Recent near-Earth supernovae probed by global deposition of interstellar radioactive ⁶⁰Fe

Abstract

The rate of supernovae (SNe) in our local galactic neighborhood within a distance of ~100 parsec from Earth (1 parsec (pc)=3.26 light years) is estimated at 1 SN every 2-4 million years (Myr), based on the total SN-rate in the Milky Way (2.0±0.7 per century^1,2). Recent massive-star and SN activity in Earth’s vicinity may be evidenced by traces of radionuclides with half-lives t_1/2 ≤100 Myr^3-6, if trapped in interstellar dust grains that penetrate the Solar System (SS). One such radionuclide is ⁶⁰Fe (t_1/2=2.6 Myr)^7,8 which is ejected in supernova explosions and winds from massive stars^1,2,9. Here we report that the ⁶⁰Fe signal observed previously in deep-sea crusts^10,11, is global, extended in time and of interstellar origin from multiple events. Deep-sea archives from all major oceans were analyzed for ⁶⁰Fe deposition via accretion of interstellar dust particles. Our results, based on ⁶⁰Fe atom-counting at state-of-the-art sensitivity⁸, reveal ⁶⁰Fe interstellar influxes onto Earth 1.7–3.2 Myr and 6.5–8.7 Myr ago. The measured signal implies that a few percent of fresh ⁶⁰Fe was captured in dust and deposited on Earth. Our findings indicate multiple supernova and massive-star events during the last ~10 Myr at nearby distances ≤100 pc.

The density and temperature distribution of the interstellar medium (ISM) is highly variable, with typical substructures of ~50–150 pc (superbubbles) having life-times of some 10 Myr. Several SN explosions over the last ~14 Myr shaped the present structure of the local superbubble (LB)^12-14. The SS, now embedded in the LB, is expected to have faced fronts of SN ejecta and accumulated material from massive stars. To enter the SS, any material from the ISM must be condensed into larger dust grains to avoid being deflected away by the solar wind and interplanetary magnetic field^3,10,11. ISM dust particles were indeed identified at Earth orbit¹⁵ and may accumulate on Earth in archives such as deep-sea sediments and ferromanganese (FeMn) crusts and nodules which retain time information over millions of years. ⁶⁰Fe as well as ²⁶Al (t_1/2=0.71 Myr) are observed^1,9 in the ISM as a result of many SNe and emission from massive stars. Direct detection of ‘live’ radionuclides^3-5,10,11 on Earth would provide insight into recent and nearby nucleosynthesis in massive stars^14,16,17, dust formation and transport into the SS. Extraterrestrial ⁶⁰Fe was in fact already observed in FeMn crusts in pioneering studies at TU Munich^10,11, and interpreted as being of SN^10,11,18 or (micro)meteoritic origin^19,20.

In the present work, the ⁶⁰Fe contents of three different deep-sea archives (four sediment cores, two FeMn-crusts and two FeMn-nodules) recovered from the Indian, Pacific and Atlantic Oceans respectively (Supplementary Figure S1) were determined. All were dated via their ¹⁰Be (t_1/2=1.39 Myr) content, complemented by ²⁶Al for the sediments²¹. All radionuclides (⁶⁰Fe, ²⁶Al and ¹⁰Be) were counted using accelerator mass spectrometry (AMS) (Supplementary Information). The sediment cores provided a record from 1.7-3.2 Myr BP (before present) with a time resolution of <30 kyr, bracketed by recent and ~5-7 Myr old samples. Pacific ‘Crust-1’ extends from present to 10.9 Myr with ~2.2 Myr time resolution and ‘Crust-2’ from 1.2–7 Myr BP (~100 kyr resolution). Two nodules covered 5.4 Myr BP (~2 Myr resolution).

In the sediment, 288 ⁶⁰Fe-events were registered for the time period 1.71-3.18 Myr (45 individual samples) with a mean isotopic ratio ⁶⁰Fe/Fe=(1.79±0.10)×10⁻¹⁵, a factor of ~40 above the measurement background of (0.042±0.015)×10⁻¹⁵. None of the recent or old sediment samples show evidence for ⁶⁰Fe above background (3 ⁶⁰Fe-events). The first two layers in Crust-1 gave ⁶⁰Fe-signals 4σ and 7σ above background; layers 3 and 5 are close to the measurement background, but layer 4, which spans the period 6.5-8.7 Myr, has a significantly higher ratio (~4σ above background, Table 2). For Crust-2 a clear ⁶⁰Fe-signal was also found at <3.5 Myr. The nodules support this finding (Table 3, Supplementary Tables S3-S5).

Table 2. ⁶⁰Fe/Fe-ratios from AMS measurements at ANU of layered samples of the two Pacific FeMn crust samples (Crust-1 and Crust-2) and of the two Atlantic FeMn nodules (no. 21 and 24).

Crust-1
layer	depth (mm)	time period (Myr)	60Fe counts detected	60Fe/Fe (10−15 at/at)a	60Fe/Fe│d.c. (10−15 at/at)b	Fe conc. (10−2 g g−1)c	60Fe conc. (106 at g−1)	60Fe incorp. rates (atoms·cm−2yr−1)	60Fe incorporation (106 at·cm−2/layer)
Layer 1	0 – 5	0 – 2.17	23	0.96±0.25	1.19±0.33	12.8±0.1	1.64±0.45	0.72±0.20	1.56±0.43
Layer 2	5 – 10	2.17 – 4.35	74	1.58±0.23	3.60±0.55	11.8±0.1	4.56±0.68	2.00±0.30	4.34±0.65
Layer 3	10 – 15	4.35 – 6.52	2	0.14±0.12	0.36±0.51	11.4±0.1	0.34±0.62	0.19±0.27	0.40±0.59
Layer 4	15 – 20	6.52 – 8.70	26	0.38±0.09	2.49±0.97	13.7±0.2	3.67±1.43	1.61±0.63	3.49±1.36
Layer 5	20 – 25	8.70 – 10.87	3	0.15±0.11	1.20±1.50	11.2±0.2	1.45±1.82	0.64±0.80	1.38±1.73
total	0–4.35 & 6.52-8.70		123		--			--	9.4±0.9
Crust-2
Layer 1-2	0 – 1.0	1.20 – 1.41	5	1.06±0.48	1.50±0.67	11.6±0.1	1.87±0.83	1.67±0.75	0.36±0.16
Layer 3-5	1.0 – 2.5	1.41 – 1.73	37	1.28±0.21	2.01±0.33	11.9±0.1	2.60±0.43	2.33±0.38	0.74±0.12
Layer 6-8	2.5 – 4.0	1.73 – 2.05	32	0.84±0.15	1.39±0.25	11.8±0.1	1.73±0.31	1.55±0.27	0.50±0.09
Layer 9-10	4.0 – 5.8	2.05 – 3.05	20	0.55±0.12	1.09±0.24	11.1±0.1	1.35±0.30	0.46±0.10	0.46±0.10
Layer 11-12	5.8 – 7.7	3.05 – 4.11	2	0.12±0.08	0.29±0.21	10.4±0.1	0.37±0.26	0.13±0.09	0.13±0.09
Layer 13-26	7.7 – 21.0	4.11 –7	1	<0.03	<0.1	9	<0.1	<0.09	<0.06
60Fe:	0 – 7.7	1.20 – 3.05	94		--				2.19±0.22
Nodule 21
Layer 1	0 – 3	0 – 1.8	3	0.16±0.11	<0.23	15±2	<0.4	<0.13	<0.23
Layer 2	3 – 6	1.8 – 3.3	13	0.47±0.16	0.60±0.22	15±2	0.97±0.36	0.40±0.15	0.55±0.21
Layer 3/4	6 – 17	3.3 – 5.4	5	0.18±0.06	<0.10	15±2	<0.16	<0.1	<0.23
60Fe:	3 – 6	1.8–3.3	13	0.47±0.16	0.60±0.22	15±2	0.97±0.36	0.40±0.15	0.55±0.21
Nodule 24
Layer 1	0 – 4	0–1.9	15	0.56±0.18	0.71±0.23	15±2	1.13±0.37	0.34±0.11	0.86±0.41
Layer 2	4 – 8	1.9–3.3	5	0.23±0.12	0.45±0.23	15±2	0.71±0.37	0.27±0.15	0.54±0.34
Layer 3/4	8 – 19	3.3–5.4	1	0.05±0.05	<0.15	15±2	<0.24	<0.1	<0.25
60Fe:	0 – 8	0–3.3	20	0.40±0.10	0.58±0.13	15±2	0.92±0.21	0.31±0.09	1.40±0.50

^ameasured ⁶⁰Fe/Fe ratios. The uncertainties from ⁶⁰Fe denote statistical uncertainties only (1σ, using Poisson statistics).

^bbackground & decay corrected (d.c.) ⁶⁰Fe/Fe data. The machine background of ⁶⁰Fe/Fe=(0.042±0.015)×10⁻¹⁵ was used for the subsequent background correction. Ages are based on the ¹⁰Be data and have an uncertainty of ±0.3 Myr (Crust-1) and ±0.5 Myr (Crust-2 and nodules).

^cstable iron content of the dissolved crust material and of the leachate in case of the nodules, as measured via ICP-MS. The mean dry density of the crust and nodule samples was 1.9·g·cm⁻³. ‘conc.’ means ‘concentration’. The Fe concentration listed for Crust-2, layers 13-26 is the average; individual data range between 5.5 and 13.1%.

Table 3.

Summary of ⁶⁰Fe-deposition into various archives as obtained in this work and given in the literature^10,11,22 (no correction for incorporation efficiency). Uncertainties are 1σ.

Deep-sea archive	cores	location	time period (Myr)	60Fe detector events	60Fe deposition (106 at·cm−2)a
Sediment	4	Indian Ocean	1.71 – 3.18	288	35.4±2.6
FeMn Crust 1	2	Pacific Ocean	0 – 4.35	97	5.9±0.8
FeMn Crust 1			6.52 – 8.70	26	3.5±1.0
FeMn Crust 2			1.2 – 3.1	94	2.2±0.2
FeMn nodules	2	Atlantic Ocean	1.8 – 3.3 0 – 3.3	13 20	0.6±0.2 1.4±0.5
FeMn Mona Pihoa10	1	Pacific ocean	0 – 5.9	21	~9b
FeMn 237KD11	1	Pacific ocean	1.74 – 2.61b	69	1.5±0.4b
Lunar material22	1	Moon	integral	--c	~10

^afor Crust-1 and Crust-2 an incorporation efficiency of 17 and 7% respectively, has to be taken into account to calculate the ⁶⁰Fe fluence from the deposition values; similarly 2 and 4% for the nodules.

^badjusted for a revised ⁶⁰Fe and ¹⁰Be half-live values^7,8

^cnot listed in Ref. 22.

In summary, two clear ⁶⁰Fe signals with a total of 538 ⁶⁰Fe-events were observed. In the sediments, the signal covers the time period 1.7–3.2 Myr. In the crusts, ⁶⁰Fe is found up to 3.5 and ~4 Myr, with a second influx between 6.5 and 8.7 Myr. The nodules confirm the presence of ⁶⁰Fe at <3.3 Myr. No ⁶⁰Fe signal is found in recent (<0.2 Myr) or older (≥5 Myr) sediments and nodules, or in crusts between 4.4–6.5 and 8.7–10.9 Myr.

Between 1.7 and 3.1 Myr, the ⁶⁰Fe deposition rate into the sediments was ~11–35 ⁶⁰Fe atoms·cm⁻²yr⁻¹ (300-kyr averages), whereas incorporation rates into crust material were significantly lower at 1–2 atoms·cm⁻²·yr⁻¹ (Figure 1, all data are decay-corrected). This suggests an incorporation-efficiency into Crust-1 and Crust-2 of 17% and 7%, respectively. The deposition in the 1.5-Myr interval covered by the signal in the sediment is (35±2)×10⁶ atoms·cm⁻². For the second ⁶⁰Fe signal (6.5-8.7 Myr, Crust-1, 17% incorporation) it is (21±6)×10⁶ atoms·cm⁻² (Tables 2-3).

Figure 1. Deposition rates for sediment (150 kyr averaged data) and incorporation rates for two crust samples.

⁶⁰Fe concentrations (⁶⁰Fe/g) for the sediment are given in the inset; they were on average 6.7×10⁴ atoms/g between 1.7 and 3.2 Myr, but 260×10⁴ atoms/g crust and 95×10⁴ atoms/g nodule, reflecting the difference in growth rate and incorporation efficiency (see Supplement). The error bars (1σ) include all uncertainties and scale with decay correction, thus upper limits are becoming larger for older samples. The absolute ages for the sediment are uncertain by 0.1 Myr, but for the 5.5-Myr sediments ~1 Myr. Ages of Crust-1 are 0.3 and of Crust-2 0.5 Myr uncertain.

Although the 1.5 Myr time-spread of ⁶⁰Fe influx measured in the present work exceeds the ~0.8 Myr previously reported for crust 237KD^11,18, the two time profiles are not inconsistent given the lower counting statistics and signal-to-background in Ref. 11. Furthermore, the marginally positive result for the same time period for an Atlantic sediment¹⁸ is consistent with our data, considering their higher sedimentation rates and stable Fe-contents. ⁶⁰Fe has also been reported in lunar material, though without time information²² and recently in Pacific sediments²³.

Clearly, our data are incompatible with a constant ⁶⁰Fe production or deposition. A terrestrial origin can be ruled out, because there is no suitable target for cosmic-ray induced production and anthropogenic input would be concentrated in the surface layer. Since ⁶⁰Fe was found in each of the major oceans, it is reasonable to assume a uniform global distribution. A micro-meteoritic or meteoritic origin can be excluded, since the measured cosmic-dust flux is 400 times lower than would be required (Supplementary Information and Figure S6). Similarly a hypothetical break-up of a single object, comparable to the asteroid invoked in relation to the K/T event 65 Myr ago, would have delivered 4,500 times less ⁶⁰Fe.

We assume that the extraterrestrial ⁶⁰Fe flux through Earth’s cross-section is homogenously distributed over Earth’s surface. Thus, the measured mean deposition of ~24.5 atoms·cm⁻²yr⁻¹ (1.7–3.2 Myr signal) corresponds to a ⁶⁰Fe-flux of 98 atoms·cm⁻²yr⁻¹ into the inner SS or integrated over 1.5 Myr to an ⁶⁰Fe-fluence of (1.46±0.15)×10⁸ atoms·cm⁻² at Earth orbit; the fluence for the older event is (1.2±0.4)×10⁸ atoms·cm⁻². Interstellar grains, filtered by the SS in size to an average of ~0.5 μm, were detected by space missions¹⁵, suggesting that (6±3)% (ε_dust) in mass of ISM dust reaches the inner SS⁶. These grains follow the flow velocity of the ISM. Assuming the ⁶⁰Fe-loaded grains follow the same mass-distribution as determined for ISM grains at Earth orbit, we deduce an interstellar ⁶⁰Fe-concentration in dust of (2.8±1.4)×10^{−11 60}Fe atoms·cm⁻³ for 1.7-3.2 Myr and integrated over the full period of 11 Myr an average concentration of ~(5-15)×10⁻¹² atoms·cm⁻³. Observations of ⁶⁰Fe-decay^1,9 and nucleosynthesis models² suggest an average Galaxy concentration of ~6×10^{−12 60}Fe atoms·cm⁻³ (Supplementary Information), in agreement with the 11-Myr local-data reported here.

⁶⁰Fe is produced in massive stars^2,24-27 in their late phases, predominantly just before SN-explosions, and then ejected into space. (Super)AGB stars also produce and eject ⁶⁰Fe through their stellar winds during ~50 kyr, leading to a time profile similar to SNe; however, their contribution to the galactic ⁶⁰Fe inventory is small²⁸.

Models suggest a travel time of ~200 kyr with a time spread of ~100–400 kyr⁵ for ejecta from a single SN at ~100pc distance. Our measured spread of ~1.5-Myr is inconsistent with the interpretation in terms of ejecta from a single SN (or AGB-star) moving across the SS (Supplementary Figure S6). It suggests multiple SN- and massive-star activities within the last ~10 Myr in Earth’s vicinity and two distinct periods 1.7–3.2 and ~6.5–8.7 Myr BP. The recent time profile would be compatible with movement across the SS of ejecta in a series of SN-fronts in short succession within 1.5 Myr. This would, however, require a high SN-frequency (~2-3 SNe/Myr) since large fluctuations were not observed in the time profile. Alternatively, the ejecta containing the ⁶⁰Fe-bearing grains could have come to rest in the ambient ISM and diffused into volumes or clouds, that were then traversed by the SS¹⁸.

The SS is currently embedded in a flow of ISM-material with interstellar grains moving parallel to the flow of neutral interstellar gas in local ISM clouds arguing for a common history or driver²⁹. Such clouds were suggested as part of an expanding superbubble-shell driven by SNe and winds from massive stars^29,12-14. Assuming the ejecta originate from a distance 70-100 pc (~limit of the LB) and ⁶⁰Fe is equally distributed into the outer shell of size 30 pc (distance representing 1.5 Myr travel), i.e. a spherical shell of mean radius 70-100 pc with a thickness of 30 pc, we deduce a total ⁶⁰Fe mass trapped in ISM dust of (5–11)×10⁻⁵ solar masses (M_⊙) in the shell volume. This number represents a lower limit as it reflects the fraction of ⁶⁰Fe condensed into dust without correction for radioactive decay and neglects the granularity of clumpy ejecta. Models predict core-collapse and electron-capture SN-nucleosynthesis yields for ⁶⁰Fe to be (0.5–14)×10⁻⁵ M_⊙ for 8–25 M_⊙-stars^24-27,2, depending on the progenitor mass with large uncertainties in the nuclear-physics input. (Super)AGB stars produce (0.003–1)×10⁻⁵ M_⊙ ⁶⁰Fe²⁸. Our observed signals therefore favor SN events. The fraction of ⁶⁰Fe in dust can be roughly estimated by a comparison of our measured ⁶⁰Fe deposition with nucleosynthesis yields. Under these assumptions and assuming reasonable distances (20–100pc) ~0.4-9% of ⁶⁰Fe would be trapped in dust (Supplementary Information, Figures S7 and S8).

Comparing our data with a similar work for ISM-²⁴⁴Pu in sediments and crust samples⁶ yields a ²⁴⁴Pu/⁶⁰Fe atom-ratio of ~3×10⁻⁵ or less during periods of elevated ⁶⁰Fe deposition over the last 10 Myr which agrees with the recently reported low ²⁴⁴Pu SN-yields⁶ (Supplementary Information).

Our broad and global ⁶⁰Fe-influx on Earth demonstrates recent (<10Myr) and wide-spread massive-star ejections in our near galactic neighborhood (<100pc), most likely from SN-explosions. Interestingly, the older event coincides with a strong increase in ³He and temperature change ~8 Myr BP³⁰, while the more recent activity starting ~3 Myr BP occurred at the same time as Earth’s temperature started to decrease during the Plio-Pleistocene transition.

Table 1. Averaged ⁶⁰Fe/Fe atom ratios from AMS measurements at ANU.

52 sediment samples from four sediment cores (Eltanin) from the Indian Ocean were analyzed (individual data are listed in the Supplementary Information) as well as a series of blank samples (commercial iron).

sediment cores	sediment samples	time period (Myr)	60Fe counts detected	60Fe/Fe (10−15 at/at)a	60Fe/Fe│d.c. (10−15 at/at)b	Fe conc. (10−2 g g−1)c	60Fe conc. (104 at g−1)	60Fe-deposition rates (at cm−2yr−1)	60Fe deposition (106 at·cm−2 layer−1)d
45-21 / 50-02	5	< 0.2	2	0.06±0.04	0.02±0.02	0.30±0.10	<0.2	<0.2	--
49-53 / 45-21	14	1.71 – 2.0	123	1.67±0.15	2.52±0.23	0.23±0.01	6.0±0.6	22.8±2.3	6.5±0.7
49-53 / 45-21 / 50-02	11	2.0 – 2.3	51	1.51±0.21	2.48±0.35	0.24±0.01	6.7±1.0	24.8±3.6	7.4±1.1
49-53 / 45-21 / 50-02	7	2.3 – 2.6	33	1.96±0.34	3.50±0.61	0.17±0.01	6.5±1.2	27.1±5.0	8.1±1.5
49-53 / 45-16	7	2.6 – 2.9	54	3.40±0.46	6.61±0.90	0.16±0.01	10.3±1.5	34.8±5.2	10.4±1.5
49-53 / 45-16	6	2.9 – 3.18	27	1.18±0.23	2.41±0.47	0.13±0.01	3.4±0.7	11.4±2.4	3.0±0.6
45-16	2	~4–7e	1	0.11±0.11	0.20±0.30	0.14±0.01	<0.4	<1	--
commercial iron	99	background	7	0.042±0.015	--

^ameasured ⁶⁰Fe/Fe ratios. The uncertainties from ⁶⁰Fe denote statistical uncertainties only (1σ, using Poisson statistics).

^bbackground & decay-corrected (d.c.) ⁶⁰Fe/Fe-data. Surface and old layers are compatible with the measurement background obtained from chemistry blank samples. The age is based on the ²⁶Al and ¹⁰Be data and tie-points of magnetic reversals.

^c‘conc’ means ‘concentration’. Stable iron content of the leached material as measured via ICP-MS (averaged values, see Methods). The average leachable Fe content for the four cores was measured to 0.18% (core Eltanin 49-53), 0.25% (45-16), 0.3% (45-21) and 0.45% (50-02). The mean dry density of the sediments was 1.16·g·cm⁻³.

^dfor sediments with 100% incorporation efficiency the Fe deposition equals the terrestrial fluence.

^euncertain by ~1 Myr.