Onoue et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-019-0267-0(2019) 6:18

RESEARCH ARTICLE

Progress in Earth andPlanetary Science

Open Access

Enhanced flux of extraterrestrial 3He acrossthe Permian–Triassic boundary
Tetsuji Onoue1*, Naoto Takahata2, Mitsutaka Miura1, Honami Sato3, Akira Ishikawa4, Katsuhito Soda1, Yuji Sano2 andYukio Isozaki5

Abstract
The ca 252 Ma Permian–Triassic boundary (PTB) represents the most severe mass extinction event of the Phanerozoic,
with the disappearance of ~ 80% of marine invertebrate species. Large-scale eruption of the Siberian Traps is thecommonly favored cause, although the link between volcanism and extinction remains debated. Here, we report
evidence for an extraterrestrial 3He influx in the PTB section of deep-sea bedded cherts from Japan. This unusual signal
indicates a significant increase in the influx of interplanetary dust particles, likely related to an asteroid shower in the
inner solar system. High-resolution stratigraphy indicates that the peak flux of dust particles occurred during the final
500 kyr of the Permian, concurrent with a pre-extinction decline in radiolarian diversity.
Keywords: Permian, Triassic, Bedded chert, Mino Belt, Helium, Platinum group element, Interplanetary dust particle,
Radiolaria

Introduction
The Permian–Triassic boundary (PTB) mass extinction
event (MEE) (Erwin 1994; Wignall 2007) was associatedwith catastrophic environmental changes including
oceanic anoxia, global carbon cycle perturbations, methane hydrate release, and global warming (Isozaki 1997;
Wignall 2007; Burgess et al. 2014). The release of volcanic and contact metamorphic carbon and sulfur gases(CO2, CH4, and SO2) from the Siberian Traps is commonly invoked as the trigger for climatic perturbations
across the PTB (Wignall 2007; Burgess et al. 2017),
which led to the end-Permian MEE. Although extraterrestrial causes have been proposed (e.g., bolide impact,
Becker et al. 2001, 2004; Basu et al. 2003), they have notbeen favored (Koeberl et al. 2004; Farley et al. 2005),
largely due to a lack of geochemical and mineralogicalevidence such as platinum group element (PGE) anomalies and shocked quartz. The accretion of extraterrestrial noble gases such as He and Ar was discussed byBecker et al. (2001), who reported high concentrationsof extraterrestrial He (3HeET) in PTB rocks from Japanand China, with a suggestion that 3HeET is trapped in* Correspondence: t_onoue@kumamoto-u.ac.jp
1
Department of Earth and Environmental Sciences, Kumamoto University,
Kumamoto 860-8555, Japan
Full list of author information is available at the end of the article

fullerenes associated with an impact event (Becker et al.
2001). However, the reported 3HeET and fullerene concentrations have not been reproduced in independent
analyses of the same samples and others from PTB sections in China, Canada, and Austria (Koeberl et al. 2004;
Farley et al. 2005).
The solar system contains abundant sub-millimeterinterplanetary dust particles (IDPs) enriched in 3He, and3
He concentrations in ancient deep-sea sediments havebeen used to constrain the IDP flux to Earth over at
least the last 100 Myr (Farley 1995; Farley et al. 2012).
However, the use of 3He in detecting IDPs is often compromised by the diffusional loss of 3He from sedimentary rocks (Farley et al. 2005; Mukhopadhyay and Farley2006), with the exception of some Ordovician samples
that record a period of unusually high extraterrestrial
3
He flux (Patterson et al. 1998).
This is the first report of preservation of 3HeET in Permian bedded chert and overlying Triassic black claystone from Waidani (Sano et al. 2010, 2012a), Japan,
which was deposited in the Panthalassa superocean(Isozaki 1997) and subsequently accreted to the Japan
margin during the Jurassic. Biostratigraphic and chemostratigraphic data indicate that this PTB section preserves
a complete pelagic PTB transition, without a temporalgap at lithological contacts between bedded chert and© The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
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(2019) 6:18

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black claystone (Sano et al. 2010, 2012a; Agematsu et al.
2014). The 3HeET flux across the PTB was calculated on
the basis of 3HeET concentrations and sedimentary massaccumulation rates estimated from the astronomicallytuned cyclostratigraphy of the bedded chert. The 3HeET
record is supported by PGE concentrations measured inbulk rock samples from the section, and their major andtrace element compositions.

unmelted micrometeorites, and cosmic spinels (Onoue etal. 2011, 2012; Uno et al. 2012).
The studied section crops out along a road cuttingnear Waidani in the Mt. Funabuseyama area of the
Mino Belt, central Japan (Additional file 2: Figure S2).
The examined rocks correspond to the Permian–
Lower Triassic Hashikadani Formation, which is characterized by pelagic, deep-sea siliceous sediments(Sano et al. 2012a). The section was referred to as
NF1212R and NF1212C in previous studies (Sano etal. 2010, 2012a), but the term “Waidani section” isused here.
The Waidani section comprises a 3.5 m thick continuous succession of gray and dark gray bedded chert overlain by black claystone, with a pyrite-rich brown
claystone layer at its base (Fig. 1). The bedded chert
comprises rhythmically alternating beds of chert andrelatively thin beds of claystone. Based onhigh-resolution radiolarian and conodont biostratigraphy(Sano et al. 2010; Agematsu et al. 2014), the bedded
chert in the section is assigned to the Upper Permian(Changhsingian), and the overlying black claystone tothe lowermost Triassic (Griesbachian). A negative excursion in organic carbon isotopic ratios (δ13Corg) occurs at
the top of the dark gray bedded chert (Sano et al.
2012a), coinciding with radiolarian fossil disappearances
correlated with the MEE horizon in other PTB sections(Feng and Algeo 2014). The Triassic base of the section
is defined by the presence of natural assemblages of theconodont Hindeodus (H. parvus and H. typicalis) in theOnoue et al. Progress in Earth and Planetary Science

Methods/Experimental
Sample collection and preparation

Samples for whole-rock He isotopic analyses were collected from the siliceous PTB section of the Mino Belt,
Japan, which is a Jurassic subduction-generated accretionary complex (Additional file 1: Figure S1). PTB siliceous rocks of the Mino Belt are considered to belong-lived Panthalassa remnants representing an allochthonous proto-Pacific Plate assemblage (Matsuda andIsozaki 1991; Isozaki 1997). The bedded cherts lack carbonate and coarse terrigenous grains, suggesting that
their primary depositional site was deeper than the calcium
carbonate compensation depth and beyond the depositional
range of terrigenous clastic grains (Matsuda and Isozaki
1991; Isozaki 1997). PTB siliceous rocks in Japan are characterized by a very low sedimentation rate (less than a few
millimeters per kyr), an absence of coarse-grained terrigenous clastics, and fine-grained sediment compositions(mainly cryptocrystalline quartz). They contain small extraterrestrial particles, including well-preserved melted andFig. 1 Lithology and geochemical profiles of the study section. Stratigraphic column showing samples, lithologies, total organic carbon contents(TOC, wt.%), and organic carbon isotopic values (δ13Corg, VPDB‰, divided into carbon isotope zones, CIZ). Modified after Sano et al. (2012a)

(2019) 6:18

Page 3 of 12basal 15 cm thick stratigraphic interval of black claystone (Agematsu et al. 2014).
To avoid variations in chemistry related to lithologicalchanges (Farley et al. 2005), all samples for whole-rockgeochemical analysis were collected from claystone beds:
13 Permian claystones and 1 brown and 5 dark gray Triassic claystones. Veins and strongly recrystallized and/or
weathered parts of the claystone beds were avoided tominimize effects of diagenetic overprinting on sedimentgeochemistry. Samples contain brown conodont elements (Agematsu et al. 2014), with a conodont color alteration index (CAI) of 2, indicating that the rocks were
exposed to burial temperatures of 60–140 °C.

a Triassic bedded chert unit from the study area identified IDPs preserved with silicate phases (olivine and pyroxene, Onoue et al. 2011). If these minerals act as
carrier phases for extraterrestrial 3He, most 3He would
be lost during acid leaching (Mukhopadhyay and Farley2006). Four bulk samples were selected that yielded high(NF-37B, 41B) and low (NF-25B, 51I) 3He concentrations. The samples were treated with a 12M HF–3M
HCl mixture for 48 h at room temperature, rinsed fourtimes with ultrapure water, and dried. He was extracted
at temperatures of 750 °C, 950 °C, and 1800 °C during
heating in a furnace.

Onoue et al. Progress in Earth and Planetary Science

Estimation of extraterrestrial 3He concentrations
He isotopic analysis

Prior to He isotopic analysis, the samples were crushed
and washed with ultrapure water; after drying, the fragments were carefully hand-picked to avoid contamination by altered and weathered material, andhand-picked fragments (< 5 mm grain size, 10–20 g)
were pulverized in an agate mortar.
Both bulk and stepped heating methods were used todetermine He concentrations and 3He/4He ratios. The
bulk method was applied to 19 samples and involved gas
extraction by melting samples at 1800 °C. The stepped
heating method (Amari and Ozima 1985; Farley et al.
2005; Mukhopadhyay and Farley 2006) was used to identify the 3He carrier in the most 3He-rich sample(NF-37B): heating began at 550 °C increasing in 100 °C
steps, with 30–50 min per step.
Cleaned and homogenized powder samples (0.25 g forthe bulk method, ~ 1 g for the stepped method) werewrapped in Al foil and placed in the loading port. Samples were preheated under vacuum at 110 °C for 11 h todrive off adsorbed gases. Samples were then fused undervacuum at a maximum temperature of 1800 °C to extract
trapped gases, which were purified using a charcoal trap
at liquid nitrogen temperature and with a hot titanium
getter. After purification, 4He and 20Ne abundances and4
He/20Ne ratios were determined using a quadrupole
mass spectrometer, and He concentrations and 3He/4He
ratios using a noble gas mass spectrometer (Helix-SFT;GV Instruments) at the Atmosphere and Ocean Research Institute, University of Tokyo, Tokyo, Japan. Meteorite samples had not previously been analyzed by thismeasurement system. Air was used as a standard fornoble gas isotopic analyses (Sano et al. 2013). He blanks(3 × 10−15 cm3 STP for 3He; 3 × 10−9 cm3 STP for 4He)
were < 20% of the sample signal.
Acid leaching procedure

Most IDPs found on Earth’s surface comprise aggregatesof small grains of olivine, pyroxene, Fe–Ni sulfides, andmagnetite (Genge et al. 2008). Previous investigations of

3

HeET concentrations in sediments were calculated using
the following equation (Marcantonio et al. 1995):
3
HeET ¼ 1  Rterrigenous =Rmeasured = 1  Rterrigenous =RIDP3 Hemeasured
ð1Þ
where Rterrigenous and RIDP are 3He/4He ratios for terrigenous (3He/4He = 0.015 Ra, Sano 2017) and extraterrestrial (3He/4He = 170 Ra, McGee and Mukhopadhyay2013) end-members, respectively (Ra = atmospheric3
He/4He ratio = 1.382 × 10−6, Sano 2017). The lack ofvolcanic material in the analyzed samples precludes thepresence of trapped mantle He. The lowest 3He/4He ratio (0.2 Ra) was measured in Triassic beds, with the fraction of extraterrestrial 3He exceeding 90%.
Platinum group element analysis

Platinum group elements (Ir, Ru, Pt, and Pd) and Re
were analyzed by isotope dilution mass spectrometry(ID–MS) after quartz tube digestion. Sample digestion,
purification, and mass spectrometry were based on procedures described by Ishikawa et al. (2014). Powderedsamples (~ 0.1 g) and spike solutions containing 185Re
and 191Ir–99Ru–196Pt–105Pd were transferred to a quartz
tube. After the addition of 2 mL of inverse aqua regia,
the tubes were frozen in dry ice–ethanol, sealed with an
oxypropane torch, and placed in an oven at 240 °C for
72 h. HF (1 mL, 38%) was added to the residue for desilicification (Ishikawa et al. 2014).
Platinum group element isotopic compositions of purified sample solutions were determined by inductively
coupled plasma mass spectrometry (ICP–MS, ThermoElement XR) at the University of Tokyo, Komaba, Japan.
Sample and standard solutions were interspersed
throughout analytical sessions to monitor and correctfor instrumental fractionation; 89Y, 90Zr, 95Mo, 97Mo,
99
Ru, 100Ru, 101Ru, 105Pd, 106Pd, 108Pd, 111Cd 178Hf,
185
Re, 187Re, 191Ir, 193Ir, 194Pt, 195Pt, 196Pt, and 202Hg
were monitored for possible mass interference. Raw

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signal intensities were corrected using measured isobaric–oxide interferences, although contributions of interferences to sample signals were insignificant (< 0.1%).
Elemental concentrations were corrected using average
total procedural blanks obtained from each batch of analyses (0.7 pg Ir, 2.3 pg Ru, 13 pg Pt, 10 pg Pd, and 0.3 pg
Re; n = 2). Uncertainties for each sample were estimatedby error propagation through ICP–MS measurements(2SE) and blank corrections (< 51%). Blank contributionsof Ir, Pt, Pd, and Re were < 12%, < 6%, < 9%, and < 7%,
respectively, while the contribution to Ru was slightly
higher (5–26%, < 41% for sample NF-21B).

bedded chert of the Hashikadani Formation is estimated to be ~ 0.3 cm kyr−1.

Onoue et al. Progress in Earth and Planetary Science

Major and trace element analyses

Major element contents of 21 samples were determinedby X-ray fluorescence spectrometry (XRF, PANalytical
Epsilon 3XLE with a Mo X-ray tube) on pressed powder
pellets, at Kumamoto University, Kumamoto, Japan.
Standard rock samples issued by the Geological Surveyof Japan were used for calibration purposes. Reproducibility, based on four repeat analyses of standard JSd-1,
was better than ± 0.5% for Mg, Al, Si, K, Ca, Ti, and Fe
and better than ± 1% for Na and Mn.
Nineteen samples were analyzed for trace and rareearth elements (REE) by ICP–MS after lithium metaborate/tetraborate fusion at Actlabs, Ancaster, Canada. Teninternational rock and mineral standards were analyzed
for quality control purposes during trace element and
REE analyses. Analytical procedures are described in
http://www.actlabs.com.
Estimation of sedimentation rate for Changhsingian
bedded chert

The average sedimentation rate for Changhsingian bedded chert of the Hashikadani Formation was estimatedfor a PTB section at Iwaidani (NF195 section; Sano et al.
2012b), cropping out ~ 3 km from the Waidani section(Additional file 2: Figure S2). Lithologically, the NF195
section is very similar to the Waidani section (Sano et al.
2010, 2012b), comprising a lower unit (~ 6.2 m thick) ofgray bedded chert, a middle unit (~ 0.7 m thick) of black
bedded chert including pyrite nodules near the top, andan upper unit (~ 0.9 m thick) characterized by black
claystone with thin black chert beds. Biostratigraphic
analysis of radiolarians reveals that the section contains
the Wuchiapingian–Changhsingian boundary in thebasal ~ 0.3 m of the lower unit and an MEE horizon at
the top of the middle unit. The stratigraphic range of
the Changhsingian is ~ 6.6 m, and its duration was
2.20 ± 0.16 Myr according to U–Pb ages for the MEE(251.941 ± 0.037 Ma, Burgess et al. 2014) and Wuchiapingian–Changhsingian boundary (254.14 ± 0.12 Ma,
Ramezani and Bowring 2017). From these data, theaverage sedimentation rate for the ChanghsingianTime-series analysis

The bedded chert in the studied section comprises
rhythmically alternating beds of chert (1–7 cm thick)and relatively thin beds of claystone (1–2 cm thick). Individual chert and claystone thicknesses were measured
throughout the section, with “silica ranks” of 0 for claystone and 1 for chert being assigned. Cyclostratigraphic
analyses of the Triassic–Jurassic bedded chert sequence
were based on bed thicknesses in the Mino Belt (Ikeda
and Tada 2014). However, Algeo et al. (2011) suggestthat other methods should be used where analyzed intervals are too short for estimating sedimentation rates or
to capture astronomical cycles such as the 405 kyr eccentricity cycles (Laskar et al. 2004). Sedimentation rates
for the Upper Permian bedded chert sequence were
therefore estimated using orbital tuning procedures involving long precession cycles (Yao et al. 2015). Periodicities and modulations in time–frequency space were
estimated using REDFIT software (Schulz and Mudelsee
2002) and continuous wavelet analyses with Morlet
mother signals (Torrence and Compo 1998). Bandpass
filters with Gaussian windows (Paillard et al. 1996) wereapplied to extract astronomical signals from binary
series.

Results
He analyses

Analyzed samples yielded bulk 3He and 4He concentrations of 0.05–0.34 × 10−12 cm3 STP g−1 and 0.09–0.47 ×
10−6 cm3 STP g−1, respectively (Table 1). 3He concentrations are highest in the topmost ~ 1.5 m of the studied
Permian deposits (Fig. 2a), with a maximum 3He concentration of 0.34 × 10−12 cm3 STP g−1, which is 4–5
times greater than that measured in the overlying Triassic unit.
The analyzed samples likely preserve primary fluctuations in 3He concentrations across the PTB because (1)the detected 4He/20Ne ratios are > 100 times greater than
the atmospheric value, thereby precluding the existence ofsignificant atmospheric He in the samples and (2) the lackof correlation (r2 = 0.3) between 3He and 4He concentrations suggests that the increase in 3He during the uppermost Permian cannot be explained by He retention alone.
3
He/4He ratios follow a similar trend to 3He concentrations, with the maximum 3He/4He ratio of 0.75 Ra being
measured in the uppermost Permian bed.
Stepped heating

To identify the source of the high 3He/4He signature, stepwise heating of the sample with the highest 3He concentration (NF-37B) was performed at 550 °C, 650 °C, 750 °C,

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Table 1 3He and 4He concentrations, and 3He/4He ratios for samples from the Waidani section
SampleTemperature (°C)
Height (m) MAR*(g cm− 2 kyr− 1)

3

He (pcc g− 1)

He (μcc g− 1)

4

3

He /4He (Ra)# min 4He/20Ne

3

3
HeET
HeET flux(pcc g− 1) (pcc cm− 2 kyr− 1)

Bulk
NF-62

0.88

1800

0.06

0.18

0.25 ± 0.04

40

0.06

NF-51 K 0.60

1800

0.05

0.17

0.22 ± 0.03

26

0.05

NF-51I

0.30

1800

0.05

0.20

0.19 ± 0.03

53

0.05

NF-51G 0.12

1800

0.08

0.32

0.17 ± 0.03

70

0.07

NF-51D 0.00

1800

0.12

0.47

0.18 ± 0.02

49

0.11

NF-51A −0.10

1800

0.08

0.21

0.27 ± 0.03

44

0.08

NF-48B −0.33

0.81 ± 0.18

1800

0.23

0.36

0.47 ± 0.02

37

0.23

0.18 ± 0.05

NF-45B −0.53

0.81 ± 0.18

1800

0.30

0.29

0.75 ± 0.02

82

0.30

0.24 ± 0.05

NF-41B −0.76

0.81 ± 0.18

1800

0.24

0.32

0.55 ± 0.02

211

0.23

0.19 ± 0.04

NF-37B −0.90

0.81 ± 0.18

1800

0.34

0.34

0.72 ± 0.02

37

0.34

0.27 ± 0.06

NF-32B −1.11

0.81 ± 0.18

1800

0.18

0.27

0.47 ± 0.02

49

0.17

0.14 ± 0.03

NF-29B −1.25

0.81 ± 0.18

1800

0.13

0.15

0.59 ± 0.02

37

0.12

0.10 ± 0.02

NF-25B −1.42

0.81 ± 0.18

1800

0.13

0.22

0.43 ± 0.02

58

0.13

0.10 ± 0.02

NF-24B −1.50

0.81 ± 0.18

1800

0.14

0.24

0.42 ± 0.02

60

0.13

0.11 ± 0.02

NF-23B −1.56

0.81 ± 0.18

1800

0.09

0.14

0.45 ± 0.03

36

0.08

0.07 ± 0.02

NF-21B −1.63

0.81 ± 0.18

1800

0.07

0.09

0.53 ± 0.03

22

0.06

0.05 ± 0.01

NF-17B −1.82

0.81 ± 0.18

1800

0.10

0.19

0.35 ± 0.03

34

0.09

0.07 ± 0.02

NF-12B −2.01

0.81 ± 0.18

1800

0.18

0.27

0.48 ± 0.02

38

0.17

0.14 ± 0.03

NF-11B −2.08

0.81 ± 0.18

1800

0.09

0.21

0.32 ± 0.03

13

0.09

0.07 ± 0.02

0.01

0.01
1.00

0.32

blank
air

Stepwise heating
NF-37B −0.90

550

0.075

0.232

0.23 ± 0.02

194

NF-37B −0.90

650

0.071

0.054

0.95 ± 0.02

48

NF-37B −0.90

750

0.067

0.009

5.22 ± 0.09

13

NF-37B −0.90

850

0.071

0.004

13.5 ± 0.2

6

NF-37B −0.90

950

0.019

0.001

13.9 ± 0.5

5

NF-37B −0.90

1800

0.006

0.001

3.80 ± 0.22

2

NF-37B −0.90

total

0.31

0.30

0.73

NF-51I

0.60

750–950

0.002

0.001

1.22 ± 0.07

143

NF-41B −0.76

750–9500.014< 0.001

107.1 ± 1.9

33

NF-37B −0.90

750–9500.007< 0.001

149.7 ± 8.8

10

NF-37B −0.90

950–1800

0.003< 0.001

16.6 ± 0.7

34

NF-25B −1.42

750–950< 0.001< 0.001

2.98 ± 0.27

45

12 M HF–3 M HCl dissolution*Based on the sedimentation rate (mean ± 2SD) derived from an astronomically tuned cyclostratigraphy age model for the Permian section#
Error assigned to the isotopic ratio is 1σ

850 °C, 950 °C, and 1800 °C (Fig. 3), resulting in extractionof He from different mineral phases, which was analyzed
for 3He/4He ratios at each step. Most 4He was extracted at
the lowest step (550 °C), while ~ 50% of 3He was released

at intermediate temperatures (750 °C and 850 °C). Thisdecoupled behavior indicates that 3He and 4He are hosted
in separate phases within the sample. Previous studies indicate that 3He hosted in IDPs is released at temperatures

Onoue et al. Progress in Earth and Planetary Sciencea(2019) 6:18

bPage 6 of 12cdFig. 2 Stratigraphic He and Ir profiles. Stratigraphic profiles of a 3He concentrations, b 3He/4He ratios, c 3He/4He ratios of acid-insoluble
residues produced during stepwise heating at temperatures of 750–950 °C, and d excess Ir in bedded cherts of the Waidani section. Red
and yellow symbols in the 3He profile represent the extraterrestrial fraction and measured 3He concentrations, respectively. The base of
the Triassic and MEE horizons in this section are defined by the occurrence of the conodont Hindeodus parvus and disappearance ofPermian radiolarian fossils, respectivelyof > 800 °C (Farley et al. 2005). Helium extracted at 850 °C
from sample NF-37B is similar to that in IDPs in terms of3
He release temperature and high 3He/4He ratios (14 Ra),
whereas the component released at 550 °C has a much
lower 3He/4He ratio (0.2 Ra). The temperature range overabFig. 3 Results of the stepwise heating experiments. 3He (a) and 4He (b)release profiles as a function of temperature for sample NF-37B. 3He/4He
ratios for NF-37B are shown in each panel for comparison. Helium
extracted at 850 °C yielded a high 3He/4He ratio (14 Ra), suggesting releasefrom IDPs. The stratigraphic level of sample NF-37B is shown in Fig. 2

which 3He was released (550–850 °C) is equivalent to that
determined for the decomposition of IDPs (Farley et al.
2005). Our results therefore indicate that the unusuallyhigh 3He concentration in the studied PTB section is ofextraterrestrial origin and is likely hosted in IDPs.
An acid leaching procedure (Mukhopadhyay and Farley 2006) was applied to the PTB samples to confirm theexistence of extraterrestrial He. High 3He/4He ratios (upto 150 Ra) were detected during the extraction of He at
750–950 °C from acid-insoluble residues produced fromsamples with high 3He concentrations (NF-37B, 41B).
Low-3He samples (NF-25B, 51I) have low 3He/4He ratios(< 3 Ra, Fig. 2c), with > 96% of 3He being removed during leaching. We therefore interpret the high 3He/4He
ratios as recording the existence of extraterrestrial He,
hosted mainly in IDPs.
HF–HCl-resistant refractory phases found in meteorites include spinel, corundum, diamond, graphite, SiC,
amorphous carbon, and organic material (Patterson etal. 1998). Previous studies excluded such phases (except
for spinel) as possible long-term carriers of 3HeET(Mukhopadhyay and Farley 2006). A Middle Ordovician(~ 470 Ma) shallow marine limestone in Sweden, which
records clear evidence of an asteroid breakup or meteorite shower event, contains 3HeET hosted primarily inchromite (Heck et al. 2008). For our samples, we consider that extraterrestrial spinel and/or relict silicate
grains may host 3He in acid-insoluble residues.
Stratigraphic distribution of extraterrestrial material

The extraterrestrial fraction of 3He across the PTB (Eq.
(1)) indicates that bulk 3He concentrations in the PTB
section are highly sensitive to extraterrestrial input.

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Furthermore, 3HeET concentrations in the topmost ~ 1.5
m of the uppermost Permian section are at least fourtimes higher those in the pre-event interval. Platinumgroup elements such as Ir and Pt are often used as
tracers of extraterrestrial material. As with concentrations of 3HeET, an increase in Pt, Pd, Ru, and Ir concentrations (Table 2) occurs in the topmost ~ 1.5 m of theuppermost Permian section (Fig. 4). Iridium is the least
mobile PGE (Evans et al. 1993), and excess Ir concentrations (IrXS) were calculated using the equation:
IrXS ¼ Irmeasured  Almeasured  Irterrigenous =Alterrigenous

throughout the Upper Permian (Changhsingian) (Isozaki
1997; Sano et al. 2012a; Takahashi et al. 2014) and anoxic-to-anoxic transition was not detected within thestudied interval (Sano et al. 2012a) (Additional file 3:
Figure S3). Indeed, there is no strong correlation between Ir and redox-sensitive elements such as Fe, Mn, V,
Ni, Co, Mo, or U (Additional file 4: Figure S4).

Onoue et al. Progress in Earth and Planetary Science

ð2Þ
where “terrigenous” indicates the average value for theupper continental crust (McLennan 2001). This calculation subtracts the terrigenous Ir (supplied primarily byaeolian dust) from the total Ir content of PTB sediments,
allowing the extraterrestrial contribution to be determined. The similar durations and magnitudes (4–6
times) of increases in IrXS and 3HeET (Fig. 2), and thegood positive correlation (r2 = 0.7) between them (Fig. 5),
support our interpretation of enhanced extraterrestrial
3
He in the uppermost Permian and its change across the
PTB. Previous studies indicate that Fe–Mn oxyhydroxides contain significant amounts of Ir, which may beremobilized when they are reduced (Colodner et al.
1992). However, reducing conditions were prominent

Discussion
Our results provide no evidence of elevated 3HeET levels
at the PTB itself, consistent with previous work on other
PTB sections (Farley and Mukhopadhyay 2001; Farley etal. 2005). However, concentrations of 3HeET in the PTB
section here indicate an abrupt and significant increasein the topmost ~ 1.5 m of the uppermost Permian section. Although He isotopic compositions may be influenced by provenance composition (Winckler andFischer 2006), the uniform values of terrigenous elemental ratios (e.g., constant Mg/Al, Ti/Al, and Zr/Hf ratios; Additional file 5: Table S1 and Additional file 6:
Table S2) indicate no effects on the stratigraphic increase of 3He by changes of provenance compositionof the Permian and Triassic deposits. The contribution of cosmogenic 3He can also be ignored because
the studied rocks were excavated some years previously from a shielded location (2–3 m underground)
during road cutting.

Table 2 PGE concentrations in claystone samples from the Waidani section
SampleHeight (m)Ir (ppb ± 2SE)

Ru (ppb ± 2SE)

Pt (ppb ± 2SE)

Pd (ppb ± 2SE)

Re (ppb ± 2SE)

NF-62

0.88

0.085 ± 0.021

0.117 ± 0.007

2.92 ± 0.14

3.09 ± 0.17

12.5 ± 0.3

NF-51 K

0.60

0.062 ± 0.023

0.114 ± 0.009

3.97 ± 0.19

3.07 ± 0.20

44.9 ± 1.7

NF-51I

0.30

0.052 ± 0.022

0.089 ± 0.007

2.64 ± 0.12

2.05 ± 0.11

18.5 ± 0.4

NF-51G

0.12

0.114 ± 0.021

0.094 ± 0.008

3.15 ± 0.13

1.85 ± 0.35

10.7 ± 0.2

NF-51D

0.00

0.154 ± 0.022

0.151 ± 0.009

2.65 ± 0.13

1.13 ± 0.17

1.01 ± 0.02

NF-51A− 0.100.076 ± 0.024

0.123 ± 0.007

3.84 ± 0.18

1.72 ± 0.43

1.42 ± 0.03

NF-48B−0.33

0.339 ± 0.042

0.365 ± 0.009

11.3 ± 0.3

7.71 ± 0.28

0.366 ± 0.009

NF-45B−0.53

0.479 ± 0.031

0.180 ± 0.007

14.7 ± 0.4

6.28 ± 0.25

0.079 ± 0.006

NF-41B−0.76

0.484 ± 0.035

0.242 ± 0.007

15.4 ± 0.5

6.97 ± 0.32

0.179 ± 0.008

NF-37B−0.90

0.290 ± 0.033

0.155 ± 0.008

16.9 ± 0.4

7.16 ± 0.25

0.081 ± 0.005

NF-32B−1.11

0.186 ± 0.023

0.188 ± 0.008

19.8 ± 0.5

10.9 ± 0.5

0.106 ± 0.006

NF-29B− 1.250.082 ± 0.021

0.075 ± 0.007

9.04 ± 0.28

6.47 ± 0.23

0.074 ± 0.005

NF-25B− 1.420.074 ± 0.020

0.093 ± 0.006

11.4 ± 0.4

9.46 ± 0.37

0.072 ± 0.005

NF-24B−1.50

0.143 ± 0.021

0.159 ± 0.007

5.25 ± 0.18

6.41 ± 0.24

0.077 ± 0.006

NF-23B−1.56

0.083 ± 0.026

0.085 ± 0.007

4.23 ± 0.18

4.34 ± 0.21

0.098 ± 0.007

NF-21B−1.63

0.057 ± 0.024

0.032 ± 0.007

1.91 ± 0.11

1.84 ± 0.10

0.048 ± 0.005

NF-17B− 1.82

0.091 ± 0.020

0.064 ± 0.008

3.54 ± 0.15

2.35 ± 0.18

0.055 ± 0.005

NF-12B−2.01

0.126 ± 0.022

0.118 ± 0.006

4.61 ± 0.15

3.67 ± 0.14

0.045 ± 0.005

NF-11B−2.08

0.090 ± 0.019

0.093 ± 0.007

3.69 ± 0.13

3.43 ± 0.14

0.041 ± 0.006

Onoue et al. Progress in Earth and Planetary Science(2019) 6:18

Page 8 of 12

Fig. 4 Stratigraphic profiles of PGE contents of bedded cherts of the Waidani section. See Table 2 for data

There are two possibilities to explain this significant
increase in 3HeET concentrations: (1) 3HeET concentrations reflect an approximately fourfold decrease in sedimentation rates or (2) the change reflects a significant
increase in 3HeET influx. These possibilities were examined by establishing the floating astronomical timescaleof the Upper Permian bedded chert, based on astronomical tuning. The whole-section power spectrum exhibits
a dominant peak at 6.0 cm above the 99% confidence
level (Fig. 6a). Assuming an average sedimentation rate
for Changhsingian bedded chert of ~ 0.3 cm kyr−1, 6.0
cm cycles may represent ~ 20 kyr precession cycles at
the PTB. This is consistent with previous studies that
demonstrated rhythmic alternations of chert and claystone beds over precession cycles of ~ 20 kyr (Ikeda andTada 2014). A 20 kyr tuned astrochronology, anchoredat the MEE horizon (251.942 Ma, Burgess et al. 2014),
allows us to constrain the sedimentation rate for eachbed of the Permian section. Results indicate that sedimentation rates varied from 0.2 to 0.5 cm kyr−1 (Fig. 6b),with no significant decrease for bedded chert in the topmost ~ 1.5 m of the Permian deposits. Therefore, we
conclude that variations of 3He in the Permian section
record changes in the extraterrestrial 3He flux with timerather than sedimentation rate. Spectral analysis of theprecession-tuned time-series also reveals that the interval characterized by elevated 3HeET has a duration of ~500 kyr, with such a duration precluding large-impact
and meteorite debris origins.
The extraterrestrial 3He flux was calculated by multiplying the 3HeET concentration by the sedimentary massaccumulation rate (MAR, g cm−2 kyr−1):
Flux3 HeET ¼

Fig. 5 Bivariate plot of extraterrestrial 3He (3HeET) versus excess Ir in
samples from the Waidani section. Dashed line represents linear
regression between the two. See Table 1 for 3HeET and Table 2 forexcess Ir data

3HeET  MAR =R

ð3Þ

where R is the fractional retention of 3He after deposition on the sea floor (Farley 1995), ranging from 1 (total
retention) to 0 (total loss). Using an approximate density
for shale of 2.6 g cm−3, we estimate an MAR value for
the Permian section of 0.81 ± 0.18 g cm−2 kyr−1 (Table 1).
Ignoring possible diffusive loss (R = 1), we estimate amaximum 3HeET flux of 0.27 ± 0.06 pcc cm−2 kyr−1(1 pcc = 10−12 mL STP) before the MEE (Fig. 7). For a
scenario where the MAR remained stable after the MEE,
the 3HeET flux is tentatively estimated as 0.04–
0.06 pcc cm−2 kyr−1 in the Triassic section. These values
are consistent with those reported for Cenozoic andCretaceous units (Farley et al. 1998, 2012).
We propose two possible mechanisms to explain theunique IDP signal recorded in the latest Permian samples:
a cometary shower or collisions in the asteroid belt. Interplanetary dust particles are generally regarded as having

Onoue et al. Progress in Earth and Planetary Science(2019) 6:18

aPage 9 of 12bFig. 6 Cyclostratigraphy and floating astronomical time scale (ATS) of the Waidani section. a Silica rank and frequency–amplitude modulations of
the Upper Permian (Changhsingian) bedded chert sequence. The average sedimentation rate for the Changhsingian bedded chert is ~ 0.3
cm kyr−1, so the extracted 6 cm cycle was converted to a long precession cycle (~ 20 kyr) for orbital tuning (Yao et al. 2015). b Variations in
sedimentation rate for each bed, calculated from the Changhsingian ATS and frequency–amplitude modulations of orbital-tuned silica rank. Red,
orange, and blue lines in the REDFIT spectra delineate 99%, 95%, and 90% confidence levels (CL) against red-noise background from a first-order
autoregressive process, respectively (Schulz and Mudelsee 2002). P and SP at the spectral peaks represent precession and semi-precession (Berger
et al. 2006; Soda et al. 2015), respectively. Black contours and shaded regions in the wavelet spectra indicate 95% CL against the red noise andcone of influence due to zero padding, respectively (Torrence and Compo 1998)

originated from dust-producing objects such as comets
and asteroids (Kortenkamp and Dermott 1998). The particles migrate toward the Sun, driven by Poynting–Robertson and solar wind drag, and are continuously depositedon Earth. The Poynting–Robertson lifetime formicrometer-sized IDPs is of the order of 104–105 years(Burns et al. 1979), so most decay products from comets
and asteroids are rapidly lost from the solar system. Cometary showers or collisions in the asteroid belt (Farley etal. 1998, 2006) result in episodes of elevated IDP flux toEarth over a few million years or less. Our estimate of theduration of peak 3HeET flux immediately before the PTB is
similar in magnitude to those of two well-documented
Cenozoic 3HeET events: a major asteroid collision in thelate Miocene (Farley et al. 2006) and a dust shower generated from comets or asteroids in the late Eocene (Farley etal. 1998). These events were associated with longer periods (~ 1.5 Myr) of 3HeET flux decay than is observed
across the PTB (Farley et al. 2006, 2012). However, theobserved short duration of enhanced 3HeET flux (~ 500 kyr)is consistent with the dynamic lifetime (104–105 years) offine-grained dust liberated by a single asteroid disruption
event. The timing of collision of the Erigone family of asteroids in the inner main asteroid belt is constrained to220þ60−80 Ma, close to the PTB (Spoto et al. 2015). Our data
are presently insufficient to identify an individual asteroid
shower episode.
The present study indicates that an enhanced IDP flux
event occurred simultaneously with a marked decline in
the diversity of Permian radiolarians from ~ 500 kyr prior
to the PTB (Fig. 7). A similar substantial pre-MEE declineof radiolarians has been reported in the Upper Permian(late Changhsingian) in South China (Feng et al. 2007;
Feng and Algeo 2014). Furthermore, a recent statisticalanalysis of Permian ammonoids revealed that high turnover rates and extinction pulses occurred during the last
~ 700 kyr of the Permian (Kiessling et al. 2018). As a first
approximation, it seems reasonable to assume that theOnoue et al. Progress in Earth and Planetary Science(2019) 6:18

Page 10 of 12abFig. 7 Extraterrestrial 3He (3HeET) flux and radiolarian species diversity across the PTB. a 3HeET flux calculated from extraterrestrial 3He concentrations(Table 1), a mean dry bulk density of 2.6 g cm−3 for claystone, and mean sedimentation rates calculated using astronomically tuned cyclostratigraphy.
Error bars are based on two standard deviations calculated for the MAR of the Permian section. The 3HeET flux after the MEE horizon was tentatively
estimated assuming sedimentation rates remained stable after the MEE. See Table 1 for data. b Stratigraphic variation of the number of Permian
radiolarian species in the study section (Sano et al. 2012a). Marked decline in the diversity of Permian radiolarians began at ~ 500 kyr prior to the PTB.
Permian radiolarians in the Triassic section are regarded as reworked species (Sano et al. 2010)

pre-MEE declines of latest Permian radiolarians and ammonoids were related to an extraterrestrial event indicatedby the 3He record in the deep-sea PTB section of Japan.
Accretion and ablation of IDPs in Earth’s ionosphere may
affect the chemistry and aerosol distribution of the upper
atmosphere (Grebowsky et al. 2002), providing a possible
link between IDP accretion rate and global climate change(Muller and MacDonald 1997; Patterson and Farley 1998).
The present study provides the first clear evidence of an
extraterrestrial influence immediately prior to the PTB, anda re-evaluation of the cause of the pre-MEE decline is required to confirm the link between extraterrestrial material
flux and extinction-related global climate change.

Conclusions
Extraterrestrial 3He is preserved in deep-sea bedded chertfrom a continuous PTB section at Waidani, Japan, which
was deposited in the Panthalassa superocean. High3
He/4He ratios of up to 150 Ra were determined foracid-insoluble residues from the uppermost Permian(2019) 6:18

Page 11 of 12

deposits, indicating the occurrence of extraterrestrial He
hosted mainly in IDPs. The extraterrestrial 3He fraction
varies across the PTB with 3HeET concentrations being
higher in the topmost ~ 1.5 m of the studied Permian deposits (4–5 times higher than in the overlying Triassic unit).
The 3HeET flux to Earth across the PTB was estimated fromthe 3HeET concentration and sedimentary mass accumulation rate of the Waidani PTB section, with the flux increasing over the last 500 kyr of the Permian. This unusualsignal suggests a significant increase in the influx of IDPs,
likely related to an asteroid shower in the inner solar system. High-resolution stratigraphy indicates that peak IDP
flux occurred during the final 500 kyr of the Permian, concurrent with a pre-MEE decline in radiolarian diversity.

participated in the discussions to interpret the analytical results and aided in
the writing of the manuscript. All authors read and approved the final manuscript.

Onoue et al. Progress in Earth and Planetary Science

Additional files
Additional file 1: Figure S1. Map showing the location of the study area
in central Japan. The deep-sea PTB section discovered in a radiolarian chert
succession within the Mino Belt (Sano et al. 2012a) represents a Jurassic
subduction-generated accretionary complex. (PDF 228 kb)
Additional file 2: Figure S2. Geological map of the Mt. Funabuseyama
area. Map showing the location of the Waidani section (35°41′11″N, 136°
40′27″E). (PDF 2034 kb)
Additional file 3: Figure S3. Mo and U enrichment factors and theirratios (MoEF/UEF) for the studied section. Mo/U ratios are typicallymoderate (~ 3, similar to modern seawater) under weakly anoxic
conditions and high (~ 9, three times that of modern seawater) underhighly anoxic–euxinic bottom-water conditions (Takahashi et al. 2014;
Algeo and Tribovillard 2009). See Additional file 2: Table S2 for data.
(PDF 222 kb)
Additional file 4: Figure S4. Correlation coefficients for pairs of theelements/Al ratios determined in this study. Concentrations of theelements were normalized using Al concentrations to compensate theeffect of biogenic SiO2-dilution. Correlation coefficients (r) > 0.75 areindicated by colors according to the ranges of values. ∑REE represents
total REE concentrations. (PDF 56 kb)
Additional file 5: Table S1. Major element compositions of claystone
samples from the Waidani section. (XLSX 13 kb)
Additional file 6: Table S2. Trace element and REE compositions ofclaystone samples from the Waidani section. (XLSX 15 kb)
Abbreviations
CAI: Conodont alteration index; IDP: Interplanetary dust particle; MEE: Mass
extinction event; PGE: Platinum group element; PTB: Permian–Triassic
boundary
Acknowledgements
We thank S. Maruyama, J. Kirschvink, T. Nakamura, and D. Nakashima for
discussions. H. Sano assisted with fieldwork and sample collection, and D.
Yamashita assisted with sample processing.
Funding
This work was supported by the Japan Society for the Promotion of Science,
grant number 26106005 to TO and NT.
Availability of data and materials
Please contact the corresponding author regarding data requests.
Authors’ contributions
TO, NT, YS, and YI designed the study. TO, NT, MM, and YS performed thehelium isotopic analyses. HS and AI performed the PGE analyses. TO
performed the XRF analyses. KS performed the spectral analysis. All authors

Competing interests
The authors declare that they have no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims in publishedmaps and institutional affiliations.
Author details
1
Department of Earth and Environmental Sciences, Kumamoto University,
Kumamoto 860-8555, Japan. 2Atmosphere and Ocean Research Institute, The
University of Tokyo, Chiba 277-8564, Japan. 3Ocean Resources ResearchCenter for Next Generation, Chiba Institute of Technology, Chiba 275-0016,
Japan. 4Department of Earth and Planetary Sciences, Tokyo Institute ofTechnology, Tokyo 152-8550, Japan. 5Department of Earth Science andAstronomy, The University of Tokyo, Tokyo 153-8902, Japan.
Received: 6 January 2019 Accepted: 23 January 2019

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