Uncertainty in seasonal snow reconstruction: Relative impacts of model forcing and image availability

There are many areas of uncertainty when solving the inverse problems of snow water equivalent (SWE) reconstruction. These include (i) the ability to infer the Final Date of the Seasonal Snow (FDSS) cover, particularly from remote sensing; (ii) errors in model forcing data (such as air temperature or radiation fluxes); and (iii) weaknesses in the snow model used for the reconstruction, associated with both the fidelity of the equations used to simulate snow processes (structural uncertainty) and the parameter values selected for use in the model equations. We investigate the trade-offs among these sources of uncertainty using 10,000 station-years worth of data from the western US SNOTEL network. Model structural and parameter uncertainty are eliminated by using a perfect model scenario i.e. comparing results to modelled control runs. The model was calibrated for each station-year to ensure that the model simulations reflect reality. Results indicate that for a temperature index model, a ±5 days error in FDSS gives a median −25%/+32% error in maximum SWE. A 1 °C air temperature bias produces a SWE error larger than a 5 days error in the FDSS for 50% of the 10,000 cases. Similarly, a 5 days error in FDSS could be accounted for by a net radiation error of 13 W m⁻² or less during the melt period, in 50% of cases. Mean absolute errors of 1 °C or more are typically reported in the literature for air temperature interpolations at high elevations. Observed solar radiation during the melt season can differ by 30 W m⁻² over relatively short distances, while estimates from reanalysis (NARR, ERA-Interim, MERRA, CFSRR) and GOES satellites typically span more than 40 W m⁻². Using data from both MODIS sensors (Terra & Aqua) at all snow covered points in the western US, a consecutive 5 days gap in imagery at time of FDSS is likely to occur only 5–10% of the time. This work shows that errors in model forcing data are at least as important, if not more, than image availability when reconstructing SWE.     

Variability of equatorial ionospheric electron density at fixed heights below the F2 peak

A study on variability of the equatorial ionosphere was carried out at fixed heights below the F2 peak for two different levels of solar activity. The study covered height range of 100 km up to the peak of F2 layer using a real height step increase of 10 km. The variability index used is the percentage ratio of standard deviation over the average value for the month. Daytime minimum variability of between 3% and 10% was observed at height range of about 150–210 km during low solar activity and between 2% and 7% at height range of 160–220 km during high solar activity. The nighttime maximum of between 70% and 187% was observed at height range of about 210–250 km during low solar activity and between 42% and 127% at height range of 210–250 km during high solar activity. The height range at which daytime minimum was observed falls within the F1 height of the ionosphere. The result obtained is consistent with previous works carried out in the low latitude locations for American sector.     

Modeling of the river ecological status with macrophytes using artificial neural networks

Biomonitoring methods based on macrophytes have been used mandatorily in the assessment of freshwaters since the implementation of the Water Framework Directive (WFD). The Macrophyte Index for Rivers (MIR) was developed in Poland for the monitoring of running waters under the WFD requirements. This index shows the degree of river degradation under the influence of water pollutants, especially nutrients. The aim of the present study was to determine the relationship between the MIR and various hydrochemical parameters using artificial neural networks (ANNs). Physico-chemical parameters of water (monthly results for the whole year), which were derived from 147 lowland river survey sites, all located in Poland, were applied to model the MIR values. Water quality variables were determined over three timeframes: the annual average; the average for the vegetation period; and the average for the summer period. Quality of the networks was assessed using coefficient of determination (R²), Nash-Sutcliffe efficiency (NSE) and root mean square error (RMSE). The best modeling quality was obtained for yearly average values of water quality parameters. The quality statistics were: R² = 0.722, NSE = 0.721 and RMSE = 0.056 (training dataset); R² = 0.555, NSE = 0.533 and RMSE = 0.101 (validation dataset); R² = 0.650. NSE = 0.600 and RMSE = 0.089 (testing dataset). This indicates that macrophytes reflect the whole year impact of pollution, whereas summer.     

Evolution of erythemal and total shortwave solar radiation in Valladolid,Spain: Effects of atmospheric factors

Erythemal ultraviolet (UVER; 280–400 nm) and total shortwave (SW; 305–2800 nm) solar irradiances were recorded from 2000 to 2009 in Valladolid, Spain. UVER and SW values under cloudless conditions are simulated by radiative transfer (TUV 4.6) and empirical models. These model estimations are tested with experimental measurements showing a great agreement (root mean square error around 7%). The aerosol effect on UVER irradiance is determined through a model study. UVER radiation and total ozone column (TOC) temporal evolutions show a negative relationship. TOC accounts for 80% of UVER variance and its radiation amplification factor is 1.1 at zenith of 65°. Cloud effects on solar radiation are shown and quantified by the cloud modification factor. Moreover the enhancement effect cases are analysed. SW radiation proves more sensitive to clouds than UVER. Clouds are seen to attenuate and enhance solar radiation by up to 93% and 22% in the UVER range, respectively.     

A temperature-dependent multi-relaxation spectroscopic dielectric model for thawed and frozen organic soil at 0.05–15 GHz

A dielectric model for thawed and frozen Arctic organic-rich soil (50% organic matter) has been developed. The model is based on soil dielectric measurements that were collected over ranges of gravimetric moisture from 0.03 to 0.55 g/g, dry soil density from 0.72 to 0.87 g/cm³, and temperature from 25 to −30 °C (cooling run) in the frequency range of 0.05–15 GHz. The refractive mixing dielectric model was applied with the Debye multi-relaxation equations to fit the measurements of the soil’s complex dielectric constant as a function of soil moisture and wave frequency. The spectroscopic parameters of the dielectric relaxations for the bound, transient bound, and unbound soil water components were derived and were complimented by the thermodynamic parameters to obtain a complete set of parameters for the proposed temperature-dependent multi-relaxation spectroscopic dielectric model for moist soils. To calculate the complex dielectric constant of the soil, the following input variables must be assigned: (1) density of dry soil, (2) gravimetric moisture, (3) wave frequency, and (4) temperature. The error of the dielectric model was evaluated and yielded RMSE_ε′ values of 0.348 and 0.188 for the soil dielectric constant and the loss factor, respectively. These values are on the order of the dielectric measurement error itself. The proposed dielectric model can be applied in active and passive microwave remote sensing techniques to develop algorithms for retrieving the soil moisture and the freeze/thaw state of organic-rich topsoil in the Arctic regions.     

Solar activity imprints in tree ring width from Chile (1610–1991)

We have investigated the solar activity signal in tree ring data from two locations in Chile. The tree ring time series extended over a period of ∼400 yr. Spectral and wavelet analysis techniques were employed. We have found evidence for the presence of the solar activity Schwabe (∼11 yr), Hale (∼22 yr), fourth-harmonic of the 208-yr Suess cycle (∼52 yr) and Gleissberg (∼80 yr) cycles. The Gleissberg cycle of tree ring data is in anti-phase with solar activity. Wavelet and cross-wavelet techniques revealed that the periods found are intermittent, possibly because solar activity signals observed in tree rings are mostly due to solar influence on local climate (rainfall, temperature, and cloud cover) where trees grow up. Further, cross-wavelet analysis between sunspot and tree ring time series showed that the cross power around the 11 yr solar cycle is more significant during periods of high solar activity (grand maximum) than during periods of low solar activity (grand minimum). As Glaciar Pio XI is practically at the Pacific Ocean level, the tree-ring response may be stronger due to the heating of the Pacific Ocean water following an increase of the solar radiation incidence rather than at the higher altitudes of Osorno region.     

The statistical and numerical study of the global distribution of coronal plasma and magnetic field near 2.5 Rs over a 10-year period

The basic characteristics of the global distribution for the corona plasma and magnetic field near 2.5 Rs are analyzed with the statistical and numerical methods for 136 Carrington Rotations (CRs) covering four different phases of solar activity. By using the observational data and the velocity distribution model in the corona, the statistical average distribution of the magnetic field, density and the coronal mass outputs are analyzed for the four different phases. Then, a numerical study of the global distribution near 2.5 Rs has been made by solving a self-consistent MHD system. Finally, the solar wind speed at 1 AU is given by mapping the speed at 2.5 Rs to that near 1 AU, and the comparison of the numerical results with the observational measurements and the simulation result of the Wang–Sheeley–Arge (WSA) model are made during more than 5 years. The numerical results indicate that the global distributions on the source surface of 2.5 Rs at different phases of solar activity could be used to predict the change of the solar wind in interplanetary space.     

Long-term thermospheric neutral wind observations over the northern polar cap

We study the solar dependence of the thermospheric dynamics based on more than 20 years Fabry–Perot interferometer O 6300 Å emission observation of polar cap thermospheric wind from three stations: Thule (76.53°N, 68.73°W, MLAT 86N), Eureka (80.06°N, 86.4°W, MLAT 89N), and Resolute (74.72°N, 94.98°W, MLAT 84N) in combination with the National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model (NCAR-TIEGCM). All three stations showed a dominant diurnal oscillation in both the meridional and zonal components, which is a manifestation of anti-sunward thermospheric wind in the polar cap. The three-station observations and the TIEGCM simulation exhibit varying degree of correlations between the anti-sunward thermospheric wind and solar F10.7 index. The diurnal oscillation is stronger at Eureka (∼150 m/s) than that at Resolute (∼100 m/s) according to both observations and TIEGCM simulation. The semidiurnal oscillation is stronger at Resolute (∼20 m/s) than that at Eureka based (∼10 m/s) on data and model results. These results are consistent with a two-cell convection pattern in the polar cap thermospheric winds. The Thule results are less consistent between the model and observations. The simulated meridional wind diurnal and semidiurnal oscillations are stronger than those observed.     

The origins and concentrations of water,carbon, nitrogen and noble gases on Earth

The isotopic compositions of terrestrial hydrogen and nitrogen are clearly different from those of the nebular gas from which the solar system formed, and also differ from most of cometary values. Terrestrial N and H isotopic compositions are in the range of values characterizing primitive meteorites, which suggests that water, nitrogen, and other volatile elements on Earth originated from a cosmochemical reservoir that also sourced the parent bodies of primitive meteorites. Remnants of the proto-solar nebula (PSN) are still present in the mantle, presumably signing the sequestration of PSN gas at an early stage of planetary growth. The contribution of cometary volatiles appears limited to a few percents at most of the total volatile inventory of the Earth. The isotope signatures of H, N, Ne and Ar can be explained by mixing between two end-members of solar and chondritic compositions, respectively, and do not require isotopic fractionation during hydrodynamic escape of an early atmosphere.The terrestrial inventory of ⁴⁰Ar (produced by the decay of ⁴⁰K throughout the Earth's history) suggests that a significant fraction of radiogenic argon may be still trapped in the silicate Earth. By normalizing other volatile element abundances to this isotope, it is proposed that the Earth is not as volatile-poor as previously thought. Our planet may indeed contain up to ~ 3000 ppm water (preferred range: 1000–3000 ppm), and up to ~ 500 ppm C, both largely sequestrated in the solid Earth. This volatile content is equivalent to an ~ 2 (± 1) % contribution of carbonaceous chondrite (CI-CM) material to a dry proto-Earth, which is higher than the contribution of chondritic material advocated to account for the platinum group element budget of the mantle. Such a (relatively) high contribution of volatile-rich matter is consistent with the accretion of a few wet planetesimals during Earth accretion, as proposed by recent dynamical models.The abundance pattern of major volatile elements and of noble gases is also chondritic, with two notable exceptions. Nitrogen is depleted by one order of magnitude relative to water, carbon and most noble gases, which is consistent with either N retention in a mantle phase during magma generation, or trapping of N in the core. Xenon is also depleted by one order of magnitude, and enriched in heavy isotopes relative to chondritic or solar Xe (the so-called “xenon paradox”). This depletion and isotope fractionation might have taken place due to preferential ionization of xenon by UV light from the early Sun, either before Earth's formation on parent material, or during irradiation of the ancient atmosphere. The second possibility is consistent with a recent report of chondritic-like Xe in Archean sedimentary rocks that suggests that this process was still ongoing during the Archean eon (Pujol et al., 2011). If the depletion of Xe in the atmosphere was a long-term process that took place after the Earth-building events, then the amounts of atmospheric ¹²⁹Xe and ^131–136Xe, produced by the short-lived radioactivities of ¹²⁹I (T_1/2 = 16 Ma) and ²⁴⁴Pu (T_1/2 = 82 Ma), respectively, need to be corrected for subsequent loss. Doing so, the I–Pu–Xe age of the Earth becomes ≤ 50 Ma after start of solar system formation, instead of ~ 120 Ma as computed with the present-day atmospheric Xe inventory.     

Numerical modeling of the light ion trough and heat balance of the topside ionosphere in quiet geomagnetic conditions

The results from the numerical calculations of the global distribution of topside ionospheric parameters such as H⁺ ions and ion and electron temperatures up to 1500 km height are presented for equinoctial conditions at solar minimum. Calculations are carried out using the Global Self-consistent Model of Thermosphere, Ionosphere and Protonosphere (GSM TIP) developed in WD IZMIRAN, and using a new calculation block for electric fields due to dynamo and of magnetospheric origin. A comparison of two sets of calculations of magnetospheric convection electric field for a given potential difference is carried out, one through polar caps and other through field aligned currents of first zone. It is shown that the distribution of the electric potential obtained through field aligned currents of first zone is more self-consistent than that through polar caps. The light ion trough in H⁺ ions is deeper and occupies larger region for the potential difference through polar cap. For a given potential difference through field aligned current, at 1500 km, the maximum ion temperature is 150 K higher, minimum ion temperature is 200 K lower and maximum electron temperature is 100 K higher than those obtained for the same potential difference through polar caps. It is concluded that for modeling the electric field of magnetospheric origin, it is necessary to use the potential difference through field aligned current of first zone instead of through polar caps.     

Temperature trends in the middle atmosphere as seen by historical Russian rocket launches: Part 2, Heiss Island (80.6°N, 58°E)

Long-term changes of the temperature of the middle atmosphere are investigated using a data bank obtained by Russian rocketsondes at Heiss Island (80.6°N, 58°E). The major interest of the data series is that it is one of the longest and uninterrupted records obtained at high latitudes in the northern hemisphere over 25 years, from 1969 to 1994. Previous estimates using this dataset has shown the largest trends. The revised analysis performed here took into account all possible discontinuities in the data series, such as a change in the time-of-measurement, T₀, and in the type of sensor. For this purpose, some data were filtered out, and a statistical model based on multiple regression analyses included step functions to take into account such discontinuities. The temperature responses to different sources of variability (solar activity, volcanic aerosols) were retrieved for summer and winter periods. The response to the 11-year solar cycle in the winter period is found to be largely positive in the stratosphere (∼+4 K) and largely negative in the mesosphere (∼−8 K), with a smaller and opposite response in summer. This response depends on the phase of the QBO, as already shown by previous studies. The response to volcanic aerosols is found to be significantly positive in the upper mesosphere, in good agreement with numerical simulations and with observations above France. The long-term trend resulting from this reanalysis indicates a cooling of the middle atmosphere, increasing with altitude from −2 K/decade at 40 km to a maximum of −6 K/decade around 65 km. This result is slightly larger than the trend observed at mid-latitude but quite smaller than previous estimates.     

The JB2006 empirical thermospheric density model

A new empirical atmospheric density model is developed using the CIRA72 (Jacchia 71) model as the basis for the diffusion equations. New solar indices based on orbit-based sensor data are used for the solar irradiances in the extreme and far ultraviolet wavelengths. New exospheric temperature and semiannual density equations are employed to represent the major thermospheric density variations. Temperature correction equations are also developed for diurnal and latitudinal effects, and finally density correction factors are used for model corrections required at high altitude (1500–4000 km). The new model, Jacchia–Bowman 2006, is validated through comparisons of accurate daily density drag data previously computed for numerous satellites. For 400 km altitude the standard deviation of 16% for the standard Jacchia model is reduced to 10% for the new JB2006 model for periods of low geomagnetic storm activity.     

Radiogenic heat production in the lithosphere of Sulu ultrahigh-pressure metamorphic belt

The Chinese Continental Scientific Drilling (CCSD) project is located at the Sulu ultrahigh-pressure metamorphic (UHPM) belt. It offers a unique opportunity for studying the radiogenic heat production of both shallower and deeper rocks. Based on the concentrations of radiogenic elements U, Th and K on 349 samples from main hole of CCSD (CCSD MH), pilot holes and exposures, we determined radiogenic heat productions of all major rock types in the Sulu UHPM belt. Results show the mean values of orthogneiss and paragneiss are respectively 1.65 ± 0.81 and 1.24 ± 0.61 µW m^? 3. Due to different composition and grade of retrogressive metamorphism, the eclogites display significant scatter in radiogenic heat production, ranging from 0.01 to 2.85 µW m^? 3, with a mean of 0.44 ± 0.55 µW m^? 3. The radiogenic heat production in ultramafic rocks also varies within a large range of 0.02 to 1.76 µW m^? 3, and the average turns out to be 0.18 ± 0.31 µW m^? 3. Based on the measurements and crustal petrologic model, the vertical distribution model of heat production in Sulu crust is established. The resulting mean heat production (0.76 µW m^? 3) contributes 24 mW m^? 2 to the surface heat flow. 1-D thermal model indicates that the temperature at the Moho reaches above 750 °C, and the thermal thickness of the lithosphere is ~ 75 km, in good agreement with the geophysical results. The high teat flow (~ 75 mW m^? 2) together with thin lithosphere presents strong support for the extension events during the late Cretaceous and Cenozoic.     

Solar irradiance and total ozone over El Arenosillo (Spain) during the solar eclipse of 3 October 2005

The ground track of the annular eclipse of 3 October 2005 crossed the Iberian Peninsula. The main objective of this work was to analyze the variability of the solar irradiance and the total ozone column during the course of this event at El Arenosillo (Southwestern Spain). For achieving this goal, two Kipp & Zonen broadband radiometers (one for measuring total solar irradiance and other for measuring ultraviolet erythemal solar irradiance), one NILU-UV multi-band instrument and one Brewer spectroradiometer were used in this work. Total irradiance (310–2800 nm), and ultraviolet erythemal radiation (UVER) were recorded at a high frequency of 5 s, showing a strong reduction (higher than 80%) of the irradiance at the maximum solar obscuration which was of 79.6%. The irradiance decrease during the course of the eclipse was positively correlated with the percentage of eclipse obscuration, showing a very high agreement (R²～0.99). The irradiance recorded at selected wavelengths from the NILU-UV instrument shows a more pronounced decrease in the UV irradiance at the lower wavelengths during the solar eclipse. Finally, the evolution of the total ozone column (TOC) derived from Brewer and NILU instruments during the eclipse presented an opposite behavior: while the Brewer derived TOC values increase about 15 DU, the NILU derived TOC values decrease about 11 DU. This opposite behavior is mainly related to an artifact in the spectral irradiances recorded by the two instruments.     

Modeling the responses of the middle latitude ionosphere to solar flares

In this paper, we investigate the solar flare effects of the ionosphere at middle latitude with a one-dimensional ionosphere theoretical model. The measurements of solar irradiance from the SOHO/Solar EUV Monitor (SEM) and GOES satellites have been used to construct a simple time-dependent solar flare spectrum model, which serves as the irradiance spectrum during solar flares. The model calculations show that the ionospheric responses to solar flares are largely related to the solar zenith angle. During the daytime most of the relative increases in electron density occur at an altitude lower than 300 km, with a peak at about 115 km, whereas around sunrise and sunset the strongest ionospheric responses occur at much higher altitudes (e.g. 210 km for a summer flare). The ionospheric responses to flares in equinox and winter show an obvious asymmetry to local midday with a relative increase in total electron content (TEC) in the morning larger than that in the afternoon. The flare-induced TEC enhancement increases slowly around sunrise and reaches a peak at about 60 min after the flare onset.     

Hf and Nd isotopes in marine sediments: Constraints on global silicate weathering

The combined use of Lu–Hf and Sm–Nd isotope systems potentially offers a unique perspective for investigating continental erosion, but little is known about whether, and to what extent, the Hf–Nd isotope composition of sediments is related to silicate weathering intensity. In this study, Hf and Nd elemental and isotope data are reported for marine muds, leached Fe-oxide fractions and zircon-rich turbidite sands collected off the Congo River mouth, and from other parts of the SE Atlantic Ocean. All studied samples from the Congo fan (muds, Fe-hydroxides, sands) exhibit indistinguishable Nd isotopic composition (ε_Nd ~ ? 16), indicating that Fe-hydroxides leached from these sediments correspond to continental oxides precipitated within the Congo basin. In marked contrast, Hf isotope compositions for the same samples exhibit significant variations. Leached Fe-hydroxide fractions are characterized by ε_Hf values (from ? 1.1 to + 1.3) far more radiogenic than associated sediments (from ? 7.1 to ? 12.0) and turbidite sands (from ? 27.2 to ? 31.6). ε_Hf values for Congo fan sediments correlate very well with Al/K (i.e. a well-known index for the intensity of chemical weathering in Central Africa). Taken together, these results indicate that (1) silicate weathering on continents leads to erosion products having very distinctive Hf isotope signatures, and (2) a direct relationship exists between ε_Hf of secondary clay minerals and chemical weathering intensity.These results combined with data from the literature have global implications for understanding the Hf–Nd isotope variability in marine precipitates and sediments. Leached Fe-hydroxides from Congo fan sediments plot remarkably well on an extension of the ‘seawater array’ (i.e. the correlation defined by deep-sea Fe–Mn precipitates), providing additional support to the suggestion that the ocean Hf budget is dominated by continental inputs. Fine-grained sediments define a diffuse trend, between that for igneous rocks and the ‘seawater array’, which we refer to as the ‘zircon-free sediment array’ (ε_Hf = 0.91 ε_Nd + 3.10). Finally, we show that the Hf–Nd arrays for seawater, unweathered igneous rocks, zircon-free and zircon-bearing sediments (ε_Hf = 1.80 ε_Nd + 2.35) can all be reconciled, using Monte Carlo simulations, with a simple weathering model of the continental crust.     

Prediction of sunspot number amplitude and solar cycle length for cycles 24 and 25

The prediction of solar activity strength for solar cycles 24 and 25 is made on the basis of extrapolation of sunspot number spectral components. Monthly sunspot number data during the 1850–2007 interval (solar cycles 9–23) are decomposed into several levels and searched for periodicities by iterative regression in each level. For solar cycle 24, the peak is predicted in November 2013 with a sunspot number of 113.3. The cycle is expected to be weak, with a length of 133 mo (months) or 11.1 yr. The sunspot number maximum in cycle 25 is predicted to occur in April 2023 with a sunspot number 132.1 and a solar cycle length of 118 mo or 9.8 yr. Thus, solar cycle 24 is predicted to have an intensity 23% lower than cycle 23, and cycle 25 will be 5% lower than cycle 23.     

Slope angle and aspect as influencing factors on the accuracy of the SRTM and the ASTER GDEM databases

Presently, the application of digital elevation or surface models have increasing relevance in all areas of scientific research and in practical engineering applications. The ASTER GDEM and SRTM databases are the most widely used digital surface models, due to their free accessibility and global coverage. The SRTM model was produced using a radar-based technique and the ASTER GDEM was developed using optical stereo image-pairs. Therefore, as all models contain errors (i.e. differences stemming from real surface or vertical biases), errors in these models will also differ. Our aim was to examine these vertical biases and to calculate the rate of error variance. A TIN (Triangulated Irregular Network) model was used as a reference surface, derived from the contour lines of a large scale topographic map. Errors were evaluated with statistical and geoinformation techniques. We discovered significant differences between the surfaces. The mean difference between topographic elevations minus the SRTM-V2 is +2.6 ± 4 m, while the mean difference between topographic elevations minus the SRTM-V3 is +2.7 ± 2.5 m. With the GDEM, the mean difference was 2.7 ± 9.1 m. Furthermore, we found that in the case of SRTM, the differences were significant considering the aspects and the steepness of the slopes: southern and eastern directions and larger slope angles showed greater differences compared to the reference data. The GDEM V2 DEM had a larger error variance, but the error did not vary significantly with slope angle.     

Luminescence dating of archaeosediments: A comparison of K-feldspar and plagioclase IRSL ages

IRSL single-aliquot dating of three feldspar fractions from a Howieson's Poort (SA) industry site has been investigated. In the case of the plagioclase fraction (ρ > 2.62 g cm^?3) equivalent dose is similar to that of the other fractions (i.e. ρ < 2.58 g cm^?3, high-K-feldspar; and 2.58 < ρ < 2.62 g cm^?3; low-K-feldspar). However, the g values measured for this fraction is such that the fading-corrected age is overestimated. The K content of each fraction was estimated by SEM. The DRC-corrected ages for the two lighter feldspar fractions are consistent with the archaeological interpretation. This study shows that reliable single-aliquot luminescence ages can be obtained using K-feldspar and Na-plagioclase.     

Primitive neon and helium isotopic compositions of high-MgO basalts from the Kerguelen Archipelago,Indian Ocean

The geochemical characteristics of mildly alkalic basalts (24–25 Ma) erupted in the southeastern Kerguelen Archipelago are considered to represent the best estimate for the composition of the enriched Kerguelen plume end-member. A recent study of picrites and high-MgO basalts from this part of the archipelago highlighted the Pb and Hf isotopic variations and suggested the presence of mantle heterogeneities within the Kerguelen plume itself. We present new helium and neon isotopic compositions for olivines from these picrites and high-MgO basalts (6–17 wt.% MgO) both to constrain the enriched composition of the Kerguelen plume and to determine the origin of isotopic heterogeneities involved in the genesis of Kerguelen plume-related basalts. The olivine phenocrysts have extremely variable ⁴He / ³He compositions between MORB and primitive values observed in OIB (∼90,000 to 40,000; i.e., R / R_a ∼8 to 18) and they show primitive neon isotopic ratios (average ²¹Ne / ²¹Ne_ext ∼0.044). The neon isotopic systematics and the ⁴He / ³He ratios that are lower than MORB values for the Kerguelen basalts clearly suggest that the Kerguelen hotspot belongs to the family of primitive hotspots, such as Iceland and Hawaii. The rare gas signature for the Kerguelen samples, intermediate between MORB and solar, is apparently inconsistent with mixing of a primitive component with a MORB-like source, but may result from sampling a heterogeneous part of the mantle with solar ³He / ²²Ne and with a higher (U, Th) / ³He ratio compared to typically high R / R_a hotspot basalts such as those from Iceland and Hawaii.