Description: Available online 22 January 2013 Publication year: 2013 Source: Quaternary Geochronology We present results of thermal neutron flux measurements in experimental granite piles that were tailored to study the effect of hydrogen-rich covers on that flux. We find that hydrogen-rich covers (polyethylene, water), used as proxies for snow, dead and/or live plant matter, increase the thermal neutron flux in an underlying rock surface significantly, as compared to the state without cover. The rock serves as the main source for thermal neutrons, the hydrogen-rich cover as a neutron reflector. In situations where the thickness of such a cover would be negligible in terms of high-energy neutron (〉10 MeV) attenuation, e.g. 2-3 cm water equivalent cover, a significant enhancement of the thermal neutron flux (factor 〉2.5±0.5) can be achieved. This increase is made up of three components ( Masarik et al., 2007 ): (1) reflected thermal neutrons (albedo neutrons), (2) moderated fast neutrons from the ground, and (3) moderated fast neutrons from the atmospheric cascade ( Masarik et al., 2007 ). The higher thermal neutron flux increases the production rates of those cosmogenic nuclides that have a significant thermal neutron production pathway ( 3 He, 36 Cl, 41 Ca). Ignoring this effect in situations where target nuclei ( 6 Li, 35 Cl, 40 Ca) are abundant will severely underestimate production rates. The effect of hydrogen-rich ground cover on the thermal neutron flux has the potential to be used for studies that are aimed at reconstructing the persistence of past plant/snow cover. Isotopic ratios of spallogenic versus predominantly thermal neutron produced nuclides, would reveal the presence or absence of hydrogen-rich cover in the past as compared to the present-day situation. Highlights ► The effects of hydrogen-rich covers on the thermal neutron flux are investigated. ► hydrogen-rich covers more than double the thermal neutron flux in the ground. ► Production rates of cosmogenic nuclides have to be corrected for hydrogen-rich cover. ► The sensitivity to hydrogen-rich covers can be used to infer the nature of groundcover in the past.

Description: Available online 28 January 2013 Publication year: 2013 Source: Quaternary Geochronology We report on the status and capabilities of the Lamont-Doherty Earth Observatory in situ 14 C extraction laboratory. In late 2006 we began, in collaboration with the AMS group at the University of Arizona, construction of a new laboratory to extract in situ cosmogenic 14 C from terrestrial silicates. Long-term measurements of the process blank over the last two years give an arithmetic mean and standard deviation of 125±43x10 3 atoms 14 C ( n =9) and show significant improvement in the number of atoms, as well as stability compared to initial measurements of the process blank. We report long-term measurements of the intercomparison material CRONUS-A, which has been developed as part of the CRONUS-Earth effort to characterize inter- and intra-laboratory variability. We interpret the standard deviation (5%) of six replicate measurements of CRONUS-A as the reproducibility of in situ 14 C extractions in our laboratory. Highlights ► Detail the development of the LDEO in situ 14 C Laboratory. ► Laboratory yields low, stable process blanks. ► Measurement of intercomparison material yields low levels of scatter, ∼5%

Description: Available online 31 January 2013 Publication year: 2013 Source: Quaternary Geochronology

Description: Available online 4 February 2013 Publication year: 2013 Source: Quaternary Geochronology

Description: Available online 1 February 2013 Publication year: 2013 Source: Quaternary Geochronology Proxy records from recently accumulated peats provide valuable information about past environmental change, but they depend on high quality chronological information to calculate rates and timing of change. However, there is uncertainty in the accuracy and consistency of the methodologies used for dating recent peats. This study compares results from Spheroidal Carbonaceous Particles (SCPs) and natural and anthropogenic fallout radionuclides ( 210 Pb, 137 Cs and 241 Am) used to date three replicate cores from three contrasting sites. Data are used to test the consistency of dating techniques within and between sites, and to assess the impact of local conditions on geochronological results. There is broad consistency in results, but there is also a significant disagreement between dates in a number of cores, both within and between sites. A relatively dry site that had been affected by past burning and erosion showed the greatest consistency between methods and replicate cores. Wetter, less degraded sites showed least consistency. Using patterns of (dis)agreement between dating techniques we assess the potential causes of dating inaccuracy. The data support previous suggestions that 210 Pb is mobile in wetter conditions, and suggests that 241 Am can be considered an increasingly valuable radionuclide. Finally, our data suggest the current estimates for SCP-based ages in the region are incorrect and require further regional calibration. Using several techniques on replicate cores from three sites in the same area has provided a more robust evaluation of the likely reliability of individual techniques and the processes that may adversely affect them. We conclude that until advances are made in understanding the processes behind the variable quality of SCP and fallout radionuclide dating, using two or more dating techniques will greatly improve understanding of the validity of a peatland chronology, especially in wetter locations. Highlights ► Multiple peat cores dated using SCPs and FRN in variable conditions to test accuracy. ► SCPs may need more local calibration. ► Dating more consistent in drier peat, supports suggestions 210 Pb is mobile in wet conditions. ► Multiple techniques are required for robust chronology in recent peat deposits.

Description: Available online 1 February 2013 Publication year: 2013 Source: Quaternary Geochronology Al Wahbah is a large (∼2.2 km diameter, ∼250 m deep) maar crater in the Harrat Kishb volcanic field in western Saudi Arabia. It cuts Proterozoic basement rocks and two Quaternary basanite lava flows, and is rimmed with an eroded tuff ring of debris from the phreatomagmatic explosion that generated the crater. A scoria cone on the northern wall of the crater was dissected by the explosion and exposes a dolerite plug that was intruded immediately prior to crater formation. The dolerite plug yields a 40 Ar/ 39 Ar age of 1.147 ± 0.004 Ma. This is the best possible estimate of the time Al Wahbah crater formed. It is a few tens of thousand years younger than the age of the lower and upper basalt flows, 1.261 ± 0.021 Ma and 1.178 ± 0.007 Ma respectively. A dolerite dyke exposed within the basement in the wall of the crater is dated at 1.886 ± 0.008 Ma. This is the most precise age so far determined for the initiation of basaltic volcanism of Harrat Kishb, and confirms that it is significantly younger than the other post-rift volcanic provinces in the region. This study provides constrains the timing of humid climatic conditions in the region and suggests that the Quaternary basaltic volcanism that stretches the length of the western side of the Arabian peninsula may prove to be useful for establishing palaeoclimatic conditions. Highlights ► We have mapped studied the geology of Al Wahbah maar crater in Saudi Arabia. ► We identify a volcanic plug that was intruded simultaneously with the phreatomagmatic explosion that formed the crater. ► We date the intrusion, and therefore time of crater formation, at 1.14 million years ago. ► We date several other volcanic rocks that provide precise constraints on the regional volcanic activity.

Description: Available online 1 February 2013 Publication year: 2013 Source: Quaternary Geochronology Basin-wide erosion rates can be determined through the analysis of in situ-produced cosmogenic nuclides. In transient landscapes, and particularly in mountain catchments, erosion and transport processes are often highly variable and consequently the calculated erosion rates can be biased. This can be due to sediment pulses and poor mixing of sediment in the stream channels. The mixing of alluvial sediment is one of the principle conditions that need to be verified in order to have reliable results. In this paper we perform a field-based test of the extent of sediment mixing for a ∼42 km 2 catchment in the Alps using concentrations of river-born 10 Be. We use this technique to assess the mechanisms and the spatio-temporal scales for the mixing of sediment derived from hillslopes and tributary channels. The results show that sediment provenance and transport, and mixing processes have a substantial impact on the 10 Be concentrations downstream of the confluence between streams and tributary channels. We also illustrate that the extent of mixing significantly depends on: the sizes of the catchments involved, the magnitude of the sediment delivery processes, the downstream distance of a sample site after a confluence, and the time since the event occurred. In particular, continuous soil creep and shallow landsliding supply high 10 Be concentration material from the hillslope, congruently increasing the 10 Be concentrations in the alluvial sediment. Contrariwise, a high frequency of mass-wasting processes or the occurrence of sporadic but large-magnitude events results in the supply of low-concentration sediment that lowers the cosmogenic nuclide concentration in the channels. The predominance of mass-wasting processes in a catchment can cause a strong bias in detrital cosmogenic nuclide concentrations, and therefore calculated erosion rates may be significantly over- or underestimated. Accordingly, it is important to sample as close as possible to the return-period of large-size sediment input events. This will lead to an erosion rate representative of the “mass-wasting signal” in case of generally high-frequency events, or the “background signal” when the event is sporadic. Our results suggest that a careful consideration of the extent of mixing of alluvial sediment is of primary importance for the correct estimation of 10 Be-based erosion rates in mountain catchments, and likewise, that erosion rates have to be interpreted cautiously when the mixing conditions are unknown or mixing has not been achieved. Highlights ► Hillslope creep yields congruent in-channel mixing. ► Sporadic events perturb cosmogenic nuclide concentrations in channels. ► High-frequency landsliding drives sustained lower concentrations. ► Sediment mixing should be confirmed both in space and time.

Description: Available online 21 December 2012 Publication year: 2012 Source: Quaternary Geochronology New Ar/Ar data for alkaline basaltic rocks on the West African island of São Tomé document a Pleistocene eruptive episode that saw the formation of cinder cones and lava flows between 0.036±0.004 and 0.86±0.02 Ma (2σ). The cinder cones trend north-northeast along the long axis of the island, interpreted as a rift zone. This rift zone appears to be the source of several lava flows that reach the coastline, including one that underlies the airport of São Tomé. Estimates of magma production rates, both from recent lava flows and from consideration of the total edifice history suggest low magma production rates (10 −4 to 10 −2 m 3 /s), compared to other ocean island settings. Assuming a background of low overall volcanic activity, it is expected that any future eruptions would most likely occur along the north-northeast rift, resulting in cinder cone and lava flow emplacements that may impact the densely populated northeast region of the island. Highlights ► Ar/Ar data for 9 samples are presented, 6 gave meaningful ages. ► Basalts from São Tomé give ages of 0.036±0.004 and 0.86±0.02 Ma (2s). ► Magma production rates suggest low activity (10 -4 to 10 -2 m 3 /s). ► Future volcanic activity may be centered on the long axis of the island. ► Future volcanic activity may generate cinder cones, ash and lava flows.

Description: December 2012 Publication year: 2012 Source: Quaternary Geochronology, Volume 13 This paper reports on the first investigations into the potential of luminescence dating to establish a chronological framework for fluvial processes in the Tisa River Valley in Vojvodina, N Serbia. A total of 18 samples were collected from a terrace exposure near Mužlja, which comprised non-fluvial deposits at the top and fluvial deposits at the base. The luminescence characteristics of sand-sized quartz and potassium feldspar grains were investigated using a single-aliquot regenerative dose (SAR) protocol. The quartz separates exhibit a complex behaviour, making this dosimeter less suited for optical dating. Investigations into the behaviour of infrared stimulated luminescence (IRSL) measured at 50 °C (IR 50 ) from K-rich feldspar extracts showed that preheating in excess of 60 s at 225 °C leads to significant sensitivity changes. The effect of IR stimulation on the thermoluminescence (TL) signal and that of preheating on IRSL intensity were examined; the results are inconclusive with respect to thermal stability, but are not inconsistent with the idea that the data do not reflect depletion of an electron trap. The IR 50 -signal measured after a preheat of 60 s at 115 °C behaves well in the SAR protocol, although it suffers from anomalous fading. Assuming that the signal is thermally stable, the fading-corrected ages suggest that deposition occurred during the end of the Late Pleniglacial and the early Late Glacial, which is consistent with geological expectations. Our observations demonstrate that IRSL dating of feldspar holds potential for establishing a chronology of fluvial dynamics of the Tisa River in N Serbia. Further methodological investigations are desirable; however, as issues relating to the stability and resetting of the IRSL signal remain to be resolved. Highlights ► We investigate the potential of luminescence dating of fluvial sediments in Serbia. ► We examined two dosimeters using a SAR protocol for equivalent dose determination. ► The quartz separates exhibit a complex behaviour, making feldspar the mineral of choice. ► We demonstrate that IRSL dating of feldspar holds potential for establishing a chronology of fluvial dynamics in Serbia. ► Calculated fading-corrected ages are consistent with geological expectations.

Description: December 2012 Publication year: 2012 Source: Quaternary Geochronology, Volume 13 The Lake Suigetsu 2006 Varved Sediment Core Project (SG06 Project) aims to contribute to the international terrestrial radiocarbon calibration model, extending it to 〉50,000 cal years BP using the new SG06 sediment record, which shows annual laminations (varves) for most of this period. For varve counting, a novel approach using high resolution X-ray fluorescence (μXRF) and X-radiography was applied, described here in detail for the first time, and applied to the Late Glacial sediments of core SG06. This new technique was carried out alongside conventional varve counting by thin-section microscopy ( Schlolaut et al., 2012 ). This dual approach allows comparison of results from the two independent counting methods on metre to sub-mm scales, enabling the identification and characterisation of differences between the techniques, and quantification of their weaknesses. Combining the results produces a more robust chronology than either counting method could produce alone. The reliability of this dual approach is demonstrated by comparison of the combined chronology with the radiocarbon dataset of SG06, calibrated with the tree-ring derived Late Glacial section of IntCal09. Highlights ► A novel approach to varve counting using high resolution XRF and X-radiography. ► Results compared and combined with those produced using thin-section microscopy. ► New dual approach produces a more robust final chronology. ► Reliability demonstrated by comparison of final chronology with 14 C dataset of SG06.