Nuclide Production Rates Research Articles

Numerical simulations of in situ production of terrestrial cosmogenic nuclides

A variety of geologic and environmental events and processes can be studied with the cosmogenic nuclides accumulated in terrestrial rocks. Reliable interpretation of the measured in situ-produced cosmogenic nuclides requires a good understanding of the involved nuclear processes and factors influencing them. The production rates of nuclides depend on many parameters. In this paper, calculations are reported and discussed on the influence of the air/ground interface and snow cover on production rates of in situ produced cosmogenic nuclides.

Atmospheric scaling of cosmogenic nuclide production: Climate effect

Absorption of cosmic rays by atmospheric mass varies temporally due to a redistribution of atmospheric pressure by ice sheets during glaciations, the compression and expansion of the atmosphere due to cooling and warming, and changes in katabatic winds near large ice masses. These atmospheric processes can result in changes in production rates of cosmogenic nuclides which, when integrated over long exposure durations may result in 0% to >5% adjustments in site production rates depending on location. Combining a CCM3 model with imbedded ice sheets for 20 ka, we show that production rates changes (relative to today) are greatest at high elevations (6–7% at 5 km altitude) due to atmospheric compression from decreased temperature. Production rates at specific times for sites near ice sheet margins can be reduced more than 10% due to a combination of katabatic winds draining off the ice sheet margins and atmospheric cooling. Nunatak settings may be significantly affected by the climate effect due to persistent glacial atmospheric conditions. Atmospheric variability may explain some of the disparities among cosmogenic nuclide production rate calibrations.

Benefits of extended capabilities of the driver accelerator for EURISOL

Possibilities are studied for the optimization of EURISOL rare nuclide yields in specific regions of the nuclear chart by building the driver accelerator in a way that enables accelerating several additional beam species, to specific energies, besides the baseline 1 GeV proton beam. Nuclide production rates with these driver beams are compared to the production rates expected with the 1 GeV proton beam in the direct-production and the high-power-converter scenarios. Arguments are presented to show that several additional driver-beam scenarios could provide substantial benefit for the production of nuclides in specific regions of the nuclear chart. The quantitative values in this report are preliminary in the sense that they depend on assumptions on the values of some key parameters which are subject to technical development, e.g., maximum beam intensities or limits on the target heat load. The different scenarios are compared from the aspect of nuclide yields. The arguments presented here, when complemented by the corresponding technological and financial considerations, are intended to serve as a part of the basis for decisions on the design of the driver accelerator of EURISOL.

Systematic comparison of ISOLDE-SC yields with calculated in-target production rates

Recently, a series of dedicated inverse-kinematics experiments performed at GSI, Darmstadt, has brought an important progress in our understanding of proton and heavy-ion induced reactions at relativistic energies. The nuclear reaction code ABRABLA that has been developed and benchmarked against the results of these experiments has been used to calculate nuclide production cross-sections at different energies and with different targets and beams. These calculations are used to estimate nuclide production rates by protons in thick targets, taking into account the energy loss and the attenuation of the proton beam in the target, as well as the low-energy fission induced by the secondary neutrons. The results are compared to the yields of isotopes of various elements obtained from different targets at CERN-ISOLDE with 600 MeV protons, and the overall extraction efficiencies are deduced. The dependence of these extraction efficiencies on the nuclide half-life follows a simple pattern in many different cases. A universal function is proposed to parameterize this behavior in a way that quantifies the essential properties of the extraction efficiency for the element and the target–ion-source system in question.

Elevation dependence of cosmogenic 36Cl production in Hawaiian lava flows

We measured an elevation profile of cosmogenic 36Cl in two well-preserved lava flows on Mauna Kea, Hawaii (19.8° N, 155.5° W) in order to directly constrain the elevation dependence of cosmogenic nuclide production rates. The flows are vertically-extensive hawaiites erupted at 40.1 ± 0.6 and 62.2 ± 1.0 ka from point-vents on the upper flanks of Mauna Kea. The average paleo cutoff rigidity (a measure of geomagnetic shielding of cosmic rays) for these flows is 11 GV and their paleo-elevation range is 2100–3700 m. Production of 36Cl is dominated by neutron reactions, with the high-energy 39K( n, x) and 40Ca( n, x) mechanisms accounting for nearly half of the 36Cl production and the low-energy reaction 35Cl( n, γ) responsible for the remaining half. Production by negative muons is small at the elevations of our samples, accounting for less than 2% of the total production in the lowest elevation samples. The elevation dependence of 36Cl production measured in these lava flows is described by an effective attenuation length of 138 ± 5 g cm − 2 . This result is close to the value of 140 g cm − 2 determined from neutron monitor surveys of high-energy nucleon fluxes, but significantly below the value of 149 g cm − 2 determined from measurements of low-energy neutrons. The predicted atmospheric attenuation length for these lava flows, incorporating both high- and low-energy mechanisms, is 144 g cm − 2 . The good agreement between the 36Cl elevation profile and cosmic-ray surveys validates the use of neutron flux measurements to scale 36Cl production rates when production by muons is negligible.

Extended scaling factors for in situ cosmogenic nuclides: New measurements at low latitude

Production rates of cosmogenic nuclides at the earth's surface are controlled by the intensity of energetic cosmic-ray nucleons, which changes rapidly with elevation. An incomplete knowledge of how nucleon fluxes vary with elevation remains a major obstacle to utilizing cosmogenic nuclides as geochronometers in applications requiring highly accurate ages. One problem is that attenuation characteristics depend on nucleon energy. Measurements of high-energy (> 50 MeV) nucleon fluxes tend to give shorter attenuation lengths than low-energy (< 1 MeV) fluxes, but these differences are not well characterized due to a lack of data at lower energies. Another problem is that the atmospheric attenuation length for nucleon fluxes varies with the geomagnetic cutoff rigidity (a parameter related to geomagnetic latitude), RC, and that there has been an incomplete mapping of nucleon fluxes at high RC (low geomagnetic latitude). We report new measurements of nucleon fluxes from altitude transects in Hawaii (RC = 12.8 GV) and Bangalore, India (RC = 17.3 GV). Our measurements in Hawaii of low-energy neutrons (median energy 1 eV) and energetic nucleons (median energy 140 MeV) confirm that nucleon scaling functions are energy-dependent in the range of energies at which cosmogenic nuclides are produced. Our measurements in southern India extend our previously reported scaling model for spallation reactions [D. Desilets, M. Zreda, Spatial and temporal distribution of secondary cosmic-ray nucleon intensity and applications to in situ cosmogenic dating. Earth Planet. Sci. Lett. 206 (2003) 21–42] from RC = 13.3 GV to RC = 17.3 GV, nearly the highest cutoff rigidity on earth. The anomalously high cutoff rigidity over India provides a geomagnetic shielding condition that is effectively the same as would be observed at the geomagnetic equator in a dipole field with an intensity 1.2 times the modern value. This makes it possible to scale low-latitude production rates to paleomagnetic fields that are stronger than the present dipole field.

Cosmogenic 3He production rates from Holocene lava flows in Iceland

We measured cosmogenic 3He production rates in olivine phenocrysts from four radiocarbon-dated postglacial basaltic lava flows in the Western Volcanic Zone of Iceland. These measurements provide important new calibrations of cosmogenic nuclide production rates near sea level at high latitudes. Calibration sites from Lambahraun (4040 ± 250 cal yr BP; n = 4), Leitahraun (5210 ± 110 cal yr BP; n = 5), Búrfellshraun (8060 ± 120 cal yr BP; n = 6), and Þingvallahraun (10,330 ± 80 cal yr BP; n = 4) yield a mean production rate of 132 ± 5 atoms 3He g − 1 yr − 1 (± 1 σ; normalized to sea level at high latitudes with the standard atmosphere). Mean production rates from the four flows agree within uncertainty, indicating that these calibrations establish a reproducible local 3He production rate that will significantly increase the accuracy of exposure dating in Iceland. The 3He production rate in Iceland is ∼ 17% higher than the mean of normalized Holocene values previously determined in the western USA. The high production rates in Iceland are attributed to the influence of persistent low atmospheric pressure over Iceland (the Icelandic Low) through the Holocene, which yielded higher cosmic ray fluxes. The Icelandic calibrations thus support previous suggestions that cosmogenic isotope production rates should be adjusted for regional variations in long-term atmospheric pressure. By extending the latitudinal extent of previous calibration studies, these results are also useful for evaluating scaling models.

Where does sediment come from? Quantifying catchment erosion with detrital apatite (U-Th)/He thermochronometry

We present a new method for tracing sediment using detrital apatite (U-Th)/He (AHe) thermochronometry, and use this to quantify the spatial distribution of catchment erosion in the eastern Sierra Nevada, California. Well-developed age-elevation relationships permit detrital AHe ages to track the elevations where sediment grains were shed from bedrock. We analyzed sediment exiting nonglaciated Inyo Creek and adjacent (formerly) glaciated Lone Pine Creek. Statistical comparison of measured AHe age probability density functions (PDFs) with predicted PDFs based on catchment hypsometries suggests that Inyo Creek is eroding uniformly, consistent with field observations of weathered hillslopes tightly coupled to the fluvial system. In contrast, significant mismatch between measured and predicted PDFs from Lone Pine Creek reveals that sediment derives primarily from the lower half of the catchment. The dearth of older ages is likely due to sediment storage in cirques and moraines and/or focused erosion at intermediate elevations, both potential consequences of glacial modification. Measured PDFs can also improve cosmogenic nuclide-based erosion rates by more accurately scaling nuclide production rates. Our results demonstrate the utility of detrital AHe thermochronometry for quantifying erosion in fluvially and glacially sculpted catchments.

Landscape development in an hyperarid sandstone environment along the margins of the Dead Sea fault: Implications from dated rock falls

In this study, we explored the spatial and temporal relations between boulders and their original in-situ locations on sandstone bedrock cliffs. Thiswas accomplished bycombining field observations withdating methods using cosmogenicisotopes ( 10 Be and 14 C) and optically stimulated luminescence (OSL). Our conclusions bear both on the landscape evolution and cliff retreat process in the hyperarid region of Timna and on the methodology of estimating exposure ages using cosmogenic isotopes. We recognize three discrete rock fall events, at 31 ka, 15 ka, and 4 ka. In this hyperarid region, the most plausible triggering mechanism for rock fall events is strong ground acceleration caused by earthquakes generated by the nearby Dead Sea fault (DSF). Our record, however, under represents the regional earthquake record implying that ongoing development of detachment cracks prior to the triggering event might be slower than the earthquake cycle. Cliff retreat rates calculated using the timing of rock fall events and estimated thickness of rock removed in each event range between 0.14 m ky � 1 and 2 m ky � 1 . When only full cycles are considered, we derive a more realistic range of 0.4 m ky � 1 to 0.7 mk y � 1 . These rates are an order of magnitude faster than the calculated rate of surface lowering in the area. We conclude that sandstone cliffs at Timna retreat through episodic rock fall events that preserve the sharp, imposing, landscape characteristic to this region and that ongoing weathering of the cliff faces is minor. A 10%–20% difference in the 10 Be concentrations in samples from matching boulder and cliff faces that have identical exposure histories and are located only a few meters apart indicates that cosmogenic nuclide production rates are sensitive to

Geomagnetic field variations and the accumulation of in-situ cosmogenic nuclides in an eroding landform

The analysis of cosmogenic nuclides in an eroding surface yields information concerning erosion rates and exposure times. Such analysis requires accurate knowledge of the nuclide production rate at the site in question. Here we develop a relatively simple analytic model for the time variation of the production rate caused by fluctuations in the geomagnetic field. This model contains three parameters, which have a clear physical meaning and which provide a convenient way to quantify information on the production rate variations. We use this model to compute the buildup of nuclides in an eroding surface, and derive an analytic formula for the nuclide concentration as a function of the exposure time and the erosion rate. Several representative applications of our results are presented.

Numerical ages for Plio-Pleistocene glacial sediment sequences by 26Al/ 10Be dating of quartz in buried paleosols

We describe a method for dating Plio-Pleistocene sediments by analysing 26Al and 10Be in quartz from buried paleosols. The method amounts to a generalization of the technique of “burial dating,” which has previously been used to date river sands deposited in caves as well as fluvial terraces. We (a) measure nuclide concentrations at multiple depths in the paleosol; (b) use the geologic context of the sample to construct an exposure/burial history (an “exposure model”) that can be used to predict the nuclide concentrations in the samples as a function of whatever unknown parameters are of geological interest (e.g., the ages of geologic units); and then (c) find the parameters in the exposure model that result in the best fit between predicted and observed nuclide concentrations. We apply the method to a paleosol developed on the uppermost till in eastern Nebraska, which is buried by several middle to late Pleistocene loess units, the ages of some of which are known independently. The lowest loess unit, whose age was not previously known, was deposited 0.58±0.12 Ma, and the till was emplaced 0.65±0.14 Ma, which agrees with existing age constraints from paleomagnetic measurements and the position of the Lava Creek ash in the regional stratigraphy. The most important source of uncertainty in these age determinations is the analytical uncertainty in 26Al and 10Be measurements; uncertainties in either the nuclide production rates or the geologic model are comparatively unimportant.

Simulation of snow shielding corrections for cosmogenic nuclide surface exposure studies

Snow cover reduces cosmogenic nuclide production rates in bedrock. Corrections for snow cover can be more than 10% in mountainous, mid-latitude regions where many glacial chronologies have been constructed using cosmogenic nuclide surface dating of landforms. Most published snow corrections use historic climate data of limited duration that are not likely to reflect adequately the full range of snow conditions over the time of exposure. We present a model for describing the impact of snow burial on long-term exposure histories of landforms. The model applies an energy balance approach to snowpack evolution and incorporates both historic and long-term climate proxy data. Attenuation of cosmogenic fast neutrons is modeled alternatively as a simple exponential decrease with increased shielding or as a thin surface layer with constant production followed by an exponential decrease with increasing depth. The choice of attenuation model has little effect on the modeled results for the Cairngorms but will have a more significant effect in regions characterized by thinner, less dense snowpacks. Spatial variability in snow cover is modeled as a function of elevation only, ignoring local variability in snow accumulation as a result of slope aspect, wind redistribution and local topography. Thus, model results reveal general spatial and temporal trends in snow shielding effects, rather than site-specific corrections. Applications to data from the Cairngorm Mountains of Scotland show that the constant-plus-exponential (CPE) production rate-depth profile reduces but does not eliminate snow-shielding effects. Under present-day conditions, snow at 900 m in the Cairngorm Mountains reduces average production rates by 6% using the CPE profile and 9% with the exponential profile (EP). Long-term climate simulations from 15.5 ka through today produce larger snow shielding effects, mainly because they predict an increased proportion of precipitation as snowfall during the Younger Dryas. At 900 m, this long-term simulation reduces average cosmogenic isotope production rates by 12% (CPE) and 14% (EP). These results indicate that snow-shielding corrections based on historic climate records may be a potential source of systematic error in midlatitude mountainous regions.

Production rates of cosmogenic nuclides in boulders

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.

Oral histories in meteoritics and planetary science: XI. Masatake Honda

Abstract— Masatake Honda majored in inorganic chemistry at the University of Tokyo and then pursued graduate studies in geochemistry. In 1943, he completed his first research project, which yielded new data on the behavior of strontium in carbonates. He then spent the next two years as a technical officer in the Japanese Imperial Navy. While on duty, he gained expertise in the important new field of ion exchange methods, which he ultimately chose as the topic for his Ph.D. thesis and then expanded into a book. In 1955, Honda traveled to Switzerland and spent a year in research laboratories at Bern and Zürich. He then joined Professor James R. Arnold at Princeton University and soon began focusing his research on cosmic‐ray produced nuclides in meteorites. Two years later, he accompanied Dr. Arnold to the University of california at La Jolla where they joined the research group of Professor Harold C. Urey. Honda developed techniques for measuring terrestrial ages of meteorites and showed that most of them have survived weathering for vastly longer periods than had been anticipated. After spending nearly eight years abroad, he returned to Japan in 1962 as a full professor at the University of Tokyo. During the Apollo missions, he performed research on cosmogenic nuclides in lunar rocks, surface soils, and deep drill cores. In the same period, he studied terrestrial histories of numerous Antarctic meteorites. In 1992, he retired from his professorship but he continues to carry on his research and to publish papers. In 1987, the Meteoritical Society presented its Leonard Medal to Masatake Honda for his pioneering work in establishing the abundances and production rates of stable, long‐lived, and short‐lived nuclides by cosmic irradiation of meteorites and lunar samples.

Spatial and temporal distribution of secondary cosmic-ray nucleon intensities and applications to in situ cosmogenic dating

Cosmogenic nuclide production rates depend critically on the spatio-temporal distribution of cosmic-ray nucleon fluxes. Since the 1950s, measurements of the altitude, latitude and solar modulation dependencies of secondary cosmic-ray fluxes have been obtained by numerous investigators. However, until recently there has been no attempt to thoroughly evaluate the large body of modern cosmic-ray literature, to explain systematic discrepancies between measurements or to put these data into a rigorous theoretical framework appropriate for cosmogenic dating. The most important parameter to be constrained is the dependence of neutron intensity on atmospheric depth. Our analysis shows that effective nucleon attenuation lengths measured with neutron monitors over altitudes 0–5000 m range from 128 to 142 g cm−2 at effective vertical cutoff rigidities of 0.5 and 14.9 GV, respectively. Effective attenuation lengths derived from thermal neutron data are somewhat higher, ranging from 134 to 155 g cm−2 at the same cutoff rigidities and over the same altitudes. We attribute the difference to a combination of two factors: the neutron monitor is more sensitive to the higher end of the nucleon energy spectrum, and the shape of the nucleon energy spectrum shifts towards lower energies with increasing atmospheric depth. We have derived separate scaling models for thermal neutron reactions and spallation reactions based on a comprehensive analysis of cosmic-ray survey data. By assuming that cosmic-ray intensity depends only on atmospheric depth and effective vertical cutoff rigidity, these models can be used to correct production rates for temporal changes in geomagnetic intensity using paleomagnetic records.

Variation in glacial erosion near the southern margin of the Laurentide Ice Sheet, south-central Wisconsin, USA: Implications for cosmogenic dating of glacial terrains

We measured the abundance of cosmo- genic 10 Be and 26 Al in 22 samples collected from five striated granite, metarhyolite, and quartzite outcrops in south-central Wiscon- sin that were covered by the late Wisconsin Laurentide Ice Sheet. In two outcrops, mea- sured nuclide abundances are consistent with the existing radiocarbon chronology of ice retreat. In three outcrops, nuclide abun- dances were up to eight times higher than predicted by the radiocarbon chronology. At these three sites, several thousand years of ice flow eroded only centimeters to deci- meters of rock, allowing a significant quan- tity of nuclides (10 5 two samples carry the equivalent of .150,000 yr of surface exposure, even though they were covered by ice during the last-glacial-maximum advance. Samples highest on the landscape or in plucked ar- eas have less inheritance than those from the lee sides of large hills or lower in the landscape. Three quartzite samples collect- ed ;10 km up-ice from the margin contain three to four times the expected nuclide abundance (10 5 to 10 6 atoms per gram of quartz). In contrast, eight other quartzite and granite samples from two outcrops .50 km up-ice from the former margin contain only 10 5 atoms of 10 Be per gram of quartz, consistent with late Pleistocene ex- posure and little, if any, nuclide inheri- tance. This relationship between glacial erosion and distance from the former ter- minus is consistent with a marginal zone of minimal subglacial erosion; the ice was ei- ther frozen to its bed there, or the ice thick- ness and duration of ice cover were less near the terminus. These data, together with simple model- ing of nuclide production by deeply pene- trating muons, suggest that many meters of rock must be removed to reduce inheri- tance to negligible levels (,1000 yr) in con- tinental terrains with low long-term erosion rates. Our results indicate that cosmogenic dating of exposed bedrock surfaces near former ice margins or in areas where ice was frozen to the bed may be uncertain, and in some cases impossible, because nu- clides are inherited from prior periods of cosmic-ray exposure. Unfortunately, striat- ed glacial outcrops that can be used to dem- onstrate most easily the assumption of ''no postglacial erosion'' are also the most likely to have undergone little glacial erosion. This finding suggests that cosmogenic- nuclide production rates based on glacially striated surfaces may include cosmogenic nuclides inherited from prior exposure.

Production of krypton and xenon isotopes in thick stony and iron targets isotropically irradiated with 1600 MeV protons

Abstract— Two spherical targets made of gabbro with a radius of 25 cm and of steel with a radius of 10 cm were irradiated isotropically with 1600 MeV protons at the SATURNE synchrotron at Laboratoire National Saturne (LNS)/CEN Saclay, in order to simulate the production of nuclides in meteorites induced by galactic cosmic‐ray protons in space. These experiments supply depth‐dependent production rate data for a wide range of radioactive and stable isotopes in up to 28 target elements. In this paper, we report results for 78Kr, 80–86Kr isotopes in Rb, Sr, Y and Zr and for 124Xe, 126Xe, 128–132Xe, 134Xe, 136Xe isotopes in Ba and La. Krypton and xenon concentrations have been measured at different depths in the spheres by using conventional mass spectrometry. Based on Monte‐Carlo techniques, theoretical production rates are calculated by folding depth‐dependent spectra of primary and secondary protons and secondary neutrons with the excitation functions of the relevant nuclear reactions. The comparison of the model calculation results with experimental data in the thick target experiments performed at LNS and previously at CERN have allowed adjustments of the poorly known excitation functions of neutron‐induced reactions. Thus, for the two experiments at SATURNE, excellent agreement is obtained between experimental and calculated production rates for most Kr and Xe isotopes in all investigated target elements. Only Xe production in Ba in the gabbro is underestimated by the calculations by ˜25%. This work validates the approach of the thin‐target model calculations of cosmogenic nuclide production rates in the attempt of modeling the interaction of galactic cosmic‐ray protons with stony and iron meteorites in space as well as with lunar samples.

Geomagnetic field variations in northern Sweden during the Holocene quantified from varved lake sediments and their implications for cosmogenic nuclide production rates

Palaeomagnetic analyses were conducted on two varved lake-sediment sequences in northern Sweden. The magnetic properties of the sediment sequences are dominated by stable single-domain magnetite with characteristics typical of bacterial magnetosomes. Alternating field demagnetization measurements indicate that the single-domain magnetite is the dominant carrier of a stable natural remanent magnetization. Temporal variations in inclination and declination were obtained from a total of four cores and the data points were stacked according to their independent calendar-year (varve) ages. Statistically significant patterns in inclination and declination form a regional palaeomagnetic secular variation (PSV) curve, which possesses features that are identical in form to the UK Holocene PSV master curve. However, the calibrated radiocarbon ages of UK features identified prior to 1500 BC are approximately 500 years older than their Swedish varve-dated equiva lents, which points to dating errors and/or drifting of the geomagnetic field. The sediments meet the uniformity criteria proposed for palaeointensity reconstruction and estimates of relative geomagnetic field intensity are calibrated against global dipole-moment compilations. A calculated nuclide production curve is derived from the reconstructed geomagnetic field intensity, which empirically demonstrates the dominant modulation of cosmogenic nuclide production by dipole-moment between 5000 BC and ad 1500.

Cosmogenic Be-10 and the Solid Earth: Studies in Geomagnetism, Subduction Zone Processes, and Active Tectonics

Since the mid-1980s, cosmogenic 10Be, with a 1.5 Myr half-life, has proven to be an extremely useful tool for studies of the solid Earth and surface processes. Measured at very low concentrations using a particle accelerator, 10Be reveals tantalizing clues to the behavior of the Earth’s geodynamo, permits “geochemical imaging” of physical and magmatic processes in subduction zones, and provides ages and uplift and incision rates essential for understanding active tectonic processes. This paper emphasizes the utility of atmospheric and in situ -produced 10Be in understanding these solid Earth processes, using igneous and sedimentary rocks, deep-sea and lacustrine sediment, and ice cores as archives. One focus of this contribution is on studies of the geodynamo and subduction zone processes that utilize 10Be produced by galactic cosmic radiation (GCR) in the atmosphere and subsequently adsorbed onto marine sediments. Following the extensive treatment of geomagnetic effects on cosmic radiation by Stormer (1955), Elsasser et al. (1956) recognized that variations in cosmogenic nuclide production rates could be used as proxies for changes in intensity of the Earth’s magnetic field. A year earlier, Peters (1955) had also proposed that 10Be may be favorable for recording Cenozoic marine sedimentation rates and other geophysical variations. High-resolution 10Be-depth profiles in marine sediments have the potential to evaluate the suggestion of asymmetric sawtooth variation in the geomagnetic field paleointensity and to assess the speculation that Milankovitch variations in Earth’s orbital parameters may influence the geodynamo. Globally coherent and systematic co-variations in paleointensity and 10Be concentrations in marine sediments and ice cores offer promise as a tool for correlating and dating marine sediments unsuited to other methods. The use of geomagnetic field paleointensity variations, derived magnetically and from cosmogenic nuclides, shows promise as an interhemispheric correlation tool at sub-Milankovitch time scales. …

On scaling cosmogenic nuclide production rates for altitude and latitude using cosmic-ray measurements

The wide use of cosmogenic nuclides for dating terrestrial landforms has prompted a renewed interest in characterizing the spatial distribution of terrestrial cosmic rays. Cosmic-ray measurements from neutron monitors, nuclear emulsions and cloud chambers have played an important role in developing new models for scaling cosmic-ray neutron intensities and, indirectly, cosmogenic production rates. Unfortunately, current scaling models overlook or misinterpret many of these data. In this paper, we describe factors that must be considered when using neutron measurements to determine scaling formulations for production rates of cosmogenic nuclides. Over the past 50 years, the overwhelming majority of nucleon flux measurements have been taken with neutron monitors. However, in order to use these data for scaling spallation reactions, the following factors must be considered: (1) sensitivity of instruments to muons and to background, (2) instrumental biases in energy sensitivity, (3) solar activity, and (4) the way of ordering cosmic-ray data in the geomagnetic field. Failure to account for these factors can result in discrepancies of as much as 7% in neutron attenuation lengths measured at the same location. This magnitude of deviation can result in an error on the order of 20% in cosmogenic production rates scaled from 4300 m to sea level. The shapes of latitude curves of nucleon flux also depend on these factors to a measurable extent, thereby causing additional uncertainties in cosmogenic production rates. The corrections proposed herein significantly improve our ability to transfer scaling formulations based on neutron measurements to scaling formulations applicable to spallation reactions, and, therefore, constitute an important advance in cosmogenic dating methodology.