A noble-gas analysis line capable of accurate and precise measurements of small absolute amounts of 4He released from crystals is a key analytical step in the production of (U-Th)/He chronologic data. He analysis lines that are custom-built in-house can be optimized for specific lab needs and facilitate continued maintenance, repair, and upgrades. However, there is little information in the published literature about the methods and approaches for building a He line. Here, we describe the design, construction, automation, and metrological calibration of a custom 4He extraction and analysis line as part of establishing laser-ablation (U-Th)/He methods in the University of Colorado Thermochronology Research and Instrumentation Lab (CU TRaIL). The line, called the Jimbochron, is designed to precisely measure very small (∼ fmol) amounts of 4He while being fully automated and easily modifiable in the future. These goals are achieved by minimizing the line volume, adopting a unique double-hexagonal manifold configuration, installing a high-sensitivity quadrupole mass spectrometer, and developing editable LabView code for instrument communication and automation that is straightforward to update. We also explain the steps used to calibrate the Jimbochron metrologically from first principles with a new in-house calibration volume to ensure high-accuracy He measurements. The Jimbochron accurately and precisely measures small gas amounts (< 0.1 fmol) and now routinely generates accurate and precise He data for laser-ablation (U-Th)/He applications.
Abstract. Cosmogenic-nuclide surface exposure ages provide empirical data for validating models simulating the timing and pace of ice-sheet response to a warming climate. Increasing emphasis is being placed on obtaining exposure ages that both accurately constrain Holocene deglaciation and are precise enough to capture ice sheet change at the sub-millennial scale. However, longer-lived nuclides such as 10Be are susceptible to cosmogenic nuclide inheritance often persisting through multiple periods of exposure and burial, which can impact the accuracy of the most recent Holocene exposure history. Shorter-lived in situ cosmogenic 14C (in situ 14C) is largely insensitive to nuclide inheritance pre-dating the last glacial maximum (LGM), and when combined with longer-lived nuclides can be used to constrain complex ice sheet histories over Holocene timescales. Here, we present new in situ 14C exposure ages from nine erratic cobbles from Mount Murphy, West Antarctica. Six of these suggest Mt Murphy deglaciated from 5–3 ka; this is inconsistent with previously measured 10Be ages of the same samples that place deglaciation from 8–6 ka. We investigate potential explanations for the conflicting exposure histories by analysing paired 14C–10Be data of Holocene age presently archived in the informal cosmogenic-nuclide exposure-age database (ICE-D, https://version2.ice-d.org/, last access: 29 March 2024). Our analysis reveals that neither variations in geologic setting nor modelled scenarios of subsurface nuclide production can explain the conflicting Mt Murphy ages. However, replicate in situ 14C measurements indicate that initial in situ 14C concentrations used to calculate the youngest exposure ages (5–3 ka) do not reproduce within stated 2σ uncertainty, whereas measurements used to calculate the older ages (8–6 ka) are reproducible. Furthermore, we observe that in situ 14C concentrations measured in 15 of 31 samples taken from ICE-D do not replicate within their nominal 2σ analytical uncertainty. Together, these results suggest that analytical uncertainty for in situ 14C measurements may currently be underestimated. We provide recommendations for improving measurement precision that will benefit future Holocene deglaciation studies, including analysis and publication of more replicate measurements and the continuation of efforts to quantify and minimise sources of scatter in blank measurements.
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.
It is generally accepted that the commonly used fission track age equation accurately calculates the cooling age for apatite minerals when the cooling rate is fast. Nevertheless, it is used when the cooling rate is gradual, for example when the age of transition through the partial annealing window is to be estimated. Added age information is here obtained by inclusion of the length distribution of fully included near horizontal tracks. The tendency that the shortest tracks are the oldest ones, and the longest ones are the youngest enables the age dating of a given track by counting the number of shorter tracks, adding one, and dividing by the volumetric track generation rate. The difficulty is that the track length-age relation is blurred by the spread in lengths due to the inherent spread in fission decay energies, crystallographic anisotropy and observational uncertainties. The blurring can be reduced by mathematical deconvolution in which the blurring of tracks in annealing experiments is used. A previously given equation for the oldest track in each histogram column is mathematically further reduced. This paper presents a method where deblurring is first performed by projecting the observed track lengths on the mineral c-axis and then by deblurring using probabilistic least squares inversion. This leads to the extraction of several track ages with deviations for each deconvolved track length histogram. This information may be used to constrain the timing of tectonic events and provide the basis for calculation of past temperature.
Cosmogenic nuclide surface-exposure dating (SED) is a rapidly growing tool in geoscience owing to its unrivalled potential for directly dating rock surfaces and thus the geomorphic and climatic events they represent. Fundamental to the efficacy of the SED method is reliable constraint of the in situ production rate, which is typically calculated via calibration experiments: cosmogenic nuclide concentrations are measured in surfaces for which the true exposure age is known independently, allowing the production rate to be derived (in atoms g-1 yr-1) for the specific calibration site. This value can then be extrapolated to distal field sites using numerical scaling methods designed to account for spatial and elevational differences in geomagnetic and atmospheric shielding. Thanks to successive and increasingly co-ordinated calibration efforts, production rate estimates for the most widely used cosmogenic nuclide, beryllium-10 (10Be), have improved in recent decades, with the majority converging on sea-level high-latitude (SLHL) values of similar to 3.8-4.1 atoms g-1 yr-1 ("St" scaling). Nonetheless, there remains sufficient variability among production rates to undermine the reliability of derived surface-exposure ages, particularly for applications to short-lived events such as the abrupt climate shifts of the last glacial termination. To help address this uncertainty, this paper reports new 10Be concentrations from deglacial surfaces on the Redpoint Peninsula in north-west Scotland that were exposed during retreat of the last British ice sheet. By comparing the surface-exposure results from eight current 10Be production rates to local radiocarbon constraint for deglaciation, we (1) evaluate the viability of each production rate for this site and (2) report a maximum SLHL value of 3.925 +/- 0.07 atoms g-1 yr-1 ("St" scaling), above which resulting surface-exposure ages will be too young with respect to the Redpoint radiocarbon chronology. This study also demonstrates that the Rannoch Moor 10Be production rate, calibrated against independently dated glacial landforms in the central Scottish Highlands, gives the best match with the 14C control and thus is appropriate for Late Pleistocene applications at these geomagnetic latitudes.
High-temperature contact metamorphism in the aureole of the 1322 Ma Makhavinekh Lake Pluton, Labrador, led to progressive consumption of 1850 Ma garnet formed during upper-amphibolite facies regional metamorphism that produced migmatitic paragneiss (Tasiyuak Gneiss). Biotite Rb-Sr isotope measurements were carried out in situ by laser ablation ICP-MS/MS allowing biotite in a variety of textural settings to be characterized. This natural laboratory provides important information about the nature of Rb-Sr closure temperature (T-c) as a function of textural setting in highgrade metamorphic rocks. Intact biotite inclusions armoured in garnet preserved in the outer aureole (> 4 km from the contact) display a range of Rb-Sr isochron ages between similar to 1850 and similar to 1322 Ma consistent with a zone of partial retention of Sr in biotite. Isotopic resetting in the outer aureole was controlled by microfractures in garnet that provided short-circuit diffusion pathways for redistribution of radiogenic Sr into plagioclase-bearing contact metamorphic assemblages; biotite inclusions isolated from microfractures retain 1850 Ma Rb-Sr isochron ages. Biotite grains falling along a similar to 1322 Ma isochron attest to efficient intra-and intercrystalline Sr diffusion at T >= 500 degrees C on timescales of >= 5 Myr. Samples in the central part of the contact aureole (3.7 to 1.1 km from the contact) contain partly resorbed biotite surrounded by contact metamorphic Opx + Crd coronal assemblages in addition to armoured inclusions in relict garnet. These display similar Rb-Sr behaviour to outer aureole samples with the exception that similar to 1322 Ma biotite domains display higher Rb/Sr due to more extreme loss of Sr. In the inner aureole, where garnet was completely consumed by contact metamorphic assemblages, a new generation of biotite neoblasts grew textural equilibrium with Opx + Crd. This biotite preserves Rb-Sr ages <= 1322 Ma with initial Sr-87/Sr-86 best interpreted as a mixture of radiogenic Sr accumulated in regional biotite and whole-rock Sr liberated from low-Rb/Sr regional metamorphic garnet, apatite, and plagioclase. This study reveals how the exact textural setting of biotite in high-grade metamorphic rocks influences the preservation of Rb-Sr ages and demonstrates that there is no universal closure temperature for biotite Rb-Sr. It also reveals that in situ Rb-Sr dating of granulite-facies rocks might provide robust chronometric data if grains isolated from intergranular diffusion are systematically evaluated to reveal zones of partial retention.
Post-infrared infrared stimulated luminescence (post-IR IRSL) signals from potassium feldspars are gaining prominence in both luminescence dating and luminescence-based sediment tracing techniques. To enhance the accuracy and reliability of these applications, it is essential to develop a comprehensive understanding of how post-IR IRSL signals undergo bleaching. While previous studies have explored post-IR IRSL bleachability using multi-grain approaches, a systematic single-grain investigation on modern analogues has not been conducted. In this study, we examined the bleaching behaviour of the post-infrared infrared stimulated luminescence signal measured at 200 degrees C (post-IR IRSL200) at the single-grain level in eleven modern floodplain samples from the tectonically active Southern Central Andes. Our study demonstrated considerable variation in the residual doses following 2 d of laboratory solar simulator bleaching across the sample set. This variability was evident not only between different samples but also among individual grains within the same sample. Thus, we evaluated the influence of bleaching duration, grain-specific geochemical composition, catchment-scale lithological variability, and the size of the natural dose on the laboratory-measured residual doses.Our laboratory bleaching experiments in which single grains were given a fixed regenerated dose of 30 Gy prior to solar simulator exposure showed similar post-IR IRSL200 signal bleaching behaviour across four different samples, reaching a plateau based on normalised luminescence signal after 2 d of exposure to solar simulator light. While individual grains exhibited a wide range of bleaching rates, this variability did not account for the spread in residual dose values. Notably, extended light exposure reduced variability in signal intensity, underscoring its role in dose homogenisation. Geochemical analysis of major oxides showed no significant correlation with either residual dose magnitude or bleaching rate, suggesting that mineral composition (including K-concentration) does not influence bleaching efficiency at the individual grain level. Furthermore, bleaching behaviour remained consistent across samples regardless of catchment lithology, with no discernible relationship between lithological units and remnant dose (defined as the natural dose remaining at the time of deposition and burial) or residual dose values. Most importantly, we identified a strong positive linear correlation (R2= 0.89) between residual dose and natural remnant dose, revealing dose-dependent bleaching efficiency and the presence of a negligible unbleachable component at the time of deposition. This relationship between residual dose and natural remnant dose also suggests that while the youngest samples (with low natural remnant dose) could reach zero residual dose, the relatively older samples (with more than 10 Gy of natural remnant dose) could show a significant amount of residual dose. By integrating insights on bleachability with the information on the unbleachable component and remnant doses derived from modern analogues, we highlight the limitations of correcting palaeodoses by directly using either residual or remnant doses and evaluate three context-sensitive correction strategies. Finally, we discuss how residual doses can be leveraged to more reliably identify well-bleached grains, enhancing the accuracy of luminescence-based sediment tracing applications.
The Christiana Islands group consists of three at present uninhabited islands 20 km SW of Santorini, Aegean Sea, Greece, that are the subaerial remnants of the Christiana volcano. The age of the Christiana Islands has been unclear and has been previously assumed to have started around the same time as the emergence of Santorini (600 ka). Other studies, based on seismic reflection, have correlated volcanic deposits of the Christiana archipelago to Pliocene sedimentary layers. Five subaerial Christiana volcanic rocks of the Upper Lava formation cluster tightly between 2.57–2.69 Ma with relatively small uncertainties (0.02–0.03 Ma). One sample dated much younger: 133 ka; this obsidian from a pyroclastic deposit is most likely derived from the Middle Pumice Plinian eruption of Santorini. The 2.5–2.7 Ma age for Christiana volcano shows that all volcanic fields of the South Aegean Volcanic Arc (SAVA) were active around 3 Ma ago and started when oceanic crust arrived at 100 km depth below the SAVA volcanic fields after a long period of continental lithosphere subduction. The Christiana volcano was constructed when the local stress field showed NNE-SSW extension. During the transition from NNE-SSW to NW-SE extension the Christiana volcano became extinct and a period of >1.0 Ma with volcanic quiescence and/or low volcanic output followed until the start of submarine volcano Poseidon and present-day volcanic centres Santorini and Kolumbo.
Cosmogenic-nuclide surface exposure ages provide empirical data for validating models simulating the timing and pace of ice-sheet response to a warming climate. Increasing emphasis is being placed on obtaining exposure ages that both accurately constrain Holocene deglaciation and are precise enough to capture ice sheet change at the sub-millennial scale. However, longer-lived nuclides such as 10Be are susceptible to cosmogenic nuclide inheritance often persisting through multiple periods of exposure and burial, which can impact the accuracy of the most recent Holocene exposure history. Shorter-lived in situ cosmogenic 14C (in situ 14C) is largely insensitive to nuclide inheritance pre-dating the last glacial maximum (LGM), and when combined with longer-lived nuclides can be used to constrain complex ice sheet histories over Holocene timescales. Here, we present new in situ 14C exposure ages from nine erratic cobbles from Mount Murphy, West Antarctica. Six of these suggest Mt Murphy deglaciated from 5-3 ka; this is inconsistent with previously measured 10Be ages of the same samples that place deglaciation from 8-6 ka. We investigate potential explanations for the conflicting exposure histories by analysing paired 14C-10Be data of Holocene age presently archived in the informal cosmogenic-nuclide exposure-age database (ICE-D, https://version2.ice-d.org/, last access: 29 March 2024). Our analysis reveals that neither variations in geologic setting nor modelled scenarios of subsurface nuclide production can explain the conflicting Mt Murphy ages. However, replicate in situ 14C measurements indicate that initial in situ 14C concentrations used to calculate the youngest exposure ages (5-3 ka) do not reproduce within stated 2 sigma uncertainty, whereas measurements used to calculate the older ages (8-6 ka) are reproducible. Furthermore, we observe that in situ 14C concentrations measured in 15 of 31 samples taken from ICE-D do not replicate within their nominal 2 sigma analytical uncertainty. Together, these results suggest that analytical uncertainty for in situ 14C measurements may currently be underestimated. We provide recommendations for improving measurement precision that will benefit future Holocene deglaciation studies, including analysis and publication of more replicate measurements and the continuation of efforts to quantify and minimise sources of scatter in blank measurements.
This study examines the feasibility of dating pluvial lake beach ridges using rock surface luminescence dating techniques. Dating pluvial lake highstands in the internally drained Great Basin of the United States helps us understand the timing of changes in precipitation and temperature patterns in western North America during the Late Pleistocene. The majority of highstand ages have relied on few radiocarbon ages of shell and/or charcoal sometimes coupled with luminescence dating of sand. Within our study area in the south-central Great Basin, luminescence ages of sand-size particles have successfully dated aeolian influxes of sand during arid intervals, but have not successfully dated the highstand beach ridges, the best preserved of which are largely gravel. Directly dating when these gravel clasts were last exposed to sunlight via luminescence is ideal but their limestone and volcanic lithologies prove challenging. Initial measurements from these lithologies show that feldspar luminescence signals are suited to single-aliquot regenerative (SAR) dose measurement protocols and show evidence for heterogeneous bleaching of rock surfaces. Polymineral extracts from dissolved limestone clast surfaces from Coal Valley that contain sufficient detrital sediment exhibit infrared signals measured at 50 degrees C (IR50) with low to moderate fading rates. Single-grain ages from detrital sediment from three clasts, calculated using the central dose model, are statistically consistent with the radiocarbon age estimate of the Pluvial Lake Coal highstand. Crushed slices from volcanic clasts from Cave Valley could be dated using a high-temperature (290 degrees C) post-infrared infrared stimulated luminescence (pIRIR) signal with a correction for fading. Many ages obtained from volcanic clast surfaces are observed to be several thousand years younger than the expected age of the similar to 18-20 ka beach ridge. This suggests that the volcanic rocks have been exposed to light long after the pluvial lake highstand, likely because of bioturbation, and that their most recent burial occurred in response to climatically driven soil formation processes. Comparisons between age-depth profile plateau ages from inside volcanic rocks and independent age control suggest that gravel-sized volcanic rocks were small enough to have been bleached throughout their entire thickness in the pluvial lake beach environment and that pIRIR signals that record the time of beach ridge formation and subsequent soil formation during the Pleistocene-Holocene transition may be preserved within the rock sub-surface. This study develops novel dating approaches for challenging rock lithologies. Rock surface dating techniques for pluvial lake beach ridges in the Great Basin should be further developed with consideration of local bedrock type(s), clast size, sample collection and preparation methods, gravel bleaching processes in pluvial lake environments and the impact of soil development and bioturbation on study sites.
Low-temperature thermochronology provides a powerful means of extracting quantitative information on the thermal evolution of different tectonic settings from rocks exposed at the surface of the Earth. Geodynamic numerical models enable tracking the entire thermal structure of simulated tectonic settings throughout their evolution. Despite the highly complementary nature of these two approaches, few geodynamic modeling studies have used the thermal information in models to predict thermochronometric ages as a means of comparing model results with observational data. Here, we present Geodynamic Thermochronology (GDTchron): an open-source Python package designed to forward model large numbers of low-temperature thermochronometric ages from time-temperature paths output by geodynamic numerical models. This package uses existing techniques to estimate apatite (U-Th)/He, apatite fission track, and zircon (U-Th)/He ages from time-temperature paths in a parallelized workflow that enables faster computation on multicore processors and high-performance computing systems. The workflow is built on typical output files from geodynamic models containing particle location, time, and temperature, and we use an interpolation scheme to allow new particles to inherit the thermal histories of their nearest neighbors. GDTchron can be applied to any tectonic setting, though for results to be comparable to nature, geodynamic models should carefully account for erosion and sedimentation. We demonstrate the functionality of this software with a highly simplified geodynamic model of exhumation and a more complicated model of rift-inversion orogenesis with the aim of encouraging community participation in broadening future development.
Bayesian modelling is often implemented in geochronology and its applications to geomorphology, archaeology, etc. The rationale behind such practices is the aim to improve robustness, precision and accuracy thanks to the use of prior knowledge regarding the studied sites, and in particular the order of samples constrained by stratigraphy. All chronological models tested in this study (OxCal, Chronomodel and BayLum) use the same mathematical model to handle stratigraphic constraints. However, this model has been shown to lead to estimation biases. First, this bias is illustrated with BayLum modelling on a high-resolution OSL dataset. Then, this paper compares statistical inferences obtained with the three above-mentioned modelling software on the Neolithic East mound of & Ccedil;atalh & ouml;y & uuml;k (Turkey). For this site, 49 radiocarbon ages were obtained with the aim to determine the start of occupations at this locality. Interestingly, age uncertainties are rather large, because of calibration curve plateaus. Therefore, the conditions for estimation biases are met. We discuss the behaviour of the different models and show that caution must be taken when modelling results are at odds with measurements. While OxCal, Chronomodel and BayLum are all affected by a spread in ages resulting from their common model of stratigraphic errors, Chronomodel suffers from a great loss of precision and OxCal, through the phase model, concentrates ages undesirably. We also conclude that the onset of occupations at & Ccedil;atalh & ouml;y & uuml;k was probably earlier than previously thought based on the OxCal model.
(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 & micro;m) in grains with a range of 4He concentrations and (U-Th-Sm) / He dates (16 to similar 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 & micro;m. Additionally, with an ablation spot diameter of 20 & micro;m, a six-spot in situ 4He profile requires a minimum grain diameter (measured perpendicular to the c-axis) of 145 & micro;m. 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.
The dating of supergene copper minerals has been widely used as a proxy to investigate the evolution and onset of hyperaridity in the Atacama Desert. However, investigation of supergene copper mineralisation in the Atacama Desert has been restricted to two physiographic units favourable for the industrial extraction of copper: the Central Depression and the Precordillera. Furthermore, these studies dated the timing of supergene mineralisation by secondary non-copper minerals like alunite. In this study, we present new results of LA-ICP-MS U-Pb dating of chrysocolla from supergene deposits hosted in the western part of the Coastal Cordillera of northern Chile. The obtained U-Pb ages range from 8.0 +/- 1.2 to 0.045 +/- 0.027 Ma. Supergene mineralisation ages point to significantly reduced precipitation, necessary for leaching and mineral precipitation process, since the Late Miocene to Pleistocene in the Coastal Cordillera, later than the secondary supergene mineralisation ages from the Precordillera. The data point to repeated phases of sufficient moisture along the Coastal Cordillera that promoted chrysocolla mineralisation during the Pliocene and Pleistocene. We propose that due to the position of the study areas near the coastal escarpment, and the predominant hyperarid environment in this part of the Coastal Cordillera since at least the Mid-Miocene, pluvial periods and/or intensification of coastal fog events caused alternating phases of supergene activity.
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).
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.
Constructing accurate age models for Pleistocene marine sediments is crucial for our understanding of glacial-interglacial cycles and other climatic processes. Benthic foraminiferal delta 18O 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 delta 18O alignment target influences the derived chronology at a given site, and divergent regional trends in benthic delta 18O highlight the need for ocean-specific benthic delta 18O 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 delta 18O 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 similar to 41 kyr obliquity cycle, while avoiding assumptions about astronomical phase relationships. This suite of stacks offers flexibility in choosing delta 18O stratigraphic alignment targets, to allow a wide range of applications in paleoceanographic hypothesis testing.
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 (f(U/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.
We present high-resolution 10Be concentration and flux records from the Talos Dome ice core (East Antarctica), covering the period from 170 to 270 ka BP, to assess the capacity of Antarctic ice cores to capture the dipole moment reductions triggered by geomagnetic excursions of different amplitudes. Three distinct geomagnetic events are identified in the 10Be flux. The dipole collapse linked to the Iceland Basin Excursion (IBE) is clearly recorded as a 10Be peak flux 1.59 to 2.08 times above background between (192.0 +/- 1.4) ka BP and (185.6 +/- 1.4) ka BP. A clear asymmetric structure is observed, with a rapid decline of the geomagnetic dipole, followed by a three-step recovery. Two dipole decreases of lower amplitude are also resolved in relation with the Pringle Falls Excursion (PFE), lasting from (218.5 +/- 1.90) to (206.0 +/- 0.8) ka BP, and the Mamaku Excursion (ME), identified at (242.0 +/- 0.3) ka BP, both showing an increase of the 10Be flux by a factor of 1.24 to 1.63. A total of 40 short-term 10Be concentration minima were also identified and are consistently associated with peaks in major ion concentrations, indicating post-depositional effects that affect concentration but not the longer-term flux signal. Comparison with Dome Fuji ice core and oceanic authigenic 10Be/9Be records reveals strong agreement in the timing and structure of the dipole moment collapses linked with these excursions. These results further support the use of 10Be for synchronizing ice and marine archives as well as to reconstruct past geomagnetic dipole moment variations and refining age models over the Pleistocene.
