Corrections for initial isotopic disequilibrium in the speleothem U-Pb dating method Publication date: October 2019 Source: Quaternary Geochronology, Volume 54 Author(s): J. Engel, J. Woodhead, J. Hellstrom, R. Maas, R. Drysdale, D. Ford Abstract The accuracy of ages produced by the U-Pb speleothem chronometer is influenced by initial disequilibrium effects resulting from weathering and transit of water through the epikarst. If some residual disequilibrium remains at the time of measurement, corrections can be made. However, once the system reaches equilibrium, the initial state has to be estimated thus propagating significant uncertainty into the age calculation. As an alternative approach, here we explore the possibility of calculating [234U/238U]i initial values by forcing concordance in the 238U to 206Pb and 235U to 207 Pb decay schemes. Whether this method can be successfully implemented relies heavily on the quality of the linear regression enacted, particularly with respect to the 235U-207 Pb decay scheme. We therefore also explore the effect of 208Pb-based normalisation in order to optimise isochron construction. An unexpected consequence of this analysis is the observation that 208Pb/204Pb ratios may aid in the identification of heterogeneous initial Pb isotopic compositions in speleothems. Both methods have the potential to generate improved isochron fits which ultimately produce improved disequilibrium corrected speleothem ages. Initial disequilibrium values obtained in this way can be applied to other samples where disequilibrium information is unavailable.

Detecting landscape transience with in situ cosmogenic 14C and 10Be Publication date: October 2019 Source: Quaternary Geochronology, Volume 54 Author(s): Daniel S. Skov, David L. Egholm, John D. Jansen, Mike Sandiford, Mads F. Knudsen Abstract Constraining past variations in rates of erosion remains a key challenge in geomorphology, as estimates of erosion rate on different timescales must be bridged. The Late Pleistocene to Holocene represents a key timescale for studying landscape transience, as climatic change, increasing anthropogenic activity, and/or tectonic activity changed the rate of surface processes in many landscapes. Few chronometers are suitable for studying surface processes and landscape transience on these timescales, but, although not yet widely used, in situ 14C holds much promise due to its appropriate half-life of 5.7 kyr. Furthermore, by pairing in situ 14C with a stable or longer-lived nuclide, such as 10Be, 21Ne or 26Al, it is possible to compare surface process rates on distinctly different timescales. In this paper, we explore how these paired chronometers can be used to study landscape transience in non-glacial landscapes on Late Pleistocene to Holocene timescales. Indeed, we find that paired measurements of in situ 14C and a long-lived cosmogenic nuclide (CN) in samples from eroding landscapes enable the detection of changes in erosion rate during the Late Pleistocene to Holocene, if the increase or decrease in erosion rate was larger than a factor of two, and the landscape is eroding at rates that are typical for fluvial landscapes (5–500 mm/kyr). Similarly, detecting changes in catchment-wide denudation rates using paired CN measurements in stream-sediment samples is also possible in catchments with suitable conditions, such as short sediment transport times, minimal erosion from deep-seated mass movement, etc. (von Blanckenburg, 2006).

Comparing the accuracy and precision of luminescence ages for partially-bleached sediments using single grains of K-feldspar and quartz Publication date: August 2019 Source: Quaternary Geochronology, Volume 53 Author(s): R.K. Smedley, J.-P. Buylaert, G. Újvári Abstract Glacial settings are considered to be the most challenging context for the application of luminescence dating. The optically stimulated luminescence (OSL) signal of quartz is often preferred for luminescence dating in partially-bleached settings as it resets (or bleaches) more rapidly in response to sunlight than the post-IR IRSL (pIRIR) signal of K-feldspar, and can therefore better characterise the well-bleached part of the partially-bleached De distribution. However, the relative bleaching extents of single grains of quartz and K-feldspar have not yet been compared for sedimentary samples from the natural environment. Here we compare the De distributions and accuracy and precision of ages determined using single grains of quartz and K-feldspar from sedimentary samples deposited in a proglacial setting with independent age control. We found that the extent of bleaching of the OSL signal of quartz and pIRIR225 signal of K-feldspar was similar (with similar over-dispersion), and therefore the pIRIR225 signal bleached to similarly low levels as the OSL signal of quartz in this partially-bleached setting. We also observed a consistent offset in over-dispersion between quartz and K-feldspar of ∼10% that can be linked to scatter arising from internal dose-rates of K-feldspar and should be included when applying age models. The results here demonstrate that the accuracy and precision of ages determined using the pIRIR225 signal of single grains of K-feldspar were similar to the OSL signal of quartz. However, K-feldspars were 5–18 times more efficient than quartz at determining the population of interest for age calculation as a larger proportion of K-feldspar grains emitted a detectable luminescence signal in comparison to quartz. These findings contradict our current understanding of the bleaching of K-feldspar and quartz grains in the natural environment, and are likely applicable to other partially-bleached settings (e.g. fluvial, alluvial).

Investigation into the OSL and TT-OSL signal characteristics of ancient (>100 ka) Tasmanian aeolian quartz and its utility as a geochronometer for understanding long-term climate-driven landscape change Publication date: August 2019 Source: Quaternary Geochronology, Volume 53 Author(s): Christina M. Neudorf, Olav B. Lian, Peter D. McIntosh, Travis B. Gingerich, Paul C. Augustinus Abstract The luminescence characteristics and age estimates of nine samples of aeolian quartz are reported from the Southwood B section in the Arve Valley of southern Tasmania, Australia. New OSL and TT-OSL ages obtained from the deepest Tasmanian aeolian section found so far, the Southwood B section (>6 m deep), range from 20 ka (MIS 2) to 180 ka (MIS 6). Here we test two previously published TT-OSL protocols on our samples: the protocol of Brown and Forman (2012) and a protocol modified from Stevens et al. (2009). Congruencies between our TT-OSL chronology, OSL chronology and previously published TL ages suggest that the modified Stevens et al. (2009) protocol is the most robust for these samples. Furthermore, detailed comparisons of the laboratory data for TL, OSL, and TT-OSL samples provide information on their signal characteristics, their utility as chronometers, and give insight into the bleaching histories of the deposits. All OSL and TT-OSL ages obtained from the Southwood B section fall below the upper dating limits of these signals, which are reported to be as high as ∼280 ka and ∼570 ka, respectively. The TT-OSL signal saturates at doses that are two to three times those of the OSL signal, and may not only provide a chronology for sediments deposited during MIS 5, but also, according to the saturation limits, for any preserved sediments that accumulated during MIS 6 through to MIS 15. TT-OSL ages from older sediments in this region, combined with further stratigraphic studies have the potential to determine the climatic history of Tasmania over multiple glacial periods.

Changes in the optically stimulated luminescence (OSL) sensitivity of single grains of quartz during the measurement of natural OSL: Implications for the reliability of optical ages Publication date: August 2019 Source: Quaternary Geochronology, Volume 53 Author(s): Naveen Chauhan, A.K. Singhvi Abstract Single Aliquot Regeneration (SAR) protocol has been routinely used to estimate palaeodose for sediment samples. However, it was observed that significant changes in the optically stimulated luminescence (OSL) sensitivity occur during read out of the natural OSL and that the current SAR protocol does not take into account these changes (Singhvi et al., 2011). It was suggested that if not corrected, this change of OSL sensitivity would lead to substantive systematic offsets in ages based on the conventional SAR protocol. To circumvent this issue of sensitivity change of OSL, a natural correction factor based SAR (NCF-SAR) measurement protocol and a correction procedure was suggested by Singhvi et al. (2011) with an implicit but a reasonable assumption that the sensitivity of 110 °C TL peak of quartz correlates with the OSL sensitivity. Use of this correction resulted in reduced over-dispersion in paleodoses and additionally provided a satisfactory way to deal with samples, where natural signal was significantly higher than the saturation value of the regenerated signal. As a logical extension of this work, similar sensitivity changes in the OSL from single grains of quartz were examined and, their impact on the accuracy and reliability of single grain based SAR ages is discussed in this contribution. About ∼200 single grains each from 9 samples of diverse depositional environments were analysed and the NCF of each grain was measured. The single grain NCF’ [=(1/NCF)] values varied from 0.7 to 20. Of these ∼50% grains had sensitivity changes in the range of 0.7–1.3. The results indicated that, 1) the brighter grains (photon counts > 1000 c/s in a standard Risoe reader) had NCF′ closer to 1, though significant outliers did exist, 2) the distribution of NCF′ was positively skewed with significant number of values greater than one and, 3) the averaged single grain and multigrain NCF’ values were different, suggesting that in the case of multigrain aliquots, sensitivity changes of dim grains, on account of their larger numbers, collectively contributed significantly to the light sum in OSL.

Timing and structure of the weak Asian Monsoon event about 73,000 years ago Publication date: August 2019 Source: Quaternary Geochronology, Volume 53 Author(s): Wenjing Du, Hai Cheng, Yao Xu, Xunlin Yang, Pingzhong Zhang, Lijuan Sha, Hanying Li, Xiaoyan Zhu, Meiliang Zhang, Nicolás M. Stríkis, Francisco W. Cruz, R. Lawrence Edwards, Haiwei Zhang, Youfeng Ning Abstract Oxygen isotope (δ18O) records of cave speleothem have played an important role in the past two decades in characterizing the Asian Monsoon (AM) variability and correlating the monsoonal events with other global climate events on millennial timescales. Of a series of millennial events occurred during the last glacial period, the Chinese Stadial-20 (CS-20, corresponding to the Greenland Stadial-20, GS-20) event around ∼73 kyr BP (thousand years before present, where present = 1950 AD) is distinctive, since it is the weakest AM event during the last glacial period and is likely linked to the Toba volcanic super-eruption. While Greenland ice core records are commonly used to correlate the last glacial millennial events, yet their absolute age uncertainties around the GS-20 is larger than 1000 years. This prohibits precise correlations of the event between global climate archives from different climate systems to investigate the underlying climatic dynamics. Here, we present three Chinese cave stalagmite δ18O records from the AM region, covering a period from 76 to 71 kyr BP, across the CS-20. All stalagmites have high uranium contents and relatively fast growth rates, allowing acquisition of high-resolution (∼10 years) δ18O records with precise 230Th age controls (≤200 years, 2σ) to precisely characterize the structure and timing of the CS-20. Our results demonstrate that the onset and termination of the CS-20 are more gradual in Chinese cave records relative to the GS-20 event in Greenland ice core records. As such, we suggest a new ‘break-point’ approach to correlate CS-20 with GS-20, at either the initial onset or initial termination shifts of the events, instead of the conventional ‘mid-point’ match. We dated the initial onset and initial termination of CS-20 to ~74.0 ± 0.2 and 72.5 ± 0.2 kyr BP, respectively, confirming the Greenland ice core chronology well within the quoted uncertainty. The ‘break-point’ correlation at the GS-20/CS-20 initial termination suggests a lagged onset of CS-20 relative to the onset of GS-20. The lagged onset is in line with a northern high-latitude forcing mechanism triggering the event and a central role of oceanic reorganizations in the propagation of the climate signal. An alternative ‘break-point’ correlation at the GS-20/CS-20 initial onset suggests that the CS-20 initial termination leads the GS-20 initial termination by a few hundred years. This apparent paradox thus calls for further empirical and theoretical studies to better understand the underlying climatic dynamics and in turn the correlation strategy.

Luminescence dating of sediments from central Atacama Desert, northern Chile Publication date: August 2019 Source: Quaternary Geochronology, Volume 53 Author(s): Ian del Río, André Oliveira Sawakuchi, Gabriel González Abstract This study examines the feasibility of applying luminescence dating methods to quartz and potassium feldspar (K-feldspar) grains from Quaternary continental deposits of the Mejillones Peninsula and Coastal Cordillera in Central Atacama, northern Chile. Sediment burial ages were calculated using single-aliquot regenerative-dose (SAR) protocols applied to the optically stimulated luminescence (OSL) signal of quartz as well as infrared stimulated luminescence (IRSL) and post-infrared infrared stimulated luminescence (post-IR IRSL) signals of K-feldspar. Considering that the target sedimentary deposits comprise the Late Pleistocene age range, K-feldspar grains were dated using fading-corrected IRSL signals measured at 50 °C (IR50) and post-IR IRSL measured at 225 °C (pIRIR225) to minimize the potential effect of residual doses on calculated ages. The results of the analytical procedures indicate that quartz grains extracted from the studied sediments present a very weak or even no fast OSL component. The combination of a low OSL sensitivity, signal instability and equivalent dose distributions with high overdispersion (>40% for most samples) hinder reliable age estimation using quartz aliquots. Bleaching test results show that the IR50 signal from K-feldspar aliquots is well reset after 20–24 h of light exposure, while the pIRIR225 signal may present residual doses corresponding to between 10 and 15% of the natural signal. IR50 fading rates are ∼6–7%/decade for most studied samples, with exception of one sample which yielded a g-value of 18.77 ± 2.06%/decade. Fading rates for the pIRIR225 signal yielded variable results among sampling sites, with g-values ranging between 0.70 ± 0.24 and 6.77 ± 1.05%/decade. The dating results point out that quartz OSL ages are largely underestimated in relation to K-feldspar fading-corrected IR50 and pIRIR225 ages. Fading-corrected pIRIR225 K-feldspar ages indicate that the alluvial sedimentation in the downthrown block of an active normal fault in Mejillones Peninsula occurred between 163.4 ± 18.4 ka and 87.4 ± 6.6 ka. K-feldspar ages also allow to constrain the age of fault scarp degradation and fault reactivation in two main branches of the Atacama Fault System. In the Naguayán Fault, pIRIR225 ages indicate that faulting occurred after 21.4 ± 3.2 ka while in the Salar del Carmen Fault, faulting is recorded after 14.7 ± 1.0 ka.

A multi-dating approach to age-modelling long continental records: The 135 ka El Cañizar de Villarquemado sequence (NE Spain) Publication date: Available online 22 June 2019 Source: Quaternary Geochronology Author(s): B.L. Valero-Garcés, P. González-Sampériz, G. Gil Romera, B.M. Benito, A. Moreno, B. Oliva-Urcia, J. Aranbarri, E. García-Prieto, M. Frugone, M. Morellón, L.J. Arnold, M. Demuro, M. Hardiman, S.P.E. Blockley, C.S. Lane Abstract We present the multidisciplinary dating approach - including radiocarbon, Uranium/Thorium series (U/Th), paleomagnetism, single-grain Optical Stimulated Luminescence (OSL), Infrared Stimulated Luminescence (IRSL) and tephrochronology - used for the development of an age model for the Cañizar de Villarquemado sequence (VIL) for the last ca. 135 ka. We describe the protocols used for each technique and discuss the positive and negative results, as well as their implications for interpreting the VIL sequence and for dating similar terrestrial records. In spite of the negative results of some techniques, particularly due to the absence of adequate sample material or inaccurate analytical precision, the multi-technique strategy employed here is essential to maximize the chances of obtaining robust age models in terrestrial sequences. The final Bayesian age model for VIL sequence includes 16 AMS 14C ages, 9 OSL ages and 5 previously published IRSL ages, and the accuracy and resolution of the model are improved by incorporating information related to changes in accumulation rate, as revealed by detailed sedimentological analyses. The main paleohydrological and vegetation changes in the sequence are coherent with global Marine Isotope Stage (MIS) 6 to 1 transitions since the penultimate Termination, although some regional idiosyncrasies are evident, such as higher moisture variability than expected, an abrupt inception of the last glacial cycle and a resilient response of vegetation in Mediterranean continental Iberia in both Terminations.