Terrestrial Cosmogenic Nuclide Research Articles

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.

Estimating the long-term slip rate of active normal faults: The case of the Paganica Fault (Central Apennines, Italy)

The footwall of the surface rupturing Paganica normal fault, the source of the 2009 L’Aquila earthquake (Mw 6.1) in the Central Apennines (Italy), was investigated using integrated geological and geomorphological approaches. The aim was to constrain the active tectonics by studying the Raiale River that orthogonally crosscuts the fault trace, where it provides a useful geomorphological marker of long-term fluvial incision and footwall uplift. Using morphostratigraphy and paleomagnetic analysis, the Plio–Pleistocene morphotectonic evolution of the area was reconstructed, comprising an ancient continental basin and paleolandforms that predate the footwall incision. Starting from the Late Early Pleistocene–Middle Pleistocene, fluvial dissection was mainly due to marked river downcutting triggered by significant activity of the Paganica Fault, which caused progressive base-level lowering. The Raiale River downcutting formed five Middle–Late Pleistocene fluvial terraces, that, along with absolute Optically Stimulated Luminescence (OSL) dating, allowed the identification of paleolongitudinal profiles with a diverging downstream configuration. Terrace dating yielded a minimum incision rate of 0.25 ± 0.02 mm/a, which only partially compensates the footwall uplift and can thus be considered as a minimum value for the Paganica Fault throw rate, which could reach up to ~0.45 mm/a. In parallel, using terrestrial cosmogenic nuclides, a denudation rate of 0.02–0.04 mm/a was measured on the summit of the footwall block. This denudation is in keeping with the drainage incision, suggesting a non-steady state for the fault footwall topography and a dominance of relief growth. Last, the analysis of the modern Raiale River longitudinal profile denoted an ungraded status, with two main knickzones that we interpret as transient forms due to tectonic perturbations, likely triggered by activity of the Paganica Fault during the end Early Pleistocene and the Late Pleistocene. Considering the 2009 L’Aquila earthquake coseismic rupture, we observe that the younger transience on the Raiale River longitudinal profile, if it is of tectonic origin, could have only been produced by much larger seismic events (i.e., Mw > 6.5) than those documented in the area by paleoseismological investigations. The collective results confirmed that in the Central Apennines, conditions of dynamic equilibrium are often not met, and that the persistence of transient perturbations induced by tectonics should be accounted for.

Reconstruction of Last Glacial Maximum glaciers and palaeoclimate in the central Taurus Range, Mt. Karanfil, of the Eastern Mediterranean

We report the first glacial chronology of Mt. Karanfil (3059 m above sea level; a.s.l.), a small mountain in the south-central Taurus Range in Turkey. Well-preserved terminal and lateral moraines are clustered in four groups; M1, M2, M3, and M4, between ∼2000 and ∼2400 m a.s.l., composed of limestone blocks. Each moraine group originated from one to three cirque areas (C1, C2, C3, and C4 cirques) on the mountain's northern face between 2400 and 2600 m a.s.l. The best estimated mean ages of moraines M1 (17.3 ± 2.4 ka), M2 (18.6 ± 1.6 ka), and M3 (20.0 ± 2.6 ka) indicate that glaciers reached their maximum extents on the global Last Glacial Maximum (LGM), and they were persistent till the Late Glacial. The last moraine group (M4), more oriented towards the north, is dated to 14.6 ± 2.4 ka, implying that the M4 glacier lasted a few thousand years more. Also, a relict rock glacier in the C1 cirque floor was dated to 9.9 ± 0.9 ka and testifies to the development of permafrost conditions at the onset of Holocene. The timing of glaciation on Mt. Karanfil is close to the global LGM, as similar terrestrial cosmogenic nuclide chronologies were obtained from other mountains in the eastern Mediterranean. We also used the Parallel Ice Sheet Model (PISM) to reconstruct the glaciers and climate during the LGM on Mt. Karanfil. Twenty-one palaeoclimate simulations were run as a function of present temperature and precipitation to reach the steady-state glacier extents, and the modelled glacial areas were compared with the field-observed ice extents. The best-fit scenarios indicate that the LGM climate on Mt. Karanfil was 8.3 ± 0.5 °C colder than today if the precipitation levels were the same as today. More humid (20% wetter) or arid (20% drier) conditions than today bring the palaeotemperature estimates to 6.9 ± 0.4 °C or 10.4 ± 0.6 °C lower than the present, respectively.

A tool for the ages: The Probabilistic Cosmogenic Age Analysis Tool (P-CAAT)

While revolutionary to the geomorphic community, the application of terrestrial cosmogenic nuclide (TCN) dating is complicated by geological uncertainties, which often lead to skewed or poorly clustered TCN age distributions. Although a range of statistical approaches are typically used to detect and remove outliers, few are optimized for analysis of TCN datasets. Many are mean- or median-based and therefore explicitly assume a single probability distribution (e.g., Mean Squared Weighted Deviates, Chauvenet's Criterion, etc.). Given the ubiquity of pre- and post-depositional modification of rock surfaces, which occur at different rates in different geomorphic settings, these approaches struggle with multimodal distributions which often characterize TCN datasets. In addition, most statistical approaches do not propagate measurement or production rate uncertainties, which become increasingly important as dataset size or clustering increases. Finally, most approaches provide arithmetic single solutions, irrespective of geologic context. To address these limitations, we present the Probabilistic Cosmogenic Age Analysis Tool (P-CAAT), a new approach for outlier detection and landform age analysis. This tool incorporates both sample age and geologic uncertainties and uses Monte Carlo simulations to eliminate dataset skewness by isolating component normal distributions from a cumulative probability density estimate for datasets with three or more samples. This approach allows geologic context to inform post-analysis interpretations, as researchers can assign landform ages based upon statistically distinct subpopulations, informed by the characteristics of geomorphic systems (e.g., exhumation of boulders as moraines degrade through time). To evaluate the effectiveness of P-CAAT, we analyzed a range of synthetic TCN datasets and compared the results to commonly used statistical approaches for outlier detection. Irrespective of dataset size or clustering, P-CAAT outperformed other approaches and returned accurate solutions that improve in precision as sample size increases. To enable more comprehensive utilization of our approach, P-CAAT is packaged with a GUI interface and is available for download at kgs. uky.edu/anorthite/PCAAT.

Terrestrial cosmogenic 10Be dating of the Última Esperanza ice lobe moraines (52°S, Patagonia) indicates the global Last Glacial Maximum (LGM) extent was half of the local LGM

Whether glaciers in the southern hemisphere were asynchronous to those in the north during the global Last Glacial Maximum (gLGM; 26.5–19 ka) is still debated. In Patagonia (South America), numerous ice lobes attained their maximum extents during diverse episodes of the last glacial cycle. To understand the variations in the gLGM vs local LGM record, we studied the Lago Aníbal Pinto area (52°00′ S, 72°40′ E; Chile), where several moraine belts were deposited by one of the eastward-flowing southern Patagonian Ice Sheet (PIS) outlet ice lobes; the Última Esperanza. We report eight 10Be terrestrial cosmogenic nuclides (TCN) surface ages from granitic moraine boulders. Our weighted average age obtained from the southern part of the Río Turbio moraine belt yields 50.7 ± 2.4 ka (oldest boulder age; 53.8 ± 5.3 ka) and confirms the greatest extent of the local Last Glacial Maximum (lLGM) during Marine Isotope Stage 3 (MIS 3) in the previously dated northern moraines from the same belt. Our 10Be TCN age (32.6 ± 2.2 ka) derived from the Dos Lagunas moraine, which makes up the northernmost margin of the Última Esperanza ice lobe's Arauco advance, also validates the MIS 3 timing. Following the formation of Arauco moraines, the Última Esperanza ice lobe was split into three main tributaries in the south, which formed three restricted and previously undated moraine complexes. We dated one of them, the Aníbal Pinto moraine complex. Whereas the highest moraine yielded the oldest age (28.3 ± 2.2 ka), lower moraine's surface that was later truncated as a lacustrine erosional platform, yielded younger boulder ages (weighted average age = 18.9 ± 1.0 ka; oldest boulder age = 19.0 ± 1.5 ka), indicating they were deposited under the Pinto Lake level. Our new age data allow us to propose a new chronology for the Aníbal Pinto moraine complex and consolidate previously published ages from other moraine belts. We attribute the Aníbal Pinto moraine complex to early gLGM (MIS 2), emphasising that the gLGM was half the extent of lLGM in the Última Esperanza ice lobe that underpins its interhemispheric asynchronous character.

Alpine rock glacier activity over Holocene to modern timescales (western French Alps)

Abstract. Active rock glaciers are some of the most frequent cryospheric landforms in midlatitude high-elevation mountain ranges. Their activity strongly influences the hydrology and geomorphology of alpine environments over short (years to decades) and long (centuries to millennia) timescales. Being conspicuous expressions of mountain permafrost and important water reserves in the form of ground ice, rock glaciers are seen as increasingly important actors in the geomorphological and hydrological evolution of mountain systems, especially in the context of current climate change. Over geological timescales, rock glaciers both reflect paleoclimate conditions and transport rock boulders produced by headwall erosion, and they therefore participate in shaping high mountain slopes. However, the dynamics of rock glaciers and their evolution over different timescales remain under-constrained. In this study, we adopt a multi-method approach, including field observations, remote sensing, and geochronology, to investigate the rock glacier system of the Vallon de la Route (Combeynot Massif, western French Alps). Remotely sensed images and correlation techniques are used to document the displacement field of the rock glacier over timescales ranging from days to decades. Additionally, to estimate displacement over periods from centuries to millennia, we employ terrestrial cosmogenic nuclide (quartz 10Be) surface-exposure dating on rock boulder surfaces located along the central flow line of the rock glacier, targeting different longitudinal positions from the headwall to the rock glacier terminus. The remote sensing analysis demonstrates that between 1960 and 2018 the two lower units of the rock glacier were motionless, the transitional unit presented an integrated surface velocity of 0.03±0.02 m a−1, and the two upper active units above 2600 m a.s.l. showed a velocity between 0.14±0.08 and 0.15±0.05 m a−1. Our results show 10Be surface-exposure ages ranging from 13.10±0.51 to 1.88±0.14 ka. The spatial distribution of dated rock glacier boulders reveals a first-order inverse correlation between 10Be surface-exposure age and elevation and a positive correlation with horizontal distance to the headwall. These observations support the hypothesis of rock boulders falling from the headwall and remaining on the glacier surface as they are transported down valley, and they may therefore be used to estimate rock glacier surface velocity over geological timescales. Our results also suggest that the rock glacier is characterized by two major phases of activity. The first phase, starting around 12 ka, displays a 10Be age gradient with a rock glacier surface velocity of about 0.45 m a−1, following a quiescent period between ca. 6.2 and 3.4 ka before the emplacement of the present-day upper two active units. Climatic conditions have favored an integrated rock glacier motion of around 0.18 m a−1 between 3.4 ka and present day. These results allow us to quantify back-wearing rates of the headwall of between 1.0 and 2.5 mm a−1, higher than catchment-integrated denudation rates estimated over millennial timescales. This suggests that the rock glacier system promotes the maintenance of high rock wall erosion by acting as debris conveyor and allowing freshly exposed bedrock surfaces to be affected by erosion processes.

Modelling the systematics of cosmogenic nuclide signals in fluvial sediments following extreme events

AbstractThe effect of punctuated mass‐wasting events on longer‐term erosion rates is not fully understood, and yet it is key to quantifying sediment generation, source‐to‐sink dynamics, and landscape evolution in active orogens. The measurement of terrestrial cosmogenic nuclides (TCNs) in river sediments is a common method for determining basin‐averaged erosion rates over centennial to millennial timescales and is often used to compare erosional processes between catchments. However, these comparisons often overlook the role of landsliding rates and their spatial distribution in the measurement and potential variability of TCN signals. While it is widely accepted that basin‐scale perturbations should temporarily dilute TCN concentrations as landsliding mobilizes new, low‐concentration material, the impact of the catastrophic release of hillslope sediment caused by a single event on TCN signatures has not yet been systematically investigated. In this modelling study, we use a catchment in central Nepal to build upon previous modelling efforts to consider how TCNs are recorded in landscapes with varying erosion rates, landsliding rates, and spatial distribution of landslides. We then use the 25 April 2015 Mw 7.8 Gorkha earthquake, Nepal, as a case study to investigate how perturbations like earthquakes are recorded in TCN time series and transferred to and ultimately archived in the sedimentary record. We find that the likelihood of a perturbation being measured by TCN dilution is based on a multitude of factors, including background erosion rates, long‐term landsliding rates, and the connectivity of newly released material to the fluvial system. Especially in landscapes like the central Himalaya with high background erosion and landsliding rates, changes in detrital TCN concentrations are not a reliable indicator of an upstream perturbation, nor should we expect a clear dilution signal following a major event. Our modelling results emphasize that TCN dilution is not a universal characteristic of high‐magnitude landslide‐triggering events.

The Pangal landslide complex, Cachapoal basin, central Chile (34°S): An example of a multi-temporal slope instability cluster in the Andes

A landslide cluster located in the Pangal river valley, Cachapoal river basin, in the Andes of central Chile is investigated. Deposits of five rock avalanche deposits of volumes varying from 1.5 to 150 million cubic metres are distinguished by detailed geomorphological mapping and dating. The landslides were originated in volcanic rocks affected by hydrothermal alteration. The largest three avalanche deposits, formerly considered as a single deposit, were dated using Terrestrial Cosmogenic Nuclide methods. The results show three distinct ages from Late Pleistocene to Holocene, confirming the hypothesis of separate events. The presence of clay minerals detected by X-ray diffraction in matrix samples confirm the presence of hydrothermal alteration in the landslide source areas detected by satellite image multispectral analyses, suggesting an important control of such alteration in the landslide generation. The present river morphology and outcrops of lacustrine deposits and outburst terraces evidence that the largest rock avalanches, with deposit thicknesses of up to about 100 m, have dammed the valley to be later eroded by the Pangal river. The repetition of large volume landslides in the valley and their potential of river damming should be considered as a hazard that may endanger critical infrastructure and human settlements downstream. This case study is an example of potential hazards of large slope collapses and related river damming in the Chilean Andes that should be addressed in more depth along with further investigations on the landslide controlling factors and failure mechanisms in the region.

Use of terrestrial cosmogenic 10Be to quantify anthropogenic sediment yield from mountainous watersheds: Application in reconstructing environmental change in the Tanakami Mountains, central Japan

This study proposes a methodology that employs a terrestrial cosmogenic nuclide to quantify the magnitude of human impacts on soil-mantled hillslopes and reconstruct the history of the environmental transition from preserved to devastated states by the anthropogenic sediment yield in a mountainous watershed. The feasibility of the approach was tested in the Tanakami Mountains, central Japan. In the target area, preserved and devastated hilly watersheds distributes adjacent each other as a result of longstanding consumption of forest resources and subsequent acceleration of hillslope erosion. 10Be concentrations in quartz grains were measured in modern fluvial sand from the contrasting watersheds and cores of accretionary legacy sediment recovered at a terrestrial sink near to the most severely devastated watershed. The anthropogenic mass loss from hillslopes was calculated based on the difference between 10Be concentrations in the fluvial sand collected from the preserved and devastated watersheds. The timing and duration of severe human-induced erosion in the devastated watersheds were determined based on 10Be profiling and 14C dating of the sediment cores. To compare the datasets from the different watersheds and the legacy sediments, we normalized the 10Be concentration by the nuclide production rate, which thus represents the residence time of the sand particles transported through the soil and shallow bedrock zones. The average apparent residence times in the preserved and devastated watersheds were 10.5 ± 1.8 and 5.4 ± 1.4 kyr, respectively, reflecting the contrasting anthropogenic impacts on the watersheds. The 10Be and 14C archives indicate the 10Be concentration in the legacy sediment has been diluted at shallower depths and shows marked fluctuations over the last 300 yr. The total mass loss from the devastated watersheds was 5.3 × 105 to 2.9 × 106 g m−2, which can be converted to remove thickness of 0.3–1.8 m by assuming the density of subsurface materials as 1.6 × 106 g m−3. This result is consistent with the actual state of devastated hillslopes in the area characterized by the complete removal of soil cover and subsequent active erosion of exposed bedrock. The fluctuations in 10Be concentration in the cores probably resulted from the mixing of sediment particles from the preserved and devastated zones within the watershed, reflecting the nature of the anthropogenic environmental transition that progressed with propagation of the devastation front. The timing and duration of watershed devastation coincided with a period of extensive forest overharvesting documented in historical records.

Late Acheulean Lithic Assemblages From Locality 010 at Gürgürbaba Hill (Eastern Anatolia)

The province of Van in north-eastern Turkey served as a land bridge between Africa and Eurasia during the Palaeolithic. The region is of particular relevance for understanding the movement of hominins between these continents. This study concerns the lithic remains from a locality at Gürgürbaba Hill, named Locality 010, north of the village of Ulupamir (Erciş district). Locality 010 was dated to 311±32 kya by terrestrial cosmogenic nuclides method, which coincides with Marine Isotope Stage 9 (MIS 9), a Middle Pleistocene interglacial period. The assemblage from this site is attributed to the Late Acheulean and resembles that of the southern Caucasus. This similarity indicates that the artefacts from Locality 010 were probably produced by late Lower Palaeolithic technology in a broad sense. These findings suggest local adaptations of late Middle Pleistocene hominins to high plateau environments.

Tectonic controls on Quaternary landscape evolution in the Ventura basin, southern California, USA, quantified using cosmogenic isotopes and topographic analyses

AbstractThe quantification of rates for the competing forces of tectonic uplift and erosion has important implications for understanding topographic evolution. Here, we quantify the complex interplay between tectonic uplift, topographic development, and erosion recorded in the hanging walls of several active reverse faults in the Ventura basin, southern California, USA. We use cosmogenic 26Al/10Be isochron burial dating and 10Be surface exposure dating to construct a basin-wide geochronology, which includes burial dating of the Saugus Formation: an important, but poorly dated, regional Quaternary strain marker. Our ages for the top of the exposed Saugus Formation range from 0.36 +0.18/-0.22 Ma to 1.06 +0.23/-0.26 Ma, and our burial ages near the base of shallow marine deposits, which underlie the Saugus Formation, increase eastward from 0.60 +0.05/-0.06 Ma to 3.30 +0.30/-0.41 Ma. Our geochronology is used to calculate rapid long-term reverse fault slip rates of 8.6–12.6 mm yr–1 since ca. 1.0 Ma for the San Cayetano fault and 1.3–3.0 mm yr–1 since ca. 1.0 Ma for the Oak Ridge fault, which are both broadly consistent with contemporary reverse slip rates derived from mechanical models driven by global positioning system (GPS) data. We also calculate terrestrial cosmogenic nuclide (TCN)-derived, catchment-averaged erosion rates that range from 0.05–1.14 mm yr–1 and discuss the applicability of TCN-derived, catchment-averaged erosion rates in rapidly uplifting, landslide-prone landscapes. We compare patterns in erosion rates and tectonic rates to fluvial response times and geomorphic landscape parameters to show that in young, rapidly uplifting mountain belts, catchments may attain a quasi-steady-state on timescales of <105 years even if catchment-averaged erosion rates are still adjusting to tectonic forcing.

Large river chronology along the Jinshaan Gorge on the Yellow River and its implications for initialization

The ancient fluvial terrace is one of the keys to understanding how a large river develops in long-term landform evolution. However, surface erosion usually dissects the once extensive terrace into discontinuous segments and makes it difficult to explore the evolution of a large river worldwide. Fortunately, an exception is found in the Jinshaan Gorge of the Yellow River (JGYR), which preserves many Late Miocene-Late Quaternary fluvial terraces and thus can help understand the origin and evolution of a large river. Detailed investigation of the high terraces along the JGYR indicates a broad valley that was later superposed by a V-shaped valley due to rapid incision. To constrain the timeframe in which the broad valley persisted, as well as the onset time of the V-shaped valley formation, we select ten typical fluvial terrace profiles, apply terrestrial cosmogenic nuclide (TCN) 26Al/10Be burial dating to yield the ages of terrace deposits, and correct the results by considering both inherited and postburial nuclide production during slow red clay and loess deposition. Integrated with previous studies of magnetostratigraphic ages, our chronological results show that the lateral erosion and valley widening of the infant JGYR formed a broad valley during ca. 8–3.7 Ma; subsequent downward incision began ca. 3.7 Ma, accelerated to form a V-shaped valley and incising meanders into the initial broad valley since ca. 2.5 Ma. Moreover, most terrace ages plot in either periodic or short-lived arid conditions during ca. 8–3.7 Ma, while a distinctive downward incision has occurred under strengthened oscillating arid conditions since ca. 2.5 Ma, suggesting that climate aridification favours terrace aggradation, and tectonic uplift mainly dominated incision, which was consistent with the onset of the extension of the Shanxi Graben System ca. 8 Ma and the enhanced uplift of the Ordos Block since ca. 3.7 Ma.

Sequential quartz purification of 125–63 µm material for in-situ cosmogenic nuclide analysis

Quartz purification is a necessary and critical step to producing robust and reproducible results in terrestrial cosmogenic nuclide studies. Previous quartz purification work has centred on coarse sample material (1 mm-500 µm) and is anecdotally effective down to ∼250 µm. However, sample material with finergrain sizes are usually discarded. The adapted purification procedure outlined here shows that very-fine-sand sized material (125–63 µm) can be reliably cleaned for use in terrestrial cosmogenic nuclide studies. The results below show that 35% mass loss in very fine-grained quartz is sufficient to remove major elements (Al, Ti, Na, K, Fe, Mg, Ca, Mn,), 9Be, and meteoric 10Be. While other precise analysis (ICP-MS and ICP-OES) can verify sample purity, mass loss is a fast, inexpensive, and easy metric to set a standard by. This work shows that meteoric 10Be is easily removed from very-fine-grained samples while Al takes more effort.

Northeastern Patagonian Glacier Advances (43°S) Reflect Northward Migration of the Southern Westerlies Towards the End of the Last Glaciation

The last glacial termination was a key event during Earth’s Quaternary history that was associated with rapid, high-magnitude environmental and climatic change. Identifying its trigger mechanisms is critical for understanding Earth’s modern climate system over millennial timescales. It has been proposed that latitudinal shifts of the Southern Hemisphere Westerly Wind belt and the coupled Subtropical Front are important components of the changes leading to global deglaciation, making them essential to investigate and reconstruct empirically. The Patagonian Andes are part of the only continental landmass that fully intersects the Southern Westerly Winds, and thus present an opportunity to study their former latitudinal migrations through time and to constrain southern mid-latitude palaeo-climates. Here we use a combination of geomorphological mapping, terrestrial cosmogenic nuclide exposure dating and glacial numerical modelling to reconstruct the late-Last Glacial Maximum (LGM) behaviour and surface mass balance of two mountain glaciers of northeastern Patagonia (43°S, 71°W), the El Loro and Río Comisario palaeo-glaciers. In both valleys, we find geomorphological evidence of glacier advances that occurred after the retreat of the main ice-sheet outlet glacier from its LGM margins. We date the outermost moraine in the El Loro valley to 18.0 ± 1.15 ka. Moreover, a series of moraine-matching simulations were run for both glaciers using a spatially-distributed ice-flow model coupled with a positive degree-day surface mass balance parameterisation. Following a correction for cumulative local surface uplift resulting from glacial isostatic adjustment since ∼18 ka, which we estimate to be ∼130 m, the glacier model suggests that regional mean annual temperatures were between 1.9 and 2.8°C lower than present at around 18.0 ± 1.15 ka, while precipitation was between ∼50 and ∼380% higher than today. Our findings support the proposed equatorward migration of the precipitation-bearing Southern Westerly Wind belt towards the end of the LGM, between ∼19.5 and ∼18 ka, which caused more humid conditions towards the eastern margins of the northern Patagonian Ice Sheet a few centuries ahead of widespread deglaciation across the cordillera.

Segmentation and Holocene Behavior of the Middle Strand of the North Anatolian Fault (NW Turkey)

AbstractThe North Anatolian Fault (NAF) in the Marmara region is composed of three parallel strands all separated by ∼50 km. The activity of the middle strand, which borders the southern edge of the Marmara Sea, is much debated because of its present‐day very low seismicity. This contrasts with historical, archeological and paleoseismological evidence, which suggest several destructive earthquakes have occurred during the last 2000 years. Our study aims to better constrain seismic hazard on the middle strand by exploring its Holocene paleoseismicity. For this, we mapped 148 km of the eastern part of the middle strand, using high‐resolution satellite imagery. A series of landforms offset by the middle strand activity have been systematically measured to recover the past ruptures. Three Late Pleistocene‐Holocene terraces have been dated with the terrestrial cosmogenic nuclide method, constraining a horizontal slip rate of mm/yr. The statistical analysis of the offsets evidences several major ruptures preserved in the landscape, with coseismic lateral displacements ranging between 3 and 6.5 m. This corresponds to Mw ∼7.3 earthquakes able to propagate along several fault segments. Historical seismicity and paleoseismology data suggest that the last large earthquakes along the middle strand of the NAF (MNAF) happened in 1065 CE and between the 14th and 18th centuries CE. Since then, the MNAF may have accumulated enough stress to generate a destructive rupture.

Moraine crest or slope: An analysis of the effects of boulder position on cosmogenic exposure age

Terrestrial cosmogenic nuclide dating of ice-marginal moraines can provide unique insights into Quaternary glacial history. However, pre- and post-depositional exposure histories of moraine boulders can introduce geologic uncertainty to numerical landform ages. To avoid geologic outliers, boulders are typically selected based on their depositional context and individual characteristics but while these criteria have good qualitative reasoning, many have not been tested quantitatively. Of these, boulder location is critical, as boulders located on moraine crests are prioritised, while those on moraine slopes are typically rejected. This study provides the first quantitative assessment of the relative utility of moraine crest and moraine slope sampling using new and published 10Be and 36Cl ages (n = 19) and Schmidt hammer sampling (SH; n = 635 moraine boulders, ∼19,050 SH R-values) in the northern and southern Pyrenees. These data show that for many of the studied moraines, the spatial distribution of “good” boulders is effectively random, with no consistent clustering on moraine crests, ice-proximal or -distal slopes. In turn, and in contrast to prior work, there is no clear penalty to either moraine crest or moraine slope sampling. Instead, we argue that landform stability exerts a greater influence on exposure age distributions than the characteristics of individual boulders. For the studied landforms, post-depositional stability is strongly influenced by sedimentology, with prolonged degradation of matrix-rich unconsolidated moraines while boulder-rich, matrix-poor moraines stabilised rapidly after deposition. While this pattern is unlikely to hold true in all settings, these data indicate that differences between landforms can be more significant than differences at the intra-landform scale. As ad hoc assessment of landform stability is extremely challenging based on geomorphological evidence alone, preliminary SH sampling, as utilised here, is a useful method to assess the temporal distribution of boulder exposure ages and to prioritise individual boulders for subsequent analysis.

Maximum extent and readvance dynamics of the Irish Sea Ice Stream and Irish Sea Glacier since the Last Glacial Maximum

ABSTRACTThe BRITICE‐CHRONO Project has generated a suite of recently published radiocarbon ages from deglacial sequences offshore in the Celtic and Irish seas and terrestrial cosmogenic nuclide and optically stimulated luminescence ages from adjacent onshore sites. All published data are integrated here with new geochronological data from Wales in a revised Bayesian analysis that enables reconstruction of ice retreat dynamics across the basin. Patterns and changes in the pace of deglaciation are conditioned more by topographic constraints and internal ice dynamics than by external controls. The data indicate a major but rapid and very short‐lived extensive thin ice advance of the Irish Sea Ice Stream (ISIS) more than 300 km south of St George's Channel to a marine calving margin at the shelf break at 25.5 ka; this may have been preceded by extensive ice accumulation plugging the constriction of St George's Channel. The release event between 25 and 26 ka is interpreted to have stimulated fast ice streaming and diverted ice to the west in the northern Irish Sea into the main axis of the marine ISIS away from terrestrial ice terminating in the English Midlands, a process initiating ice stagnation and the formation of an extensive dead ice landscape in the Midlands.

Timing and pace of ice‐sheet withdrawal across the marine–terrestrial transition west of Ireland during the last glaciation

ABSTRACTUnderstanding the pace and drivers of marine‐based ice‐sheet retreat relies upon the integration of numerical ice‐sheet models with observations from contemporary polar ice sheets and well‐constrained palaeo‐glaciological reconstructions. This paper provides a reconstruction of the retreat of the last British–Irish Ice Sheet (BIIS) from the Atlantic shelf west of Ireland during and following the Last Glacial Maximum (LGM). It uses marine‐geophysical data and sediment cores dated by radiocarbon, combined with terrestrial cosmogenic nuclide and optically stimulated luminescence dating of onshore ice‐marginal landforms, to reconstruct the timing and rate of ice‐sheet retreat from the continental shelf and across the adjoining coastline of Ireland, thus including the switch from a marine‐ to a terrestrially‐based ice‐sheet margin. Seafloor bathymetric data in the form of moraines and grounding‐zone wedges on the continental shelf record an extensive ice sheet west of Ireland during the LGM which advanced to the outer shelf. This interpretation is supported by the presence of dated subglacial tills and overridden glacimarine sediments from across the Porcupine Bank, a westwards extension of the Irish continental shelf. The ice sheet was grounded on the outer shelf at ~26.8 ka cal bp with initial retreat underway by 25.9 ka cal bp. Retreat was not a continuous process but was punctuated by marginal oscillations until ~24.3 ka cal bp. The ice sheet thereafter retreated to the mid‐shelf where it formed a large grounding‐zone complex at ~23.7 ka cal bp. This retreat occurred in a glacimarine environment. The Aran Islands on the inner continental shelf were ice‐free by ~19.5 ka bp and the ice sheet had become largely terrestrially based by 17.3 ka bp. This suggests that the Aran Islands acted to stabilize and slow overall ice‐sheet retreat once the BIIS margin had reached the inner shelf. Our results constrain the timing of initial retreat of the BIIS from the outer shelf west of Ireland to the period of minimum global eustatic sea level. Initial retreat was driven, at least in part, by glacio‐isostatically induced, high relative sea level. Net rates of ice‐sheet retreat across the shelf were slow (62–19 m a−1) and reduced (8 m a−1) as the ice sheet vacated the inner shelf and moved onshore. A picture therefore emerges of an extensive BIIS on the Atlantic shelf west of Ireland, in which early, oscillatory retreat was followed by slow episodic retreat which decelerated further as the ice margin became terrestrially based. More broadly, this demonstrates the importance of localized controls, in particular bed topography, on modulating the retreat of marine‐based sectors of ice sheets.

Late Quaternary Intraplate Deformation Defined by the Las Chacras Fault Zone, West‐Central Argentina

AbstractSeveral major (up to MW 7.5) earthquakes over the past 320 years have shaken the thick‐skinned Sierras Pampeanas region of Argentina, despite exhibiting much lower GPS‐shortening rates than across the thin‐skinned Precordillera region to its west. Whether geodetic shortening rates indicate an actual long‐term shortening gradient, and whether shortening rates translate to higher uplift rates due to steeper faults in the Sierras Pampeanas, remain uncertain due to the limited spatio‐temporal coverage and the inherently large error in the vertical component of deformation of GPS measurements. We measure geomorphic offsets and use 10Be terrestrial cosmogenic nuclide surface exposure dating to determine slip rates on the Las Chacras Fault Zone (LCFZ)—an ∼30 km long, NNW‐trending, steeply dipping (55–65°E) reverse fault that branches off of the longest, westernmost, thick‐skinned Valle Fértil range‐front fault in the western Sierras Pampeanas. Average shortening and uplift rates measured on the LCFZ are ∼0.2 and ∼0.3–0.4 mm/yr, respectively. Despite an uplift rate similar to most other faults in the region, the LCFZ shortening rate is lower than faults to its west; this is in agreement with the inferred west‐east decrease in shortening rates from GPS data, indicating consistent regional deformation patterns since the Late Pleistocene. The decrease in shortening to the east coincides spatially with the termination of the flat portion of the subducted Nazca plate between 67 and 68°W. From scaling relationships among magnitude, slip rate, and fault length, the LCFZ is capable of generating earthquakes of MW 6.7–7.1.

Late Quaternary Slip Rate and Kinematics of the Baoertu Fault, Constrained by 10Be Exposure Ages of Displaced Surfaces within Eastern Tian Shan

AbstractInvestigation on the kinematics and deformation rates about active fault interior of the Tian Shan can provide significant information for strengthening our understanding on the present tectonic evolution of this range. The Baoertu Fault (BETF) is a major E-W striking active structure within the eastern Tian Shan and separates the south and central Tian Shan. But its kinematics and slip rates in the late Quaternary have never been systematically reported before. Based on interpretations of remote sensing images, drone photography, and detailed field investigations, we propose that the BETF is characterized by left-lateral strike-slip faulting with a thrust component and provides the first late Pleistocene slip rate for this fault. At the northern margin of the Kumishi Basin, combining offset reconstructions of displaced alluvial fan surfaces with the terrestrial cosmogenic nuclide (TCN) exposure age dating, we calculate an average sinistral slip rate of 0.65±0.16 mm/yr and average vertical slip rate of 0.07±0.01 mm/yr for the BETF since 95-106 ka. The differential movement eastward between the central Tian Shan block and Yanqi-Kumishi Basin block is likely the dominant driver of the left-lateral slip of the BETF. Synthesizing other quantitative data in eastern Tian Shan, we suggest that the hinterland active faults or folds, including the BEFT, roughly accommodate ~28-45% of the total N-S convergence across the eastern Tian Shan.