There is growing interest in geochronological applications of terrestrial in situ-produced cosmogenic nuclides, with the most commonly measured being Be-10 and Al-26 in quartz. To extract and then separate these radio-nuclides from quartz and prepare them in the oxide form suitable for accelerator mass spectrometry (AMS) requires extensive and careful laboratory processing. Here we discuss the suitability of a crushed, sieved and etched, sub-aerially exposed vein quartz specimen (CoQtz-N) to act as a reference material for chemical laboratory preparation and AMS measurements. Splits of CoQtz-N were distributed to eleven target preparation laboratories. The CoQtz-N Be-10 targets were then measured at seven different AMS facilities and five of the preparation laboratories had their Al-26 targets measured at four different AMS facilities. We show that CoQtz-N splits are sufficiently homogeneous with regard to nuclide concentrations, that it has been cleaned of any atmospheric derived (i.e. meteoric) Be-10 and that it has low concentrations of the major elements that can interfere with Be and Al extraction chemistry and AMS measurements. We derive preliminary concentrations for Be-10 and Al-26 in CoQtz-N as 2.53 +/- 0.09 x 10(6) at/g and 15.6 +/- 1.6 x 10(6) at/g, respectively, at the 95% confidence limit.

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 Be-10 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 Be-10 measured in sediment from the active stream currently feeding this fan delta. The temporal trend in the Be-10 concentrations after correction for postdepositional production of Be-10 was found to be overall constant and in good agreement with the modern river Be-10 concentration. The calculated mean catchment-wide denudation rate amounts to 0.73 +/- 0.18 mm yr(-1). This fairly constant level of Be-10 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 Be-10 signatures are mixed. Therefore, presumable variations in the Be-10 signals derived from changes in denudation under a fluctuating Holocene climate are only poorly transferred to the catchment outlet and not recorded in the Be-10 record. However, despite the absence of high frequency signals, we propose that the buffered and averaged Be-10 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 Be-10 signal exported by some alpine settings that needs to be taken into account and further investigated. Copyright (C) 2016 John Wiley & Sons, Ltd.

The Marocche di Dro deposits in the lower Sarca Valley are some of the most distinctive rock avalanche deposits of the Alps. We use geomorphology and cosmogenic Cl-36 exposure dating of boulders to divide the Marocche di Dro deposits into two rock avalanche bodies; the Marocca Principale to the north and the Kas to the south. The deposits were previously undated and had been mapped as up to five different events. The largest event Marocca Principale, which comprises an estimated 1000 10(6) m(3) of predominantly Rotzo Formation limestones, occurred 5300 +/- 860 yr ago. The release area is located mainly in the alcove between Mt. Casale and Mt. Granzoline, but likely extends all the way to Mt. Brento. The Kas event took place 1080 +/- 160 yr ago with detachment below Mt. Brento. The Kas debris, with an estimated volume of 300 10(6) m(3), buried the southern third of the Marocca Principale deposit. Kas presents a barren, stark landscape dominated by house-sized Tovel Member Rotzo Formation boulders bearing distinctive chert lenses. Both the extreme relief of the rock wall (more than 1300 m) and the tectonic setting predispose the range front to massive failure. For the two events, initially translational movement likely quickly evolved into complex failure and massive collapse. Run-out across the valley of several kilometers and run-up on the opposite slope of hundreds of meters followed.A summary of all dated and large historical landslides in the Alps underlines the periods of enhanced slope activity discussed in the literature: 10-9 kyr, 5-3 kyr, and 2-1 kyr, the latter especially for the Southern Alps. No deposits of the first temporal cluster are found at Marocche di Dro. The age of Marocca Principale at 5300 +/- 860 yr suggests occurrence during the second period. Failure may have been related to the shift to wetter, colder climate at the transition from the middle to the late Holocene. Nevertheless, a seismic trigger cannot be ruled out. For the Kas rock avalanche at 1080 +/- 160 yr ago we implicate the "Middle Adige Valley" (1046 CE) earthquake as trigger. Its epicentral distance is much closer to the Sarca Valley in comparison to that of the Verona earthquake (1117 CE). (C) 2017 Elsevier Ltd. All rights reserved.

Zircon formation and modification during magmatic crystallization and high-grade metamorphism are explored using TIMS and LA-ICP-MS U–Pb geochronology, Lu–Hf isotope chemistry, trace element analysis and textural clues on zircons from the Koraput alkaline intrusion, Eastern Ghats Belt (EGB), India. The zircon host-rock is a granulite-facies nepheline syenite gneiss with an exceptionally low Zr concentration, prohibiting early magmatic Zr saturation. With zircon formation occurring at a late stage of advanced magmatic cooling, significant amounts of Zr were incorporated into biotite, nearly the only other Zr-bearing phase in the nepheline syenite gneisses. Investigated zircons experienced a multi-stage history of magmatic and metamorphic zircon growth with repeated solid-state recrystallization and partial dissolution–precipitation. These processes are recorded by complex patterns of internal zircon structures and a wide range of apparently concordant U–Pb ages between 869±7Ma and 690±1Ma. The oldest ages are interpreted to represent the timing of the emplacement of the Koraput alkaline complex, which significantly postdates the intrusion ages of most of the alkaline intrusion in the western EGB. However, Hf model ages of TDM=1.5 to 1.0Ga suggest an earlier separation of the nepheline syenite magma from its depleted mantle source, overlapping with the widespread Mesoproterozoic, rift-related alkaline magmatism in the EGB. Zircons yielding ages younger than 860Ma have most probably experienced partial resetting of their U–Pb ages during repeated and variable recrystallization events. Consistent youngest LA-ICP-MS and CA-TIMS U–Pb ages of 700–690Ma reflect a final pulse of high-grade metamorphism in the Koraput area and underline the recurrence of considerable orogenic activity in the western EGB during the Neoproterozoic. Within the nepheline syenite gneisses this final high-grade metamorphic event caused biotite breakdown, releasing sufficient Zr for local saturation and new subsolidus zircon growth along the biotite grain boundaries.

The topographic signature of a mountain belt depends on the interplay of tectonic, climatic and erosional processes, whose relative importance changes over times, while quantifying these processes and their rates at specific times remains a challenge. The eastern Andes of central Bolivia offer a natural laboratory in which such interplay has been debated. Here, we investigate the Rio Grande catchment which crosses orthogonally the eastern Andes orogen from the Eastern Cordillera into the Subandean Zone, exhibiting a catchment relief of up to 5000 m. Despite an enhanced tectonic activity in the Subandes, local relief, mean and modal slopes and channel steepness indices are largely similar compared to the Eastern Cordillera and the intervening Interandean Zone. Nevertheless, a dataset of 57 new cosmogenic 10Be and 26AI catchment wide denudation rates from the Rio Grande catchment reveals up to one order of magnitude higher denudation rates in the Subandean Zone (mean 0.8 mm/yr) compared to the upstream physiographic regions. We infer that tectonic activity in the thrusting dominated Subandean belt causes higher denudation rates based on cumulative rock uplift investigations and due to the absence of a pronounced climate gradient. Furthermore, the lower rock strength of the Subandean sedimentary units correlates with mean slopes similar to the ones of the Eastern Cordillera and Interandean Zone, highlighting the fact, that lithology and rock strength can control high denudation rates at low slopes.Low denudation rates measured at the outlet of the Rio Grande catchment (Abapo) are interpreted to be a result of a biased cosmogenic nuclide mixing that is dominated by headwater signals from the Eastern Cordillera and the Interandean zone and limited catchment sediment connectivity in the lower river reaches. Therefore, comparisons of short- (i.e., sediment yield) and millennial denudation rates require caution when postulating tectonic and/or climatic forcing without detailed studies. (C) 2015 The Authors. Published by Elsevier B.V.

To constrain the depth-dependence of in situ C-14 production we measured the cosmogenic C-14 concentration of quartz separates along a quartzite core from the Leymon High site in northwest Spain. A total of 16 quartz samples were measured over a depth range of 1-1545 cm (3-4017 g cm(-2)). The obtained C-14 profile was modeled using a neutron production rate, exponentially decreasing with depth, and a fast and negative muon production parameterized as a function of the local muon flux as derived by (Heisinger et al., 2002a, 2002b). This model yields a total negative muon capture probability of 1.72 (+0.22/-0.56) x 10(-2) and a fast muon reaction cross section of 0 (+11.8/-0.00) mu b. Rescaled to sea level high latitude using a Lal/Stone scaling scheme, these estimates yield a surface muon production rate of 3.31 (+0.43/-1.07) and 0 (+0.421-0.00) at.g(-1) yr(-1) for negative muon capture and fast muons, respectively. This is the first muon production estimate for in situ C-14 from a natural setting and is within uncertainty of the previous experimental estimates. The present contribution also provides new long-term blank and standard (PP-4, CRONUS-A & CRONUS-N) in situ C-14 data from the ETH Zurich C-14 extraction line. (C) 2015 Elsevier B.V. All rights reserved.

Abstract Determining the millennial-scale behaviour of marine-based sectors of the West Antarctic Ice Sheet (WAIS) is critical to improve predictions of the future contribution of Antarctica to sea level rise. Here high-resolution ice sheet modelling was combined with new terrestrial geological constraints (in situ 14C and 10Be analysis) to reconstruct the evolution of two major ice streams entering the Weddell Sea over 20 000 years. The results demonstrate how marked differences in ice flux at the marine margin of the expanded Antarctic ice sheet led to a major reorganization of ice streams in the Weddell Sea during the last deglaciation, resulting in the eastward migration of the Institute Ice Stream, triggering a significant regional change in ice sheet mass balance during the early to mid Holocene. The findings highlight how spatial variability in ice flow can cause marked changes in the pattern, flux and flow direction of ice streams on millennial timescales in this marine ice sheet setting. Given that this sector of the WAIS is assumed to be sensitive to ocean-forced instability and may be influenced by predicted twenty-first century ocean warming, our ability to model and predict abrupt and extensive ice stream diversions is key to a realistic assessment of future ice sheet sensitivity.

The Chauvet-Pont d'Arc Cave is one of the most important sites for the study of the earliest manifestations and development of prehistoric art at the beginning of the Upper Paleolithic. Different dating techniques have been performed thus far (AMS 14C, U/Th TIMS, 36Cl dating) to model the chronological framework of this decorated cave. The cave yielded several large charcoal fragments, which enabled the opportunity for obtaining multiple dates; thus, a First Radiocarbon Intercomparison Program (FIP) was initiated in 2004 using three charcoal pieces. The FIP demonstrated that those cross-dated samples belonged to a time period associated with the first human occupation. One of the statistical interests of an intercomparison program is to reduce the uncertainty on the sample age; thus, to further assess the accuracy of the chronological framework, the Second Intercomparison Program (SIP) involving 10 international 14C laboratories was carried out on two pieces of charcoal found inside two hearth structures of the Galerie des Mégacéros. Each laboratory used its own pretreatment and AMS facilities. In total, 21 and 22 measurements were performed, respectively, which yielded consistent results averaging ∼32 ka BP. Two strategies have currently been developed to identify statistical outliers and to deal with them; both lead to quasi-identical calibrated combined densities. Finally, the new results were compared with those of the FIP, leading to the important conclusion that five different samples from at least three different hearth structures give really tightened temporal densities, associated with one short human occupation in the Galerie des Mégacéros.

We present the improved performance of the modified in situ cosmogenic 14C extraction system at ETH Zürich. Samples are now processed faster (2 days in total) and are measured with a high analytical precision of usually <2% using the gas ion source of the MICADAS AMS facility. Measurements of the PP-4 standard sample show a good reproducibility and consistency with published values. Procedural blanks are very low at currently ∼4.0 × 104 14C atoms. Analyses of samples from a ∼300 year old rock avalanche prove that we can successfully apply in situ 14C exposure dating to very young surfaces. Additionally, we present a modified calculation scheme for in situ 14C concentrations which differs from that used for conventional radiocarbon dating. This new approach explicitly accounts for the characteristics of in situ 14C production.

We reconstruct the timing of ice flow reconfiguration and deglaciation of the Central Alpine Gotthard Pass, Switzerland, using cosmogenic Be-10 and in situ C-14 surface exposure dating. Combined with mapping of glacial erosional markers, exposure ages of bedrock surfaces reveal progressive glacier downwasting from the maximum LGM ice volume and a gradual reorganization of the paleoflow pattern with a southward migration of the ice divide. Exposure ages of similar to 16-14 ka (snow corrected) give evidence for continuous early Lateglacial ice cover and indicate that the first deglaciation was contemporaneous with the decay of the large Gschnitz glacier system. In agreement with published ages from other Alpine passes, these data support the concept of large transection glaciers that persisted in the high Alps after the breakdown of the LGM ice masses in the foreland and possibly decayed as late as the onset of the Bolling warming. A younger group of ages around similar to 12-13 ka records the timing of deglaciation following local glacier readvance during the Egesen stadial. Glacial erosional features and the distribution of exposure ages consistently imply that Egesen glaciers were of comparatively small volume and were following a topographically controlled paleoflow pattern. Dating of a boulder close to the pass elevation gives a minimum age of 11.1 +/- 0.4 ka for final deglaciation by the end of the Younger Dryas. In situ C-14 data are overall in good agreement with the Be-10 ages and confirm continuous exposure throughout the Holocene. However, in situ C-14 demonstrates that partial surface shielding, e.g. by snow, has to be incorporated in the exposure age calculations and the model of deglaciation. (C) 2013 Elsevier B.V. All rights reserved.

Catchment-wide denudation rates (CWDRs) obtained from cosmogenic nuclides are an efficient way to determine geomorphic processes quantitatively in alpine mountain ranges over Holocene time scales. These rate estimations assume steady geomorphic processes. Here we use a time series (3 yr) in the Aare catchment (central Swiss Alps) to test the impact of spatially heterogeneous stochastic sediment supply on CWDRs. Our results show that low-frequency, high-magnitude debris-flow events significantly perturb cosmogenic nuclide (Be-10, C-14) concentrations and thus CWDRs. The Be-10 concentrations decrease by a factor of two following debris-flow events, resulting in a doubling of inferred CWDRs. The variability indicates a clear time and source dependency on sediment supply, with restricted area-weighted mixing of sediment. Accordingly, in transient environments, it is critical to have an understanding of the history of geomorphic processes to derive meaningful CWDRs. We hypothesize that the size of debris flows, their connectivity with the trunk stream, and the ability of the system to sufficiently mix sediment from low- and high-order catchments control the magnitude of CWDR perturbations. We also determined in situ C-14 in a few samples. In conjunction with Be-10, these data suggest partial storage for colluvium of a few thousand years within the catchment prior to debris-flow initiation.

In the past few years a number of catchment-based studies have focused on the role of glaciers and have come to conflicting conclusions regarding the effect of the glacier on chemical weathering rates and resulting CO2 consumption. Gaining a clear picture of weathering processes in glacial regions is difficult due to dilute meltwaters, the truncated field sampling season and the influence of daily melt cycles. This work forms part of the multi-disciplinary BigLink Project which is investigating the 10.7 km granitic Damma catchment (Switzerland). The glacier has retreated rapidly, exposing fresh mineral surfaces, allowing the initial stages of granite weathering to be studied. Stream waters were sampled for six months in five different locations, in conjunction with high-resolution hydrological and meteorological measurements. Selected isotope ratios were analysed in addition to the overall chemical composition to characterise spatial and temporal variations in stream water chemistry. After correction for atmospheric inputs, daily and seasonal cycles were clearly observed in the cation concentrations. These trends were independent of dilution and indicated the mixing of at least two distinct water sources whose relative proportions changed over seasonal and daily timescales. These sources reflected differing water-rock interaction times as evinced by variable elemental ratios (e.g. Na/Ca) which were offset from bulk rock ratios. The δCa isotope ratios (measured by TIMS using a Ca-Ca double spike) were uniform and indistinguishable from bedrock samples. Thus, the dissolved Ca isotopic signature of the catchment output is that of the granitic bedrock and this indicates that Ca isotopes are not fractionated during the initial stages of granite weathering. The calculated cationic and silica fluxes are among the lowest reported from alpine glaciers and are at the low end of the range reported for non-glaciated granitic catchments. Further analyses will help to provide better constraints and improve our understanding of the chemical weathering and mixing processes that occur in glacial, granitic catchments. Short and long-term denudation rates at the Altiplano margin, La Paz region, Bolivia K. HIPPE*, F. KOBER, G. ZEILINGER, S. IVY-OCHS, P.W. KUBIK, R. WIELER Isotope Geology and Mineral Resources, ETH Zürich, 8092 Zürich, Switzerland (*correspondence: hippe@erdw.ethz.ch) Institute of Geology, ETH Zürich, 8092 Zürich, Switzerland Institute of Geology, Univ. Potsdam, D-14476 Golm, Germany Laboratory of Ion Beam Physics, ETH Zürich, 8093 Zürich, Switzerland

Extracting the tiny amounts of 14C from terrestrial quartz samples represents one challenging task in the application of in situ cosmogenic 14C analyses to quaternary geology and geomorphology. The ETH Zurich all-metal 14C extraction line is dedicated to achieve low-blank total quartz degassing and obtain pure CO2 gas after a thorough gas purification procedure. High temperatures can be reached in the extraction furnace by using a sapphire tube enclosing the platinum sample holder omitting the addition of a flux agent. Without prior graphitization the recovered CO2 is measured with the gas ion source at the MICADAS AMS system at ETH/PSI. Yield tests show a very good performance of the gas cleaning system. Consistent CO2 recovery of at least 98.8 % demonstrates that no significant amount of CO2 is lost by cryogenic trapping or passage through a Cu-Ag filled tube. Line blanks are equally encouraging. The complete extraction cycle without heating the sample combustion furnace yields at most 1.2 x 104 14C atoms, of which about 6 x 103 14C atoms are associated with the gas purification system. Heating the combustion furnace at ~1350°C for 1.5 hours results in a higher extraction blank of about 1.8 x 105 14C atoms. However, we are capable of further reducing the blank level by repeated O2 flow through the combustion furnace that acts as main contamination source. Quartz outgassing experiments will reveal the temperature needed for complete degassing of quartz. Thereupon, cosmogenic 14C will be extracted and analyzed from quartz samples with a precisely known 10Be concentration to systematically test the analytical precision and accuracy of the 14C method. Results will be presented at the meeting.