Reorganization of the upper ocean circulation in the mid-Holocene in the northeastern Atlantic

A micropaleontological investigation was conducted on two sediment cores from the Reykjanes Ridge (RR; core LO09-14; 59°12.30'N, 31°05.94'W) and the Faroe-Shetland Channel (FSC; core HM03-133-25; 60°06.55'N, 06°04.18'W) to document hydrographical changes of the North Atlantic Current (NAC) during the Holocene. Dinocyst and coccolith assemblages were analyzed, and quantitative reconstructions of sea surface temperatures (SSTs) and sea surface salinities (SSSs) were conducted based on dinocyst assemblages. Both proxies suggest a major reorganization of surface circulation patterns in the northeastern North Atlantic between 7 and 5.4 ka BP. At both sites. SSSs before 6.5-7 ka BP were lower than during the mid-late Holocene, suggesting dispersal of meltwater through the NAC. Long term trends of SSTs, however, show higher than present summer SSTs on the RR from 9.3 to ∼6 ka BP, and lower than present SSTs in the FSC until ca. 5.4 ka BP. The contrasted SST trends at the two sites suggest that decreasing summer insolation was not the only forcing behind hydrographical changes in the region. Decoupling of the NAC and the Slope Current (SC), which both influence the FSC, is proposed as a possible mechanism. We hypothesize that a strong NAC during the early to middle Holocene resulted in a SST increase on the RR and decrease in the FSC. Inversely, a weaker NAC after 5-6 ka BP, leading to decreased SSTs on the RR, would have enhanced the relative contribution of the warmer, saltier SC in the FSC, thus resulting in a regional SST and SSS increase.