Comments to Westaway and Bridgland – ‘Causes, consequences and chronology of large‐magnitude palaeoflows in Middle and Late Pleistocene river systems of northwest Europe’

Abstract

Westaway and Bridgland (Earth Surface Processes and Landforms 35: 1071-1094, 2010) discuss the causes, the consequences and the chronology of large-magnitude palaeoflows in Pleistocene river systems of northwest Europe. Based on their calculations, these authors suggest that the combined effects of meltwater from Alpine glaciers, rainfall, snowmelt and melting of permafrost during Heinrich Events (HEs) explain the large-magnitude discharges of the Fleuve Manche palaeoriver which punctuated the last glacial period.This comment identifies some approximations and inconsistencies regarding (i) the timing of the last massive Fleuve Manche palaeoriver discharge and its relation to the de-glacial pattern of the British-Irish Ice Sheet (Point 1); (ii) the palaeoclimatic conditions prevailing on land and the antagonistic forcing mechanism proposed by the authors to explain the large-magnitude palaeoflows (Point 2); (iii) Westaway and Bridgland's (2010) revised interpretation of the deep-sea records from the Bay of Biscay (Point 3); and (iv) the relationship between the offshore sedimentation since the Middle Pleistocene and the formation of the Dover Strait (Point 4). Each point is discussed.

Reply to comments by S. Toucanne, S. Zaragosi, F. Eynaud, J. F. Bourillet, G. Lericolais and P. L. Gibbard on ‘Causes, consequences and chronology of large‐magnitude palaeoflows in Middle and Late Pleistocene river systems of northwest Europe’, by Rob Westaway and David R. Bridgland ()

“…Much of the Toucanne et al () critique concerns a single paragraph in our paper, tentatively suggesting how the chronologies of sediments deposited by affluents of the Channel River, of de‐glaciation, and of marine deposition in the Bay of Biscay in the latter part of the Late Pleistocene may have been interrelated. They accuse Westaway and Bridgland () of ‘misuse’ of papers produced by their team at IFREMER.…”

“…Toucanne et al () placed this event during 18·3−17 ka, whereas Toucanne et al () placed it between 18·3 ka and either 17·5 or 17 ka, depending on which marine borehole dataset was examined. Toucanne et al () now say that it was during 18·3−17·5 ka. However, when analysing another borehole dataset, Auffret et al () reported the two most recent peaks in sediment flux, which must now be regarded as derived from the Channel River (but see also later), at 23·7 and 16·7 ka.…”

“…However, when analysing another borehole dataset, Auffret et al () reported the two most recent peaks in sediment flux, which must now be regarded as derived from the Channel River (but see also later), at 23·7 and 16·7 ka. All these publications based their chronologies on radiocarbon dates calibrated in terms of calendar ages, so discrepancies of this order have nothing to do with Westaway and Bridgland () supposedly mixing up calibrated and uncalibrated radiocarbon dates, as Toucanne et al () have claimed. It would appear that these discrepancies have instead resulted from uncertainties in the determination of the age−depth relationships for the sediments in each of the Bay of Biscay boreholes.…”

“…The discrepancies between the chronologies in the different IFREMER publications indicate that margins of uncertainty of ~1 ka are applicable to each of their age estimates. We thus regard their established practice of quoting the ages of events to the nearest 100 years, with no error margin [which Toucanne et al () have indeed adopted in their commentary], as inappropriate.…”

“…In a contribution to a field guide on part of southwest England, discussing the impact of the Pleistocene phases of high palaeoflow in the Channel River on this region's local rivers, Westaway () has emphasized the extent of agreement between Toucanne et al () and Westaway and Bridgland (), even though these different works have investigated the Channel River from completely different perspectives. We are therefore puzzled as to why Toucanne et al () should wish to emphasize points of difference, such as the evident slight differences in chronology and the different interpretations of why these high palaeoflows have occurred. We nonetheless reply to each of the points they have raised.…”