Martin Wahlen

A reassessment of the global isotopic budget of atmospheric nitrous oxide

A simple box model is developed which accounts for the natural and anthropogenic sources and sinks of tropospheric nitrous oxide. Stable isotopic terms are included as well to investigate what additional insight they might provide concerning the overall picture of the global nitrous oxide budget. It is seen that fractionation associated with ultraviolet photolysis in the stratosphere plays a key role in balancing the isotopic budget. It is also noted that increased production due to human influence should have a unique isotopic signature which should provide observable differences between preindustrial air trapped in polar firn or ice and modern air. This build-up of isotopically light, anthropogenic nitrous oxide can be thought of as an N2O “Suess effect” and should be observable in time series measurements of clean baseline atmospheric samples. High-precision records of such changes could lead to a better estimate of the preindustrial N2O isotopic signature as well as provide insight into the pathways of the anthropogenic sources.

Oxygen loss in biosphere 2

Oxygen concentrations have dropped sharply in the air of “Biosphere 2,” an enclosed experimental ecosystem located in southern Arizona. Biosphere 2 is a 3.15-acre airtight structure roofed in glass and underlain by an impermeable liner. It houses an artificial ecosystem containing soil, air, water, flora, and fauna and was built primarily as an apparatus for the experimental investigation of biogeochemical cycles, whole ecosystems, and life-support systems for space habitation [see Nelson et al., 1993]. n nO2 in Biosphere 2 decreased during the first 16 months of closure from the ambient 21% to 14%, enough to cause health problems in the human occupants. We present evidence that the O2 loss is caused by microbial respiration of the excessive amount of organic matter incorporated into the experiments soils and furthermore, that the respired CO2 is reacting with the structures concrete to form calcium carbonate.

The biogeochemical controls of the δ15N and δ18O of N2O produced in landfill cover soils

We document an enrichment of both the δ18OAtm.O2 and δ15NAtm.N2 values of soil-derived N2O collected from landfill cover soils relative to tropospheric N2O. The isotopic values of N2O vary from −5.1‰ to +l9.4‰ and from +19.0‰ to +33.5‰ for δ 15NAtm.N2 and δ18OAtm.O2,, respectively. A tight linear correlation for δ18OAtm.O2 versus δ15NAtm.N2 is apparent, reflecting coupled microbial processes that produce N2O that may be isotopically enriched or depleted in relation to tropospheric N2O. Several explanations are provided to explain this correlation, including evidence for NH3 limitation during nitrification, which would be expected to diminish isotopic fractionation and consequently result in more enriched isotopic values of N2O. Desiccation effects on nitrification were also observed, which contribute to NH3 limitation and thus could influence the isotopic signature of N2O. Our results indicate that the N2O isotopic composition from soils may vary greatly depending on the season and soil moisture conditions and may at times be enriched in 15N and 18O relative to tropospheric N2O.

An interlaboratory comparison of techniques for extracting and analyzing trapped gases in ice cores

We undertook an interlaboratory comparison of techniques used to extract and analyze trapped gases in ice cores. The intercomparison included analyses of standard reference gases and samples of ice from the Greenland Ice Sheet Project 2 (GISP2) site. Concentrations of CO2, CH4, the x7f5180 of 02, the x7f515N of N2, and the O2/N2, and Ar/N2 ratios were measured in air standards and ice core sampries. The standard reference scales for CO2 and CH 4 were consistent at the +2% level. The x7f502/N2 and x7f5180 of O2 measurements showed substantial deviations between the two laboratories able to measure these ratios. The deviations are probably related to errors associated with calibration of the working standards. ThesAr/N2 and x7f515N of N2 measurements were consistent. Five laboratories analyzed the CH4 concentration in a 4.2-m section of the GISP2 ice core. The average of 20 discrete CH 4 measurements was 748+10 parts per billion by volume (ppbv). The standard deviation of these measurements was close to the total analytical uncertainty associated with the measurements. In all cases, those laboratories employing a dry extraction technique determined higher CH 4 values than laboratories using a wet extraction technique. The origin of this difference is unclear but may involve uncertainties associated with blank corrections. Analyses of the CO2 concentration of trapped gases showed extreme variations which cannot be explained by analytical uncertainties alone. Three laboratories measured the (CO2) on 21 discrete depths yielding an average value of 283+13 parts per million by volume (ppmv). In this case, the standard deviation was roughly a factor of 2 greater than the analytical uncertainties. We believe the variability in the measured (CO2) results from impurities in the ice which may have compromised the (CO2) of trapped gases in Greenland ice.

CO 2 diffusion in polar ice: observations from naturally formed CO 2 spikes in the Siple Dome (Antarctica) ice core

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Historical biomass burning: Late 19th century pioneer agriculture revolution in northern hemisphere ice core data and its atmospheric interpretation

Ice core data from Yukon and Greenland spanning from ∼1750 to 1950 indicate that between ∼1850 and ≤1910 a clear atmospheric signal exists of an episodic biomass burning event that is referred to as the Pioneer Agriculture Revolution. This is best seen in NH 4 + ion and particulate concentrations but also in some limited black carbon concentration data, where for all three quantities maximum levels reach about 3 times the prerevolution background concentrations. Tree cellulose δ 13 C data and some early, controversial, French, air CO 2 data, occurring within the same time interval, are interpreted as providing other independent evidence for the same, mainly North American, late 19th century biomass burning event. Some hitherto problematic northern hemisphere ice core derived CO 2 concentration data may now be interpreted as containing a biomass burn signal, and these data are compared, especially as to the time of occurrence, with all the other results. A global carbon cycle model simulation of atmospheric CO 2 mixing ratios using a maximum input of 3 Gt(C)/yr at northern midlatitudes produces anomalous CO 2 levels close to some of the ice core carbon dioxide values. However, other values in this data set do not reasonably represent fully mixed atmospheric values. This suggests that these values might be transients but still tracers for biomass burning. Nevertheless, it appears possible that interhemispheric CO 2 gradients of similar magnitude to the present one could have existed briefly late last century.

Record drilling depth struck in Greenland

On July 1, 1993, after 5 years of drilling, the Greenland Ice Sheet Project (GISP2) penetrated several meters of silty ice and reached bedrock at a depth of 3053.4 m. It then penetrated 1.5 m into the bedrock, producing the deepest ice core ever recovered (Figure 1). n nIn July 1992, a nearby European ice coring effort, the Greenland Ice Core Project (GRIP), reached an ice depth of 3028.8 m, providing more than 250,000 years of record. Comparisons between these ice core records have already demonstrated the remarkable reproducibility of the upper ∼90% of the records unparalleled view of climatic and environmental change.

BOREAS TGB-06 Soil Methane Oxidation and Production from NSA BP and Fen Sites

The BOReal Ecosystem-Atmosphere Study Trace Gas Biogeochemistry (BOREAS TGB-6) team collected soil methane measurements at several sites in the Southern Study Area (SSA) and Northern Study Area (NSA). This data set contains soil methane consumption (bacterial CH4 oxidation) and associated C-13 fractionation effects in samples that were collected at various sites in 1994 and 1996 from enclosures (chambers). Methane C-13 data in soil gas samples from the NSA Young Jack Pine (YJP) and Old Jack Pine (OJP) sites for 1994 and 1996 are also given. Additional data on the isotopic composition of methane (carbon and hydrogen isotopes) produced in the NSA beaver ponds and fen bog in 1993 and 1994 are given as well. The data are stored in tabular ASCII files.