Debajyoti Paul

Thorium-uranium systematics require layered mantle convection

Two of the most important sources of information on the style of mantle convection come from geochemical constraints and mantle tomography. Constraints imposed by uranium-thorium systematics are examined in this paper. Observations require that the mid-ocean ridge basalt (MORB) source region be nearly homogeneous with a thorium-uranium ratio κ = 2.5±0.1. Simple mass balance considerations utilizing limits on the value of κ for the continental crust (κ<6) and a bulk earth value κ = 4.0±0.2 with heat production constraints show that the whole mantle has a thorium-uranium ratio greater than κ = 3.35±0.1. This leads to the conclusion that a substantial fraction of the mantle has a much more primitive value of κ (closer to 4.0) than the MORB source region. Our model assumes a near-uniform, depleted upper mantle that is the source region for MORB and has κ ≈ 2.5 and a near-primitive lower mantle with κ ≈ 4.0. Our results indicate that it is very difficult to have the upper mantle reservoir larger than 65% of the whole mantle and our preferred value is 45%. Three alternative hypotheses can be proposed: (1) A vertical stratification of κ through the mantle (since the role of whole mantle convection is to extract heat, the resulting vertical transport in the mantle would be expected to eliminate the vertical stratification of κ), (2) a mantle permeated with blobs (since the blobs would have to constitute some 65±10% of the mantle and be absent from the MORB source region, it is difficult to support this hypothesis), and (3) layered mantle convection with a compositional barrier to convection in the lower mantle. The results given here strongly favor this layered mantle hypothesis as recently set forth by Kellogg et al. [1999] and van der Hilst and Karason [1999].

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.

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 238U/204Pb (μ) and 232Th/238U (κ) 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 Earths K/U ratio is close to 10000; lower values suggested recently seem unlikely.

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.

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.The Bronze-age Indus civilisation (4.6–3.9 ka) was thought to have been linked to the development of water resources in the Himalayas. Here, the authors show that along the former course of the Sutlej River the Indus settlements developed along the abandoned river valley rather than an active Himalayan river.

Stable Carbon Isotope and Bulk Composition of Wintertime Aerosols from Kanpur

This study assesses stable carbon isotopic composition (δ13C) of total carbon (TC) in ambient aerosols (PM2.5) during wintertime (December 2014) from Kanpur (26.30 °N, 80.14 °E) in northern India. Chemical constituents viz organic carbon (OC), elemental carbon (EC) and water-soluble ions in PM2.5 have also been measured. Back trajectories of air masses arriving at the sampling site (Centre for Environmental Science and Engineering, IIT Kanpur) have been utilized to infer the air-mass transport. Most of the trajectories showed their origin from northwestern region during the study period. Average PM2.5 and TC concentrations were centered around 240 μg m−3 and 91 μg m−3, respectively. The OC + EC concentrations averaged at 58 ± 15 μg m−3. Significant linear correlation between OC and EC in conjunction with high OC/EC ratio (9 to 12) suggests dominance of anthropogenic combustion sources of organic aerosols. Concentration of anthropogenic ionic species (SO4 2− + NO3 − + NH4 +) averaged at 46.74 μg m−3. The average δ13C values of TC in the integrated 24-h samples were centered around −25‰. Integrated data analyses of chemical constituents and stable C isotope suggests the influence of mixed emission sources. Future studies are required to better constraint the observations.

Nature and composition of interbedded marine basaltic pumice in the ∼52–50 Ma Vastan lignite sequence, western India: Implication for Early Eocene MORB volcanism offshore Arabian Sea

The recognition of pyroclasts preserved in sedimentary environments far from its source is uncommon. We here describe occurrences of several centimetres-thick discontinuous basaltic pumice lenses occurring within the Early Eocene Vastan lignite mine sedimentary sequence, western India at two different levels – one at ∼5 m and the other at 10 m above a biostratigraphically constrained 52 Ma old marker level postdating the Deccan Volcanism. These sections have received global attention as they record mammalian and plant radiations. We infer the repetitive occurrence of pumice have been sourced from a ∼52–50 Ma MORB related to sea-floor spreading in the western Arabian Sea, most plausibly along the Carlsberg Ridge. Pyroclasts have skeletal plagioclase with horsetail morphologies ± pyroxene ± Fe–Ti oxide euhedral crystals, and typically comprise of circular polymodal (radii ≤10 to ≥30 μm), non-coalescing microvesicles (>40–60%). The pumice have undergone considerable syngenetic alteration during oceanic transport and post-burial digenesis, and are a composite mixture of Fe–Mn-rich clay and hydrated altered basaltic glass (palagonite). The Fe–Mn-rich clay is extremely low in SiO 2, Al 2O3, TiO 2, MgO, alkalies and REE, but very high in Fe 2O3, MnO, P, Ba, Sr contents, and palagonitization involved significant loss of SiO 2, Al 2O3, MgO and variable gain in Fe 2O3, TiO 2, Ni, V, Zr, Zn and REE. Bubble initiation to growth in the ascending basaltic magma (liquidus ∼1200–1250 ∘C) may have occured in ∼3 hr. Short-distance transport, non-connected vesicles, deposition in inner shelf to more confined lagoonal condition in the Early Eocene and quick burial helped preservation of the pumice in Vastan. Early Eocene Arabian Sea volcanism thus might have been an additional source to marginal sediments along the passive margin of western India.