Experimental studies of equilibrium iron isotope fractionation in ferric aquo-chloro complexes

Here we compare new experimental studies with theoretical predictions of equilibrium iron isotopic fractionation among aqueous ferric chloride complexes (Fe(H₂O)₆³⁺, FeCl(H₂O)₅²⁺, FeCl₂(H₂O)₄⁺, FeCl₃ (H₂O)₃, and FeCl₄^-), using the Fe-Cl-H₂O system as a simple, easily-modeled example of the larger variety of iron-ligand compounds, such as chlorides, sulfides, simple organic acids, and siderophores. Isotopic fractionation (⁵⁶Fe/⁵⁴Fe) among naturally occuring iron-bearing species at Earth surface temperatures (up to ∼3‰) is usually attributed to redox effects in the environment. However, theoretical modeling of reduced isotopic partition functions among iron-bearing species in solution also predicts fractionations of similar magnitude due to non-redox changes in speciation (i.e., ligand bond strength and coordination number). In the present study, fractionations are measured in a series of low pH ([H⁺] = 5 M) solutions of ferric chloride (total Fe = 0.0749 mol/L) at chlorinities ranging from 0.5 to 5.0 mol/L. Advantage is taken of the unique solubility of FeCl₄^- in immiscible diethyl ether to create a separate spectator phase, used to monitor changing fractionation in the aqueous solution. Δ⁵⁶Fe_aq-eth = δ⁵⁶Fe (total Fe remaining in aqueous phase)−δ⁵⁶Fe (FeCl₄^- in ether phase) is determined for each solution via MC-ICPMS analysis.Both experiments and theoretical calculations of Δ⁵⁶Fe_aq-eth show a downward trend with increasing chlorinity: Δ⁵⁶Fe_aq-eth is greatest at low chlorinity, where FeCl₂(H₂O)₄⁺ is the dominant species, and smallest at high chlorinity where FeCl₃(H₂O)₃ is dominant. The experimental Δ⁵⁶Fe_aq-eth ranges from 0.8‰ at [Cl^-] = 0.5 M to 0.0‰ at [Cl^-] = 5.0 M, a decrease in aqueous-ether fractionation of 0.8‰. This is very close to the theoretically predicted decreases in Δ⁵⁶Fe_aq-eth, which range from 1.0 to 0.7‰, depending on the ab initio model.The rate of isotopic exchange and attainment of equilibrium are shown using spiked reversal experiments in conjunction with the two-phase aqueous-ether system. Equilibrium under the experimental conditions is established within 30 min.The general agreement between theoretical predictions and experimental results points to substantial equilibrium isotopic fractionation among aqueous ferric chloride complexes and a decrease in ⁵⁶Fe/⁵⁴Fe as the Cl^-/Fe³⁺ ion ratio increases. The effects on isotopic fractionation shown by the modeling of this simple iron-ligand system imply that ligands present in an aqueous environment are potentially important drivers of fractionation, are indicative of possible fractionation effects due to other speciation effects (such as iron-sulfide systems or iron bonding with organic ligands), and must be considered when interpreting iron isotope fractionation in the geological record.     

Solar rotation measurements at Mount Wilson

This paper describes a thorough reevaluation of the procedures for reducing the data acquired at the Mt. Wilson Observatory synoptic program of solar observations at the 150-foot tower. We also describe a new program of acquiring as many scans per day as possible of the solar magnetic and velocity fields. We give a new fitting formula which removes the background velocity field from each scan. An important new feature of our reduction algorithm is our treatment of the limb shift which permits time variation in this function. We identify the difference between the limb shift along the north-south axis and the east-west axis as potentially being a result of meridional circulation. Our analysis interprets the time variation in the east-west limb shift as being the result of changes in a vertical component of the meridional circulation.The performance of the system improved in 1982 as a result of the installation of a new exit slit assembly. The amplitude of the limb shift variations found prior to 1982 is larger than is easily explained with simple ideas of meridional circulation. However, we have not been able to firmly identify instrumental causes for the variations although small changes in the band-pass of the exit slit assembly could have contributed.We have established a correlation between the observed stray light in the system and a component of the velocity field which is antisymmetric with respect to the solar central meridian. We remove this stray light effect by adding an additional term to the fitting function.Finally, we show that the inclusion of the above improvements allows us to study the torsional oscillations at high latitude using a procedure which can retain the longitude dependent information about the velocity pattern.     

29Si and 1H NMR spectroscopy of high-pressure hydrous magnesium silicates

We present NMR spectroscopic data, obtained by ¹H MAS, ¹H static spin-echo, and ²⁹Si{¹H} CP-MAS techniques, for a series of hydrous magnesium silicate samples synthesized at high pressure. This series includes chondrodite, β-Mg₂SiO₄, and phases A, B, superhydrous B, and E. Phases B and superhydrous B give very narrow ²⁹Si NMR peaks and display the most de-shielded Si^VI chemical shifts yet reported: ?170.4?ppm for B and ?166.6 for superhydrous B. The ¹H NMR spectra of B and superhydrous B confirm the presence of paired hydroxyls, as determined from refinement of the H positions from X-ray diffraction data. The ¹H MAS NMR spectra of phase B contain peaks for the two distinct hydrogen positions, with chemical shifts of +4.7 and +3.3?ppm. The static ¹H spectrum contains a powder pattern characteristic of a strongly coupled hydrogen pair, from which a dipolar coupling constant of 18.6(4)?kHz and inter-hydrogen distance of d(H–H)=1.86(2)?Å were obtained. Superhydrous B appears to give two poorly resolved ¹H MAS peaks, consistent with the presence of two distinct hydrogen pairs in the P2₁ mn crystal structure. Analysis of its spin-echo spectrum gives d(H–H)=1.83(3)?Å, slightly shorter than for phase B. β-Mg₂SiO₄, coexisting with phases B and superhydrous B, appears to give ²⁹Si{¹H} CP-MAS signal, indicating that it contains significant H concentration. The ²⁹Si chemical shifts for phases B, superhydrous B, and chondrodite, together with those reported previously for other Mg-silicates, show a good correlation with structural parameters.     

Sensitivity and uncertainty analyses for performance assessment modeling

Sensitivity and uncertainty analyses methods for computer models are being applied in performance assessment modeling in the geologic high-level radioactive-waste repository program. The models used in performance assessment tend to be complex physical/chemical models with large numbers of input variables. There are two basic approaches to sensitivity and uncertainty analyses: deterministic and statistical. The deterministic approach to sensitivity analysis involves numerical calculation or employs the adjoint form of a partial differential equation to compute partial derivatives; the uncertainty analysis is based on Taylor series expansions of the input variables propagated through the model to compute means and variances of the output variable. The statistical approach to sensitivity analysis involves a response surface approximation to the model with the sensitivity coefficients calculated from the response surface parameters; the uncertainty analysis is based on simulation. The methods each have strengths and weaknesses.     

Geomorphic effects of a large flood on fluvial fans

The response of 12 fluvial fans near Sydney, Australia to a large storm between 2 and 4 February 1990 was determined by repeating previously surveyed longitudinal profiles and by undertaking detailed field observations of erosion and deposition. Peak rainfall intensities occurred on 3 and 4 February when between 173 and 193·8 mm were recorded. Return periods for 24 h duration peak rainfall ranged between 5·7 and 11·0 years on the annual maximum series at six stations within the study area and return periods for 48 h peak rainfall ranged between 13·5 and 29·4 years. Of the 12 fans, seven were trenched and five untrenched. The most significant geomorphic effects of the storm were recorded on the proximal region of the fans. However, fan response was highly variable, with one fan exhibiting no detectable change, three fans localized deposition, two fans spatially disjunct erosion and deposition, two fans channel avulsions, and seven fans fanhead trench reworking. Some fans exhibited more than one type of response. A four-stage, tentative cyclical model of fanhead development was constructed from the field data. Stage 1 refers to the episodic aggradation of the fanhead by localized deposition, spatially disjunct erosion and deposition and/or channel avulsions. Stage 2 represents the initiation of a fanhead trench when progressive aggradation locally exceeds a threshold slope leading to localized erosion. This erosion initially creates one or more discontinuous flow-aligned scour pools. Over time, the scour pools widen, deepen and extend both up- and downfan. Stage 3 refers to the coalescence of discontinuous scour pools into a continuous trench by the removal of intervening log and boulder steps. Stage 4 represents the backfilling phase of the trench once it has been overwidened and/or slope reduced. Aggradation then continues as for stage one.     

Embeddedness in Practice,Numbers in Context: The Politics of Knowing and Doing*

In this paper, I argue that feminist geographers need to scrutinize the claims being made by the three identifiable epistemological orientations in feminist geography regarding the use of numbers, for each offers a different view of objectivity and a different way to count. I go on to pursue a refinement of a critical feminist epistemology grounded in a mediated objectivity, one located in the embeddedness of everyday life. Within this framework, I suggest that numbers are useful, but only in context.     

Analysis of chlorophyll diagenesis—II. Solvent effects on metal incorporation into chlorophyll diagenetic products

We are attempting to quantify selected transitions in the diagenetic pathway of chlorophyll in deep ocean sediments. To analyze the aromatization from phorbides (less stable) to porphyrins (more stable), we used a model system which closely resembles the compounds identified in oceanic sediments. Kinetic parameters were determined for the incorporation of copper into deoxomesopyropheophorbide a methyl ester (DOMPP) and deoxophylloerythrin methyl ester (DPE) in a series of solvents ranging from highly polar and protic to non-polar and aprotic, representing varying sedimentary environments. In the reaction of copper with these two compounds, the activation parameters for the reaction with DOMPP indicate that DOMPP is more sensitive to the environment than DPE. Also, over the temperature range studied, DOMPP always reacted faster than DPE under similar conditions. These two observations lend support to our previous hypothesis that the phorbide to porphyrin transition includes the metallophorbide as a short-lived intermediate in the reaction sequence.     

Analysis of chlorophyll diagenesis—I. Kinetics of the phorbide to porphyrin transition

The objective of this project is to quantify selected transitions in the diagenetic pathways of chlorophyll in deep ocean sediments, initially the aromatization from phorbides (unstable) to porphyrins (stable), using a model system which closely resembles the compounds identified in oceanic sediments. We determined the kinetic parameters for the aromatization of deoxomesopyropheophorbide a methyl ester (DOMPP) to deoxophylloerythrin methyl ester (DPE) in the presence of Cu(II) in a series of solvents ranging from highly polar and protic to nonpolar and aprotic, representing varying sedimentary environments. With Cu(II) in ethanol, the aromatization occurs through a pathway which involves a relatively slow initial formation of CuDOMPP, rapid aromatization to CuDPE, and finally an equilibrium reaction in the presence of clay which induces demetalation to form the free base DPE. Based on our model studies, only DOMPP, CuDPE and some DPE would be detected in significant quantities in more polar, protic sedimentary environments on a geologic time scale, although the story might be different with changed environment polarity. Our model studies may ultimately reconcile laboratory observations with those based on DSDP cores which appear to indicate that the diagenetic sequence proceeds from phorbide to free base porphyrin, followed by metalation of the porphyrin.