Theory of isotopic fractionation on facetted ice crystals

The currently used "kinetic-fractionation" (KF) model of the differential incorporation of water-molecule isotopologues into vapor-grown ice omits surface processes on crystal facets that may be important in temperature reconstructions. This article introduces the "surface-kinetic" fractionation model, a model that includes such surface processes, and shows that differences in deposition coefficients for water isotopologues can produce isotopic fractionation coefficients that significantly differ from those of KF theory. For example, if the deposition coefficient of H218O differs by just 5% from that of ordinary water (H216O), the resulting fractionation coefficient at 20% supersaturation may deviate from the KF value by up to about ±17‰, and even more at greater supersaturation. As a result, the surface-kinetic theory may significantly change how fractionation depends on supersaturation. Moreover, the model introduces possible new temperature dependencies from the deposition coefficients. These parameters need to be constrained by new laboratory measurements.

Theory of isotope fractionation on facetted ice crystals

Present models of the differential incorporation of isotopic water molecules into vapor-grown ice omit surface processes that may be important in temperature reconstructions. This article introduces a model that includes such surface processes and shows that differences in deposition coefficients for water isotopes can produce isotope fractionation coefficients that significantly differ from those of existing theory. For example, if the deposition coefficient of H218O differs by just 5 % from that of ordinary water (H216O), the resulting fractionation coefficient at 20 % supersaturation may deviate from the kinetic fractionation (KF) prediction by up to about ±17 ‰. Like the KF model, this "surface-kinetic" fractionation model generally predicts greater deviation from the equilibrium prediction at higher supersaturations; indeed, the sensitivity to supersaturation far exceeds that to temperature. Moreover, the model introduces possible new temperature dependencies from the deposition coefficients. These parameters need to be constrained by new laboratory measurements; nevertheless, the theory suggests that observed δ18O changes in ice samples are unlikely to be due solely to temperature changes.

Analyses of Forest Foliage I: Laboratory Procedures for Proximate Carbon Fractionation and Nitrogen Determination

As part of NASA's Accelerated Canopy Chemistry Program we performed analyses for the determination of carbon constituents and nitrogen content in fresh forest foliage. Foliage from three deciduous species was collected for carbon fraction analysis. Samples were analysed using a series of extractions that yielded different carbon constituents: non-polar, polar, cellulose and lignin. Results showed this to be a satisfactory method for carbon fractionation. Coefficient of variation within and between runs was less than 10%. These results were further supported by an interlaboratory comparison involving two other labs and samples collected as part of NSF's LTER LIDET experiment. Variations in laboratory results were due in part to differences in ash correction calculations. Approximately 1000 additional samples were analysed by wet chemical methods and used as a calibration set for Visible/NIR reflectance. Results showed accuracy of spectrophotometric measurements to approach that of wet chemistry techniques.