The Middle Eocene Climatic Optimum (MECO) in the high northern latitudes: biotic and climatic consequences

The so-called ‘Middle Eocene Climatic Optimum’ (MECO, ~40 Ma) represents a ~500-750 kyr period of transient climate warming followed by marked cooling superimposed on the long-term middle and late Eocene cooling trend (~50-34 Ma). The MECO, with the onset calibrated against magnetosubchron C18n.2n was originally recognized in sediments from the Southern Ocean but later also identified in records from the equatorial Atlantic Ocean and the central Tethyan Realm. Based on dinoflagellate cysts and organic geochemistry, this study provides the first evidence of the influence of the MECO in the higher northern latitudes, thereby confirming that it indeed was a truly global event. The studied middle to upper Eocene sediments from the Labrador Sea (Ocean Drilling Program, ODP, Site 647, 53ºN) and Norwegian-Greenland Sea (ODP Site 913, 75ºN) show a relative abundance peak in typical low latitude organic-walled dinocyst species and a strong drop in productivity at the base of magnetosubchron C18n.2n that are related to the MECO. The MECO coincides with carbonate dissolution while the organic molecular paleothermometer TEX86 shows a concomitant brief (~150 kyr) warming in sea surface temperatures (SSTs) at ODP Site 647. Warming during the MECO thus allowed typical low latitude dinocyst species to bloom in the high northern latitudes. The strong productivity collapse during the MECO possibly results from shifting water masses. Alternatively, since the signal recovered seems to be transported from elsewhere, changes in the source area (e.g. decreased runoff) may have caused productivity to drop during the MECO. Furthermore, post-MECO cooling (~6ºC for Site 647) allowed several typical cold-water adapted dinocyst species to occur. While this biotic change is similar to that found in the Southern Ocean, the duration and amount of warming are very different. This is possibly related to carbonate dissolution that may have had a larger influence in the high northern latitudes and caused a large part of the MECO signal to be missing at the studied high northern latitude sites