From early Miocene to present: Reconstruction of the deep Thermohaline Circulation at the Eirik Drift

Abstract

The Thermohaline Circulation (THC) distributes heat and freshwater around the global oceans, interacts with the atmosphere, and therefore is closely connected to the global climate. The deep branch of the North Atlantic THC mainly consists of deep-water formed by atmospheric cooling in the Nordic Seas, which overflows the Greenland-Scotland Ridge into the North Atlantic. The Eirik Drift south of Greenland is located closely downstream of the North Atlantic deep-water formation region and has been shaped by the Western Boundary Undercurrent (WBUC), which constitutes the main part of the deep North Atlantic THC. The sedimentary record of the Eirik Drift documents changes of the WBUC activity, which can be related to climate changes. The analysis of the sedimentary structure in combination with geological information from scientific drilling leads to a revised seismostratigraphic concept at the Eirik Drift and reveals particularly that the Eirik Drift has been influenced by the WBUC already since the early Miocene (~19 Ma). A more detailed structural analysis of the depocenter locations and their redistribution results in a temporal reconstruction of the deep paleocirculation at the Eirik Drift. The observed changes of pathways and intensity of the WBUC at the Eirik Drift were linked to the development of the Greenland-Scotland Ridge and climate changes. The onset of drift building at the Eirk Drift followed the formation of the Faroe Conduit in early Miocene, which allowed northern sourced deep-water to overflow the eastern part of the Greenland-Scotland Ridge. A separation of the WBUC at the Eirik Drift into two branches occurred contemporaneously with the onset of deep-water overflow at the Denmark Strait, the western part of the Greenland-Scotland Ridge (~7 Ma). At the Eirik Drift, strong WBUC activity is inferred to occur during warm climates and at the beginning of cooling phases, while cooling phases with enhanced ice extent are characterized by weak WBUC activity. Based on a combination of these observations with interpretations from other North Atlantic sediment drifts, a paleo flow path reconstruction for the northern North Atlantic is proposed. It is suggested that the deep-water formation regions and the main deep-water pathway shifted to the south during cold phases with enhanced ice-extent, i.e. Northern Hemisphere Glaciation. This implies that during these cool phases solely weak branches of the deep-water circulation overflowed the Eirik Drift and that the main North Atlantic deep-water route affected the Eirik Drift just during warm phases. Moreover, by applying the seismic oceanography method the present pathway and structure of the upper WBUC core at the Eirik Drift is imaged. For the first time, this method is successfully applied in water depth > 1500 m. The study confirms not only the improvement of oceanographic research by use of the seismic oceanography method but also supports the interpretation of the analysis of the sedimentary structure at the Eirik Drift.