environmental stresses (e.g., extended darkness, global cooling, or acid rain24,36) that would have been caused by a bolide impact.

Did the middle–late Norian extinction event occur uniformly on a global scale, or does it represent a regional phenomenon in the Panthalassa Ocean? The record of radiolarian faunal change across the middle–upper Norian boundary has been established at the species level in several regions37,38, showing that widespread and apparently sudden extinctions affected the Subfamily Capnodocinae and Family Capnuchosphaeridae at the boundary23. As a first approximation, it is probably reasonable to assume that a geographically widespread faunal change across the middle–upper Norian boundary was related to an impact event that triggered the radiolarian extinction in the equatorial Panthalassan Ocean. Existing radiolarian records are not sufficiently precise to constrain these relationships with biostratigraphic resolutions comparable to those presented here for the middle–upper Norian. Further biostratigraphic analyses of middle–upper Norian boundary sections will be required to validate this hypothesis.

We propose that the impact event was probably the major factor responsible for the conodont and Pacific (North American) ammonoid extinctions that occurred in the middle–upper Norian boundary39,40,41. The base of the Epigondolella bidentata conodont zone and the base of the Gnomohalorites cordilleranus ammonoid zone in western North America39,40 define the position of the middle–upper Norian boundary that is most closely aligned with the traditional base of the Sevatian41 and which can be correlated with the radiolarian extinction interval in the study section in Japan. Although ammonoids are absent in the studied section, the biostratigraphic record of conodonts suggests that a few Parvigondolella species survived across the ejecta layer, but that an important middle Norian Epigondolella species became extinct just below the impact horizon (Supplementary Fig. S11). The present data also show the first appearance of late Norian Epigondolella species in the E1 and E2 intervals. Conspicuous morphological changes occur in this genus across the ejecta layer; Epigondolella species below the ejecta layer are characterized by a wide platform, whereas those above the ejecta layer possess a longer and more narrow platform. The catastrophic collapse of the pelagic ecosystem during the E1 and E2 events was probably the major factor responsible for the conodont turnover that occurred at the end of the middle Norian.

This study has revealed that late middle Norian open-ocean ecosystems experienced profound disruptions after a large impact event (chondritic impactor of 3.3–7.8 km in diameter) and that the event was possibly related to the 90-km-diameter Manicouagan crater in Canada. Although many marine sections over the past 540 Myr have been examined, no catastrophic collapse in marine ecosystems caused by an extraterrestrial impact has yet been described, with the outstanding exception of the Cretaceous–Paleogene boundary (KPB) crisis24 and the middle Norian event reported here. Given that no large volcanic events occurred in the Norian3, the fossil record of the middle–upper Norian is key to evaluating the general importance of impacts as causes of biotic and environmental changes in pelagic ecosystems.