This dataset contains all supplementary data for: Relative sea-level trends in southern Norway during the last millennium from a multiproxy analysis of salt marsh deposits. It is organized as follows: - S1 - Tidal data October to December 2023 - S2 - Stable isotopes of surface transect and TV-1 - S3 - Grain size TV-1 - S4 - LOI and DBD TV-2 - S5 - XRF and PCA TV-1 - S6 - Chronology TV-1 - S7 - All data vs modelled age

According to relative sea-level (RSL) curves from southern Norway, RSL has been falling since the culmination of the Tapes Transgression c. 7000 cal. yr BP. Tide gauge measurements, however, indicate RSL rise since at least 1960 CE. With an age gap of c. 1400 years between the youngest geologically reconstructed sea level index point (SLIP) and installation of the oldest (and only) tide gauge in southernmost Norway, the exact nature and timing of the onset of RSL rise in southern Norway remains unknown. To fill this gap, we collected peat cores from a shallow coastal basin to reconstruct RSL changes over the past 1000 years using a multiproxy approach, including multiple (210Pb and 14C) dating methods, grain-size analysis, loss-on-ignition (LOI), geochemistry (stable carbon isotopes, carbon/nitrogen ratios and XRF), as well as a qualitative assessment of diatoms. Our reconstruction shows decreasing environmental energy and salinity over most of the last millennium, suggesting falling RSL. Sediment geochemistry also appears to vary with wetter and drier climatic periods. An increase in marine-brackish diatoms in combination with an increase in sedimentation rates after 1930 CE (1899-1954 CE) CE suggest that RSL began to rise around this time in southernmost Norway. While most of the proxy data appear to have delayed sensitivity to RSL changes and can be linked to other causal processes, they, nonetheless, provide valuable insight into the environmental response of sheltered, northern salt marshes to slow rates of RSL change. As well, they may be useful in adding more precise RSL constraints to future isolation basin studies of Late Pleistocene-Holocene RSL trends.

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