Burial Ages Research Articles

Plio-Quaternary landscape evolution in the uplifted Ardennes: New insights from 26Al/10Be data from cave-deposited alluvium (Meuse catchment, E. Belgium)

Despite a wealth of recent studies dealing with the evolution of the drainage network in the uplifted Ardennes massif (E. Belgium), especially from the Middle Pleistocene onwards, the Ardennian landscape evolution and long-term incision rates in the Meuse catchment remain poorly documented over the whole Plio-Quaternary. Alluvium-filled multilevel cave systems represent a relevant setting to unravel the Late Cenozoic history of regional river incision. We present here a dataset of 26Al/10Be concentration data obtained from fifteen pebble samples washed into the Chawresse system, one of the largest multi-level cave systems of Belgium, which developed in Devonian limestones of the lower Ourthe Valley, the main Ardennian tributary of the Meuse. The sample collection spans an elevation difference higher than 120 m and their depleted 26Al/10Be ratios yield burial ages ranging from ~0.25 to 3.28 Ma. After critical assessment of our dataset for intra-karstic reworking issues, the most striking outcome of the obtained burial ages is the acceleration by a factor five of the incision rates (from ~30 to ~150 m/Ma) during the first half of the Middle Pleistocene. Integrating this incision peak and our pre-burial denudation rates, we then revisit the existing framework of Plio-Quaternary denudation and river incision in the Ardennian Meuse catchment. Whilst our 26Al/10Be concentration data shed new light on the temporal and spatial variability of the local river and hillslope system response to coupled tectonic and climatic forcings, it simultaneously highlights sampling issues and the need for further chronological data.

Correcting MIS5e and 5a sea-level estimates for tectonic uplift, an example from southern California

Along tectonically active margins, the difference in elevations between global sea levels during highstands and uplifted marine terraces is a function of both tectonics and glacial-isostatic adjustment (GIA). However, disentangling the relative influence of these two processes remains a challenge for those trying to gain insights into either process. In this study, we outline a strategy for isolating the tectonic contribution to marine isotope stage (MIS) 5e and 5a marine terrace elevations for the southern California coast by determining the cosmogenic radionuclide burial age and elevation of the early Pleistocene (1.48 ± 0.17 Ma) Clairemont Terrace in San Diego. Using this older terrace as a datum for calculating tectonic uplift rate provides a much longer time period to average out uncertainties in past local or relative sea levels (RSL) that arise from ambiguities in GIA parameters and global meltwater volumes. The assumption of constant uplift rates is warranted for this portion of the California coast given its relatively simple tectonic setting on the rift flank of the Salton Trough. From this approach, we determine an average uplift rate of 0.066 ± 0.020 mm/yr or 0.055 ± 0.013 mm/yr, depending on the RSL model used for the time of the Clairemont Terrace formation, for much of the San Diego coastline. Correcting for this tectonic uplift rate leaves an estimate of 15.1 + 2.6/-3.1 m (16.4 + 1.9/-2.6 m) and 4.8 ± 1.9 m (5.6 ± 1.5 m) for RSL during MIS5e and MIS5a, respectively. These new estimates of MIS5e and MIS5a sea levels along the southern California coast provide important constraints on GIA parameters and former ocean and ice volumes.

The potential of portable luminescence readers in geomorphological investigations: a review

AbstractThe development of functional portable optically stimulated luminescence (OSL) readers over the last decade has provided practitioners with the capability to acquire luminescence signals from geological materials relatively rapidly, which allows for expedient preliminary chronostratigraphic insight when working with complex depositional systems of late Quaternary age. Typically, when using the portable OSL reader, infrared (IR) or blue post‐IR OSL signals are acquired from bulk unprocessed materials, in contrast to regular luminescence dating, which is usually based on measurements on pure quartz or feldspar mineral separates, or on select silt‐sized polymineralic portions. To demonstrate the utility of portable OSL measurements, this paper outlines the basic features of portable OSL readers and their constraints. Subsequently, case studies in which the instrument has been used to elucidate cryptostratigraphic variations in sedimentary sequences for geomorphological applications are reviewed. The studies can generally be grouped into three main categories. The first includes studies where the variation of portable OSL reader luminescence signal intensities with depth are plotted to generate profiles that contextualize sediment stratigraphy. In the second group, portable OSL reader luminescence signal intensities are used to interpret sediment processes that shed light on depositional histories. In the last category, luminescence signals from the portable OSL reader are calibrated to approximate numerical burial ages of depositional units. The paper concludes with a discussion of possible future directions. © 2020 John Wiley & Sons, Ltd.

New cosmogenic nuclide burial-dating model indicates onset of major glaciations in the Alps during Middle Pleistocene Transition

A set of four outwash terraces in the northern Alpine Foreland motivated Penck and Brückner's classical scheme of four great Alpine ice ages: Günz, Mindel, Riss, and Würm. While it is now established that the Würm corresponds to marine isotope stages (MIS) 5d–2 (∼117–14 ka) and the Riss type locality to MIS 6 (∼191–130 ka), there is no consensus regarding the age of the older glaciations. The two oldest terraces, known as Höhere Deckenschotter (HDS) and Tiefere Deckenschotter (TDS) in Switzerland and neighbouring Germany, contain interbedded tills that directly indicate the first arrival of glaciers into the northern Alpine Foreland. Here, we set out to constrain the timing of the HDS, which signal the first major glaciations in the Alps. To achieve this goal, we devised a new burial-dating model tailored to glaciogenic sediments: P-PINI (Particle Pathway Inversion of Nuclide Inventories). The method applies a source-to-sink framework to a cosmogenic 10Be-26Al inversion model accounting for variable cosmic-ray exposure and non-steady erosion. Taking published 10Be-26Al data from five HDS sites (Feusi, Tromsberg, Siglistorf, Irchel Steig, and Irchel Hütz) and one TDS site (Iberig), we obtain age distributions (±1σ) that are especially well constrained for Feusi (0.93 ± 0.13 Ma), Iberig (0.93 ± 0.17 Ma), and Tromsberg (0.88 ± 0.14 Ma), less well-constrained for Irchel Steig (0.69 ± 0.25 Ma) and Siglistorf (0.94 ± 0.27 Ma), and very poorly constrained for Irchel Hütz (1.39 ± 0.56 Ma). Consistent with the morphostratigraphy, which dictates that the TDS postdates the HDS, we implemented a Bayesian modelling framework, yielding an age of 0.69 ± 0.12 Ma for Iberig (TDS) and a combined age of 0.95 ± 0.07 Ma for the HDS sites. Based on the P-PINI burial ages as well as the combined, Bayesian burial age, we propose an age around 1.0–0.9 Ma for the onset of the large Alpine glaciations that triggered the accumulation of the HDS outwash sediments. This roughly accords with the first long glaciation of the Pleistocene (MIS 24–22), identified as a step-change to colder climate and larger glaciations towards the end of the Middle Pleistocene Transition. While our results challenge previously reported ages of ∼2 Ma or more for the HDS in Switzerland, they corroborate evidence from the southern Alpine retroforeland and provide quantitative support for the early hypothesis by G.J. Kukla, who ascribed the oldest glacial deposits in the northern Alpine Foreland to around MIS 22. Finally, we suggest that the source-to-sink approach of P-PINI offers a viable alternative to the established isochron burial-dating method in cases involving non-steady exposure and erosion.

A multi-technique dating study of two Lower Palaeolithic sites from the Cher Valley (Middle Loire Catchment, France): Lunery-la Terre-des-Sablons and Brinay-la Noira

We present the results of a new dating study carried out at Lunery-la Terre-des-Sablons (LTS) and Brinay-la Noira (BN), two key Lower Palaeolithic sites located in deposits associated to the Cher River (Middle Loire Catchment, France). These sites preserve abundant Mode 1 and Mode 2 lithic industries, and are considered as among the oldest evidence of hominin presence in Western Europe north of the 45°N latitude. Following a multi-technique approach combining electron spin resonance (ESR), single-grain thermally-transferred optically stimulated luminescence (TT-OSL) dating of quartz grains and palaeomagnetism, we obtained new chronological constraints for the sedimentary sequence, and the associated lithic assemblages, at the two sites. The new independent dating results derived from each method are consistent and in overall agreement with existing ESR and terrestrial cosmogenic nuclide (TCN) burial age estimates, except for the Lowermost Unit 3 at LTS. By integrating all of the previous and new dating results, we derive combined age estimates of 772-735 ka and 665 ± 29 ka for the fluvial sands at LTS (Unit 1) and BN, respectively. These two distinct aggradation phases may tentatively be correlated to interglacial/glacial transitions associated with Marine Isotope Stage (MIS) 19-18 for the former and MIS 17-16 for the latter.At BN, an age range of 638–676 ka may be proposed for the hominin occupation after combining numerical age results and geological evidence. This result is consistent with the initial chronology proposed by Moncel et al. (2013) [Moncel, M.H., Despriée, J., Voinchet, P., Tissoux, H., Moreno, D., Bahain, J. J., Courcimault, G., Falgueres, C. (2013). Early evidence of Acheulean settlement in north-western Europe – la Noira site, a 700 000 year-old occupation in the Centre of France. Plos One, 8(11): 1–22] and confirms that BN is among the oldest Acheulean occurrences in Western Europe. At LTS, the mean age of 710 ± 50 ka obtained for the sandy layer in Unit 3 provides a minimum age constraint for the archaeological level located below. The major sedimentary disconformity observed between the fluvial deposits and the underlying pebble layers hosting the lithic artefacts suggests that the true age of the artefacts might be significantly older, probably Early Pleistocene given their similarities with other Mode 1 assemblages identified in Western Europe. However, further refinement of the Mode 1 chronological inference at LTS remains difficult at this stage.Finally, these new dating results show the importance of using the Multiple Centre approach for ESR dating of quartz grains, and confirm the value of combining different dating methods in order to build more robust chronologies for Lower Palaeolithic sites in Europe. From a methodological point of view, the dating results presented here are especially encouraging for the reliability of the ESR method applied to optically bleached quartz grains. This is one of the very first studies demonstrating that quartz samples independently dated by two different laboratories may produce generally reproducible ESR age results.

Development of cosmogenic nuclide capabilities in South Africa and applications in Southern African geomorphology

ABSTRACT Cosmogenic nuclide (CN) applications offer methods to quantify rates of landscape change and of dating geomorphological deposits and surfaces. Over the past two decades, CNs have been used to determine denudation rates, constrain uplift rates and determine burial ages of sediments in different geomorphological settings across the Southern African landscape. Here, we first give an overview of the landscape evolution of Southern Africa and how CN-based studies have contributed to this subject. Then we give a compilation of CN denudation data from Southern Africa, which show that average erosion rates have decreased by an order of magnitude since the late Cenozoic. Finally, we present the state of CN infrastructure and capabilities in South Africa as well as envisaged future developments. An Accelerator Mass Spectrometer (AMS) has become operational for carbon-14, beryllium-10 and aluminium-26 measurements at the National Research Foundation (NRF) iThemba Laboratory for Accelerator Based Sciences (LABS) in Gauteng since 2017. The measurements’ accuracy and precision compare well with other AMS laboratories internationally. Local sample preparation laboratories are currently available with limited capabilities and are mostly still under development.

Analyzing Statistical Age Models to Determine the Equivalent Dose and Burial Age Using a Markov Chain Monte Carlo Method

In optically stimulated luminescence (OSL) dating, statistical age models for equivalent dose (D<sub>e</sub>) distributions are routinely estimated using the maximum likelihood estimation (MLE) method. In this study, a Markov chain Monte Carlo (MCMC) method was used to analyze statistical age models, including the central age model (CAM), the minimum age model (MAM), the maximum age model (MXAM), <i>etc.</i> This method was first used to obtain sampling distributions on parameters of interest in an age model using D<sub>e</sub> distributions from individual sedimentary samples and subsequently extended to simultaneously extract age estimates from multiple samples with stratigraphic constraints. The MCMC method allows for the use of Bayesian inference to refine chronological sequences from multiple samples, including both fully and partially bleached OSL dates. This study designed easily implemented open-source numeric programs to perform MCMC sampling. Measured and simulated D<sub>e</sub> distributions are used to validate the reliability of dose (age) estimates obtained by this method. Findings from this study demonstrate that estimates obtained by the MCMC method can be used to informatively compare results obtained by the MLE method. The application of statistical age models to multiple OSL dates with stratigraphic orders using the MCMC method may significantly improve both the precision and accuracy of burial ages.

Rapid incision events since 3.7 ka in the Anjihaihe Gorge along the northern Tianshan range, China

Young river terrace levels distributed along active tectonic regions and major river channels record the histories of the most recent tectonic activities and geomorphological evolution. There are 18 terraces in the upper reach of the Anjihaihe River, along active zones of the northern Tianshan in China. The surface of these young terrace insets is covered by Aeolian loess and fluvial deposits. Consequently, optically stimulated luminescence (OSL) dating was used to reveal the burial ages of those sediments, and to calculate the tectonic deformation and fluvial incision rate. However, the ages of young quartz sediments are difficult to determine due to the weak and dim OSL signal characteristics, and therefore geological and geomorphological information was not forthcoming. Recently, however, a “simplified procedures” methodology was developed to overcome these dating problems. Loess deposits can constrain the minimum abandonment age and fluvial sands were used to provide the age limits of formation, which correspond to the maximum and minimum incision rates. The elevation of each terrace above the modern river bed was obtained using a Digital Elevation Map with 12-m resolution, Google Earth images, and portable Global Positioning System measurements. Together with the results of the OSL dating and the elevation measurements, we conclude that the Anjihaihe River has incised the gorge to a depth of ~260 m since 3.7±0.2 ka, with a maximum average incision rate of 7.2±0.1 cm/a. The incision record can be divided into four major stages: ~3.7−3.0, ~3.0−2.4, ~2.4−1.3 ka, and from 1.3 ka to the present; the respective average incisive rates are 3.1±0.3, 15±0.3, 8.5±0.2, and 4.6±0.4 cm/a. The results indicate that the incision occurred in a series of steps which coincided with environmental changes in the region. As a result, this set of 18 strath terrace levels has recorded the aggradational-degradational surface processes of the Anjihaihe River gorge since ~3.7 ka. Before ~3.7 ka, a thickness of ~260 m of fluvial sands was aggraded above the pre-Quaternary bedrock. Subsequently, the river incised these sand layers until T11, and then, before the formation of T11, younger fluvial sands were deposited on the top of older deposits after 2.4 ka. The AJH River then continued to vertically erode these younger sands until 1.3 ka, when the incision reached the pre-Quaternary bedrock. From 1.3 ka to the present, at least five strath terraces were formed in the gorge. These surface processes and rapid incisive rates are ascribed to the specific lithological and geomorphological characteristics of the area, under the influence of climate change. First, the bedrock of the terrace levels is composed of pre-Quaternary sandstones, and more homogeneous, less consolidated siltstones and sandy mudstones. In addition, the upper terraces are covered by mixed cobbles and sands with a thickness of ~100 m above the bedrock, and the lower terraces by a thin layer of fluvial sands above the pre-Quaternary bedrock. Second, the variations in the flux of river water and sediments during episodes of climate change influenced the slope of the terrace surface, resulting in changes in aggradational-degradational processes and in the energy of the incision. Climate change was also a driving force, because the abrupt changes in incision rate correspond well to anomalous climatic events. Overall, the specific geomorphological characteristics and location of the Anjihaihe River have resulted in its recording an unusually rapid incision rate within the Anjihaihe Gorge.

Testing the potential of K-feldspar pIR-IRSL and quartz ESR for dating coastal alluvial fan complexes in arid environments

Coastal alluvial fans (CAF) are important geo-archives due to their sensitivity to both tectonic activity and climatic changes. Thus, they can give key insights for geomorphic and sedimentary processes. In this study we test the potential of K-feldspar post infrared-infrared stimulated luminescence (pIR-IRSL) and quartz electron spin resonance (ESR) methods for dating alluvial fan complexes in arid environments. The existing chronological data from marine terraces that interact with CAF make the Atacama Desert in northern Chile an excellent setting for this research. Samples have been collected from alluvial, marine and aeolian sediments embedded in the CAF complexes, allowing evaluation of the different signal properties and bleaching characteristics of the pIR-IRSL and ESR signals over Late Pleistocene time scales.pIR-IRSL dose distributions of clast-rich alluvial fan samples are characterised by higher scatter and demonstrate heterogeneous bleaching, while matrix-rich alluvial fan deposits show rather homogeneous poor bleaching in the dose dispersion as indicated by a modern analogue sample. In contrast, marine and aeolian deposits are homogeneously well bleached, supported by a modern littoral sample. Following the quartz multiple centre (MC) ESR dating approach (Al and Ti centres), bleaching of the different centres prior to deposition has been achieved. While the Ti–H centre provides mostly lower doses than the Ti–Li centre, in most cases the Al centre provides the highest dose values. This pattern is consistent with their respective bleaching kinetics and suggests that the Ti centre signals most likely provide the closest estimate to the true burial dose for samples with doses >200 Gy.ESR and pIR-IRSL ages are consistent at 2σ for the marine, aeolian and clast-rich debris-flow deposits, which is in agreement with existing chronological data in this area. It appears that the mode of sediment transport on alluvial fans, either as matrix- or clast-rich flows, plays an important role in sediment bleaching. While clast-rich alluvial fan deposits are likely better bleached, we cannot exclude insufficient bleaching during matrix-rich alluvial fan flows; our dating results suggest that both pIR-IRSL and ESR dating overestimate the true burial age. The combination of pIR-IRSL and MC ESR dating can be considered as a promising tool for deciphering alluvial fan formation over (Late) Pleistocene time scales.

Initial incision of the Jinshan Gorge of the Yellow River, China, constrained by terrestrial in situ cosmogenic nuclides chronology

How post-burial terrestrial in situ cosmogenic nuclides (TCN) production is build-up in slow burial condition remains uncertain. In this study, we dated high fluvial terrace sediments that overlain by loess with a slow deposition rate that is based on the assumption of steady accumulation rate via 26Al/10Be burial dating method. For sample in deep and rapid buried condition prior to slow overlying, post-burial production shows low component (4–6%) in measured concentration. However, post-burial production occupied 39–79% of measured 26Al and 10Be concentrations, which suggests approximately 0.2–0.3 Ma underestimation in simple burial dating mode if post-burial production is neglected. Corrected by post-burial production, we yielded burial ages of two top terraces (ca. 1.7–2.5 Ma and 1.5 Ma, respectively) below the Tangxian planation surface along the Jinshan Gorge of the Yellow River, implying that the Jinshan Gorge initially incised at ca. 1.7–2.5 Ma, corresponding well to regional tectonic activities, such as extension in the Fenwei Graben, accelerative uplift of the Tangxian planation surface, vigorous dissection of North China, and significant tectonic deformation in northeastern Tibetan Plateau. The slow burial mode is probably a potential method for plateau paleo-elevation estimation taking advantage of height sensitivity of TCN production rate if overlying loess magnetostratigraphic age is known.

Determining the Plio-Quaternary uplift of the southern French Massif Central; a new insight for intraplate orogen dynamics

Abstract. The evolution of intraplate orogens is still poorly understood. Yet, it is of major importance for understanding the Earth and plate dynamics, as well as the link between surface and deep geodynamic processes. The French Massif Central is an intraplate orogen with a mean elevation of 1000 m, with the highest peak elevations ranging from 1500 to 1885 m. However, active deformation of the region is still debated due to scarce evidence either from geomorphological or geodetic and seismologic data. We focus our study on the southern part of the Massif Central, known as the Cévennes and Grands Causses, which is a key area to study the relationship between the recent geological deformation and landscape evolution. This can be done through the study of numerous karst systems with trapped sediments combined with the analysis of a high-resolution digital elevation model (DEM). Using the ability of karst to durably record morphological evolution, we first quantify the incision rates. We then investigate tilting of geomorphological benchmarks by means of a high-resolution DEM. We finally use the newly quantified incision rates to constrain numerical models and compare the results with the geomorphometric study. We show that absolute burial age (10Be∕26Al on quartz cobbles) and the paleomagnetic analysis of karstic clay deposits for multiple cave system over a large elevation range correlate consistently. This correlation indicates a regional incision rate of 83 +17/-5 m Ma−1 during the last ca. 4 Myr (Pliocene–Quaternary). Moreover, we point out through the analysis of 55 morphological benchmarks that the studied region has undergone a regional southward tilting. This tilting is expected as being due to a differential vertical motion between the northern and southern part of the studied area. Numerical models show that erosion-induced isostatic rebound can explain up to two-thirds of the regional uplift deduced from the geochronological results and are consistent with the southward tilting derived from morphological analysis. We presume that the remaining unexplained uplift is related to dynamic topography or thermal isostasy due to the Massif Central Pliocene–Quaternary magmatism. Integrating both geochronology and morphometrical results into lithospheric-scale numerical models allows a better understanding of this intraplate–orogen evolution and dynamic. We assume that the main conclusions are true to the general case of intraplate deformation. That is to say, once the topography has been generated by a triggering process, rock uplift is then enhanced by erosion and isostatic adjustment leading to a significant accumulation of mainly vertical deformation.

Empirical assessment of beta dose heterogeneity in sediments: Implications for luminescence dating

Abstract Optically stimulated luminescence (OSL) dating of single grains is often required to determine an accurate age for partially-bleached sediment by identifying those grains with OSL signals that were well bleached prior to burial. However, single-grain De distributions are typically characterised by greater amounts of scatter in comparison to multiple grains. Here we investigate the scatter in single-grain De distributions of quartz from 56 proglacial samples associated with the retreat of the last British-Irish Ice Sheet. Our findings provide the first empirical dataset showing that beta-dose heterogeneity can impact the extrinsic scatter in single-grain De distributions, in addition to partial bleaching in nature. The additional scatter in single-grain De distributions caused by beta-dose heterogeneity suggests that it is inappropriate to apply a fixed threshold to determine between well-bleached and partially-bleached De distributions, but the skewness of the De distributions could alternatively be used. Autoradiography and QEMSCAN analyses show that there was a negative relationship between the relative standard deviation (RSD) of the beta-dose heterogeneity and the beta dose-rate. This relationship offers the opportunity to infer the RSD of the beta-dose heterogeneity for each sample using just the beta dose-rate, instead of acquiring empirical data for every sample. For this large suite of sedimentary samples, we observe a minimum OD of 20% arising from the effects of beta-dose heterogeneity (Fig. 3e), which should be added (in quadrature) to the intrinsic OD to determine σb for the minimum age model (MAM) to calculate accurate OSL ages and prevent underestimation of the burial age.

Late Miocene to Quaternary slip history across the Qiulitag anticline in the southern Tianshan piedmont

AbstractIn this study, we reconstruct the Miocene to Quaternary shortening history across the Qiulitag anticline, a complex fault‐bend fold located in southern Tianshan. We studied the Yaha and Kuche sections, where we combined surface structural measurements and seismic imaging to model the stratigraphic horizons. The history of folding was reconstructed based on magnetostratigraphic analyses and eight cosmogenic burial ages in Kuche. Pleistocene deformation rates were also quantified in Yaha based on a deformed fluvial terrace that we dated to ~67 ka using a cosmogenic depth profile. Our results suggest that the fold grew at a mean slip rate of 0.9–1.3 mm/a in both sections but accelerated to ~2.5 mm/a during the Pleistocene in Kuche. These results support a migration of the deformation towards the basin during the Pleistocene and suggest that most of the present deformation in the Tianshan is accommodated across the external structures of the range.

Electron spin resonance dating of the culminant allostratigraphic unit of the Mondego and Lower Tejo Cenozoic basins (W Iberia), which predates fluvial incision into the basin-fill sediments

Abstract The Cenozoic basins of western Iberia have a culminant allostratigraphic unit (designated UBS13), which records the beginning of Atlantic drainage and predates the fluvial incision that led to the development of the present drainage networks. However, the available numerical dating is quite limited and mainly restricted to the lower-level terrace deposits. Therefore, this study uses for the first time the electron spin resonance (ESR) method to date this culminant unit in the Mondego and Lower Tejo Cenozoic basins of Portugal. The depositional age of this unit is supposed to lie between ~3.7 Ma (basal deposits) and ~1.8 Ma (uppermost deposits). The Al-centre provided reliable ESR data, but the dates obtained by using the Ti Li centre clearly underestimate the burial ages. With reference to the existing independent dating of the Vale Farpado site (3.7–3.6 Ma) at the lowermost basal level of the UBS13 deposits, the ESR (Al-centre) ages of 3.0 to 2.3 Ma obtained for the UBS13 basal and middle deposits give reliable estimates of the burial age. The ESR ages (Al-centre) obtained for the UBS13 uppermost deposits indicate a probable age of ~1.8 Ma. Thus, these results are of international significance, in that they constitutes the first numerical ages obtained for the uppermost levels of the Cenozoic basin-fills of western Iberia, which predate the fluvial incision that took place in response to lower Quaternary base levels.

Pressure-temperature-time paths from the Funeral Mountains, California, reveal Jurassic retroarc underthrusting during early Sevier orogenesis

Abstract New metamorphic pressure-temperature (P-T) paths and Lu-Hf garnet ages reveal a temporal correlation between Middle to Late Jurassic retroarc underthrusting and arc magmatism in southwestern North America. P-T paths were determined for 12 garnet porphyroblasts from six samples from the Chloride Cliff area of the Funeral Mountains in southeastern California. The composite path shows a pressure increase from 4.2 to 6.5 kbar as temperature increased from 550 to 575 °C, followed by a pressure decrease to 5.1 kbar during a further increase in temperature to 590 °C. Lu-Hf garnet ages from a pelitic schist (167.3 ± 0.7 Ma) and a garnet amphibolite (165.1 ± 9.2 Ma) place these P-T paths in the Middle Jurassic. We interpret the near-isothermal pressure increase portion of the P-T path to have developed during thrust-related burial, similar to lower grade rocks at Indian Pass, 8 km to the southeast, where garnet P-T paths show a pressure increase dated by the Lu-Hf method at 158.2 ± 2.6 Ma. We interpret the pressure decrease portion of the composite P-T path from the Chloride Cliff area to reflect exhumation contemporaneous with cooling in the Indian Pass area documented from muscovite 40Ar/39Ar step-heating ages of 152.6 ± 1.4 and 146 ± 1.1 Ma. The conditions and timing of metamorphism determined for the Indian Pass and Chloride Cliff areas, and isogradic surfaces that cut across stratigraphy, support the interpretation that the strata were dipping moderately NW during metamorphism, parallel to the thrust ramp that buried the rocks. Burial likely resulted from top-SE motion along the Funeral thrust, which was later reactivated as a low-angle normal fault with opposite motion to become the currently exposed Boundary Canyon detachment that was responsible for Miocene and possibly older exhumation. The part of the burial history captured by garnet growth occurred ∼6 m.y. before the 161 Ma peak of high-flux magmatism in the arc. Burial was contemporaneous with metamorphic ages from the western Sierra Nevada metamorphic belt, with the possible timing of accretion of arc terranes in northern California, and with the initiation of Franciscan subduction. Burial ages are also similar in timing with generally E-W crustal shortening in the retroarc that produced the East Sierra thrust system, the Luning-Fencemaker fold and thrust belt, the possible early history of the Central Nevada thrust belt, and the western thrusts of the southern Sevier belt. The timing of tectonic burial documented in this study and of high-flux magmatism in the arc supports the interpretation that the development of a coherent arc-trench system in the Early Jurassic resulted in the underthrusting of melt-fertile material beneath the arc along west- to northwest-dipping faults such as the Funeral thrust in the Jurassic, which penetrated the basement to the west as well as the roots of the magmatic arc, leading to increased magmatism.

Effects of long soil surface residence times on apparent cosmogenic nuclide denudation rates and burial ages in the Cradle of Humankind, South Africa

AbstractIn situ cosmogenic nuclides are an important tool for quantifying landscape evolution and dating fossil‐bearing deposits in the Cradle of Humankind (CoH), South Africa. This technique mainly employs cosmogenic 10‐Beryllium (10Be) in river sediments to estimate denudation rates and the ratio of 26‐Aluminium (26Al) to 10Be (26Al/10Be), to constrain ages of sediment burial. Here, we use 10Be and 26Al concentrations in bedrock and soil above the Rising Star Cave (the discovery site of Homo naledi) to constrain the denudation rate and the exposure history of soil on the surface. Apparent 10Be‐derived denudation rates obtained from pebble‐ to cobble‐sized clasts and coarse‐sand in soil (on average 3.59 ± 0.27 m/Ma and 3.05 ± 0.25 m/Ma, respectively) are 2‐3 times lower than the bedrock denudation rates (on average 9.46 ± 0.68 m/Ma). In addition, soil samples yield an average 26Al/10Be ratio (5.12 ± 0.27) that is significantly lower than the surface production ratio of 6.75, which suggests complex exposure histories. These results are consistent with prolonged surface residence of up to 1.5 Ma in vertically mixed soils that are up to 3 m thick. We conclude that the 10Be concentrations accumulated in soils during the long near‐surface residence times can potentially cause underestimation of single‐nuclide (10Be) catchment‐wide denudation rates in the CoH. Further, burial ages of cave sediment samples that consist of an amalgamation of sand‐size quartz grains could be overestimated if a pre‐burial 26Al/10Be ratio calculated from the surface production is assumed. © 2019 John Wiley & Sons, Ltd.

Seeking enlightenment of fluvial sediment pathways by optically stimulated luminescence signal bleaching of river sediments and deltaic deposits

Abstract. Reconstructing sediment pathways in fluvial and deltaic systems beyond instrumental records is challenging due to a lack of suitable methods. Here we explore the potential of luminescence methods for such purposes, focusing on bleaching of the optically stimulated luminescence (OSL) signal of quartz sediments in a large fluviodeltaic system across time and space. We approach this by comparing residual doses of sand and silt from the modern Mississippi River channel with estimated residual doses of sand isolated from Late Holocene Mississippi Delta mouth bar and overbank deposits. Further insight is obtained from a comparison of burial ages of paired quartz sand and silt of Mississippi Delta overbank deposits. In contrast to some previous investigations, we find that the bleaching of the OSL signal is at least as likely for finer sediment as for coarser sediment of the meandering Mississippi River and its delta. We attribute this to the differences in light exposure related to transport mode (bedload vs. suspended load). In addition, we find an unexpected spatiotemporal pattern in OSL bleaching of mouth bar sand deposits. We suggest this may be caused by changes in upstream pathways of the meandering channel belt(s) within the alluvial valley or by distributary channel and coastal dynamics within the delta. Our study demonstrates that the degree of OSL signal bleaching of sand in a large delta can be highly time- and/or space-dependent. Silt is shown to be generally sufficiently bleached in both the modern Mississippi River and associated paleo-deposits regardless of age, and silt may therefore provide a viable option for obtaining OSL chronologies in megadeltas. Our work contributes to initiatives to use luminescence signals to fingerprint sediment pathways within river channel networks and their deltas and also helps inform luminescence dating approaches in fluviodeltaic environments.

230Th/U burial dating of ostrich eggshell

Obtaining precise and accurate dates for late Quaternary terrestrial sequences containing Middle Stone Age or Middle Paleolithic archaeological materials (ca. 30–300 ka) can be challenging. Many such sequences lie beyond the limit of radiocarbon dating (∼50 ka) and lack volcanic ashes suitable for 40Ar/39Ar dating. We report a dating approach with about ten times the range of radiocarbon dating (∼500 ka) based on a novel protocol for 230Th/U dating of ostrich eggshell, a common component of African, Near Eastern and Asian archaeological sequences. Uranium in ostrich eggshell, typically at levels of tens to hundreds of ppb, is acquired after burial. However, accurate burial ages can be determined from measured 230Th/U ages when corrected for uranium uptake via diffusion. Using ostrich eggshells from a Pleistocene-Holocene east African site, we have: (1) characterized the spatial distribution of uranium and common thorium (232Th) concentrations in ostrich eggshell by laser ablation ICP-MS, (2) determined apparent 230Th/U ages on outer and inner layers of eggshells by solution ICP-MS analyses of selectively abraded eggshells, and (3) calculated 230Th/U burial ages using a simple model for diffusive uptake of uranium. We assess our method by comparing the 230Th/U burial ages (median uncertainty 2.3%, 1 SD) with radiocarbon dates on carbonate from the same eggshells. We find good agreement in seven out of nine eggshells, confirming the accuracy of their 230Th/U model ages. Eggshells that fail to yield reliable 230Th/U ages may be recognized by anomalous patterns of apparent 230Th/U ages, providing a reliability criterion innate to the 230Th/U data. Ostrich eggshells are resistant to diagenesis simplifying 230Th/U dating compared to bones and teeth. Eggshells of ostrich, emu, rhea and other living and extinct ratites found on six continents share similar structure and composition, suggesting our new approach to 230Th/U dating of eggshell may be applicable in diverse late Quaternary terrestrial settings.

Chlorine-36∕beryllium-10 burial dating of alluvial fan sediments associated with the Mission Creek strand of the San Andreas Fault system, California, USA

Abstract. We apply cosmogenic-nuclide burial dating using the 36Cl-in-K-feldspar∕10Be-in-quartz pair in fluvially transported granitoid clasts to determine the age of alluvial sediment displaced by the Mission Creek strand of the San Andreas Fault in southern California. Because the half-lives of 36Cl and 10Be are more different than those of the commonly used 26Al∕10Be pair, 36Cl∕10Be burial dating should be applicable to sediments in the range ca. 0.2–0.5 Ma, which is too young to be accurately dated with the 26Al∕10Be pair, and should be more precise for Middle and Late Pleistocene sediments in general. However, using the 36Cl∕10Be pair is more complex because the 36Cl∕10Be production ratio varies with the chemical composition of each sample. We use 36Cl∕10Be measurements in samples of granodiorite exposed at the surface at present to validate calculations of the 36Cl∕10Be production ratio in this lithology, and then we apply this information to determine the burial age of alluvial clasts of the same lithology. This particular field area presents the additional obstacle to burial dating (which is not specific to the 36Cl∕10Be pair, but would apply to any) that most buried alluvial clasts are derived from extremely rapidly eroding parts of the San Bernardino Mountains and have correspondingly extremely low nuclide concentrations, the majority of which most likely derive from nucleogenic (for 36Cl) and post-burial production. Although this precludes accurate burial dating of many clasts, data from surface and subsurface samples with higher nuclide concentrations, originating from lower-erosion-rate source areas, show that the age of upper Cabezon Formation alluvium is 260 ka. This is consistent with stratigraphic age constraints as well as independent estimates of long-term fault slip rates, and it highlights the potential usefulness of the 36Cl∕10Be pair for dating Upper and Middle Pleistocene clastic sediments.

A homogeneous liquid reference material for monitoring the quality and reproducibility of in situ cosmogenic 10Be and 26Al analyses

Analysis of the in situ produced cosmogenic nuclides 10Be and 26Al has been used for several decades to study Earth surface processes; yet, no reference material has been widely adopted and used by the community to ensure the quality and comparability of sample preparation and isotopic analyses. Such a reference material could facilitate inter-laboratory comparison and serve as a benchmark for quantifying the accuracy, precision, and long-term reproducibility of terrestrial cosmogenic nuclide analyses. Here, we describe a liquid reference material, UVM-A, which is freely available to the community. We created four liters of solution by dissolving CRONUS-A granular reference material [1], and spiking the solution with stable 9Be and 27Al, yielding a solution containing 236.7 ± 0.6 μg g−1 Be and 932.6 ± 1.6 μg g−1 Al (average, 1SD). Initial analyses of UVM-A indicate that the nuclide ratios of the reference material (10Be/9Be 1.43 ± 0.02 × 10−13; 26Al/27Al 4.44 ± 0.05 × 10−13; average, standard error) are similar to those of late-glacial exposure samples, are tightly clustered, and match the nuclide concentrations expected from the CRONUS-A quartz used to prepare the reference material. Use of this reference material will help to quantify uncertainty in cosmogenic exposure ages, burial ages, and erosion rates resulting from sample preparation and isotopic analysis.