Tracing continental weathering and water mass mixing in the Atlantic sector of the Southern Ocean with neodymium and hafnium isotopes

Final Report Abstract

The first combined distribution patterns of radiogenic Hf and Nd isotope composition and concentration in the Southern Ocean have been determined on samples obtained during RV Polarstern cruise ANTXXIV/3 in the frame of the IPY 2008 and the international GEOTRACES programme. These data provide valuable new insights into the weathering inputs and their provenance, as well as into the distribution and mixing of water masses in the Atlantic sector of the Southern Ocean on the basis of their isotopic signatures. To carry out this project a chemical separation scheme was developed that allowed the separation and purification of Hf, Nd, Ac, Pa, and Th from the same large volume water samples. Concentrations of both Hf and Nd are generally low in the surface waters and increase with water depth as a function of adsorption and remineralisation processes, whereby the data indicate that Hf is more efficiently removed into the sediments with biogenic opal than Nd. Hafnium and Nd isotope signatures in the surface layer largely reflect local weathering inputs. Enhanced release of Nd and Hf from volcanic rocks near the coast implies that those rocks are an important source for radiogenic Hf and Nd in the ocean whereas old cratonic rocks appear to be not as important. The deeper water column is surprisingly invariant for Hf isotopes and thus does not allow tracing of water masses. In contrast, Nd isotopes clearly reflect different deep water masses originating from the North Atlantic and the Weddell Sea. Deep water mixing calculations reveal that Nd is only removed by 10 to 20 % during mixing between northern and southern sourced Atlantic waters. The entire Nd budget in the Atlantic sector of the Southern Ocean can be explained by Pacific (~ 30 %) and Atlantic (~ 70%) contributions, implying that local contributions from Antarctica are quantitatively small. These data will serve as a basis for the extraction of seawater signatures of the two radiogenic isotope systems from marine sediments in the Southern Ocean and their use as proxy tracers for past water mass mixing and ocean circulation, as well as weathering inputs under different climatic regimes.

Publications

• 2009. Hafnium and neodymium isotope composition of filtered particles from the Drake Passage, Vortrag, Goldschmidt Conference, Davos, Switzerland
  Stichel, T., Frank, M., Haley, B. A., Rickli, J. und Venchiarutti, C.
  
• 2009. Hafnium and neodymium isotope composition of seawater and filtered particles from the Southern Ocean, Vortrag, AGU Fall Meeting, San Francisco, USA
  Stichel, T., Frank, M., Haley, B., Rickli, J. und Venchiarutti, C.,
  
• 2010. Nd and Hf isotopic compositions in the Atlantic sector of the Southern Ocean, Vortrag, Ocean Sciences Meeting,Portland, Oregon, USA
  Stichel, T., Frank, M., Haley, B., Rickli, J. und Venchiarutti, C.
  
• 2011. Sources and input mechanisms of hafnium and neodymium in surface waters of the Atlantic sector of the Southern Ocean, Vortrag, Goldschmidt Conference, Prague, Czech Republic
  Stichel, T., Frank, M., Rickli, J., Hathorne, E., Haley, B. A., Jeandel, C. und Pradoux, C.
  
• 2011. Tracing water masses and continental weathering by neodymium and hafnium isotopes in the Atlantic sector of the Southern Ocean. PhD. thesis, University of Kiel, 149 pp.
  Stichel, T.
  
• 2012. The hafnium and neodymium isotope composition of seawater in the Atlantic sector of the Southern Ocean. Earth Planet. Sci. Lett. 317-318, 282-294
  Stichel, T., Frank, M., Rickli, J., and Haley, B.A.