Piyali Chanda

Bio: Piyali Chanda is an academic researcher from Northern Arizona University. The author has contributed to research in topics: Equilibrium fractionation & Isotope fractionation. The author has an hindex of 5, co-authored 8 publications receiving 76 citations. Previous affiliations of Piyali Chanda include NASA Astrobiology Institute & Pennsylvania State University.

High Precision Iron Isotope Measurements With High Mass Resolution MC-ICPMS

Abstract: Abstract This study presents a new technique for high precision measurement of iron isotope ratios using high mass resolution MC-ICPMS on the ThermoFinnigan Neptune. A mass resolving power of about 8000–9000 was used to resolve the mass interferences of isobaric polyatomic ions (e.g., 40 Ar 16 O + on 56 Fe + , 40 Ar 16 OH + on 57 Fe + and 40 Ar 14 N + on 54 Fe + ) from the Fe isotopes and to produce “flat top” peak shapes with a plateau width of about 100 ppm in mass (Δm/m). The abundance sensitivity was determined to 50 ppm contribution of the 40 Ar 16 O + peak tail on the 56 Fe + peak. This enables Fe isotope measurements with a Fe-isotope/interference ratio of up to 1, without significant contribution from the tail of the interference peak. The performance of the technique was tested by determining the reproducibility and accuracy of synthetic Fe samples and spiked standards against the IRMM014 standard. Various interface setups, including wet and dry plasma techniques, and a wide range of concentrations (from 50 ppb to 10 ppm) have been used to demonstrate the flexibility, robustness and sensitivity of the technique. An external precision of ca. 0.10‰ for δ 56 Fe and ca. 0.15‰ for δ 57 Fe (2 S.D.) was routinely achieved. Measured and calculated delta values between spiked standards and the IRMM014 standard agree within uncertainties. In the investigated concentration range, precision and accuracy are independent of both the sample introduction system and the sample concentration. The new technique presented in this study is a robust and simple method to perform high precision Fe isotope measurements, and it allows to measure much lower sample concentrations compared to previous techniques.

Kinetic Isotopic Fractionation During Diffusion of Ionic Species in Water

Abstract: Kinetic isotopic fractionation during diffusion of ionic species in water Frank M . Richter , Ruslan A . Mendybaev , John Christensen , Ian D. Hutcheon , Ross W. Williams , Neil C. Sturchio and Abelardo D. Beloso Jr . 'The University of Chicago, Department of the Geophysical Sciences, 5734 South Ellis Avenue, Chicago, IL 60637. Lawrence Berkeley National Laboratory, Berkeley, C A . Glenn T. Seaborg Institute, Lawrence Livermore National Laboratory, Livermore, CA. University of Illinois at Chicago, Chicago, I L 60607 June 9, 2005 Corresponding author: Frank M . Richter (richter@geosci.uchicago.edu) Abstract Experiments specifically designed to measure the ratio of the diffusivities of ions dissolved in water were used to determine D / D , D7 /D6 , D25 /D24 , D26 /D25 , and D37 /D35 . The measured ratio of the diffusion coefficients for L i and K in water ( D / D =0.6) is in good agreement with published data, providing evidence that the experimental design being used resolves the relative mobility of ions with adequate precision to also be used for determining the fractionation of isotopes by diffusion in water. In the case of L i we found measurable isotopic fractionation associated with the L i K Li Li Mg Mg Mg Mg a C1 Li K diffusion of dissolved L i C l ( D 7Li /D 6Li This difference in the diffusion coefficient of L i compared to L i is significantly less than reported in an earlier study, a difference we attribute to the fact that in the earlier study L i diffused through a membrane separating the water reservoirs. Our experiments involving M g diffusing in water found no measurable isotopic fractionation (D25M /D24 = 1.00003±0.00006). CI isotopes were fractionated during diffusion in water (D37 /D35ci = 0.99857±0.00080) whether or not the co-diffuser (Li or Mg) was isotopically fractionated. The isotopic fractionation associated with the diffusion of ions in water is much smaller than values we found previously for the isotopic fractionation of L i and Ca isotopes by diffusion in molten silicate liquids. A major distinction between water and silicate liquids is that water, being a polar liquid, surrounds dissolved ions with hydration shells, which very likely play an important but still poorly understood role in reducing isotopic fractionation associated with diffusion. g Mg C1

Application of Benthic Foraminiferal Mg/Ca Ratios to Questions of Cenozoic Climate Change

Abstract: We investigate the evolution of Cenozoic climate and ice volume as evidenced by the oxygen isotopic composition of seawater (δ18Osw) derived from benthic foraminiferal Mg/Ca ratios to constrain the temperature effect contained in foraminiferal δ18O values. We have constructed two benthic foraminiferal Mg/Ca records from intermediate water depth sites (Ocean Drilling Program sites 757 and 689 from the subtropical Indian Ocean and the Weddell Sea, respectively). Together with the previously published composite record of Lear et al. [Science 287 (2002) 269–272] and the Neogene record from the Southern Ocean of Billups and Schrag [Paleoceanography 17 (2002) 10.1029/2000PA000567], we obtain three, almost complete representations of the δ18Osw for the past 52 Myr. We discuss the sensitivity of early Cenozoic Mg/Ca-derived paleotemperatures (and hence the δ18Osw) to assumptions about seawater Mg/Ca ratios. We find that during the middle Eocene (∼49–40 Ma), modern seawater ratios yield Mg/Ca-derived temperatures that are in good agreement with the oxygen isotope paleothermometer assuming ice-free conditions. Intermediate waters cooled during the middle Eocene reaching minimum temperatures by 40 Ma. The corresponding δ18Osw reconstructions support ice growth on Antarctica beginning by at least 40 Ma. At the Eocene/Oligocene boundary, Mg/Ca ratios (and hence temperatures) from Weddell Sea site 689 display a well-defined maximum. We caution against a paleoclimatic significance of this result and put forth that the partitioning coefficient of Mg in benthic foraminifera may be sensitive to factors other than temperature. Throughout the remainder of the Cenozoic, the temporal variability among δ18Osw records is similar and similar to longer-term trends in the benthic foraminiferal δ18O record. An exception occurs during the Pliocene when δ18Osw minima in two of the three records suggest reductions in global ice volume that are not apparent in foraminiferal δ18O records, which provides a new perspective to the ongoing debate about the stability of the Antarctic ice sheet. Maximum δ18Osw values recorded during the Pleistocene at Southern Ocean site 747 agree well with values derived from the geochemistry of pore waters [Schrag et al., Science 272 (1996) 1930–1932] further highlighting the value of the new Mg/Ca calibrations of Martin et al. [Earth Planet. Sci. Lett. 198 (2002) 193–209] and Lear et al. [Geochim. Cosmochim. Acta 66 (2002) 3375–3387] applied in this study. We conclude that the application of foraminiferal Mg/Ca ratios allows a refined view of Cenozoic ice volume history despite uncertainties related to the geochemical cycling of Mg and Ca on long time scales.

Phosphate Imposed Limitations onBiological Reduction and Alterationof Ferrihydrite

Abstract: Biogeochemical transformation (inclusive of dissolution) of iron (hydr)oxides resulting from dissimilatory reduction has a pronounced impact on the fate and transport of nutrients and contaminants in subsurface environments. Despite the reactivity noted for pristine (unreacted) minerals, iron (hydr)oxides within native environments will likely have a different reactivity owing in part to changes in surface composition. Accordingly, here we explore the impact of surface modifications induced by phosphate adsorption on ferrihydrite reduction by Shewanella putrefaciens under static and advective flow conditions. Alterations in surface reactivity induced by phosphate changes the extent, decreasing Fe(Ill) reduction nearly linearly with increasing P surface coverage, and pathway of iron biomineralization. Magnetite is the most appreciable mineralization product while minor amounts of vivianite and green rust-like phases are formed in systems having high aqueous concentrations of phosphate, ferrous iron, and bicarbonate. Goethite and lepidocrocite, characteristic biomineralization products at low ferrous-iron concentrations, are inhibited in the presence of adsorbed phosphate. Thus, deviations in iron (hydr)oxide reactivity with changes in surface composition, such as those noted here for phosphate, need to be considered within natural environments.