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[SGL19-04] Osmium isotopic records and abundances of platinum group elements of the Cretaceous-Paleogene boundary in the Nemuro Group
Keywords:the K-Pg boundary, Osmium isotope, the Nemuro Group
The Cretaceous-Paleogene boundary (K-Pg boundary) at 66 million years ago marks one of the most severe mass extinctions through Phanerozoic. Many fossil records show that up to 75% of species including non-avian dinosaurs became extinct at the K-Pg boundary. A clear peak in abundances of platinum group elements (PGEs) such as iridium (Ir) and osmium (Os) has been observed in clay beds at the K-Pg boundary around the world, and is one of the most important pieces of evidence of a massive meteorite impact that could have triggered the mass extinction. A sharp decline in osmium isotope ratio (187Os/188Os) can also be used to detect an extraterrestrial impact.
In Japan, upper Cretaceous and lower Paleogene sedimentary successions have been reported in the Nemuro Group in Hokkaido (Saito et al., 1986), but this section lacks a clear anomaly of Ir. More recently, based on
micropaleontological investigations and radiometric dating (zircon U-Pb) of tuff, sedimentary succession that includes upper Cretaceous and lower Paleogene has been identified in the Kawaruppu Formation of the Nemuro Group, which is near the section reported in Saito et al.(1986). In this study, we measured mass fractions of platinum group elements and Os isotopic compositions of the Kawaruppu Formation to further constrain the stratigraphic position of the K-Pg boundary. We sampled two sets of sedimentary rocks at the Kawaruppu tributary, one for low resolution and long-time range analysis, and the other for high-resolution analysis near the presumed K-Pg boundary.
The 187Os/188Os ratios in the uppermost Cretaceous interval were approximately 0.6, which are consistent with the global seawater values for Maastrichtian reconstructed from pelagic sedimentary rocks (e.g. Ravizza and Peucker-Ehrenbrink, 2003 and Robinson et al., 2009). The isotopic ratios of the lowest Paleogene interval were ~0.4, which are also consistent with the Paleogene values for pelagic sedimentary rocks. A temporary increase in Os concentration and a slight increase in Ir concentration were also observed in the stratigraphic levels where the lowest 187Os/188Os ratio was observed. These results allow me to identify the stratigraphic levels that contain the K-Pg boundary. In addition, a detailed examination of the Os/Ir ratio reveals the presence of short-term hiatus in the Early Paleogene.
According to Ravizza and Peucker-Ehrenbrink (2003) and Robinson et al. (2009), a two-step decrease in the 187Os/188Os ratio at the K-Pg boundary has been observed at sites worldwide. The first decline (from 0.6 to 0.4) began at the C29R/C30N magnetic reversal boundary just before the K-Pg boundary (about 66.3 Ma), which roughly coincides with the beginning of the Deccan volcanic activity. The second decline (from 0.4 to 0.2) is attributed to K-Pg meteorite impact. However, in Kawaruppu Formation, the 187Os/188Os ratio is stable at about 0.6 even in the C29R interval, where Deccan volcanism had already occurred, and the 187Os/188Os ratio suddenly decreases at the K-Pg boundary. This is most likely due to the difference in how PGEs from meteoric and volcanic sources were supplied. While the sites in the previous studies are located in the pelagic area, the Kawaruppu Formation was deposited in the forearc basin close to the continental shelf or upper continental slope. Therefore, the flux of PGEs in the Kawaruppu Formation may have been different from that in the open ocean.
[References]
Ravizza, G., and Peucker-Ehrenbrink, B., 2003. Chemostratigraphic evidence of Deccan volcanism from the marine osmium isotope record. Science. 302, 1392-1395.
Robinson, N., Ravizza, G., Coccioni, R., Peucker-Ehrenbrink, B., and Norris, R., 2009. A high-resolution marine 187Os/188Os record for the late Maastrichtian: Distinguishing the chemical fingerprints of Deccan volcanism and the KP impact event. Earth Planet. Sci. Lett. 281, 159-168.
Saito, T., Yamanoi, T. and Kaiho, K., 1986. End-Cretaceous devastation of terrestrial flora in the boreal Far East. Nature. 323, 253-255.
In Japan, upper Cretaceous and lower Paleogene sedimentary successions have been reported in the Nemuro Group in Hokkaido (Saito et al., 1986), but this section lacks a clear anomaly of Ir. More recently, based on
micropaleontological investigations and radiometric dating (zircon U-Pb) of tuff, sedimentary succession that includes upper Cretaceous and lower Paleogene has been identified in the Kawaruppu Formation of the Nemuro Group, which is near the section reported in Saito et al.(1986). In this study, we measured mass fractions of platinum group elements and Os isotopic compositions of the Kawaruppu Formation to further constrain the stratigraphic position of the K-Pg boundary. We sampled two sets of sedimentary rocks at the Kawaruppu tributary, one for low resolution and long-time range analysis, and the other for high-resolution analysis near the presumed K-Pg boundary.
The 187Os/188Os ratios in the uppermost Cretaceous interval were approximately 0.6, which are consistent with the global seawater values for Maastrichtian reconstructed from pelagic sedimentary rocks (e.g. Ravizza and Peucker-Ehrenbrink, 2003 and Robinson et al., 2009). The isotopic ratios of the lowest Paleogene interval were ~0.4, which are also consistent with the Paleogene values for pelagic sedimentary rocks. A temporary increase in Os concentration and a slight increase in Ir concentration were also observed in the stratigraphic levels where the lowest 187Os/188Os ratio was observed. These results allow me to identify the stratigraphic levels that contain the K-Pg boundary. In addition, a detailed examination of the Os/Ir ratio reveals the presence of short-term hiatus in the Early Paleogene.
According to Ravizza and Peucker-Ehrenbrink (2003) and Robinson et al. (2009), a two-step decrease in the 187Os/188Os ratio at the K-Pg boundary has been observed at sites worldwide. The first decline (from 0.6 to 0.4) began at the C29R/C30N magnetic reversal boundary just before the K-Pg boundary (about 66.3 Ma), which roughly coincides with the beginning of the Deccan volcanic activity. The second decline (from 0.4 to 0.2) is attributed to K-Pg meteorite impact. However, in Kawaruppu Formation, the 187Os/188Os ratio is stable at about 0.6 even in the C29R interval, where Deccan volcanism had already occurred, and the 187Os/188Os ratio suddenly decreases at the K-Pg boundary. This is most likely due to the difference in how PGEs from meteoric and volcanic sources were supplied. While the sites in the previous studies are located in the pelagic area, the Kawaruppu Formation was deposited in the forearc basin close to the continental shelf or upper continental slope. Therefore, the flux of PGEs in the Kawaruppu Formation may have been different from that in the open ocean.
[References]
Ravizza, G., and Peucker-Ehrenbrink, B., 2003. Chemostratigraphic evidence of Deccan volcanism from the marine osmium isotope record. Science. 302, 1392-1395.
Robinson, N., Ravizza, G., Coccioni, R., Peucker-Ehrenbrink, B., and Norris, R., 2009. A high-resolution marine 187Os/188Os record for the late Maastrichtian: Distinguishing the chemical fingerprints of Deccan volcanism and the KP impact event. Earth Planet. Sci. Lett. 281, 159-168.
Saito, T., Yamanoi, T. and Kaiho, K., 1986. End-Cretaceous devastation of terrestrial flora in the boreal Far East. Nature. 323, 253-255.