ALBERT

Description: During the Last Glacial Maximum (LGM), a very cold and dry period around 26.5–19 kyr BP, permafrost was widespread across Europe. In this work, we explore the possible benefit of using regional climate model data to improve the permafrost representation in France, decipher how the atmospheric circulation affects the permafrost boundaries in the models, and test the role of ground thermal contraction cracking in wedge development during the LGM. With these aims, criteria for possible thermal contraction cracking of the ground are applied to climate model data for the first time. Our results show that the permafrost extent and ground cracking regions deviate from proxy evidence when the simulated large-scale circulation in both global and regional climate models favours prevailing westerly winds. A colder and, with regard to proxy data, more realistic version of the LGM climate is achieved given more frequent easterly winds conditions. Given the appropriate forcing, an added value of the regional climate model simulation can be achieved in representing permafrost and ground thermal contraction cracking. Furthermore, the model data provide evidence that thermal contraction cracking occurred in Europe during the LGM in a wide latitudinal band south of the probable permafrost border, in agreement with field data analysis. This enables the reconsideration of the role of sand-wedge casts to identify past permafrost regions.

Description: While numerous high-resolution studies concerning Last Glacial aeolian sequences are available for Europe, the approach of the penultimate glacial in this geographical area is still poorly developed. In order to bridge this gap, this study focuses on the Bulgarian sequence of Harletz, along the Danube River, where extremely high sedimentation rates allow the depiction of high-resolution signals during MIS 6. At Harletz in NW Bulgaria on the western bank of the Ogosta River (tributary of the Danube) a 20m thick loess-palaeosols section was cleaned and sampled for a multi-disciplinary study and detailed pedostratigraphic approach. High-resolution continuous bulk sampling (5 cm) was carried out to characterise sedimentary grain size, magnetic properties (including magnetic susceptibility and its frequency dependence), colour reflectance (1 cm), and organic carbon. Geochronological control is based on 16 samples collected for OSL and MET-pIRIR dating. Using a cyclo-stratigraphic approach of the sequence combined with dating constraints provided by both MET-pIRIR dates and the age of a tephra layer occurring at a depth of 12m within the main loess unit, we can demonstrate that the Harletz section exhibits a 10m thick Late Saalian (Marine Isotope Stage 6, MIS 6) loess accumulation unique in Europe. The lower part of the main loess unit is 4m thick and overlies a basal brown soil complex allocated to MIS 7, which includes an exceptionally thick (4 m) and detailed succession of loess and four incipient soil horizons never described in European loess until now. The closest and best-dated high-resolution palaeoenvironmental archive suitable for comparison comes from Lake Ohrid located about 400 km to the SW of Harletz. The Ohrid palynological record shows a progressive step-by-step evolution in climate and in environmental change during the transition between MIS 7 and MIS 6 from which a parallel with the Harletz pedosedimentary succession can be proposed. During the younger part of MIS 6 (160-129ka), steppe vegetation with abundant herbs (Artemisia) is dominant in the Lake Ohrid record, in good accordance with a global enhancement of aeolian dynamics, especially well recorded in sections located close to the Danube River from Serbia to Bulgaria and Romania (L2 loess). According to interpretations stemming from this study, the silts and fine sands building the Harletz loess section would have been transported from the Danube braided river system located (at that time) at about 4.5 km to the NW. Based on our data, the main loess units are characterised by a very low to a total absence of coarse sand particles. By contrast, during the Eemian interglacial (MIS 5e), and to a lesser extent throughout MIS 5 and during MIS 3 interstadials, the long distance transport of silt and fine sand particles is stopped and a weak aeolian sedimentation is likely driven by north-easterly winds transporting coarse sand grains from the proximal Ogosta River sandy banks. Finally, the weak development of Last Glacial loess (4m max.) likely results from a rapid infilling of the sedimentary trap during the Saalian, then followed by a strong anthropogenic erosion of the topsoil and of the upper part of the loess profile since the Early Holocene (Neolithic).

Description: The high-resolution colour reflectance data were acquired with a Minolta CM 2600d over the extended visible wavelength domain from 360 to 740 nm and operating with a 6504 K light source.

Description: Loess sequences are a particular record of paleoenvironments and paleoclimates and show regional peculiarities. Among those, European loess sequences show the occurrence of paleosols and other pedogenic units that have been demonstrated to correspond to the Greenland Interstadials (GIS) or Dansgaard-Oeschger events (DO), for the last climate cycle (Moine et al. 2017), of GIS-like for the penultimate climate cycle (Rousseau et al. 2020). During the last climate cycle, these paleosols developed synchronously over Europe along a wide longitude transect eastward in Ukraine (Rousseau et al., 2017). More interesting the development of these paleosols or pedogenic units, occurred during a stop of the dust deposition from the top of the most recently deposited eolian unit. Taking into consideration this point in our manuscript, we revisited the stratigraphy of the European loess sequences by considering the paleodust units, equivalent to Greenland Stadials (GS), as associating the lower loess unit and the overlying paleosol or pedogenic unit. Moreover, the close correlation that we established between the paleosols or pedogenic units with GIs, allows us to consider that the paleosol development occurred during the related GI in Greenland (Rousseau et al., 2017). Having the GI durations published by Rasmussen et al (2014), we propose therefore new timescales for the European loess sequences. Moreover, we have assigned the paleosol-loess units doublets to the corresponding Bond cycles defined by Broecker (1994). These cycles group several DO events, of increasing cold amplitude, and end with a Heinrich event that some literature interpreted as the coldest and dustiest time interval over Europe, an interpretation that we are testing in our paper. In our manuscript, we demonstrate our new method by applying it to the reference sequence of Nussloch that we have investigated for decades. We present a revised detailed record of sedimentation and mass accumulation rates over the 60 ka to 15 ka b2k time interval (TAB. 1). We also apply our method to other key European sequences that we investigated previously at high resolution, allowing us to propose new estimates for the SR and MARs of the most recent Bond cycles, i.e. e. between GI4 and GS3 (29 to 23.2ka b2k) and between GI8 and GS5 (38.2 to 29ka b2k) (TAB. 2). We conclude the LGM as the dustiest interval with the highest values, and presenting a longitudinal pattern along the studied European transect, with the highest values westward. Another finding is that for every Bond cycle, the dustiest interval always happened in the GS prior the last ones corresponding to Heinrich stadials. Expanding the comparison with high-resolution sequences from the Chinese loess plateau (TAB. 3) for the same Bond cycles, our study shows that Europe was dustier than China. A final test of our new method is by considering the SR and MARs for the various grain size categories measured in three key reference sequences. Considering the finest grain size category, which can be assimilated as the closest the mineral aerosols, our estimates fit the dust deposition reconstructed for the LGM in Europe by Earth System models opening new perspectives for future data-model comparisons (TAB. 4).

Description: Our study focuses on European loess sequences, particularly the eolian intervals in between the observed pedogenic units. The classical concept of soil formation from parent material is reformulated to estimate of the duration and the associated sedimentation rate (SR) and mass accumulation rate (MAR) of these paleodust intervals. We show that the Greenland Stadial (GS) duration in European loess deposits includes the thickness of the overlying pedogenic unit, which in fact developed downward into the upper part of the eolian unit. The lower stratigraphical limit of the eolian unit overlying the pedogenic unit corresponds to the restart of the dust sedimentation of the younger GS. We illustrate this interpretation first by computing both SRs and MARs first for the Nussloch key sequence, the most complete European series. The correlation between Nussloch and other European loess sequences, located along a 1,800 km longitudinal transect, allows computation of SR and MAR for several identified GS events. Comparing GS from marine and ice core records, our study shows that the two last Bond cycles are preserved in every European eolian record. Bulk SR and MAR are estimated and compared for these two Bond cycles, showing the highest SRs and MARs in western Europe. These indices also indicate that the last stadials, embedding an Henrich event, were not the dustiest in every Bond cycle. Our estimated MAR also differ from previously published computations, which did not take into account the various pedogenic units present in the studied loess sequences. The bulk SR and MAR estimates computed for the two last Bond Cycles from Chinese sequences from the Loess Plateau indicate lower atmospheric dust than in Europe during the Last Glacial Maximum. SR and MAR estimates computed from the fine-grained material for European records fit with Earth System model reconstructions.