Abstract. (U-Th-Sm) / He is a thermochronometric method used to reconstruct the rates and timing of geological processes. Recent developments in analytical approaches, specifically laser ablation (in situ) measurements, allow quantifying the distribution of parent isotopes (U, Th, and, in apatites, Sm) and decay products (4He) within individual mineral grains. This is particularly important to understand potential date over-dispersion, which can arise from the heterogeneous distribution of parent isotopes, and to develop thermal history modelling for single-grain (U-Th-Sm) / He techniques. We build on previous studies and combine in situ 4He concentration profile measurements with parent nuclide distribution mapping in natural apatites to explore analytical and modelling strategies for single-grain thermal history reconstructions. Specifically, we investigate the effects of laser ablation spot size, the number and location of ablation spots in a grain, and grain size on data resolution and suitability for thermal history modelling. In doing so, we introduce the calculation of Caw, which is the concentration of parent nuclides at each ablation site weighted by alpha-particle stopping distances to account for the redistribution of 4He in the crystal from high-energy alpha decay. We present stacked U, Th, and Sm maps measured at different ablation depths in two apatite grains from South Germany (one with homogeneous and one with zoned parent isotope distribution) and one apatite from the McClure Mountain Syenite age standard. Furthermore, we show in situ 4He profiles of the two South German apatites and inversions for thermal histories. Our results indicate that, for our study and instrument set-up (a RESOchron system (Applied Spectra) consisting of a He-line and an excimer laser), four to six spot measurements at various distances from the grain rim enable measuring an in situ 4He profile. We tested different laser ablation spot sizes (10–30 µm) in grains with a range of 4He concentrations and (U-Th-Sm) / He dates (16 to ∼ 200 Ma) and determined that the optimal spot diameter for in situ 4He profile measurements for apatite grains with (U-Th-Sm) / He dates as young as 16 Ma is 20–30 µm. Additionally, with an ablation spot diameter of 20 µm, a six-spot in situ 4He profile requires a minimum grain diameter (measured perpendicular to the c-axis) of 145 µm. Combined with information from detailed parent nuclide maps, the in situ 4He profiles offer a possibility to reconstruct the thermal histories of single grains, potentially including zoned and irregularly shaped crystals.

Abstract. The rate at which ice sheets erode rock and produce sediment is poorly known. Here, we use paired cosmogenic nuclides in both deglacial and modern sediment to understand better the efficacy with which the Quebec-Labrador Ice Dome (QLID) of the Laurentide Ice Sheet eroded bedrock and generated sand and boulders across the landscape of eastern Canada. We sampled deglacial sediment (esker and delta sand, n= 10), sediment from modern streams (n= 11), one bedrock outcrop, and a bedrock depth profile (n= 7), measuring concentrations of 10Be and 26Al in quartz isolated from all samples. We also collated published cosmogenic nuclide measurements of boulders and bedrock from eastern Canada (n= 237 samples), and using independent estimates of deglaciation timing, calculated initial nuclide concentrations when the material was exposed by the most recent deglaciation, between 6.3 to 15.2 ka. At the time of deposition, all 10 deglacial sand samples contained 10Be and 26Al, on average equivalent to several thousand years of surface exposure. The ubiquitous presence of 10Be and 26Al in eastern Quebec deglacial sediment is consistent with older-than-expected exposure ages for bedrock outcrops (n= 26 of 46 samples) and boulders (n= 65 of 192 samples) once covered by the QLID. Error-weighted averages of 26Al / 10Be ratios for both deglacial (6.1 ± 0.3, all uncertainties 1 SD) and modern sediment samples (6.6 ± 0.5) are lower than the measured production ratio at high latitudes (Greenland, 7.3 ± 0.3), suggesting cumulative burial of at least some sediment grains for at least hundreds of thousands of years. This burial history suggests that ice at the middle of the QLID either survived some interglacials and/or that the average sediment residence time on the landscape is several times longer than a 100 kyr glacial cycle, allowing storage and burial of sediment over multiple glacial cycles, either under ice and/or in thick deposits such as deltas and moraines. Modern river sand contains on average only slightly higher nuclide concentrations than deglacial sediment, suggesting that contemporary river sand is predominately recycled from glacial deposits. Together, the new sediment data (which amalgamate across large areas of the landscape), and our compilation of bedrock and boulder point data, suggest that the average depth of bedrock erosion by ice and the speed of glacial sediment transport in eastern Canada were insufficient to remove material containing cosmogenic nuclides produced during prior interglacial(s).

Abstract. Fission track thermochronology is based on the visual analysis of optical images. This visual process is prone to observer bias. Fission track datasets are currently reported as numerical summary tables. The interpretation of these tables requires a high degree of trust between the fission track analyst and the user of the data. geochron@home is software that removes this requirement of trust. It combines a browser-based “virtual microscope” with an online database to provide FAIR (Findable, Accessible, Interoperable and Reproducible) access to fission track data. geochron@home serves four different purposes. It can be used (1) to count fission tracks in “private mode”, i.e. hidden from other users on the internet; (2) to archive fission track images and counts for inspection by other users; (3) to create tutorials for new students of the fission track method; and (4) to serve randomly selected selections of images to citizen scientists. We illustrate these four applications with examples that demonstrate (1) geochron@home's ability to compare and combine fission track counts for multiple users within a lab group; (2) the value of the geochron@home archive in the peer review system; (3) the use of simple tutorials in teaching novice users how to count fission tracks; and (4) the opportunities and challenges of crowd-sourced fission track analysis. geochron@home was written in Python and Javascript. Its code is freely available for inspection and modification, allowing users to set up their own geochron@home server. Alternatively, users who would like to upload data to the archive, but do not have the facilities to set up their own server, may use the server at University College London free of charge. The archive accepts image stacks acquired on any type of digital microscope, and accommodates fission track data (counts and length measurements) from external fission track analysis suites such as Fission Track Studio and TrackFlow. We anticipate that the introduction of FAIR workflows will make fission track data more accurate and more future proof. Storing fission track data online will benefit future developments in fission track thermochronology. For example, archival datasets of peer reviewed fission track counts can be used to train and improve machine learning algorithms for automated fission track analysis. We invite other geochronological methods to follow the fission track community's lead in FAIR data processing. This would benefit all the Earth Science disciplines that depend on geochronological data.

Abstract. Constructing accurate age models for Pleistocene marine sediments is crucial for our understanding of glacial-interglacial cycles and other climatic processes. Benthic foraminiferal δ18O stacks, a proxy for ice sheet and climate evolution, are often used for stratigraphic alignment and chronology development in deep-sea sedimentary records, in combination with biostratigraphy, paleomagnetism, and radioisotopic constraints. Selection of an appropriate benthic δ18O alignment target influences the derived chronology at a given site, and divergent regional trends in benthic δ18O highlight the need for ocean-specific benthic δ18O stacks. The specific scientific question to be addressed by a study may also influence whether the alignment target should include astronomical tuning. Here, we introduce three benthic δ18O stacks – Atlantic, Pacific, and global – with three distinct chronologies for the global stack that incorporate astronomical forcing constraints to various degrees. The new global stack utilizes data from 221 cores and includes 45 % more data than the previous “ProbStack” (Ahn et al., 2017). Hand-tuned regional and global stacks, intended as updates to the “LR04” stack (Lisiecki and Raymo, 2005), incorporate chronologies transferred from absolutely dated archives during 0–654 thousand years ago (ka) and an astronomically forced ice sheet model during 654–2700 ka. Due to the heterogeneous nature of the age constraints used for these stacks, we call them BIGSTACKmixed, BIGSTACKmixedA, and BIGSTACKmixedP. For applications where astronomical tuning should be minimized, we present a global stack primarily constrained by geomagnetic reversal age estimates, BIGSTACKmagrev. We also develop a third age model, BIGSTACKauto, which uses an automated optimization algorithm to “minimally tune” the stack to the pervasive ∼ 41 kyr obliquity cycle, while avoiding assumptions about astronomical phase relationships. This suite of stacks offers flexibility in choosing δ18O stratigraphic alignment targets, to allow a wide range of applications in paleoceanographic hypothesis testing.

Abstract. Quantifying timescales and establishing robust eruption chronologies is critical for understanding the evolution and hazards of volcanic systems. U-Th disequilibrium dating on zircon is especially valuable for young and active systems (< 300 ka). However, there is no consensus on how to calculate U-Th crystallization ages. To address this, we applied an optimized LA-ICP-MS U-Th-Pb double-dating strategy that simultaneously retrieves U-Th and U-Pb ages from the same zircon ablation volume. This dating routine increases confidence in crystallization ages across 150–300 ka, where the resolution of either dating technique alone is limited. We applied this strategy to the Kos Plateau Tuff, which spans this critical interval, and compared U-Th model age calculation approaches against the well-established U-Pb age calculations. U-Th model ages calculated using the two endmember approaches, either using a constant melt composition or a constant zircon-melt U / Th fractionation factor (fU/Th), yield similar age spectra when well-estimated values are used. In this context, it is essential to evaluate whether the measured groundmass glass or whole-rock composition truly reflects the zircon-forming melt. This can be assessed by comparison with the youngest isochron intercept on the secular equilibrium line, which provides an independent melt composition estimate. We also evaluated eruption age estimation methods using synthetic U-Th datasets, with increasing uncertainty toward older ages. Bayesian models, particularly those with uniform priors, consistently outperformed weighted mean methods in terms of accuracy and precision and are therefore recommended for eruption age estimates in volcanic U-Th zircon datasets.

Abstract. Quartz Thermoluminescence (TL), Optically Stimulated Luminescence (OSL) and Electron Spin Resonance (ESR) offer valuable quantitative tools both for understanding sediment provenance and surface processes. However, the variability of quartz sensitivity remains an issue, attributed either to the intrinsic properties of source bedrock, to processes during sediment transport and deposition, or to both. This study addresses these questions by investigating quartz from magmatic, metamorphic, and sedimentary formations in the Strengbach catchment (Vosges Massif, France). Using a combination of ESR, TL, OSL, and LA-ICPMS (Laser-Ablation Inductively Coupled Plasma Mass Spectrometry) trace element analyses, our study reveals significant relationships between quartz TL-OSL/ESR sensitivities and source bedrock characteristics, such as lithology, crystallization conditions, and deformation histories. ESR Ti-centre and TL-OSL signals are notably influenced by trace elements like Al, Li, and Ti. Quartz that underwent high pressure during metamorphism along with those located in the tectonic shear zone show both lowest TL-OSL and ESR intensities, while higher sensitivities are observed in quartz from plutonic rocks and sandstones. This suggests that (i) pressure can be one of the prevailing factors driving changes in quartz TL-OSL/ESR sensitivities (ii) enhanced quartz TL-OSL sensitivity in mature and recycled sediments (sandstones) highlight the importance of sedimentary transport and reworking on TL-OSL and ESR signals. Our results highlight the need for careful interpretation of ESR and TL-OSL signals, both for dating or sourcing, particularly in sediments derived from metamorphic terrains.

Abstract. We describe a “virtual mineral separation” method for measuring the cosmogenic 3He concentration in pyroxene in mafic rocks that consist mainly of plagioclase and pyroxene, without physically separating the minerals. This approach is significantly faster and more cost-effective than the conventional method, which requires physical separation and purification of pyroxene grains by time-consuming and labor-intensive crushing, acid cleaning, magnetic separation, HF etching, and handpicking under a microscope. The premise of the method is that helium diffusivity is much higher in plagioclase than in pyroxene, so controlled preheating of a mixed whole-rock sample can degas 3He from plagioclase while retaining all 3He in pyroxene. A second heating step releases all 3He from pyroxene for measurement. To then obtain a 3He concentration in pyroxene rather than the whole rock, we determine the pyroxene weight fraction in the sample using X-ray computed tomography (CT). A comparison of 3He concentrations in pyroxene measured using virtual mineral separation with those measured in the same samples by physical mineral separation in previous work shows no evidence of systematic bias between the methods. Virtual mineral separation greatly simplifies the workflow for 3He exposure-dating of mafic rocks, reduces time, effort, and cost, and permits measurements on very small samples. This enables new emerging applications of exposure dating, such as quantifying stochastic surface processes, ecosystem studies, and potential subglacial bedrock exposure dating.

Abstract. Quantifying environmental radiation dose rates is an essential step in age calculation using trapped charge dating methods. A means of rapid dose rate estimation would therefore be useful for a variety of reasons, especially in contexts where rapid equivalent dose estimates are available. For instance, for informing sampling strategy, providing initial age estimates, or supporting portable luminescence studies. However, high-precision methods often used to calculate dose rates are typically time consuming and expensive and are impractical for such “range-finder” applications. Portable X-ray fluorescence (pXRF) offers a rapid means of measuring the potassium (K) concentration of sediment, although the other radionuclides typically used to calculate dose rates, uranium (U) and thorium (Th), fall beneath its detection limits at the quantities at which they are usually present in sediments. In this study, we investigate whether pXRF measurements of K concentration alone can be used to estimate total environmental dose rates. A large, global dataset of 1473 radionuclide samples is used to generate a set of regression relationships between (1) K concentration and external beta dose rate; (2) external beta and gamma dose rates; and (3) external gamma and alpha dose rates. We test the utility of these relationships by measuring the K contents of 67 sediment samples with independent, high-precision radionuclide data from a variety of contexts using pXRF. The resulting K concentrations are then converted to external dose rate estimates using these equations. A simplified set of attenuation parameters are used to correct infinite matrix dose rate estimates, and these are combined with cosmic ray and internal contributions to rapidly calculate total environmental dose rates for a range of theoretical, common luminescence-dating scenarios (such as 180-250 µm quartz that has undergone etching). Results show that pXRF can accurately measure K concentrations in a laboratory setting. The regression equations can predict external beta dose rates to a good degree of accuracy based on K content alone, whilst external gamma dose rates are predicted less accurately, and external alpha dose rates are predicted the least accurately. In combination, total estimated dose rates show good agreement with their counterparts calculated from high-precision methods, with 95 % of our results lying within uncertainties of ±10 % of unity for scenarios where the alpha dose rate contribution is assumed to be negligible. Whilst alpha dose rate contributions are predicted the least accurately, scenarios including an alpha component result in at least 80 % of predictions lying within uncertainties of ±10 % of unity. The use of simplified attenuation factors to correct estimated infinite matrix dose rates does not contribute significantly to resulting scatter. This study serves as a proof of concept that pXRF measurements, along with a set of regression equations and a simplified correction procedure, can be used to rapidly calculate range-finder environmental dose rates.

Abstract. Chemical abrasion (CA), a two-step process of annealing and partial dissolution, is routinely applied to zircon grains prior to U–Pb geochronology to dissolve portions of the grains affected by Pb loss prior to analysis. Despite the utility of the technique, it is not clear what the more HF-soluble material produced in the annealing step is, what degree of lattice damage causes it to form instead of zircon, how to predict if a specific sub-volume of a zircon will survive CA, or how any of these processes relate to Pb mobility. In this study, we use secondary ion mass spectrometry (SIMS), Raman spectroscopy, and atom probe tomography (APT) to constrain what happens to both concordant and discordant zircon during each step of the CA process. We find that zircon in SIMS sputter craters which have undergone Pb loss generally have more heterogeneous Raman band widths than in those sputter craters where Pb has been retained. Annealing drastically reduces Raman band widths, but some heterogeneity is still present in discordant sputter craters. APT results from all samples which successfully ran were homogeneous in U, Pb, Th, and most other elements in all cases. This makes it hard to link Pb loss and lattice damage at the submicrometre scale by direct imaging in this study. However, as the zircon sputter craters with Pb loss show homogeneous APT results, we recommend against using homogeneous APT results as an indicator of closed-system U–Pb behaviour.

Abstract. The Llanos de Moxos, in the Bolivian Amazon, preserves a remarkable archaeological record, featuring thousands of forest islands. These anthropogenic sites emerged as a result of activities of the earliest inhabitants of Amazonia during the Early and Middle Holocene. Excavations conducted to date on the forest islands have revealed that many assemblages contain a high number of ancient freshwater snail remains. In these shell middens, the most represented mollusc taxon, and in most cases the sole one, is Pomacea spp., a genus that inhabits inland shallow lakes and wetlands. Although human burials and faunal remains are typically recovered from these sites, their collagen is often not preserved or is of poor quality, and shell carbonates from Pomacea shells, along with carbonised plant remains, are often used for 14C measurements. However, it remains undetermined if these measurements are subject to radiocarbon reservoir effect (RRE). To determine if a freshwater RRE could affect the age estimations of Amazonian archaeological and other paleoecological deposits, we collected modern coeval Pomacea shells and tree leaves from four locations across the Llanos de Moxos area for AMS radiocarbon dating. The radiocarbon results combined with the environmental history of Llanos de Moxos during the Holocene, do not reveal any significant RREs, and support the continued use of freshwater molluscs as viable material for radiocarbon dating in the region.