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Discovery of the Miocene impact ejecta layer from deep-sea sediments―Clue to unraveling the cause of mass extinction event in 11.6 Ma
1. Key Points
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- We have discovered the Miocene impact ejecta layer from deep-sea sediments at Minamitorishima Island offshore, Northwest Pacific. Existence of the impact ejecta layer was confirmed by negative Os-isotope excursion, anomalous enrichments of platinum group elements (PGEs), and occurrences of numerous spherules having Ni-rich spinels.
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- Sedimentary age of the ejecta layer was dated to be ca. 11 million years ago (Ma). Since there has been reported no synchronous large craters on land, it is the most plausible that this is the second discovery of a deep-sea impact event in the world.
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- Depositional age of the ejecta layer overlaps with the most recent mass extinction event in ca. 11.6 Ma of the Miocene period within the error, and this impact event possibly caused the Miocene mass extinction event.
2. Overview
A research group led by Dr. Tatsuo Nozaki, Scientist of the Submarine Resources Research Center, Research Institute for Marine Resources Utilization, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), conducted detailed petrographic observation and chemical analyses on piston core samples (*1) (Figs. 1 and 2) collected from the Minamitorishima Island offshore in October 2014 using the oceanographic research vessel MIRAI in collaboration with the University of Tokyo, Kobe University, Chiba Institute of Technology, Kyushu University, Tokyo Institute of Technology, and Waseda University, leading the discovery of an ejecta layer (*2) from the Miocene impact event.
This ejecta layer exhibits a conspicuous negative Os-isotope (187Os/188Os) (*3) excursion of ~0.2 (Figs. 3 and 4). Maximum Os and Ir concentrations are ca. 2.2 and 3.2 ppb, respectively, and several tens of times higher than those of average upper continental crust (UCC), indicating the anomalous enrichment of platinum group elements (PGEs) (*4) (Figs. 3 and 4). In addition, the piston core samples contain numerous spherules (spherical particles). These spherules are composed of clay minerals having olivine pseudomorphs (*5) and spinel (*6) grains with the highest NiO concentration of 23.3% (Figs. 5 and 6). These petrographic and geochemical lines of evidence can be explained well by impact event onto the Earth and its concomitant partial melting of meteorite and target projectile.
Sedimentary age of the discovered ejecta layer was yielded to be ca. 11 Ma by Os- isotope stratigraphy (*7) (Fig. 3). Since there has been reported no large crater structures on land within the errors of the sedimentary age, the ejecta layer was formed by new impact event which has not been recognized so far. Based on the no evidence of an impact structure at that time on land, it is the most plausible that this is the second deep-sea impact event in the world, after the Eltanin impact event of ca. 2.51 Ma in the Bellingshausen Sea. Moreover, the cause of the most recent mass extinction event (*8) in ca. 11.6 Ma has long been enigmatic, but this discovery provides a clue to its cause. We plan to investigate deep-sea sediments in other marine areas to unravel details of the newly discovered Miocene impact event.
The results will be published on 20th November 2019 in Scientific Reports issued by the UK Nature Publishing Group.
Title：A Miocene impact ejecta layer in the pelagic Pacific Ocean
Authors：Tatsuo Nozaki 1,2,3,4, Junichiro Ohta 4,2,5,6, Takaaki Noguchi 7, Honami Sato 4,1, Akira Ishikawa 8,1,4, Yutaro Takaya 9,1,4, Jun-Ichi Kimura 6, Qing Chang 6, Kazuhiko Shimada 10, Jun-ichiro Ishibashi 10, Kazutaka Yasukawa 2,5,4, Katsunori Kimoto 11, Koichi Iijima 1, Yasuhiro Kato 2,5,4,1
Affiliations：
1. Submarine Resources Research Center, Research Institute for Marine Resources Utilization, JAMSTEC
2. Frontier Research Center for Energy and Resources (FRCER), School of Engineering, The University of Tokyo
3. Department of Planetology, Kobe University
4. Ocean Resources Research Center for Next Generation, Chiba Institute of Technology
5. Department of Systems Innovation, School of Engineering, The University of Tokyo
6. Volcanoes and Earth’s Interior Research Center, Research Institute for Marine Geodynamics, JAMSTEC
7. Division for Experimental Natural Science, Faculty of Arts and Science, Kyushu University
8. Department of Earth and Planetary Sciences, Tokyo Institute of Technology
9. Department of Resources and Environmental Engineering, School of Creative Science and Engineering, Waseda University
10. Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University
11. Earth Surface System Research Center, Research Institute for Global Change, JAMSTEC
*1 Piston core: A sedimentary core sample collected by piston corer. When a piston corer falls freely and penetrates into a seafloor, piston part stays on a seafloor and long pillar-shaped deep-sea core sample can be collected by negative pressure produced by piston part. Piston core samples used in this study were collected using a 15 m-length pipe.
*2 Ejecta layer: At the end of the meteorite impact event, materials released from the crater structure with a low-angle is mixed with a sedimentary layer on the Earth’s surface. This mixed sedimentary layer formed by the impact event is called as an ejecta layer.
*3 Os isotope ratio (187Os/188Os）: There are seven different mass numbers for Os of 184, 186, 187, 188, 189, 190, and 192. Proportion of these two Os isotopes is an Os isotope ratio. Since 187Os/188Os is well known that this value is one order of magnitude different among reservoirs, 187Os/188Os is often used to detect extraterrestrial materials and size estimation of meteorite impactor.
*4 Platinum group element (PGE); A PGE is a generic term for six elements of Ru, Rh, Pd, Os, Ir, and Pt. As PGEs are enriched in the Earth’s interior and primitive material such as a meteorite, PGEs geochemistry is effective to detect the extraterrestrial materials.
*5 Pseudomorph: A pseudomorph is a mineral or mineral compound formed by chemical or structures changes. While the appearance or crystal form is retained, the original mineral is removed and/or replaced by another one.
*6 Spinel: A spinel is used for both a mineral name and a group name of minerals having a wide chemical composition due to the solid solution. In a broad sense as a group name of minerals, spinel has a chemical composition of AB2O4 and is classified to be a spinel group (B is Al and A is Mg, Fe, Zn, and Mn), magnetite group (B is Fe3+ and A is Mg, Fe2+, Zn, Mn, and Ni), and chromite group (B is Cr and A is Mg or Fe2+). A spinel group in a narrow sense is also called as an Al spinel and is composed of spinel (MgAl2O4), hercynite (FeAl2O4), Zn spinel (ZnAl2O4), and Mn spinel (MnAl2O4).
*7 Os-isotope (187Os/188Os) stratigraphy: Marine 187Os/188Os is controlled by relative balances in three fluxes of radiogenic riverine water, unradiogenic hydrothermal fluid and extraterrestrial material into the ocean, which are changed as with the time along with the global environmental changes. Since REY-rich mud and Fe-Mn crust preserves a marine 187Os/188Os at that time, a sedimentary age of the piston core sample can be determined by fitting its 187Os/188Os with the seawater evolution curve. This dating method is called as an Os-isotope stratigraphy.
*8 From the 300 Ma to present, it is known that at least eleven mass extinction events occurred. Caused of these mass extinctions events are considered to be as follows;
| Approximate age | Cause of mass extinction |
| 11.6 Ma | - |
| 36 Ma | Meteorite impact | 66 Ma | Meteorite impact, LIPs eruption, and OAE? |
| 94.2 Ma | LIPs eruption and OAE |
| 116 Ma | LIPs eruption and OAE |
| 145 Ma | Meteorite impact |
| 182.7 Ma | LIPs eruption and OAE |
| 201.3 Ma | LIPs eruption and OAE |
| 215 Ma | Meteorite impact |
| 252.2 Ma | LIPs eruption and OAE |
| 259.8 Ma | LIPs eruption and OAE |
LIPs: Large igneous provinces, OAE: Ocean anoxic event
Figure 1 Location of the piston core sample (MR14-E02 PC11) used in this study.
Figure 2 Half-sectional image of the piston core sample used in this study. The ejecta layer is contained in the center part of Section 4.
Figure 3 (a) Re-Os concentrations, isotope ratios, and (b) Os-isotope age of the piston core sample. An anomalous enrichment of Os and negative Os-isotope excursion were detected 320 - 360 cm below seafloor (cmbsf). The Os-isotope stratigraphy yielded a sedimentary age of the ejecta layer to be ca. 11 Ma.
Figure 4 (a) Re-Os geochemistry, PGE concentrations, and (b) CI chondrite-normalized PGE patterns of the piston core sample. Peaks of anomalous Os enrichment and negative Os-isotope excursion are concurrent with PGE enrichment. The PGE patterns show platinum subgroup of PGE (PPGE)-enriched patterns and iridium subgroup of PGE (IPGE) are more enriched than PPGE compared with the average UCC.
Figure 5 (a) 3D micro X-ray computed tomographic image of the piston core sample with the highest Os concentration and most unradiogenic Os isotope ratio. Many spherules several tens to hundreds μm in size were observed; (b and c) Enlargement images of the largest spherule in Fig. 5 (a) with different color scales.
Figure 6 (a–c) Microscopic photographs under reflected light and (d–i) back-scattered electron images of a polished section made from sieved coarse-grained fractions (more than 62 µm) of piston core samples with anomalous Os enrichment and negative Os-isotope excursion. (a) The outer surface of the spherules is covered with pelagic sediment, but (a, d) the interior is dominated by hexagonal plate-shaped clay minerals showing olivine pseudomorphs. (a–i) A plenty of euhedral, dendritic, and spherical spinel grains occur in the clay minerals. (c, f–h) Dendritic spinel grains were observed to grow often from the edges of euhedral spinel grains. (i) Rare spherical spinel grains (white dotted circles) were observed, having the highest NiO concentration of 23.29 wt%.
Figure 7 (a–e) Scanning electron microscopic image of spinel grain in the spherule of the piston core sample, elemental mapping (Cr and Fe) by transmission electron microscope, and geochemical composition of spinel grains determined by electron probe microanalyzer and transmission electron microscope-energy dispersive X-ray spectrometer. (a) The spinel grains are composed of a Cr-rich core and Fe-rich rim parts, and the growth of the magnetite-rich dendritic spinel grains was observed from the rim part. (b–e) The geochemical composition of spinel grains in the spherules is clearly distinguished from that of the Eltanin meteorite and is similar to that of spinel contained in carbonaceous or ordinary chondrites.
Contacts:
- (For this study)
- Tatsuo Nozaki, Scientist, Submarine Resources Research Center, Research Institute for Marine Resources Utilization, JAMSTEC
- (For press release)
- Public Relations Section, Marine Science and Technology Strategy Department , JAMSTEC