Cosmogenic Radionuclide Concentrations Research Articles

Late Miocene or older canyon incision in the northern U.S. Cordillera shown by erosion rates, incision models, and basalt flow ages

Deep canyons along the Salmon, Snake, and Clearwater rivers in central Idaho, USA suggest long-lasting transient incision, but the timing and drivers of this incision are not well understood. The perturbation of the Yellowstone hotspot, eruption of flood basalts, and drainage of Lake Idaho all occurred within or near to this region, but the relationship among these events and incision is unclear. Here, we utilized in situ 10Be cosmogenic radionuclide concentrations for 46 samples (17 new) of fluvial sediment across the region to quantify erosion rates, calibrate stream power models, and estimate incision timing. We estimate that transient incision along the Salmon River began prior to ca. 10 Ma. However, canyon age decreases to ca. 5 Ma or earlier farther to the north. For a group of tributaries underlain by basalt, we use the age of the basalt to estimate that local transient incision began between ca. 11.5 and 5 Ma. Based on these timing constraints, the canyons along the Salmon and Clearwater rivers predate the drainage of Lake Idaho. We argue that canyon incision was triggered by events related to the Yellowstone hotspot (e.g., basalt lava damming, subsidence of the Columbia Basin, reactivation of faults, and/or lower crustal flow). Furthermore, our models suggest basalt may be more erodible than the other rock types we study. We show that lithology has a significant influence on fluvial erosion and assumptions regarding river incision model parameters significantly influence results. Finally, this study highlights how geodynamic processes can exert a significant influence on landscape evolution.

The impact of earthquakes on orogen-scale exhumation

Abstract. Individual, large thrusting earthquakes can cause hundreds to thousands of years of exhumation in a geologically instantaneous moment through landslide generation. The bedrock landslides generated are important weathering agents through the conversion of bedrock into mobile regolith. Despite this, orogen-scale records of surface uplift and exhumation, whether sedimentary or geochemical, contain little to no evidence of individual large earthquakes. We examine how earthquakes and landslides influence exhumation and surface uplift rates with a zero-dimensional numerical model, supported by observations from the 2008 Mw 7.9 Wenchuan earthquake. We also simulate the concentration of cosmogenic radionuclides within the model domain, so we can examine the timescales over which earthquake-driven changes in exhumation can be measured. Our model uses empirically constrained relationships between seismic energy release, weathering, and landsliding volumes to show that large earthquakes generate the most surface uplift, despite causing lowering of the bedrock surface. Our model suggests that when earthquakes are the dominant rock uplift process in an orogen, rapid surface uplift can occur when regolith, which limits bedrock weathering, is preserved on the mountain range. After a large earthquake, there is a lowering in concentrations of 10Be in regolith leaving the orogen, but the concentrations return to the long-term average within 103 years. The timescale of the seismically induced cosmogenic nuclide concentration signal is shorter than the averaging time of most thermochronometers (>103 years). However, our model suggests that the short-term stochastic feedbacks between weathering and exhumation produce measurable increases in cosmogenically measured exhumation rates which can be linked to earthquakes.

Estimating the disequilibrium in denudation rates due to divide migration at the scale of river basins

Abstract. Basin-averaged denudation rates may locally exhibit a wide dispersion, even in areas where the topographic steady state is supposedly achieved regionally. This dispersion is often attributed to the accuracy of the data or to some degree of natural variability of local erosion rates which can be related to stochastic processes such as landsliding. Another physical explanation of this dispersion is local and transient disequilibrium between tectonic forcing and erosion at the scale of catchments. Recent studies have shown that basin divide migration can potentially induce such perturbations, and they propose metrics to assess divide mobility based on cross-divide contrasts in headwater topographic features. Here, we use a set of landscape evolution models assuming spatially uniform uplift, rock strength and rainfall to assess the effect of divide mobility on basin-wide denudation rates. We propose using basin-averaged aggressivity metrics based on cross-divide contrasts (1) in channel χ, an integral function of position in the channel network; (2) in channel local gradient; and (3) in channel height, measured at a reference drainage area. From our simulations, we show that the metric based on differences in χ is the most reliable to diagnose local disequilibrium. The other metrics are more suitable for relatively active tectonic regions such as mountain belts, where contrasts in local gradient and elevation are more important. We find that the ratio of basin denudation associated with drainage migration to uplift can reach a factor of 2, regardless of the imposed uplift rate, erodibility, diffusivity coefficient or critical hillslope gradient. A comparison with field observations in the Great Smoky Mountains (southern Appalachians, USA) underlines the difficulty of using the metric based on χ, which depends on the – poorly constrained – elevation of the outlet of the investigated catchment. Regardless of the considered metrics, we show that observed dispersion is controlled by catchment size: a smaller basin may be more sensitive to divide migration and hence to disequilibrium. Our results thus highlight the relevance of divide stability analysis from digital elevation models as a fundamental preliminary step for basin-wide denudation rate studies based on cosmogenic radionuclide concentrations.

Cosmogenic soil production rate calculator

To understand the rates at which soils form from bedrock, it is important to know the rates at which the bedrock surface lowers (the apparent erosion rate, which is assumed to be constant). Previous models that calculate apparent erosion rates using measured concentrations of cosmogenic radionuclides rely on the assumption that the bulk density of the soil which forms as a product of bedrock erosion either equals that of the bedrock itself or is constant with depth down the soil profile. This assumption fails to recognise that soils have significantly lower densities that might not be constant with depth. The model presented here allows for the calculation of isotopically-derived soil production rates, considering the bulk density profile of the soil overlying the bedrock surface. This calculator, which can be run both in MATLAB® and GNU Octave©, represents a novel and significant contribution to the derivation of soil production rates.

Determination of 7Be, 210Pb and 22Na Activity in Air and Rainwater samples by Gamma-ray Spectroscopy

The activity concentration of cosmogenic radionuclides in atmospheric aerosol has been used in many applications, such as air mass transport models and solar activity reconstructions. One of the most studied cosmogenic radionuclides is 7Be which is being monitored at the Nuclear Engineering Department of the National Technical University of Athens (NED- NTUA) since many years, with high accuracy and precision. Sodium-22 is another cosmogenic radionuclide met in such studies, however less reported due to its very low activity concentration, of the order of 1 μBq/m3 [1], [2] – roughly four orders of magnitude lower than that of 7Be. Besides cosmogenic radionuclides, radionuclides of terrestrial origin like the 222Rn progeny 210Pb are also used in atmospheric aerosol studies. Lead-210 serves as an ideal tracer of continental air masses because 222Rn emission from oceans and lakes is negligible (<1%) compared to land sources. However, its deposition exhibits geographic and seasonal variations depending on 226Ra content of the top soils, soil-porosity and meteorology of a particular region [3]. Values reported for 210Pb activity in the air range from 0.1 to 3.4 mBq/m3 [4]. During this research, an investigation regarding the detection of 210Pb and 22Na in atmospheric aerosol and the determination of 7Be/22Na ratio has been conducted at NED-NTUA. Sampling and analysis methods were appropriately selected to allow for the detection of all three nuclides in aerosol samples. In order to study the role of precipitation in surface air 7Be, 210Pb and 22Na activity concentration, a procedure for collecting and analyzing rainwater was also developed. From the results obtained so far it is concluded that with the sampling and analysis procedures followed, 210Pb can be easily determined with reasonable accuracy. As far as 22Na is concerned, it was concluded that it can be detected although with high uncertainty, since its activity concentration is within the limits of detection.

Late Quaternary evolution of the Kumkol Basin at the northeastern margin of the Tibetan Plateau revealed by tectonic geomorphology and the analysis of in situ cosmogenic nuclides

The Tibetan Plateau was formed by the collision between the Indian and Eurasian plates, and it continues to grow laterally while keeping its elevation constant. The mechanism of the lateral expansion at the northeastern margin of the plateau, however, is highly debated owing to the scarcity of tectonic research. We conducted tectonic geomorphological analyses in the Kumkol Basin, where lateral growth is ongoing. This basin encloses a large active anticlinorium, the Kumkol Anticlinorium (KA), which would provide a quantitative constraint on the lateral growth of the plateau if the tectonically deformed landforms were dated. Our geomorphological mapping revealed six terrace steps, designated T1 to T6 in descending order, along the Pitileke River. In situ cosmogenic radionuclide concentrations in subsurface and surface samples from T1, T2, T3, T4, and T6 showed their formation ages to be 252 ± 24 ka, 140–160 ka, 103 ± 23 ka, 98 ± 11 ka, and 0–23 ka, respectively. Thus, T1 formed during marine isotope stage (MIS) 8, T2 during MIS 6, and T3 and T4 during MIS 5. Our results are quantitative evidence that degradation–aggradation cycles in the basin are correlated with global climatic fluctuations. To estimate the geometry of faults contributing to the evolution of the anticlinorium, we used a dislocation fault model and the overall surface deformation across the KA. The results suggest that the KA has been produced principally by slip on a detachment fault at 15 to 10 km depth, which is within or just above the brittle–ductile transition zone. We then determined the maximum uplift rate to be ~1 mm/yr and the horizontal shortening rate to be 2.5–3.2 mm/yr from the formation ages of the terraces and their deformation. These results suggest that there is a large-scale detachment fault below a basement-involved fold-and-thrust belt and that the concentration of stress at the plateau margin likely facilitated the development of crustal-scale deformation underlying the Kumkol Basin, and this deformation then accelerated the lateral growth of the Tibetan Plateau.

Approximately 2400-Year Cycle in the Concentration of Cosmogenic Radionuclides: Sources of Variations

Cosmogenic isotopes, including 14C, 10Be, and 7Be, are produced in the Earth’s atmosphere under the effect of cosmic rays. The rate of their production is determined by several factors, such as the intensity of primary galactic cosmic rays, the level of solar activity, and the strength of the Earth’s magnetic field. Changes in the isotope concentrations and distributions receive contributions from mixing processes proceeding in the surrounding medium: the atmosphere, biosphere, and oceans. The isotopes 14C and 10Be are the most important for studying solar activity and climate. Investigation of isotope concentrations reveal that there are both long-term trends and cyclic components. As for 14C, the long-term component caused by the change in the magnetic dipole moment of the Earth with a characteristic time of about 104 years is the most commonly known. It is well known that the concentrations of cosmogenic isotopes change cyclically with time. The ~2400-year cycle (Hallstatt cycle) and the ~210-year cycle (de Vries cycle) are the most famous. In the present article, we discuss the possible origin of the ~2400-year cycle.

10Be-inferred paleo-denudation rates imply that the mid-Miocene western central Andes eroded as slowly as today

Terrestrial cosmogenic nuclide concentrations of detrital minerals yield catchment-wide rates at which hillslopes erode. These estimates are commonly used to infer millennial scale denudation patterns and to identify the main controls on mass-balance and landscape evolution at orogenic scale. The same approach can be applied to minerals preserved in stratigraphic records of rivers, although extracting reliable paleo-denudation rates from Ma-old archives can be limited by the target nuclide’s half-life and by exposure to cosmic radiations after deposition. Slowly eroding landscapes, however, are characterized by the highest cosmogenic radionuclide concentrations; a condition that potentially allows pushing the method’s limits further back in time, provided that independent constraints on the geological evolution are available. Here, we report 13–10 million-year-old paleo-denudation rates from northernmost Chile, the oldest 10Be-inferred rates ever reported. We find that at 13–10 Ma the western Andean Altiplano has been eroding at 1–10 m/Ma, consistent with modern paces in the same setting, and it experienced a period with rates above 10 m/Ma at ~11 Ma. We suggest that the background tectono-geomorphic state of the western margin of the Altiplano has remained stable since the mid-Miocene, whereas intensified runoff since ~11 Ma might explain the transient increase in denudation.

Cosmic‐ray exposure ages of six chondritic Almahata Sitta fragments

AbstractThe Almahata Sitta strewn field is dominated by ureilites, but contains a large fraction of chondritic fragments of various types. We analyzed stable isotopes of He, Ne, Ar, Kr, and Xe, and the cosmogenic radionuclides 10Be, 26Al, and 36Cl in six chondritic Almahata Sitta fragments (EL6 breccia, EL6, EL3‐5, CB, LL4/5, R‐like). The cosmic‐ray exposure (CRE) ages of five of the six samples have an average of 19.2 ± 3.3 Ma, close to the average of 19.5 ± 2.5 Ma for four ureilites. The cosmogenic radionuclide concentrations in the chondrites indicate a preatmospheric size consistent with Almahata Sitta. This corroborates that Almahata Sitta chondrite samples were part of the same asteroid as the ureilites. However, MS‐179 has a lower CRE age of 11.0 ± 1.4 Ma. Further analysis of short‐lived radionuclides in fragment MS‐179 showed that it fell around the same time, and from an object of similar size as Almahata Sitta, making it almost certain that MS‐179 is an Almahata Sitta fragment. Instead, its low CRE age could be due to gas loss, chemical heterogeneity that may have led to an erroneous 21Ne production‐rate, or, perhaps most likely, MS‐179 could represent the true 4π exposure age of Almahata Sitta (or an upper limit thereof), while all other samples analyzed so far experienced exposure on the parent body of similar lengths. Finally, MS‐179 had an extraordinarily high activity of neutron‐capture 36Cl, ~600 dpm kg−1, the highest activity observed in any meteorite to date, related to a high abundance of the Cl‐bearing mineral lawrencite.

Isotopic Evidence for Multi-stage Cosmic-ray Exposure Histories of Lunar Meteorites: Long Residence on the Moon and Short Transition to the Earth

Abstract It is known that most lunar meteorites have complicated cosmic-ray exposure experiences on the Moon and in space. In this study, cosmic-ray irradiation histories of six lunar meteorites, Dhofar 489, Northwest Africa 032 (NWA 032), NWA 479, NWA 482, NWA 2995, and NWA 5000, were characterized from neutron-captured isotopic shifts of Sm and Gd, and from the abundances of long-lived cosmogenic radionuclides like 10Be, 26Al, 36Cl, and 41Ca. Sm and Gd isotopic data of all of six meteorites show significant isotopic shifts of 149Sm–150Sm and 157Gd–158Gd caused by accumulation of neutron capture reactions due to cosmic-ray irradiation, corresponding to the neutron fluences of (1.3–9.6) × 1016 n cm−2. In particular, very large Sm and Gd isotopic shifts of NWA 482 are over those of a lunar regolith 70002, having the largest isotopic shifts among the Apollo regolith samples, corresponding to cosmic-ray exposure duration over 800 million years in the lunar surface (2π irradiation). Meanwhile, the concentrations of cosmogenic radionuclides for individual six meteorites show the short irradiation time less than one million years as their bodies in space (4π irradiation). Our data also support the results of previous studies, revealing that most of lunar meteorites have long exposure ages at shallow depths on the Moon and short transit times from the Moon to the Earth.

A practical tool for examining paleoerosion rates from sedimentary deposits using cosmogenic radionuclides: Examples from hypothetical scenarios and data

AbstractWe provide a MatlabTM‐based algorithm that calculates paleoerosion rates based on measured Cosmogenic Radionuclide (CRN) concentrations from sedimentary deposits and additional geological constraints provided by the user. Based on the input data, the algorithm models CRN concentrations accumulated due to sediment burial and the subsequent exhumation of these deposits. Description of the methods and applications of the algorithm are provided, which includes single or coupled CRNs (i.e., 10Be or 10Be + 26Al). Results from the literature are reproduced. Further applicability of this algorithm is demonstrated with hypothetical scenarios of paleoerosion rates. The tool can be used toward the exploration of necessary assumptions, feasibility of case‐studies, and method limits. This contribution accompanies an overview of the paleoerosion rate method, its applications, and necessary assumptions.

LONG-TERM VARIATION IN THE SUN’S ACTIVITY CAUSED BY MAGNETIC ROSSBY WAVES IN THE TACHOCLINE

Long-term records of sunspot number and concentrations of cosmogenic radionuclides (10Be and 14C) on the Earth reveal the variation of the Sun's magnetic activity over hundreds and thousands of years. We identify several clear periods in sunspot, 10Be, and 14C data as 1000, 500, 350, 200 and 100 years. We found that the periods of the first five spherical harmonics of the slow magnetic Rossby mode in the presence of a steady toroidal magnetic field of 1200-1300 G in the lower tachocline are in perfect agreement with the time scales of observed variations. The steady toroidal magnetic field can be generated in the lower tachocline either due to the steady dynamo magnetic field for low magnetic diffusivity or due to the action of the latitudinal differential rotation on the weak poloidal primordial magnetic field, which penetrates from the radiative interior. The slow magnetic Rossby waves lead to variations of the steady toroidal magnetic field in the lower tachocline, which modulate the dynamo magnetic field and consequently the solar cycle strength. This result constitutes a key point for long-term prediction of the cycle strength. According to our model, the next deep minimum in solar activity is expected during the first half of this century.

Quantifying modern erosion rates and river-sediment contamination in the Bolivian Andes

We use petrographic, mineralogical and geochemical data on modern river sediments of the Tupiza basin in the Bolivian Andes to investigate the relationships among human activity, heavy-metal contamination of sediments and modern erosion rates in mountain fluvial systems. Forward mixing model was used to quantify the relative contributions from each main tributary to total sediment load of the Tupiza River. The absolute sediment load was estimated by using the Pacific Southwest Inter Agency Committee model (PSIAC, 1968) after two years of geological field surveys (2009; 2010), together with data obtained from the Instituto Nacional del Agua public authority (INA, 2007), and suspended-load data from Aalto et al. (2006).Our results indicate that the sediment yield in the drainage basin is 910 ± 752 ton/km2year and the mean erosion rate is 0.40 ± 0.33 mm/year. These values compare well with erosion rates measured by Insel et al. (2010) using 10Be cosmogenic radionuclide concentrations in Bolivian river sediments. More than 40% of the Tupiza river load is produced in the upper part of the catchment, where highly tectonized and weathered rocks are exposed and coupled with sporadic land cover and intense human activity (mines). In the Rio Chilco basin strong erosion of upland valleys produce an increase of erosion (∼10 mm/year) and the influx of large amounts of sediment by mass wasting processes. The main floodplain of the Tupiza catchment represents a significant storage site for the heavy metals (∼657 ton/year). Fluvial sediments contain zinc, lead, vanadium, chromium, arsenic and nickel. Since the residence time of these contaminants in the alluvial plain may be more than 100 years, they may represent a potential source of pollution for human health.

Using cosmogenic nuclides to quantify pebble abrasion in rivers

Sediment transport drives channel erosion and shaping of rivers, so understanding sediment dynamics is a key factor in understanding landscape evolution. Rates of river bedrock erosion depend on the properties of sediment transport, including the grain size distribution of transported sediment and the speed at which sediment moves. However, scientists' knowledge of sediment transport in rivers over long time periods or long distances is limited because there are few methods of observing grain transport over long distances or time scales. Carretier and Regard show that the concentrations of cosmogenic radionuclides in pebbles vary during river transport.

Jesenice—A new meteorite fall from Slovenia

Abstract– On April 9, 2009, at 3:00 CEST, a very bright fireball appeared over Carinthia and the Karavanke Mountains. The meteoroid entered the atmosphere at a very steep angle and disintegrated into a large number of objects. Two main objects were seen as separate fireballs up to an altitude of approximately 5 km, and witnesses reported loud explosions. Three stones were found with a total weight of approximately 3.611 kg. The measured activity of short‐lived cosmogenic radionuclides clearly indicates that two specimens result from a very recent meteorite fall. All cosmogenic radionuclide concentrations suggest a rather small preatmospheric radius of <20 cm; a nominal cosmic‐ray exposure age based on 21Ne is approximately 4 Ma, but the noble gas and radionuclide results in combination indicate a complex irradiation. Jesenice is a highly recrystallized rock with only a few relic chondrules visible in hand specimen and thin section. The texture, the large grain size of plagioclase, and the homogeneous compositions of olivines and pyroxenes clearly indicate that Jesenice is a L6 chondrite. The bulk composition of Jesenice is very close to the published average element concentration for L ordinary chondrites. The chondrite is weakly shocked (S3) as indicated by the undulatory extinction in olivine and plagioclase and the presence of planar fractures in olivine. Being weakly shocked and with gas retention ages of >1.7 Ga (4He) and approximately 4.3 Ga (40Ar), Jesenice seems not to have been strongly affected by the catastrophic disruption of the L‐chondrite parent body approximately 500 Ma ago.

Reorganization of ice sheet flow patterns in Arctic Canada and the mid‐Pleistocene transition

Evidence for the evolution of Laurentide Ice Sheet (LIS) basal thermal regime patterns during successive glaciations is poorly preserved in the geologic record. Here we explore a new approach to constrain the distribution of cold‐based ice across central Baffin Island in the eastern Canadian Arctic over many glacial‐interglacial cycles by combining till geochemistry and cosmogenic radionuclide (CRN) data. Parts of the landscaped with geomorphic evidence for limited glacial erosion are covered by till characterized by high chemical index of alteration (CIA) values and CRN concentrations requiring complicated burial‐exposure histories. Till from regions scoured by glacial erosion have CIA values indistinguishable from local bedrock and CRN concentrations that can be explained by simple exposure following deglaciation. CRN modeling results based on these constraints suggest that the weathered tills were deposited by 1.9 to 1.2 Ma, and by that time the fiorded Baffin Island coastline must have developed close to its modern configuration as piracy of ice flow by the most efficient fiord systems resulted in a major shift in the basal thermal regime across the northeastern LIS. The resultant concentration of ice flow in fewer outlet systems may help explain the cause of the mid‐Pleistocene transition from 41‐ to 100‐kyr glacial cycles.

Constraining the evolution of river terraces with integrated OSL and cosmogenic nuclide data

We present a framework for consistent incorporation of optically-stimulated luminescence dates with cosmogenic radionuclide concentrations measured in a single alluvial section, and apply it on two late Quaternary terraces in the Negev Desert, Israel, to derive constraints on their evolution. We solve an integrated, self-consistent and co-dependant set of equations to reproduce either dataset based on three model parameters of (i) uniform cosmogenic inheritance, (ii) constant terrace aggradation rate and (iii) subsequent exposure time. A subset of all possible parameter combinations, which approximates the measurements within the uncertainties of their best-fits, yields a better constraint of the alluvial history than that obtained by any of these dating proxies alone. The derived set of common solutions indicates similar paleo-inheritances of ∼1 × 10 5 10Be atoms g −1 in both terraces; progressive deposition of the higher terrace (T2) began at ∼250 ka, lasted for a period of ∼80 ka, was abandoned at ∼170 ka and since that time has remained intact; the lower terrace (T1) was most likely deposited at ∼3 ka and rapidly abandoned by ∼2 ka. In an alternative interpretation for T2, which treats the two geochronometers separately, this terrace might have experienced discrete events of aggradation at (216 ± 13) ka and deep truncation at 118 − 29 + 42 ka, significantly separated in time. In the currently active channel of the Neqarot canyon, contemporary stripping of the lowest terrace located along the stream is suggested by decreasing values of 10Be downstream along the river channel.

Numerical modeling of cosmogenic deglaciation records, Front Range and San Juan mountains, Colorado

We use cosmogenic radionuclide (CRN) exposure ages from polished, striated bedrock to constrain numerical simulations of deglaciation in the Middle Boulder Creek Valley, Colorado Front Range, and the Animas River Valley, San Juan Mountains, Colorado. In both valleys, the cosmogenic ages suggest initiation of deglaciation ∼20 ka and ongoing retreat until 12–13 ka. While the first‐order trend in CRN concentrations in each valley suggests a monotonic glacial retreat, we evaluate other retreat scenarios with different implications for post‐Last Glacial Maximum regional climate. We use a 2‐D numerical glacier simulation with a CRN layer to investigate how CRN‐based deglaciation records are affected by retreat histories that are punctuated by periods of glacier readvance. The CRN layer simulates both production during periods of exposure and reduction by glacial erosion during readvances. We simulate glacial occupation of the valleys as they respond to equilibrium line altitude (ELA) histories characterized by stepwise change, gradual rise, or a rise punctuated by short periods of lowering. Each scenario generates a distinct spatial pattern of concentrations in the CRN layer. These results and the spatial pattern of measured concentrations in bedrock constrain the range of ELA histories that reproduce the CRN pattern in each valley. In the Animas River Valley, the exposure ages are well explained by a linear ELA rise from full glacial to deglacial conditions. Ages in Middle Boulder Creek Valley are best explained by a deglaciation history including a stillstand or partial readvance between 16 and 14 ka, followed by rapid retreat.

Numerical and analytical models of cosmogenic radionuclide dynamics in landslide‐dominated drainage basins

In many tectonically active regions on Earth, landslides dominate sediment delivery to channels. While cosmogenic radionuclides (CRN) represent a valuable tool to document basin‐averaged erosion rates, a better understanding of CRN dynamics in these systems is required before erosion rates can be reliably inferred from CRN concentrations. The stochastic nature of landslides results in variability of CRN concentrations in both alluvial and hillslope samples that is both difficult to document and poorly understood. To guide sampling strategies, we developed a 2‐D plan view model to simulate the influence of landslides on CRN dynamics. We employ a sediment mixing model, in which concentrations of CRNs from material eroded by both landslides and steady processes are combined to evaluate how best to infer erosion rates from CRN concentrations. This model is further used to test a new analytical solution for the distribution of CRN concentrations on hillslopes in a landslide‐prone system. Results suggest that for landscapes dominated by deep (>5 m) landslides, only large drainage basins (≥100 km2) provide consistently reliable estimates of erosion rates from CRN concentrations. For landscapes dominated by shallow landslides (0.7–2 m), smaller basins can produce reliable estimates. Results further suggest that the distribution of concentrations from hillslope samples could be used to deduce the relative contribution of material derived from landslide and steady processes. These findings underscore the need to consider carefully the sources of natural variability in sampling and analyzing CRNs; if properly done, the variability can reveal useful information about geomorphic processes in landslide‐prone areas.

The complex exposure histories of the Pitts and Horse Creek iron meteorites: Implications for meteorite delivery models

Abstract— The concentrations of cosmogenic radionuclides and noble gases in Pitts (IAB) and Horse Creek (ungrouped) provide unambiguous evidence that both irons have a complex exposure history with a first‐stage irradiation of 100–600 Myr under high shielding, followed by a second‐stage exposure of ˜1 Myr as small objects. The first‐stage exposure ages of ˜100 Myr for Horse Creek and ˜600 Myr for Pitts are similar to cosmic‐ray exposure ages of other iron meteorites, and most likely represent the Yarkovsky orbital drift times of irons from their parent bodies in the main asteroid belt to one of the nearby chaotic resonance zones. The short second‐stage exposure ages indicate that collisional debris from recent impact events on their precursor objects was quickly delivered to Earth. The short delivery times suggests that the recent collision events occurred while the precursor objects of Horse Creek and Pitts were either very close to the chaotic resonance zones or already in Earth‐crossing orbits. Since the cosmogenic noble gas records of Horse Creek and Pitts indicate a minimum radius of a few meters for the precursor objects, but do not exclude km‐sized objects, we conclude that these irons may represent fragments of two near‐Earth asteroids, 3103 Eger and 1986 DA, respectively. Finally, we used the cosmogenic nuclide concentrations in Horse Creek, which contains 2.5 wt% Si, to test current model calculations for the production of cosmogenic 10Be, 26Al, and neonisotopes from iron, nickel, and silicon.