Jozef Masarik

Terrestrial cosmogenic-nuclide production systematics calculated from numerical simulations

Abstract Calculations for the production of cosmogenic nuclides in the Earths atmospheric and in-situ in the surface are reported and discussed. We calculated production rates of 3 H, 7 Be, 10 Be, 14 C, and 36 Cl in the atmosphere by both galactic cosmic rays and solar protons, and our calculated production rates for 14 C agree well with previous results. Production of atmospheric 7 Be and 36 Cl by solar protons in polar regions is not negligible. Our produciton rates and depth dependences for in-situ 3 He, 10 Be, 14 C, 21 Ne, 26 Al, and 36 Cl agree well with experimental measurements for certain minerals in surface samples. The altitude dependence of in-situ production rates was also calculated.

Presence of the Solar de Vries Cycle (∼205 years) during the Last Ice Age

Certain characteristic periodicities in the Δ14C record from tree rings, such as the well-known 11-yr Schwabe cycle, are known to be of solar origin. The origin of longer-period cycles, such as the 205-yr de Vries cycle, in the Δ14C record was less certain, and it was possible to attribute it either to solar or climatic variability. Here, we demonstrate that the de Vries cycle is present in 10Be data from the GRIP ice core during the last ice age (25 to 50 kyr BP). Analysis of the amplitude of variation of this cycle shows it to be modulated by the geomagnetic field, indicating that the de Vries cycle is indeed of solar, rather than climatic, origin.

10BE AND 26AL PRODUCTION RATES DEDUCED FROM AN INSTANTANEOUS EVENT WITHIN THE DENDRO-CALIBRATION CURVE, THE LANDSLIDE OF KOFELS, OTZ VALLEY, AUSTRIA

Surface exposure dating requires the knowledge of cosmogenic nuclide production rates. When determining time-integrated production rates the exposure ages of the calibration samples need to be accurately known. The landslide of Kofels (Austria) is very well suited for this purpose. It is the largest landslide in the crystalline Alps of Austria dating back to 7800±100 years BC (AMS 14C dating of buried wood), which is well within the 14C dendro calibration curve. Exposed quartz veins were sampled from the tops of large boulders from the toe of the landslide for analysis of 10Be and 26Al. To calculate sea level, high geomagnetic latitude (≥60°), open sky radionuclide production rates, corrections were applied for altitude and latitude, for shielding by surrounding mountains, for sample geometry, vegetation and snow cover, and for sample thickness. The production rates for an exposure age of 10,000 years are 5.75±0.24 10Be atoms/yr g SiO2 and 37.4±1.9 26Al atoms/yr g SiO2. A 26Al/10Be ratio of 6.52±0.43 can be calculated. The influence of the geomagnetic field on these production rates has been estimated using two different geomagnetic field records. Our production rates should be a good approximation for the use of surface exposure dating between about 5000 and 13,000 years BP.

Effects of bulk composition on nuclide production processes in meteorites

Abstract The bulk chemical composition of meteorites is a major factor influencing the production of cosmogenic nuclides. Numerical simulations using Monte Carlo particle production and transport codes were used to investigate particle fluxes, 38Ar elemental production ratios and 21 Ne 22 Ne ratios in meteorites with a wide range of compositions. The calculations show that enhanced fluxes of low-energy secondary particles in metal-rich phases explain certain experimentally observed differences in nuclide production in various meteorite classes.

Chlorine-36 evidence for the Mono Lake event in the Summit GRIP ice core

Abstract A distinct peak has been discovered in the 36 Cl data from the GRIP ice core between the Dansgaard Oeschger (D–O) events 6 and 7 at approximately 32 kyr BP. This peak can be attributed to a minimum of the geomagnetic dipole field associated with the Mono Lake event. Since the 36 Cl peak reflects a higher production rate of all cosmogenic radionuclides, it has an impact on the 14 C dating of the last ice age. Furthermore, it provides an additional time marker similar to a peak found earlier corresponding to the Laschamp event at approximately 39 kyr BP.

Correction of in situ cosmogenic nuclide production rates for geomagnetic field intensity variations during the past 800,000 years

We present integrated relative production rates for cosmogenic nuclides in rock surfaces, which take into account reported variations of the geomagnetic field intensity during the past 800,000 yr. The calculations are based on the model simulating cosmic ray particle interactions with the Earth’s atmosphere given by Masarik and Beer [“Simulation of particle fluxes and cosmogenic nuclide production in the Earth’s atmosphere,” J. Geophys. Res. 104(D10), 12099–12111, 1999]. Corrections are nearly independent on altitude between sea level and at least 5000 m. The correction factors are essentially identical for all stable and radioactive cosmogenic nuclides with half-lives longer than a few hundred thousand years. At the equator, integrated production rates for exposure ages between ∼40,000 to 800,000 yr are 10 to 12% higher than the present-day values, whereas at latitudes >40°, geomagnetic field intensity variations have hardly influenced in situ cosmogenic nuclide production. Correction factors for in situ 14C production rates differ from those of longer-lived nuclides. They are always smaller than ∼2% because the magnetic field intensity remained rather constant during the past ∼10 kyr, when the major fraction of the 14C extant today was produced.

Reconstruction of the geomagnetic field between 20 and 60 kyr BP from cosmogenic radionuclides in the GRIP ice core

A pure physical model for the simulation of cosmic ray particle interactions with the Earth’s atmosphere was used to investigate the effects of a changing geomagnetic field on the production rates of cosmogenic nuclides. Analytical dependencies of the production rates of 3H, 7Be, 10Be, 14C and 36Cl on geomagnetic field intensity were developed. Applying those relations to the 10Be and 36Cl fluxes measured in the GRIP ice core, the geomagnetic field intensity for the period between 20 and 60 kyr BP was reconstructed. Comparison with remnant magnetism records from marine sediment cores shows excellent agreement. This validates the use of cosmogenic nuclides in ice cores to reconstruct geomagnetic field variations.

Gamma ray production and transport in Mars

The elemental composition of the Martian surface can be determined by gamma ray spectroscopy from landers or orbiting spacecraft. We used the well-tested Los Alamos High Energy Transport (LAHET) Code System (LCS) to numerically simulate the galactic-cosmic-ray-induced processes producing secondary particles and γ rays in Mars. For γ ray lines, LCS was used only for the calculation of the particle fluxes in the Martian soil and atmosphere ; these fluxes were then integrated with cross sections to calculate the production rates of γ rays as a function of depth. The fluxes of γ ray lines were then calculated both at the Martian surface and at an altitude of 378 km. The water content of the Martian soil and the atmospheric thickness were varied to establish the dependence of the particle distributions and γ ray fluxes on these parameters. Most attention is devoted to γ rays from neutron-induced reactions in the Martian soil, but contributions from the atmosphere, natural radioactivity, and from proton interactions are also mentioned. Our results are compared with those from previous studies.

Production rates of cosmogenic nuclides in boulders

Abstract We study how the production rates of cosmogenic nuclides in solid targets at the Earth’s surface depend on the shape and size of a sampled rock and the position of a sample. We use a physical model simulating the interaction of galactic cosmic ray particles with matter. Production rates at boulder surfaces may be up to 10–12% lower than values at the surface of an infinite flat target of the same chemical composition, even when the former sample sees cosmic rays from the full 2π solid angle. This is because cosmic ray neutrons are more easily lost back to the atmosphere from within a non-flat sample than from a flat surface. Therefore, the shape and size of a boulder need to be considered when taking samples, and production rates may have to be corrected accordingly.

Some results relevant to the discussion of a possible link between cosmic rays and the Earth's climate

Based on a 16-year observation period (1980–1995), it was claimed recently that Earths climate was linked to variations in the flux of cosmic rays penetrating into the atmosphere via their postulated effect on global cloud cover. Data from three independent studies yield information relevant to the ongoing discussion of the likelihood of the existence of such a link. (1) Model calculations show that the relative change in the ion production rate from a solar maximum to a solar minimum is of the same order as, or even greater than, the corresponding change in global cloud cover. (2) However, the smoothed combined flux of 10Be and 36Cl at Summit, Greenland, from 20–60 kyr B.P. (proportional to the geomagnetically modulated cosmic ray flux) is unrelated to the corresponding δ18O and CH4 data (interpreted as supraregional climate proxies). (3) Furthermore, although a comparison of the incoming neutron flux with cloud cover in Switzerland over the last 5 decades shows a significant correlation at times during the 1980s and 1990s, this does not occur during the rest of the period.