Radionuclides (²³¹Pa, Th isotopes) in size-fractionated particulate and seawater samples in relation to other trace elements in the Southern Ocean

Natural radionuclides are powerful tracers to study particle dynamics and to estimate the rate of transport processes in the ocean. ²³¹Pa (T_1/2= 32760 y) and ²³⁰Th (T_1/2=75380 y) are both produced by the constant decay of soluble and homogeneously distributed Uranium. They are very particle reactive, i.e they adsorb onto particles and sink to the bottom with the sinking particles. This substraction of radionuclides from the water column is known as “scavenging”. This scavenging is “reversible”, i.e there is a constant exchange of the radionuclides between dissolved and particulate phases. They are therefore really efficient particle tracers and their distributions in the ocean are controlled by particle flux and boundary scavenging.

²³²Th, the most abundant isotope of thorium, has a lithogenic origin and is used to trace sediment-resuspension or terrigenous inputs.

²³¹Pa and Th isotopes (²³⁰Th, ²³²Th and ²³⁴Th) were investigated in seawater and size-fractionated particulate samples along 3 transects (Figure 1): Zero-Meridian, Weddell Sea and Drake Passage during Polarstern ANTXXIV-3 (Feb-April 2008), as part of the IPY project GEOTRACES. These radionuclides, sampled in different environments (shelf, open-ocean, ice formation areas…), and for the first time in 3 different sizes of particles, give more insights into the particle dynamics in the water column.



Figure 1. Polarstern ANTXXIV-3 cruise track in blue and the Bonus-GoodHope cruise track in red. An inter-comparative exercise was realised at common stations to the two cruises.

The project is structured in several objectives:

1) Analysis and interpretation of samples collected during the Zero, Weddell and Drake research cruise onboard the Polarstern (ANTXXIV-3).
2) Application of suitable reversible scavenging models based on Th isotopes to estimate exchange between suspended and sinking particle pools (aggregation/disaggregation, adsorption/desorption constants, settling velocities) and ventilation or upwelling rates (especially around the Antarctic peninsula or North of Drake Passage).
3) Comparison of ²³²Th and ²³¹Pa/²³⁰Th data with other trace elements such as Al and Mn (Prof. Hein de Baar and Rob Middag, NIOZ, Netherlands), ²²⁷Ac (Nd and Hf isotopic composition (Prof. Martin Frank and Torben Stichel, Ifm-GEOMAR, Kiel) and CFC and ³He (Dr. Oliver Huhn, University of Bremen). These two last projects are part of the joint DFG proposal).
4) Incorporation of radionuclide data in numerical models of ocean circulation and biogeochemical cycling (collaboration with Reiner Schlitzer, AWI, Bremerhaven). The overall aim of this work is to combine the results of the objectives listed above into a comprehensive model of the key processes influencing the biogeochemical cycling of particle-reactive trace elements in the Southern Ocean.

Presently, ²³¹Pa and Th distributions in the Drake Passage (4 stations over 5) and in the Weddell Sea (1 station close to the Antarctic Peninsula) have already been determined in seawater and size-fractionated particulate samples. These data fit nicely with other trace element data, such as Al, Mn and Hf isotopic composition at shallow stations and at the bottom of the southernmost station along Drake Passage, indicating terrigenous inputs from the Antarctic shelf. Preliminary Pa/Th results indicate a significant influence of opal or MnO₂ along Drake transect.

Scientists

Celia Venchiarutti
Alfred Wegener Institut für Polar und Meeresforschung
Dept of Marine Geochemistry
Am Handelshafen 12, Room C-412
D-27570 Bremerhaven, Germany
Tel: +49 (471) 4831 1030
Email: cvenchia@awi.de

Michiel Rutgers van der Loeff
Alfred Wegener Institut für Polar und Meeresforschung
Dept of Marine Geochemistry
Postfach 120161
27515 Bremerhaven
Tel: +49 (471) 1831 1259
Email: mloeff@awi.de

Research areas

Greenwich Meridian, Weddell Sea and Drake Passage

Publications

Rutgers v d Loeff M, Cai P, Stimac I, Bracher A, Middag R, Klunder M, van Heuven S, 2011. 234Th in surface waters: distribution of particle export flux across the Antarctic Circumpolar Current and in the Weddell Sea during the GEOTRACES expedition ZERO and DRAKE. Deep Sea Research II - Topical Studies in Oceanography. doi:10.1016/j.dsr2.2011.02.004

Venchiarutti C, Rutgers v d Loeff M, Stimac I, 2011. Scavenging of 231Pa and thorium isotopes based on dissolved and size-fractionated particulate distributions at Drake Passage (ANTXXIV-3). Deep-Sea Research II - Topical Studies in Oceanography. doi:10.1016/j.dsr2.2010.10.040

Rutgers v d Loeff MM, Geibert W, 2008. U/Th series nuclides as tracers of particle dynamics, scavenging and biogeochemical cycles in the oceans. In: U/Th series radionuclides in aquatic systems (ed. Krishnaswami S, Cochran JK), Chapter 7.1.

Venchiarutti C, Jeandel C, Roy-Barman M, 2008. Particle dynamics study in the wake of Kerguelen Island using thorium isotopes. Deep Sea Research Part I - Oceanographic Research Papers 55, 1343.

Venchiarutti C, 2007. Approche de la dynamique des particules dans le sillage des Kerguelen, à l'aide des traceurs géochimiques: ²³⁰Th et ₂₃₁Pa. Université Paul Sabatier Toulouse III.

Rutgers v d Loeff MM, Friedrich J, Geibert W, Hanfland C, Höltzen H, Vöge I, Walter HJ, 2004. Radionuclides as tracers for particle flux and transport of water masses in the Atlantic sector of the Southern Ocean. In: The South Atlantic during the Late Quaternary: Reconstruction of Material Budget and Current Systems, Vol. SFB 261 Synthesis Volume (ed. Wefer G, Mulitza S, Ratmeyer V), pp. 47-63. Springer.

Rutgers v d Loeff MM, Buesseler K, Bathmann U, Hense I, Andrews J, 2002. Comparison of carbon and opal export rates between summer and spring bloom periods in the region of the Antarctic Polar Front, SE Atlantic. Deep Sea Research Part II - Topical Studies in Oceanography 49, 3849-3869.

Rutgers v d Loeff MM, 2001. Uranium-Thorium decay series in the water column. In: Encyclopedia of Ocean Sciences, Vol. MS 168 (ed. Steele J, Thorpe S, Turekian K), pp. 3135-3145. Academic Press.

Rutgers v d Loeff MM, Moore WS, 1999. Determination of natural radioactive tracers. Chapter 13. In: Methods of Seawater Analysis, third Edition (ed. Grasshoff K, Ehrhardt M, Kremling K), pp. 365-397. Verlag Chemie.

Rutgers v d Loeff MM, Friedrich J, Bathmann UV, 1997. Carbon export during the spring bloom at the southern Polar Front, determined with the natural tracer ²³⁴Th. Deep-Sea Research II 44, 457-478.

Rutgers v d Loeff MM, Berger GW, 1993. Scavenging of ²³⁰Th and ²³¹Pa near the Antarctic Polar Front in the South Atlantic. Deep-Sea Research I 40, 339-357.

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Research funding organisation

German Research Foundation

Project number: RU 712/7-1
Funding period: July 2007 - February 2009