This paper presents new results of terrestrial cosmogenic nuclide dating of erratics in northern Poland. We report the first exposure ages of erratics located on the pre-Local Last Glacial Maximum ...and the Local Last Glacial Maximum moraines in Poland. Published radiocarbon ages are calibrated and used as a background indicator of the possible time window for the Local Last Glacial Maximum.
The terrestrial cosmogenic nuclide exposure ages presented in this study indicate that: (1) exposure ages of erratics located on the pre-Local Last Glacial Maximum moraines (15.9 ± 1.4 to 101.7 ± 8.5 ka) are not clustered around any specific time interval, (2) the age of the ice sheet retreat from the Local Last Glacial Maximum ice limit in western Poland is 20.7 ± 0.8 ka and the probable duration of the Local Last Glacial Maximum in western and central Poland is between ∼25 ka and ∼21 ka, (3) the age of the ice sheet retreat from the Local Last Glacial Maximum ice limit in eastern Poland is 17.3 ± 0.5 ka and the probable duration of the Local Last Glacial Maximum in eastern Poland is between ∼22 ka and ∼18 ka. Our results show that the Local Last Glacial Maximum ice limit in western and central Poland is probably ∼3 ka older than in eastern Poland.
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•A new set of 34 TCN exposure ages of erratics in northern Poland.•SIS retreat from the Leszno (Brandenburg) Phase at 20.7 ± 0.8 ka.•SIS retreat from the Poznań (Frankfurt) Phase at 17.3 ± 0.5 ka.•The Local LGM in western and central Poland (Leszno Phase) 25–21 ka.•The Local LGM in eastern Poland (Poznań Phase) 22–18 ka.
High-resolution records of beryllium (10Be) production and relative paleointensity have been obtained across the Matuyama–Brunhes (M–B) reversal from the equatorial Indian Ocean (Maldives area). Both ...magnetic and geochemical analyses were performed from the same discrete samples to avoid any artificial depth offset. The authigenic 10Be concentrations were normalized with respect to 9Be in order to correct for potential environmental effects, while the relative paleointensity was derived from the remanent magnetization intensity after accounting for changes in magnetic concentration within the sediment. The relative paleointensity and the 10Be/9Be records are both characterized by large deviations, which culminate in the middle of the reversal. In contrast to most previous studies, and despite relative high deposition rate (4.7 cm/ka), we observed a perfect synchronism between the 10Be/9Be peak, the lowest value of relative paleointensity and the switch in direction, which indicates that bioturbation and post-depositional processes did not affect the magnetic record. This leaves no ambiguity for the stratigraphic position of the reversal located within Marine Isotopic Stage 19 as revealed by the planktonic δ18O record from the same core. The magnetic data depict a two-phase process with a precursory event preceding the rapid polarity switch, while only the second phase is present in the 10Be record, similarly to other low latitude records from the Indonesian area. Using an orbitally-tuned age model, we obtain an age of 772 ka±5 ka for the middle of the transition, while the precursory event occurred almost 20 ka before. We believe that the bimodal distribution emerging from the compilations of the ages of the M-B reversal results from the succession of these two events. Microtektites from the Australasian impact were found at 0.6 m below the transition (790 ka±5 ka B.P.) and confirm that this large event occurred 12 ka prior to the polarity transition. The distribution of tektite abundance was used to deconvolve the 10Be/9Be signal. The results confirm that the beryllium changes are concentrated during the transitional period, thus likely in presence of a multipolar geomagnetic field (or in the vicinity of a geomagnetic pole) that favored the penetration of cosmic rays and consequently increased the 10Be production. The absence of 10Be during the precursor indicates that the present site and the Indonesian ones were far away from a geomagnetic pole and that interlatitudinal atmospheric mixing was limited. The geomagnetic pole positions above the Indonesian sites during the precursor would thus be incompatible with the corresponding inclined dipolar field during this period, and suggest the dominance of low-degree harmonics.
•Synchronism of beryllium, paleointensity and directional signals during the reversal.•10Be peak during the transition but not during the precursor.•Ages of precursor and transition reconcile radiometric and astrochronological ages.•Australasian microtektites indicate bioturbation over 20 cm at most.
The Central Apennines are among the most seismically active regions in Italy. This region is affected by an extension accommodated by active normal faults. Those faults with high seismogenic ...potential hosted Mw > 6 shallow damaging earthquakes, such as the Mw 6.5 event of Oct. 30, 2016. Although fault slip rates are crucial to seismic hazard assessments, in the Central Apennines, slip rate assessments encompassing several seismic cycles (> 10 ka) are sparse. This is particularly true for the Mt. Vettore-Mt. Bove Fault system that ruptured during the mainshocks of the 2016–2017 seismic sequence. In this study, we present new geochronological constraints of offset geomorphological markers along the northern portion of the fault system on the Mt. Porche Fault segment using in-situ produced 36Cl cosmogenic nuclides. Offset measurements were made using a 5-cm resolution DEM obtained through a drone survey and constrained a fault scarp height of 15.5 ± 1.4 m and a maximal cumulative offset estimated between 32 and 40.5 m. Samples were collected from the Valle Lunga terminal moraine at 1710 m asl and yielded 36Cl exposure ages of 12.7 + 2.2/−1.9 ka, while the abraded surface, located on top of the tectonic scarp yielded 36Cl exposure ages of 23.4 + 5.3/−4.3 ka. Using the fault scarp height and the exposure age of this abraded surface, we constrained a minimum vertical fault slip rate of 0.7 + 0.2/−0.1 mm/yr. Assuming the offset started to accumulate when climate conditions allowed the scarp preservation, we constrained a maximal vertical slip rate of 1.2 ± 0.2 mm/yr along the main fault of the Mt. Vettore-Mt. Bove Fault system. This rate is higher than those previously obtained from trenches along secondary branches of the Mt. Vettore-Mt. Bove Fault system. Besides, the yielded chronology for the last glacial maximum in that area at ~23 ka is in good agreement with the timing previously proposed for the LGM in the Apennines.
•The last advance of the Valle Lunga glacial (Dryas), is dated at ~12.7 + 2.2/−1.9 ka.•Local LGM glacier retreat occurred ~23.4 + 5.3/−4.3 ka ago in the Valle Lunga.•The Mt. Vettore main fault has a minimum vertical slip rate of 0.7 + 0.2/−0.1 mm/yr.
Understanding topographic evolution requires integrating elementary processes acting at the hillslope scale into the long-wavelength framework of landscape dynamics. Recent progress has been made in ...the quantification of denudation of eroding landscapes and its links with topography. Despite these advances, data is still sparse in carbonate terrain, which covers a significant part of the Earth's surface. In this study, we measured both long-term denudation rates using in situ-produced 36Cl concentrations in bedrock and regolith clasts and surface convexity at 12 sites along ridges of the Luberon carbonate range in Provence, Southeastern France. Starting from ∼30 mm/ka for the lowering of the summit plateau surface, denudation linearly increases with increasing hilltop convexity up to ∼70 mm/ka, as predicted by diffusive mass transport theory. Beyond this point denudation rates appear to be insensitive to the increase in hilltop convexity. We interpret this constant denudation as indicating a transition from a regime where hillslope evolution is primarily controlled by diffusive downslope regolith transport, toward a situation in which denudation is limited by the rate at which physical and chemical weathering processes can produce clasts and lower the hilltop. Such an abrupt transition into a weathering-limited dynamics may prevent hillslope denudation from balancing the rate of base level fall imposed by the river network and could potentially explain the development of high local relief in many Mediterranean carbonate landscapes.
•We measure the evolution of curvature with denudation on hilltops of a carbonate range.•For low curvature ridges we observe a linear increase of denudation with curvature.•For high curvature ridges denudation becomes nearly constant and independent of curvature.•This suggests a transition from transport to weathering limited dynamics.
Abstract
Soil organic carbon is one of the largest surface pools of carbon that humans can manage in order to partially mitigate annual anthropogenic CO
2
emissions. A significant element to assess ...soil sequestration potential is the carbon age, which is evaluated by modelling or experimentally using carbon isotopes. Results, however, are not consistent. The
14
C derived approach seems to overestimate by a factor of 6–10 the average carbon age in soils estimated by modeling and
13
C approaches and thus the sequestration potential. A fully independent method is needed. The cosmogenic chlorine nuclide,
36
Cl, is a potential alternative.
36
Cl is a naturally occurring cosmogenic radionuclide with a production that increased by three orders of magnitude during nuclear bomb tests. Part of this production is retained by soil organic matter in organochloride form and hence acts as a tracer of the fate of soil organic carbon. We here quantify the fraction and the duration of
36
Cl retained in the soil and we show that retention time increases with depth from 20 to 322 years, in agreement with both modelling and
13
C-derived estimates. This work demonstrates that
36
Cl retention duration can be a proxy for the age of soil organic carbon.
Located at the southern boundary of the Alpine chain in Morocco, the deformation front of the Southern Rif Mountains is a region of moderate tectonic activity, which makes it a good natural ...laboratory to understand whether, and how, low compressional strains are located on specific structures. Along the ≈80 km‐long left‐lateral, transpressive and reverse fault zone that runs at the toe of the Pre‐Rif Ridges, an analysis of high‐resolution digital topography provides new geomorphic lines of evidence supporting Quaternary activity along, 20 km‐long fault segments. The fault zone can be divided into the Meknès and the Fès segments, which are constrained at depth by reactivated, NE‐trending basement faults, delimitating paleo‐grabens associated with the Late Triassic‐Jurassic opening of the Atlantic Ocean. For selected sites, we used in situ‐produced 36Cl, 10Be, and 26Al and high‐resolution topography to infer the timing of abandonment of fluvial markers, which suggest incision rates on the order of 0.6–2 mm/yr. Given their lengths, scaling laws suggest that the identified fault segments should root at about 7–12 km‐depth, possibly reactivating former basement normal faults and making them potential seismogenic sources capable of generating Mw6+ earthquakes, with return times of the order of several hundreds of years. Our new morphochronological data set confirms that the Southern Rif deformation front is a key structure that may have accommodated most of the lateral extrusion of the Rif between the Nubia and Iberia tectonic plates.
Plain Language Summary
In Morocco, the population may suffer from the dramatic consequences of earthquakes. Far from a tectonic plate boundary, it is not so easy to localize in the landscape the faults that might be responsible for a strong earthquake. In this article, we used high‐resolution satellite imagery to study the active faults along the Pre‐Rif Ridges, in Northern Morocco. We also applied a dating technique that uses the accumulation of some rare isotopes in the rock exposed to cosmic particles at the surface of the Earth. The combination of the two methods permits to estimate rates of river incisions and to convert them into rates of shortening and fault slip. This study contributes to the seismic hazard assessment of the faults close to Meknès and Fès cities.
Key Points
High resolution digital topography helps locate morphological indexes of active tectonics in Morocco
Cosmogenic nuclides allow placing time constraints for landscape development in the Southern Rif Front
The Southern Rif Front is an important geodynamic boundary with a non‐negligible seismogenic potential
The dynamics of carbonate-dominated landscapes is still debated, and first-order questions pertaining to the respective importance of chemical and mechanical weathering processes or the influence of ...climatic variability on denudation rates are largely open. To address these issues, we investigate the denudation pattern across a climatic gradient in Provence, SE France. We focused on a series of major carbonate mountain ranges with elevation spanning 200 to 2000 m and associated with >2-fold variations in mean annual temperature and precipitation. We collected 25 samples from 10 different mountain ranges and quantified denudation rates through the measurement of their 36Cl concentrations in order to explore tectonic and climatic controls on these carbonate landscapes evolution. We observe that most of our samples experience denudation rates in the 30–60 mm/ka range (averaged over 5 to 40 ka), with no evident dependence on variations of mean annual temperature or precipitation. One range (Lure mountain) stands out as a significant outlier, with much higher denudation rates (>100 mm/ka) than adjacent sites (Ventoux mountain) at the same elevation and experiencing similar present-day environmental conditions. We explore in details the climatic, tectonic and geomorphological contexts of these two ranges in order to understand the reasons for this apparent 3-fold difference in denudation rates. A regionally-documented major Holocene hillslope destabilization event appears to be a likely candidate to explain the observed lower 36Cl concentrations at the top of the Lure mountain. At longer wavelengths, our regional dataset shows no clear control of modern precipitation gradients on denudation rates, consistent with previous observations in landscapes underlain by other lithologies.
•Carbonate denudation rates in Provence are comparable to those in other settings.•Precipitation distribution has little influence on carbonate denudation.•Denudation variability is linked to Holocene hillslope destabilization events.•For similar climate denudation of carbonates is higher than for other lithologies.
The Main Himalayan Thrust (MHT) is the source of great earthquakes that have been documented along the range. Its geometry is a key parameter that influences accommodation of tectonic loading and ...earthquake magnitudes along the Himalayan Arc. Although seismic images are available for both the western and the central part of the range, this geometry remains poorly constrained for the Bhutanese Himalayas. Here we address this issue using a 10Be cosmogenic nuclides denudation transect across western Bhutan. We observe a wide low denudation rate domain between 50 km and 110 km from the front followed by a strong northward increase. Using a joint inversion of denudation rates, GPS data, and Holocene uplift rates, we interpret this pattern as a consequence of a flat‐ramp transition along the MHT. Compared to central Nepal and Sikkim, this location of the ramp suggests a wider décollement, with implications for greater seismogenic potential of the MHT in western Bhutan.
Key Points
Assessment of denudation rates in Himalayas
Robust constrain on the location of the flat‐ramp transition of the MHT
Evidence of a strong lateral variation in the MHT geometry
The growing number of studies based on data provided by the accelerator mass spectrometry (AMS) technique from increasingly diverse matrices implies that developing very effective and reproducible ...sample processing techniques is crucial to meet the demand generated.
After being dissolved, the isolation of the cosmogenic nuclide to be measured and its chemical purification imply complex processes throughout successive stages The presented automatic system was designed to perform the final steps of the 10Be and 26Al chemical extraction protocol from natural matrices (sediments, pure mineral phases,…), previously purified, decontaminated and dissolved in the case of in situ-produced cosmogenic nuclides, or leached or dissolved in the case of meteoric cosmogenic nuclides. Driven by Labview software, the developed automatic system, in addition to freeing the operator from tedious tasks, allows reproducibly processing simultaneously up to eight samples of different volumes in roughly 40 min. This article presents the design of the automated extraction bench and its performance based on the recovery efficiency of the stable element and the reproducibility of the measurements of the cosmogenic nuclide concentrations by AMS.
Recent synoptic studies have summarized the former spatial extent and chronology of the last Scandinavian Ice Sheet (SIS) and provide exceptional compilations of empirical datasets. The latter ...represent the primary source of information for the modelling community to test and develop thermomechanical ice sheet model, which are then further embedded into climatic and general circulation models. The Last Glacial Maximum (LGM) extent and chronology of the SIS across continental Europe were highlighted as being the least well-known thus generating the biggest uncertainties in the reconstruction of the ice sheet. While early geomorphological studies of the ice sheet marginal belts are numerous and detailed in western Russia, geochronological data are virtually inexistent. In an attempt to fill in this gap, we conducted a series of sampling campaigns for surface exposure dating using 10Be in the western and northwestern regions of Russia. We present here the first results from the Valday Heights where glacial deposits constitute well-preserved margins of the SIS. A total of 16 samples firmly establish the timing of the LGM in the Valday Heights at 20.1 ± 0.4 ka. The Valday Heights acted as a natural wall for the last maximum advance of the SIS during MIS2, constraining the eastern flank of the Ladoga-Ilmen-Lovat ice stream to the west of the hills. The Valday Heights were only overran by previously more extensive glacial advances as suggested by older exposure ages obtain on erratic boulders (31.2 ± 1.4 to 47.1 ± 1.5 ka) located east of the LGM limit.
•We conducted a series of sampling campaigns for surface exposure dating using 10Be in W and NW Russia.•A total of 16 samples firmly establish the timing of the LGM in the Valday Heights at 20.1 ± 0.4 ka.•The Valday Heights acted as a natural wall for the last maximum advance of the SIS during MIS2.•Older exposure ages east of the Valday Heights suggest that the region was overran by more a extensive ice sheet prior the LGM.
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