Transport of Late Pleistocene loess particles by katabatic winds during the lowstands of the English Channel

Abstract

Studies of the submerged Pleistocene conglomerate cored and dredged in the English Channel show that it resulted from the cementation of stony beaches under a loess blanket. Although the loess cover was later washed out by younger transgressions, the northern limit of the conglomerate corresponds to the original offshore extension of the loess deposits. Compilation of offshore and onshore altitudes of the limits of these deposits shows that loess was deposited by low-level wind fields never thicker than 200 m. Mapping surveys show that the present limits of loess are probably close to their original boundaries. The past proximity of the British Ice Sheet, the accumulation of dusty sediments coating north-facing cliffs of Brittany originating in the outwash of the British–Irish Ice Sheet and in the palaeo-rivers of the English Channel, as well as the southeastward orientation of the palaeo-winds deduced from particle size analysis and heavy minerals distribution, suggest that large volumes of loess were transported by strong katabatic winds blowing from the northern ice-covered regions towards Brittany and Normandy. The existence of katabatic winds is also consistent with the drift of ice rafts that transported Icelandic basalts now grounded in Brittany and possibly with the orientation of Homo neandertalensis shelters.

Impact of katabatic winds on the environment of Neanderthals during the Middle Palaeolithic in westernmost Europe

“…By contrast, loess deposited on the north-facing slopes would be equivalent to the deposits of northern Brittany and those of Portelet would represent leeward deposits associated with the katabatic winds. • The absence of Neanderthals during the thicker loess sedimentation (Scott et al, 2014) confirms the previous observations made onshore in Brittany (Lefort et al, 2019b) since it is during these episodes that katabatic winds were stronger and Neanderthals were absent (Fig. 5).…”

“…This approach is an entirely new one when compared to the former models. In the present review, the physical parameters used in the study by Lefort et al (2019b) will only be summarized; details of this study can be found in the full article.…”

“…Because the previous offshore loess deposits no longer exist on the bottom of the sea, having been washed out during the various Pleistocene transgressive episodes, their initial extent can only be recognized thanks to the existence of remnant cemented conglomerates that formed at their base (Lefort et al, 2019b).…”

“…In summary, cold katabatic winds were generated by the British-Irish ice sheet and propagated over long distances at a very low altitude ranging up to between 170 and 220 m and transporting thick clouds of loess particles (Lefort et al, 2019b). The only information which cannot be extracted from the field data is that they were able to reach a velocity of 300 km h −1 and above (Parish and Cassano, 2003).…”

“…The assumption that katabatic winds may not travel over long distances and are limited to a displacement of 20-100 km (Adolphs and Wendler, 1995) is contradicted by NASA infrared satellite photos (Bromwich et al, 1992) which show that they may propagate over distances of more than 1000 km above the sea. In Brittany the results of analysis of augites and other heavy minerals show that these winds were still operating at up to 450 km from the Ice Sheet (Lefort et al, 2019b). In the Western Channel most of the remnants of the initial loess cover (Lefort et al, 2019b) were deposited at about 250 km from the British Ice Sheet (Chiverrell and Thomas, 2010) and 200 km from the Celtic ice floe (Praeg et al, 2013).…”