Historic (1900) seafloor composition in the Belgian–Dutch part of the North Sea: A reconstruction based on calibrated visual sediment descriptions

Concerns about human-induced degradation of marine ecosystems are presently being translated into European Directives aimed at a restoration of healthier seas. However, anthropogenic alterations are ancient and data on the pristine ecosystem are lacking, making the evaluation of their cumulative effects in the long run challenging. In the Belgian and Dutch parts of the North Sea, a historic data-set (the Gilson’s archive) exists to describe various compartments of the marine ecosystem in the first decade of the 20th century (1899–1914). The bulk of samples were acquired in the nearshore (up to 10 nautical miles off the coast) and further offshore in the Hinder banks area. In this contribution, we focus on the sediment information, which consists of 2200 visual descriptions of geo-referenced seafloor samples and 690 archived sub-samples, to reconstruct the seafloor composition 100 years ago. Estimates of mud levels, sand grain size, shell debris levels as well as occurrence of gravel, pebbles and cobbles were derived from the descriptions and standardized. Mud levels and sand grain-size categories were calibrated by means of grain-size analysis of archived samples (n=383 analyses).Large patches of very high mud levels (50–100%) used to occur close to the shore, along the central and eastern coast. The visual sand grain-size categories display a significant trend in their median grain-size, enabling us to identify distinct patches of fine (<250 μm) and medium (250