Description: The natural uranium decay series provide a suite of tracers to study transport processes in the ocean. We have used nuclides of the particle-reactive elements Th, Pa, Pb and Po for studies of particle flux in the Southern Ocean, whereas isotopes of the elements Ra and Ac served as tracers for the transport of water masses. Here we summarize the specific aspects of the behaviour of these nuclides in the Southern Ocean and give some examples of their application. We review the important influence of exchange between ocean basins by advection and upwelling on the long-lived nuclides. We show how the distribution of 234Th in surface waters across the ACC represents the export production, whereas in the benthic nepheloid layer this tracer is used to illustrate how the resuspension regime in the ACC is linked to the position of the oceanographic fronts.

Description: 227Ac is a naturally occurring radioisotope with a unique combination of properties that make it suitable for the determination of deep ocean mixing and upwelling rates. Here, we present a method for the determination of 227Ac in sea water on sample sizes of 20–80 L. The measurement is based on co-precipitation of 227 Ac with MnO2, followed by chemical isolation of actinium in the presence of an artificial Ac isotope. Actinium is then electrodeposited onto silver discs. In two alpha-spectrometric counting periods, first the artificial 225Ac isotope is counted, then after N100 days five daughters of 227Ac. The first counting period gives a total yield for the procedure, integrating chemical recovery and detector efficiency. The total yield was found here to be on average 15±5%, the chemical yield on average about 50%. The counting of five decay products of 227Ac in the second period makes the method particularly sensitive. Using appropriate decay corrections, the initial 227Ac activity can be determined to better than 10% relative error for concentrations b10,000 atoms/L. We compare data acquired by the new method to a data set from in-situ pumps, from a parallel sampling campaign in the Eastern Weddell Gyre, and we can show excellent agreement. Repeated determinations of 227Ac in a uranium reference material (UREM-11) demonstrate the accuracy of the method.

Description: Die Polargebiete stellen Schlüsselgebiete im heutigen globalen Klimageschehen dar und bergen wertvolle Informationen über die Klimaentwicklung der jüngeren geologischen Vergangenheit (z.B. Sedimentkerne). Wichtige Fragen bezüglich des Zirkulationsgeschehens oder der Primärproduktion in den polaren Breiten werden jedoch nur ansatzweise verstanden.Die drei natürlich vorkommenden Zerfallsreihen (238U-206Pb, 235U-207Pb, und 232Th-208Pb) liefern mit ihren radioaktiven Zwischenprodukten wichtige Tracer für Prozeßstudien im marinen Milieu. Gemäß ihres geochemischen Verhaltens im Meerwasser lassen sich eher partikelreaktive (z.B. Thorium und Protactinium) und eher lösliche Isotope (z.B. Uran, Radium, Radon, Actinium) unterscheiden, die entsprechend im Sediment bzw. in der Wassersäule akkumulieren. Aktivitätsungleichgewichte innerhalb eines Mutter-Tochter-Paares einer Zerfallsreihe erlauben Rückschlüsse auf die zugrundeliegenden Prozesse wie z.B. laterale oder vertikale Mischung, Auftrieb von Tiefenwasser, Grundwassereinträge, Bestimmung der Exportproduktion aus der Deckschicht oder Partikelfluß ins Sediment. Die unterschiedlichen Halbwertszeiten (Tage bis Millionen Jahre) der Zerfallsprodukte ermöglichen die Untersuchung von Prozessen auf unterschiedlichen Zeitskalen.Anhand der Isotope 227Ac, 228Ra, 222Rn, 234Th und 230Th sollen mit Beipielen aus der europäischen Arktis und dem Atlantischen Sektor des Südozeans die vielfältigen Einsatzmöglichkeiten von natürlich vorkommenden Radionukliden für marine Fragestellungen illustriert werden.

Description: The short-lived thorium isotope 234Th (half-life 24.1 days) has been used as a tracer for a variety of transport processes in aquatic systems. Its use as a tracer of oceanic export on sinking particles has stimulated a rapidly increasing number of studies that require analyses of 234Th, both in marine and in freshwater systems. The original 234Th method is labour intensive. Thus, there has been a quest for simpler techniques that require smaller sample volumes. Here, we review current methodologies in the collection and analysis of 234Th from the water column, discuss their individual strengths and weaknesses, and provide an outlook on possible further improvements and future challenges. Also included in this review are recommendations on calibration procedures and on the production of standard materials as well as a flow chart designed to help researchers find the most appropriate 234Th analytical technique for a specific aquatic regime and known sampling constraints.

Description: During the expedition ANTXXIV-3 (Feb-April 2008, GEOTRACES/IPY), Pa and Th isotopes (230Th, 232Th and 234Th) were determined in seawater and size-fractionated particles along 3 transects: Zero-Meridian, Weddell Sea and Drake Passage. 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. Pa and Th distributions are controlled by particle flux and boundary scavenging. Therefore, based on reversible scavenging models and distributions of these radionuclides, particle settling velocities, adsorption/desorption rates and aggregation/disaggregation rates were further investigated. At open ocean stations, most of the excess 230Th distributions display a linear increase with increasing depth down to 1000 m. These distributions are sometimes modified from the linear trend, in surface or deeper water masses, reflecting ice melting, downwelling of surface waters and ventilation of bottom waters. Terrigenous inputs are observed near the shelf. Our results will be compared with the distributions of other trace elements, analysed in the same samples (Nd, 227Ac) or in parallel sampling (CFC).