Rising stable nitrogen isotope ratios (δ15N) in dated sediment records of the German Bight/SE North Sea track river-induced coastal eutrophication over the last 2 centuries. Quantitative assessments of the Zn and Pb content in the sediments suggest that since the onset of enhanced Zn and Pb deposition, the anthropic Zn and Pb input in the HMA amounts to ~ 12,000 t and ~ 4000 t, respectively. Zn and Pb enrichment began in the south-eastern HMA and progressed subsequently with an average of 10 m per year north-westward, most likely triggered by variations in river discharge and by the hydrodynamic setting. Both regions are directly connected to the HMA by the Elbe and Weser rivers. Beginning already during medieval times, the enhanced input of Zn and Pb seemingly is related to silver and zinc mining in the Harz Mountains and the Erzgebirge, well-known mining areas since the Bronze Age. Stratigraphic data indicate that the onset of heavy metal enrichment is diachronous progressing north-westward over the depocentre, paralleled by a decrease in the thickness of the enriched layer. Zinc (Zn) and lead (Pb) contents in the sediments show a distinct increase towards the youngest most sediments with the thickness of the heavy metal enriched sediments ranging from 15 to 103 cm. Radiocarbon and ²¹⁰Pb analyses revealed continuous sedimentation over the last several centuries. The Helgoland Mud Area (HMA) in the German Bight, covering an area of approximately 500 km², is one of a few depocentres for finer sediments in the North Sea. Pre-industrial levels of 15N modeled with 28% of the modern annual (1990-1999) atmospheric loads and 10% of the modern annual river loads agree best with levels of 15N (~6) observed in sediments of the cores dated to 1860. Our results suggest that eutrophication of the German Bight predates the 1960 period of documented rapidly increasing river loads. Sensitivity tests suggest that loads and isotopic ratios of nitrogen forms other than nitrate (ammonium and organic nitrogen) have minor impact on the modeled surface sediments, despite their higher abundance in the riverborne loads in the past. The increasing trend reflects changes in the abundance and isotopic composition of riverine nitrate loads caused by anthropogenic activities. Value of 15N in sediment cores increased from approximately 1860 to 2000 by 2.5. Modeled 15N for 1860 conditions are consistent with 15N values observed in dated sediment cores that span the last 900 years. The modeled spatial distribution of 15N in sediments for 1860 conditions (pre-industrial) showed a moderate 15N gradient from the Elbe river mouth (15N9) to the open sea (15N<7). #HELGOLAND X SERIAL KEY ARCHIVE#The modeled spatial distribution of 15N in sediments for 1960 conditions and the observed spatial pattern of 15N in archive sediment samples (1950-1969) represent a period of moderate eutrophication. We also modeled the 15N in past situations (19) using an N-isotope-tracking ecosystem model. To better constrain pre-1970s conditions, we determined 15N in archive sediment samples (1950-1969) and dated cores from the Helgoland depositional area. Stable nitrogen isotope ratios (15N) in sediment records can be used to decipher historical river nitrate contributions. The German Bight/SE North Sea is considered a hot-spot of river-induced eutrophication, but the scarce observational data of river nitrate loads prior to the 1970s complicate the assessment of target conditions for environmental management and legislation. After the shift in sedimentation, a continuous and highly resolved paleoenvironmental record reflects natural events, such as regional storm-flood activity, as well as human impacts at work at local to global scales, on sedimentation in the Helgoland mud area. According to historical maps, Helgoland used to be substantially bigger at around 800a.d. Among a number of major environmental changes in this region during the Middle Ages, the disintegration of the island of Helgoland appears to be the most likely factor which caused the very high sedimentation rates prior to 1250a.d. Despite the shallowness of the setting (13 to ~1.6mm/year. The Helgoland mud area in the German Bight is one of the very few sediment depocenters in the North Sea.
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