Beschreibung: Publication date: Available online 24 December 2016 Source: Quaternary Geochronology Author(s): Matthew A. Kosnik, Quan Hua, Darrell S. Kaufman, Michal Kowalewski, Katherine Whitacre Amino acid racemization (AAR) is widely used as a cost-effective method to date molluscs in time-averaging and taphonomic studies, but it has not been attempted for echinoderms despite their paleobiological importance and distinct biomineralization. Here we demonstrate the applicability of AAR geochronology for dating Holocene Peronella peronii (Echinodermata: Echinoidea) collected from Sydney Harbour (Australia). Using standard HPLC methods we determined the extent of AAR in 74 Peronella tests and performed replicate analyses on 23 tests. Replicate analyses from the outer edge of 23 tests spanning the observed range of D/L values yielded median coefficients of variation 〈4% for Asp, Glu, and Phe D/L values, which is comparable to the analytical precision. Correlations between THAA D/L values across 178 independently prepared subsamples of 74 individuals are also very high (Spearman ρ ≥ 0.95) for these three amino acids. The ages of 20 individuals spanning the observed range of D/L values were determined using 14 C analyses, and Bayesian model averaging was used to determine the best AAR age model. Only three models fit to Glu D/L contributed to the final averaged age model. Modeled ages ranged from 14 to 5496 years, and the median 95% confidence interval for the 54 AAR ages was ±29% of the modelled age. In comparison, the median 2σ confidence interval for nine graphite target 14 C ages was ±8% of the median age estimate and the median 2σ confidence interval for 20 carbonate target 14 C ages was ±26% of the median age estimate. Overall Peronella yield high-quality D/L values and appear to be a good target for AAR geochronology.

Beschreibung: Publication date: Available online 3 December 2016 Source: Quaternary Geochronology Author(s): L.C.P. Martin, P.-H. Blard, G. Balco, J. Lavé, R. Delunel, N. Lifton, V. Laurent Over the last decades, cosmogenic exposure dating has permitted major advances in many fields of Earth surface sciences and particularly in paleoglaciology. Yet, exposure age calculation remains a complicated and dense procedure. It requires numerous choices of parameterization and the use of an accurate production rate. This study describes the CREp program and the ICE-D production rate online database. This system is designed so that the CREp calculator will automatically reflect the current state of this global calibration database production rate, ICE-D. ICE-D will be regularly updated in order to incorporate new calibration data and reflect the current state of the available literature. CREp is a Octave/Matlab © online code that computes Cosmic Ray Exposure (CRE) ages for 3 He and 10 Be, available at crep.crpg.cnrs-nancy.fr. A stand-alone version of the CREp code is also released with the present article. Note however that only the online version is connected to the online database ICE-D. The CREp program offers the possibility to calculate ages with two scaling models: i.e. the empirical Lal-Stone time-dependent model (Balco et al., 2008; Lal, 1991; Stone, 2000) with the muon parameters of Braucher et al. (2011), and the Lifton-Sato-Dunai (LSD) theoretical model (Lifton et al., 2014). The default atmosphere model is the ERA-40 database (Uppala et al., 2005), but one may also use the standard atmosphere for comparison (N.O.A.A, 1976) to apply the atmospheric correction. To perform the time-dependent correction, users may import their own geomagnetic database for paleomagnetic corrections or opt for one of the three proposed datasets (Lifton, 2016; Lifton et al., 2014; Muscheler et al., 2005). For the important choice of the production rate, CREp is linked to a database of production rate calibration data that is part of the ICE-D (Informal Cosmogenic-nuclide Exposure-age Database) project ( http://calibration.ice-d.org ). This database includes published empirical calibration rate studies that are publicly available at present, comprising those of the CRONUS-Earth and CRONUS-EU projects, as well as studies from other projects. In the present study, the efficacy of the different scaling models has also been evaluated looking at the statistical dispersion of the computed Sea Level High Latitude (SLHL) production rates. Lal/Stone and LSD models have comparable efficacies, and the impact of the tested atmospheric model and the geomagnetic database is also limited. Users however have several possibilities to select the production rate: 1) using a worldwide mean value, 2) a regionally averaged value (not available in region s with no data), 3) a local unique value, which can be chosen among the existing dataset or imported by the user, or 4) any combination of single or multiple calibration data. If a global mean is chosen, the 1σ uncertainty arising from the production rate is about 5% for 10 Be and 10% for 3 He. If a regional production rate is picked, these uncertainties are potentially lower. CREp is able to calculate a large number of ages in a reasonable time (typically 〈 30 s for 50 samples). The user may export a summary table of the computed ages and the density probability function associated with each age (in the form of a spreadsheet).

Beschreibung: Publication date: Available online 1 December 2016 Source: Quaternary Geochronology Author(s): M.D. Tomkins, J.M. Dortch, P.D. Hughes

Beschreibung: Publication date: Available online 23 November 2016 Source: Quaternary Geochronology Author(s): James K. Feathers

Beschreibung: Publication date: Available online 18 November 2016 Source: Quaternary Geochronology Author(s): Marine Frouin, Sébastien Huot, Sebastian Kreutzer, Christelle Lahaye, Michel Lamothe, Anne Philippe, Norbert Mercier Measuring the infrared radiofluorescence (IR-RF) signal of K-feldspars, which is thought to be not affected by any anomalous fading potentially provides a promising alternative dating approach to the more studied IR and pIRIR signals. Here we report a series of experiments aimed at characterising the IR-RF signal, which led us to propose an improved IR-SAR (Infrared Radiofluorescence Single-Aliquot Regeneration) protocol. In comparison with the original protocol from Erfurt and Krbetschek (2003a), the proposed one relies on IR-RF signal measurements and bleaching experiments conducted at elevated temperatures. Stabilising the temperature at 70 °C during the measurements helps keeping the very shallow traps empty during the process. Internal protocol tests and dating applications are presented using known age samples. We show that the improved protocol (named RF 70 ) is capable to reproduce known age results within uncertainties. However, considering the slow bleaching rate of the RF 70 signal, the multi-grain approach is likely not the most suitable for the D e determination of complex samples.

Beschreibung: Publication date: Available online 17 November 2016 Source: Quaternary Geochronology Author(s): Shuhei Sakata, Shinsuke Hirakawa, Hideki Iwano, Tohru Danhara, Marcel Guillong, Takafumi Hirata We have measured 238 U– 206 Pb, 235 U– 207 Pb, and 232 Th– 208 Pb ages on Quaternary zircons by laser ablation, single-collector, magnetic sector inductively coupled plasma mass spectrometry (LA-ICP-MS). To obtain reliable ages for Quaternary zircons, corrections for initial disequilibrium associated with deficits and excesses of both 230 Th and 231 Pa relative to secular equilibrium resulting from differential partitioning during zircon crystallization or source melting must be made. In contrast, the 232 Th– 208 Pb decay system is clearly advantageous for samples affected by disequilibrium because the 232 Th decay system lacks long-lived intermediate daughter isotopes. Conventionally, the initial disequilibrium for the 238 U and 235 U decay series has been determined by the distribution ratio between the melt and zircon (i.e., ƒ Th/U  = (Th/U) Zircon /(Th/U) Melt and ƒ Pa/U  = (Pa/U) Zircon /(Pa/U) Melt ). In our study, these correction factors were determined from comparison of the measured 238 U– 206 Pb and 235 U– 207 Pb ages with 232 Th– 208 Pb ages obtained for three zircons of known eruption and, in some cases, zircon crystallization ages (Kirigamine Rhyolite, Bishop Tuff, and Toga Pumice). The resulting correction factors are ƒ Th/U  = 0.19 ± 0.14 and ƒ Pa/U  = 3.66 ± 0.89 (Kirigamine Rhyolite), ƒ Th/U  = 0.24 ± 0.20 and ƒ Pa/U  = 3.1 ± 1.2 (Bishop Tuff), and ƒ Th/U  = 0.28 ± 0.17 and ƒ Pa/U  = 3.04 ± 0.99 (Toga Pumice). Although the uncertainties of these f values are relatively large, our results support the adequacy of the conventional approach for correction of initial disequilibrium. A recent study published results that apparently show zircon crystallization ages are younger than the eruption age of Bishop Tuff. It seems to be difficult to eliminate these discrepancies, even if the Th/U partitioning and disequilibrium generated during partial melting are taken into account for recalculation of its zircon age. However, magma chamber process and history of Bishop Tuff are too complex to obtain accurate zircon ages by U–Pb method. To overcome this, therefore, the Th–Pb zircon dating method is a key technique for understanding complex, pre-eruptive magma processes, and further efforts to improve its precision and accuracy are desirable.

Beschreibung: Publication date: Available online 15 November 2016 Source: Quaternary Geochronology Author(s): Tabito Matsu'ura, Jun-Ichi Kimura, Qing Chang, Junko Komatsubara We refine the tephrostratigraphy and cryptotephrostratigraphy of the Middle Pleistocene deep-sea sediments in the C9001C cores and reconstruct the age model of the cores to provide a standard stratigraphy for the NW Pacific Ocean area. In the lower half of the cores (corresponding to marine isotopic stages [MISs] 9–18), we newly identify nine concentrations (spikes) of glass shards (numbered G17–G25 from top to bottom) below sixteen previously reported spikes (numbered G1–G16) in the upper half of the cores (MISs 1–9). We correlated two spikes (both cryptotephras) with known widespread tephras: G18 with Naruohama-IV (MIS 10d; 357 ka) and G19 with Kasamori 5 (MIS 12b–c; 434–458 ka). Although the two cryptotephra spikes cannot be distinguished by the major- and trace-element compositions of glass shards, these correlations are based upon their stratigraphic occurrence within separate glacial periods (MISs 10 and 12) as indicated by the δ 18 O stratigraphy of the C9001C sequence. Based on the stratigraphic positions of the Naruohama-IV and Kasamori 5 tephras, we assign the sediment units corresponding to MISs 10, 11, and 12, and revise the previous assignment of core segments corresponding to MISs 7, 8, and 9. The resulting tephra-based age model is consistent with the last occurrence of Proboscia curvirostris , but it is inconsistent with the first occurrence of Emiliania huxleyi . According to the new age model, the sedimentation rate during MISs 18–2 has ranged from 0.21 m/ka (during MIS 17) to 0.74 m/ka (during MISs 8 and 9), however, the sedimentation rate was never as high as the rate of 1.1 m/ka indicated by an earlier age model.

Beschreibung: Publication date: Available online 31 October 2016 Source: Quaternary Geochronology Author(s): Peter Wilson

Beschreibung: Publication date: Available online 28 October 2016 Source: Quaternary Geochronology Author(s): Kristina J. Thomsen, Andrew S. Murray, Jan-Pieter Buylaert, Mayank Jain, Jakob H. Hansen, Thierry Aubry, Guillaume Guérin In the note “A response to some unwarranted criticisms of single-grain dating” Feathers raises many issues with both the approach and the conclusions of Thomsen et al. (2016). After careful consideration, we find we disagree with Feather's analysis and conclusions, and stand by the original conclusions of Thomsen et al. (2016). We reiterate that, for these samples, the multi-grain measurements are demonstrably in better agreement with the independent age control than are the standard single-grain measurements. In our view, Feathers’ most important criticisms are that the 14 C age control is reported incorrectly and that Thomsen et al. (2016) cannot conclude that standard single-grain methods are in poorer agreement with the independent age control than the multi-grain methods. We acknowledge the presence of a minor presentation error in Figure 3 of Thomsen et al. (2016), but we demonstrate that this detail has no bearing on the conclusions of Thomsen et al. (2016). We respond below in detail to the main issues raised by Feathers. We have retained his structure for ease of cross-comparison.

Beschreibung: Publication date: Available online 19 October 2016 Source: Quaternary Geochronology Author(s): A.E.K. Ojala, T.P. Luoto, J.J. Virtasalo The uppermost 50 cm of sediments from the 23 m deep lacustrine basin of Nurmijärvi, a lake in south-central Finland, are composed of the clastic-biogenic type of varves. We use multiple dating methods to render a sediment chronology that is as precise and accurate as possible for future paleoenvironmental research on this high-fidelity sedimentary archive. The 250-year-long varve chronology contains a chronological error of ±2%, as estimated based on repeated varve counts. The varve-based chronology was verified with the vertical distribution of 137 Cs by comparing variations in the paleomagnetic relative declination of the surface sediments with instrumental observations and a calculated representation of declination features in Finland. A comparison of the sediment cesium content using two gamma counters, different sediment pretreatments, and normalization of the 137 Cs concentration with masses and accumulation rates of different sediment components revealed that the AD 1986 fallout peak from the Chernobyl accident is the dominant feature in the studied sediments, regardless of the sample pretreatment or normalizing procedure. The vertical distribution of the AD 1986 peak in sediments with clastic-biogenic varves is fairly narrow, thus providing an accurate age-depth marker for the recent deposits. However, the Nurmijärvi section also reveals a ca. 10-year-long period of catchment-to-basin 137 Cs mobilization and redeposition after the AD 1986 fallout, as well as significant downwards diffusion of the Chernobyl-derived cesium in the sediment. This study highlights the ways in which the quality of surface sediment chronology needs to be considered and secured prior to comparisons between paleoproxy records and instrumental observations.