Incised Pleistocene valleys in the Western Belgium coastal plain: Age, origins and implications for the evolution of the Southern North Sea Basin

The Belgian coastal plain occupies a key position as it is located at the transition between the Southern North Sea Basin and the Strait of Dover. It is characterized by thick sequences (> 20 m) of Pleistocene terrestrial and littoral sediments. Yet the wider stratigraphical and palaeo-environmental significance of these sediments received little attention. In this paper we draw on the results of a recent sedimentological study based on > 100 drillings that spans the Pleistocene sequence, and present new biostratigraphical (pollen, foraminifera, ostracods) data, all revealing a complex history of deposition. The record includes evidence of the development of incised-valley systems that were initiated in the late Middle and Late Pleistocene. Five phases of fluvial incision can be identified. The majority of the infills are deposited in an estuarine environment that passes into a fluvial environment land inward, except the Weichselian infill which has a predominant fluvial origin. The greatest part of the most seaward located zone of the western coastal plain was free of valley incisions, there, shallow marine sediments built up the record. Local biostratigraphical investigations provide a timeframe. The result is placed in a regional context.

Genetic evidence supports recolonisation by Mya arenaria of western Europe from North America

The softshell clam Mya arenaria (L.) is currently widespread on the east and west coasts of North America. This bivalve also occurs on western European shores, where the post-Pleistocene origin of the species, whether introduced or relict, has been debated. We collected 320 M. arenaria from 8 locations in Europe and North America. Clams (n = 84) from 7 of the locations were examined for mitochondrial DNA variation by sequencing a section of the cytochrome oxidase 1 (COX1) gene. These were analysed together with 212 sequences, sourced from GenBank, from the same gene from 12 additional locations, chiefly from eastern North America but also 1 site each from western North America and from western Europe. Ten microsatellite loci were also investigated in all 320 clams. Nuclear markers showed reduced levels of variation in certain European samples. The same common COX1 haplotypes and microsatellite alleles were present throughout the range of M. arenaria, although significant differences were identified in haplotypic and allelic composition between many samples, particularly those from the 2 continents (Europe and North America). These findings support the hypothesis of post-Pleistocene colonisation of European shores from eastern North America (and the recorded human transfer of clams from the east to the west coast of North America in the 19th century).