The start of a major sea-level rise indicates that icesheet expansion in western Norway commenced before the Younger Dryas
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The relative sea level rose 10 m on Sotra, western Norway, during the Younger Dryas (YD). Based on dating of isolation basins, the transgression started in late Allerød at ~13 110 and ended in late YD at ~11 780 cal yr BP, with an average sea-level rise of ~7 mm yr-1. Between ~11 780 and ~11 560 cal yr BP the relative sea level was stable. Shortly after the YD/Holocene boundary the sea level rapidly fell 37 m at ~23 mm yr-1. The shorelines for the regression minimum in Allerød and the transgression maximum in YD are almost parallel with tilt of 1.2-1.4 m km-1, indicating that no isostatic tilting, and thus neither uplift or depression occurred during the sea-level rise. We conclude that the transgression was caused by a YD ice-sheet re-advance mapped in the same area. This stopped the isostatic uplift and increased the gravitational attraction on the sea elevating the geoid in this area. There may also been a contribution from rising glacio-eustatic sea level. Our results show that the transgression started around 13 110 cal yr BP. Thus, we conclude that the YD ice-sheet advance in western Norway started before the onset of the YD.