The North Atlantic Oscillation forcing through the last 2000 years: Spatial variability as revealed by high-resolution marine diatom records from N and SW Europe

The Tagus pro-delta (Portuguese Margin) and the Skagerrak (NE of the North Sea) are two marine systems controlled by atmospheric changes, which at present are mainly determined by fluctuations of the North Atlantic Oscillation (NAO). On the basis of diatom records from marine sediment cores, environmental changes (primary productivity and salinity) are reconstructed for the last 2000 years for both regions. These sites are investigated focusing on the regional response to changing NAO forcing.Both studied sites are characterized by sedimentation rates in the order of 0.12 cm/year and 0.47 cm/year for the most recent deposits off the Tagus pro-delta, allowing high-resolution paleoceanographic reconstructions (8.3 and 2.1 years represented per sample). The last 2000 years are a period covering in Europe the historical climatic periods known as the Dark Ages (DA), the Medieval Warm Period (MWP) and the Little Ice Age (LIA). In the Skagerrak, the cold periods of the DA and LIA are marked by diatom dissolution stages, whereas at the Tagus pro-delta, the DA were associated with increased diatom production and possible upwelling and the onset of the LIA corresponded to enhanced flow of the Tagus River. During the MWP, better diatom preservation in the Skagerrak, related to stronger advection of salty Atlantic waters, is paralleled by dominant upwelling conditions at the Tagus pro-delta. The two most intense upwelling periods at the Tagus pro-delta, at ∼ AD 600 and ∼ AD 900, correspond to a dissolution stage and a slight change in salinity in the Skagerrak, respectively.Although the comparison of the two study sites suggests a common forcing such as the NAO, the different inferred behaviors for each main climatic period in each region demonstrate that the NAO by its own is not sufficient to explain the climatic variability at a regional scale.