Quaternary Geochronology

Assessing the dead carbon proportion of a modern speleothem from central Brazil Publication date: June 2019 Source: Quaternary Geochronology, Volume 52 Author(s): K.D. Macario, N.M. Stríkis, F.W. Cruz, I. Hammerschlag, E.Q. Alves, V.F. Novello, L. Edwards, H. Cheng, F.R.D. Andrade, P.F.S.M. Buarque, J.A.S. Garbelim Abstract Geographic and temporal variations in atmospheric carbon isotopic ratios are recorded in environmental proxies. In temperate regions, this may assist in the construction of datasets for the calibration of radiocarbon ages. Over the tropics, the high growth rate of speleothems combined with high precision 230Th dating provides potential records for atmospheric 14C reconstruction. In this preliminary work, we investigate the isotopic composition (δ18O and δ13C) of a speleothem from central Brazil, precisely dated by the multi-collector inductively coupled plasma mass spectrometry technique (MC-ICP-MS). Radiocarbon Accelerator Mass Spectrometry (AMS) measurements were compared to the 14C concentration in the atmosphere during the nuclear tests period based on the Bomb13SH1-2 curve. Our results show that the speleothem dead carbon proportion is less than 20% between 1932 and 1992 AD while δ13C values vary between −14 and −9‰. Given that the empirical radiocarbon calibration curves for the Southern Hemisphere were constructed based on limited records, we discuss the potential and limitations of Brazilian speleothems for the evaluation of atmospheric 14C concentrations over the last millennia.

Geochemical characterisation of the Late Quaternary widespread Japanese tephrostratigraphic markers and correlations to the Lake Suigetsu sedimentary archive (SG06 core) Publication date: June 2019 Source: Quaternary Geochronology, Volume 52 Author(s): Paul G. Albert, Victoria C. Smith, Takehiko Suzuki, Danielle McLean, Emma L. Tomlinson, Yasuo Miyabuchi, Ikuko Kitaba, Darren F. Mark, Hiroshi Moriwaki, SG06 Project Members, Takeshi Nakagawa Abstract Large Magnitude (6–8) Late Quaternary Japanese volcanic eruptions are responsible for widespread ash (tephra) dispersals providing key isochrons suitable for the synchronisation and dating of palaeoclimate archives across East Asia, the NW Pacific and beyond. The transfer of geochronological information using these eruption deposits demands robust tephra correlations underpinned by detailed and precise volcanic glass geochemical data. Presented here is a major (electron microprobe; EMP) and trace element (Laser ablation inductively coupled plasma mass spectrometry; LA-ICP-MS) characterisation of near-source deposits from a series of large magnitude Japanese eruptions spanning approximately the last 150 ka. These data offer new insights into diagnostic compositional variations of the investigated volcanic sources spanning the Japanese islands. Whilst in the case of the highly productive Aso caldera (Kyushu), we are able to explore compositional variations through successive large magnitude eruptions (50–135 ka). These near-source volcanic glass data are used to validate and refine the visible tephrostratigraphy of the intensely dated Lake Suigetsu sedimentary record (SG06 core), Honshu Island, whilst also illustrating key tephrostratigraphic tie points to other East Asian palaeoclimate records (e.g. Lake Biwa). The identification of widespread Japanese tephrostratigraphic markers in the SG06 sediment record enables us to place chronological constraints on these ash dispersals, and consequently explosive volcanism at source volcanoes situated along the Kyushu Arc, including Kikai, Ata and Aso calderas. The proximal Aso-4 Ignimbrite (Magnitude 7.7) deposit is dated here by 40Ar/39Ar at 86.4 ± 1.1 ka (2σ), and provides a chronological anchor (SG06-4963) for the older sediments of the Lake Suigetsu record. Finally, trace element glass data verify visible ash fall layers derived from other compositionally distinct source regions of Japanese volcanism, including activity along the northern Izu-Bonin arc and North East Japan Arcs. These findings underline the Lake Suigetsu record as central node in the Japanese tephrostratigraphic framework.

Cosmogenic and nucleogenic 21Ne in quartz in a 28-meter sandstone core from the McMurdo Dry Valleys, Antarctica Publication date: June 2019 Source: Quaternary Geochronology, Volume 52 Author(s): Greg Balco, Pierre-Henri Blard, David L. Shuster, John O.H. Stone, Laurent Zimmermann Abstract We measured concentrations of Ne isotopes in quartz in a 27.6-m sandstone core from a low-erosion-rate site at 2183 m elevation at Beacon Heights in the Antarctic Dry Valleys. Surface concentrations of cosmogenic 21Ne indicate a surface exposure age of at least 4.1 Ma and an erosion rate no higher than ca. 14 cm Myr−1 21Ne concentrations in the upper few centimeters of the core show evidence for secondary spallogenic neutron escape effects at the rock surface, which is predicted by first-principles models of cosmogenic-nuclide production but is not commonly observed in natural examples. We used a model for 21Ne production by various mechanisms fit to the observations to distinguish cosmic-ray-produced 21Ne from nucleogenic 21Ne produced by decay of trace U and Th present in quartz, and also constrain rates of subsurface 21Ne production by cosmic-ray muons. Core samples have a quartz (U-Th)/Ne closure age, reflecting cooling below approximately 95 °C, near 160 Ma, which is consistent with existing apatite fission-track data and the 183 Ma emplacement of nearby Ferrar dolerite intrusions. Constraints on 21Ne production by muons derived from model fitting are consistent with a previously proposed value of 0.79 mb at 190 GeV for the cross-section for 21Ne production by fast muon interactions, but indicate that 21Ne production by negative muon capture is likely negligible.

U-Th and radiocarbon dating of calcite speleothems from gypsum caves (Emilia Romagna, North Italy) Publication date: June 2019 Source: Quaternary Geochronology, Volume 52 Author(s): Andrea Columbu, Russell Drysdale, John Hellstrom, Jon Woodhead, Hai Cheng, Quan Hua, Jian-xin Zhao, Paolo Montagna, Edwige Pons-Branchu, R. Lawrence Edwards Abstract High-resolution U-Th and 14C dating of two calcite flowstones (RTf: Last Interglacial; RTy: Late Holocene) from the Re Tiberio – Monte Tondo karst system (North Italy) is presented to investigate the palaeoclimate potential of speleothems from gypsum caves. To date, there is a lack of information regarding the dating potential of calcite speleothems that have grown in such gypsum karst environment (e.g. no pseudokarst). High-resolution U-Th dating, aimed at establishing if these speleothems can provide robust radiometric age sequences, is first presented. Although both samples show promise for reliable radiometric dating, about 20% of the ages in the RTf dataset were excluded from the age-model as outliers. These outliers are best explained by post-depositional diagenetic processes affecting the U-Th system rather than anything specific to gypsum karst environment conditions. In contrast, outliers were not detected in the RTy sample, but U-Th analysis was not able to constrain its relatively young age (∼600 years before present, B.P.). Consequently, radiocarbon measurements were undertaken, and combined with the U-Th ages to build an age-model. The latter allowed the estimation of “dead carbon fraction” (DCF), which yields a constant value through time, and a magnitude comparable to other sites where climate is similar to Monte Tondo. Soils were considered the main source of carbon, because bedrock dissolution acted predominantly under open-system conditions, and soil organic matter (SOM) turnover rate appears above average compared to sites studied elsewhere.

Evaluating the potential of tree-ring methodology for cross-dating of three annually laminated stalagmites from Zoolithencave (SE Germany) Publication date: June 2019 Source: Quaternary Geochronology, Volume 52 Author(s): Dana F.C. Riechelmann, Jens Fohlmeister, Tobias Kluge, Klaus Peter Jochum, Detlev K. Richter, Michael Deininger, Ronny Friedrich, Norbert Frank, Denis Scholz Abstract Three small stalagmites from Zoolithencave (southern Germany) show visible laminae, which consist of a clear and a brownish, pigmented layer pair. This potentially provides the opportunity to construct precise chronologies by counting annual laminae. The growth period of the three stalagmites was constrained by the 14C bomb peak in the youngest part of all three stalagmites and 14C-dating of a piece of charcoal in the consolidated base part of stalagmite Zoo-rez-2. These data suggest an age of AD 1970 for the top laminae and a lower age limit of AD 1973–1682 or AD 1735–1778. Laminae were counted and their thickness determined on scanned thin sections of all stalagmites. On stalagmites Zoo-rez-1 and -2, three tracks were measured near the growth axes, each separated into three sections at prominent anchor laminae (I, II, III). Each section was replicated three times (a, b, c). For Zoo-rez-3, only one track was measured. The total number of laminae counted for Zoo-rez-1 ranges from 138 to 177, for Zoo-rez-2 from 119 to 145, and for Zoo-rez-3 from 159 to 166. The numbers agree well with the range constrained by the bomb peak and the age of the charcoal, which supports the annual origin of the laminae. The replicated measurements of the different tracks as well as the three different tracks on the stalagmites Zoo-rez-1 and -2 were cross-dated using the TSAP-Win® tree-ring software. This software is very useful for cross-dating because it enables to insert or delete missing or false laminae as well as identifying common pattern by shifting the series back and forth in time. However, visual inspection of the thin sections was necessary to confirm detection of missing or false laminae by TSAP-Win®. For all three Zoo-rez speleothems, cross-dating of the mean lamina thickness series was not possible due to a missing common pattern. The cross-dating procedure results in three refined chronologies for the three Zoo-rez stalagmites of ranging from AD 1821–1970 (Zoo-rez-1), AD 1835–1970 (Zoo-rez-2), and AD 1808–1970 (Zoo-rez-3).

Guidelines for reporting and archiving 210Pb sediment chronologies to improve fidelity and extend data lifecycle Publication date: June 2019 Source: Quaternary Geochronology, Volume 52 Author(s): Colin J. Courtney Mustaphi, Janice Brahney, Marco A. Aquino-López, Simon Goring, Kiersten Orton, Alexandra Noronha, John Czaplewski, Quinn Asena, Sarah Paton, Johnny Panga Brushworth Abstract Radiometric dating methods are essential for developing geochronologies to study Late Quaternary environmental change and 210Pb dating is commonly used to produce age-depth models from recent (within 150 years) sediments and other geoarchives. The past two centuries are marked by rapid environmental socio-ecological changes frequently attributed to anthropogenic land-use activities, modified biogeochemical cycles, and climate change. Consequently, historical reconstructions over this recent time interval have high societal value because analyses of these datasets provide understanding of the consequences of environmental modifications, critical ecosystem thresholds, and to define desirable ranges of variation for management, restoration, and conservation. For this information to be used more broadly, for example to support land management decisions or to contribute data to regional analyses of ecosystem change, authors must report all of the useful age-depth model information. However, at present there are no guidelines for researchers on what information should be reported to ensure 210Pb data are fully disclosed, reproducible, and reusable; leading to a plethora of reporting styles, including inadequate reporting that reduces potential reusability and shortening the data lifecycle. For example, 64% of the publications in a literature review of 210Pb dated geoarchives did not include any presentation of age uncertainty estimates in modeled calendar ages used in age-depth models. Insufficient reporting of methods and results used in 210Pb dating geoarchives severely hampers reproducibility and data reusability, especially in analyses that make use of databased palaeoenvironmental data. Reproducibility of data is fundamental to further analyses of the number of palaeoenvironmental data and the spatial coverage of published geoarchives sites. We suggest, and justify, a set of minimum reporting guidelines for metadata and data reporting for 210Pb dates, including an IEDA (Interdisciplinary Earth Data Alliance), LiPD (Linked Paleo Data) and generic format data presentation templates, to contribute to improvements in data archiving standards and to facilitate the data requirements of researchers analyzing datasets of several palaeoenvironmental study sites. We analyse practices of methods, results and first order interpretation of 210Pb data and make recommendations to authors on effective data reporting and archiving to maximize the value of datasets. We provide empirical evidence from publications and practitioners to support our suggested reporting guidelines. These guidelines increase the scientific value of 210Pb by expanding its relevance in the data lifecycle. Improving quality and fidelity of environmental datasets broadens interdisciplinary use, lengthens the potential lifecycle of data products, and achieves requirements applicable for evidenced-based policy support.

Increased radiocarbon dating resolution of ombrotrophic peat profiles reveals periods of disturbance which were previously undetected Publication date: June 2019 Source: Quaternary Geochronology, Volume 52 Author(s): Piotr Kołaczek, Mariusz Gałka, Mariusz Lamentowicz, Katarzyna Marcisz, Katarzyna Kajukało-Drygalska, Monika Karpińska-Kołaczek Abstract We tested the robustness of three previously published age-depth models by supplementing them with five new 14C AMS dates. The age-models were calculated for profiles retrieved from ombrotrophic peatlands in northern Poland (Bagno Kusowo, Gązwa, and Mechacz Wielki bogs). For the Bagno Kusowo and Mechacz Wielki bogs, the addition of new dates did not significantly change the shape of the age-depth models. In the topmost sections of these peat profiles the new dates did not conform with previously published models. The updated age-depth model from the Gązwa bog profile revealed the presence of 14C date inversions, which led to multiple chronology scenarios. The section of the Gązwa profile with the outliers was characterised by (i) a higher amplitude of reconstructed water table fluctuations in comparison with the lower peat layers and (ii) a higher frequency of layers with macrocharcoal (indicating local fires). However, no relationship was found between the presence/absence of Arcella discoides, Eriophorum vaginatum (hydrological disturbances), and coprophilous fungi (presence of herbivores or omnivores) and peat layers under and above the section with 14C date inversions. Moreover, a section with outlying 14C dates did not differ in terms of disturbance indicator content compared with the profiles from Bagno Kusowo and Mechacz Wielki. Although our study does not give a clear explanation for the presence of the section rich in outlying dates in the Gązwa profile, it proves that with low-resolution dating it might be very difficult to detect sections of peat disturbance. In addition, obtaining the most accurate reconstruction of the vertical depth range of post-bomb dates in a peat core may improve age-depth models for the youngest peat layers.

Seismic history from in situ 36Cl cosmogenic nuclide data on limestone fault scarps using Bayesian reversible jump Markov chain Monte Carlo Publication date: June 2019 Source: Quaternary Geochronology, Volume 52 Author(s): J. Tesson, L. Benedetti Abstract Constraining the past seismic activity and the slip-rates of faults over several millennials is crucial for seismic hazard assessment. Chlorine 36 (36Cl) in situ produced cosmogenic nuclide is increasingly used to retrieve past earthquakes histories on seismically exhumed limestone normal fault-scarps. Here we present a new methodology to retrieve the exhumation history based on a Bayesian transdimensional inversion of the 36Cl data and using the latest muon production calculation method. This procedure uses the reversible jump Markov chains Monte-Carlo algorithm (RJ-MCMC, Green 1995) which enables 1-exploring the parameter space (number of events, age and slip of the events), 2-finding the most probable scenarios, and 3- quantifying the associated uncertainties. Through a series of synthetic tests, the algorithm revealed a great capacity to constrain event slips and ages in a short computational time (several days) with a precision that can reach 0.1 ky and 0.5 m for the age and slip of exhumation event, respectively. In addition, our study show that the amount of 36Cl accumulated when the sampled fault-plane was still buried under the colluvial wedge, prior its exhumation, might represents up to 35% of the total 36Cl. Additional sampling under the colluvial is necessary to constrain this contribution.

Temporal variability of 14C reservoir effects and sedimentological chronology analysis in lake sediments from Chibuzhang Co, North Tibet (China) Publication date: June 2019 Source: Quaternary Geochronology, Volume 52 Author(s): Hao Chen, Liping Zhu, Jianting Ju, Junbo Wang, Qingfeng Ma Abstract Chronological analysis is the basic premise for paleoenvironmental research on lake sediment cores. It mainly employs bulk organic matter as a mixed dating material, although terrestrial plant remains are believed to provide reliable radiocarbon (14C) ages. 14C reservoir effects (REs) are generally problematic in establishing a final chronology and are usually recorded in aquatic macrophytes by the incorporation of dissolved inorganic carbon in high-altitude lakes of the Tibetan Plateau. We present a typical case of a 5.48 m-long core sampled from Chibuzhang Co, a lake in north Tibet, China, which contained two obvious obstacles: high REs and age reversals. The 14C RE was recently as much as ∼3500 years in the upper part according to total organic carbon (TOC) 14C ages and related dating from 210Pbex activity. It notably changed during the climatic transition to the Holocene due to carbonate input from catchment erosion and glacial meltwater, resulting in reversed 14C ages in the lower section of the core. 14C-dead carbon (or old carbon) dissolved in lake water was incorporated in the dating materials, as inferred from the 14C ages of plant samples. By combining data on grain size and carbonate content changes, two sedimentary units were identified with a boundary at 390 cm depth. These were used to estimate individual sediment accumulation rates (SARs) for the final age-depth models, instead of using a constant RE-corrected chronology. Our results provide an environmental history of ∼12.7 kyr in a long sediment core, based on an optimized SAR model of conventional 14C ages, which is supported by related lacustrine records and Bacon models. Use of separate 14C SAR analyses can solve the problem of age inversion in sediment cores, as sedimentological chronology analysis.

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: Available online 20 May 2019 Source: Quaternary Geochronology 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 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.