Exposure Dating Research Articles

Deglaciation and postglacial evolution of the Cère Valley (Cantal, French Massif Central) based on geomorphological mapping, 36Cl surface exposure dating and glacier modelling

ABSTRACTThe landform assemblage in the Cère Valley (Cantal, France) provides one of the most complete sequences for Late Pleistocene glacial fluctuations in the French Massif Central. However, the associated glacial chronology has been debated since the 1980s. This paper aims to improve the glacial chronology of the Cère Valley using 36Cl surface exposure ages. Geomorphological results define two glacier stadials with reconstructed equilibrium line altitudes of 1078 ± 43 and 1152 ± 34 m above sea level. These results are comparable to those obtained in the Alps or the Pyrenees during the Last Glacial Maximum (26–19.5 ka). However, 36Cl surface exposure ages are centred around the Younger Dryas (YD), between 13 and 11 ka (n = 4). We suggest that these 36Cl ages are not related to a standstill during the YD but rather to the effects of the postglacial evolution of the Cère Valley. We investigate two geomorphological end‐member scenarios to explain the postponed exposure of sampled boulders: the Aurillac Lake scenario and the later fluvial incision scenario. While the nature of the geomorphological events leading to the boulder exhumation is not fully resolved, we highlight a long phase of postglacial evolution in the Cère Valley.

Early glacier advance in New Zealand during the Antarctic Cold Reversal

ABSTRACTGlacial landscapes preserve records of past climate change. Investigating the glacier–climate system over the Late Quaternary provides information about past climate change and context for present‐day glacier response to climate warming. Using 28 beryllium‐10 (10Be) surface exposure dates and snowline reconstructions, we present glacier fluctuations and climate changes for the Antarctic Cold Reversal in the Ahuriri River catchment, Southern Alps of New Zealand (44°7′50″S, 169°38′29″E). Prominent terminal and lateral moraine features from the upper right tributary of the Ahuriri River valley have exposure ages of 14.5 ± 0.3, 13.6 ± 0.3 and 12.6 ± 0.2 ka, suggesting retreat of the glacier during the Antarctic Cold Reversal. Maximum elevation of lateral moraines (MELM) and accumulation area ratio (AAR) suggest snowline elevations at these ages were ≤700, ≤630 and ~360 m lower than today, respectively. This equates to air temperatures ≤3.9, ≤3.5 and 2.3 ± 0.7 °C lower than today (1981–2010), assuming no changes in past precipitation. Ice‐sculpted bedrock surfaces bound by a lateral moraine at nearby Canyon Creek have an age of 13.1 ± 0.3 ka, indicating the moraine correlates with those in the Ahuriri upper right tributary. MELM and AAR reconstructions from the Canyon Creek suggest that snowline elevations at 14.5–13.6 ka were ≤500 or ~380 m lower than today, corresponding to air temperatures ≤2.8 or 2.4 ± 0.7 °C lower than the present‐day (1981–2010). Our results provide insight into the structure of the Antarctic Cold Reversal in the Southern Alps, showing that the largest glacier advance occurred at the start of this interval at c. 14.5 ± 0.3 ka and was followed by gradual retreat. We hypothesize that the early cooling and glacier readvance in New Zealand at the onset of the Antarctic Cold Reversal were triggered by a latitudinal shift of the Southern Hemisphere westerly wind belt.

5α-reductase inhibitors and the risk of bladder cancer in a large, population-based cohort.

The ability of 5α-reductase inhibitors (5ARI) to reduce the risk of new onset bladder cancer (BC) has been studied with variable results. Our objective was to conduct a retrospective cohort population-based study to evaluate the association between 5ARI use, BC diagnosis, and BC mortality. We used routinely collected health care data from Ontario, Canada. Men ≥66 years of age with a prescription for a 5ARI were matched to non-5ARI users. Matching was done using a propensity score of selected covariates to make 96 different covariates comparable. We measured 5 additional baseline variables which may have impacted the risk of future BC diagnosis: prior cystoscopy, urine cytology, urinalysis, gross hematuria episodes, and transurethral resection of a bladder lesion. Only the first period of continuous usage of 5ARIs was considered. The prespecified at-risk period for outcomes started 1 year after initiating therapy and ended at the last date of 5ARI exposure + 1 year. We identified 93,197 men who initiated 5ARI therapy (52% dutasteride, and 48% finasteride) between 2003 and 2013 and matched them 1:1 to men who did not start a 5ARI. The median at-risk period for the 5ARI group was 1.68 years (interquartile range 1.00, 4.27). With adjustment for the variables related to prior BC investigations there was no significant difference in BC diagnosis (hazard ratio [HR] 1.05, 95% confidence interval [CI] 0.82-1.32) during the period of 0 to <2 years of 5ARI use; however, after ≥2 years of 5ARI use, the risk of BC diagnosis was significantly lower among the 5ARI group (HR 0.82, 95% CI 0.79-0.94). In a similarly adjusted model, BC mortality was lower among 5ARI users, but no longer statistically significant (HR 0.82, 95% CI 0.65, 1.02). When stratified by type of 5ARI, finasteride significantly reduced the risk of BC diagnosis after ≥2 years of continuous use (HR 0.86, 95% CI 0.76, 0.96); however, dutasteride did not (HR 0.92, 95% CI 0.83, 1.03). In a large cohort of men, the use of a 5ARI was associated with a significantly decreased the risk of BC diagnosis after more than 2 years of continuous therapy.

Estimation of the incubation period and generation time of SARS-CoV-2 Alpha and Delta variants from contact tracing data.

Quantitative information on epidemiological quantities such as the incubation period and generation time of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants is scarce. We analysed a dataset collected during contact tracing activities in the province of Reggio Emilia, Italy, throughout 2021. We determined the distributions of the incubation period for the Alpha and Delta variants using information on negative polymerase chain reaction tests and the date of last exposure from 282 symptomatic cases. We estimated the distributions of the intrinsic generation time using a Bayesian inference approach applied to 9724 SARS-CoV-2 cases clustered in 3545 households where at least one secondary case was recorded. We estimated a mean incubation period of 4.9 days (95% credible intervals, CrI, 4.4-5.4) for Alpha and 4.5 days (95% CrI 4.0-5.0) for Delta. The intrinsic generation time was estimated to have a mean of 7.12 days (95% CrI 6.27-8.44) for Alpha and of 6.52 days (95% CrI 5.54-8.43) for Delta. The household serial interval was 2.43 days (95% CrI 2.29-2.58) for Alpha and 2.74 days (95% CrI 2.62-2.88) for Delta, and the estimated proportion of pre-symptomatic transmission was 48-51% for both variants. These results indicate limited differences in the incubation period and intrinsic generation time of SARS-CoV-2 variants Alpha and Delta compared to ancestral lineages.

1895. SingHealth COVID-19 Disease Registry

Abstract Background One of Singapore’s national strategies for the COVID-19 pandemic was containment. Efforts included a fourteen-day quarantine of close contacts, were subjected to an entry and exit SARS-CoV-2 PCR test, the latter being done between 11-14 days post exposure. Additionally, symptomatic contacts were tested for SARS-CoV-2. We aim to determine the trend in COVID-19 incubation periods during three distinct pandemic waves corresponding to different SARS-CoV-2 variants. Incubation Period Incubation period of the prevalent SARS-CoV-2 variant in circulation Methods This is an ecological study and information collected from the SingHealth COVID-19 Registry, a database of all inpatients admitted to any of the SingHealth hospitals. For patients under quarantine, the start date of the quarantine period was assumed to be the last date of exposure to the index case. Incubation period was determined by the duration between date of exposure and date of the first positive SARS-CoV-2 PCR test. The prevalent strain in circulation was identified from the Singapore database in the GISAID collection. Only variants of concern, as categorized by WHO, Alpha (23rd Jan 2020 – 1st Mar 2021), Delta (5th May 2021 – 31st Oct 2021) and Omicron (1st Jan 2022 – Present) were considered. For the Omicron variant, quarantine was discontinued, hence the last date of arrival from international travel was assumed to be the date of exposure. Results From January 2020 to March 2022, there were 19,905 patients in the COVID-19 registry, of whom 11,235 were under quarantine and 8,612 had preceding international travel. Of the 11,235 patients under quarantine, 8,189 patients were infected when SARS-CoV-2 Alpha variant and 3,046 patients were infected when SARS-CoV-2 Delta variant were in circulation. Of the 8,612 patients with preceding travel, 6,503 patients were infected when SARS-CoV-2 Omicron variant was in circulation. The median incubation period for the Alpha variant was 11 days (IQR: 7-14 days) versus 3 days (IQR: 2-4 days) for the Delta variant versus 3 days (IQR: 0-5 days) for the Omicron variant. Pairwise comparisons between the variants were (p-value = &amp;lt; .001) Conclusion The significant differences between incubation periods of the SARS-CoV-2 variants in circulation poses a challenge to containment efforts and has emphasize the importance of dynamic national strategies. Disclosures All Authors: No reported disclosures.

Technical note: Evaluating a geographical information system (GIS)-based approach for determining topographic shielding factors in cosmic-ray exposure dating

Abstract. Cosmic-ray exposure (CRE) dating of boulders on terminal moraines has become a well-established technique to reconstruct glacier chronologies. If topographic obstructions are present in the surroundings of sampling sites, CRE ages need to be corrected for topographic shielding. In recent years, geographical information system (GIS)-based approaches have been developed to compute shielding factors with elevation data, particularly two toolboxes for the ESRI ArcGIS software. So far, the output of the most recent toolbox (Li, 2018) has only been validated with a limited number of field-data-based shielding factors. Additionally, it has not been systematically evaluated how the spatial resolution of the input elevation data affects the output of the toolbox and whether a correction for vegetation leads to considerably more precise shielding factors. This paper addresses these issues by assessing the output of the toolbox with an extensive set of field-data-based shielding factors. Commonly used elevation data with different spatial resolutions were tested as input. To assess the impact of the different methods on CRE ages, ages of boulders with different 10Be concentrations at sites with varying topography and 10Be production rates were first recalculated with GIS-based shielding factors and then with field-data-based shielding factors. For sampling sites in forested low mountainous areas and in high Alpine settings, the shielding factors were independent of the spatial resolution of the input elevation data. Vegetation-corrected elevation data allowed more precise shielding factors to be computed for sites in a forested low mountainous area. In most cases, recalculating CRE ages of the same sampling sites with different shielding factors led to age shifts between 0 % and 2 %. Only one age changed by 5 %. It is shown that the use of elevation data with a very high resolution requires precise x and y coordinates of sampling sites and that there is otherwise a risk that small-scale objects in the vicinity of sampling sites will be misinterpreted as topographic barriers. Overall, the toolbox provides an interesting avenue for the determination of shielding factors. Together with the guidelines presented here, it should be more widely used.

The collapse of the Cordilleran–Laurentide ice saddle and early opening of the Mackenzie Valley, Northwest Territories, Canada, constrained by 10Be exposure dating

Abstract. Deglaciation of the northwestern Laurentide Ice Sheet in the central Mackenzie Valley opened the northern portion of the deglacial Ice-Free Corridor between the Laurentide and Cordilleran ice sheets and a drainage route to the Arctic Ocean. In addition, ice sheet saddle collapse in this section of the Laurentide Ice Sheet has been implicated as a mechanism for delivering substantial freshwater influx into the Arctic Ocean on centennial timescales. However, there is little empirical data to constrain the deglaciation chronology in the central Mackenzie Valley where the northern slopes of the ice saddle were located. Here, we present 30 new 10Be cosmogenic nuclide exposure dates across six sites, including two elevation transects, which constrain the timing and rate of thinning and retreat of the Laurentide Ice Sheet in the area. Our new 10Be dates indicate that the initial deglaciation of the eastern summits of the central Mackenzie Mountains began at ∼15.8 ka (17.1–14.6 ka), ∼1000 years earlier than in previous reconstructions. The main phase of ice saddle collapse occurred between ∼14.9 and 13.6 ka, consistent with numerical modelling simulations, placing this event within the Bølling–Allerød interval (14.6–12.9 ka). Our new dates require a revision of ice margin retreat dynamics, with ice retreating more easterly rather than southward along the Mackenzie Valley. In addition, we quantify a total sea level rise contribution from the Cordilleran–Laurentide ice saddle region of ∼11.2 m between 16 and 13 ka.

Novel luminescence kinetic models for rock surface exposure dating

Luminescence dating of rock surfaces is an emerging absolute chronological technique that has the potential to determine how long a rock surface has been exposed to daylight and/or how long it has been buried. By measuring the OSL signal as a function of depth into the rock surface, it is possible to determine how long the rock surface has been exposed to daylight by fitting a luminescence kinetic model to the measured luminescence-depth profile. Despite increasing use of this technique, the possible dependence of derived ages on the chosen model has not been examined. Existing studies are based on the assumption that first-order kinetics apply and a first order model (F.O.M.) has been used. In this study, we use (i) a general-order kinetics model G.O.M., (McKeever and Chen, 1997) and (ii) a localised recombination model here referred to as the fading model, F.M., (Jain et al., 2015) to develop new analytical expressions for luminescence-depth profiles in rocks. These models are tested using both simulated and experimental data based on the IR50 signal. Based on these tests it is concluded that the derived apparent exposure ages are dependent on the chosen model. This model dependence occurs despite the fact that the speed of progression of the bleaching front into the rock surface is model independent. The reasons for this apparent discrepancy are discussed in details. Calibration with a single known-age sample (Sohbati et al., 2012b) gives inaccurate exposure ages for all investigated models unless the appropriate model is applied and/or the age of the calibration profile is of the same order of magnitude as the age of the unknown profile, but using the G.O.M. improves the accuracy of age estimates for IRSL profiles. An alternative approach to exposure dating is presented here as “Exposure Response Curve” calibration. We conclude that using inflection points, to avoid the difficulties caused by fitting parameter correlation, proves model-independent, and is a promising method for further investigation in Rock Surface Luminescence Exposure dating.

Real World Outcomes of Brentuximab Vedotin Maintenance after Autologous Stem Cell Transplant in Relapsed/Refractory Classical Hodgkin Lymphoma: An Time-Variable Analysis of the Effect of Dose on Progression Free Survival

Background: In the AETHERA trial, brentuximab vedotin (BV) maintenance after autologous stem cell transplant (ASCT) in patients (pts) with high-risk relapsed/refractory classical Hodgkin lymphoma (r/r cHL) demonstrated an improvement in 2-year progression free survival (PFS) over placebo [Moskowitz 2015]. However, in clinical practice, pts are rarely able to complete all 16 cycles of BV at full dose due to toxicity. The aim of this retrospective, multicenter study was to investigate the effect of the cumulative maintenance BV dose on 2-year PFS in pts with high-risk r/r cHL. In an earlier analysis of this study, we found that 50-75% of the total cumulative BV dose attains a similar PFS advantage as >75% dose [Wagner 2022], we now report further findings with additional pts and a modified statistical analysis where total cumulative dose was treated as a time varying exposure. Methods: Individual patient data were collected from those who received at least one cycle of BV maintenance after ASCT from 13 institutions across the United States. Pts were required to have at least one high-risk disease feature as defined in AETHERA: primary refractory disease (PRD), extra-nodal disease (END), or relapse < 12 months from the end of frontline therapy (RL <12). Pts were split in to 3 different cohorts based on total cumulative dose of BV maintenance. Cohort 1 (C1) included those who received > 75% of the planned total cumulative dose, cohort 2 (C2) included those who received 51 - 75%, and cohort 3 (C3) included those who received ≤ 50%. In order to avoid bias from conditioning on future exposure, the total cumulative dose was treated as a time varying exposure. We linearly interpolated when each dose was administered based on when the first and last dose of BV maintenance were administered. Pts were excluded if we did not have a date of first or last BV exposure. Adverse events were defined using CTCAE v. 5 criteria. A multivariate analysis was performed assessing the effect of total cumulative BV dose, RL <12, PRD, achieving less than a CR prior to ASCT, receiving two or more salvage therapies and prior BV exposure on 2-year PFS. Descriptive statistics were utilized, and survival outcomes were estimated using Kaplan-Meier. Results: A total of 123 pts were screened for inclusion; 5 did not have documented dates of first or last BV administration and were excluded, for a total of 118 evaluable pts. The median age was 36 (27 - 42) years, 53% being male, and the majority (95%) receiving ABVD as frontline therapy. High risk features were as follows: 50% of pts had PRD, 29% had RL<12 and the remaining 21% relapsed ≥ 12 months after initial therapy with 39% of total pts having END. Twenty-three percent had more than one line of salvage before ASCT. The majority of pts achieved a CR (65%) prior to ASCT and half (49%) had BV exposure prior to ASCT (majority during salvage) (Table 1). The median follow up for the entire cohort was 2.96 years. The median number of BV maintenance cycles completed was 12 with the majority (69/118 = 59%) of pts discontinuing BV maintenance prior to completing all 16 cycles. Reasons for early discontinuation were as follows: 45 for toxicity, 10 for progression, 8 for patient preference, 2 for cost and 4 for other reasons. Forty-eight pts (41%) had a dose-reduction prior to discontinuation or completion of maintenance. Only 14% of pts (16/118) received the full cumulative dose of BV. One patient continued past 16 cycles and received a total of 25 cycles. Grade ≥3 adverse events for neuropathy, neutropenia, and infections were 17%, 7%, and 4% respectively. The 2-year PFS was 89.2% for C1, 86.2% for C2, and 77.9% for C3 (p = 0.70, Figure 1). No variables were significant in the multivariate analysis for PFS (Table 1). Conclusions: The 2-year PFS was robust regardless of cumulative dose of BV maintenance. The 2-year PFS was superior for all groups compared to the original AETHERA trial, which had a 2-year PFS of 65% [Moskowitz 2015]. This difference could be due to higher rates of CR achieved with novel salvage agents prior to ASCT in this population versus the pre-ASCT CR reported in the AETHERA trial (37-38%) [Moskowitz 2015]. PFS was not affected by prior BV exposure during salvage therapy. These data are reassuring for pts who require dose reductions or discontinuation to manage toxicity and suggest that the PFS advantage may still be attained. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Regional sea-level highstand triggered Holocene ice sheet thinning across coastal Dronning Maud Land, East Antarctica

The East Antarctic Ice Sheet stores a vast amount of freshwater, which makes it the single largest potential contributor to future sea-level rise. However, the lack of well-constrained geological records of past ice sheet changes impedes model validation, hampers mass balance estimates, and inhibits examination of ice loss mechanisms. Here we identify rapid ice-sheet thinning in coastal Dronning Maud Land from Early to Middle Holocene (9000–5000 years ago) using a deglacial chronology based on in situ cosmogenic nuclide surface exposure dates from central Dronning Maud Land, in concert with numerical simulations of regional and continental ice-sheet evolution. Regional sea-level changes reproduced from our refined ice-load history show a highstand at 9000–8000 years ago. We propose that sea-level rise and a concomitant influx of warmer Circumpolar Deep Water triggered ice shelf breakup via the marine ice sheet instability mechanism, which led to rapid thinning of upstream coastal ice sheet sectors.

COVID-19 Case Investigation and Contact Tracing in New York City, June 1, 2020, to October 31, 2021

Contact tracing is a core strategy for preventing the spread of many infectious diseases of public health concern. Better understanding of the outcomes of contact tracing for COVID-19 as well as the operational opportunities and challenges in establishing a program for a jurisdiction as large as New York City (NYC) is important for the evaluation of this strategy. To describe the establishment, scaling, and maintenance of Trace, NYC's contact tracing program, and share data on outcomes during its first 17 months. This cross-sectional study included people with laboratory test-confirmed and probable COVID-19 and their contacts in NYC between June 1, 2020, and October 31, 2021. Trace launched on June 1, 2020, and had a workforce of 4147 contact tracers, with the majority of the workforce performing their jobs completely remotely. Data were analyzed in March 2022. Number and proportion of persons with COVID-19 and contacts on whom investigations were attempted and completed; timeliness of interviews relative to symptom onset or exposure for symptomatic cases and contacts, respectively. Case investigations were attempted for 941 035 persons. Of those, 840 922 (89.4%) were reached and 711 353 (75.6%) completed an intake interview (women and girls, 358 775 [50.4%]; 60 178 [8.5%] Asian, 110 636 [15.6%] Black, 210 489 [28.3%] Hispanic or Latino, 157 349 [22.1%] White). Interviews were attempted for 1 218 650 contacts. Of those, 904 927 (74.3%) were reached, and 590 333 (48.4%) completed intake (women and girls, 219 261 [37.2%]; 47 403 [8.0%] Asian, 98 916 [16.8%] Black, 177 600 [30.1%] Hispanic or Latino, 116 559 [19.7%] White). Completion rates were consistent over time and resistant to changes related to vaccination as well as isolation and quarantine guidance. Among symptomatic cases, median time from symptom onset to intake completion was 4.7 days; a median 1.4 contacts were identified per case. Median time from contacts' last date of exposure to intake completion was 2.3 days. Among contacts, 30.1% were tested within 14 days of notification. Among cases, 27.8% were known to Trace as contacts. The overall expense for Trace from May 6, 2020, through October 31, 2021, was approximately $600 million. Despite the complexity of developing a contact tracing program in a diverse city with a population of over 8 million people, in this case study we were able to identify 1.4 contacts per case and offer resources to safely isolate and quarantine to over 1 million cases and contacts in this study period.

Late Holocene initiation of a deep rock slope failure in an alpine valley revealed by 10Be surface exposure dating (Chamonix, France)

We studied a newly identified, multiple-kilometer-long rock slope failure in the Aiguilles Rouges massif (Chamonix valley, France). Owing to a high-resolution light detection and ranging (LiDAR) digital elevation model (DEM) and field work, we mapped morphostructures, including scarps, open fractures, and counterscarps. In some places, vertical offsets can reach tens of meters and crevasses can be meters wide. The evidence of gravitational activity (boulder displacements from analyses of archival satellite images) and the sharpness of the scarp outcrops together suggest very recent movements. These observations agree with ground displacement rates of a few millimeters per year estimated by interferometric synthetic aperture radar (InSAR) time series between 2014 and 2018.We sampled two vertical profiles along the top scarps to define the chronology of the slope failure using beryllium-10 (10Be) surface exposure dating. Glacially polished surfaces cut by these gravitational scarps were also sampled to determine glacial retreat timing as well as to constrain the pre-exposure 10Be inheritance. In total, 11 samples were studied.Our results highlight a significant time lag (approximately 15 ka) between the first evidence of nonglacial activity and the initiation of the slope failure that happened 1.3–2.5 ka ago, depending on the inheritance schemes. This suggests that the delayed opening of the crevasse is only one stage of a process that began when the valley was deepened and the glacial debuttressing is not the unique driving factor. This process of progressive failure of an excessively steep slope may continue, and the evolution of this slope failure may constitute a hazard for the upper part of the Chamonix valley.

Discriminating between impact or nonimpact origin of small meteorite crater candidates: No evidence for an impact origin for the Tor crater, Sweden

AbstractCompared to intensive research on km‐sized meteorite impact craters, fewer studies focus on smaller craters. The small craters are often hard or impossible to recognize using “classical” criteria like the presence of shatter cones, shocked quartz, and geochemical indicators. Therefore, a long list of candidate structures awaiting approval/disapproval of their origin has been formed over the last decades. One of them is the Tor structure in central Sweden. To test a hypothesis of an impact origin of this structure, we have performed topographical analysis, geophysical studies, 10Be exposure dating of boulders, and 14C dating of Tor‐associated charcoal. None of the methods gave us a reason to claim the Tor structure is of impact origin. Thus, we support a recently suggested idea of Tor being formed by a grounded iceberg within a glacial lake.

Drill-site selection for cosmogenic-nuclide exposure dating of the bed of the Greenland Ice Sheet

Abstract. Direct observations of the size of the Greenland Ice Sheet during Quaternary interglaciations are sparse yet valuable for testing numerical models of ice-sheet history and sea level contribution. Recent measurements of cosmogenic nuclides in bedrock from beneath the Greenland Ice Sheet collected during past deep-drilling campaigns reveal that the ice sheet was significantly smaller, and perhaps largely absent, sometime during the past 1.1 million years. These discoveries from decades-old basal samples motivate new, targeted sampling for cosmogenic-nuclide analysis beneath the ice sheet. Current drills available for retrieving bed material from the US Ice Drilling Program require &lt; 700 m ice thickness and a frozen bed, while quartz-bearing bedrock lithologies are required for measuring a large suite of cosmogenic nuclides. We find that these and other requirements yield only ∼ 3.4 % of the Greenland Ice Sheet bed as a suitable drilling target using presently available technology. Additional factors related to scientific questions of interest are the following: which areas of the present ice sheet are the most sensitive to warming, where would a retreating ice sheet expose bare ground rather than leave a remnant ice cap, and which areas are most likely to remain frozen bedded throughout glacial cycles and thus best preserve cosmogenic nuclides? Here we identify locations beneath the Greenland Ice Sheet that are best suited for potential future drilling and analysis. These include sites bordering Inglefield Land in northwestern Greenland, near Victoria Fjord and Mylius-Erichsen Land in northern Greenland, and inland from the alpine topography along the ice margin in eastern and northeastern Greenland. Results from cosmogenic-nuclide analysis in new sub-ice bedrock cores from these areas would help to constrain dimensions of the Greenland Ice Sheet in the past.

Timing and climatic drivers for the MIS 6 glaciation in the central Himalaya: 10Be surface exposure dating of hummocky moraine northwest of Mt. Gang Benchhen, Paiku Gangri

Quantifying the timing and extent of Quaternary mountain glaciations can help us in understanding the cause of ice-age cycles. As such, impressive achievements have been made in glacial dynamics across many mountain ranges around the world, such as the Himalayas. Yet, the Marine Isotope Stage 6 (MIS 6) glacial histories have not been well determined across the Himalayan regions due to a lack of adequate chronological controls. Here, we present a 10Be surface-exposure chronology northwest of Mt. Gang Benchhen, Paiku Gangri. Twelve 10Be exposure-ages show that the outermost hummocky moraine outside of the Laqu Valley was formed around ∼165.7 ka, reflecting the MIS 6 glacial maximum in the central Himalaya. The minimum age constraint is consistent with the low atmospheric CO2 levels as well as low summer solar insolation intensity at northern high-latitude, which indicate that glaciers in the Laqu Valley culminated during cold climate condition. Also, we identified a potential correlation between the MIS 6 glacial advance in the central Himalaya and Heinrich-like event. We thus hypothesize that there seems to be a response of Himalayan glaciers to the combined forcing of the boreal summer solar insolation intensities, atmospheric CO2 concentrations, and North Atlantic climate oscillations during the MIS 6 glaciation.

Cosmogenic 36Cl surface exposure dating of glacial landforms on Mt. Barla (SW Turkey)

Investigating Quaternary glaciations by terrestrial cosmogenic nuclide dating provides unique insights into glacial landscapes. However, postglacial processes may reshape these topographies via weathering, erosion and deposition, especially after the glaciers have entirely disappeared. Understanding the timing of Quaternary glaciations in the areas developed under different geomorphological processes may be challenging. In this study, we produced a detailed geomorphological map of Mt. Barla in the Western Taurus Mountains of Turkey. We found that geomorphological processes such as karstification and mass movements provided favourable settings for the valleys to be carved by glaciers during the Late Pleistocene. We applied in-situ cosmogenic 36Cl surface exposure dating to moraine boulders to reveal the glacial chronology and the geomorphological evolution. We dated moraine boulders (n = 11) from two glacial valleys that show at least two and possibly three deglaciations during the Late Pleistocene. For Gedik Valley, the left lateral moraine marks the Late-glacial glacier retreat by 16.6 ± 2.6 ka (MIS 2). On the other hand, the terminal moraine indicates that the most extensive glaciation occurred at 42.1 ± 5.2 ka (pre-global LGM) in this valley. The outer terminal moraine age (45.5 ± 6.8 ka) in Kapıdere Valley confirms this MIS 3 extension. Our results also imply even a possible earlier glaciation in Kapıdere Valley at 78.5 ± 17.6 ka (MIS 5), even though more data are needed to confirm this extent.

Low-volume magmatism linked to flank deformation on Isla Santa Cruz, Galápagos Archipelago, using cosmogenic 3He exposure and 40Ar/39Ar dating of fault scarps and lavas

Isla Santa Cruz is a volcanic island located in the central Galápagos Archipelago. The island’s northern and southern flanks are deformed by E–W-trending normal faults not observed on the younger Galápagos shields, and Santa Cruz lacks the large summit calderas that characterize those structures. To construct a chronology of volcanism and deformation on Santa Cruz, we employ 40Ar/39Ar geochronology of lavas and 3He exposure dating of fault scarps from across the island. The combination of Ar–Ar dating with in situ-produced cosmogenic exposure age data provides a powerful tool to evaluate fault chronologies. The 40Ar/39Ar ages indicate that the island has been volcanically active since at least 1.62 ± 0.030 Ma (2SD). Volcanism deposited lavas over the entire island until ~ 200 ka, when it became focused along an E–W-trending summit vent system; all dated lavas < 200 ka were emplaced on the southern flank. Structural observations suggest that the island has experienced two major faulting episodes. Crosscutting relationships of lavas indicate that north flank faults formed after 1.16 ± 0.070 Ma, but likely before 416 ± 36 ka, whereas the faults on the southern flank of the island initiated between 201 ± 37 and 32.6 ± 4.6 ka, based on 3He exposure dating of fault surfaces. The data are consistent with a model wherein the northeastern faults are associated with regional extension owing to the young volcano’s location closer to the Galápagos Spreading Center at the time. The second phase of volcanism is contemporaneous with the formation of the southern faults. The expression of this younger, low-volume volcanic phase was likely related to the elongate island morphology established during earlier deformation. The complex feedback between tectonic and volcanic processes responsible for southward spreading along the southern flank likely generated persistent E-W-oriented magmatic intrusions. The formation of the Galápagos Transform Fault and sea-level fluctuations may be the primary causes of eruptive and deformational episodes on Santa Cruz.

Constraining a model of punctuated river incision for Quaternary strath terrace formation

In the small fraction of Earth's surface with the highest erosion rates such as the Alps and Himalayas quantifying rates of incision, rock uplift and inferring climatic controls on the landscape can be relatively straightforward once the ages of river terraces cut in bedrock (strath terraces) are constrained. However, in many mid to lower relief settings that are more typical of mountain belts worldwide, periods of net river incision and riverbed lowering are relatively short (punctuated), interrupted by long periods of sediment aggradation or stasis. We define a conceptual model of punctuated river incision and strath terrace formation for the calculation of incision and rock uplift rates, and recommend strategies for geochronological sampling and interpretation. An approach using OSL dating of terrace gravels allows us to constrain a detailed ~150 kyr history of punctuated river incision and strath terrace formation spanning two stratigraphic landform levels in the High Atlas Mountains (NW Africa). Extensive preservation and exposure of strath-top gravels, within a post-orogenic setting unaffected by eustatic influences, enables the derivation of rates of base-level fall, integrated over periods of strath-top aggradation and incision, that are consistent with independently constrained regional rock uplift rates. Combining a punctuated river incision model with our well-constrained terrace formation history allows us to demonstrate how assumptions concerning Quaternary river incision and aggradation can lead to the problematic Sadler Effect, an apparent dependence of incision rates on measured time interval. Subsequently, we demonstrate that an approach to reinterpreting previously published data using the punctuated incision model, even when combined with limited terrace age data, results in more consistent conclusions about rates of river incision, rock uplift and base-level lowering across the mountain belt. Our recommendations for sampling strategies to constrain rock uplift rates require samples to be taken just above the strath surface, and in addition towards the top of the deposit for river incision rates. In a setting with punctuated river incision and strath terrace formation, both rock uplift and incision rates require burial dates, as exclusive use of abandonment ages will not yield constraints on accurate rates of rock uplift or incision. Furthermore, we find that only with multiple along-stream locations and multiple burial dates in each terrace deposit, could a reliable climatic signal be extracted: this signal would not have shown up in terrace abandonment ages such as those derived from cosmogenic exposure dates. The demonstrated effects of assumptions about strath terrace formation, and the recommended approaches for sampling and interpretation, have implications for those attempting to constrain palaeoclimatic, tectonic, and geomorphic histories from strath terrace records in regions exhibiting punctuated river incision.

Paleo-glacial reconstruction of the Thajwas glacier in the Kashmir Himalaya using 10Be cosmogenic radionuclide dating

Quantitative glacial chronologies of past glaciations are sparse in the Himalaya, and mostly absent in the Kashmir Himalaya. We used cosmogenic 10Be exposure dating, and geomorphological mapping to reconstruct glacial advances of the Thajwas Glacier (TG) in the Great Himalayan Range of the Kashmir Himalaya. From 10Be exposure dating of ten moraine boulders, four glacial stages with ages ∼20.77 ± 2.28 ka, ∼11.46 ± 1.69 ka, ∼9.12 ± 1.39 ka and ∼4.19 ± 0.78 ka, were identified. The reconstructed cosmogenic radionuclide ages confirmed the global Last Glacial Maximum (gLGM), Younger Dryas, Early Holocene, and Neoglaciation episodes. As per area and volume change analyses, the TG has lost 51.1 km2 of its area and a volume of 2.64 km3 during the last 20.77 ± 2.28 ka. Overall, the results suggested that the TG has lost 64% of area and 73% of volume from the Last glacial maximum to Neoglaciation and about 85.74% and 87.67% of area and volume, respectively, from Neoglaciation to the present day. The equilibrium line altitude of the TG fluctuated from 4238 m a.s.l present to 3365 m a.s.l during the gLGM (20.77 ± 2.28 ka). The significant cooling induced by a drop in mean ambient temperature resulted in a positive mass balance of the TG during the gLGM. Subsequently the melting accelerated due to the continuing rise of the global ambient temperature. Paleo-glacial history reconstruction of the Kashmir Himalaya, with its specific geomorphic and climatic setting, would help close the information gap about the chronology of past regional glacial episodes.

Chlorine-36 Surface Exposure Dating of Late Holocene Moraines and Glacial Mass Balance Modeling, Monte Sierra Nevada, South-Central Chilean Andes (38°S)

The development of robust chronologies of Neoglaciation from individual glaciers throughout the high-altitude Andes can provide fundamental knowledge of influences such as regional temperature and precipitation variability, and aid in predicting future changes in the Andean climate system. However, records of Late Holocene glaciation from the Central Chilean Andes are sparse, and often poorly constrained. Here, we present 36Cl surface exposure ages, dendrochronologic constraints, and glacial mass balance modeling simulations of Late Holocene glacier fluctuations in the Central-South Chilean Andes. A series of concentric moraine ridges were identified on Monte Sierra Nevada (38°S), where exposure dating of basaltic boulders was used to establish a chronology of ice recession. We infer that moraine abandonment of the most distal ridge in the valley commenced by ∼4.2 ka, and was followed by glacier margin retreat to an up-valley position. Exposure ages of the oldest Late Holocene boulders (∼2.5–0.8 ka) along the marginal extents of the moraine complex indicate fluctuations of the glacier terminus prior to ∼0.65 ka. A final expansion of the ice margin reoccupied the position of the 4.2 ka moraine, with abatement from the outermost composite moraine occurring by ∼0.70 ka, as constrained by tree-ring data from live Araucaria araucana trees. Finally, a series of nested moraines dating to ∼0.45–0.30 ka, formed from a pulsed ice recession during the latest Holocene when the lower reaches of the glacial snout was most likely debris mantled. A distributed temperature index model combined with a glacier flow model was used to quantify an envelope of possible climatic conditions of Late Holocene glaciation. The glacial modeling results suggest conditions were ∼1.5°C colder and 20% wetter during peak Neoglaciation relative to modern conditions. These records also suggest a near-coeval record of Late Holocene climate variability between the middle and high southern latitudes. Furthermore, this study presents some of the youngest 36Cl exposure ages reported for moraines in the Andes, further supporting this method as a valuable geochronologic tool for assessing Late Holocene landscape development.