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The measurement of erosion and weathering rates in different geomorphic settings and over diverse temporal and spatial scales is fundamental to the quantification of rates and patterns of earth surface processes. A knowledge of the rates of these surface processes helps one to decipher their relative contribution to landscape evolution – information that is crucial to understanding the interaction between climate, tectonics and landscape. Consequently, a wide range of techniques has been developed to determine short- ( < 10 2 a ) and long-term ( > 10 4 a ) erosion rates. However, no method is available to quantify hard rock erosion rates at centennial to millennial timescales. Here we propose a novel technique, based on the solar bleaching of luminescence signals with depth into rock surfaces, to bridge this analytical gap. We apply our technique to glacial and landslide boulders in the Eastern Pamirs, China. The calculated erosion rates from the smooth varnished surfaces of 7 out of the 8 boulders sampled in this study vary between < 0.038 ± 0.002 and 1.72 ± 0.04 mm ka − 1 (the eighth boulder gave an anomalously high erosion rate, possibly due to a recent chipping/cracking loss of surface). Given this preferential sampling of smooth surfaces, assumed to arise from grain-by-grain surface loss, we consider these rates as minimum estimates of rock surface denudation rates in the Eastern Pamirs, China. [ABSTRACT FROM AUTHOR]

Abstract: Observations of long-term West Antarctic Ice Sheet (WAIS) behavior can be used to test and constrain dynamic ice sheet models. Long-term observational constraints are however, rare. Here we present the first constraints on long-term (Miocene–Holocene) WAIS elevation from the interior of the ice sheet near the WAIS divide. We use geologic observations and measurements of cosmogenic 21Ne and 10Be in bedrock surfaces to constrain WAIS elevation variations to <160m above the present-day ice levels since 7Ma, and <110m above present-day ice levels since 5.4Ma. The cosmogenic nuclide data indicate that bedrock surfaces 35m above the present-day ice levels had near continuous exposure over the past 3.5Ma, requiring average interior WAIS elevations to have been similar to, or lower than present, since the beginning of the Pliocene warm period. We use a continental ice sheet model to simulate the history of ice cover at our sampling sites and thereby compute the expected concentration of the cosmogenic nuclides. The ice sheet model indicates that during the past 5Ma interior WAIS elevations of >65m above present-day ice levels at the Ohio Range occur only rarely during brief ice sheet highstands, consistent with the observed cosmogenic nuclide data. Furthermore, the model''s prediction that highstand elevations have increased on average since the Pliocene is in good agreement with the cosmogenic nuclide data that indicate the highest ice elevation over the past 5Ma was reached during the highstand at 11ka. Since the simulated cosmogenic nuclide concentrations derived from the model''s ice elevation history are in good agreement with our measurements, we suggest that the model''s prediction of more frequent collapsed-WAIS states and smaller WAIS volumes during the Pliocene are also correct. [Copyright &y& Elsevier]

Abstract: When calculated with the commonly accepted average Northern Hemisphere production rate, 10Be dates of surface boulders on moraines in the Lago Argentino area of Patagonia are younger than minimum-limiting 14C ages for the same landforms. This disagreement could result from the lack of a regional 10Be production-rate calibration site. To assess this possibility, we here present high-precision measurements of 10Be in samples collected from surface boulders on the Herminita and Puerto Bandera moraine complexes deposited alongside Lago Argentino on the eastern flank of the Andes at 50°S in Patagonia. Together with maximum- and minimum-limiting 14C ages for the two moraine systems, these measurements confine the local 10Be production rate to between 3.60 and 3.82atoms/g/yr (midpoint=3.71±0.11atoms/g/yr) when using a time-dependent scaling method that incorporates a high-resolution geomagnetic model. This range includes upper and lower error bounds of acceptable production rates derived from both the Herminita and the Puerto Bandera sites. The upper limit of this range is more than 12% below the average Northern Hemisphere production rate, as calculated using the same scaling method, given in Balco et al. [Quat. Geochron 3 (2008) 174–195]. Other scaling models yield production rates with similarly large offsets from the Balco et al. (2008) rate. On the other hand, the range of acceptable production rate values determined from Patagonia overlaps at 1σ with, and encompasses, the production rate recently derived in Macaulay valley in the Southern Alps of New Zealand [A. Putnam et al., Quat. Geochron. 5 (2010a) 392–409]. Within uncertainties (i.e., overlap at 1 sigma) this Patagonian production rate range also agrees with a recently determined production rate from low-elevation sites in northeastern North America and northern Norway. When the Macaulay production rate is used to calculate Patagonian exposure dates, 14C and 10Be chronologies are mutually compatible for late-glacial moraine systems. Both chronologies then indicate that outlet glaciers of the Southern Patagonian Icefield achieved a late-glacial maximum in the western reaches of Lago Argentino at 13,000cal. yr BP at the end of the Antarctic Cold Reversal (14,500–12,900cal. yr BP). Outlet glaciers subsequently receded to near present-day ice margins during the Younger Dryas stadial (12,900–11,700cal. yr BP). This general retreat was interrupted about 12,200cal. yr BP when Upsala Glacier constructed an interlobate complex of moraine ridges on Herminita Peninsula. Mountain glaciers in Patagonia and New Zealand, on both sides of the South Pacific Ocean, exhibited a coherent pattern of late-glacial ice-margin fluctuations. [Copyright &y& Elsevier]

Abstract: Knowledge of the timing and extent of past West Antarctic Ice Sheet (WAIS) elevation changes provides insight to the WAIS response to sea level and climate forcing, as well as constraints for ice sheet models. Trimlines and erratics indicate that maximum ice elevation at the Ohio Range, located near the WAIS divide, was 125m above the present ice surface and was reached at ~10ka. However, 3He and 10Be exposure ages of most granitic erratics and supra-glacial boulders in adjacent blue ice areas are older than the last highstand and provide constraints on earlier interior WAIS elevation changes. Although diffusive loss of 3He occurs in quartz, an excellent correlation between 3He and 10Be concentrations in samples up to 400ka is observed in the Ohio Range, demonstrating the utility of 3He measurements in Antarctic quartz. The correlation is used to calculate “equivalent 10Be exposure ages” from the larger 3He data set. The exposure ages occur in distinct clusters 10–20ka, 70–80ka, 130–140ka and 180–190ka that we relate to pulses in the emergence of englacial debris in localized blue ice ablation areas. A combined ice sheet/ice shelf model predicts that WAIS elevations near the Ohio Range have varied only ~125m over the last 200kyr with highstands at ~10ka and ~130ka, in good agreement with observations of trimlines and peaks in the exposure age distribution at the Ohio Range. In the model, interior WAIS elevations are controlled by the interaction of accumulation rates and ice dynamics related to ice sheet/ocean interactions at the grounding line. WAIS highstands occur early in interglacial periods during intervals of warmer temperatures and high accumulation rates, prior to the arrival of a wave of thinning. We hypothesize that the correspondence of rising regional WAIS elevation with debris pulses relates to increased ice (debris) flux and ablation in the local blue ice areas resulting from the warmer temperatures. The occurrence of clasts with exposure ages up to 200ka on the WAIS surface indicates elevation changes near the Ohio Range were limited in extent during stage 5e. The geologic observations, combined with model results, place constraints on WAIS geometry during stage 5e and limit the WAIS contribution to the higher sea levels observed during the last interglacial to ~3m. [Copyright &y& Elsevier]

Abstract: The Altai range (western Mongolia) accommodates NNE–SSW shortening across the northern India–Eurasia collision zone by dextral slip on faults trending NNW–SSE, and anticlockwise, vertical-axis rotations of fault-bounded blocks. However, fault slip-rates and the way in which faulting evolves over time are poorly understood, and form the motivation for this study. We focussed on the Har-Us-Nuur fault, a major transpressional fault bounding the eastern margin of the Altai. Three abandoned alluvial fan surfaces, each displaced right-laterally by the fault, were targeted for dating with cosmogenic 10Be and quartz optically stimulated luminescence (OSL). The first surface (A2) shows an exponential decrease in 10Be with increasing depth, with a significant inherited component. Modelling this profile yielded a minimum age of 74.1 ka. Material from the same sampling pit was dated at ~19 ka with OSL, but we consider this younger age to be incorrect, possibly due to feldspar contamination or abnormal quartz OSL characteristics. The A2 surface is displaced by 175 m, implying a (maximum) dextral slip-rate of 2.4±0.4 mm yr−1. A second fan surface (F1) was dated at ~6 ka with OSL and shows little variation in 10Be with depth, consistent with this young age. The inherited component is higher than for A2, indicating contrasting levels of inheritance for different periods of fan aggradation. A final surface (F2) shows scattered 10Be concentrations and lacks material suitable for OSL, so cannot be dated precisely. Using the total vertical displacement across the fault, we place the initiation of movement on the fault at ~2 Ma, significantly later than the late Oligocene to Miocene (28–5 Ma) onset of shortening in the Altai region. This suggests that deformation in the Altai has widened over time to incorporate new faults at the range margins (such as Har-Us-Nuur), possibly because older faults in the range interior have rotated about vertical axes into orientations that require work to be done against gravity. [Copyright &y& Elsevier]

Abstract: Surface exposure dating is a new, but already widely used tool in geomorphological studies, aiming, e.g. at the establishment of glacial chronologies. Sometimes, however, results from samples taken from one and the same moraine scatter widely and thus interpretation turns out to be difficult. Here we assess sample-specific effects (topographic shielding, sample geometry and erosion) on 17 surface exposure ages obtained by analysis of in situ produced cosmogenic 10Be in boulders from moraines in the Pamir Mountains (Tajikistan). A wide scatter of exposure ages is observed only for hummocky moraines (>45 to 10 ka BP), but not for distinct lateral moraines (∼20 and 60 ka BP, respectively). It is hypothesized that inheritance (prior exposure) and moraine degradation may be responsible for the wide scatter of exposure ages on the hummocky moraines. To proof this hypothesis, a data set of 72 boulders from the Pamir Mountains is used. We conclude that most likely hummocky moraines experienced long-lasting landform surface instability, probably due to buried glacier ice that is protected from melting several ten thousand years by a thick debris cover. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

• 3He concentrations in Miocene-age basalt decrease exponentially to a depth of ∼100 m. • Concentration-depth profile is diagnostic of 3He production by cosmic ray muons. • 3He production by muons must be considered when dating erosional events in old bedrock. Cosmic ray muons penetrate deeply into rock where they interact with atoms to produce cosmogenic nuclides. Incorporation of the muon contribution to the production rates of cosmogenic nuclides increases the accuracy of exposure dates, burial ages, and erosion rates inferred from measured nuclide concentrations. In the absence of empirical evidence, it is generally assumed that muons do not produce 3He, a cosmogenic nuclide commonly used for exposure dating. Here we assess whether muons produce 3He by measuring He isotope concentrations in pyroxene and ilmenite from a ∼300 m deep drill core and other subsurface samples of the mid-Miocene Columbia River Basalt in Washington, USA. 3He concentrations in our samples exhibit an exponential decline with depth with an e -folding length of 32.4 m, which corresponds to an attenuation length for 3He production of 8780 g cm−2. The deeply penetrating exponential is diagnostic of 3He production by cosmic ray muons. Assuming no erosion, we constrain the minimum surface muonogenic production rate to be 0.23 atom g−1 pyroxene yr−1, whereas when incorporating erosion the production rate is 0.45 atom g−1 pyroxene yr−1. 3He concentrations in samples deeper than ∼100 meters are consistent with model-based estimates of depth-independent nucleogenic production from the capture by 6Li of neutrons produced by alpha particle reactions on light elements. Measurements in other subsurface samples indicate that muon-produced 3He is prevalent across the Columbia Plateau. The penetration depth of muonogenic 3He production is substantially deeper, and the ratio of muon- to spallation-produced 3He is substantially lower, than found for other cosmogenic nuclides. Our results provide the first definitive empirical evidence for 3He production by muons, which has several implications for quantifying the timing and rates of Earth surface change and interpreting He isotope ratios. Importantly, despite the low production rates, landforms in the Channeled Scablands, which were formed by incision of the Columbia River Basalt by the late-Pleistocene Missoula floods, have high concentrations of 3He inherited from post-Miocene muon exposure. Hence 3He production by muons must be considered, particularly when dating rapid erosional events in old bedrock. Our findings indicate samples with less than several tens of meters of shielding by overlying rock will contain cosmogenic 3He that elevates 3He/4He ratios. Hence caution should be used when using 3He/4He ratios from samples at shallower depths to infer mantle sources of basalt. [ABSTRACT FROM AUTHOR]

• Two new cosmogenic isotope analyses were performed on rocks from Mount Sharp. • Low 36Ar abundance suggests recent (≲1 Ga) water-rock interactions. • These rocks likely were exposed >1 Ga, compared to <100 Ma on the crater floor. • Episodic intense erosion likely drives landscape change in Gale crater. • Ongoing erosion rates suggest Mount Sharp formed during the Hesperian. The erosion rates and mechanisms operating on Mount Sharp in Gale crater, Mars were assessed via experiments performed by the SAM instrument to determine the cosmogenic noble gas contents of Murray mudstone formation samples Mojave 2 and Quela. Previous measurements of samples from the Aeolis Palus depression between Mount Sharp and the north rim of Gale crater indicate that scarp retreat-generated surfaces formed within the last 100 Ma. In contrast, Mojave 2 yielded exposure ages of 1,320±240 (3He), 910±420 (21Ne), and 310±60 Ma (36Ar). Quela gave a 3He age of 1,460±200 Ma; 21Ne and 36Ar from this sample could not be quantified due to isobaric interferences. The discordant and young 36Ar exposure age in Mojave 2 is likely the result of interaction with water which dissolved the chlorine-bearing host phases of this nuclide. The most probable exposure scenario is that both Mojave 2 and Quela have been at the surface for the most recent ∼1 Ga after the overlying few meters of rock were removed in a geologically rapid exhumation episode. Based on local geomorphology, scarp retreat is the most likely mechanism for the exposure at these two sites. The exposure ages measured throughout Curiosity's traverse indicate that the net removal of rock has proceeded more recently on Aeolis Palus than on the lower slopes of Mount Sharp. The implied differential erosion rate is insufficient to explain how Mount Sharp formed, even over billions of years. Instead, given that the surfaces on Mount Sharp have existed for >1 Ga, the mountain must have formed early, likely during the Hesperian. This study provides direct quantitative support for inferences based on crater counts that Mount Sharp had eroded to close to its current form before onset of the Amazonian. [ABSTRACT FROM AUTHOR]

• OSL measurements of stochastic bedrock erosion are accurate over a large range. • Heavy-tailed distributions of removal length or hiatus duration are unlikely. • At high erosion rates OSL profile is sensitive to individual rock chip removal events. • True erosion rates at boulder tops are greater than or equal to ground level rates. • OSL profiles can be used to resolve erosion frequency or length-scale characteristics. Exposed bedrock is ubiquitous on terrestrial and planetary landscapes, yet little is known about the rate of granular-scale bedrock erosion on timescales longer than the instrumental record. As recently suggested, using the bleaching depth of luminescence signals as a measure of bedrock erosion may fit these scales. Yet this approach assumes constant erosion through time, a condition likely violated by the stochastic nature of erosional events. Here we simulate bedrock luminescence bleaching in response to power-law distributions of removal lengths and hiatus durations. We compare simulation results with previously measured luminescence profiles from boulder surfaces (Sohbati et al., 2018) to illustrate that prolonged hiatuses are unlikely and that typical erosion scales are sub-granular with occasional loss at mm scales, consistent with ideas about microflaws governing bedrock detachment. For a wide range of erosion rates, measurements are integrated over many removal events, producing reasonably accurate estimates despite the stochastic nature of the simulated process. We hypothesize that the greater or equal erosion rates atop large boulders compared to rates at ground level suggest that subcritical cracking may be more influential than aeolian abrasion for boulder degradation in the Eastern Pamirs, China. [ABSTRACT FROM AUTHOR]