The transgressive Early-Middle Holocene boundary – the case for a GSSP at Rotterdam, Rhine Delta, North Sea Basin.

Postglacial sea-level rise at the start of the Holocene continued to drown continental shelves around the world. By the early–middle Holocene transition, deltas and other coastal systems had begun to stabilize their positions, which have since been maintained. The last major acceleration of sea-level rise occurred between 8.5 and 8.2 ka, due to the largest meltwater pulse from a single source area, released from the thawing Laurentian ice sheet in the Hudson Bay area. This event left a marked transgressive impact on river-mouth sedimentary sequences around the globe, exemplified in the Rhine Delta (North Sea, The Netherlands) from boreholes and underground exposures in the city of Rotterdam and its megaharbour. What ended as the 8.2 ka climatic event actually began as a freshwater release at ~8.45 ka: it should therefore be properly regarded as an 8.5–8.2 ka event. In contrast to the 8.2 ka climatic event, which was temporary, globally highly variable, and commonly insignificantly registered, and which only indirectly affected sedimentation, the sea-level imprint of the freshwater release was permanent, circumoceanic, and predictably spatially variable, and had direct impacts on sedimentation on both sides of the migrating coastline. Consequently, the water release left lithostratigraphic- and environmental-event boundaries in coastal sequences around the world, in the zone where Holocene accumulations are thickest and functional subdivision is architecturally most important. For these reasons, the sea-level signal of the 8.5–8.2 ka event should be considered as the beginning of a formalized Middle Holocene, and not the somewhat-later 8.2 ka cold spell maximum over Greenland, as is currently being proposed elsewhere. In that context, the transgressive contact found at the base of the Rhine Delta at Rotterdam is presented as a potential GSSP (8450 ± 44 cal BP).