Devonian-Carboniferous extension and Eurekan inversion along an inherited WNW-ESE-striking fault system in Billefjorden, Svalbard.

Background: The Billefjorden area in central Spitsbergen hosts thick Lower-lowermost Upper Devonian, late-post-Caledonian collapse deposits presumably deformed during the Late Devonian Svalbardian Orogeny. These rocks are juxtaposed against Proterozoic basement rocks along the Billefjorden Fault Zone and are overlain by uppermost Devonian-early Permian deposits of the Billefjorden Trough, a N-S-trending Carboniferous rift basin bounded by the Billefjorden Fault Zone. Methods: We interpreted seismic reflection (also depth-converted), bathymetric, and exploration well data. Results: The data show abundant Early Devonian, WNW-ESE-striking (oblique-slip) normal faults segmenting the Billefjorden Trough, and a gradual decrease in tectonic activity from the Early Devonian (collapse phase) to early Permian (post-rift phase). Early Devonian-Middle Pennsylvanian WNW-ESE-striking faults were mildly reactivated and overprinted and accommodated strain partitioning and decoupling in the early Cenozoic. This resulted in intense deformation of Lower Devonian sedimentary rocks and in the formation of bedding-parallel décollements, e.g., between the Lower Devonian Wood Bay and the uppermost Pennsylvanian-lowermost Permian Wordiekammen formations. This suggests that intense deformation within Devonian rocks in Dickson Land can be explained by Eurekan deformation alone. Eurekan deformation also resulted in the formation of WNW-ESE- and N-S- to NNE-SSW-trending, kilometer-wide, open folds such as the Petuniabukta Syncline, and in inversion and/or overprinting of Early Devonian to Early Pennsylvanian normal faults by sinistral-reverse Eurekan thrusts. WNW-ESE-striking faults merge at depth with similarly trending and dipping ductile shear zone fabrics in Proterozoic basement rocks, which likely formed during the Timanian Orogeny. Conclusions: A NNE-dipping shear zone, which is part of a large system of Timanian thrusts in the Barents Sea, controlled the formation of WNW-ESE-striking Devonian-Mississippian normal faults and syn-tectonic sedimentary rocks in Billefjorden. Eurekan strain partitioning and decoupling suggest that the Svalbardian Orogeny did not occur in Svalbard.

Implications of Timanian thrust systems in the Barents Sea and Svalbard on using paleontological constraints for plate tectonics reconstructions.

Background: The Svalbard Archipelago is commonly believed to have been located at comparable latitude and, possibly, to have been attached to Laurentia in the early Paleozoic (500-420 Ma) based on trilobite assemblage similarities. Trilobite assemblage differences and lack of mixing between Laurentia-Svalbard and Baltica were further used to propose that these continents were separated by the Iapetus Ocean at that time. However, recent structural correlation of Timanian (650-550 Ma) thrust systems throughout the Barents Sea show that Svalbard was already attached to Baltica in the latest Neoproterozoic and remained so during the Phanerozoic. Methods: The present study presents a new interpretation of seismic reflection data from the DISKOS database, which were tied to nearby exploration wells. The study uses recently acquired knowledge of the seismic facies of intensely deformed pre-Caledonian rocks and principles of sequence stratigraphy to interpret the data. Results: The present study reconciles the proximity of Svalbard and Laurentia with the early accretion of Svalbard to Baltica in the latest Neoproterozoic. It also describes the influence of Timanian thrust systems on paleoenvironments and possible effects on trilobite assemblages, e.g., the lack of mixing between those of Laurentia-Svalbard and Baltica. Conclusions: The results suggest that paleontological constraints are robust markers to discuss continent amalgamation but should be considered with greater care when discussing continent separation since other factors, such as major thrust systems, may create major, linear, topographical boundaries, which may act as major faunal barriers within a single tectonic plate. Other factors to consider include paleoclimatic belts.

Previous paleontological studies have used the differences in faunas (in the present case, now extinct fossil occurrences of trilobites) between two or more continental blocks (presently the Svalbard Archipelago, North America, and Scandinavia) to infer separation of these blocks by large distances up to several thousands of kilometers ca. 550 to 420 million years ago due to global plate tectonics processes. The present study shows that this method is biased because previous studies undermined factors such as climatic belts and (topographic) faunal barriers such as large, mountain-building cracks in the Earth's crust. The study builds on previous work on seismic data in the Barents Sea and Svalbard, which identified continuous, thousands of kilometers long, tens of kilometers thick networks of cracks extending from northwestern Russia to Svalbard and potentially northern Greenland, which formed 650 to 550 million years ago, therefore demonstrating a connection between all these continental blocks at 550-420 Ma.

Tritium and radiocarbon levels in the Rhône river delta and along the French Mediterranean coastline.

The Rhône is characterised by a heavy concentration of nuclear-based industries including nuclear power stations and nuclear sites housing civilian and military facilities. Here, we report the results of a four-year survey (2010-2013) of tritium and radiocarbon levels in a variety of matrices within the Rhône delta and along the French Mediterranean coastline. The aim of the study is to create a spatial reference framework of environmental levels of these two radionuclides, which are the most prevalent in radioactive effluents from nuclear power stations. Although both tritium and radiocarbon levels in the samples analysed are very low and can only be detected using ultra-sensitive analytical techniques, they clearly show the influence of the tritium and radiocarbon discharges carried by the Rhône plume along the Mediterranean coast. The tritium content of suspended matter and sediments of the Rhône is a special case, which shows elevated tritium values not seen in other French rivers with similar nuclear facilities. The north-south spatial distribution of this tritium anomaly shows that these trace values are at their highest in the upper Rhône, close to the Swiss border and upstream of Creys Malville, the northernmost nuclear power station on the Rhône. This points to a legacy of past tritium releases by the watchmaking industry. A dedicated study would be needed to clearly identify the source and the exact nature of this contamination.

Impact of the Fukushima accident on tritium, radiocarbon and radiocesium levels in seawater of the western North Pacific Ocean: A comparison with pre-Fukushima situation.

Tritium, radiocarbon and radiocesium concentrations in water column samples in coastal waters offshore Fukushima and in the western North Pacific Ocean collected in 2011-2012 during the Ka'imikai-o-Kanaloa (KoK) cruise are compared with other published results. The highest levels in surface seawater were observed for 134Cs and 137Cs in seawater samples collected offshore Fukushima (up to 1.1 Bq L-1), which represent an increase by about three orders of magnitude when compared with the pre-Fukushima concentration. Tritium levels were much lower (up to 0.15 Bq L-1), representing an increase by about a factor of 6. The impact on the radiocarbon distribution was measurable, but the observed levels were only by about 9% above the global fallout background. The 137Cs (and similarly 134Cs) inventory in the water column of the investigated western North Pacific region was (2.7 ± 0.4) PBq, while for 3H it was only (0.3 ± 0.2) PBq. Direct releases of highly contaminated water from the damaged Fukushima NPP, as well as dry and wet depositions of these radionuclides over the western North Pacific considerably changed their distribution patterns in seawater. Presently we can distinguish Fukushima labeled waters from global fallout background thanks to short-lived 134Cs. However, in the long-term perspective when 134Cs will decay, new distribution patterns of 3H, 14C and 137Cs in the Pacific Ocean should be established for future oceanographic and climate change studies in the Pacific Ocean.

Measurement of the French national tritiated-water standard by helium-3 mass spectrometry.

The (3)He ingrowth technique is based on the detection of the tritium radioactive daughter, (3)He, by mass spectrometry. Over the last three decades it has been used extensively in oceanography and groundwater studies for measurements of very low to ultra-low levels of tritium. To compare it with the best available methods of radioactive counting, we applied this method to measure the massic activity of a tritiated-water primary standard prepared by the French Laboratoire National Henri Becquerel (LNHB). One liter of a low-level tritium solution was prepared from a small aliquot of the LNHB standard by two-step gravimetric dilution with tritium-free groundwater. Sixteen samples of this solution were analyzed at the LSCE-Saclay noble gas facility, using the (3)He ingrowth method. The massic activity of the prepared solution was also measured by Liquid Scintillation Counting using the Quantulus LNHB counter and an internal calibration method with the LNHB tritiated water standard. All results agree within their standard uncertainty.

Modelling tritium flux from water to atmosphere: application to the Loire River.

Tritium (³H or T) is one of the major radionuclides released by nuclear power plants (NPP) into rivers. However, tritiated water (HTO) flux from water to air is seldom considered when assessing health effects of such releases. The aim of this paper is to present the result of a research program, called LORA, conducted on the Loire River (France). To improve our understanding of HTO flux from surface water to air, three field campaigns were organised during the NPP's radioactive releases to measure simultaneously the activity concentrations in air on the riverbank, using an innovative system, and in river water. The measurements showed that during radioactive releases, water vapour was enriched in ³H. These results were used to calibrate exchange velocities. The average of these estimated exchange velocities was more than one order of magnitude higher than those calculated in the literature from indoor experiments. The variability of these values was also larger, showing that outdoor studies cover a wide range of conditions influencing HTO flux. No correlation was observed between exchanges velocities and meteorological conditions. However, there was a significant difference between day and night with a higher value observed during the day. Two approaches used to calculate HTO evaporation from water (i.e. the approach based on water evaporation and the approach considering that HTO follows its own concentration gradient) were included in a hydrodynamic model, which was used to evaluate HTO air activity along the Loire River. In conclusion, only the approach considering that HTO follows its own gradient led to a good agreement between measurements and predictions. A one-year simulation was done to estimate the contribution of this process to the dose. Its contribution can be considered as negligible in this case compared to the other pathways such as ingestion of water or foodstuffs.

(3)He mass spectrometry for very low-level measurement of organic tritium in environmental samples.

The design, setup and performance of a mass spectrometric system for the analysis of low to very low-level tritium in environmental samples are described. The tritium concentration is measured indirectly by the (3)He ingrowth from radioactive decay after complete initial degassing of the sample. The analytical system is fully computer-controlled and consists in a commercial helium isotope mass spectrometer coupled with a high vacuum inlet system. A detection limit of 0.15 Bq/kg is routinely obtainable for sample sizes of 20g of water equivalent and an accumulation time of three months. Larger samples (and/or longer accumulation time) can be used to obtain lower detection limits. In addition to the benefit of a lower detection limit, another advantage of this non-destructive method lies in the simplicity of the analytical procedure which strongly limits the risk of contamination. An inter-comparison was successfully performed with the conventional beta counting technique on lyophilized grass samples, in a range of tritium concentrations of environmental interest. It shows that the (3)He mass spectrometry method yields results that are fully consistent with the conventional liquid scintillation technique over a wide range of tritium concentrations.

The distribution of tritium in the terrestrial and aquatic environments of the Creys-Malville nuclear power plant (2002-2005).

The Creys-Malville nuclear plant, located on the left bank of the Rhône, was shut down in 1998. The facilities are currently in their initial stage of dismantling. In order to establish a baseline for tritium in the vicinity of the site prior to the main dismantling phase, we carried out a monitoring program between 2002 and 2005 in the main terrestrial and aquatic compartments of the local environment. Tritium levels in the groundwaters and in the Rhône waters correspond to the regional tritium concentration in precipitation. The data obtained for the terrestrial environment are also in good agreement with the regional background and do not show any specific signature linked to the nuclear plant. The various aquatic compartments of the Rhône (fish, plant, sediment) are significantly enriched in tritium both upstream and downstream of the power plant: although Tissue-Free Water Tritium concentrations are in equilibrium with the river water, the non-exchangeable fraction of organic bound tritium in plants and fishes shows values which outpace the river water background by one to two orders of magnitude, and up to four to five orders of magnitude in the sediments. This tritium anomaly is not related to the nuclear plant, as it is already present at the Swiss border 100km upstream of the site. Although fine particles of tritiated polystyrene entering the composition of the luminous paints used by the clock industry have been suspected on several occasions, the exact nature and the origin of this tritium source remain unknown and require further investigations.

Constraints on hydrothermal processes and water exchange in Lake Vostok from helium isotopes.

Lake Vostok, the largest subglacial lake in Antarctica, is covered by the East Antarctic ice sheet, which varies in thickness between 3,750 and 4,100 m (ref. 1). At a depth of 3,539 m in the drill hole at Vostok station, sharp changes in stable isotopes and the gas content of the ice delineate the boundary between glacier ice and ice accreted through re-freezing of lake water. Unlike most gases, helium can be incorporated into the crystal structure of ice during freezing, making helium isotopes in the accreted ice a valuable source of information on lake environment. Here we present helium isotope measurements from the deep section of the Vostok ice core that encompasses the boundary between the glacier ice and accreted ice, showing that the accreted ice is enriched by a helium source with a radiogenic isotope signature typical of an old continental province. This result rules out any significant hydrothermal energy input into the lake from high-enthalpy mantle processes, which would be expected to produce a much higher 3He/4He ratio. Based on the average helium flux for continental areas, the helium budget of the lake leads to a renewal time of the lake of the order of 5,000 years.

Design and performance of a mass spectrometric facility for measuring helium isotopes in natural waters and for low-level tritium determination by the 3He ingrowth method.

The design and performance of a mass spectrometric system for the measurement of helium isotopes and very low tritium concentrations in natural waters are described and discussed in the light of analytical precision and accuracy. The system consists of a VG 3000 mass spectrometer with a fully automated inlet system for preparation and purification of the samples. Along with this mass spectrometric system, different custom-fabricated units are described, especially designed for taking samples, extracting helium or degassing tritium samples prior to the mass spectrometric analysis. The 3He detection limit of the system is close to 10(-16) cm3 STP corresponding to a tritium level of 0.003 TU for a 500 g water sample stored six months for 3He regrowth. A vertical oceanic tritium profile from the south hemisphere is presented as an illustration of the system's capability to detect very low tritium concentrations in the environment.