Diatom of the Month – October 23: Palaeoenvironmental reconstruction of a medieval embankment at the lost harbour of Mude (Zeeland, the Netherlands) based on palynology and diatom analysis
The coastal plains along the North Sea underwent significant societal and environmental changes during medieval times. From the tenth century onwards, the massive construction of hydraulic structures such as dikes and dams transformed the saltmarshes and mudflats into polders, which still characterise the region nowadays. The coastal plains of what is now called West Flanders in Belgium and Zeelandic-Flanders in the southern Netherlands were both part of the medieval County of Flanders. The region was a very important economic and cultural centre because it was connected to the North Sea by a large tidal inlet called the Zwin, which pierced into the hinterland after the storm of 1134, along which a network of ports developed. However, after its heyday the ports fell in disuse and disappeared due to the collapse of the economy and politics, and the progressive silting up of the Zwin. A multidisciplinary project aimed to reconstruct the paleoenvironment during the Roman and medieval times of the former Zwin region.
Marine diatom Paralia sulcata
Pollen of Plumbaginaceae, a halophytic plant
Mude, known today as Sint Anna ter Muiden (Zeeland, the Netherlands), was one of the ports that developed along the Zwin. Palynological (organic-wall microfossils) and diatom analyses were carried out on sediments retrieved from two profiles, starting from undisturbed mudflat sediments evolving into anthropogenic deposits (12th-14th centuries), which were part of a dike, in order to reconstruct the terrestrial and aquatic palaeoenvironment at a local and regional scale. The analyses of the mudflat sediments point to a woodland-dominated landscape in the vicinity and a direct marine influence, reflected by dinoflagellate cysts and marine tychoplanktonic diatoms (e.g., Cymatosira belgica and Paralia sulcata). A layer of manure was laid on top of the mudflat sediments to solidify the soil and initiate the construction of the dike. Then, sticky sods with similar environmental assemblages to the mudflat were used to build the dike, as it was the preferred building technique. The shell layer on top and the waste layer along the dike record palynomorphs associated with anthropogenic activities such as the use of peat for fuel and the use of hemp for rope making. In the shell layer, marine-brackish tychoplanktonic diatoms are dominant and most probably reworked from the lower sods or from the shells. Brackish-freshwater benthic species (e.g., Navicula cincta and Hantzschia amphyoxis) lived locally on top of the dike and indicate dry conditions. In the waste layer alongside the dike, epontic and benthic brackish-freshwater species (e.g., Halamphora coffeiformis and Nitzschia capitellata) are dominant and reflect wetter conditions in comparison with the diatoms living in the shell layer. The diatoms from both waste layers indicate a decrease in marine influence, reflecting the silting up of the Zwin. Finally, arable soil developed on top of the dike and was possibly used as pathways for sheep herding, as the region was also famous for its wool production. Dikes are elevated archaeological structures that underwent oxidation and usually are considered sterile in microorganisms. However, our palynological and diatom results allowed the distinction of the terrestrial and aquatic paleoenvironments between natural sediments and waste layers.
About Coralie André
I am currently PhD student at the Geology department of Ghent University in Belgium. My thesis is part of the CRA GOA-project ‘High-tide, Low-tide’. An interdisciplinary team including archaeology, history, palaeoecology, and geology aims at better understanding the evolution and dynamics of the maritime-cultural landscape of Bruges (West Flanders, Belgium) that developed along the Zwin tidal inlet during the medieval times. My research focuses on the reconstruction of the terrestrial and aquatic palaeoenvironments from retrieved sediments from three sites in the former Zwin region based on the ecology of palynomorphs and diatoms in archaeological context.