Late-glacial palaeoenvironments in Aquitaine, Southwestern France: the “La Brunetière” alluvial sequence (Bergerac, France)

Permafrost & Periglacial

Couchoud

Charlier

et al. 2023

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The origin of white calcite silts forming 0.5 to 3‐cm‐thick lenses in alluvial fan deposits 14C‐dated to the Last Glacial Maximum in the Dronne Valley (Dordogne, southwest France) is investigated using microscopic imagery, chemistry, and O and C stable isotopes. The calcite silts, composed mainly of aggregates of 3–5‐μm euhedral crystals, do not resemble secondary precipitations of pedological origin because of the strata‐like pattern and the lack of clearly identifiable biological structures. Their association with evidence of ice formation in the soil (platy structure, involutions) suggests that they were deposited in a deep seasonal frost context. Their isotopic composition differs significantly from those of detrital carbonates and of Holocene bioprecipitation and seems to be best explained by precipitation under closed‐system conditions. Calculation of the isotopic composition of calcite that would have formed in equilibrium with groundwater of regional LGM aquifers provides values that are in the range of the composition of the calcite silts for a precipitation temperature close to 0°C. Therefore, these deposits are interpreted as cryogenic calcite precipitated from waters close to saturation with respect to calcite freezing at the base of/within icings or within the ground, possibly from frost blisters. Similar calcite precipitation at the outlet of karstic springs may have been abundant in the calcareous terrains of southwest France during the LGM, although still unrecognized in the geological record.

Section: Discussionmentioning

confidence: 99%

Last Glacial Maximum cryogenic calcite deposits in an alluvial fan at Villetoureix, southwest France

Permafrost & Periglacial

Couchoud

Charlier

et al. 2023

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“…Dolines of Lateglacial or Holocene age in palaeochannels of the Dordogne River produced peaty accumulations of similar thickness. The formation of these deposits has been estimated to require between 0.5 and 2 ka based on radiocarbon dates (Bertran et al, 2009(Bertran et al, , 2013. Secondly, as plant communities respond quickly to climatic fluctuations, the homogeneity of the pollen spectra equally suggests a rapid sedimentation that did not span several stadials and interstadials (see Sánchez-Goñi et al, 2013, for the last glacial-interglacial transition in southwest France).…”

Section: Discussionmentioning

confidence: 99%

Last Glacial palaeoenvironments at Lascaux, southwest France, with special emphasis on MIS 4 (Ognon II interstadial)

Palaeogeography, Palaeoclimatology, Palaeoecology

Allenet

Brenet

et al. 2016

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International audienceA deep sequence of Last Glacial peats and slope deposits at the foot of the Lascaux hill was investigated using a large array of methods (geology, geophysics, C-14 and OSL dating, palynology, macro-remain analysis, entomology). The peaty levels, OSL dated to between similar to 73 and 60 ka, are found Within a doline developed in a fluvial terrace of the Vezere River. These gyttja and chalky gyttja deposits covered by fen peat yielded abundant beetle and plant (seeds, pollen) remains. Palaeoecological reconstructions suggest a cool steppe with willow stands compatible with an interstadial of marine isotopic stage (MIS) 4, probably Ognon II (GI-192). Aquatic and hygrophilous plants (sedges), which reflect local vegetation, and their dependent insects are equally well represented in the record. The peats are covered by a sequence of slope deposits: a coarse-grained lower unit deposited during late MIS 4, overlain by a heavily decalcified sandy unit with a boreal soil complex correlated with early MIS 3, and finally, a coarse grained, calcareous unit formed during late MIS 3 and MIS 2 in connection with slope dynamics typical of periglacial environments. The palaeosols yielded a small Middle Palaeolithic lithic assemblage. (C) 2016 Elsevier B.V. All rights reserved

“…1), ca. 3 km north-west from the study area, yielded a sequence of bedded clay and gyttja extending over the entire Bølling (Bertran et al, 2009), which may have formed in the context of a small lake due to meander cutoff or channel avulsion.…”

Section: Discussionmentioning

confidence: 99%

Architecture of the lower terraces and evolution of the Dordogne River at Bergerac (south‐west France) during the last glacial–interglacial cycle

Frouin

Mercier

et al. 2013

J Quaternary Science

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The lower terraces of the Dordogne River at Bergerac (south‐west France) were studied in detail using cores, trenches, ground penetrating radar profiles, and 14C, optically stimulated luminescence and infrared stimulated luminescence dating. This study shows that the lowest terrace (Fx) is made up of two major lithostratigraphic units: (i) dominantly horizontal gravel strata interpreted as compound bar deposits in a braided river, which pre‐date 18–17 ka, and (ii) thick lateral accretion gravel units (point bars) formed in an anabranching river during the Late Pleniglacial and Lateglacial. Most of the anabranching channels were abandoned at ca. 11–10 ka following shrinkage of the river bed into a single meandering channel and were partially filled by gyttjas during the Boreal chronozone. Finally, the channels were plugged by overbank fines until about 5 ka. Floods seldom reached the Fx terrace during the late Holocene and accretion became negligible. The main phase of bedrock incision spanned the Holocene, whereas the period of channel adjustment to change in river regime during late Marine Isotope Stage (MIS) 2 only led to a redistribution of the Pleniglacial gravels. The higher terrace (Fw3) formed before the Eemian and was covered by colluvial fans, mainly during MIS 2 and 3. Copyright © 2013 John Wiley & Sons, Ltd.