Investigation of sources and atmospheric transformation of carbonaceous aerosols from Shyamnagar, eastern Indo-Gangetic Plains: Insights from δ13C and carbon fractions.

This study reports the chemical characterization of the carbonaceous component of PM2.5 (particulate matter with aerodynamic diameter ≤2.5 µm) collected over a year-long campaign from a regional site in Shyamnagar, West Bengal, in the Indo-Gangetic Plains (IGP), India. The carbonaceous fractions (elemental and organic carbon), mass concentrations, and stable carbon isotopic composition (δ13C value) of aerosols were measured and utilized to characterize the sources and understand the atmospheric processing of aerosols. Cluster analysis, Potential Source Contribution Function (PSCF) modeling, and fire count data were analyzed to decipher the pattern of air masses, source contributions, and extent of burning activities. The PM2.5 mass concentrations were significantly higher during winter (168.3 ± 56.3 µg m-3) and post-monsoon (109.8 ± 59.1 µg m-3) compared to the monsoon (29.8 ± 10.7 µg m-3) and pre-monsoon (55.1 ± 23.0 µg m-3). Organic carbon (OC), elemental carbon (EC), and total carbon (TC) concentrations were also several factors higher during winter and post-monsoon compared to monsoon and pre-monsoon. The winter and post-monsoon experienced the impact of air masses from upwind IGP. On the other hand, long-range transported air masses from the South-West direction dominated during monsoon and pre-monsoon, which are also relatively cleaner periods. The average δ13C during post-monsoon and winter was ∼1‰ higher compared to monsoon and pre-monsoon. The vehicular exhaust and biomass/biofuel burning contributed dominantly in winter and post-monsoon. In comparison, lower δ13C in pre-monsoon and monsoon might be attributed to the dominance of biomass/biofuel combustion. Photochemical-induced aging of the anthropogenic aerosols resulted in a higher δ13C of TC in winter and post-monsoon, whereas the mixing of different local sources in pre-monsoon and monsoon resulted in lower δ13C values. These findings benefit policymakers in strategizing proper and effective management of biomass/biofuel burning in the IGP to minimize air pollution.

Tracing the predominant sources of carbon in PM2.5 using δ13C values together with OC/EC and select inorganic ions over two COALESCE locations.

As part of the COALESCE (Carbonaceous Aerosol Emissions, Source apportionment and Climate Impacts) campaign, ambient PM2.5 was collected at two regional sites (Bhopal and Mysuru) in India during 2019. We utilized organic carbon (OC), elemental carbon (EC) and water-soluble inorganic ions together with δ13C values, to better understand total carbon (TC) sources at these locations. The annual average δ13C values (-26.2 ± 0.6‰) at Mysuru and Bhopal (-26.6 ± 0.6‰) were comparable. However, at Mysuru, except during winter, day-to-day variability was much lower (narrow range of -26.8 to -26.0‰) than that at Bhopal (range: -28.1 to -24.7‰), suggesting that TC was contributed by few sources, likely dominated by vehicular emissions. Seasonal average δ13C values at Bhopal increased slightly (-25.8 ± 0.5‰) during the winter (Jan-Feb) and decreased (-27.0 ± 0.3‰) during the monsoon (Jun-Sep) season compared to the annual average. The decrease in δ13C values during the monsoon season was likely driven by enhanced secondary organic aerosol formation. Further, based on MODIS derived fire spots and back trajectories, we infered that the δ13C values (-27.5 to -26.0‰) in Bhopal during the post-monsoon season (Oct-Dec) were indicative of dominant biomass burning contributions. The inorganic ions/TC ratio during this season suggested that biomass burning aerosol was aged and may have been transported from crop residue burning in the Indo-Gangetic plains. At Mysuru, like the trend at Bhopal, the δ13C values during the monsoon season were lower than those during the winter season. Finally, δ13C values were input to a Bayesian model-MixSIAR to demonstrate the usefulness of such models in apportioning TC. In its simplest implementation, the model separated TC sources into fossil fuel emissions and non-fossil fuel sources . Fossil fuel combustion emissions accounted for 47 ± 19% and 62 ± 22% of the TC at Bhopal and Mysuru, respectively.

Variabilities of δ13C and carbonaceous components in ambient PM2.5 in Northeast India: Insights into sources and atmospheric processes.

A year-long sampling campaign of ambient PM2.5 (particulate matter with aerodynamic diameter ≤2.5 mm) at a regional station in the North-Eastern Region (NER) of India was performed to understand the sources and formation of carbonaceous aerosols. Mass concentration, carbon fractions (organic and elemental carbon), and stable carbon isotope ratio (δ13C) of PM2.5 were measured and studied along with cluster analysis and Potential Source Contribution Function (PSCF) modelling. PM2.5 mass concentration was observed to be highest during winter and post-monsoon seasons when the meteorological conditions were relatively stable compared to other seasons. Organic carbon (OC) concentration was more than two times higher in the post-monsoon and winter seasons than in the pre-monsoon and monsoon seasons. Air mass back trajectory cluster analysis showed the dominance of local and regional air masses during winter and post-monsoon periods. In contrast, long-range transported air masses influenced the background site in pre-monsoon and monsoon. Air mass data and PSCF analysis indicated that aerosols during winter and post-monsoon are dominated by freshly generated emissions from local sources along with the influence from regional transport of polluted aerosols. On the contrary, the long-range transported air masses containing aged aerosols were dominant during pre-monsoon. No significant variability was observed in the range of δ13C values (-28.2‰ to -26.4‰) during the sampled seasons. The δ13C of aerosols indicates major sources to be combustion of biomass/biofuels (C3 plant origin), biogenic aerosols, and secondary aerosols. The δ13C variability and cluster/PSCF modelling suggest that aged aerosols (along with enhanced photo-oxidation derived secondary aerosols) influenced the final δ13C during the pre-monsoon. On the other hand, lower δ13C in winter and post-monsoon is attributed to the freshly emitted aerosols from biomass/biofuels.

Understanding the origin of carbonaceous aerosols during periods of extensive biomass burning in northern India.

Post-harvest crop residue burning is extensively practiced in North India, which results in enhanced particulate matter (PM) concentrations. This study explores the PM2.5 (particulate matter with aerodynamic diameter ≤ 2.5 µm) emissions during various time periods (pre-monsoon, monsoon, and post-monsoon) over the biomass burning source region in Beas, Punjab. The PM2.5 concentrations during the pre-monsoon period (106-458 µg m-3) and the post-monsoon period (184-342 µg m-3) were similar but much higher than concentrations during the monsoon season (23-95 µg m-3) due to enhanced wet deposition. However, the carbonaceous aerosol fraction in PM2.5 was nearly double in the post-monsoon season (∼27%) than the pre-monsoon period (∼15%). A higher contribution of secondary organic carbon (SOC) observed during the pre-monsoon season can be attributed to enhanced photochemical activity in dry conditions. Stable carbon isotope ratio (δ13C value) of ambient PM allowed elucidation of contributing sources. δ13CTC correlation with SOC during post-monsoon and pre-monsoon periods suggests significant influence of secondary formation processes during both time periods. The concentrations of carbon fractions in sampled sources and aerosols suggests contribution of biofuels, resulting in enhanced PM concentration at this location. δ13CTC values of pre- and post-monsoon samples show dominance of freshly emitted aerosols from local sources. Impact of biomass and biofuel combustion was also confirmed by biomass burning K+BB tracer, indicating that major agriculture residue burning occurred primarily during nighttime. C3 plant derived aerosols dominated at the sampling location during the entire sampling duration and contributed significantly during the pre-monsoon season. Whereas, both fossil fuel and C3 plant combustion contributed to the total mass of carbonaceous aerosols during the post-monsoon and monsoon seasons.

Chemical characterization and stable nitrogen isotope composition of nitrogenous component of ambient aerosols from Kanpur in the Indo-Gangetic Plains.

Measurements of water-soluble total nitrogen (WSTN), water-soluble inorganic nitrogen (WSIN), water-soluble organic nitrogen (WSON) and ẟ15NTN (total N) was carried out on PM2.5 aerosol samples during wintertime to understand the major sources of ambient nitrogenous species at a heavily polluted location of Kanpur in north India. During the nighttime sampling campaign, WSON and NH4+_N contributed dominantly to the WSTN. Ammonium-rich condition persisted during sampling (NH4+/SO42- average equivalent mass ratio = 3.1 ± 0.7), suggesting complete neutralization of SO42- and formation of NH4NO3, which is stable in winter due to low temperature and high relative humidity (RH). Stagnant atmospheric conditions during wintertime enhanced concentrations of ionic species (SO42-, NH4+, and NO3-) at this location. Good correlations between NO3-_N, NH4+_N and biomass burning tracer K+BB (and also between NO3-_N, NH4+_N and SO42-) suggests a strong impact of biomass burning activities. Multi-linear regression (MLR) analysis shows a strong dependence of ẟ15N on NO3-_N, SO42- and WSON in night-1 (10:00 pm to 2:00 am) and on NO3-_N and SO42- in night-2 (2:00 am to 6:00 am) depicting different formation and removal mechanism of aerosols during both the time-periods. ẟ15NTN in PM2.5 varied from +8.8 to +15.5‰ (10.8 ± 1.3), similar to the variability observed for many urban locations in India and elsewhere. NH4+_N and WSON control the final ẟ15N value of nitrogenous aerosols. High relative humidity during nighttime enhanced the secondary organic aerosols formation due to aqueous-phase formation and gas to particle-phase partitioning. Isotopic fractionations associated with multi-phase reactions during gas to particle conversion of NH3 would result in an increase in ẟ15N by ~48‰ to 51‰ (at T of 5.4 °C to 15.4 °C) than that of the emission source(s), which indicates the most likely N-emission sources at Kanpur to be from agriculture activities and waste generation.

Root architecture traits variation and nitrate-influx responses in diverse wheat genotypes under different external nitrogen concentrations.

In order to identify the genetic variations in root system architecture traits and their probable association with high- and low-affinity nitrate transport system, we performed several experiments on a genetically diverse set of wheat genotypes grown under two external nitrogen levels (optimum and limited nitrate conditions) at two growth points of the seedling stage. Further, we also examined the nitrate uptake and its transport under different combinations of nitrate availability in the external media using 15N-labelled N-source (15NO3-), and gene expression pattern of different high- and low-affinity nitrate transporters. We observed that nitrate starvation invariably increases the total root size in all genotypes. However, the variation of component traits of total root size under nitrate starvation is genotype-specific at both stages. Further, we also observed genotypic variation in both nitrate uptake and translocation depending on the growth stage, external nitrate concentration and growing conditions. The expression of the TaNRT2.1 gene was invariably up-regulated under low external nitrate concentration; however, it gets reduced after a longer period (21 days) of starvation than the early stage (14 days). Among the four NRT1.1 orthologs, TaNPF6.3 and TaNPF6.4 consistently showed higher expression than TaNPF6.1 and TaNPF6.2 at higher nitrate concentration at both the growth stages. TaNPF6.3 and TaNPF6.4 apparently showed a feature of typical low-affinity nitrate transporter gene at higher external nitrate concentration at 14 and 21 days growth stages, respectively. The present study reveals the complex root system of wheat that has genotype-specific N-foraging along with highly coordinated high- and low-affinity nitrate transport systems for nitrate uptake and transport.

Tsunami records of the last 8000 years in the Andaman Island, India, from mega and large earthquakes: Insights on recurrence interval.

As many as seven tsunamis from the past 8000 years are evidenced by sand sheets that rest on buried wetland soils at Badabalu, southern Andaman Island, along northern part of the fault rupture of the giant 2004 Aceh-Andaman earthquake. The uppermost of these deposits represents the 2004 tsunami. Underlying deposits likely correspond to historical tsunamis of 1881, 1762, and 1679 CE, and provide evidence for prehistoric tsunamis in 1300-1400 CE, in 2000-3000 and 3020-1780 BCE, and before 5600-5300 BCE. The sequence includes an unexplained hiatus of two or three millennia ending around 1400 CE, which could be attributed to accelerated erosion due to Relative Sea-Level (RSL) fall at ~3500 BP. A tsunami in 1300-1400, comparable to the one in 2004, was previously identified geologically on other Indian Ocean shores. The tsunamis assigned to 1679, 1762, and 1881, by contrast, were more nearly confined to the northeast Indian Ocean. Sources have not been determined for the three earliest of the inferred tsunamis. We suggest a recurrence of 420-750 years for mega-earthquakes having different source, and a shorter interval of 80-120 years for large magnitude earthquakes.

Sorption and recovery of platinum from simulated spent catalyst solution and refinery wastewater using chemically modified biomass as a novel sorbent.

In this study, Lagerstroemia speciosa biomass modified by polyethylenimine (PEI-LS) was developed as a potential biosorbent for sorption and recovery of platinum(II) from platinum bearing waste solutions. Batch experiments were conducted to study the effect of various parameters on the sorption and recovery of platinum(II) using PEI-LS. The equilibrium time for platinum(II) sorption process was found to be 6 h. Both the sorption kinetics and sorption isotherm data fits pseudo second-order kinetic model and Langmuir isotherm, respectively. The maximum sorption capacity of platinum(II) onto PEI-LS at pH 2 for the studied temperature range (25-45 °C) is in the range of 122-154 mg/g. Evaluation of thermodynamic parameters suggests that the platinum(II) sorption is spontaneous and endothermic in nature. The regeneration of PEI-LS can be achieved using acidic thiourea as an eluent for recovery of platinum from the biosorbent. Fourier transform infrared (FT-IR) analysis suggests many functional groups were involved in platinum(II) sorption onto PEI-LS. Both the scanning electron microscope/energy dispersive spectroscopy (SEM/EDS) and X-ray photoelectron spectroscopy (XPS) analysis suggest a successful modification of raw biomass with PEI. The XPS analysis further concludes that platinum(II) sorption is governed by ion-exchange and co-ordination reaction. Finally, the PEI-LS was shown to recover ≥ 90% of platinum from two simulated solutions: the acid-leached spent catalyst solution and refinery wastewater. The biosorbent developed in this study is a low-cost and eco-friendly media that can be effectively used for platinum recovery from industrial wastewater.

Counter-intuitive influence of Himalayan river morphodynamics on Indus Civilisation urban settlements.

Urbanism in the Bronze-age Indus Civilisation (~4.6-3.9 thousand years before the present, ka) has been linked to water resources provided by large Himalayan river systems, although the largest concentrations of urban-scale Indus settlements are located far from extant Himalayan rivers. Here we analyse the sedimentary architecture, chronology and provenance of a major palaeochannel associated with many of these settlements. We show that the palaeochannel is a former course of the Sutlej River, the third largest of the present-day Himalayan rivers. Using optically stimulated luminescence dating of sand grains, we demonstrate that flow of the Sutlej in this course terminated considerably earlier than Indus occupation, with diversion to its present course complete shortly after ~8 ka. Indus urban settlements thus developed along an abandoned river valley rather than an active Himalayan river. Confinement of the Sutlej to its present incised course after ~8 ka likely reduced its propensity to re-route frequently thus enabling long-term stability for Indus settlements sited along the relict palaeochannel.

Removal of hexavalent chromium upon interaction with biochar under acidic conditions: mechanistic insights and application.

Chromium pollution of soil and water is a serious environmental concern due to potential carcinogenicity of hexavalent chromium [Cr(VI)] when ingested. Eucalyptus bark biochar (EBB), a carbonaceous black porous material obtained by pyrolysis of biomass at 500 °C under oxygen-free atmosphere, was used to investigate the removal of aqueous Cr(VI) upon interaction with the EBB, the dominant Cr(VI) removal mechanism(s), and the applicability to treat Cr(VI)-contaminated wastewater. Batch experiments showed complete removal of aqueous Cr(VI) at pH 1-2; sorption was negligible at pH 1, but ~55% of total Cr was sorbed onto the EBB surface at pH 2. Detailed investigations on unreacted and reacted EBB through Fourier transform infrared spectroscopy and X-ray photoelectron spectrometry (XPS) indicate that the carboxylic groups in biochar played a dominant role in Cr(VI) sorption, whereas the phenolic groups were responsible for Cr(VI) reduction. The predominance of sorption-reduction mechanism was confirmed by XPS studies that indicated ~82% as Cr(III) and ~18% as Cr(VI) sorbed on the EBB surface. Significantly, Cr(VI) reduction was also facilitated by dissolved organic matter (DOM) extracted from biochar. This reduction was enhanced by the presence of biochar. Overall, the removal of Cr(VI) in the presence of biochar was affected by sorption due to electrostatic attraction, sorption-reduction mediated by surface organic complexes, and aqueous reduction by DOM. Relative dominance of the aqueous reduction mechanism depended on a critical biochar dosage for a given electrolyte pH and initial Cr(VI) concentration. The low-cost EBB developed here successfully removed all Cr(VI) in chrome tanning acidic wastewater and Cr(VI)-contaminated groundwater after pH adjustment, highlighting its potential applicability in effective Cr(VI) remediation.

Photocatalytic reduction of organic pollutant under visible light by green route synthesized gold nanoparticles.

We report a rapid method of green chemistry approach for synthesis of gold nanoparticles (AuNPs) using Lagerstroemia speciosa leaf extract (LSE). L. speciosa plant extract is known for its effective treatment of diabetes and kidney related problems. The green synthesis of AuNPs was complete within 30min at 25°C. The same could also be achieved within 2min at a higher reaction temperature (80°C). Both UV-visible spectroscopy and transmission electron microscopy results suggest that the morphology and size distribution of AuNPs are dependent on the pH of gold solution, gold concentration, volume of LSE, and reaction time and temperature. Comparison between Fourier transform infrared spectroscopy (FT-IR) spectra of LSE and the synthesized AuNPs indicate an active role of polyphenolic functional groups (from gallotannins, lagerstroemin, and corosolic acid) in the green synthesis and capping of AuNPs. The green route synthesized AuNPs show strong photocatalytic activity in the reduction of dyes viz., methylene blue, methyl orange, bromophenol blue and bromocresol green, and 4-nitrophenol under visible light in the presence of NaBH4. The non-toxic and cost effective LSE mediated AuNPs synthesis proposed in this study is extremely rapid compared to the other reported methods that require hours to days for complete synthesis of AuNPs using various plant extracts. Strong and stable photocatalytic behavior makes AuNPs attractive in environmental applications, particularly in the reduction of organic pollutants in wastewater.

Error propagation in normalization of stable isotope data: a Monte Carlo analysis.

A higher analytical precision of a stable isotope ratio mass spectrometer does not automatically guarantee accurate determination of the true isotope composition (delta-value) of samples, since estimates of true delta-values are obtained from the normalization of raw isotope data. We performed both Monte Carlo simulations and laboratory experiments to investigate aspects of error propagation during the normalization of carbon stable isotope data. We found that increasing both the number of different reference standards and the number of repetitions of each of these standards reduces the normalization error. A 50% reduction in the normalization error can be achieved over the two-point normalization by either analyzing two standards four times each, or four standards two times each. If the true delta-value of a sample is approximately known a priori, the normalization error may then be reduced through a targeted choice of locally optimal standards. However, the difference in improvement is minimal and, therefore, a more practical strategy is to use two or more standards covering the whole stable isotope scale. The selection of different sets of standards by different laboratories or for different batches of samples in the same laboratory may lead to significant differences in the normalized delta-values of the same samples, leading to inconsistent results. Hence, the same set of standards should always be used for a particular element and a particular stable isotope analytical technique.

Presence and severity of COPD among patients attending cardiology OPD of a tertiary healthcare centre.

Chronic obstructive pulmonary disease (COPD) and ischaemic heart disease (IHD) are two very common entities, which often coexist due to common risk factors notably smoking. Though both are common causes of chronically poor health, only cardiovascular disease has got major health priority and government research funding. COPD is largely underdiagnosed and even unsuspected among cases of IHD. The present study wants to address this relatively unexplored area of magnitude of COPD among cases of IHD. A total of 86 (male-65, female-21) consecutive stable and ambulatory IHD patients diagnosed by cardiologist and fulfilling the criteria for inclusion, were selected from cardiology outpatient department of IPGME&R, Kolkata from January 2005 to August 2006. Associated COPD was found in 51.2% (n = 44) patients of the study group (males-36, females-8) according to GOLD criteria; 90.9% of cases of COPD had moderate to severe disease. This was much higher than the prevalence of COPD among general population. A positive correlation was found between severity of COPD and impaired left ventricular ejection fraction (EF). This study also shows that general perception about COPD is poor among patients and their physicians. Most of the COPD cases (81.8%) of IHD were newly detected in this study by spirometric evaluation. Use of inhaled bronchodilator among the previously diagnosed cases is also very low (15.9%). Awareness regarding coexistence of the two diseases may be helpful in management and reduction of mortality and morbidity of COPD in IHD.

Electron paramagnetic resonance (EPR) and stable isotope records of paleoenvironmental conditions during peat formation.

EPR spectroscopy was performed on four peat cores (1-2.5 m depth) collected from Yellowstone National Park (USA), Scotland (UK) and Lower Silesia (Poland) to study peat formation process. The stable free radicals identified in all investigated samples are semiquinone type and g-parameters range from 2.0030 to 2.0048. The highest g-values are characteristic of upper well-aerated peat layers and gradually decrease with depth. The lowest g-values are typical of relatively old fens and bogs where anaerobic conditions are expected and carbonization processes are advanced. The decrease in g-parameter value is connected with conjugation of semiquinone units with gradually augmented polyaromatic units in the peat substance. Generally the radical concentration increases with depth (0.05-5x10(17) spins/gram). However the g-values, line width parameters, and spin concentrations exhibit strong variations in some peat layers. Variation of these parameters observed for certain peat horizons correlate with the variation of carbon stable isotopic composition. For the old well-conserved peat deposits (e.g. Scotland/UK, approximately 5600 BP), variation of EPR parameters may be used to study paleo redox conditions.

Normalization of measured stable isotopic compositions to isotope reference scales--a review.

In stable isotope ratio mass spectrometry (IRMS), the stable isotopic composition of samples is measured relative to the isotopic composition of a working gas. This measured isotopic composition must be converted and reported on the respective international stable isotope reference scale for the accurate interlaboratory comparison of results. This data conversion procedure, commonly called normalization, is the first set of calculations done by the users. In this paper, we present a discussion and mathematical formulation of several existing routinely used normalization procedures. These conversion procedures include: single-point anchoring (versus working gas and certified reference standard), modified single-point normalization, linear shift between the measured and the true isotopic composition of two certified reference standards, two-point and multi-point linear normalization methods. Mathematically, the modified single-point, two-point, and multi-point normalization methods are essentially the same. By utilizing laboratory analytical data, the accuracy of the various normalization methods (given by the difference between the true and the normalized isotopic composition) has been compared. Our computations suggest that single-point anchoring produces normalization errors that exceed the maximum total uncertainties (e.g. 0.1 per thousand for delta(13)C) often reported in the literature, and, therefore, that it must not be used for routinely anchoring stable isotope measurement results to the appropriate international scales. However, any normalization method using two or more certified reference standards produces a smaller normalization error provided that the isotopic composition of the standards brackets the isotopic composition of unknown samples.

Delta13C analyses of calcium carbonate: Comparison between the GasBench and elemental analyzer techniques.

Measurements of stable carbon isotopic composition (delta13C) of carbonates or carbonate-rich soils are seldom performed in a continuous-flow isotope ratio mass spectrometer (IRMS) using an elemental analyzer (EA) as an online sample preparation device. Such analyses are routinely carried out with an external precision better than 0.1 per thousand using a GasBench II (GB) sample preparation device coupled online with a continuous-flow IRMS. In this paper, we report and compare delta13C analyses (86 total analyses) of calcium carbonates obtained by using both the GB and the EA. Using both techniques, the delta13C compositions of two in-house carbonate standards (MERCK carbonate and NR calcite) and ten selected carbonate-rich paleosol samples (of variable CaCO3 content) were analyzed, and data are reported in the VPDB scale calibrated against international standards, NBS 18 and 19. For the in-house standards analyzed by both techniques, a precision better than 0.08 per thousand is achieved. The analytical errors (1sigma) computed from multiple analyses of the delta13C of both the MERCK and NR obtained by the above two techniques are nearly identical. In general, the 1sigma (internal error) of paleosol analyses obtained in the GB is better than 0.06 per thousand, whereas that for the analyses in the EA (three repetitive analyses of the same sample) varies in the range 0.05-0.21 per thousand. However, for paleosols having more than 85% CaCO3, 1sigma is better than 0.15 per thousand (similar to the instrument precision), and in this case the delta13C(VPDB) of samples obtained by the GB is similar to that obtained by the EA. Our results suggest that the delta13C of pure calcium carbonate samples can also be analyzed using the EA technique.

Flushing time and storage effects on the accuracy and precision of carbon and oxygen isotope ratios of sample using the Gasbench II technique.

Continuous-flow isotope ratio mass spectrometry interfaced with a Gasbench II is used for automated and faster analyses of delta(13)C and delta(18)O in water, carbonate, and air samples that are accurate and highly precise. Prior to online chemistry and measurement using the Gasbench technique, rubber septa-capped glass vials are routinely flushed to remove air. Due to the small amounts of sample gas required for isotope analyses using current techniques, care should be taken to properly flush these vials to avoid contamination of sample gas with air. Our results indicate that isotopic composition of sample CO(2) gas remains constant when 10 mL vials are flushed (rate of 100 mL/min) for > or =600 s, whereas for vials flushed <600 s, the isotopic composition becomes substantially lighter with decreasing time of flushing, which affects the accuracy of analyses. This largely depends on the isotopic composition (and volume) of air that still remains after flushing. This effect is more pronounced on delta(18)O than on delta(13)C of sample CO(2) gas because there is very little carbon in the air. After 24 h storage in vials with punctured septa, both delta(13)C and delta(18)O of CO(2) become isotopically heavier compared with first day analyses, suggesting time-dependent changes in isotopic composition. The magnitude of shift depends on the concentration and the isotopic composition of CO(2) in laboratory air as well as on fractionation due to outflow of sample gas or inflow of air via punctured septa. Contamination of sample gas with air can be observed as a secondary peak on chromatograms that precedes sample peaks, and the intensity of these peaks depends on the amount of air. Such peaks are always present with short flushing times. For accuracy and better precision, irrespective of the magnitude of the secondary peaks, the analyses should be discarded if these appear in the chromatograms.

Modelling the isotopic evolution of the Earth.

We present a flexible multi-reservoir (primitive lower mantle, depleted upper mantle, upper continental crust, lower continental crust and atmosphere) forward-transport model of the Earth, incorporating the Sm-Nd, Rb-Sr, U-Th-Pb-He and K-Ar isotope-decay systematics. Mathematically, the model consists of a series of differential equations, describing the changing abundance of each nuclide in each reservoir, which are solved repeatedly over the history of the Earth. Fluxes between reservoirs are keyed to heat production and further constrained by estimates of present-day fluxes (e.g. subduction, plume flux) and current sizes of reservoirs. Elemental transport is tied to these fluxes through 'enrichment factors', which allow for fractionation between species. A principal goal of the model is to reproduce the Pb-isotope systematics of the depleted upper mantle, which has not been done in earlier models. At present, the depleted upper mantle has low (238)U/(204)Pb (mu) and (232)Th/(238)U (kappa) ratios, but Pb-isotope ratios reflect high time-integrated values of these ratios. These features are reproduced in the model and are a consequence of preferential subduction of U and of radiogenic Pb from the upper continental crust into the depleted upper mantle. At the same time, the model reproduces the observed Sr-, Nd-, Ar- and He-isotope ratios of the atmosphere, continental crust and mantle. We show that both steady-state and time-variant concentrations of incompatible-element concentrations and ratios in the continental crust and upper mantle are possible. Indeed, in some cases, incompatible-element concentrations and ratios increase with time in the depleted mantle. Hence, assumptions of a progressively depleting or steady-state upper mantle are not justified. A ubiquitous feature of this model, as well as other evolutionary models, is early rapid depletion of the upper mantle in highly incompatible elements; hence, a near-chondritic Th/U ratio in the upper mantle throughout the Archean is unlikely. The model also suggests that the optimal value of the bulk silicate Earth's K/U ratio is close to 10000; lower values suggested recently seem unlikely.