Recent nucleosynthesis in the solar neighbourhood, detected with live radionuclides

In this article we try to summarize all information, gathered in the last three decades, on short-lived (order of Myr) radionuclides found in the Solar System with interstellar origin, most probably due to stellar processes like supernovae. The most important isotope is 60\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{60}$$\end{document}Fe, but we discuss also information on 26\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{26}$$\end{document}Al, 244\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{244}$$\end{document}Pu and 53\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{53}$$\end{document}Mn. We describe the environment of the Solar System during the past ≈10\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\approx 10$$\end{document} Myr as well as the likely locations where the supernovae occured. Confirming evidence has been found in the composition and energy distribution of galactic cosmic rays. Finally, we discuss the effects that the recent supernova activity might have had on Earth’s climate and biosphere.


Introduction
Practically all atomic nuclei heavier than oxygen, found in the Solar System (SS), have been formed in earlier stellar processes, i.e. more than 4.5 Gyr ago. The fact that stellar nucleosynthesis still occurs in our Galaxy has been proven by the observation of γ -radiation from radioactive nuclei, like 26 Al, 60 Fe, or 44 Ti (e.g. [1,2]). Since Earth's atmosphere is not transparent for γ -rays, these observations have been made by instruments on satellites like INTEGRAL. The time scale here is given by the half-lives of the radionuclides, not longer than a few Myr. In 1996, the first sentence of this paragraph was somehow questionned. In two publications [3,4] the idea was brought forward, to search on Earth for live (not yet decayed) radioactivities which were produced in core collapse supernovae (SNe) rather close to the SS and entered Earth. Ellis et al. [3] suggested to search for the nuclides with half-lives in the Myr region 10 Be, 26 Al, 36 Cl, 53 Mn, 60 Fe, 129 I, 146 Sm, and 244 Pu. They discussed also the background due to cosmic ray production of these nuclides in extraterrestrial material and of fission of actinide nuclei. The a e-mail: korschin@tum.de (corresponding author) suggestion from Munich [4] only discussed 60 Fe because for lighter nuclides and those with 80 ≤ A ≤ 150 the backgrounds from cosmic rays and fission appeared to be dominant, whereas 60 Fe cannot be strongly produced with proton induced reactions on the dominant extraterrestrial target elements (ranging up to Ni). There also the detection method is clearly proposed, namely instead of the detection of the decay radiation the direct determination of the nuclide by nuclear charge and mass with the method of accelerator mass spectrometry (AMS). The Munich AMS setup with a gas filled magnet "GAMS" [5] allowed to search for SN formed 60 Fe in Earths' deposits for the first time.
Here we want to review the ample experimental information which has been obtained since then, mainly by the labs in Munich, Vienna, Canberra and Sydney. In addition to 60 Fe, also 244 Pu and 53 Mn of extra-solar origin have been detected. We describe also the environment of the Sun in the past millions of years and possible consequences of the recent SN activity on Earth.

Detection method: accelerator mass spectrometry (AMS)
AMS is the only method to determine the ultra-low ratios of 60 Fe/Fe and 53 Mn/Mn of SN material as deposited, and found on Earth and on the Moon [6][7][8][9][10][11]. In case of the long-living radioisotopes 60 Fe and 53 Mn solely two AMS setups at tandem accelerators were able to achieve the required sensitivities, the one situated near Munich (jointly operated by both major Munich universities LMU and TUM -meanwhile shut down) and the other at the Australian National University (ANU) in Canberra. Both were able to deliver high enough energy in conjunction with dedicated detection systems, as needed to suppress any possible background events. Details of the Munich and ANU (Canberra) system can be found in reference [5] and references [12,13], respectively. For 244 Pu the labs in Vienna (VERA) [14] and Sydney (ANSTO) [15] with their small accelerators could compete and efficiently determine concentrations, because for 244 Pu the determination of the mass number is sufficient for identification, since 244 Pu is the only long lived iosobar. Thus 244 Pu can be detected as well by other AMS laboratories which are able to magnetically transport these high-rigidity particles and are equipped with appropriate detector systems.

60 Fe
One of the most promising isotopes to search for SN produced radioactivity on Earth is 60 Fe. It is predicted to be produced in significant amounts by SNe [16,17] and the natural abundance on Earth is far below any SN induced signal. There are other long-lived isotopes nearly free of terrestrial background (e.g. 146 Sm, 182 Hf, 244 Pu, 247 Cm), however they are formed in SNe in much smaller quantities. In addition 60 Fe has a half-life of 2.61 ± 0.04 Myr [18,19], long enough to survive transport to Earth. It is believed that 60 Fe and other radioisotopes are delivered to Earth in form of dust of grain size larger than 0.3 μm; radiation pressure prevents smaller interstellar dust grains to reach the innermost part of the solar system [20]. There are only few deposits which are suitable to reveal a clear signal for long time-periods in Earth's history like hydrogenetic ferromanganese crusts [21] with growth rates of only a few mm/Myr. The analysis of dated depth layers can reveal an isotopic anomaly of 60 Fe/Fe. A first measurement [6] in a deep ocean ferromanganese crust showed an excess of 60 Fe. A highly significant increase of 60 Fe, 2.8 Myr ago, has been observed in a temporally resolved ferromanganese crust in a follow up study [7]. Figure 1 shows the measured 60 Fe/Fe ratios versus the age of the layers without any background or decay correction. It depicts a clear enhancement of the 60 Fe/Fe ratio around 2.8 Myr ago indicating a deposit of 60 Fe at that time. In Fig. 1 the error bars directly reflect the total number of identfied 60 Fe events for a sample. E.g. for the most significant point at 2.8 Myr a total of 43 events of 60 Fe was identified. Similarly as in the mangenese crust, 60 Fe should show up as well in deep ocean sediments. A first measurement in a marine sediment showed a broad distribution covering the range from about 2 to 3 Myr. [22]. The results did not reveal a signal as expected with the traversal of the SS by a young SN shock wave. The broad distribution points however to an interaction of the SS with a SN ejecta that has considerably slowed down. To achieve a better resolved signal in a   [7] deep ocean sediment, measurements in fractions of two independent Pacific Ocean sediment cores have been performed. The fractions were magnetofossils, the fossilized chains of magnetite crystals produced by magnetotactic bacteria. The selectively extracted Fe from magnetofossils was expected to be minimized by any dilution from large-grained mineral phases. The results show that the 60 Fe signal onset occurs around 2.6 Myr to 2.8 Myr, and terminates around 1.7 Myr ago, and peaks at about 2.2 Myr [9].
Confirming and extended studies [8] have been done on deep-sea archives, two Pacific FeMn crust samples, two Atlantic FeMn nodules, and four sediment cores (Eltanin). Their results, Fig. 2, show 60 Fe interstellar influxes onto Earth at 1.5-3.2 Myr ago. In addition to previous measurements, they claim another weak signal between 6.5-8.7 Myr ago in FeMn Crust-1.
Work by Basu et al. [23] questioned the SN origin of the 60 Fe increase in [7]. They measured the 3 He/ 4 He ratio in samples from the same crust and found a strong increase in the ratio 4-5 Myr ago and interpreted this increase to an enhanced deposition of micrometeorites which had been exposed to the flux of galactic cosmic rays (GCR). They argued that the 60 Fe increase, as well as the 3 He increase, was due to GCR induced reactions on the Ni content of these micrometeorites. In Ref. [22] we argued against this hypothesis because this would require a higher Ni/Fe ratio in the crust than measured. Very recently, Graham and Konrad [24] measured the 3 He/ 4 He ratio in samples from the same Eltanin sediment cores as were analyzed for 60 Fe by Wallner et al. [8]. They find no correlation between the 3 He and 60 Fe content and conclude that this supports the hypothesis of a SN origin of the 60 Fe.
A summary of locations where enhanced 60 Fe/Fe ratios have been measured on Earth is depicted in Fig. 3. All sam-

26 Al
Searches for different long-living radioisotopes of SN origin are hampered strongly either by their cosmogenic background or from very low abundance. Thus a study to search for 26 Al (T 1/2 = 0.717 ± 0.024 Myr [25]) in deep-sea sediments where 60 Fe has been previously detected, could not find an excess over the cosmogenic background [26]. With the assumption, that there was no alteration of the 60 Fe/ 26 Al  [34], it depicts the different measurements in deep-sea crusts and sediments. It compares also with galactic chemical evolution models (shown as green area). Details in Ref. [34] ratio during the journey until the final deposition on Earth, a lower limit of 0.18 +0. 15 −0.08 for the local interstellar 60 Fe/ 26 Al isotope ratio has been stated. It compares with ratios from nucleosynthesis models which are within the range of 0.05 ≤ 60 Fe/ 26 Al ≤ 0.15.

244 Pu
An important question in nucleosynthesis concerns the site of actinide production. Besides other scenarios a candidate source for the r-process have been core-collapse SNe [27]. To find actinides in correlated (or uncorrelated) layers where 60 Fe has been found could support (or challenge) this hypothesis. Best suited in this case is 244 Pu as it has a long half-life of 81.3 ± 0.3 Myr [28] and interference of primordial 244 Pu is not expected. Previously reported detection [29] of primordial 244 Pu on Earth has not been confirmed [30]. First measurements in deep sea manganese encrustations and of pre-bomb depth showed already that the 244 Pu signal is weak, but still consistent with a SN signal suggested by the 60 Fe data [31,32]. An upper limit of 244 Pu in a 1-kg deep-sea dry sediment has been reported by Paul et al. [33]. An elaborated study [34] showed for the first time a clear detection of live interstellar 244 Pu in Earth's deep-sea floor combined over the last 25 Myr. The abundances were lower than expected from continuous production in the Galaxy by about 2 orders of magnitude. This large discrepancy could point to a rarity of actinide r-process nucleosynthesis sites. Possible sources could be neutron-star mergers with a small subset of actinideproducing SNe [34]. Figure 4 depicts the different measurements in deep-sea crusts and sediments. It compares also with galactic chemical evolution models (shown as green area). Details in reference [34].
In a follow-up study [35] 60 Fe and 244 Pu concentrations in further deposits have been determined. While 60 Fe is mainly produced in massive stars and ejected in SN explosions, 244 Pu is produced in r-process events only. Importantly, the measured 244 Pu influx was lower than expected if SNe dominate r-process nucleosynthesis. It has been concluded, that the data were compatible with the Local Bubble (see chapter 4) being a local disturbance of a large-scale Galactic steady-state (from SN enrichment of the ISM occurring more frequently than the radioactive half-life), with less-frequent injections from rarer r-process sources that nevertheless dominate the production of r-process elements, such as neutronstar mergers. The data are also consistent with the hypothesis of a nearby rare event before the time of SS formation that supplied the majority of the SS known inventory of the primordial actinides [35,36]. Also from observations the evidence became compelling during the last decade that the r-process occurs in kilo-novae [37] which are the result of neutron star mergers [38].

53 Mn
Several indications support the SN origin of the 60 Fe [39,40]. However, there is also the possibility of 60 Fe being formed in asymptotic giant branch (AGB) stars [41][42][43] (for a recent review on cosmic nucleosynthesis we also refer to Diehl et al. [44]). Despite of a lower nucleosynthesis yield, they could be the origin of the observed 60 Fe, as well. Therefore, a solely SN formed radionuclide, such as 53 Mn (T 1/2 = 3.7 ± 0.4 Myr) [45], detected in the same samples as the 60 Fe, would be a compelling support for the SN origin of this 60 Fe. Until now, 53 Mn, formed by nucleosynthesis, has not been detected in the interstellar space because 53 Mn decays by electron capture directly to the ground state of 53 Cr, hence only low-energy X-rays are emitted, not detectable by space borne detectors. Different to 60 Fe, the dominant fraction of the 53 Mn in the SS is produced by cosmic rays in dust that originates from asteroid collisions or comets. Compared to interplanetary 53 Mn influx on Earth the 53 Mn influx via interstellar dust [46] represents only a very small contribution. To evaluate this very small contribution four different crusts, all of them of hydrogenetic origin, and from different locations in the Pacific Ocean have been analyzed for their 53 Mn content [11]. All 53 Mn/Mn ratios have been calculated at the time of incorporation by means of their age (i.e., corrected for radioactive decay). In a second step the data from the four crusts have been merged as shown in Fig. 5.
For the time period from 1.5 to 4 Myr before present an 53 Mn excess concentration in terms of 53 Mn/Mn over that expected for cosmogenic production has been found. It confirms the SN origin of the 60 Fe. It is also for the first time that SN formed 53 Mn has been detected and it is the second positively identified radioisotope from the same SN. Also, the

Interstellar 60 Fe on the surface of the Moon
The remnant of a SN which passed the SS and has been detected on Earth [6][7][8][9] should leave a signature, as well, on the solar planetary system and also on Earths' Moon. Hence the detection of a 60 Fe signal on another SS body than Earth was of crucial importance to confirm the SN origin. There are significant differences, however, between a signal on the Moon and those on Earth. The 60 Fe deposition on Earth until it finally ends in ocean sediments or ferromanganese crusts at different locations is a rather complex process, involving atmospheric and ocean current processes as well as ocean chemistry, to mention only a few. This is rather different for lunar samples. We can assume that the dominant part of the interstellar dust is of larger size ≥ 0.2 µm [47]. This compares well to lunar dust which comprises also similar size particles. In addition, the impact velocity of interstellar particles can vary over the year as the orbital velocity of the Earth is around 29 km/s which have to be considered with respect to the initial velocity of the particles, depending on the direction of the impact. There are several estimations on the trapping of dust particles on the Moon [48,49]. We should point out however that any experimental data input is based on a detector where the impact of dust particles happened on a solid target. This is diverse at the dust layer on the Moon which is of different composition and morphology. The dust projectiles tend to interact with individual grains and not with bulk regolith. Hence the impact of dust particles with lunar  Table I in [10]. The shaded bar indicates the error band for cosmogenically produced 53 Mn and 60 Fe activities in meteorites. The error bars indicate the 1σ confidence level [50]. Figure and captions adopted from [10] dust is rather complex, and unfortunately no experimental data are available. This hampers theoretical estimations.
Thus, experimental information and a confirmation of the SN origin of 60 Fe in measurements of lunar samples are of essential importance. Therefore, 60 Fe and 53 Mn ratios from selected samples from different Apollo missions [10] have been determined. Besides of SN origin, 60 Fe is also formed in nuclear reactions by solar and galactic cosmic rays (GCR) with the heavy Ni isotopes in lunar soil. This adds to interstellar 60 Fe. 53 Mn is formed dominantly by GCRs through the (p,α) reaction on 56 Fe whereas the interstellar contribution is negligible. Figure 6 shows the measured activities of lunar and meteoritic samples. The lunar samples are labeled by number from 1 to 11. The data points (3,8,11) have a complicated history, as described in [10], they have activities comparable to the meteorite samples along the shaded area. All others show an enhanced 60 Fe activity relative to the Ni content (dpm/kg Ni). The meteoritic radionuclides are cosmogenically produced. In both cases the radionuclides 60 Fe and 53 Mn are formed by nuclear reactions of cosmic rays with Ni and Fe respectively. The shaded bar indicates the error band for the cosmogenically produced 53 Mn and 60 Fe activities in meteorites. The variation in the meteorite samples is because of the differences in the preatmospheric sizes of the precursor meteoroids and the depths of the samples.
As the surface of the Moon is constantly mixed and stirred by the impact of micrometeorites and larger objects [51], a process called "gardening", the deposited radionuclides are distributed in a layer of a few mm thickness a few Myr after deposition. The origin of the lunar samples was of different depth which is reflected in the scatter of the data of the lunar samples. Considering the downward distribution of the 60 Fe signal due to gardening a decay corrected local interstellar fluence of 60 Fe at the time of deposition has been estimated between 0.8 × 10 8 and 4 × 10 8 atoms/cm 2 [10]. The data are comparable to other measurements in Earths' reservoirs [7][8][9], and they support the SN origin.

Radionuclides today
Information on the most recent deposition on Earth of interstellar material has been obtained by Koll et al. [52]. In this work 500 kg of Antarctic snow, not older than 20 yr, were analyzed with respect to the radionuclides 60 Fe and 53 Mn. The molten snow water was filtered with a pore size of 2-3 µm. In the filtrate amounts of 5 × 10 4 atoms 60 Fe and 3 × 10 6 atoms 53 Mn were found by AMS. The 53 Mn content served, as in the lunar samples (Chapter 3.2), as reference to the cosmic ray produced radioactivities. For the atomic ratio in meteorites 60  Wallner et al. [54] have also searched for 60 Fe in youngcompared to the strong input 2-3 Myr ago-sediment cores from the Indian Ocean. The layers covered ages between 1 and 33 kyr and yield 60 Fe deposition rates of about 3.5 atoms/cm 2 /year. Within the large error bars the five data points are consistent, about three times the present deposition rate in Antarctica [52] and an order of magnitude smaller than the maximum deposition in sediments 2-3 Myr ago [8]. They as well as Koll et al. [55] discuss different scenarios, how the 60 Fe could enter the SS. One possibility could be that 60 Fe containing dust from earlier SNe resides in the Local Interstellar Cloud (LIC) which the SS entered some 40 kyr ago. To distinguish between the scenarios a determination of the 60 Fe content in 50-300 kyr old sediment samples with good time resolution would be important.

The surroundings of the Sun during the last millions of years
The Sun is presently inside the Local Bubble (LB), a nearly spherical, almost empty structure surrounded by a shell of dense interstellar medium of atoms, ions, molecules, and dust particles [56,57]. According to Breitschwerdt et al. [39] its origin are explosions of several SNe, starting around 14 Myr ago. The remnants and the swept material from the ambient interstellar medium formed the shell. Presently, the SS is positioned almost in the center of the LB [58] which has a diameter of about 300 pc. To find the position of the Sun relative to the LB when it passed the remnant of the last SN [39], we can do a crude estimation. With a relative velocity of 15.4 km/s [58] of the SS, and the time of the deposition of the 60 Fe on Earth around 2.5 Myr ago [7][8][9], we estimate the distance to the present position of around 40 pc. These can be compared with Zucker et al. [58] where it is stated that the SS entered the LB around 5 Myr ago. We conclude that the 2.5 Myr event happened inside of the preformed bubble. This seems different in case of the earliest detected 60 Fe deposition between around 6.5 Myr until 8.7 Myr ago [8,35]. In this case we find a distance of 120 pc if we consider a mean value of 7.6 Myr. If we compare this with the entrance of the SS into the bubble 5 Myr ago, we see an indication that the 60 Fe deposition at about 7.6 Myr should have happened outside of the LB. Origin for this 60 Fe signal could be that the SS has collected remnant and sweptup material from the star forming regions indicated as Taurus old and Taurus young [58]. As noted above, a confirmation of the older influx was published in [35] together with a time profile of the 7.6 Myr peak.
There are cloudlets of dense interstellar medium that move inside of the LB. Their origin is discussed in refs. [57,59]. A possible scenario addresses the interaction of the LB with the adjacent loop I bubble [57]. The nearest cloudlets to the SS are the Local Interstellar Cloud (LIC) and the G-Cloud as shown in Fig. 7. The illustration shows the solar neighborhood with the nearby clouds, the LIC and the G-Cloud, and the relative motions, indicated by blue arrows. The SS is located on the margin of the LIC close to the G-Cloud, its' motion is indicated by a yellow arrow. It entered the LIC around 40 kyr ago and will leave it in about 4 kyr [59]. To detect 60 Fe deposited from the LIC, as found in deep ocean crusts and sediments [7][8][9], and even on the Moon [10] could help to establish the origin of the LIC. This question was addressed by Koll et al. [52] by searching for 60 Fe in Antarctic snow (see chapter 3.3). The reader is also referred to the later work [54,55].

Possible locations of the SN activity
Already in 2002, Benitez and colleagues [60] suggested that SN explosions in the Scorpius-Centaurus OB association could be responsible for the 60 Fe observed on Earth [6]. Fry, Fields and Ellis [61] discussed the transport of 60 Fe, condensed in dust grains, from a SN origin through the interstellar medium, the solar heliosphere and the atmosphere onto Earth (and Moon). They estimate an 8-10 M SN at a distance around 50 pc as responsible for the 60 Fe on Earth. A group around D. Breitschwerdt reacted to the observations of live 60 Fe and calculated [39] possible trajectories and masses of possible SN progenitors within a moving group of stars whose surviving members are now in the Scorpius-Centaurus stellar association. They conclude that the closest SN occured 2.3 Myr ago with a progenitor mass of 9.2M , the next closest 1.5 Myr ago with 8.8M , at distances of 91 pc and 96 pc, respectively. Later work by the same group [62,63] discusses also new data and the creation of the LB by earlier SNe (see also chapter 4). Neuhäuser et al. [64] searched in the Scorpius-Centaurus-Lupus group of young stars for runaway stars which had belonged to a binary system, where the heavier partner underwent a SN and only a neutron star remained, not able to bind the partner. They identify from their relative velocities one pair of a runaway star (called ζ Oph) and a radiopulsar which were less than 0.5 pc apart 1.8 Myr ago at a distance of 107 pc from the Sun. The estimated mass of the SN progenitor is 16-18 M . Briceños-Morales and Chanamé [65] studied the SN activity in the OB association Upper Scorpius and attribute this runaway star also to the Lupus I cloud and a SN 1.8 Myr ago.

Cosmic rays
SNe not only eject huge amounts of matter (> 2M ), containing also freshly produced radionuclides in a rapidly expanding shock front, but they also accelerate nuclei, mostly protons, in these shock fronts to relativistic velocities. A number of spacecrafts is equipped with instruments to detect these cosmic ray particles and to measure their energy and even their mass and charge number, thus uniquely identfying the detected nucleus. On board of NASA's Advanced Composition Explorer (ACE), which orbits around the Lagrange point L1, 1.5 × 10 6 km from the Earth, operates the Cosmic Ray Isotope Spectrometer (CRIS). In 2016 its result for an observation time period of 17 years was published [66]. Indeed, in this paper Binns et al. report on a clean identification of 15 60 Fe nuclei in an energy range between 200 and 500 MeV/nucleon or velocities 0.56 ≤ v/c ≤ 0.76. Because the neighbouring isotope 59 Fe has a half-life of only 44 d it does not appear in the spectrum and allows for that clean identification. They determined the number ratio relative to the most abundant stable iron isotope as 60 Fe/ 56 Fe = 7.5(2.9) × 10 −5 at the source. In view of the half-life of 60 Fe of 2.6 Myr [18,19] they conclude an upper limit for the time between nucleosynthesis and acceleration of a few Myr. The responsible SNe should have occured < 620 pc from the Sun and they see the Sco-Cen OB association and the Orion OB1 association as the main contributors to the observed 60 Fe nuclei. In fact, for the production of these cosmic rays two SNe are required, one for the nucleosynthesis and one for the acceleration, because no 59 Ni (T 1/2 = 76 kyr [67]) was observed in the cosmic rays, meaning that it had decayed before acceleration [68].
Just recently, Boschini et al. [69] have analyzed cosmic ray spectra of heavier nuclei including recent data of the Alpha Magnetic Spectrometer (AMS-02) on board of the ISS [70]. They find an excess in the spectrum of stable iron nuclei in the energy range below 2 GeV/nucleon which they ascribe to the same origin as the excess in the isotope 60 Fe, namely the past SN activity in the LB.
In addition, already the energy spectra of protons (in the energy range TeV to PeV), positrons (30-300 GeV) and antiprotons (30-300 GeV) differ from those of the average Galactic spectra. According to Kachelrieß et al. [40] these deviations can best be understood if they assume a local cosmic ray source between 2 and 4 Myr old which ejected about 10 50 erg and happened on a galactic magnetic field line which passed the Sun at a distance not more than 100 pc. The only plausible assumption for this source are SNe.

Effects on the climate and biosphere
One of the first discussions on biological effects of a nearby SN has been done by Terry and Tucker [71]. In a different approach to this problem, following the line of Ruderman [72], who had studied the depletion of the ozone layer, Ellis and Schramm [73] discussed the influence of a close SN on the Earth. For a distance of about 10 pc the direct thermal influence would be negligible. The main effect would be the ozone destruction by γ radiation and cosmic rays. With the shielding ozone layer missing, the solar UV radiation would damage life on Earth and could lead to a mass extinction.
More recently, Melott, Thomas and coworkers [74][75][76][77] discuss possible consequences on Earth of SN activity at distances between about 50 and 150 pc with the putative SNe [7,8] between 2 and 10 Myr in mind. In Fig. 8 we show their calculation of the increase of ionization in the atmosphere due to a SN at a distance of 100 pc. They see an effect for life on Earth mainly due to the inrease of radiation, muon irradiation on the ground will increase by a factor of 20. Another effect of the increased atmosperic ionization could be an increase in the frequency of lightning. Lightning plays a strong role for the deposition of nitrate out of the atmosphere. Lightnings are also a cause for wildfires and thus for a conversion of woodland into savanna.
Marsh and Svensmark [78] discuss a different mechanism: the correlation between galactic cosmic radiation and cloud formation below a height of 3.2 km. Ionizing radiation in the atmosphere could help to form aerosol particles that can act as cloud condensation nuclei. The cloud conditions naturally have a significant impact on the global radiation budget. This correlation of cosmic rays and climate is further dicussed by Usoskin and Kovaltsov [79] and also by Kirkby [80] for different geological time scales up to 500 Myr. Kirkby also discusses the CLOUD experiment that has been set up at CERN which uses protons with energies up to 26 GeV from the proton synchrotron to experimentally study the effect of high-energy protons on the formation of aerosols or on cloud droplets directly. In Fig. 9 he showed us how strongly cosmic ray intensity and global temperature are correlated following an analysis of Shaviv and Veizer [81]). Just recently, Orgueira et al. [82] have tested with a statistical analysis the hypothesis that an increase of cosmic rays due to SN activity, in addition to a lowering of the geomagnetic field intensity, could have led to the global cooling about 2.5 Myr ago, just at the Pliocene-Pleistocene boundary. They find such a scenario quite probable. In a recent preprint [83] Opher and Loeb describe a little different scenario for a change of Earth's climate: When the solar system was passing through a cold cloud (e.g. the Local Leo Cold Cloud) some 2 Myr ago, the heliosphere shrunk to a scale smaller than the Earth's orbit. Therefore Earth was exposed to a neutral hydrogen density of up to 3000 cm −3 . This could have had drastic effects on Earth's climate.
When we look at the temperature changes during the past 500 Myr (Fig. 10, taken from δ 18 O data) and look at the beginning of the Pleistocene (the geologists put it at 2.588 Myr ago), we see the drop in global temperature, i.e. the period of glaciation. It just coincides with the beginning of 60 Fe deposition on Earth which we ascribe to SN activity in the neighbourhood of the Sun. These colder temperatures are seen as the cause of the conversion in Africa of wood land into savanna or grassland [84]. Already in our first report on the time resolved input of 60 Fe onto Earth [7] and later on [85,86] we speculated on a correlation of SNe with the Pliocene-Pleistocene climate change and possible consequences on the terrestrial fauna.
Another cause for a larger climate change at that time could have been the closing of the Isthmus of Panama which for a long time was dated as 3.5 Myr ago. However, a detailed analysis of molecular and fossil data [87] comes to the conclusion that the closure was completed by 6 Myr ago, i.e. long before the Pliocene-Pleistocene boundary.
Paleoanthropologists agree that this change of climate and habitat in Eastern or Southern Africa is responsible for the evolution of the genus Homo [88,89]. Just about 2.5 Myr ago the Homo Habilis and Homo Rudolfensis appeared [90], the first species that used speech, tools and developed culture. Likewise, in recent work by Timmermann et al. [91], a team of climatologists and paleoanthropologists links the appearance of later species of Homo: Homo heidelbergensis, Homo neanderthalensis and even Homo sapiens to climate changes in Africa and Eurasia and the adaptation of Homo to different climates.
Of course, there is no solid proof, but the idea is appealing that SNe have triggered the climate change at the Pliocene- Fig. 10 Global temperature for the past 500 Myr with differing time scales (from Wikipedia Commons, author: Glen Fergus). The arrow marks the beginning of the Pleistocene. Inserted is, on the same time scale (upside-down), a copy of the 60 Fe profile of Ludwig et al. [9] Pleistocene boundary and, thus, the development of the genus Homo.
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