Dating of sediments from Lake Zurich (Switzerland) with210Pb and137Cs

²¹⁰Pb- and¹³⁷Cs-measurements, and varve counting have been used to date sediment cores from Lake Zurich (Switzerland). Two cores from different water depths were dated with²¹⁰Pb/²¹⁰Po and revealed sediment accumulation rates of (0.055±0.015) g·cm^?2·y^?1 and (0.09±0.03) g·cm^?2·y^?1, respectively. A comparable rate of (0.07±0.01) g·cm^?2·y^?1 has been obtained from¹³⁷Cs measurements. These rates were confirmed by annual layer (varve)-counts which lead to rates of 0.07 g·cm^?2·y^?1. Constant²¹⁰Pb activities were observed in the top 6 cm of the sediment cores. This constancy is generally explained in the literature by mixing processes caused by bioturbation and by distortion during coring operations of the uppermost water-rich fluffy sediments. However the distinct¹³⁷Cs-maxima and the regular and undisturbed varve lamination of the top sediment observed in the cores of Lake Zurich contradict this assumption. In addition, measurements of⁷Be at the water/sediment interaces proved complete sediment core recovery and mechanically undisturbed sediments. Remobilization processes are assumed to cause the observed constant²¹⁰Pb activities. Remobilization may also be the reason for an incomplete²¹⁰Pb inventory in the sediments which contain only about 50% of the fallout from atmosphere. The results of the²¹⁰Pb dating should therefore be considered with some care. If existent, varve counting represents the easiest and most reliable means for dating lake sediments.     

The half life of32Si determined from a varved Gulf of California sediment core

Excess²¹⁰Pb measurements and varve chronology were used to establish a sediment accumulation rate of 0.19 cm/yr in a 95-cm-long box core raised from the Gulf of California. Varve thickness is unchanged over the entire length of the core, indicating a constant rate of sediment accumulation. The³²Si specific activity of biogenic silica shows an exponential decrease with depth in this core. The half life of³²Si, calculated from these data and the 0.19-cm/yr sediment accumulation rate, is276 ± 32 years. As most of the silica and³²Si supplied to the Gulf of California is a result of upwelling of deep ocean water, this half life determination should be relatively insensitive to secular variations in the atmospheric supply of³²Si.     

10Be in Arctic Ocean sediments

The¹⁰Be deposition rate in two Arctic Ocean cores, ～80°N, was measured to be ≤0.6 atoms cm^?2 sec^?1, much less than the¹⁰Be deposition rate reported for other oceanic regions. Possible causes for the low¹⁰Be deposition in the Arctic are: (1) latitudinal variation in the¹⁰Be delivery rate from the stratosphere; (2) the presence of an ice cover on the Arctic, which may inhibit the transfer of atmospherically supplied¹⁰Be from sea surface to sediments; and (3) locally low scavenging efficiency of sedimentary particles for¹⁰Be in the Arctic.     

In-phase anomalies in Beryllium-10 production and palaeomagnetic field behaviour during the Iceland Basin geomagnetic excursion

Increases in the production rate of cosmogenic radionuclides associated with geomagnetic excursions have been used as global tie-points for correlation between records of past climate from marine and terrestrial archives. We have investigated the relative timing of variations in ¹⁰Be production rate and the corresponding palaeomagnetic signal during one of the largest Pleistocene excursions, the Iceland Basin (IB) event (ca. 190 kyr), as recorded in two marine sediment cores (ODP Sites 1063 and 983) with high sedimentation rates. Variations in ¹⁰Be production rate during the excursion were estimated by use of ²³⁰Th_xs normalized ¹⁰Be deposition rates and authigenic ¹⁰Be/⁹Be. Resulting ¹⁰Be production rates are compared with high-resolution records of geomagnetic field behaviour acquired from the same discrete samples. We find no evidence for a significant lock-in depth of the palaeomagnetic signal in these high sedimentation-rate cores. Apparent lock-in depths in other cores may sometimes be the result of lower sample resolution. Our results also indicate that the period of increased ¹⁰Be production during the IB excursion lasted longer and, most likely, started earlier than the corresponding palaeomagnetic anomaly, in accordance with previous observations that polarity transitions occur after periods of reduced geomagnetic field intensity prior to the transition. The lack of evidence in this study for a significant palaeomagnetic lock-in depth suggests that there is no systematic offset between the ¹⁰Be signal and palaeomagnetic anomalies associated with excursions and reversals, with significance for the global correlation of climate records from different archives.     

36Cl and53Mn in Antarctic meteorites and10Be-36Cl dating of Antarctic ice

Cosmic-ray-produced⁵³Mn (t_1/2 = 3.7 × 10⁶years) has been measured in twenty Antarctic meteorites by neutron activation analysis.³⁶Cl (t_1/2 = 3.0 × 10⁵years) has been measured in fourteen of these objects by tandem accelerator mass spectrometry. Cosmic ray exposure ages and terrestrial ages of the meteorites are calculated from these results and from rare gases.¹⁴C (t_1/2 = 5740years) and²⁶Al (t_1/2 = 7.2 × 10⁵years) data. The terrestrial ages range from 3 × 10⁴ to 5 × 10⁵ years. Many of the L3 Allan Hills chrondrites seem to be a single fall based on these results. In addition,¹⁰Be (t_1/2 = 1.6 × 10⁶years) and³⁶Cl have been measured in six Antarctic ice samples. The first measurements of¹⁰Be/³⁶Cl ratios in the ice core samples demonstrate a new dating method for ice.     

Sedimentation rates in the central lake constance determined with210Pb and137Cs

Sediment cores from central Lake Constance were dated with²¹⁰Pb and¹³⁷Cs. A sedimentation rate of (0.11±0.02) g·cm⁻²·y⁻¹ was determined with the²¹⁰Pb method.¹³⁷Cs measurements revealed sedimentation rates of (0.11±0.01) g·cm⁻²·y⁻¹ and (0.08±0.01) g·cm⁻²·y⁻¹ respectively for two different cores sampled at the same location. The lower Cs-dated value indicates incomplete core recovery and demonstrates the sensitivity of this simple dating method to small losses of material at the water/sediment interface. An unambiguous application of the¹³⁷Cs method is, therefore, only possible if complete core recovery is ensured. Sedimentation rates based on particulate matter, collected in sediment traps at various water depths, agree with the results of the radioisotope methods. Estimates of 30–125 days residence times for suspended particulate matter were calculated from⁷Be measurements.     

Effects of long soil surface residence times on apparent cosmogenic nuclide denudation rates and burial ages in the Cradle of Humankind,South Africa

In situ cosmogenic nuclides are an important tool for quantifying landscape evolution and dating fossil-bearing deposits in the Cradle of Humankind (CoH), South Africa. This technique mainly employs cosmogenic 10-Beryllium (¹⁰Be) in river sediments to estimate denudation rates and the ratio of 26-Aluminium (²⁶Al) to ¹⁰Be (²⁶Al/¹⁰Be), to constrain ages of sediment burial. Here, we use ¹⁰Be and ²⁶Al concentrations in bedrock and soil above the Rising Star Cave (the discovery site of Homo naledi) to constrain the denudation rate and the exposure history of soil on the surface. Apparent ¹⁰Be-derived denudation rates obtained from pebble- to cobble-sized clasts and coarse-sand in soil (on average 3.59 ± 0.27 m/Ma and 3.05 ± 0.25 m/Ma, respectively) are 2-3 times lower than the bedrock denudation rates (on average 9.46 ± 0.68 m/Ma). In addition, soil samples yield an average ²⁶Al/¹⁰Be ratio (5.12 ± 0.27) that is significantly lower than the surface production ratio of 6.75, which suggests complex exposure histories. These results are consistent with prolonged surface residence of up to 1.5 Ma in vertically mixed soils that are up to 3 m thick. We conclude that the ¹⁰Be concentrations accumulated in soils during the long near-surface residence times can potentially cause underestimation of single-nuclide (¹⁰Be) catchment-wide denudation rates in the CoH. Further, burial ages of cave sediment samples that consist of an amalgamation of sand-size quartz grains could be overestimated if a pre-burial ²⁶Al/¹⁰Be ratio calculated from the surface production is assumed. © 2019 John Wiley & Sons, Ltd.     

P–PINI: A cosmogenic nuclide burial dating method for landscapes undergoing non-steady erosion

Existing methods of cosmogenic nuclide burial dating perform well provided that sediment sources undergo steady rates of erosion and the samples experience continuous exposure to cosmic rays. These premises exert important limitations on the applicability of the methods. And yet, high mountain sediment sources are rife with transient processes, such as non-steady erosion by glacial quarrying and/or landsliding, or temporary cosmic-ray shielding beneath glaciers and/or sediment. As well as breaching the premises of existing burial dating methods, such processes yield samples with low nuclide abundances and variable ²⁶Al/¹⁰Be ratios that may foil both isochron and simple burial-age solutions. P–PINI (Particle-Pathway Inversion of Nuclide Inventories) is a new dating tool designed for dating the burial of sediments sourced from landscapes characterized by abrupt, non-steady erosion, discontinuous exposure, and catchments with elevation-dependent ²⁶Al/¹⁰Be production ratios. P–PINI merges a Monte Carlo simulator with established cosmogenic nuclide production equations to simulate millions of samples (¹⁰Be–²⁶Al inventories). The simulated samples are compared statistically with ¹⁰Be–²⁶Al measured in field samples to define the most probable burial age. Here, we target three published ¹⁰Be–²⁶Al datasets to demonstrate the versatility of the P–PINI model for dating fluvial and glacial sediments. (1) The first case serves as a robust validation of P–PINI. For the Pulu fluvial gravels (China), we obtain a burial age of 1.27 ± 0.10 Ma (1σ), which accords with the isochron burial age and two independent chronometers reported in Zhao et al. (2016) Quaternary Geochronology 34, 75–80. The second and third cases, however, reveal marked divergence between P–PINI and isochron-derived ages. (2) For the fluvial Nenana Gravel (USA), we obtain a minimum-limiting burial age of 4.5 ± 0.7 Ma (1σ), which is compatible with unroofing of the Alaska Range starting ∼ 6 Ma, while calling into question the Early Pleistocene isochron burial age presented in Sortor et al. (2021) Geology 49, 1473–1477. (3) For the Bünten Till (Switzerland), we obtain a limiting burial age of <204 ka (95th percentile range), which conforms with the classical notion of the most extensive glaciation in the northern Alpine Foreland assigned to the Riss glaciation (sensu marine isotope stage 6) contrary to the isochron burial age presented in Dieleman et al. (2022) Geosciences, 12, 39. Discrepancies between P–PINI and the isochron ages are rooted in the challenges posed by the diverse pre-burial ²⁶Al/¹⁰Be ratios produced under conditions characteristic of high mountain landscapes; i.e., non-steady erosion, discontinuous cosmic-ray exposure, and elevation-dependent ²⁶Al/¹⁰Be production ratios in the source region, which are incompatible with the isochron method, but easily accommodated by the stochastic design of P–PINI.     

Transport ofBe andBe in the ocean

Beryllium isotopes (¹⁰Be and⁹Be) have been measured in suspended particles of < 1 mm size collected by mid-water sediment traps deployed in the eastern Pacific at MANOP sites H (6°32′N, 92°50′W, water depth 3600 m) and M (8°50′N, 104°00′W, 3100 m). For comparison, surface sediments from box cores taken from the two sites were also studied. The concentrations of¹⁰Be and⁹Be in sediment-trap particles are about an order of magnitude smaller than those in the bottom sediments which contain about 8 × 10⁹ and 6 × 10¹⁶ atoms g⁻¹ of¹⁰Be and⁹Be, respectively. The sediment trap samples collected from 50 m off the bottom showed significant (26–63%) contributions from resuspended bottom sediments. The¹⁰Be/⁹Be ratio in trap samples varies from 3 to 20 × 10⁻⁸. The variation may partly result from varied proportion of authigenic/detrital material. The fluxes of both isotopes exhibit a very strong seasonality. The fluxes of¹⁰Be into the traps at about 1500 m are estimated as 9 × 10⁵ and 4 × 10⁵ atoms cm⁻² a⁻¹ at sites H and M respectively. These values are to be compared with the fluxes into the sediments of 4–5 × 10⁵ atoms cm⁻² a⁻¹ at both locations. Good correlations exist between¹⁰Be,⁹Be and²⁷Al indicating that the primary carrier phase(s) for the beryllium isotopes in the water column may be aluminosilicates.     

The impact of storm-triggered landslides on sediment dynamics and catchment-wide denudation rates in the southern Central Range of Taiwan following the extreme rainfall event of Typhoon Morakot

Extreme erosion events can produce large short-term sediment fluxes. Such events complicate erosion rates estimated from cosmogenic nuclide concentrations in river sediment by providing sediment with a concentration different from the long-term basin average. We present a detrital ¹⁰Be study in southern Taiwan, with multiple samples obtained in a time sequence bracketing the 2009 Typhoon Morakot, to assess the impact of landslide sediment on ¹⁰Be concentrations (N¹⁰Be) in river sediment. Sediment samples were collected from 13 major basins, two or three times over the last decade, to observe the temporal variation of N¹⁰Be. Landslide inventories with time intervals of 5–6 years were used to quantify sediment flux changes. A negative correlation between N¹⁰Be and landslide areal density indicates dilution of N¹⁰Be by landslide sediment. Denudation rates estimated from the diluted N¹⁰Be can be up to three times higher than the lowest rate derived from the same basins. Observed increases imply that, 3 years after the passage of Typhoon Morakot, fluvial channels still contain a considerable amount of sediment provided by hillslope landslides during the event. However, higher N¹⁰Be in 2016 samples indicate that the contribution from landslide sediment at the sampled grain size has decreased with time. The correlation between changes in N¹⁰Be and landslide area and volume is not strong, likely resulting from the stochastic and complex nature of sediment transport. To simultaneously evaluate the volume of landslide-derived sediment and estimate the background denudation rate, associated with less impulsive sediment supply, we constructed a sediment-mixing model with the time series of N¹⁰Be and landslide inventories. The spatial pattern of background erosion rate in southern Taiwan is consistent with the regional tectonic framework, indicating that the landscape is evolving mainly in response to the tectonic forcing, and this signal is modified, but not obscured by impulsive sediment supply. © 2019 John Wiley & Sons, Ltd.     

Atmospherically-derived radionuclides as tracers of sediment mixing and accumulation in near-shore marine and lake sediments: Evidence from7Be,210Pb,and239,240Pu

Cosmogenic⁷Be(t_1/2 = 53.3days) has been used to estimate particle-mixing rates in the upper layers of lacustrine and near-shore marine sediments. Excess²¹⁰Pb and/or^239,240Pu have provided limits on rates of sediment accumulation in these environments and indices of the efficiency of the sediments as collectors of reactive nuclides over longer time scale.In sediment cores from Long Island Sound (marine) and Lake Whitney (fresh-water)⁷Be was measurable in the top 2–3 cm. Diffusion-analog particle-mixing coefficients calculated from these data are in the range of 10^?7 cm²/s. For Long Island Sound the coefficients are lower by factors of 3–6 than those estimated from the depth distributions of excess²³⁴Th at the same stations [14]. For Lake Whitney the calculated mixing coefficient is an upper limit because of the possibility of a sampling artifact.Measurements of total (wet + dry) atmospheric deposition of⁷Be in New Haven give an average flux of 0.07 dpm/cm² day during March-November, 1977; this is equivalent to a steady-state inventory of 5.4 dpm/cm² in a perfect collector. Sediment cores from Long Island Sound contain about half this⁷Be inventory, consistent with either a mean residence time for⁷Be in the water column of about one half-life or with post-depositional loss of⁷Be from Long Island Sound sediments. The Lake Whitney cores contain about 5 dpm/cm², much nearer the atmospheric delivery. A higher inventory of⁷Be in fresh-water, as compared to marine, sediments could be due either to a shorter mean residence time for⁷Be in fresh water or to lateral transport processes in the lake or its catchment. High inventories of excess²¹⁰Pb and^239,240Pu in Lake Whitney sediments demonstrate the importance of lateral transport on longer time scales at least.     

Rates of rockwall slope erosion in the upper Bhagirathi catchment,Garhwal Himalaya

Rockwall slope erosion is defined for the upper Bhagirathi catchment using cosmogenic Beryllium-10 (¹⁰Be) concentrations in sediment from medial moraines on Gangotri glacier. Beryllium-10 concentrations range from 1.1 ± 0.2 to 2.7 ± 0.3 × 10⁴ at/g SiO₂, yielding rockwall slope erosion rates from 2.4 ± 0.4 to 6.9 ± 1.9 mm/a. Slope erosion rates are likely to have varied over space and time and responded to shifts in climate, geomorphic and/or tectonic regime throughout the late Quaternary. Geomorphic and sedimentological analyses confirm that the moraines are predominately composed of rockfall and avalanche debris mobilized from steep relief rockwall slopes via periglacial weathering processes. The glacial rockwall slope erosion affects sediment flux and storage of snow and ice at the catchment head on diurnal to millennial timescales, and more broadly influences catchment configuration and relief, glacier dynamics and microclimates. The slope erosion rates exceed the averaged catchment-wide and exhumation rates of Bhagirathi and the Garhwal region on geomorphic timescales (10³−10⁵ years), supporting the view that erosion at the headwaters can outpace the wider catchment. The ¹⁰Be concentrations of medial moraine sediment for the upper Bhagirathi catchment and the catchments of Chhota Shigri in Lahul, northern India and Baltoro glacier in Central Karakoram, Pakistan show a tentative relationship between ¹⁰Be concentration and precipitation. As such there is more rapid glacial rockwall slope erosion in the monsoon-influenced Lesser and Greater Himalaya compared to the semi-arid interior of the orogen. Rockwall slope erosion in the three study areas, and more broadly across the northwest Himalaya is likely governed by individual catchment dynamics that vary across space and time. © 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons, Ltd.     

Surface exposure dating of Holocene basalt flows and cinder cones in the Kula volcanic field (Western Turkey) using cosmogenic 3He and 10Be

The Kula volcanic field in Western Turkey comprises about 80 cinder cones and associated basaltic lava flows of Quaternary age. Based on geomorphological criteria and K-Ar dating, three eruption phases, β2–β4, were distinguished in previous studies. Human footprints in ash deposits document that the early inhabitants of Anatolia were affected by the volcanic eruptions, but the age of the footprints has been poorly constrained. Here we use ³He and ¹⁰Be exposure dating of olivine phenocrysts and quartz-bearing xenoliths to determine the age of the youngest lava flows and cinder cones. In the western part of the volcanic field, two basalt samples from a 15-km-long block lava flow yielded ³He ages of 1.5 ± 0.3 ka and 2.5 ± 0.4 ka, respectively, with the latter being in good agreement with a ¹⁰Be age of 2.4 ± 0.3 ka for an augen gneiss xenolith from the same flow. A few kilometers farther north, a metasedimentary xenolith from the top of the cinder cone Çakallar Tepe gave a ¹⁰Be age of 11.2 ± 1.1 ka, which dates the last eruption of this cone and also the human footprints in the related ash deposits. In the center of the volcanic field, a basalt sample and a metasedimentary xenolith from another cinder cone gave consistent ³He and ¹⁰Be ages of 2.6 ± 0.4 ka and 2.6 ± 0.3 ka, respectively. Two β4 lava flows in the central and eastern part of the volcanic province yielded ³He ages of 3.3 ± 0.4 ka and 0.9 ± 0.2 ka, respectively. Finally, a relatively well-preserved β3 flow gave a ³He age of ∼13 ka. Taken together, our results demonstrate that the penultimate eruption phase β3 in the Kula volcanic field continued until ∼11 ka, whereas the youngest phase β4 started less than four thousand years ago and may continue in the future.     

Constant denudation rates in a high alpine catchment for the last 6 kyrs

Terrestrial cosmogenic nuclides (TCN) have widely been used as proxies in determining denudation rates in catchments. Most studies were limited to samples from modern active streams, thus little is known about the magnitude and causes of TCN variability on millennial time scales. In this work we present a 6 kyrs long, high resolution record of ¹⁰Be concentrations (n = 18), which were measured in sediment cores from an alluvial fan delta at the outlet of the Fedoz Valley in the Swiss Alps. This record is paired with a 3‐year time series (n = 4) of ¹⁰Be measured in sediment from the active stream currently feeding this fan delta. The temporal trend in the ¹⁰Be concentrations after correction for postdepositional production of ¹⁰Be was found to be overall constant and in good agreement with the modern river ¹⁰Be concentration. The calculated mean catchment‐wide denudation rate amounts to 0.73 ± 0.18 mm yr^?1. This fairly constant level of ¹⁰Be concentrations can be caused by a constant denudation rate over time within the catchment or alternatively by a buffered signal. In this contribution we suggest that the large alluvial floodplain in the Fedoz Valley may act as an efficient buffer on Holocene time scales in which sediments with different ¹⁰Be signatures are mixed. Therefore, presumable variations in the ¹⁰Be signals derived from changes in denudation under a fluctuating Holocene climate are only poorly transferred to the catchment outlet and not recorded in the ¹⁰Be record. However, despite the absence of high frequency signals, we propose that the buffered and averaged ¹⁰Be signal could be meaningfully and faithfully interpreted in terms of long‐term catchment‐averaged denudation rate. Our study suggests that alluvial buffers play an important role in regulating the ¹⁰Be signal exported by some alpine settings that needs to be taken into account and further investigated. Copyright © 2016 John Wiley & Sons, Ltd.     

Cosmic-ray-produced53Mn in thirty-one meteorites

Cosmic-ray-produced⁵³Mn (t_½ = 3.7 × 10⁶years) has been determined by neutron activation analysis in twenty-two chondrites including three Antarctic meteorites: Yamato-7301 (j), -7305 (k) and -7304 (m).⁵³Mn was also measured in four mesosiderites, three iron meteorites, Bencubbin (unique) and Udei Station (iron with silicate inclusions). In addition, preliminary results for¹⁰Be (1.6 × 10⁶ years) were obtained in the Yamato meteorites using a low-background needle GM counter. Based on published values of rare gas ages, corrections were made for undersaturation; the average specific saturation activities of⁵³Mn were found in the range 450 ± 90dpm⁵³Mn/kg Fe in most of the chondrites and 490 ± 75dpm⁵³Mn/kg Fe in the mesosiderites. Two meteorites had extremely low contents of⁵³Mn: 102 ± 6 in Yamato-7301 and 48 ± 3dpm⁵³Mn/kg Fe in Bondoc. The Bondoc mesosiderite was already known to have a low concentration of cosmogenic radionuclides due to its large pre-atmospheric size. Several possible mechanisms are discussed to explain the low⁵³Mn activity in Yamato-7301: (1) long terrestrial age of about 7 m.y.; (2) low production rate of⁵³Mn due to heavy pre-atmospheric shielding (>70cm); (3) multi-stage irradiation history resulting in an undersaturation of⁵³Mn; and (4) a mechanism in which two or three of the above factors are combined. The ratio of⁵³Mn production rate in Ni to that in Fe has been estimated to be 1/3, based on the measurements of⁵³Mn in the metallic and silicate phases of St. Séverin meteorite, as well as on published results of some high-energy bombardment experiments.     

10Be and Th isotopes in manganese nodules and adjacent sediments: Nodule growth histories and nuclide behavior

The concentration profiles of ⁹Be, ¹⁰Be, ²³⁰Th, ²³²Th, ²³¹Pa (via ²²⁷Th) and ²³⁸U have been measured in three manganese nodules, one each from the North Pacific (A47-16(4)), the South Pacific (TF-5) and the Indian Ocean (R/V Vitiaz). In addition the ¹⁰Be concentration in deep water from the GEOSECS reoccupation station 500 of the North Pacific, and in ? cores raised from the manganese nodule field in the North Pacific have been measured. The ¹⁰Be concentration in nodule and seawater samples was measured by the accelerator masss spectrometric technique employing the Yale Tandem Van de Graaff accelerator.The concentrations of ¹⁰Be, ²³⁰Th_exc and ²³¹Pa_exc and ratios of ¹⁰Be/⁹Be and ²³⁰Th_exc/²³²Th all decrease with depth in the nodules. This decrease, interpreted in terms of nodule growth, yields “average” growth rates of a few millimeters per million years for the nodules. The growth rates of the nodules exhibit temporal variations, both on short time (～ 50,000 years) and long time (several million years) scales. Of the three nodules studied, only in TF-5 is the short-term average growth rate based on ²³⁰Th_exc in the top 0–0.5 mm the same as the long-term average rate based on Be isotope data for the 0.5–17 mm interval. For the other two nodules, the recent average growth rates based on²³⁰Th_exc data differ significantly from the long-term average growth rates based on Be isotopes. In A47-16(4) the ¹⁰Be based rate is less than the ²³⁰Th_exc rate and in R/V Vitiaz the ¹⁰Be based rate is greater than the ²³⁰Th_exc rate. This observation, coupled with measurable changes in growth rates even during the past few hundred thousand years, suggests, but does not prove, that the discordant growth rates deduced from ²³⁰Th_exc and ¹⁰Be profiles document changes in nodule growth rate with time rather than mixing effects on ²³⁰Th_exc profiles.The ¹⁰Be concentration in the GEOSECS North Pacific deep water is 6100±1200 atoms/g. This value coupled with the average surface ¹⁰Be/⁹Be ratio of North Pacific nodules predicts a ⁹Be concentration within the limits of measured values. The inventory of ¹⁰Be and ²³⁰Th_exc in the nodules is only ～ 10% of the total, the remaining being in sediments. The ¹⁰Be concentrations in the upper portions of two adjacent cores studied are nearly the same, but the deposition fluxes of both ¹⁰Be and ²³⁰Th based on ²³⁰Th dating vary by a factor of two. This difference is attributable to local redistribution of sediment at the time of deposition prior to accumulation.     

Discordance between cosmogenic nuclide concentrations in amalgamated sands and individual fluvial pebbles in an arid zone catchment

Based on cosmogenic ¹⁰Be and ²⁶Al analyses in 15 individual detrital quartz pebbles (16–21 mm) and cosmogenic ¹⁰Be in amalgamated medium sand (0.25–0.50 mm), all collected from the outlet of the upper Gaub River catchment in Namibia, quartz pebbles yield a substantially lower average denudation rate than those yielded by the amalgamated sand sample. ¹⁰Be and ²⁶Al concentrations in the 15 individual pebbles span nearly two orders of magnitude (0.22 ± 0.01 to 20.74 ± 0.52 × 10^{6 10}Be atoms g⁻¹ and 1.35 ± 0.09 to 72.76 ± 2.04 × 10^{6 26}Al atoms g⁻¹, respectively) and yield average denudation rates of ∼0.7 m Myr⁻¹ (¹⁰Be) and ∼0.9 m Myr⁻¹ (²⁶Al). In contrast, the amalgamated sand yields an average ¹⁰Be concentration of 0.77 ± 0.03 × 10⁶ atoms g⁻¹, and an associated mean denudation rate of 9.6 ± 1.1 m Myr⁻¹, an order of magnitude greater than the rates obtained for the amalgamated pebbles. The inconsistency between the ¹⁰Be and ²⁶Al in the pebbles and the ¹⁰Be in the amalgamated sand is likely due to the combined effect of differential sediment sourcing and longer sediment transport times for the pebbles compared to the sand-sized grains. The amalgamated sands leaving the catchment are an aggregate of grains originating from all quartz-bearing rocks in all parts of the catchment. Thus, the cosmogenic nuclide inventories of these sands record the overall average lowering rate of the landscape. The pebbles originate from quartz vein outcrops throughout the catchment, and the episodic erosion of the latter means that the pebbles will have higher nuclide inventories than the surrounding bedrock and soil, and therefore also higher than the amalgamated sand grains. The order-of-magnitude grain size bias observed in the Gaub has important implications for using cosmogenic nuclide abundances in depositional surfaces because in arid environments, akin to our study catchment, pebble-sized clasts yield substantially underestimated palaeo-denudation rates. Our results highlight the importance of carefully considering geomorphology and grain size when interpreting cosmogenic nuclide data in depositional surfaces.     

Cosmogenic-nuclide burial ages for Pleistocene sedimentary fill in Unaweep Canyon,Colorado, USA

We applied both single-sample and isochron methods of cosmogenic-nuclide burial dating to determine the age of the sedimentary fill in Unaweep Canyon, western Colorado, USA. This stratigraphic sequence is of interest because it documents capture and diversion of the ancestral Gunnison River by the Colorado River during late Cenozoic incision of the Colorado Plateau. Seven ²⁶Al–¹⁰Be burial ages from sedimentary infill penetrated by a borehole in central Unaweep Canyon, as well as a ²⁶Al–¹⁰Be burial isochron age formed by multiple clasts and grain-size separates in a sample from the stratigraphically lower Gateway gravels, indicate that canyon blockage, initiation of lacustrine sediment accumulation, and presumed river capture, took place 1.41 ± 0.19 Ma. Lacustrine sedimentation ceased 1.34 ± 0.13 Ma.     

High-frequency time-series of the dynamic sedimentation processes on the western shelf of the Mississippi River Delta

Multiple box cores were collected on the continental shelf in the Mississippi Deltaic Region adjacent to Southwest Pass and analyzed for particle reactive radionuclides ²³⁴Th and ⁷Be to examine seasonal sediment dynamics associated with variations of river discharge and hydrodynamics. Three stations located along a line west of Southwest Pass were cored and reoccupied in October, November, and December of 2003 and March, April, and May of 2004. High-frequency sampling (∼monthly) comparable to the short half-life of the radiotracers (²³⁴Th t_1/2=24.1 d; ⁷Be t_1/2=53.3) enabled us to isolate the relative influence that various forcing agents (river discharge, waves, currents) had on sediment inventories of ⁷Be and ²³⁴Th. In addition, the primary source of ⁷Be (fluvial) differs from ²³⁴Th (marine), providing further insight into processes affecting sediment transport and supply. Monthly ⁷Be inventories showed a significant positive relationship to river discharge (P=0.03) proximal to Southwest Pass. Sites further from Southwest Pass exhibited little to no relationship between ⁷Be inventories and river flow. At these sites, monthly ⁷Be inventories demonstrated a significant positive relationship with average wave orbital velocity (P<0.01). During our sampling period, the transport of ⁷Be-rich sediments to sites located on the middle to outer shelf were dependent on sea conditions not river discharge. Relatively high wave orbital velocities potentially allow particles to remain in suspension longer and travel further distances before initial deposition. In addition, ²³⁴Th inventories showed evidence of sediment focusing during periods of high wave orbital velocities.     

Quantifying periglacial erosion: insights on a glacial sediment budget,Matanuska Glacier,Alaska

Glacial erosion rates are estimated to be among the highest in the world. Few studies have attempted, however, to quantify the flux of sediment from the periglacial landscape to a glacier. Here, erosion rates from the nonglacial landscape above the Matanuska Glacier, Alaska are presented and compare with an 8‐yr record of proglacial suspended sediment yield. Non‐glacial lowering rates range from 1·8 ± 0·5 mm yr^?1 to 8·5 ± 3·4 mm yr^?1 from estimates of rock fall and debris‐flow fan volumes. An average erosion rate of 0·08 ± 0·04 mm yr^?1 from eight convex‐up ridge crests was determined using in situ produced cosmogenic ¹⁰Be. Extrapolating these rates, based on landscape morphometry, to the Matanuska basin (58% ice‐cover), it was found that nonglacial processes account for an annual sediment flux of 2·3 ± 1·0 × 10⁶ t. Suspended sediment data for 8 years and an assumed bedload to estimate the annual sediment yield at the Matanuska terminus to be 2·9 ± 1·0 × 10⁶ t, corresponding to an erosion rate of 1·8 ± 0·6 mm yr^?1: nonglacial sources therefore account for 80 ± 45% of the proglacial yield. A similar set of analyses were used for a small tributary sub‐basin (32% ice‐cover) to determine an erosion rate of 12·1 ± 6·9 mm yr^?1, based on proglacial sediment yield, with the nonglacial sediment flux equal to 10 ± 7% of the proglacial yield. It is suggested that erosion rates by nonglacial processes are similar to inferred subglacial rates, such that the ice‐free regions of a glaciated landscape contribute significantly to the glacial sediment budget. The similar magnitude of nonglacial and glacial rates implies that partially glaciated landscapes will respond rapidly to changes in climate and base level through a rapid nonglacial response to glacially driven incision. Copyright © 2009 John Wiley & Sons, Ltd.