Flow-through time resolved analysis (FT-TRA) involves subjecting small mineral samples (< 10 mg) inserted in a miniature flow-through cell (50 L) to controlled flows of eluent analyzed on-line by ICP-MS. In this study, FT-TRA is used to empirically determine the dissolution regimes for the two wellstudied minerals forsterite and calcite, representing minerals with relatively slow and fast dissolution kinetics. A proportional increase in steady-state effluent [Mg, Si] concentrations with increasing flowthrough cell eluent residence times confirms a dominantly surface-controlled dissolution regime for a powdered forsterite sample at pH 2.3, implying that transport limitations are negligible. In contrast, the relationship between flow rates and dissolution rates for single grain calcite samples at pH 2.3-4 reveals that transport limitations affect the rate of calcite dissolution. To provide a quantitative and
Stable isotopes and trace elements in ostracod shells have been used widely in paleolimnological investigations of past lake hydrochemistry and climate because they provide insights into past water balance and solute evolution of lakes. Regional differences in lake characteristics and species-specific element fractionation, however, do not permit generalization of results from other regions or ostracod species to the southern Tibetan Plateau, in part because most common taxa from the southern Tibetan Plateau are endemic to the area. This study evaluated relations between present-day environmental conditions and the geochemical composition of modern ostracod shells from the southern Tibetan Plateau, to assess the suitability of using shell chemistry to infer hydrological conditions. We studied nine lakes and their catchments, located along a west–east transect in the south-central part of the Tibetan Plateau. Stable oxygen and carbon isotope values and trace element concentrations in recent shells from the four most abundant ostracod species (Leucocytherella sinensis, ?Leucocythere dorsotuberosa, Limnocythere inopinata, Tonnacypris gyirongensis) were measured, together with hydrochemical properties of host waters at the time of sampling. Results revealed significant between-species differences in stable isotope fractionation and trace element incorporation into shell calcite. Stable oxygen and carbon isotope values of ostracod shells were correlated significantly with the stable isotope composition of the respective water body (δ18OH2Oand δ13CH2O), reflecting salinity and productivity, respectively. Offsets between δ18Oshell and δ13Cshell and inorganic calcite, the latter representing isotopic equilibrium, suggest shell formation of T. gyirongensis during spring melt. L. sinensis reproduces throughout the monsoon season until September and shows several consecutive generations, and L. inopinata molts to the adult stage after the monsoon season in August/September. The influence of pore water δ13C was displayed by L. inopinata, suggesting shell calcification within the sediment. Mg/Cashell is primarily influenced by water Mg/Ca ratios and salinity and confirms the use of this shell ratio as a proxy for precipitation-evaporation balance and lake level. In addition, Sr/Ca and Ba/Ca can be used to infer changes in salinity, at least in closed-basin lakes with calcite saturation. Observed effects of water Sr/Ca and salinity on Sr/Ca incorporation are biased by the presence of aragonite precipitation in the lakes, which removes bioavailable Sr from the host water, resulting in low Sr/Cashell values. Changes in carbonate mineralogy affect the bioavailability of trace elements, a process that should be considered in paleoclimate reconstructions. Oxygen isotopes and Mg/Cashell ratios were unaffected by water temperature. Positive correlations among Fe/Ca, Mn/Ca and U/Ca in ostracod shells, and their negative correlation with δ13C, which reflects organic matter decay, show the potential to infer changes in redox conditions ...
We explore the applicability of on-line flow-through time-resolved analysis (FT-TRA) to measure mineral dissolution rates, determine dissolution rate parameters, and monitor dissolution stoichiometry under both constant and transient eluent conditions. A custom-built automated flow-through system is used to subject mineral samples to controlled eluent flows of constant or varying composition, and the dissolution products are measured online with a quadrupole ICP-MS. Because forsterite dissolution has been extensively studied, this mineral provides an ideal benchmark to test the approach. FT-TRA has several advantages compared to the mixed-flow reactors conventionally used to measure mineral dissolution. The mineral dissolution regime (surface vs transport-controlled) can be readily established prior to conducting dissolution experiments, which is important if the goal of the experiment is to determine dissolution rate parameters. Because of the small volume (25 μL) of the flow-through reactor, the time to reach steady state concentration in the effluent is shorter and is limited by the intrinsic properties of the mineral, instead of the residence time of the effluent in the reactor. For minerals reaching steady-state dissolution rapidly, dissolution rate parameters (rate constant(s) and reaction order(s)) are established in a few hours, allowing for replications, statistical analysis of the results, and investigation of the underlying causes of variability. The experimental setup can be adapted for minerals that require longer periods of time to reach steady-state dissolution. In addition, FT-TRA lends itself particularly well for detailed monitoring of dissolution rates and dissolution stoichiometry under transient conditions. For forsterite dissolution under acidic conditions, preferential release of Mg when pH transits to lower values, and preferential release of Si when pH transits to higher values is clearly documented and can be interpreted as indicating changes in the mean depth of the Si-rich surface layer in response to changes in eluent acidity. Episodes of exfoliation of the Si-rich surface layer can also be identified, providing a means to study one of the potential rate limiting steps for CO 2 sequestration by carbonation of olivine. The results presented here indicate that FT-TRA with online ICP-MS analysis could be a useful and multifaceted addition to the toolbox available to study mineral dissolution.
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