Age-depth models form the backbone of most palaeoenvironmental studies. However, procedures for constructing chronologies vary between studies, they are usually not explained sufficiently, and some are inadequate for handling calibrated radiocarbon dates. An alternative method based on importance sampling through calibrated dates is proposed. Dedicated R code is presented which works with calibrated radiocarbon as well as other dates, and provides a simple, systematic, transparent, documented and customizable alternative. The code automatically produces age depth models, enabling exploration of the impacts of different assumptions (e.g., model type, hiatuses, age offsets, outliers, and extrapolation). (C) 2010 Elsevier B.V. All rights reserved.
We codify previously published means of calculating exposure ages and erosion rates from Be-10 and Al-26 concentrations in rock surfaces, and present a single complete and straightforward method that reflects currently accepted practices and is consistent with existing production rate calibration measurements. It is intended to enable geoscientists, who wish to use cosmogenic-nuclide exposure age or erosion rate measurements in their work to: (a) calculate exposure ages and erosion rates; (b) compare previously published exposure ages or erosion rate measurements on a common basis; (c) evaluate the sensitivity of their results to differences between published production rate scaling schemes. The method is available online at http://hess.ess.washington.edu. (c) 2008 Published by Elsevier Ltd.
All Quaternary dating methods involve the measurement of one or more variables to estimate the age of a sample. Each measured quantity has an associated error and uncertainty, and may also be subject to natural variation. We review the statistical estimation of such uncertainties and variation for comparing and interpreting age estimates, with specific reference to the estimation of equivalent dose (D-e) values in the optically stimulated luminescence (OSL) dating of sediments. We discuss statistical aspects of OSL signal and background estimation, the determination of D-e values for multi-grain aliquots and individual mineral grains from the same and different samples, and the extent of variation commonly observed among such estimates. Examples are drawn from geological and archaeological contexts. We discuss the strengths and weaknesses of various graphical methods of displaying multiple, independent estimates of D-e, along with statistical tests and models to compare and appropriately combine them. Many of our recommendations are applicable also to the clear presentation of data obtained using other Quaternary dating methods. We encourage the use of models and methods that are based on well established statistical principles and, ideally, are validated by appropriate numerical simulations; and we discourage the adoption of ad hoc methods developed using a particular set of measurement conditions and tested on a limited number of samples, as these may not be applicable more generally. We emphasise that the choice of statistical models should not be made solely on statistical grounds (or arbitrary rules) but should take into account the broader scientific context of each sample and any additional pertinent information. (C) 2012 Elsevier B.V All rights reserved.
Tephrochronology (from tephra, Gk 'ashes') is a unique stratigraphic method for linking, dating, and synchronizing geological, palaeoenvironmental, or archaeological sequences or events. As well as utilizing the Law of Superposition. tephrochronology in practise requires tephra deposits to be characterized (or 'fingerprinted') using physical properties evident in the field together with those obtained from laboratory analyses. Such analyses include mineralogical examination (petrography) or geochemical analysis of glass shards or crystals using an electron microprobe or other analytical tools including laser-ablation-based mass spectrometry or the ion microprobe. The palaeoenvironmental or archaeological context in which a tephra occurs may also be useful for correlational purposes. Tephrochronology provides greatest utility when a numerical age obtained for a tephra or cryptotephra is transferrable from one site to another using stratigraphy and by comparing and matching inherent compositional features of the deposits with a high degree of likelihood. Used this way, tephrochronology is an age-equivalent dating method that provides an exceptionally precise volcanic-event stratigraphy. Such age transfers are valid because the primary tephra deposits from an eruption essentially have the same short-lived age everywhere they occur, forming isochrons very soon after the eruption (normally within a year). As well as providing isochrons for palaeoenvironmental and archaeological reconstructions, tephras through their geochemical analysis allow insight into volcanic and magmatic processes, and provide a comprehensive record of explosive volcanism and recurrence rates in the Quaternary (or earlier) that can be used to establish time-space relationships of relevance to volcanic hazard analysis. The basis and application of tephrochronology as a central stratigraphic and geochronological tool for Quaternary studies are presented and discussed in this review. Topics covered include principles of tephrochronology, defining isochrons, tephra nomenclature, mapping and correlating tephras from proximal to distal locations at metre- through to sub-millimetre-scale, cryptotephras, mineralogical and geochemical fingerprinting methods, numerical and statistical correlation techniques, and developments and applications in dating including the use of flexible depositional age-modelling techniques based on Bayesian statistics. Along with reference to wide-ranging examples and the identification of important recent advances in tephrochronology, such as the development of new geo-analytical approaches that enable individual small glass shards to be analysed near-routinely for major, trace, and rare-earth elements, potential problems such as miscorrelation, erroneous-age transfer, and tephra reworking and taphonomy (especially relating to cryptotephras) are also examined. Some of the challenges for future tephrochronological studies include refining geochemical analytical methods further, improving understanding of cryptotephra distribution and preservation patterns, improving age modelling including via new or enhanced radiometric or incremental techniques and Bayesian-derived models, evaluating and quantifying uncertainty in tephrochronology to a greater degree than at present, constructing comprehensive regional databases, and integrating tephrochronology with spatially referenced environmental and archaeometric data into 3-D reconstructions using GIS and geostatistics. (C) 2010 Elsevier B.V. All rights reserved.
As cosmogenic nuclide applications continue to expand, the need for a common basis for calculation becomes increasingly important. In order to accurately compare between results from different nuclides, a single method of calculation is necessary. Calculators exist in numerous forms with none matching the needs of the CRONUS-Earth project to provide a simple and consistent method to interpret data from most commonly used cosmogenic nuclides. A new program written for this purpose, CRONUScalc, is presented here. This unified code presents a method applicable to Be-10, Al-26, Cl-36, He-3, and C-14, with Ne-21 in testing. The base code predicts the concentration of a sample at a particular depth for a particular time in the past, which can be used for many applications. The multi-purpose code already includes functions for performing production rate calibrations as well as calculating erosion rates and surface exposure ages for single samples and depth profiles. The code is available under the GNU General Public License agreement and can be downloaded and modified to deal with specific atypical scenarios. (C) 2015 Elsevier B.V. All rights reserved.
Accurate calculation of the environmental radiation dose rate ((D) over dot) is an essential part of trapped charge dating methods, such as luminescence and electron spin resonance dating. Although the calculation of ((D) over dot) is not mathematically complex, the incorporation of multiple variables and the propagation of uncertainties can be challenging. The Dose Rate and Age Calculator (DRAC) is an open access, web-based program which enables rapid ((D) over dot) calculation for trapped charge dating applications. Users can select from recently published attenuation and conversion factors to make mathematically robust, reproducible ((D) over dot) calculations. comparison of DRAC calculated ((D) over dot) values against the published ((D) over dot) determinations of 422 samples from 32 studies results in a reproducibility ratio of 1.01 +/- 0.05. It is anticipated that DRAC will facilitate easier inter-laboratory comparisons and will provide greater transparency for ((D) over dot) calculations. DRAC will be updated to reflect the latest advances in ((D) over dot) calculation and is freely accessible at www.aber. ac.uk/alrl/drac. The code for DRAC is available from github at https://github.com/DRAC-calculator/DRAC-calculator. (C) 2015 Elsevier B.V. All rights reserved.
Geological surface-exposure dating using cosmogenic-nuclide accumulation became a practical geochronological endeavor in 1986, when the utility of Be-10, Al-26, Cl-36, and He-3 were all demonstrated. In response to the lack of a common basis for quantifying analytical consistency and calibrating cosmogenic-nuclide production, the CRONUS-Earth Project in the U.S. was started in 2005, along with a European partner project, CRONUS-EU. The goal of the CRONUS-Earth Project was to improve the accuracy and precision of terrestrial cosmogenic nuclide dating in general, focusing especially on nuclide production rates and their variation with altitude, latitude, and time, and to attempt to move from empirically based methods to ones with a stronger basis in physics. The CRONUS-Earth Project conducted extensive intercomparisons of reference materials to attempt to quantify analytical reproducibility at the community level. We found that stated analytical uncertainties nearly always underestimate the actual degree of variability, as quantified by the over-all coefficient of variation of the intercalibration data. The average amount by which the actual coefficient of variation exceeded the analytical uncertainty was a factor of two (100%), but ranged from 15% to 300% depending on the nuclide and material. Coefficients of variation ranged from 3-4% for Be-10 to 6-8% for Cl-36, C-14, and Ne-21, to 5-11% for Al-26. Both interlaboratory bias and within-laboratory excess spread of the data played a role in increasing variability above the stated analytical uncertainties. The physical basis for cosmogenic nuclide production was investigated through numerical modeling and the measurement of energy-dependent neutron cross sections for nuclide interactions. We formulated new, physically based, scaling models, denoted LSD and LSDn, by generalizing global numerical simulations of cosmic-ray processes. The CRONUS-Earth Project identified new geological calibration sites, including one at low latitude and high elevation (Huancane, Peru), and replicated nuclide measurement at numerous laboratories. At many sites multiple nuclides were measured, providing much more confidence in the equivalence of surface-exposure ages calculated from differing nuclides. The data were interpreted using an original cosmogenic-nuclide calculator, CRONUScalc, that incorporates the new physically based scaling. The new data and model produced significantly better fits than previous efforts, but do not fully resolve apparent spatial variations in production rates. The CRONUS-Earth and CRONUS-EU Projects have provided a firm foundation for assessing the strengths and weaknesses of cosmogenic-nuclide analytical methods, adjusted the AMS standards for Be-10 and consequently revised the half-life, and have provided improved calibration data sets and interpretative tools. (C) 2015 Elsevier B.V. All rights reserved.
Reversals and excursions of Earth's geomagnetic field create marker horizons that are readily detected in sedimentary and volcanic rocks worldwide. An accurate and precise chronology of these geomagnetic field instabilities is fundamental to understanding several aspects of Quaternary climate, dynamo processes, and surface processes. For example, stratigraphic correlation between marine sediment and polar ice records of climate change across the cryospheres benefits from a highly resolved record of reversals and excursions. The temporal patterns of dynamo behavior may reflect physical interactions between the molten outer core and the solid inner core or lowermost mantle. These interactions may control reversal frequency and shape the weak magnetic fields that arise during successive dynamo instabilities. Moreover, weakening of the axial dipole during reversals and excursions enhances the production of cosmogenic isotopes that are used in sediment and ice core stratigraphy and surface exposure dating. The Geomagnetic Instability Time Scale (GITS) is based on the direct dating of transitional polarity states in lava flows using the Ar-40/Ar-39 method, in parallel with astrochronologic age models of marine sediments in which oxygen isotope and magnetic records have been obtained. A review of data from Quaternary lava flows and sediments gives rise to a GITS that comprises 10 polarity reversals and 27 excursions that occurred during the past 2.6 million years. Nine of the ten reversals bounding chrons and subchrons are associated with Ar-40/Ar-39 ages of transitionally-magnetized lava flows. The tenth, the Gauss-Matuyama chron boundary, is tightly bracketed by Ar-40/Ar-39 dated ash deposits. Of the 27 well-documented geomagnetic field instabilities manifest as short-lived excursions, 14 occurred during the Matuyama chron and 13 during the Brunhes chron. Nineteen excursions have been dated directly using the Ar-40/Ar-39 method on transitionally-magnetized volcanic rocks and these form the backbone of the GITS. Excursions are clearly not the rare phenomena once thought. Rather, during the Quaternary period, they occur nearly three times as often as full polarity reversals. (C) 2013 Elsevier B.V. All rights reserved.
The major uncertainty in relating cosmogenic-nuclide exposure ages to ages measured by other dating methods comes from extrapolating nuclide production rates measured at globally scattered calibration sites to the sites of unknown age that are to be dated. This uncertainty can be reduced by locating production rate calibration sites that are similar in location and age to the sites to be dated. We use this strategy to reconcile exposure age and radiocarbon deglaciation chronologies for northeastern North America by compiling Be-10 production rate calibration measurements from independently dated late-glacial and early Holocene ice-marginal landforms in this region. 10Be production rates measured at these sites are 6-12% lower than predicted by the commonly accepted global 10Be calibration data set used with any published production rate scaling scheme. In addition, the regional calibration data set shows significantly less internal scatter than the global calibration data set. Thus, this calibration data set can be used to improve both the precision and accuracy of exposure dating of regional late-glacial events. For example, if the global calibration data set is used to calculate exposure ages, the exposure-age deglaciation chronology for central New England is inconsistent with the deglaciation chronology inferred from radiocarbon dating and varve stratigraphy. We show that using the regional data set instead makes the exposure age and radiocarbon chronologies consistent. This increases confidence in correlating exposure ages of ice-marginal landforms in northeastern North America with glacial and climate events dated by other means. (c) 2008 Elsevier Ltd. All rights reserved.
A protocol for optical dating of potassium-rich feldspar (K-feldspar) is proposed. It utilizes the infrared stimulated luminescence (IRSL) signal measured by progressively increasing the stimulation temperature from 50 to 250 degrees C in step of 50 degrees C, so-called multi-elevated-temperature post-IR IRSL (MET-pIRIR) measurements. Negligible anomalous fading was observed for the MET-pIRIR signals obtained at 200 and 250 degrees C. This was supported by equivalent dose (D-e) measurements using the IRSL and MET-pIRIR signals. The De values increase progressively from 50 degrees C to 200 degrees C, but similar D-e values were obtained for the MET-pIRIR signal at 200 and 250 degrees C. Measurement of modern samples and bleached samples indicates that the MET-pIRIR signals have small residual doses less than 5 Gy equivalent to about 1-2 ka. We have tested the protocol using various sedimentary samples with different ages from different regions of China. The MET-pIR IRSL ages obtained at 200 and 250 degrees C are consistent with independent and/or quartz OSL ages. (C) 2011 Elsevier B.V. All rights reserved.
Here we present a new algorithm (StalAge), which is designed to construct speleothem age models. The algorithm uses U-series ages and their corresponding age uncertainty for modelling and also includes stratigraphic information in order to further constrain and improve the age model. StalAge is applicable to problematic datasets that include outliers, age inversions, hiatuses and large changes in growth rate. Manual selection of potentially inaccurate ages prior to application is not required. StalAge can be applied by the general, non-expert user and has no adjustable free parameters. This offers the highest degree of reproducibility and comparability of speleothem records from different studies. StalAge consists of three major steps. Firstly, major outliers are identified. Secondly, age data are screened for minor outliers and age inversions, and the uncertainty of potential outliers is increased using an iterative procedure. Finally, the age model and corresponding 95%-confidence limits are calculated by a Monte-Carlo simulation fitting ensembles of straight lines to sub-sets of the age data. We apply StalAge to a synthetic stalagmite 'sample' including several problematic features in order to test its performance and robustness. The true age is mostly within the 95%-confidence age limits of StalAge showing that the calculated age models are accurate even for very difficult samples. We also apply StalAge to three published speleothem datasets. One of those is annually laminated, and the lamina counting chronology agrees with the age model calculated by stalAge. For the other two speleothems the resulting age models are similar to the published age models, which are both based on smoothing splines. Calculated uncertainties are in the range of those calculated by combined application of Bayesian chronological ordering and a spline, showing that StalAge is efficient in using stratigraphic information in order to reduce age model uncertainty. The algorithm is written in the open source statistical software R and available from the authors or as an electronic supplement of this paper. (C) 2011 Elsevier B.V. All rights reserved.
We present a Be-10 production-rate calibration derived from an early Holocene debris-flow deposit at about 1000 m above sea level in the central Southern Alps, New Zealand, in the mid-latitude Southern Hemisphere. Ten radiocarbon ages on macrofossils from a soil horizon buried by the deposit date the deposit to 9690 +/- 50 calendar years before AD2008. Surface Be-10 concentrations of seven large boulders partially embedded in the stable surface of the deposit are tightly distributed, yielding a standard deviation of similar to 2%. Conversion of the Be-10 measurements to sea level/high-latitude values using each of five standard scaling methods indicates Be-10 production rates of 3.84 +/- 0.08, 3.87 +/- 0.08, 3.83 +/- 0.08, 4.15 +/- 0.09, and 3.74 +/- 0.08 atoms g(-1) a(-1), relative to the '07KNSTD' Be-10 AMS standard, and including only the local time-integrated production-rate uncertainties. When including a sea level high-latitude scaling uncertainty the overall error is similar to 2.5% (1 sigma) for each rate. To test the regional applicability of this production-rate calibration, we measured Be-10 concentrations in a set of nearby moraines deposited before 18060 +/- 200 years before AD2008. The Be-10 ages are only consistent with minimum-limiting C-14 age data when calculated using the new production rates. This also suggests that terrestrial in situ cosmogenic-nuclide production did not change significantly from Last Glacial Maximum to Holocene time in New Zealand. Our production rates agree well with those of a recent calibration study from northeastern North America, but are 12-14% lower than other commonly adopted values. The production-rate values presented here can be used elsewhere in New Zealand for rock surfaces exposed during or since the last glacial period. (C) 2009 Elsevier B.V. All rights reserved.
In quartz optically stimulated luminescence (OSL) dating protocols, an initial integral of the OSL decay curve is used in the calculation of equivalent dose, once a background integral has been subtracted. Because the OSL signal commonly contains a number of exponentially decaying components, the exact choice of time intervals used for the initial-signal and background integrals determines the composition of the net signal. Here we investigate which combination of time intervals will produce the net signal most dominated by the fast OSL component, while keeping an acceptable level of precision. Using a three-component model of OSL decay, we show that for a specified level of precision, the net signal most dominated by the fast component can be obtained when the background integral immediately follows the initial signal and is approximately 2.5 times its length. With this 'early-background' approach, the contribution of slow components to the net signal is virtually zero. We apply our methods to four samples from relatively young deposits. Compared to the widely used 'late-background' approach, in which the background integral is taken from the last few seconds of OSL, we find less thermal transfer, less recuperation and a higher proportion of aliquots yielding an equivalent dose in agreement with expectations. We find the use of an early background to be a simple and effective way of improving the accuracy of OSL dating, and suggest is should be used in standard protocols. (C) 2010 Elsevier B.V. All rights reserved.
Chlorine-36 production rates obtained from different geological calibration studies (e.g. Evans et al., 1997; Phillips et al., 2001; Schimmelpfennig et al., 2011; Stone et al., 1996; Swanson and Caffee, 2001) vary significantly, principally because of the many reactions contributing to the production of this nuclide. The CRONUS-Earth Project has provided high-quality geological calibration sites, including Lake Bonneville, Peru, and Scotland, for a large-scale calibration of Cl-36 production rates. Three sites were used to calibrate the K and Ca spallation pathways for Cl-36 production yielding production rates of 56.0 +/- 4.1 at Cl-36 (g Ca)(-1) yr(-1) and 155 +/- 11 at Cl-36 (g K)(-1) yr(-1) respectively, using Lifton-Sato-Dunai scaling (LSDn). The low-energy production parameter, P-f(0), was calibrated separately using CRONUS-Earth data from the Bonneville and Baboon Lakes sites where Cl concentrations were higher, and yielded a value of 759 +/- 180 neutrons (g air)(-1) yr(-1). There is significant uncertainty associated with this pathway due to the sensitivity of this reaction to environmental conditions. The uncertainties associated with the calibrated production parameters were estimated based on the variance of calculated ages from independent ages for an independent secondary dataset. (C) 2015 Elsevier B.V. All rights reserved.
A number of recent optically stimulated luminescence (OSL) studies have cited post-depositional mixing as a dominant source of equivalent dose (D-e) scatter across a range of sedimentary environments, including those previously considered 'best suited' for OSL dating. The potentially insidious nature of sediment mixing means that this problem may often only be identifiable by careful statistical analysis of D-e data sets. This study aims to address some of the important issues associated with the characterisation and statistical treatment of mixed D-e distributions at the multi-grain scale of analysis, using simulated D-e data sets produced with a simple stochastic model. Using this Monte Carlo approach we were able to generate theoretical distributions of single-grain D-e values, which were then randomly mixed together to simulate multi-grain aliquot D-e distributions containing a known number of mixing components and known corresponding burial doses. A range of sensitivity tests were undertaken using sediment mixtures with different aged dose components, different numbers of mixing components, and different types of dose component distributions (fully bleached, heterogeneously bleached and significantly overdispersed D-e distributions). The results of our modelling simulations reveal the inherent problems encountered when dating mixed sedimentary samples with multi-grain D-e estimation techniques. 'Phantom' dose components (i.e. discrete dose populations that do not correspond to the original single-grain mixing components) are an inevitable consequence of the 'averaging' effects of multi-grain D-e analysis, and prevent the correct number of mixing components being identified with the finite mixture model (FMM) for all of the multi-grain mixtures tested. Our findings caution against use of the FMM for multi-grain aliquot De data sets, even when the aliquots consist of only a few grains. (C) 2008 Elsevier Ltd. All rights reserved.