Dynamics of the Ocean Floor
MSM 19/3 Agulhas
The research project AGULHAS is conducted in close co-operation of the Alfred Wegener Institute for Polar and Marine Research (AWI) and GEOMAR. The project began with two expeditions of the German research vessel Maria S. Merian in autumn/winter 2011 in the South Atlantic Ocean. During Leg MSM19/2 geophysical investigations (reflexion seismics), sediment echosounding and bathymetric mapping were carried out by the AWI at the Agulhas Ridge. The following leg MSM19/3 (conducted by GEOMAR) focuses on bathymetric mapping and comprehensive hard rock sampling at the Agulhas Ridge, adjacent Seamounts, and the Discovery Rise (Fig. 1).
The Agulhas Ridge is part of the Agulhas Falkland Fracture Zone, which has been formed during the Gondwana break-up in early Cretaceous by the separation of South America and Africa. The ridge, being c. 1,100 km long and more than 2,000 m high, represents a barrier for the spreading of water masses and hence the exchange of energy and heat between low and high latitudes. It remains, however, unclear why the ridge did not subside thermally. Previous studies of the AWI indicate reactivation of the Agulhas Ridge and younger volcanic features to the north and the south of the ridge (Fig. 2; Uenzelmann-Neben and Gohl 2005). The Discovery Rise extends over an area of c. 250 x 350 km and consists of several huge seamounts, which elevate up to > 4,000 m above the surrounding abyssal plain. Although they rise close to the water surface, these seamounts have not been discovered until 1936. Up till now only a few samples exist from 5 sites at the Discovery Rise. The Agulhas Ridge has only been sampled at its northeastern tip at 3 sites. Based on geochemical analyses of these samples, a deep magma source (mantle plume) has been suggested for the Discovery Rise (e.g., Douglass et al. 1995, LeRoex et al. 2010). Seismic tomography records, however, do not provide any clear evidence for plume structures beneath the Discovery Rise (Montelli et al. 2004). The Agulhas Ridge is considered as part of the Shona hotspot track by Le Roex et al. (2010), whereas Douglass et al. (1999) postulate migration of material from the Discovery plume into the Agulhas Falkland Fracture Zone.
Taken together, too little is known about the ages and the geochemical composition of the magmatic rocks forming the Agulhas Ridge and the Discovery Rise to understand their nature and formation. Based on representative hard rock sampling of all geomorphological units of these features, MSM19/3 (and the subsequent laboratory studies on land) aims to reconstruct the age, origin, composition, and evolution of the Agulhas Ridge and the Discovery Rise using volcanological, petrological, geochemical, and geochronological methods. In combination with the geophysical investigations of MSM19/2, MSM19/3 addresses the following major questions:
(1) Has the Agulhas Ridge been reactivated tectono-magmatically in the Cenozoic and is there a relation between the tectonic-magmatic activity of the ridge and the hotspots in the South Atlantic (e.g. Discovery, Shona) and the thus changing patterns of the deep sea currents?
(2) What is the origin of the Dupal Anomaly in oceanic basalts of the southern hemisphere and of the enriched mantle components EM-I and EM-II? The Dupal Anomaly (named after the French geochemists Dupré and Allègre) extends in the southern hemisphere around the entire globe as an up to 60° broad belt (Fig. 3) and is characterized by anomalous, enriched Sr-, Nd- und Pb isotope signatures. The Agulhas Ridge is predestinated for studies of the Dupal Anomaly and of the EM components since the maximum of this anomaly in the South Atlantic is located in the area of the Agulhas Ridge.
(3) What is the origin of intraplate volcanism and which role do mantle plumes play in it? ("Great Plume Debate")
Weekly reports of the cruise:
Principle investigators: Prof. Kaj Hoernle, Dr. Folkmar Hauff, Dr. Reinhard Werner
Funding source: DFG
Dr. Gabriele Uenzelmann-Neben, Alfred Wegener Institute Foundation for Polar and Marine Research (AWI), Am Alten Hafen 26, 27515 Bremerhaven (http://www.awi.de).
Dr. Cornelia Claas, Lamont-Doherty Earth Observatory (LDEO), 61 Route 9W, Palisades, NY 10964-1000 USA (http://www.ldeo.columbia.edu).
Dr. Michael Bizimis, Dept. of Earth and Ocean Sciences, 701 Sumter Street, EWS 617, University of South Carolina, Columbia, SC 29208, USA (http://www.geol.sc.edu).
Douglass J, Schilling J-G, Kingsley RH, Small C (1995). Influence of the Discovery and Shona mantle plumes on the southern Mid-Atlantic Ridge: rare earth evidence. Geophys Res Lett 22: 2893-2896
Douglass J, Schilling J-G, Fontignie D (1999), Plume-ridge interactions of the Discovery and Shona mantle plumes with the southern Mid-Atlantic Ridge (40°-55°S), J Geophys Res 104 no. B2: 2941-2962
Hart SR (1984) A large-scale isotope anomaly in the Southern Hemisphere mantle: Nature 309: 753-757
Le Roex A, Class C, O’Connor J, Jokat W (2010) Shona and Discovery aseismic ridge systems, South Atlantic: Trace element evidence for enriched mantle sources. J Petrol 51 no 10: 2089 - 2120
Uenzelmann-Neben G, Gohl K (2005) The Agulhas Ridge, South Atlantic: the peculiar structure of a fracture zone. Mar Geophys Res 25: 305-319