Protactinium and thorium scavenging and advection in the Southern Ocean, Atlantic Sector

During the expedition ANTXXIV-3 (Feb-April 2008, GEOTRACES/IPY, RV Polarstern), Pa and Th isotopes (230Th, 232Th and 234Th) were determined for the first time in seawater and size-fractionated particle samples along the Zero-Meridian, in the Weddell Sea and Drake Passage. Pa and Th distributions in these different environments (shelf, open-ocean, ice formation areas ) are mainly controlled by particle flux and boundary scavenging but also by advection of water masses. Thus, at Drake Passage, using -as a first approach- a simple scavenging-mixing model, we show that the vertical distribution of 230Th is mainly driven by the upwelling of deep water from North to South whereas the 231Pa distribution only reflects this upwelling in the upper 1500 m of the water column. Below this depth, Pa is more influenced than Th by advection of deep water masses with different origins flowing across Drake Passage. Thus, in the northern part of Drake Passage, deep water with high 231Pa concentrations (suggesting reduced Pa scavenging during transport) was inferred to come from the Central Pacific (in agreement with CFC data) while in the southern part of Drake Passage, the observed low Pa concentrations were attributed to deep water coming from a distant source around the opal-rich Antarctic shelf (as inferred also from Hf isotopes) with strong Pa scavenging and a correspondingly low fractionation factor FTh/Pa<<10. In the Central Weddell Sea, enhanced Pa and Th concentrations strengthen the concept of a reduced scavenging of these radionuclides in this area. Hence, accumulated dissolved Pa and Th seem to exit the Weddell Sea to the north either via the Weddell Gyre (30-60 Sv) to be then incorporated into the ACC (140 Sv) toward the Zero-Meridian (as shown in the high radionuclide concentrations found close to Maud Rise), or flow westward with the deep waters along the Antarctic Peninsula through the Drake Passage. Enhanced dissolved 232Th concentrations are typically observed in the vicinity of the Antarctic shelf and Peninsula. Based on 230Th and 234Th in size-fractionated particles and using a reversible scavenging model, particle settling velocities, adsorption/desorption rates and when possible, aggregation/disaggregation rates, were investigated. This study pointed out that in this area, specific scavenging-mixing models should be applied to describe the radionuclide distributions and include advection of water masses from the open ocean toward the Antarctic shelf (which may contribute to boundary scavenging) or advection of water masses with different histories

Changes in 231Pa/230Th signatures in the bottom water of the ocean

There has recently been much development in the description and modeling of the particulate 231Paxs/230Thxs ratio in the ocean as proxy for the meridional ocean circulation, especially to study the situation in the glacial Atlantic Ocean. Many studies have investigated the effects of ventilation, mass flux and particle composition on 231Paxs/230Thxs ratios. The interpretations as paleoproxy rely usually on the assumption that the isotopic signal stored in the sediment is determined by the composition of suspended or sinking particles when these arrive at the respective water depth. This composition is controlled by exchange with the deep water column [1]. At depths >2500m, Scholten et al. [2] found agreement between 231Paxs/230Thxs ratios in suspended material and surface sediments. Chase et al. [3] observed no difference in activity ratio between surface sediment and a fluff layer present on top of their cores. However, there are several processes that may cause the ratio in surface sediments to differ from the ratio in sinking particles. Bottom currents transport and redistribute the sediment and fractionate grain size [4] and isotopes [5,6]. As a result of this transport and of early diagenetic reactions in the sediment, surface sediments may have a chemical composition and reactivity that is different from sinking particles. We will discuss the possible bias that these processes can give to the signals measured in bottom waters and stored in the sediment. Whereas Pa/Th ratios have mostly been studied in relation to deep water formation in the North Atlantic, the formation of deep water in the Weddell Sea (Weddell Sea Bottom Water, WSBW) also effects the distribution of 230Th and 231Pa. We will present some new water column data confirming the strong effect of ventilation on the distribution of 231Pa and 230Th in the Atlantic sector of the Southern Ocean. Both nuclides accumulate at intermediate depth in the Weddell Sea while concentrations are appreciably lower in the newly formed Weddell Sea Bottom Water. [1] Thomas et al. (2006) Earth Planet. Sci. Lett. 241, 493-504. [2] Scholten et al. (2008) Earth Planet. Sci. Lett. 271, 159-169. [3] Chase et al. (2003) Deep-Sea Res. II 50, 739-768. [4] McCave, I. N. and Hall, I. R. (2006) Geochem. Geophys. Geosyst. 7, Q10N05. [5] Kretschmer et al. (2010) Earth Planet. Sci. Lett. 294, 131-142. [6] Kretschmer et al. (2011) Geochim. Cosmochim. Acta 75, 6971-6987

Particle cycling and scavenging of protactinium and thorium isotopes based on dissolved and size-fractionated particulate distributions (ANTXXIV-3, Southern Ocean)

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)

Review of Long-Term Trends in the Equatorial Ionosphere Due the Geomagnetic Field Secular Variations and Its Relevance to Space Weather

The Earth’s ionosphere presents long-term trends that have been of interest since a pioneering study in 1989 suggesting that greenhouse gases increasing due to anthropogenic activity will produce not only a troposphere global warming, but a cooling in the upper atmosphere as well. Since then, long-term changes in the upper atmosphere, and particularly in the ionosphere, have become a significant topic in global change studies with many results already published. There are also other ionospheric long-term change forcings of natural origin, such as the Earth’s magnetic field secular variation with very special characteristics at equatorial and low latitudes. The ionosphere, as a part of the space weather environment, plays a crucial role to the point that it could certainly be said that space weather cannot be understood without reference to it. In this work, theoretical and experimental results on equatorial and low-latitude ionospheric trends linked to the geomagnetic field secular variation are reviewed and analyzed. Controversies and gaps in existing knowledge are identified together with important areas for future study. These trends, although weak when compared to other ionospheric variations, are steady and may become significant in the future and important even now for long-term space weather forecasts

Quality control tools for age dating in nuclear safeguards and forensics

Nuclear safeguards conclusions are based to a large extent on comparison of measurement results between operator and safeguards laboratories. Nuclear forensics deals with consistency of information, coherence between materials or samples, conformity of findings with declared processes and comparison of external and internal data. Representative samples are measured by respective networks of laboratories. For nuclear safeguards these are the Euratom safeguards and the IAEA Network of Analytical laboratories (IAEA-NWAL). For nuclear forensics, the laboratories are mainly members of the Nuclear Forensics International Technical Working Group (ITWG). Metrological quality control tools are a means to establish accurate, traceable and comparable measurement results. The European Commission-Joint Research Centre (EC-JRC) has therefore put effort into the development of certified reference materials and the provision of conformity assessment tools, particularly for "age-dating" of uranium and plutonium samples, which is of importance to both communities. In this context the new IRMM-1000a and IRMM-1000b, certified for the production date based on the 230Th/234U radiochronometer, will be presented, and the results from the Regular European Inter-laboratory Measurement Evaluation Programme (REIMEP-22) on 'U Age Dating' will be discussed. Furthermore the status of the on-going development of a 243Am spike reference material in the frame of the recently signed collaboration agreement between the EC-JRC and the CEA/DEN will be presented. This spike is intended for use in the determination of the last separation date of plutonium samples by Isotope Dilution Mass Spectrometry (IDMS) using the 241Pu/241Am radiochronometer.JRC.G.2-Standards for Nuclear Safety, Security and Safeguar