An excess of 

60

Fe

 in 2.4–

3.2

×

10

6

 year old ferromanganese crust (237KD) from the deep Pacific Ocean has been considered as evidence for the delivery of debris from a nearby supernova explosion to Earth. Extremely high 

3

He

/

4

He

 (up to 

6.12

×

10

−

3

) and 

3

He

 concentrations (up to 

8

×

10

9

 

 

atoms

/

g

) measured in 237KD cannot be supernova-derived. The helium is produced by galactic cosmic rays (GCR) and delivered in micrometeorites that have survived atmospheric entry to be trapped by the crust. 

60

Fe

 is produced by GCR reactions on Ni in extraterrestrial material. The maximum 

3

He

/

60

Fe

 of 237KD (80–850) is comparable to the GCR 

3

He

/

60

Fe

 production ratio (400–500) predicted for Ni-bearing minerals in iron meteorites. The excess 

60

Fe

 can be plausibly explained by the presence of micrometeorites trapped by the crust, rather than injection from a supernova source

Basu, S

Klemm, V

Schnabel, C

Stuart, FM

UCL Discovery

Galactic-Cosmic-Ray-Produced 3He in a Ferromanganese Crust:Any Supernova 60Fe Excess on Earth?S. Basu,1 F. M. Stuart,1 C. Schnabel,1 and V. Klemm21Isotope Geosciences Unit, Scottish Universities Environmental Research Centre, East Kilbride G75 0QF, United Kingdom2Institute for Isotope Geochemistry and Mineral Resources, Department of Earth Sciences, ETH-Zu¨rich, ETH Zentrum,Clausiusstrasse 25, CH 8092, Zu¨rich, Switzerland(Received 6 October 2006; published 2 April 2007)An excess of 60Fe in 2.4–3:2 106 year old ferromanganese crust (237KD) from the deep PacificOcean has been considered as evidence for the delivery of debris from a nearby supernova explosion toEarth. Extremely high 3He=4He (up to 6:12 103) and 3He concentrations (up to 8 109 atoms=g)measured in 237KD cannot be supernova-derived. The helium is produced by galactic cosmic rays (GCR)and delivered in micrometeorites that have survived atmospheric entry to be trapped by the crust. 60Fe isproduced by GCR reactions on Ni in extraterrestrial material. The maximum 3He=60Fe of 237KD (80–850) is comparable to the GCR 3He=60Fe production ratio (400–500) predicted for Ni-bearing minerals iniron meteorites. The excess 60Fe can be plausibly explained by the presence of micrometeorites trapped bythe crust, rather than injection from a supernova source.DOI: 10.1103/PhysRevLett.98.141103 PACS numbers: 97.60.Bw, 91.50.Jc, 96.30.ZaNearby supernova (SN) explosions may have profoundimplications for the Earth’s biosphere. The enhanced fluxof cosmic rays produced by a SN explosion, depending onduration and intensity, may deplete atmospheric ozonelayer and produce a ‘‘cosmic-ray winter’’ due to increasedcloud cover [1–3]. SN explosions that are close enough toEarth to have influence on climate, i.e., within approxi-mately 30 pc from the sun, occur only a few times every108 years [4]. The ability to detect nearby SN events in thegeological record provides a test of SN frequency and mayallow the effect they have on the Earth’s climate to beinvestigated.Ancient SN explosion debris can be detectable fromanomalously high concentrations of radioactive isotopeslike 10Be, 26Al, 36Cl, 53Mn, 59Ni, and 60Fe produced duringthe explosion or in dust swept up by the ejecta as ittraverses the interstellar medium and then deposited asdebris on Earth [5]. Excesses of 60Fe have been measuredin slowly accumulating ferromanganese (FeMn) crustsfrom the deep Pacific Ocean [3,6]. The largest 60Fe ex-cesses have been recorded in a layer in crust 237KDdeposited between 2.4 and 3.2 Ma [3]. This has beenattributed to the deposition of ejecta from a type-II SN(M  30 solar masses) at approximately 40 pc [3].Confirming this observation with other radioisotopes hasproved challenging as the intrinsically low concentrationsmake detection difficult [3,7]. For instance, a small excessof 244Pu has been measured in 237KD [7], but the uncer-tainty is large, leaving the 60Fe excess in 237KD as thestrongest evidence for deposition of material of ancient SNdebris.In addition to SN, 60Fe is produced by reactions ofgalactic cosmic rays (GCR) with Ni in extraterrestrialmaterial during exposure in space [8]. Production ratesare high enough to generate measurable 60Fe excesses inNi-bearing minerals in iron meteorites [9,10]. It has beenknown for decades that ocean floor FeMn nodules incor-porate extraterrestrial particles [11]. It is possible to esti-mate the contribution of GCR-produced 60Fe by measuringthe concentration of nuclides that are exclusively producedby GCR. The helium isotope record of 237KD results fromincorporation of micrometeorites that have been exposedin space for 100’s of millions of years [12]. Here wecompile the 16 published analyses (in the original study,the crust was named VA13/2) with 52 new He measure-ments in order to quantify the GCR-He contribution [13].By comparing production rates in possible extraterrestrialmaterials, we estimate the contribution of GCR-60Fe frommicrometeorites and test the supernova hypothesis.Helium isotopes in oceanic sediments trace the contri-bution of solid extraterrestrial material to Earth [14–16].This is evident from the 3He=4He of ferromanganesecrusts. It is unlikely that seafloor sediments incorporateSN He. If 36 1052 3He atoms (9 104 solar masses)are produced during SN explosion [17], the flux distributedover a sphere with radius 40 pc for 1 kyr is 2109 atoms=cm2 per year. A trivial fraction of this willpenetrate the heliosphere and make it to the Earth as isevident from the dominance of the solar cosmic ray-derived 3He flux to Earth despite it being lower than theGCR 3He flux in the interstellar medium by 20 times [18].Further, He solubility in seawater is extremely low (e.g.,Pacific deep water  1:2 1012 atoms=g [19]), andseawater-dissolved noble gases do not partition signifi-cantly into growing minerals [15]; therefore, high-energy3He ions that make it to Earth will not be incorporatedinto growing FeMn crusts. The predicted SN 3He=4He(1:0 104, or 73RA, where RA is the atmosphere valueof 1:39 106) [17] is significantly lower than in 237KDwhere values exceed 4400RA [Fig. 1(a)].PRL 98, 141103 (2007) P H Y S I C A L R E V I E W L E T T E R S week ending6 APRIL 20070031-9007=07=98(14)=141103(4) 141103-1 © 2007 The American Physical SocietyCrust 237KD was dredged from a depth of 4.8 km in theCentral Pacific Ocean (9180N, 146030W) [20]. The av-erage growth rate is estimated to be 2:5 mm=Myr over thelast 16 Myr [21]. We have measured He isotopes in 5–10 mg samples from 23, 1 mm-thick layers back to 15 Ma.Each layer integrates approximately 0.4 Myr of crustgrowth. 3He=4He range from 5:5 0:8 to 4440 89RA.4He concentrations show limited variation (0.4 to 2:31012 atoms=g) compared to 3He (0.8 to 800107 atoms=g) (Fig. 1). This is consistent with mixingbetween an extraterrestrial component (essentially pure3He) delivered in extraterrestrial dust and a radiogenicHe component (essentially pure 4He) delivered in wind-borne terrestrial dust [12]. 3He=4He variation is governedlargely by variation in 3He concentrations. The extrater-restrial 3He flux to Earth recorded by the crust is approxi-mately 100 times lower than in bulk oceanic sediments[12]. This reflects the low trapping efficiency of the crustwhich probably results from the high water current speedand steep slope at the site of accumulation.The He isotope record of 237KD shows a significantchange at 4–5 Ma (Fig. 1). Prior to this time, 3He concen-trations varied by little over an order of magnitude, and no3He=4He was higher than the solar wind value (290RA;[22]). In the last 4 Myr, 3He concentrations increasedsignificantly and displayed more than 100-fold variation[Fig. 1(b)]. The average 3He=4He (330RA; n  32) wasconsiderably higher than the average of >4 Myr samples(54RA; n  36). Six samples have 3He=4He equal to, orhigher than, solar wind-He, indicating the dominance ofGCR-He (1:2 105RA [23]). The highest 3He=4He(4440RA) is greater than the maximum measured in strato-spheric interplanetary dust particles (IDPs) (2220RA) [24].It is the highest ratio measured in a terrestrial sample andcan only be due to the incorporation of GCR-He. The 3Heenhancement is not due to an increase in the cosmic rayintensity or the flux of extraterrestrial dust to Earth as itnot recorded by extensive studies of the oceanic sedimentrecord [e.g. [25]]. The enhanced 3He (and 4He [Fig. 1(c)])is probably due to increased efficiency of trapping densemicrometeorites that settle through the oceans. The 3Heand 4He increase may be due to a decrease of water currentvelocity. This is consistent with the decreased deep watermass exchange between the Pacific and Atlantic Oceansduring closure of the Panama gateway between 8 and 5 Ma[26].The trapping of more extraterrestrial particles after 4–5 Ma is associated with a greater variability of 3He=4He inreplicate measurement of the individual crust layers[Fig. 1(a)]. The statistical variability of He concentrationsbetween replicate analyses of the same sample is estimatedby the fractional difference (FD) following: FD  ja b	jab	2 where a and b are the He concentration in subsamples ofcomparable masses (5–10 mg) from the same layer. FDvalues depend on the ratio a=b [12]. Samples of 237KDolder than 4 Myr have a broadly Gaussian distribution ofFD peaking at 0.5. Only 9% of values are less than 0.2, andnone exceeds 1.5 [Fig. 2(a)]. This is consistent with thepattern predicted for oceanic sediments [27] and impliesthat subsamples tend to have a similar number of particles FIG. 1. Plot of sample age versus (a) 3He=4He, (b) 3He, and(c) 4He. Solid lines depict smooth curves for sample proportionof 0.4 over 20 intervals. 3He=4He, 3He, and 4He increase at 4–5 Ma. The increase in 3He and 4He concentrations is due toincorporation of more extraterrestrial and detrital grains, proba-bly due to decrease in current strength. The presence of GCR Hein the extraterrestrial dust is evident from the very high 3He=4Herecorded for many post-4 Ma samples. The 3He increase is notcoupled with the 60Fe-rich supernova layers ( gray shaded bar)predicted by [3].PRL 98, 141103 (2007) P H Y S I C A L R E V I E W L E T T E R S week ending6 APRIL 2007141103-2with similar 3He concentrations. The skew to high values isdue to under-sampling of He-rich particles. Samples thatare less than 4 Ma have a large component of extreme FDvalues [Fig. 2(b)]; 16% have FD< 0:2, and 20% haveFD> 1:6. This is highly unlikely to be sampled by normaldistribution. The likeliest explanation is that high FD val-ues are generated by the incorporation of a small number ofextremely 3He-rich particles. The association of high 3Heconcentrations and GCR-3He=4He implies that the occa-sional particles have long exposure durations in space.The occasional incorporation of GCR-He-rich particleshas important implications for the origin of excess 60Fe in237KD [3]. The production rate of 60Fe (P60) has beendetermined in pure Ni targets in a 10 cm-radius iron sphereisotropically irradiated with 1.6 GeV protons [28]. P60(normalized to 1 primary proton=cm2=s) varies little withdepth and integrated over the radius of the sphere corre-sponds to 0:6 108 atoms=g per Myr [28]. Although thisdiffers from GCR production rates in space, it can becompared to the production rate of 3He (P3) determinedin the same experiment [28]. P3 (2:8–3:5 1010 atoms=gper Myr) shows little discernible change with depth and,importantly, there is no difference in the 3He productionrate from Ni and Fe [29]. Consequently, the experimentpredicts that P3=P60	GCR in a pure Ni phase in iron mete-orites is 450–550. This will scale linearly with the Niconcentration. The P3=P60	GCR in chondritic meteoriteswill be significantly higher as P3 is approximately twicethat of iron meteorites [29], and Ni contents are commonlyless than 1% [30].3He and 60Fe have been determined in the iron me-teorites Odessa and Tlacotepec [10,31]. 3He rangesfrom 4:21013 atoms=g (Odessa) to 1:5 1014 atoms=g(Tlacotepec). For cosmic ray exposure ages of 875 Ma and945 Ma for Odessa and Tlacotepec, respectively [31], 3Heproduction rates are 4:8 1010 atoms=g per Myr (Odessa)and 1:5 1011 atoms=g per Myr (Tlacotepec). For Nicontents of 7 and 18% and 60Fe of 2:0 dpm=kg Niand 1:2 dpm=kg Ni for Odessa and Tlacotepec, respec-tively [10], P60 ranges from 7:6 107 atoms=g per Myr(Odessa) to 1:1 108 atoms=g per Myr (Tlacotepec). Theresultant P3=P60	GCR (630–1360) are broadly consistentwith the experimentally-determined values and the lowerNi concentration.The 3He concentration in the four crust samples whereGCR-He dominates (i.e., 3He=4He> 290RA) is 1:5–8:0109 atoms=g. Excess 60Fe was measured in three of the 14layers spanning the last 5 Myr of 237KD [3]. The average60Fe concentration in these three layers (after backgroundcorrection of 2:4 1016 [3] and assuming 20% Fe in thecrust) is 3:7 106 atoms=g. Radioactive decay of 60Fe(t1=2  1:49 Myr [9]) has reduced this by 67–77% since2:8 0:4 Ma [3], leading to an initial 60Fe concentrationof 0:9–1:9 107 atoms=g. If the 3He and 60Fe are deliv-ered in the same particle, the 3He=60Fe is 80–850. Thisoverlaps the predicted 3He=60Fe	GCR of FeNi alloy min-erals such as taenite (40–50% Ni) that are common in theataxite class of iron meteorites. Further, it is similar to theproduction rate ratio in iron meteorites and provides strongevidence for a significant, if not dominant, contribution ofGCR-produced 60Fe in crust 237KD. The 3He=60Fe of the237KD is a low estimate of the true value as He is sub-stantially degassed from incoming micrometeorites duringatmospheric entry [27]. For a cutoff temperature of 600 Cfor He loss, only 0.5% of the mass of the incoming particlesstill retain He by arrival at Earth surface [27].Using measured 60Fe in Dermbach [10], the 60Fe ex-cess in 237KD can be accounted by one 500 m-diameteror greater micrometeorite, or 2–4, 320–360 m particles.Although 500 m micrometeorites are not common,magnetic cosmic spherules up to 700 m diameter havebeen extracted from seafloor sediments as well as fromAntarctic and Greenland ice (e.g., [32,33]). Importantly,FeNi alloys are the densest common minerals in meteorites(taenite > 8 g=cc) and a 500 m-diameter micrometeoritewill settle approximately 5 times faster than equivalent- FIG. 2. Fractional differences (FD) of 3He concentration be-tween replicate analysis of crust layers (a) >4 Ma and(b) <4 Ma. Samples >4 Ma have a broadly Gaussian distribu-tion approximated by the solid curve. The peak between 1.8–2for FD distribution of post-4 Ma samples indicate that the Hebudget is dominated by occasional, extremely GCR-He-richparticles.PRL 98, 141103 (2007) P H Y S I C A L R E V I E W L E T T E R S week ending6 APRIL 2007141103-3sized chondritic fragments, and 7500 times faster than the<8 m-diameter zircons that contribute the 4He [34]. TheFD distribution of 4He in both the post- and pre-4 Ma crustlayers are largely between 0 and 0.4. This implies that 4Hewas transported by a large number of particles with ratherconstant 4He concentrations (probably micron-sized zircon[34]) and contrasts strongly with 3He record. This supportsthe notion that the 3He FD variation is more sensitive tosubtle changes in ocean circulation.Support for the micrometeorite origin of the GCR-60Fecomes from the presence of small excesses of 53Mn in237KD that can be accounted for by spallation of Fe andNi in meteorites [6]. The experimentally-determinedGCR-53Mn production rate (P53) ranges from 3:1–4:81010 atoms=g per Myr [34]. The equivalent P3=P53 rangesfrom 1 to 5 [28,29]. The 53Mn flux in 237KD (5–10 mm)(6:4 108=cm2=Myr [6]) corresponds to 1 109 atoms53Mn=g crust. For t1=2  3:7 Myr [6] and a terrestrialresidence time of 2–4 Myr, 3He=53Mn ranges from 0.6 to4.3. Again, this overlaps the theoretical value of GCRproduction and implies that the crust 53Mn is contributedby fragments of iron meteorite. Additionally, the outerlayers of 237KD have Os isotopic compositions that aresignificantly lower than seawater values, strongly indica-tive of a meteoritic component [35].In summary, we find that over the last 4–5 Myr, FeMncrust 237KD has incorporated micrometeorites with ex-tremely high GCR-He concentrations. Consequently, the60Fe excesses in the crust are probably due to the incorpo-ration of Ni-rich extraterrestrial particles that were exposedin space for a few 100’s Myr. The delivery of Ni-richmicrometeorites may be more sporadic than the deliveryof He-rich particles if they originate as ablation debris fromNi-rich iron meteorites. These results strongly questionwhether 60Fe excesses provide evidence for nearby super-nova explosion, but imply that it may find use as a tracer ofthe origin of meteoritic debris on Earth.We thank M. Frank, J. Hein, G. Korschinek and twoanonymous journal reviewers for comments that helped toimprove this work. The SUERC noble gas laboratory isfunded by the Scottish Universities, and this work wasfacilitated by NERC (No. NE/B504306).*To whom all correspondence should be addressed:S.Basu@suerc.gla.ac.uk[1] B. D. Fields and J. Ellis, New Astron. Rev. 4, 419 (1999).[2] N. Benı´tez, J. Maı´z-Appela´niz, and M. Canelles, Phys.Rev. Lett. 88, 081101 (2002).[3] K. Knie et al., Phys. Rev. Lett. 93, 171103 (2004).[4] B. D. Fields, New Astron. Rev. 48, 119 (2004).[5] J. Ellis, B. D. Fields, and D. N. Schramm, Astrophys. J.470, 1227 (1996).[6] K. Knie et al., Phys. Rev. Lett. 83, 18 (1999).[7] C. Wallner et al., New Astron. Rev. 48, 145 (2004).[8] R. Michel et al., Meteoritics 26, 221 (1991).[9] W. Kutschera, Nucl. Instrum. Methods Phys. Res., Sect. B17, 377 (1986).[10] K. Knie et al., Meteoritics and Planetary Science 34, 729(1999).[11] J. Jedwab, Geochim. Cosmochim. Acta 34, 447 (1970).[12] S. Basu et al., Geochim. Cosmochim. Acta 70, 3996(2006).[13] See EPAPS Document No. E-PRLTAO-98-023715 forsupplementary table. This document can be reached viathe EPAPS homepage. For more information on EPAPS,see http://www.aip.org/pubservs/epaps.html.[14] C. Merrihue, Ann. N.Y. Acad. Sci. 119, 351 (1964).[15] K. A Farley, in Accretion of Extraterrestrial MatterThroughout Earth’s History, edited by B. Ehrenbrinkand B. Schmitz (Kluwer, New York, 2001), pp. 179–204.[16] D. Lal and A. J. T. Jull, Earth Planet. Sci. Lett. 235, 375(2005).[17] M. Limongi and A. Chieffi, Astrophys. J. 592, 404(2003).[18] R. C. Reedy, J. Geophys. Res. 92, E697 (1987).[19] M. Ozima and F. A. Podosek, in Noble Gas Geochemistry(Cambridge University Press, Cambridge, 2002), pp. 102.[20] G. Friedrich and A. Schmitz-Wiechowsky, Mar. Geol. 37,71 (1980).[21] M. Segl et al., Nature (London) 309, 540 (1984).[22] J. P. Benkert, H. Baur, P. Signer, and R. Weiler,J. Geophys. Res. 98, 13147 (1993).[23] R. Wieler, in Noble Gas Geochemistry and Cosmo-chemistry, edited by D. Porcelli, C. J. Ballentine, andR. Wieler (The Mineralogical Society of America,Washington DC, 2002), pp. 125–170.[24] R. O. Pepin, R. L. Palma, and D. J. Schlutter, Meteoriticsand Planetary Science 35, 495 (2000).[25] K. Farley, Nature (London) 378, 600 (1995).[26] M. Frank, Rev. Geophys. 40, 1001 (2002).[27] K. A. Farley, S. Love, and D. B. Patterson, Geochim.Cosmochim. Acta 61, 2309 (1997).[28] S. Merchel et al., Nucl. Instrum. Methods Phys. Res., Sect.B 172, 806 (2000).[29] I. Leya et al., Meteoritics and Planetary Science 39, 367(2004).[30] H. Palme and H. Beer, in Landolt-Bo¨rnstein, New SeriesVol VI/3a (Springer-Verlag, Berlin, Heidelberg, 1994),pp. 196–221.[31] H. Voshage and H. Feldmann, Earth Planet. Sci. Lett. 45,293 (1979).[32] M. T. Murrell, P. A. Davis, Jr., K. Nishiizumi, and H. T.Millard, Jr., Geochim. Cosmochim. Acta 44, 2067 (1980).[33] M. Maurette, C. Hammer, and M. Pourchet, in FromMantle to Meteorites, edited by K. Gopalan, V. K. Gaur,B. L. K. Somayajulu, and J. D. Macdougall (IndianAcademy of Sciences, Bangalore, 1990) pp. 87–126.[34] D. B. Patterson, K. A. Farley, and M. D. Norman,Geochim. Cosmochim. Acta 63, 615 (1999).[35] K. W. Burton et al., Earth Planet. Sci. Lett. 171, 185(1999).PRL 98, 141103 (2007) P H Y S I C A L R E V I E W L E T T E R S week ending6 APRIL 2007141103-4

Galactic-Cosmic-Ray-Produced in a Ferromanganese Crust: Any Supernova Excess on Earth

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Galactic-Cosmic-Ray-Produced in a Ferromanganese Crust: Any Supernova Excess on Earth

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