Ramananda Chakrabarti

Interlaboratory comparison of magnesium isotopic compositions of 12 felsic to ultramafic igneous rock standards analyzed by MC-ICPMS

To evaluate the interlaboratory mass bias for high-precision stable Mg isotopic analysis of natural materials, a suite of silicate standards ranging in composition from felsic to ultramafic were analyzed in five laboratories by using three types of multicollector inductively coupled plasma mass spectrometer (MC-ICPMS). Magnesium isotopic compositions from all labs are in agreement for most rocks within quoted uncertainties but are significantly (up to 0.3‰ in 26Mg/24Mg, >4 times of uncertainties) different for some mafic samples. The interlaboratory mass bias does not correlate with matrix element/Mg ratios, and the mechanism for producing it is uncertain but very likely arises from column chemistry. Our results suggest that standards with different matrices are needed to calibrate the efficiency of column chemistry and caution should be taken when dealing with samples with complicated matrices. Well-calibrated standards with matrix elements matching samples should be used to reduce the interlaboratory mass bias.

7.10 Chemical Characteristics of Sediments and Seawater

The transition from an anoxic to oxygenated atmosphere was arguably the most dramatic change in the history of the Earth. This “Great Oxidation Event” (Holland 2006) transformed the biogeochemical cycles of the elements by imposing an oxidative step in the cycles, creating strong redox gradients in the terrestrial and marine realms that energised microbial metabolism. Although much past research was focused on establishing when the rise of atmospheric oxygen took place, recognition that substantial mass-independent fraction (MIF) of the sulphur isotopes is restricted to the time interval before 2.45 Ga and requires an anoxic atmosphere (Farquhar et al. 2000, 2007; Mojzsis et al. 2003; Ono et al. 2003; Bekker et al. 2004) argues the atmosphere became permanently oxygenated at this time (Pavlov and Kasting 2002). A false-start to the modern aerobic biosphere and a “whiff” of atmospheric oxygen (Anbar et al. 2007) may have occurred in the latest Archaean, as reflected in a transient enrichment in the redox-sensitive element molybdenum in marine shales and a reduction in the extent of MIF precisely coincident with the peak in Mo and FeS2 enrichment (Kaufman et al. 2007). Geochemical proxies are imperfect, and an earlier (c. 3 Ga) appearance of atmospheric oxygen is possible (Ohmoto et al. 2006) but disputed (Farquhar et al. 2007; Buick 2008).Ancient rocks record the redox conditions of the oceanatmosphere system through the distribution of iron (Fe) between oxidised and reduced minerals, which can be formulated into a suite of Fe palaeoredox proxies. The balance between Fe and S in a given system reflects the variance in a range of highand low-temperature sources and sinks. Iron can be delivered by hydrothermal, diagenetic or clastic fluxes and can be buried and removed as Fe-oxide phases, Febearing carbonates such as siderite or ankerite, relatively unreactive silicate phases, which often pass through the system in detrital form, or as a constituent of pyrite (FeS2) using sulphide sourced by sulphate reduction. Sulphate is delivered to the ocean primarily from continental weathering, which requires that a surface oxidative cycle exists, and rates of sulphate delivery and Fe removal as pyrite should thus depend on ocean-atmosphere redox. Among other successes, the iron proxies discussed here have proven their value in studies of the 2.5 Ga Mt. McRae Formation and specifically in delineating subtle increases in atmospheric oxygen prior to the Great Oxidation Event, or ‘GOE’. (Anbar et al. 2007; Kaufman et al. 2007; Reinhard et al. 2009). These Fe proxies are our most effective inorganic proxy for ancient euxinia (anoxic and H2S-rich conditions) on the local scale and are an essential independent backdrop for meaningful application of Mo isotopes to address extents of euxinia on ocean scales (Arnold et al. 2004; Gordon et al. 2009). Thus, in addition to being informative on their own, Fe-based palaeoredox indicators are a crucial component of multi-proxy approaches for distinguishing among oxic, anoxic and Fe (II)-rich (ferruginous), and euxinic depositional conditions. The quantity and speciation of highly reactive iron (FeHR) in sediments and sedimentary rocks can provide crucial insight into the redox state of the local depositional environment. The total pool of FeHR consists of mineral phases that have the potential to react with dissolved H2S when exposed on short timescales (within the water column or during earliest diagenesis) plus Fe that has already reacted and is present as FeS2 (Raiswell and Canfield 1998). Such minerals include ferrous carbonates (siderite, FeCO3; ankerite, Ca(Fe,Mg,Mn)(CO3)2), crystalline ferric oxides (haematite, Fe2O3; goethite, FeOOH), and the mixed-valence Fe oxide magnetite (Fe3O4). These phases are separated by means of a well-calibrated sequential extraction scheme described in detail elsewhere (Poulton et al. 2004; Poulton and Canfield 2005; Reinhard et al. 2009). Briefly, ~100 mg of sample powder is first treated with a buffered sodium acetate solution for 48 h to mobilise ferrous carbonate phases. A split of the extract is removed for analysis, the sample is centrifuged, and the remaining supernatant is discarded. The sample is then treated with a sodium dithionite solution for 2 h to dissolve crystalline ferric oxides and processed as before. Finally, the sample is treated with an ammonium oxalate solution for 6 h to mobilise magnetite. All extractions are performed at room temperature in 15 mL centrifuge tubes under constant agitation. The sequential extracts are analysed on an Agilent 7500ce ICP-MS after 100-fold dilution in trace-metal grade HNO3 (2 %). Pyrite iron is calculated separately based on weight percent pyrite sulphur extracted during a 2-h, hot chromous chloride distillation followed by iodometric titration (Canfield et al. 1986), assuming a stoichiometry of FeS2. For measurement of total Fe (FeT), sample powders are ashed overnight at 450 C (in order to remove organic matter but preserve volatile metals, such as rhenium) and digested using sequential HNO3-HFHCl acid treatments (see, for example, Kendall et al. 2009). After digestion, samples are reconstituted in trace-metal grade HNO3 (2 %), diluted, and analysed by ICP-MS In modern oxic sediments deposited across a wide range of environments, FeHR comprises 6–38 % of total sedimentary Fe (i.e. FeHR/FeT 1⁄4 0.06–0.38), with an average value for FeHR/FeT of 0.26 0.08 defining the modern siliciclastic baseline (Raiswell and Canfield 1998). Enrichments in FeHR that are in excess of this detrital background ratio indicate a source of reactive Fe that is decoupled from the siliciclastic flux and thus reflect the transport, scavenging and enrichment (see below) of Fe within an anoxic basin (Canfield et al. 1996; Wijsman et al. 2001). In this context, ratios of FeHR/FeT exceeding the siliciclastic range point to anoxic deposition, and the ratio FePY/FeHR can then be used to establish whether the system was Fe(II)or H2S-buffered. An anoxic system with a relatively small amount of FeHR converted to pyrite indicates a depositional environment in which reactive Fe supply was greater than the titrating capacity of available H2S produced microbially by sulphate reduction, and thus no dissolved H2S was accumulating in pore fluids or the water column. Importantly, this is true even if microbial sulphate reduction and pyrite formation was occurring in the system (Canfield 1989) because the preponderance of Fe precludes the accumulation of free H2S. In contrast, if the vast majority of FeHR is present as pyrite in an anoxic system, euxinic depositional conditions are indicated – a consequence of the C.T. Reinhard (*) Department of Earth Sciences, University of California, Riverside, CA 92521, USA 10 7.10 Chemical Characteristics of Sediments and Seawater 1483

A novel sample loading method and protocol for monitoring sample fractionation for high precision Ca stable isotope ratio measurements using double-spike TIMS

The external reproducibility (2σSD) of Ca stable isotope ratio measurements (δ44/40Ca) using double-spike thermal ionization mass spectrometry (TIMS) shows a large range from <0.1‰ to 0.5‰. We demonstrate that using a 43Ca–48Ca double spike, which allows simultaneous measurements of δ44/40Ca and δ44/42Ca, and analyses at moderate signal strengths (40Ca ranging from 6 to 10 V), high precision δ44/40Ca (external reproducibility better than ±0.08‰, 2σSD) as well as δ44/42Ca can be obtained if samples and standards are analyzed under similar fractionation conditions. To monitor the fractionation conditions, the usage of a parameter β is proposed, which measures the deviation of the 43Ca/48Ca ratio of the sample–double spike mixture from that of the pure double spike. A novel, low-cost sample loading technique using a combination of Re and Ta filaments and a Ta2O5 activator is presented which results in a steady signal. We report the δ44/40Ca and δ44/42Ca values, calculated w.r.t. NIST SRM 915a and reported as deviations in parts per mil (‰), of NIST standards SRM 915a (0.01, −0.02) and SRM 915b (0.73, 0.32), NASS 6 (seawater, 1.80, 0.89), USGS silicate rock standards BHVO-2 (0.86, 0.47) and BCR-2 (0.89, 0.48), Geological Survey of Japan carbonate standards JCp-1 (coral, 0.85, 0.45) and JCt-1 (clam shell, 0.83, 0.46) and ECRM 752-1 (limestone, alternative name BCS-CRM 393) (0.83, 0.46) from the Bureau of Analyzed Samples Ltd. UK. The Ca stable isotopic compositions of JCt-1 and ECRM 752-1 are reported for the first time.

Economically viable rare earth element deposits along beach placers of Andhra Pradesh, eastern coast of India

A comprehensive study was carried out in order to determine the radioelement and rare earth element (REE) concentrations in beach placer deposits at selected locations along the eastern coast of Andhra Pradesh in India. This was done to evaluate the economic value of these deposits. The findings of this study suggest that high Th and low K concentrations delineate the prospective regions having REE deposits. The beach placers, in general, can be characterized by high thorium and moderate uranium concentrations. The concentrations of REEs vary in the following order: Ce > La> Nd > Pr > Sm > Gd > Dy. Rapid in situ thorium prospectivity coupled with laboratory-based techniques like ICP-MS, as proposed in this study, would help in the identification of prospective REE sources along the coastal placers. The development of indigenous resources of light rare earth elements (LREEs), medium rare earth elements (MREEs), and heavy rare earth elements (HREEs) would decrease the dependence on imports, which have a strategic hold on the production and supply of the REEs, globally.

Ferromagnetism in α-Mn nanorods

The present investigation reports the first experimental evidence of ferromagnetism in the cryomilled pure α-Mn nano-rods. Cryomilling of Mn powder at liquid nitrogen temperature leads to the formation of long nanorods of α-Mn. The detailed electron microscopy reveals that the nanorods grow along [ 1 1 ¯ 2 ] directions with surfaces bounded by {110} planes of FCC α-Mn. The magnetic measurements indicate ferromagnetic hysteresis loops, suggesting typical ferromagnetic order. The ab-initio density functional theory calculations indicate that the ferromagnetic response originates from the under coordinated surface atoms.

Submarine groundwater discharge derived strontium from the Bengal Basin traced in Bay of Bengal water samples

Evaluating the submarine groundwater discharge (SGD) derived strontium (Sr) flux from the Bengal Basin to the Bay of Bengal (BoB) and determining its isotopic composition is crucial for understanding the marine Sr isotopic evolution over time. Measurements of spatially and temporally distributed water samples collected from the BoB show radiogenic 87Sr/86Sr, high Sr, calcium (Ca) concentrations and high salinity in samples collected dominantly from 100–120 m depth, which can be explained only by the contribution of saline groundwater from the Bengal Basin. These results provide a direct evidence of the SGD-Sr flux to the BoB. This SGD-Sr flux is however, spatially heterogeneous and using conservative hydrological estimates of the SGD flux to the BoB, we suggest a SGD Sr flux of 13.5–40.5 × 105 mol/yr to the BoB. Mass balance calculations using Sr concentrations and 87Sr/86Sr suggest up to 7% contribution of SGD to the 100–120 m BoB water samples. The identification of SGD at 100–120 m depth also provides an explanation for the anomalous variations in barium (Ba) concentrations and the δ18O-salinity relationship in intermediate depths of the BoB.