Revised Quaternary glacial succession and post-LGM recession, southern Wind River Range, Wyoming, USA

We present here a more complete cosmogenic chronology of Pleistocene glacial deposits for the Wind River Range, Wyoming, USA. Fifty-one new and thirty-nine re-calculated 10Be and 26Al exposure ages from Sinks and North Fork canyons, Stough Basin, Cirque of the Towers and the Temple Lake valley allow us to more tightly constrain the timing and sequence of glacial alloformations in the southern portion of the range. Moraines, diamicts and bedrock exposures here have previously been correlated with as many as five Pleistocene and four Holocene glacial events. Exposure ages from Pleistocene alloformations associated with trunk glaciers in Sinks Canyon and North Fork Canyon generally confirm earlier age estimates. Cosmogenic radionuclide (CRN, 10Be and 26Al) ages from moraines and striated bedrock surfaces previously mapped as Pinedale correspond to MIS2, while boulder exposure ages from moraines mapped as Bull Lake correspond generally to MIS5-MIS6. Geomorphic data from a moraine previously mapped as Younger pre-Sacagawea Ridge appears to correspond most closely to the Sacagawea Ridge glacial episode (MIS-16), but the uncertainty of a single 10Be exposure age suggests the unit could be as young as MIS-10 or as old as MIS-18. Boulders from a diamict on Table Mountain previously reported as Older pre-Sacagawea Ridge yield two 10Be exposure ages that suggest the presence of Early Pleistocene glacial activity here possibly older than 1–2 Ma (>MIS-30). Bedrock exposure ages within Sinks Canyon suggest the Pinedale valley glacier had retreated from the floor of Sinks Canyon to above PopoAgie Falls by ca. 15.3 ka. Cirque glaciers in Stough Basin appear to have retreated behind their riegels by ca. 16 ka, which suggests the cirque glaciers were decoupling across their riegels from the valley glaciers below at this time, prior to their readvance to form Lateglacial moraines. New 10Be boulder exposure ages from moraines previously correlated to the Temple Lake and Alice Lake allostratigraphic units in the cirques of Stough Basin and Cirque of the Towers show general equivalence to the stadial event just prior to the onset of the Bølling interstadial (17.5–14.7 ka) and to the Intra-Allerød Cold Period-Younger Dryas stadial phase (13.9–11.7 ka), respectively. From this evidence, the Temple Lake Alloformation of the Wind River Mountains now should correspond to the INTIMATE GS-2.1a (Oldest Dryas) stadial event while the Alice Lake Alloformation should correspond to the INTIMATE GS-2 stadial (IACP-Younger Dryas). Thus, we consider that evidence no longer exists for early-to mid-Holocene glacial events in the southern Wind River Range

Mass fluxes and clay mineral formation in soils developed on slope deposits of the Kowarski Grzbiet (Karkonosze Mountains, Czech Republic/Poland)

Weathering, mineral formation, and transformation processes along slopes are complex. In cool mountainous regions, undisturbed soil development with a strong vertical leaching element may abruptly change as a result of erosion, accumulation, lateral water fluxes and aeolian input. We investigated soils in the eastern Karkonosze Mountains that have developed on silicatic slope deposits. To date, illite, vermiculite and chlorite are the minerals that have been detected in the clay fraction. Although the climate and parent material should be favourable for the formation of smectites, expandable phases were not verified so far. We investigated if expandable phases could be detected and how they related to elemental fluxes along a short slope sequence (1142–1268 m a.s.l. on the border between the Czech Republic and Poland). Mass balance calculations indicated intensive mineral weathering together with a significant leaching of Mg, Al, Ca and Mn on the shoulder and foot slope positions. In the middle zone, which has a concave or undulating surface shape, however, the mass balances of several elements (Na, K, Al, P) revealed a less pronounced leaching (corresponding to a lower degree of podsolization) and in some cases even accumulation. At all sites, mass balance calculations and detected soil minerals (e.g. the increase in illite towards the surface together with an increase in Al and K) indicate some aeolian input. Kaolinite was detected in all soil horizons. Its concentration slightly increased towards the soil surface. Together with the pronounced leaching of Ca, part of kaolinite originates from plagioclase weathering. Besides being a weathering product of primary minerals, part of the kaolinite is inherited from the parent material and probably is also due to aeolian input. In all soils, illite was being transformed into vermiculite and smectites (through regularly-interstratified illite-smectite phases). In addition, the content of chloritic components which increases with depth indicated their concurrent weathering and transformation into smectites. Amphibole also may have acted as a source of smectites. Not all smectite is being actively formed in the soil. Most likely due to slope processes (cover beds) that affected even the subsoil, some smectite has been transferred along the slope. Part of the smectite also seems to derive from the parent material. Active formation of expandable clay minerals is related to convex and planar landscape forms. This relationship suggests intense element leaching, inheritance from the parent material and cover bed mixing processes have contributed to the presence of smectite. Along the slope, zones with predominant vertical transport (shoulder, foot slopes) may repeatedly be interchanged with zones dominated by lateral transport (undulating slope, concave forms)

Revised Quaternary glacial succession and post-LGM recession, southern Wind River Range, Wyoming, USA

We present here a more complete cosmogenic chronology of Pleistocene glacial deposits for the Wind River Range, Wyoming, USA. Fifty-one new and thirty-nine re-calculated 10Be and 26Al exposure ages from Sinks and North Fork canyons, Stough Basin, Cirque of the Towers and the Temple Lake valley allow us to more tightly constrain the timing and sequence of glacial alloformations in the southern portion of the range. Moraines, diamicts and bedrock exposures here have previously been correlated with as many as five Pleistocene and four Holocene glacial events. Exposure ages from Pleistocene alloformations associated with trunk glaciers in Sinks Canyon and North Fork Canyon generally confirm earlier age estimates. Cosmogenic radionuclide (CRN, 10Be and 26Al) ages from moraines and striated bedrock surfaces previously mapped as Pinedale correspond to MIS2, while boulder exposure ages from moraines mapped as Bull Lake correspond generally to MIS5-MIS6. Geomorphic data from a moraine previously mapped as Younger pre-Sacagawea Ridge appears to correspond most closely to the Sacagawea Ridge glacial episode (MIS-16), but the uncertainty of a single 10Be exposure age suggests the unit could be as young as MIS-10 or as old as MIS-18. Boulders from a diamict on Table Mountain previously reported as Older pre-Sacagawea Ridge yield two 10Be exposure ages that suggest the presence of Early Pleistocene glacial activity here possibly older than 1–2 Ma (>MIS-30). Bedrock exposure ages within Sinks Canyon suggest the Pinedale valley glacier had retreated from the floor of Sinks Canyon to above PopoAgie Falls by ca. 15.3 ka. Cirque glaciers in Stough Basin appear to have retreated behind their riegels by ca. 16 ka, which suggests the cirque glaciers were decoupling across their riegels from the valley glaciers below at this time, prior to their readvance to form Lateglacial moraines. New 10Be boulder exposure ages from moraines previously correlated to the Temple Lake and Alice Lake allostratigraphic units in the cirques of Stough Basin and Cirque of the Towers show general equivalence to the stadial event just prior to the onset of the Bølling interstadial (17.5–14.7 ka) and to the Intra-Allerød Cold Period-Younger Dryas stadial phase (13.9–11.7 ka), respectively. From this evidence, the Temple Lake Alloformation of the Wind River Mountains now should correspond to the INTIMATE GS-2.1a (Oldest Dryas) stadial event while the Alice Lake Alloformation should correspond to the INTIMATE GS-2 stadial (IACP-Younger Dryas). Thus, we consider that evidence no longer exists for early-to mid-Holocene glacial events in the southern Wind River Range

Denudation variability of the Sila Massif upland (Italy) from decades to millennia using 10Be and 239+240Pu

Landscapes and soils evolve in non‐linear ways over millennia. Current knowledge is incomplete as only average denudation (or erosion) rates are normally estimated, neglecting the temporal discontinuities of these processes. The determination of regressive and progressive phases of soil evolution is important to our understanding of how soils and landscapes respond to environmental changes. The Sila Massif (Italy) provides a well‐defined geomorphological and geological setting to unravel temporal variations in soil redistribution rates. We used a combination of in situ cosmogenic radionuclide measurements (10Be) along tor (residual rock) height profiles, coupled with fallout radionuclides (239+240Pu) in soils, to model soil denudation rates over the last 100 ka. We measured rates prior to the Last Glacial Maximum (LGM) of ≤30 t km−2 yr−1 (~0.036 mm yr−1). Following the LGM, during the transition from the Pleistocene to the Holocene, these rates increased to ~150–200 t km−2 yr−1 and appeared to be above soil production rates, causing regressive soil evolution. For the last ~50 years, we even describe erosion rates of ≥1,000 t km−2 yr−1 (~1.23 mm yr−1) and consider human impact as the decisive factor for this development. Consequently, the natural soil production rates cannot cope with the current erosion rates. Thus, a distinct regressive phase of soil formation exists, which will give rise to shallowing of soils over time. Overall, our multimethod approach traced denudation and erosion histories over geologic and human timescales and made a new archive to soil science and geomorphology accessible

Climate and relief‐induced controls on the temporal variability of denudation rates in a granitic upland

How soil erosion rates evolved over the last about 100 ka and how they relate to environmental and climate variability is largely unknown. This is due to a lack of suitable archives that help to trace this evolution. We determined in situ cosmogenic beryllium‐10 (10Be) along vertical landforms (tors, boulders and scarps) on the Sila Massif to unravel their local exhumation patterns to develop a surface denudation model over millennia. Due to the physical resistance of tors, their rate of exhumation may be used to derive surface and, thus, soil denudation rates over time. We derived soil denudation rates that varied in the range 0–0.40 mm yr‐1. The investigated boulders, however, appear to have experienced repositioning processes about ~20–25 ka bp and were therefore a less reliable archive. The scarps of the Sila upland showed a rapid bedrock exposure within the last 8–15 ka. Overall, the denudation rates increased steadily after 75 ka bp but remained low until about 17 ka bp. The exhumation rates indicate a denudation pulse that occurred about 17–5 ka bp. Since then the rates have continuously decreased. We identify three key factors for these developments – climate, topography and vegetation. Between 75 and 17 ka bp, climate was colder and drier than today. The rapid changes towards warmer and humid conditions at the Pleistocene–Holocene transition apparently increased denudation rates. A denser vegetation cover with time counteracted denudation. Topography also determined the extent of denudation rates in the upland regime. On slopes, denudation rates were generally higher than on planar surfaces. By determining the exhumation rates of tors and scarps, soil erosion rates could be determined over long timescales and be related to topography and particularly to climate. This is key for understanding geomorphic dynamics under current environmental settings and future climate change

Evolution of soil erosion rates in alpine soils of the Central Rocky Mountains using fallout Pu and δ13 C

Data from soil chronosequences have been widely used to quantify soil formation and weathering rates, but are less used to determine erosion rates and the stabilisation of moraines over time. We hypothesise that soil erosion rates on moraine hillslopes decrease over time as soils evolve and slopes stabilise. We selected a sequence of moraines in the Wind River Range (Central Rocky Mountains) to study these processes over time. Moraine ages were based on 10Be surface exposure dating of moraine boulders. Quantitative soil erosion and accumulation rates along slopes with similar exposures, lengths and gradients were determined from profile patterns of 239+240Pu radionuclides. We used stable carbon isotopes (δ13C) in relation with the total soil organic carbon (SOC) content for qualitative information about soil erosion. The 10Be boulder exposure ages revealed that the moraines were deposited during the Younger Dryas and the pre Bølling–Allerød episodes of the late Pleistocene. The morphology of the soils suggests a complex history of development and shows that both erosion and aeolian deposition have affected the soils. The 239+240Pu measurements revealed that erosion rates strongly decrease with time as soils develop. A weakly developed soil (Cambisol) is found on the youngest moraine (11.8 ka) that exhibits an erosion rate, depending on the calculation procedure, in the range of 260 to 520tkm−2a−1. With time the erosion rate rapidly decreases to almost zero, presumably as a full vegetation cover develops. Bioturbation and/or dust influx is increasingly obvious with increasing age of the soils, as evidenced by the comparison of δ13Cand SOC. The mass balance of the oldest soil (15.8 ka) indicates that the slopes have reached a geomorphological stability with little or no net erosion. Aeolian influx appears to be the primary factor to account for mass changes in the oldest soil

Palaeoclimate, glacier and treeline reconstruction based on geomorphic evidences in the Mongun-Taiga massif (south-eastern Russian Altai) during the Late Pleistocene and Holocene

Little is known about the extent of glaciers and dynamics of the landscape in south-eastern Russian Altai. The effects of climate-induced fluctuations of the glaciers and the upper treeline of the Mongun-Taiga mountain massif were, therefore, reconstructed on the basis of in-situ, multiannual observations, geomorphic mapping, radiocarbon and surface exposure dating, relative dating (such as Schmidthammer and weathering rind) techniques and palaeoclimate-modelling. During the maximal advance of the glaciers, their area was 26-times larger than now and the equilibrium line of altitude (ELA) was about 800m lower. Assuming that the maximum glacier extent took place during MIS 4, then the average summer temperatures were 2.7℃ cooler than today and the amount of precipitation 2.1 times higher. Buried wood trunks by a glacier gave ages between 60 and 28 cal ka BP and were found 600-700m higher than the present upper treeline. This evidences a distinctly elevated treeline during MIS 3a and c. With a correction for tectonics we reconstructed the summer warming to have been between 2.1 and 3.0℃. During MIS 3c, the glaciated area was reduced to less than 0.5 km² with an increase of the ELA of 310-470m higher than today. Due to higher precipitation, the glaciated area during MIS 3a was close to the current ELA. Exposure dating (¹⁰Be) would indicate that the maximum glacier extension was 24 ka BP, but the results are questionable. From a geomorphic point of view, the maximum extent can more likely be ascribed to the MIS4 stage. We estimate a cooling of summer temperature of - 3.8 to - 4.2℃ and a decrease in precipitation of 37-46% compared to the present-day situation. Samples of wood having an age of 10.6-6.2 cal ka BP were found about 350m higher than the present treeline. It seems that the summer temperature was 2.0-2.5℃ higher and annual precipitation was double that of the present-day. For that period, the reconstructed glaciation area was 1 km² less than today. Three neoglacial glacier advances were detected. The glaciers covered about double the area during the Little Ice Age (LIA), summer cooling was 1.3℃ with 70% of the present-day precipitation. The reconstructed amplitude of climatic changes and the shift of the altitudinal zones show that the landscape has reacted sensitively to environmental changes and that dramatic changes may occur in the near future

The effect of permafrost on time-split soil erosion using radionuclides (¹³⁷Cs, ²³⁹ ⁺ ²⁴⁰Pu, meteoric ¹⁰Be) and stable isotopes (δ¹³C) in the eastern Swiss Alps

Purpose: Global warming is expected to change the thermal and hydrological soil regime in permafrost ecosystems which might impact soil erosion processes. Erosion assessment using radionuclides can provide information on past and ongoing, i.e. time-split, processes. The focus of this work was to find out if permafrost soils in the Swiss Alps differ in their medium- and long-term erosion rates from non-permafrost soils and if rates have accelerated during the last few decades. Materials and methods: Using cosmogenic (meteoric ¹⁰Be) and anthropogenic radionuclides (¹³⁷Cs, ²³⁹ ⁺ ²⁴⁰Pu), a time-split approach was achieved by determining erosion activities on the long (millennia; ¹⁰Be) and medium term (decades; ¹³⁷Cs, ²³⁹ ⁺ ²⁴⁰Pu). Additionally, the stable isotope δ¹³C signature in soil organic matter was used as a qualitative indicator for soil disturbance patterns. We compared soil erosion processes in permafrost soils and nearby unfrozen soils in the alpine (sites at 2,700 m asl, alpine tundra) and the subalpine (sites 1,800 m asl, natural forest) range of the Swiss Alps (Upper Engadine). ¹³⁷Cs, ²³⁹ ⁺ ²⁴⁰Pu and δ¹³C measurements were performed at the alpine sites only. Results and discussion: Depending on the calculation procedure (profile distribution model or inventory method), the ¹³⁷Cs measurements revealed soil accumulation rates of 1–3 t/km²/year in permafrost soils and 34–52 t/km²/year in non-permafrost soils. However, due to snow cover and subsequent melt-water runoff during ¹³⁷Cs deposition after the Chernobyl accident, caesium does not seem to be an appropriate soil erosion tracer on the investigated alpine sites. With ²³⁹ ⁺ ²⁴⁰Pu, more reliable results were achieved. ²³⁹ ⁺ ²⁴⁰Pu measurements provided erosion rates of 31–186 t/km²/year in permafrost soils and accumulation rates of 87–218 t/km²/year in non-permafrost soils. Erosion and accumulation were relatively low and related to the vegetation community. The long-term (¹⁰Be) soil redistribution rates (erosion rates up to 49 t/km²/year and accumulation rates up to 4 t/km²/ year) were low with no significant differences between permafrost and non-permafrost sites. The δ¹³C signature indicated soil disturbances in permafrost and non-permafrost soils compared to the reference site. Conclusions: Our results highlight that soil redistribution rates have increased during the last few decades. However, whether the higher medium-term erosion rates obtained for the last decades are the result of the ongoing climate warming and related accelerated soil erosion or if other factors (e.g. measurement uncertainties) have been responsible for such an increase could not fully be clarified

Soil denudation rates in an old‐growth mountain temperate forest driven by tree uprooting dynamics, Central Europe

Tree uprooting may distinctly affect landscape dynamics and slope denudation. Little is known, however, about the corresponding soil redistribution rates (erosion and accumulation) on either a long‐term (millennia; 10Be) or a short‐term (decades; 239+240Pu) scale. We determined these rates in a well‐investigated forest reserve (Zofinsky primeval forest, Czech Republic) using complementary techniques: nuclides in soils and tors to derive short‐ to long‐term rates and monitoring data (43 years) of repeated tree censuses using tree uprooting data. Temporal trends of soil erosion rates were obtained by dating the timing of exhumation (10Be) of tors. The average long‐term denudation rates were about 30–40 t km−2 yr−1. It seems that these rates varied over time with probably a maximum during the Pleistocene/Holocene transition (58–91 t km−2 yr−1). 239+240Pu activities in the soils identified soil redistribution rates of 50 to >100 t km−2 yr−1 for the last decades and agree with results from the tree uprooting monitoring (<92 t km−2 yr−1). In‐situ 10Be in soils gave similar denudation rates (58–76 t km−2 yr−1). Meteoric 10Be provided a mean residence time of a soil particle of 33–100 ka supporting the measured average long‐term erosion rates. Soil aggregates indicated stable physical conditions meaning that soil mass redistribution occurs only sporadically. It seems that the main driving factors of denudation changed over time. An erosion peak at the Pleistocene/Holocene transition (climate change) seems likely but needs further proof. Over the last few millennia, tree uprooting seems the main driver of soil erosion