Constraining groundwater modeling with magnetic resonance soundings

Magnetic resonance sounding (MRS) is a noninvasive geophysical method that allows estimating the free water content and transmissivity of aquifers. In this article, the ability of MRS to improve the reliability of a numerical groundwater model is assessed. Thirty-five sites were investigated by MRS over a ～5000 km(2) domain of the sedimentary Continental Terminal aquifer in SW Niger. Time domain electromagnetic soundings were jointly carried out to estimate the aquifer thickness. A groundwater model was previously built for this section of the aquifer and forced by the outputs from a distributed surface hydrology model, to simulate the observed long-term (1992 to 2003) rise in the water table. Uncertainty analysis had shown that independent estimates of the free water content and transmissivity values of the aquifer would facilitate cross-evaluation of the surface-water and groundwater models. MRS results indicate ranges for permeability (K = 1 × 10(-5) to 3 × 10(-4) m/s) and for free water content (w = 5% to 23% m(3) /m(3) ) narrowed by two orders of magnitude (K) and by ～50% (w), respectively, compared to the ranges of permeability and specific yield values previously considered. These shorter parameter ranges result in a reduction in the model's equifinality (whereby multiple combinations of model's parameters are able to represent the same observed piezometric levels), allowing a better constrained estimate to be derived for net aquifer recharge (～22 mm/year).     

Seasonal variability of element fluxes in two Central Siberian rivers draining high latitude permafrost dominated areas

In order to constrain the origin and fluxes of elements carried by rivers of high latitude permafrost-dominated areas, major and trace element concentrations as well as Sr and U isotopic ratios were analyzed in the dissolved load of two Siberian rivers (Kochechum and Nizhnyaya Tunguska) regularly sampled over two hydrological cycles (2005-2007). Large water volumes of both rivers were also collected in spring 2008 in order to perform size separation through dialysis experiments. This study was completed by spatial sampling of the Kochechum watershed carried out during summer and by a detailed analysis of the main hydrological compartments of a small watershed. From element concentration variations along the hydrological cycle, different periods can be marked out, matching hydrological periods. During winter baseflow period (October to May) there is a concentration increase for major soluble cations and anions by an order of magnitude. The spring flood period (end of May-beginning of June) is marked by a sharp concentration decrease for soluble elements whereas dissolved organic carbon and insoluble element concentrations strongly increase.When the spring flood discharge occurs, the significant increase of aluminum and iron concentrations is related to the presence of organo-mineral colloids that mobilize insoluble elements. The study of colloidal REE reveals the occurrence of two colloid sources successively involved over time: spring colloids mainly originate from the uppermost organic-rich part of soils whereas summer colloids rather come from the deep mineral horizons. Furthermore, U and Sr isotopic ratios together with soluble cation budgets in the Kochechum river impose for soluble elements the existence of three distinct fluxes over the year: (a) at the spring flood a surface flux coming from the leaching of shallow organic soil levels and containing a significant colloidal component (b) a subsurface flux predominant during summer and fall mainly controlled by water-rock interactions within mineral soils and (c) a deep groundwater flux predominant during winter which enters large rivers through unfrozen permafrost-paths. Detailed study of the Kochechum watershed suggests that the contribution of this deep flux strongly depends on the depth and continuous nature of the permafrost.     

Sr and Nd isotope data of apatite,calcite and dolomite as indicators of source,and the relationships of phoscorites and carbonatites from the Kovdor massif,Kola peninsula,Russia

A detailed Sr−Nd isotopic study of primary apatite, calcite and dolomite from phoscorites and carbonatites of the Kovdor massif (380 Ma), Kola peninsula, Russia, reveals a complicated evolutionary history. At least six types of phoscorites and five types of carbonatite have been identified from Kovdor by previous investigators based on relative ages and their major and accessory minerals. Isotopic data from apatite define at least two distinct groups of phoscorite and carbonatite. Apatite from the earlier phoscorites and carbonatites (group 1) are characterized by relatively low⁸⁷Sr/⁸⁶Sr (0.70330–0.70349) and¹⁴³Nd/¹⁴⁴Nd initial ratios (0.51230–0.51240) with F=2.01–2.23 wt%, Sr=2185–2975 ppm, Nd=275–660 ppm and Sm=31.7–96.2 ppm. Apatite from the second group has higher⁸⁷Sr/⁸⁶Sr (0.70350–0.70363) and¹⁴³Nd/¹⁴⁴Nd initial ratios (0.51240–0.51247) and higher F (2.63–3.16 wt%), Sr (4790–7500 ppm), Nd (457–1074 ppm) and Sm (68.7–147.6 ppm) contents. This group corresponds to the later phoscorites and carbonatites. One apatite sample from a carbonatite from the earlier group fits into neither of the two groups and is characterized by the highest initial⁸⁷Sr/⁸⁶Sr (0.70385) and lowest¹⁴³Nd/¹⁴⁴Nd (0.51229) of any of the apatites. Within both groups initial⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/¹⁴⁴Nd ratios show negative correlations. Strontium isotope data from coexisting calcite and dolomite support the findings from the apatite study. The Sr and Nd isotopic similarities between carbonatites and phoscorites indicate a genetic relationship between the two rock types. Wide variations in Sr and Nd isotopic composition within some of the earlier carbonatites indicate several distinct intrusive phases. Oxygen isotopic data from calcite and dolomite (δ¹⁸O=+7.2 to +7.7‰ SMOW) indicate the absence of any low-temerature secondary processes in phoscorites and carbonatites, and are consistent with a mantle origin for their parental melts. Apatite data from both groups of phoscorite plot in the depleted quadrant of an ε_Nd versus ε_Sr diagram. Data for the earlier group lie along the Kola Carbonatite Line (KCL) as defined by Kramm (1993) and data from the later group plot above the KCL. The evolution of the phoscorites and carbonatites cannot be explained by simple magmatic differentiation assuming closed system conditions. The Sr−Nd data can best be explained by the mixing of three components. Two of these are similar to the end-members that define the Kola Carbonatite Line and these were involved in the genesis of the early phoscorites and carbonatites. An additional component is needed to explain the isotopic characteristics of the later group. Our study shows that apatite from rocks of different mineralogy and age is ideal for placing constraints on mantle sources and for monitoring the Sr−Nd evolution of carbonatites. Editorial responsibility: W. Schreyer     

The Fe–C–O–H–N system at 6.3–7.8 GPa and 1200–1400 °C: implications for deep carbon and nitrogen cycles

Interactions in a Fe–C–O–H–N system that controls the mobility of siderophile nitrogen and carbon in the Fe⁰-saturated upper mantle are investigated in experiments at 6.3–7.8 GPa and 1200–1400 °C. The results show that the γ-Fe and metal melt phases equilibrated with the fluid in a system unsaturated with carbon and nitrogen are stable at 1300 °C. The interactions of Fe₃C with an N-rich fluid in a graphite-saturated system produce the ε-Fe₃N phase (space group P6₃/mmc or P6₃22) at subsolidus conditions of 1200–1300 °C, while N-rich melts form at 1400 °C. At IW- and MMO-buffered hydrogen fugacity (fH₂), fluids vary from NH₃- to H₂O-rich compositions (NH₃/N₂?>?1 in all cases) with relatively high contents of alkanes. The fluid derived from N-poor samples contains less H₂O and more carbon which mainly reside in oxygenated hydrocarbons, i.e., alcohols and esters at MMO-buffered fH₂ and carboxylic acids at unbuffered fH₂ conditions. In unbuffered conditions, N₂ is the principal nitrogen host (NH₃/N₂?≤?0.1) in the fluid equilibrated with the metal phase. Relatively C- and N-rich fluids in equilibrium with the metal phase (γ-Fe, melt, or Fe₃N) are stable at the upper mantle pressures and temperatures. According to our estimates, the metal/fluid partition coefficient of nitrogen is higher than that of carbon. Thus, nitrogen has a greater affinity for iron than carbon. The general inference is that reduced fluids can successfully transport volatiles from the metal-saturated mantle to metal-free shallow mantle domains. However, nitrogen has a higher affinity for iron and selectively accumulates in the metal phase, while highly mobile carbon resides in the fluid phase. This may be a controlling mechanism of the deep carbon and nitrogen cycles.     

Survey of Geomagnetic Observations Made in the Northern Sector of Russia and New Methods for Analysing Them

An overview of the geomagnetic observations made in the northern part of Russia is presented from a historical perspective. Several stations were deployed on the territory of the former Soviet Union during the International Geophysical Year, 1957–1958, with the active participation and guidance of the Interagency Geophysical Committee which is inherited by the Geophysical Center of the Russian Academy of Sciences (GC RAS). In the 1990s, the majority of these stations, especially those in the remoter regions, were closed. Nowadays, the geomagnetic network, including the observatories of the INTERMAGNET program, has been restored. Examples of high-latitude geomagnetic variations in the Russian longitudinal sector are shown, and maps and trends of the secular variation over the territory of Russia presented. Particular attention is paid to the automated processing of data and to the analysis methods used. To process the growing amount of high-resolution geomagnetic data, sophisticated mathematical methods based on the fuzzy logic approach and new discrete mathematical analysis algorithms have been developed. The formal methods and algorithms for recognizing both artificial and natural disturbances in the magnetograms are described.     

Paleomagnetism of Khatyrka and Maynitsky superterranes, Koryak highlands, far eastern Russia

We have completed a paleomagnetic reconnaissance study of sedimentary and volcanic extrusive rocks collected from two major tectonic zones in northeastern Russia. Paleomagnetic sites were sampled within the fault-bounded structural units of the Khatyrka and Maynitsky superterranes and an overlap sequence of the Khatyrka superterrane. These sampling localities were chosen to allow both within-site and between-site fold tests. Stepwise thermal demagnetization within the temperature range 200–640°C showed a characteristic linear demagnetization path between thermal demagnetization steps of 400°C and 530°C. For thermal steps above 550°C, the magnetic intensity of many samples began to increase rapidly with magnetic directions, which were random between heating steps, suggesting the formation of new magnetic phases in these samples. Paleomagnetic samples collected from basalts and sediments of the Khatyrka superterrane and basalts and gabbros of the Maynitsky superterrane pass fold tests and show significant poleward motion of these superterranes since the formation of their rocks. The observed paleomagnetic paleolatitudes between 24°N or S and 32°N or S can be compared with expected paleolatitudes of 57°N to 79°N. Paleomagnetic results from sites collected from overlapping Senonian rocks pass a fold test at the 99% confidence level and give a pole position not significantly different from that expected from the apparent polar wander path for the Eurasia or North America plates, suggesting that these sedimentary units overlapping the Khatyrka superterrane were deposited along the ancient northeast margin of the Eurasian plate. The declination, in stratigraphie coordinates, shows a maximum clockwise rotation of about 20° when compared with the Eurasian APWP.     

IR-OSL dating of uranium-rich deposits from the new late Pleistocene section at the Voka site, North-Eastern Estonia

This paper presents a series of results obtained from the 22 m thick deposits of the late Pleistocene section at the Voka site, NE Estonia. The section has been intensively studied during the past few years to elucidate the dynamics of the late Weichselian palaeoenvironmental changes in the Baltic region. According to the results obtained, the sediments at the Voka site were deposited in presumably freshwater sedimentary basin, which existed at least during the whole pre-Late Glacial Maximum (LGM) period of late Pleistocene (MIS 5 to 3). One of the characteristic features of the deposits is the high uranium content (up to 23 ppm) in the upper part of the section studied. This study aimed to focus on the influence of potentially unstable environmental radioactivity on dosimetric dating results due to possible disequilibrium in the U decay chain. It was found that despite the high uranium content in the deposits, the dispersion of the dates obtained is surprisingly low. At the depths with the highest uranium content, only a moderate increase of the dispersion of the ages is observed.     

Variation Trend and Characteristics of Anthropogenic CO Column Content in the Atmosphere over Beijing and Moscow

The anthropogenic CO column content in the atmosphere is derived from measurements with infrared grating spectrometers in Beijing, China, and Moscow, Russia, during 1992-2012. Some specific variation char- acteristics and long-term variation trends of the CO col- umn content in the atmosphere in these regions are dis- cussed. An evident variation trend of anthropogenic CO in the atmosphere for the Beijing region is not observed during 1992-2012, while for the Moscow region, it de- creases yearly by about 1.4% for the same period. High CO concentrations appear quite frequently in Beijing, but much less frequently in Moscow, except during the natu- ral fire events in summer 2010. From back trajectory analysis, the high CO concentration observed in Beijing can be attributed to the intensive CO emission sources in its surrounding areas.     

Joint use of TEM and MRS methods in a complex geological setting

Transient Electromagnetic (TEM), known also as Time Domain Electromagnetic (TDEM) and Magnetic Resonance Sounding (MRS) methods were applied jointly to investigate variations in lithology and groundwater salinity in the Nahal Hever South area (Dead Sea coast of Israel). The subsurface in this area is highly heterogeneous and composed of intercalated sand and clay layers over a salt formation, which is partly karstified. Groundwater is very saline, with a chloride concentration of 100–225 g/l. TEM is known as an efficient tool for investigating electrically conductive targets like saline water, but it is sensitive to both the salinity of groundwater and the porosity of rocks. MRS, however, is sensitive primarily to groundwater volume, but it also allows delineating of lithological variations in water-saturated formations. MRS is much less sensitive to variations in groundwater salinity in comparison with TEM. We show that MRS enables us to resolve the fundamental uncertainty in TEM interpretation caused by the equivalence between groundwater resistivity and lithology. Combining TEM and MRS, we found that the sandy Dead Sea aquifer filled with Dead Sea brine is characterized by a bulk resistivity of ρ_x > 0.4 Ωm, whereas zones with silt and clay in the subsurface are characterized by a bulk resistivity of ρ_x < 0.4 Ωm. These observations are confirmed by calibration of the TEM method performed near 18 boreholes.