Holocene Sea Surface Conditions in the Nordic Seas According to Dinocyst Assemblages

Abstract:

The Holocene evolution of the upper ocean in the Nordic Seas is assessed based on the qualitative and quantitative evaluation of 16 dinoflagellate cyst (dinocyst) records retrieved from the warm and saline Atlantic Domain in the east, across the seasonally sea-ice covered Arctic Domain, to the Arctic-outflow influenced Polar Domain in the west. First signs of interglacial conditions are observed from ~11.5 ka BP onwards in the Atlantic Domain, and expanded time-transgressively westward, with deglacial processes appearing to have persisted ~2 kyr longer in the west. No clear peak warming can be observed in the dinocyst data during the interval that is generally considered to correspond to the Holocene climatic optimum, and the disparity between the dinocyst and other phytoplankton records suggests a pronounced seasonality at that time. A slightly freshened upper ocean appears to have facilitated seasonal sea ice formation even at the Vøring Plateau. Despite the strongly contrasting environmental conditions across the Nordic Seas, a basin-wide uniform change in the assemblage compositions is seen between ~7 and 6.1 ka BP, and appears to be linked to the establishment of the modern surface circulation pattern. Potential density estimates for the surface water close to modern convection sites reveals values that would imply an increased likeliness of dense enough surface waters to permit sinking from that time onwards. The changes in the Nordic Seas appear to follow a similar reorganisation in the Labrador Sea and the onset of strong winter convection there. Finally, a gradual eastward expansion of the Arctic Domain can be observed from ~4.5 ka BP onwards, with a delayed consequent reaction of the Atlantic-sourced water inflow at the Vøring Plateau showing a slight recovery of cool taxa from ~2.4 ka BP onwards.