Terrestrial organic matter biomarkers as tracers of Hg sources in lake sediments

Terrestrial organic matter (TOM) plays a key role in mercury (Hg) dynamics between watersheds and lakes. In this study we attempts to determine the role of TOM source and quality and not only quantity, in the fate and transport of total Hg (T-Hg) to boreal lakes. Integrating the watershed complexity is a daunting task. Within the scope of this project, we characterized this organic matter at a molecular level in order to determine Hg transfer conditions to the sediments. We sampled ten lakes in the Quebec boreal forest. In each lake, we took a sediment core at the deepest point in addition to analyzing T-Hg and a set of terrigenous biomarkers in recent sediments. Our results show no relationship between TOM quantity and T-Hg concentration in lake sediments. However, [T-Hg] variation is well explained by the increase of 3,5Bd/V ratios (R2 = 0.84; p\0.0002) and the decrease of C/V ratios (R2 = 0.5; p\0.0227). Our study shows that TOM source and quality are determinant for Hg loadings in lake sediments. More precisely, increasing TOM derived from humified soil horizons explains most of Hg level variation within sediments

Integrated transfers of terrigenous organic matter to lakes at their watershed level: A combined biomarker and GIS analysis

Terrigenous organic matter (TOM) transfer from a watershed to a lake plays a key role in contaminants fate and greenhouse gazes emission in these aquatic ecosystems. In this study, we linked physiographic and vegetation characteristics of a watershed with TOM nature deposited in lake sediments. TOM was characterized using lignin biomarkers as indicators of TOM sources and state of degradation. Geographical information system (GIS) also allowed us to integrate and describe the landscape morpho-edaphic characteristics of a defined drainage basin. Combining these tools we found a significant and positive relationship (R2 = 0.65, p < 0.002) between mean slope of the watershed and the terrigenous fraction estimated by Λ8 in recent sediments. The mean slope also correlated with the composition of TOM in recent sediments as P/(V + S) and 3,5Bd/V ratios significantly decreased with the steepness of the watersheds (R2 = 0.57, p < 0.021 and R2 = 0.71, p < 0.004, respectively). More precisely, areas with slopes comprised between 4° and 10° have a major influence on TOM inputs to lakes. The vegetation composition of each watershed influenced the composition of recent sediments of the sampled lakes. The increasing presence of angiosperm trees in the watershed influenced the export of TOM to the lake as Λ8 increased significantly with the presence of this type of vegetation (R2 = 0.44, p < 0.019). A similar relationship was also observed with S/V ratios, an indicator of angiosperm sources for TOM. The type of vegetation also greatly influenced the degradation state of OM. In this study, we were able to determine that low-sloped areas (0–2°) act as buffer zones for lignin inputs and by extension for TOM loading to sediments. The relative contribution of TOM from the soil organic horizons also increased in steeper watersheds. This study has significant implications in our understanding of the fate of TOM in lacustrine ecosystems

Electrically-tunable hole g-factor of an optically-active quantum dot for fast spin rotations

We report a large g-factor tunability of a single hole spin in an InGaAs quantum dot via an electric field. The magnetic field lies in the in-plane direction x, the direction required for a coherent hole spin. The electrical field lies along the growth direction z and is changed over a large range, 100 kV/cm. Both electron and hole g-factors are determined by high resolution laser spectroscopy with resonance fluorescence detection. This, along with the low electrical-noise environment, gives very high quality experimental results. The hole g-factor g_xh depends linearly on the electric field Fz, dg_xh/dFz = (8.3 +/- 1.2)* 10^-4 cm/kV, whereas the electron g-factor g_xe is independent of electric field, dg_xe/dFz = (0.1 +/- 0.3)* 10^-4 cm/kV (results averaged over a number of quantum dots). The dependence of g_xh on Fz is well reproduced by a 4x4 k.p model demonstrating that the electric field sensitivity arises from a combination of soft hole confining potential, an In concentration gradient and a strong dependence of material parameters on In concentration. The electric field sensitivity of the hole spin can be exploited for electrically-driven hole spin rotations via the g-tensor modulation technique and based on these results, a hole spin coupling as large as ~ 1 GHz is expected to be envisaged.Comment: 8 pages, 4 figure

High resolution coherent population trapping on a single hole spin in a semiconductor

We report high resolution coherent population trapping on a single hole spin in a semiconductor quantum dot. The absorption dip signifying the formation of a dark state exhibits an atomic physics-like dip width of just 10 MHz. We observe fluctuations in the absolute frequency of the absorption dip, evidence of very slow spin dephasing. We identify this process as charge noise by, first, demonstrating that the hole spin g-factor in this configuration (in-plane magnetic field) is strongly dependent on the vertical electric field, and second, by characterizing the charge noise through its effects on the optical transition frequency. An important conclusion is that charge noise is an important hole spin dephasing process

Transform-limited single photons from a single quantum dot

A semiconductor quantum dot mimics a two-level atom. Performance as a single photon source is limited by decoherence and dephasing of the optical transition. Even with high quality material at low temperature, the optical linewidths are a factor of two larger than the transform-limit. A major contributor to the inhomogeneous linewdith is the nuclear spin noise. We show here that the nuclear spin noise depends on optical excitation, increasing (decreasing) with increasing resonant laser power for the neutral (charged) exciton. Based on this observation, we discover regimes where we demonstrate transform-limited linewidths on both neutral and charged excitons even when the measurement is performed very slowly

Autocorrelation analysis for the unbiased determination of power-law exponents in single-quantum-dot blinking

We present an unbiased and robust analysis method for power-law blinking statistics in the photoluminescence of single nano-emitters, allowing us to extract both the bright- and dark-state power-law exponents from the emitters' intensity autocorrelation functions. As opposed to the widely-used threshold method, our technique therefore does not require discriminating the emission levels of bright and dark states in the experimental intensity timetraces. We rely on the simultaneous recording of 450 emission timetraces of single CdSe/CdS core/shell quantum dots at a frame rate of 250 Hz with single photon sensitivity. Under these conditions, our approach can determine ON and OFF power-law exponents with a precision of 3% from a comparison to numerical simulations, even for shot-noise-dominated emission signals with an average intensity below 1 photon per frame and per quantum dot. These capabilities pave the way for the unbiased, threshold-free determination of blinking power-law exponents at the micro-second timescale

Action du sulfite de sodium sur la concentration en composés organohalogénés et sur l'activité mutagène de solutions chlorées de substances humiques

Cette étude a eu pour but de déterminer l'effet d'un traitement par le sulfite de sodium sur la concentration en composés organohalogénés totaux (TOX) et sur l'activité mutagène de solutions chlorées de substances humiques d'origine aquatique (SHA), après avoir cherché à préciser l'influence du pH et du temps sur la concentration en TOX.Les résultats obtenus à partir d'échantillons chlorés de SHA en absence de chlore résiduel ont permis de mettre en évidence une diminution de la concentration en composés organohalogénés totaux, soit par stockage en milieu neutre ou basique, soit par addition de sulfite de sodium. L'intensité de cette réduction de la concentration en TOX augmente avec le pH, le temps de réaction et la dose de sulfite de sodium introduite.Les résultats obtenus à partir d'échantillons contenant du chlore libre indiquent que seule une déchloration totale avec un excès de sulfite de sodium peut conduire, en milieu neutre, à une diminution de l'activité mutagène et de la concentration en TOX des solutions diluées de SHA. La comparaison des pourcentages d'abattement obtenus sur le paramètre TOX et sur l'activité mutagène indique que la diminution de la génotoxicité par déchloration totale est due à l'action du sulfite sur des composés mutagènes non chlorés ou sur des composés chlorés fortement mutagènes et ne représentant qu'une très faible fraction du TOX.If is a well known tact that mimerous organohalogenated compounds are formed during the chlorination (preoxidation or final disinfection) of drinking water. Some of these compounds have been shown to be mutagenic. Recent studies have suggested that a treatment with oxygenated derivatives of SIV (SO2, NaHSO3 and Na2SO3) could reduce the genotoxicity of chlorinated drinking water.The general aim of Ibis study was to determine the effect of dechlorination treatments on the mutagenic activity of chlorinated drinking water. The following experiments were carried out in order to point out the effect of a treatment with sodium sulfite on the concentration of total organohalogenated compounds (TOX) and on the mutagenic activity of chlorinated dilute solutions of Aquatic Humic Substances (AHS).At first, the affects of pH, sodium sulfite dose and contact time on TOX concentration were investigated. Then, the importance of the dechlorination rate (partial or complete) on TOX concentration and also on the mutagenic activity could be studied.ExperimentalAquatic Humic Substances (natural mixture of fulvic and humic acids) were dissolved in phosphate-buffered ultra-pure water at 5 and 15 mg l-1 concentrations (pH 6.1 and 6.9 respectively). Stock solutions of chlorine were prepared in the laboratory and titrated by iodometry. Chlorination and dechlorination treatments were carried out in headspace-free baffles, at 20± 1 °C in the dark. Residual chlorine was determined by spectrophotometric measurements at 510 nm, following the calorimetric method using N,N-diethylphenylene-1,4-diamine (DPD). To avoid the slow oxidation of Slv into Svl by dissolved oxygen, the sodium sulfite solutions were prepared freshly before use. TOX concentrations were measured using a DOHRMAN DX-20 TOX analyser equipped with a MC-1 microcoulometric cell and with an AD-2 adsorption module. Before analysis, the residual chlorine was neutralized with sodium thiosulfate and samples were acidified to pH 1.4.The mutagenic activity was determined using acetone-dichloromethane extracts (AMBERLITE XAD-8 and XAD-2 resins) of the aqueous samples of chlorinated and dechlorinated solutions of AHS, acidified to pH 2.0 before extraction. The mutagenicity tests were carried out on TA 98 and TA 100 tester strains, following the method described by MARON and AMES (1983).Results-Effect of pH, addition of sodium sulfite and storage time on the TOX concentrationThe experiments carried out with dilute solutions of AHS ([AHS] = 5 mg 1-1; DOC = 2.5 mg Cl-1; pH = 6.1) showed a linear relationship between TOX production and chlorine consumption in the range 0-2.0 mg Cl2 l-1 (fig. 2).15 % of the chlorine demand was incorporated as organic chlorine in molecules.Experiments performed on solutions containing no residual free chlorine showed that organohatogenated compounds could be partially destroyed upon storage at neutral or basic pH (table 1). Reductions in TOX concentrations of 10 % at pH 6.1-8.5 in 24 hours and of 20 % at pH 11.5 in 2 hours were observed. This was enhanced by increasing the storage time.The addition of sodium sulfite (100 µmol l-1) in solutions containing no residual free chlorine significantly reduced the TOX concentration (10 % in 2 hours at pH 6.1-8.5; table 1). This reduction was enhanced by increasing sulfite dose and storage time and by increasing pH (30 % in 2 hours at pH 11.5). Furthermore, at a given pH value and for a reaction time of 2 hours, the decrease in TOX concentration was larger in presence of sulfite.- Effect of a dechlorination treatment on the TOX concentrationAs shown in figure 3, a dechlorination treatment (reduction of the residual free chlorine concentration) with sodium sulfite could significantly reduce the TOX concentration of the dilute solutions of AHS at pH 6.1 only if an excess of the dechlorinating agent was added. This effect was enhanced by increasing the excess of sulfite but nevertheless seemed to be limited (less than 15 % of reduction for the highest doses used; table 2).The free chlorine residuals measured after a 2 hours partial dechlorination confirmed the stoichiometric factor of 1 mole/mole for the reaction between chlorine and sodium sulfite.- Effect of a dechlorination treatment on the mutagenic activity and on the TOX concentrationThe dechlorination treatments were carried out on chlorinated dilute solutions of AHS ([AHS] = 15 mg l-1; DOC 7.5 mg C l-1; pH = 6.9). The TOX concentrations were measured on aqueous solutions and mutagenicity tests were performed on the corresponding acetone-dichloromethane extracts following a solvent exchange (dimethylsulfoxide). The results obtained showed again that only a total dechlorination treatment could reduce the TOX concentration of the aqueous chlorinated solutions and was able to destroy a significant part of the mutagenic activity of the extracts (table 3 and fig. 4).Although the effect of sulfite on TOX concentration seemed limited (less than 7 % reduction for the highest sulfite dose tested), the reduction in the genotoxicity was more important when the excess of sulfite was increased. No correlation between the TOX concentration and the mutagenic activity could be established. The mutagenic compounds destroyed by sodium sulfite do not appear to be organohalogenated ones. If they are, they are present at trace levels and thus are extremely patent and account for a very little part of the TOX concentration

Sub-wavelength surface IR imaging of soft-condensed matter

Outlined here is a technique for sub-wavelength infrared surface imaging performed using a phase matched optical parametric oscillator laser and an atomic force microscope as the detection mechanism. The technique uses a novel surface excitation illumination approach to perform simultaneously chemical mapping and AFM topography imaging with an image resolution of 200 nm. This method was demonstrated by imaging polystyrene micro-structures

Decoupling a hole spin qubit from the nuclear spins

A huge effort is underway to develop semiconductor nanostructures as low-noise hosts for qubits. The main source of dephasing of an electron spin qubit in a GaAs-based system is the nuclear spin bath. A hole spin may circumvent the nuclear spin noise. In principle, the nuclear spins can be switched off for a pure heavy-hole spin. In practice, it is unknown to what extent this ideal limit can be achieved. A major hindrance is that p-type devices are often far too noisy. We investigate here a single hole spin in an InGaAs quantum dot embedded in a new generation of low-noise p-type device. We measure the hole Zeeman energy in a transverse magnetic field with 10 neV resolution by dark-state spectroscopy as we create a large transverse nuclear spin polarization. The hole hyperfine interaction is highly anisotropic: the transverse coupling is <1% of the longitudinal coupling. For unpolarized, randomly fluctuating nuclei, the ideal heavy-hole limit is achieved down to nanoelectronvolt energies; equivalently dephasing times up to a microsecond. The combination of large and strong optical dipole makes the single hole spin in a GaAs-based device an attractive quantum platform