Global thermolhaline circulation, which is responsible for the transport of heat from the equator to the poles. The major formation regions of deep and bottom water are the Nordic Seas, the Weddell and the Ross Seas. The Eirik Drift south of Greenland and the Argentine continental margin (red circles) form ideals location to study modifications in strength and pathway of the Western Boundary Undercurrent, here mainly built up by North Atlantic Deepwater NADW.
Traces of NADW and its precursor at the Eirik Drift
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 palaeocirculation 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 palaeo 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 Deepwater 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.
Reconstructing sediment transport via numerical simulation
Transport of sediments numerically simulated for Paleo-Oceans and reconstructed from cores of the Eirik Drift (TRANSPORTED)
Seismic profiles imaging the Eirik Drift indicate a high variability in velocities and flowpath of the Western Boundary Undercurrent (WBUC) since the early Miocene and provide indications on the area of deep water formation (DWF) from the Miocene to present day. We aim at identifying the mechanisms involved in shifts of DWF sites and redirection of the WBUC. Grain size data is available for ODP Leg 105 Site 646 and IODP Expedition 303 Sites U1305-1307 drilled at the Eirik Drift (iodp.tamu.edu). The distinction into clay (< 0.004 mm), silt (0.004-0.063 mm), and sand (> 0.063 mm) is sufficient to deduce resolvable velocities of the WBUC for the different periods. Three-dimensional velocities and sediment transports will be simulated with the Regional Ocean Modelling System (ROMS). ROMS will be localized to the North Atlantic and therefore yield detailed information about DWF sites and ocean currents. Seismic profiles collected in the area of the Eirik Drift provide horizon depths, thicknesses of sedimentary units, and location/orientation of depocentres. In combination with the grain size data ground truth data is provided for the sediment transport patterns modelled by ROMS. With the numerical approach we can single out or neglect effects and thereby perform sensitivity studies and test several hypotheses. To our knowledge this is the first project aimed at understanding climate and ocean variability in the Neogene North Atlantic combining drill core analyses with numerical ocean simulations.
Müller-Michaelis and Uenzelmann-Neben (2014) attributed variable sediment deposition to changes in strength and pathway of the WBUC. The WBUC is part of the thermohaline circulation and a product of DWF in the Greenland and Nordic Seas. Fluctuations in the amount or location of DWF would therefore directly affect the WBUC and hence sediment deposition in Eirik Drift. Müller-Michaelis and Uenzelmann-Neben (2014) concluded that variations in sea ice cover over the Nordic and Greenland Seas and the Greenland Ice Sheet led to modifications in DWF. This is supported by Hunter et al. (2007a) who identified a connection between sediment thickness in Eirik Drift and glacial-interglacial periodicity.
We want to test the following hypotheses by simulations with the Regional Ocean Modeling System ROMS coupled to a sediment transport mode:
- Glacial events resulted in a switch in deep water formation between the Greenland, Labrador and Norwegian Seas.
- Tectonic events, e.g., subsidence of the Greenland Scotland Ridge, closing of the Central American Seaway, strongly affected the pathway of the WBUC.
- ROMS represents sedimentation rates in Eirik Drift in accordance with the one observed at ODP Leg 105 Site 646 and IODP Expedition Sites U1305-1307.
- The regional simulation with ROMS represents DWF, path and strength of the WBUC and sedimentation rates in the present day simulation better than the global simulation with ROMS.
Imaging of NADW using seismic data
Identify and track cores of the Western Boundary Undercurrent WBUC
Using the method of seismic oceanography we further studied the present day structure of the Western Boundary Undercurrent WBUC forming NADW to check the hypothised pathway for the past 800000 years. This methods makes use of the fact that different water masses have different physical properties, e.g. Salinities and Temperatures and hence can lead to fine structure at their boundaries, which can be imaged using seismic reflection methods. The high spatial resolution of the seismic reflection data bears the ability to create detailed images of thermohaline structure in the ocean compared to traditional oceanographic measurements. Traditional oceanographic investigations are mainly based on widespaced CTD measurements, which are interpolated to rough, two-dimensional watermass distribution sections.
We were able to identify and track the WBUC cores via the combination of CTD and seismic reflection data. The pathways of the WBUC cores suggested by Müller-Michaelis and Uenzelmann-Neben (2014) for the period <800,000 years were confirmed by our observations. With the CTD data we could prove the hypothesis of Müller-Michaelis and Uenzelmann-Neben (2014) that the deeper core transports DSOW and the upper core ISOW. Additionally, we revealed another shallow branch of ISOW, which is fed by upper ISOW spilling over the main ridge in the vicinity of the secondary peak in the NE of the study area in water depths between ∼1900 and 2400 m. Our interpretation of the seismic oceanography data thus concurs with the interpretation of the distribution of sedimentary strata and we can state that the observed WBUC core is guided by the topography. The structural analysis also showed a concentration of the WBUC core at uniform slopes and there a lesser lateral extent. We could further show that complex topography also has an important impact on the structure of the WBUC cores. Our study confirmed that seismic oceanography provides an important supplement to conventional oceanographic CTD measurements, as the small-scale structures of the deep-water masses cannot always be resolved properly by discrete CTD measurements due to their large spacing.
Documents of NADW activity in the western South Atlantic
Impact of evolving NADW on the circulation in the South Atlantic
Geochemical evidence from boreholes suggests enhanced transport of Northern Component Water (NCW) to southern latitudes from about 6 Ma onwards. However, information on how this change in transport influenced the intensity and position of current systems is sparse. A combined interpretation of seismic reflection profiles and bathymetric data have been used investigate current derived deposits at the central Argentine Margin. Upslope migrating mudwaves overlying a late Miocene erosional unconformity, provide evidence that Circumpolar Deepwater (CDW) flow slowed down with the onset of NCW inflow. During the last ~3 Ma changes in dimensions and migration rates of the waves are small indicating continuous bottom current flow conditions similar to today with only minor variations in flow speed, suggesting that the Deep Western Boundary Current (DWBC) in the western south Atlantic as observed today, has been a pervasive feature of the global thermohaline circulation system during the Plio-/Pleistocene.
Bathymetric chart with location of mudwave fields and a contourite drift at the Argentine margin. Arrows indicate bottom water flow: AABW = Antarctic Bottom Water; CDW = Circumpolar Deep Water, NCW = Northern Component Water. Canyons are shown in red. Contouritic channels are shown in orange.