The Galactic Cosmic Ray Intensity over the Past 106–109 Years as Recorded by Cosmogenic Nuclides in Meteorites and Terrestrial Samples

Abstract

Concentrations of stable and radioactive nuclides produced by cosmic ray particles in meteorites allow us to track the long term average of the primary flux of galactic cosmic rays (GCR). During the past ∼10 Ma, the average GCR flux remained constant over timescales of hundreds of thousands to millions of years, and, if corrected for known variations in solar modulation, also during the past several years to hundreds of years. Because the cosmic ray concentrations in meteorites represent integral signals, it is difficult to assess the limits of uncertainty of this statement, but they are larger than the often quoted analytical and model uncertainties of some 30%. Time series of concentrations of the radionuclide 10 Be in terrestrial samples strengthen the conclusions drawn from meteorite studies, indicating that the GCR intensity on a ∼0.5 million year scale has remained constant within some ±10% during the past ∼10 million years. The very long-lived radioactive nuclide 40 K allows to assess the GCR flux over about the past one billion years. The flux over the past few million years has been the same as the longer-term average in the past 0.5-1 billion years within a factor of ∼1.5. However, newer data do not confirm a long-held belief that the flux in the past few million years has been higher by some 30-50% than the very long term average. Neither does our analysis confirm a hypothesis that the iron meteorite data indicate a ∼150 million year periodicity in the cosmic ray flux, possibly related to variations in the long-term terrestrial climate.

Cosmogenic nuclide systematics and the CRONUScalc program

“…These incoming particles have a wide range of energies, with typical energies ranging from a few MeV to 10 20 eV (Dunai, 2010). Over long periods (10 Ma), the integrated cosmicray flux has been shown to be constant (to within 10%) when averaged over 500 kyr timescales, with the uncertainties becoming larger (up to 30% or more) when averaged over timescales of hundreds of thousands to millions of years (Wieler et al, 2013).…”

The constancy of galactic cosmic rays as recorded by cosmogenic nuclides in iron meteorites

“…A second and motivating rationale for the present study are questions about the interpretation of the meteorite data by Shaviv (2002Shaviv ( , 2003. For example, Rahmstorf et al (2004) argued that Shaviv misinterpreted the iron meteorite data (see also Wieler et al 2013). Going one step further, Alexeev (2016) used the same data set as Shaviv (2002Shaviv ( , 2003 but reinterpreted the data by fully considering uncertainties for the cosmic ray exposure (CRE) ages and using the now accepted chemical grouping for iron meteorites.…”

The Moon as a Recorder of Nearby Supernovae

“…1) and, more particularly, the Local Bubble. More recently, however, Wieler et al (2013) have reanalysed the meteoritic data and have not been able to confirm the evidence for an enhanced GCR flux within this time period.…”

“…As stressed by Wieler et al (2013), the usefulness of meteorites in attempting to identify variations in the flux of GCRs is constrained, as it is for all extraterrestrial samples studied to-date, by the fact that they can only record an integrated GCR flux since they became exposed to the space environment. For this reason, Wieler et al combined their meteoritic analyses with those of terrestrial sedimentary samples for which independent exposure histories are available.…”

“…For this reason, Wieler et al combined their meteoritic analyses with those of terrestrial sedimentary samples for which independent exposure histories are available. As they explained: "because of the limited sensitivity of the timeintegrated GCR signals provided by meteorites, it is wise to consider … also the differential GCR flux signals provided by terrestrial sediment samples" Wieler et al (2013). This is potentially a much more powerful approach, but it relies on a terrestrial sedimentary record of cosmogenic isotopes within only the last few million years (for which ice and/or ocean sediment cores preserve an independent control of the age and depositional environment).…”