Biogeochemical, sedimentological and dinoflagellate‐based reconstruction of the Holocene palaeoenvironment of the east Limfjord, Denmark

To assess how the eastern part of the Limfjord system has changed over the Holocene, relative to the wider Limfjord and western Baltic region, a sedimentary succession spanning c . 9600–150 cal. a BP was collected from Nibe Bredning, near Sebbersund, from a shallow coastal inlet along the eastern connection between the Limfjord and the Kattegat (north Jutland, Denmark). Palaeoenvironmental conditions were reconstructed using biogeochemical, sedimentological and biological analyses within a chronological framework provided by accelerator mass spectrometry (AMS) 14 C dating of terrestrial plant macrofossils. The Sebbersund succession exhibits brackish conditions prior to 9000 cal. a BP, with substantial input of nutrients and terrestrially sourced organic matter from the catchment. The timing of the marine transgression at Sebbersund beginning around ~9400–9300 cal. a BP agrees well with the general Limfjord development, and key changes in salinity largely correspond with regional sea‐level changes. After ~9300 cal. a BP, salinity increased, reaching levels higher than present day during the Mid‐Holocene (according to diatom‐inferred salinities between 7400 and 4300 cal. a BP), which together with strong marine‐sourced organic matter signatures suggest marine conditions within the fjord, implying a greater connection to adjacent marine systems than at present. Salinities likely remained high until at least 3000 cal. a BP and perhaps later, before declining to modern‐day levels. Sedimentary conditions within the fjord changed after ~3000 cal. a BP as sedimentation rate declined and grain size increased, likely related to shallower conditions following long‐term infilling and falling sea levels. Radiocarbon dating of bulk sediment samples showed the presence of old carbon in the system, creating an age offset between 1640±500 and 2650±170 calibrated 14 C years compared with the age‐depth curve based on the terrestrial macrofossils. This study provides new and important data on Holocene environmental change in the eastern Limfjord, which is crucial for improving our understanding of long‐term marine ecological and geochemical responses to sea‐level and climate change.