Fossil ostracod assemblages were investigated in five AMS 14 C-dated cores from various water depths of the Laptev and Kara seas ranging from the upper continental slope (270m) to the present-day shelf depth (40m). Six fossil assemblages were distinguished. These represent the varying environmental conditions at the North Siberian continental margin since about 18ka. In the cores from the shelf the ostracod assemblages reflect the gradual transition from an estuarine brackish-water environment to modern marine conditions since 12.3ka, as induced by the regional early Holocene transgression. The core from the upper continental slope dates back to c. 17.6ka and contains assemblages that are absent in the shelf cores. The assemblage older than 10ka stands out as a specific community dominated by relatively deep-water Arctic and North Atlantic species that also contains euryhaline species. Such an assemblage provides evidence for past inflows of Atlantic-derived waters from as early as c. 17.2ka, probably facilitated by upwelling in coastal polynyas, and a considerable riverine freshwater influence with enhanced surface water stratification owing to the proximity of the palaeocoastline until early Holocene times. In all studied cores, relative increases in euryhaline species dominant in the inner-shelf regions are recorded in the mid-late Holocene sediments (<7ka), which otherwise already contain modern-like ostracod assemblages with relatively deep-water species. This observation suggests euryhaline species to be largely sea-ice- and/or iceberg-rafted and therefore may provide evidence for a climate cooling trend. © 2012 The Author. Boreas © 2012 The Boreas Collegium.