Geology topics

Greg Michalski

Publications and source records attributed to Greg Michalski.

3 recordsLinked to original sources

USGS2004

Preparation and analysis of nitrogen-bearing compounds in water for stable isotope ratio measurement

This chapter reviews methods for analyzing "natural abundance" nitrogen-bearing compounds in water for δ 15 N, δ 18 O, and δ 17 O. The techniques that are reviewed are Kjeldahl distillation and digestion, diffusion, micro diffusion, ion exchange, and microbial denitrifier methods. It overlaps the contents of the preparation of ecological and biogeochemical samples for isotope analysis and also covers sections on particulate organic nitrogen and biological samples. Nitrogen comprises 78% of the atmosphere, oxygen 21% and argon 1%. There are two stable isotopes of N are 14 N and 15 N and a wide range of oxidation numbers exhibited by nitrogen compounds. Developments of continuous-flow mass spectrometry makes it possible to determine both δ15N and δ18O values of nitrate samples. Such techniques are particularly useful for small-size samples. With decreasing sample size, possible isotopic fractionations associated with extraction from the matrix, contaminants in the reagents, and leaks will become more significant. Due to the interest in distinguishing different species of organic-N compounds, the diffusion method is applied to differentiate types of organic-N.

Book chapter↗

USGS2004

Long term atmospheric deposition as the source of nitrate and other salts in the Atacama Desert, Chile: New evidence from mass-independent oxygen isotopic compositions

Isotopic analysis of nitrate and sulfate minerals from the nitrate ore fields of the Atacama Desert in northern Chile has shown anomalous 17 O enrichments in both minerals. Δ 17 O values of 14–21 ‰ in nitrate and 0.4 to 4 ‰ in sulfate are the most positive found in terrestrial minerals to date. Modeling of atmospheric processes indicates that the Δ 17 O signatures are the result of photochemical reactions in the troposphere and stratosphere. We conclude that the bulk of the nitrate, sulfate and other soluble salts in some parts of the Atacama Desert must be the result of atmospheric deposition of particles produced by gas to particle conversion, with minor but varying amounts from sea spray and local terrestrial sources. Flux calculations indicate that the major salt deposits could have accumulated from atmospheric deposition in a period of 200,000 to 2.0 M years during hyper-arid conditions similar to those currently found in the Atacama Desert. Correlations between Δ 17 O and δ 18 O in nitrate salts from the Atacama Desert and Mojave Desert, California, indicate varying fractions of microbial and photochemical end-member sources. The photochemical nitrate isotope signature is well preserved in the driest surficial environments that are almost lifeless, whereas the microbial nitrate isotope signature becomes dominant rapidly with increasing moisture, biologic activity, and nitrogen cycling. These isotopic signatures have important implications for paleoclimate, astrobiology, and N cycling studies.

Geochimica et Cosmochimica Acta↗

USGS2002

Determination of the total oxygen isotopic composition of nitrate and the calibration of a Δ17Ο nitrate reference material

A thermal decomposition method was developed and tested for the simultaneous determination of δ 18 O and δ 17 Ο in nitrate. The thermal decomposition of AgNO 3 allows for the rapid and accurate determination of 18 O/ 16 O and 17 O/ 16 O isotopic ratios with a precision of ±1.5‰ for δ 18 O and ±0.11‰ for Δ 17 Ο (Δ 17 Ο = δ 17 Ο − 0.52 × δ 18 O). The international nitrate isotope reference material IAEA-NO3 yielded a δ 18 O value of +23.6‰ and Δ 17 Ο of −0.2‰, consistent with normal terrestrial mass-dependent isotopic ratios. In contrast, a large sample of NaNO 3 from the Atacama Desert, Chile, was found to have Δ 17 Ο = 21.56 ± 0.11‰ and δ 18 O = 54.9 ± 1.5‰, demonstrating a substantial mass-independent isotopic composition consistent with the proposed atmospheric origin of the desert nitrate. It is suggested that this sample (designated USGS-35) can be used to generate other gases (CO 2 , CO, N 2 O, O 2 ) with the same Δ 17 Ο to serve as measurement references for a variety of applications involving mass-independent isotopic compositions in environmental studies.

Analytical Chemistry↗