A reconstruction of the last glacial maximum (LGM) ice-surface geometry in the western Swiss Alps and contiguous Alpine regions in Italy and France is based on detailed field mapping of glacial trimlines, ice-erosional features and periglacial forms. Field data provide evidence of LGM ice-surface elevations and ice-flow directions. The LGM ice surface is portrayed as a grid-format digital elevation model (DEM) using geographic information system (GIS) software.LGM ice-surface areas and ice volumes in selected regions are calculated using a DEM of the present land topography. The reconstruction described in this paper is presented in conjunction with a previously determined LGM ice-surface reconstruction for the central and eastern Swiss Alps.The LGM ice cap in the western Swiss Alps and contiguous Alpine regions in Italy and France was characterized by transection glaciers. Four main centers of ice accumulation that influenced the transection glaciers include the Rhône ice dome, the Aletsch icefield, the southern Valais icefield, and the Mt. Blanc region. Major ice diffluences were located north of Simplon Pass, on Gd. St. Bernard Pass and north of present-day Glacier d'Argentière. Estimates of LGM ice volumes in selected regions show that the largest input of ice into the Rhône Valley was from the southern Valais icefield. Centered in the southern Mattertal, the LGM southern Valais icefield had a surface elevation of at least 3010 m and an ice thickness of at least 1400 m. The LGM ice-surface reconstruction and calculated ice volumes for selected regions are the basis for a hypothesis as to how erratic boulders from the southern Valais and Mt. Blanc regions were transported to the northern Alpine foreland. Certain LGM centers of ice accumulation and ice-flow directions presented in this paper are also examined for possible paleo-atmospheric circulation information.ZUSAMMENFASSUNG Die Rekonstruktion der Eisoberflächengeometrie während des Letzten Glazialen Maximums (LGM) in den westlichen Schweizer Alpen und in angrenzenden Regionen in Italien und Frankreich basiert auf einer detaillierten Kartierung von Gletscherschliffgrenzen, Eiserosionserscheinungen und periglazialen Formen. Diese Geländedaten geben Hinweise auf die Höhe der Eisoberfläche und die Eisflussrichtungen. Die Eisoberfläche wird durch ein gitterbasiertes digitales Höhenmodel (DHM) unter Benutzung eines Geographischen Informations Systems (GIS) dargestellt. Die Eisoberflächenerstreckung und die Eismächtigkeiten werden auf Basis des DHM der gegenwärtigen Landestopographie berechnet. Die in dieser Veröffentlichung beschriebene Rekonstruktion wird mit früheren Eisoberflächenrekonstruktionen für das LGM der zentralen und östlichen Schweizer Alpen verglichen.Die Eiskappe während des LGM in den westlichen Schweizer Alpen und in angrenzenden Regionen in Italien und Frankreich war durch ein Eisstromnetz charakterisiert. Die vier Zentren der Eisakkumulation, welche das Eisstromnetz beeinflussten, waren der Rhône Eisdom, das Aletsch Eisfeld, das Eisfeld im süd...

Subglacial deformation is crucial to reconstructing glacier dynamics. Sediments associated with the Late Ordovician ice sheet in the Djado Basin, Niger, exhibit detailed structures of the subglacial shear zone. Three main types of subglacial shear zones (SSZ) are discriminated. The lowermost SSZ, developed on sandstones, displays Riedel macrostructures and cataclastic microstructures. These resulted from brittle deformation associated with strong glacier/bed coupling and low porewater pressure. Where they developed on a clay-rich bed, the overlying SSZ display S-C to S-C 0 fabrics, sheath folds, and dewatering structures. These features indicate high ductile shear strain and water overpressure. On finegrained sand beds, the SSZ exhibit homogenized sand units with sand stringers, interpreted as fluidized sliding beds. The succession of subglacial deformation processes depends on fluid-pressure behavior in relation to subglacial sediment permeability. Fluid overpressure allows subglacial sediment shear strength and ice/bed coupling to be lowered, leading to ice streaming.

Investigation of Holocene extents of mountain glaciers along with the related naturally-driven climate conditions helps improve our understanding of glacier sensitivity to ongoing climate change. Here, we present the first Holocene glacial chronology in the Mont-Blanc massif (Argentière glacier) in the French Alps, based on 25 in situ-produced cosmogenic 10 Be dates of moraines and glacial bedrocks. The obtained ages from mapped sequences of moraines at three locations reveal that the glacier was retreating from its Lateglacial extent and oscillating several times between ~11.7 ka and ~10.4 ka, i.e. during the Younger Dryas/Early Holocene (YD/EH) transition, before substantially retreating at ~10.4 ka. Climate conditions corresponding to the past extents of Argentière glacier during the YD/EH transition (~ 11 ka) and the Little Ice Age (LIA) were modelled with two different approaches: by determining summer temperature differences from reconstructed ELA-rises and by using a Positive Degree Day (PDD) mass-balance model coupled with a dynamic ice flow model. The ELArise reconstructions yield a possible range of summer temperatures for the YD/EH transition that were cooler by between 3.0 and 4.8°C compared to the year 2008, depending on the choice of the ELA sensitivity to summer temperature. The results from the PDD model indicate temperatures cooler by ~3.6 to 5.5°C during the YD/EH transition than during the 1979-2002 period. For the LIA, our findings highlight that the role of local precipitation changes, superimposed on the dominant temperature signal, is important in the detailed evolution of the glacier. Overall, this study highlights the challenge that remains in accurately inferring paleoclimate conditions from past glacier extents.

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Abstract. The Ordovician is a particular Period during Earth History highlighted by abundant evidence for continental-size polar ice-sheets. Modelling studies published so far require a sharp CO2 drawdown to initiate this glaciation. They mostly used non-dynamic slab mixed-layer ocean models. Here, we use a general circulation model with coupled components for ocean, atmosphere and sea ice to examine the response of Ordovician climate to changes in CO2 and paleogeography. We conduct experiments for a wide range of CO2 (from 16 to 2 times the preindustrial atmospheric CO2 level (PAL)) and for two continental configurations (at 470 Ma and at 450 Ma) mimicking the Middle and the Late Ordovician conditions. We find that the temperature–CO2 relationship is highly non-linear when ocean dynamics is taken into account. Two climatic modes are simulated as radiative forcing decreases. For high CO2 concentrations (≥ 12 PAL at 470 Ma and ≥ 8 PAL at 450 Ma), a relative hot climate with no sea ice characterises the warm mode. When CO2 is decreased to 8 PAL and 6 PAL at 470 and 450 Ma, a tipping-point is crossed and climate abruptly enters a runaway icehouse leading to a cold mode marked by the extension of the sea ice cover down to the mid-latitudes. At 450 Ma, the transition from the warm to the cold mode is reached for a decrease in atmospheric CO2 from 8 to 6 PAL and induces a ~ 9 °C global cooling. We show that the tipping-point is due to the existence of a quasi-oceanic Northern Hemisphere, which in turn induces a minimum in oceanic heat transport located around 40° N. The peculiar shape of the oceanic heat transport in the Northern Hemisphere explains the potential existence of the warm and of the cold climatic modes. This major climatic instability potentially brings a new explanation to the sudden Late Ordovician Hirnantian glacial pulse that does not require any large CO2 drawdown.

Reconstructing the spatial and temporal response of mountain glaciers to rapid climate change in the past provides access to the effects of current climate change. Yet, the spatial and temporal variability of past glacier fluctuations is not fully understood. In this study, we focus on the timing of glacier fluctuations in the European Alps during the Younger Dryas/Early Holocene (YD/EH) transition. In an effort to elucidate whether glacier fluctuations were synchronous during this period, we present a new chronology of the Alpine Tal efre glacier, based on 14 new 10 Be ages of moraines and roches moutonn ees. The retreat of Tal efre glacier was initiated during the mid-YD (~12.4 ka), then it experienced a gradual retreat punctuated by at least three oscillations until $ 11 ka before shrinking substantially within its Little Ice Age limits (13thÀ19th centuries). Comparison of our findings with published glacier chronologies in the Alpine region highlights broadly synchronous behaviour of glaciers across the Alps between 12 and 10 ka. The coeval glacier fluctuations at a regional scale suggest that common regional climate conditions had a major impact on Alpine glacier variations during the YD/EH transition. The similarity of glacier behaviour and independent temperature records in both the Alpine region and the northern high latitudes suggests a teleconnection between these regions, but differences in the amplitude of the mean annual temperature signals relative to summer temperature indicate pronounced changes in seasonality between the YD and the EH.

Pluridisciplinary fieldwork highlights features generated by an extended ice-sheet in the Djado Basin during the Hirnantian. Two glacial palaeovalley systems associated with glacial pavements and separated by thin glaciomarine interstadial series are revealed. Rigid glacial pavements characterised by abrasion erosion are differentiated from soft glacial pavements characterised by soft-bed deformation. Glacial pavements are associated with subglacial bedforms such as megaflutes, flutes and meltwater channels. They are also associated with clastic dykes and glaciotectonic structures such as deformed flutes, subglacial folds and duplex structures. This record demonstrates that ice was warm-based and flowed rapidly on the highfluid-pressure soft substrate, as for ice streams. The erosional glacial landscape is typical of areal scouring, and the depositional sediment-landform assemblage corresponds to subglacial processes. These data afford a reconstruction of glacial events which is consistent with the two polyphased low-frequency glacial cycles inferred in previous studies. During interstadial and postglacial stages, grabens, normal faults, radial extensional microfaults and extensional dihedrons were generated by extensional tectonics during glacio-isostatic rebound. In sectors highly affected by this tectonics, doleritic dykes reflect a basal crust fusion increase induced by adiabatic decompression.