[S-CG55] Ocean Floor Geoscience

[SCG55-05] Tracing Miocene impact events: Age estimation using the Re-Os isotope for pelagic clay in the Minamitorishima EEZ

*Mindaleva Diana¹, Tatsuo Nozaki^1,2, Kazuhide Mimura³, Moei Yano⁴, Junichiro Ohta², Koichi Iijima⁵, Kazutaka Yasukawa², Kentaro Nakamura^2,4, Yasuhiro Kato^2,4 (1.Waseda Univ., 2.Univ. of Tokyo, 3.AIST, 4.Chiba Inst. of Tech., 5.JAMSTEC)

Keywords:Ejecta layer, Impact event, Miocene, Re-Os isotope, Pelagic clay, Minamitorishima EEZ

Pelagic clay containing high rare-earth elements (REEs) concentrations (REE-rich mud) in the Minamitorishima Exclusive Economic Zone (EEZ) are considered promising future REE resources (Kato et al., 2011; Takaya et al., 2018). However, so as to understand the genesis and formation mechanism of the REE-rich mud, an accurate and precise depositional age estimation is essential. Accurate and precise determination on a depositional age of pelagic clay is often challenging due to the lack of age-diagnostic calcareous or siliceous microfossils. The marine Os isotope composition is one of the useful stratigraphic indicators owing to its high sensitivity to global environmental changes. Moreover, Os concentration and isotope composition can be used to track extraterrestrial impact events within characteristic sedimentary layers of the piston core samples. The clay layer enriched in the Os concentration and a conspicuous negative Os isotope anomaly (¹⁸⁷Os/¹⁸⁸Os as low as ~0.2), indicating an influx of extraterrestrial material, was discovered from the sediment piston core MR14-E02 PC11 (hereafter, PC11) by Nozaki et al. (2019). Here, we report the second discovery of the ejecta layer from the sediment core KR13-02 PC04 (hereafter, PC04) in the southern part of the Minamitorishima EEZ.
We used the sediment piston core PC04 (21°56.11’N, 152°39.51’E) obtained from a water depth of 5720 mbsl and compared with core PC11 previously reported from 5647 mbsl, ca 200 km northeast of PC04. Both cores contain layers with distinctly high Os concentration with negative excursion of the ¹⁸⁷Os/¹⁸⁸Os ratio. The PC04 samples contain 31.9–613.1 ppt Os, which exceeds that of the average upper continental crust (31 ppt) up to 19.8 times. The ¹⁸⁷Os/¹⁸⁸Os ratios varied from 0.213 to 0.905. The highest Os concentration (613.1 ppt) was recorded at 221.5 cmbsf, with additional positive peaks observed at 449.5 cmbsf (193.2 ppt) and 829 cmbsf (204.1 ppt), corresponding negative ¹⁸⁷Os/¹⁸⁸Os excursions of 0.213, 0.315, and 0.212, respectively.
In the previous study, an ejecta layer in the core PC11 was identified and dated as ~11 Ma (middle Miocene; Nozaki et al., 2019). To estimate the deposition age of the ejecta layer in PC04, we applied different fitting methods, linear extrapolation and L-BFGS-B (limited-memory Broyden-Fletcher-Goldfarb-Shanno algorithm with box constraints) algorithm (Byrd et al., 1995) using the ¹⁸⁷Os/¹⁸⁸Os ratio profiles. The L-BFGS-B algorithm provided a better fit with a smaller error, suggesting that a depositional age of the layer showing negative ¹⁸⁷Os/¹⁸⁸Os excursions is ca. 12 Ma, consistent with the previously discovered ejecta layer from PC11. The presence of abundant spherules with dendritic spinel due to quenching from the ejecta layer of PC04 also supports our interpretation. Our findings confirm the occurrence of the second ejecta layer in core PC04, providing additional geochemical evidence of an extraterrestrial impact event during the Miocene epoch.

References
[1] Kato et al. (2011) Nat. Geosci., 4, 535-539.
[2] Takaya et al. (2018) Sci. Rep., 8, 5763.
[3] Nozaki et al. (2019) Sci. Rep., 9, 16111.
[4] Byrd et al. (1995) SIAM J. Sci. Comput., 16, 1190-1208.