The sedimentary evolution of a mud depocenter in the southern North Sea since the Medieval Warm Period

Today, Helgoland is a small rocky island with an exposed rocky shore located in the German Bight (SE North Sea). Old maps show that only 1200 years ago Helgoland was much larger and muddy tidal flats surrounded the island. According to earlier studies this land mass disintegrated before AD 1250 and the muddy sediments appear to have filled a depression in the southeast of the island. This area, the "Helgoland Mud Area" (HMA, 20-30 m of water depth), covers about 500 km2 forming one of the few muddy sediment spots in the North Sea. It can be anticipated that these sediments carry a record that is instrumental in paleoenvironment reconstructions of the past millennium. This study is based on 1 cm-resolution granulometric data of 5 sediment cores (5 m core length, covering the past millennium) taken from the west, the center, and the east of the HMA, data of a parametric subbottom profiler, a RoxAnn seafloor-classification system, and AMS radiocarbon data.The sediment cores reveal high sedimentation rates (11 mm/a) during the Medieval Warm Period that decrease by 50% at the beginning of the Little Ice Age (LIA, AD 1350-1900). At the same time the sortable-silt mean grain size increased suggesting a higher energetic level with regard to currents and waves. At the end of the LIA the sediments show a fining trend again. The changes in grain size likely mirror the frequency and strength of severe storms during the LIA including a calmer period during the Maunder Sun-Spot Minimum (around AD 1700) that was most likely characterized by an increased number of calmer negative NAO situations.In order to detect the source of the fine sediments in a likewise classical sandy North-Sea environment, morphologic features such as the adjacent "Helgoland Hole" west of the HMA that exceeds the ambient water depth by 100%, and the sediment geometries as revealed by shallow seismics were studied. In many transects shot with the SES2000 parametric echosounder, vertically interrupted layers suggest gas in the sediment that locally extends up to the surface. Transects from the SW of the working area reveal north-inclined layers suggesting sediment discharge from the south, which would also explain why the "Helgoland Hole" is still an open depression. Transects from the southern part of the HMA show large sandwaves that likely were buried and fossilized during a dramatic discharge of sediment. All evidence including the general water-mass circulation in this part of the North Sea thus points to a southern rather than a northern (i.e. former mud flats of Helgoland) source for the muddy sediments. It is concluded that the sediment that fills the HMA originated mostly from rivers such as the Elbe, and the Wadden Sea. Variations in grain size are due to fluctuations in the predominant wind direction and speed, and due to anthropogenic action such as dike-construction measures during the past centuries