Carbon and hydrogen isotope composition and C-14 concentration in methane from sources and from the atmosphere: Implications for a global methane budget

The topics covered include the following: biogenic methane studies; forest soil methane uptake; rice field methane sources; atmospheric measurements; stratospheric samples; Antarctica; California; and Germany

Stable isotope fractionation during ultraviolet photolysis of N_2O

The biogeochemical cycling of nitrous oxide plays an important role in greenhouse forcing and ozone regulation. Laboratory studies of N_2O:N_2 mixtures irradiated between 193–207 nm reveal a significant enrichment of the residual heavy nitrous oxide isotopomers. The isotopic signatures resulting from photolysis are well modeled by an irreversible Rayleigh distillation process, with large enrichment factors of ε_(15,18)(193 nm) = −18.4,‐14.5 per mil and ε_(15,18)(207 nm) = −48.7,‐46.0 per mil. These results, when combined with diffusive mixing processes, have the potential to explain the stratospheric enrichments previously observed

Profiles of alpha 13 C and alpha D in methane from the lower stratosphere

Methane is an important greenhouse gas of biogenic and anthropogenic origin for which global budgets are being constructed from a variety of data. One approach to the global methane budget is the use of the stable isotopes C-13 and D, and the radionuclide C-14 as tracers. The authors measured the isotopic composition of methane from various sources and in tropospheric air for a number of locations. Here, the authors report on the isotopic composition of methane from the lower stratosphere. Measurements of this concentration in the stratosphere can yield estimates for the kinetic isotope effects in the methane destruction reactions. These effects have to be known for quantitative isotopic methane budgets

Reconstruction of glacial/interglacial changes in the global carbon cycle from CO2 and d13 CO2 in Antarctic ice cores (scientific paper)

High-resolution CO2 and δ13CO2 records from the Taylor Dome ice core, Antarctica, reveal significant changes in the global carbon cycle over the last 30,000 yrs. CO2 concentrations increase from 180 ppmv at 20 kyr before present to 275 ppmv at the start of the Holocene with an intermittent decline during the Antarctic Cold Reversal. δ13C shows a net increase from the average glacial level of -6.7‰ to mean Holocene values of around -6.5‰. However, the transition period is initiated by an 0.5‰ drop at 20 kyr before present followed by an 0.7 ‰ increase which is interrupted by an 0.2‰ decline during the Antarctic Cold Reversal. Isotope budget considerations show that during the Holocene and the Last Glacial Maximum changes in the size of terrestrial biosphere can account for the observed changes in CO2 and δ13C. In contrast changes in the atmosphere/ocean system are dominant during termination I. The size of the CO2 increase cannot be explained by variations in ocean temperature, salinity or net transfer of carbon into the terrestrial biosphere alone. The strong temporal correlation of CO2 and Antarctic isotope temperatures is in conflict with hypotheses to account for the missing CO2 which are connected to sea level change or a dust induced change in the biological productivity in the Southern Ocean. Changes in the Southern Ocean sea ice cover, however, may provide a direct link to temperature which appears to be able to account for the observed changes in atmospheric CO2 and δ13C

The Cl-36 in the stratosphere

Initial measurements of the cosmogenic radionuclide, Cl-36, in the lower stratosphere were made by accelerator mass spectrometry. Samples were obtained using the large volume LASL air sampling pods on a NASA WB-57F aircraft. Untreated (for collection of particulates only) and tetrabutyl ammonium hydroxide treated (for collection of particulates and HCl) IPC-1478 filters were flown on three flights in the lower stratosphere. Chlorine (Cl) and Cl compounds are important trace constituents for stratospheric chemistry, in particular with respect to O3 destruction. Stratospheric Cl chemistry has recently received increased attention with the observation of strong O3 depletion in the Antarctic winter vortex and in the weaker and more complex Arctic winter vortices. Cosmogenic (Cl-36) is produced by spallation reactions from Ar mainly in the stratosphere, and has had several applications as a geochemical tracer. The large amounts of Cl-36 introduced by nuclear weapon testing have been removed from the stratosphere by now, and measurements in the stratosphere to obtain cosmogenic production rates and concentration distributions is now possible. The use of cosmogenic Cl-36 as a tracer for stratospheric Cl chemistry and for stratospheric/tropospheric exchange processes is investigated. A first attempt to determine stratospheric and tropospheric production rates, the partitioning of Cl-36 among particulate and gaseous Cl compounds, and the respective inventories and removal rates is being made. Results from a flight at 13.7 km, 30-33 degrees N, 97-107 degrees W, and from a second flight at 17.7 km, 43-45-36 degrees N, 92-94 degrees W, for the untreated and treated filters respectively are presented

CO(2) Diffusion in Polar Ice: Observations from Naturally Formed CO(2) Spikes in the Siple Dome (Antarctica) Ice Core

One common assumption in interpreting ice-core CO(2) records is that diffusion in the ice does not affect the concentration profile. However, this assumption remains untested because the extremely small CO(2) diffusion coefficient in ice has not been accurately determined in the laboratory. In this study we take advantage of high levels of CO(2) associated with refrozen layers in an ice core from Siple Dome, Antarctica, to study CO(2) diffusion rates. We use noble gases (Xe/Ar and Kr/Ar), electrical conductivity and Ca(2+) ion concentrations to show that substantial CO(2) diffusion may occur in ice on timescales of thousands of years. We estimate the permeation coefficient for CO(2) in ice is similar to 4 x 10(-21) mol m(-1) s(-1) Pa(-1) at -23 degrees C in the top 287 m (corresponding to 2.74 kyr). Smoothing of the CO(2) record by diffusion at this depth/age is one or two orders of magnitude smaller than the smoothing in the firn. However, simulations for depths of similar to 930-950m (similar to 60-70 kyr) indicate that smoothing of the CO(2) record by diffusion in deep ice is comparable to smoothing in the firn. Other types of diffusion (e.g. via liquid in ice grain boundaries or veins) may also be important but their influence has not been quantified

A Record of Atmospheric Co2 During the Last 40,000 Years from the Siple Dome, Antarctica Ice Core

We have measured the CO2 concentration of air occluded during the last 40,000 years in the deep Siple Dome A ( hereafter Siple Dome) ice core, Antarctica. The general trend of CO2 concentration from Siple Dome ice follows the temperature inferred from the isotopic composition of the ice and is mostly in agreement with other Antarctic ice core CO2 records. CO2 rose initially at similar to 17.5 kyr B. P. ( thousand years before 1950), decreased slowly during the Antarctic Cold Reversal, rose during the Younger Dryas, fell to a local minimum at around 8 kyr B. P., and rose continuously since then. The CO2 concentration never reached steady state during the Holocene, as also found in the Taylor Dome and EPICA Dome C ( hereafter Dome C) records. During the last glacial termination, a lag of CO2 versus Siple Dome isotopic temperature is probable. The Siple Dome CO2 concentrations during the last glacial termination and in the Holocene are at certain times greater than in other Antarctic ice cores by up to 20 ppm (mumol CO2/mol air). While in situ production of CO2 is one possible cause of the sporadic elevated levels, the mechanism leading to the enrichment is not yet clear