Showing papers by "Martin Heimann published in 1980"

14C Variations Caused by Changes in the Global Carbon Cycle

Abstract: A box-diffusion model for the carbon cycle is used to estimate the magnitude of 14C variations caused by changes of reservoir sizes and exchange fluxes in the global carbon system. The influence of changes in atmospheric CO2 concentration, biomass, CO2 exchange rate between atmosphere and ocean, and ocean mixing is considered. Steady-state 14C concentrations as well as the transients are calculated. For changing biomass, atmospheric CO2 levels and 13C/12C ratios are also calculated. Carbon-cycle-induced 14C variations may have been significant in the transition period from Glacial to Postglacial when drastic changes in environmental conditions took place within short time periods, while they were probably less important during the climatically more stable Postglacial. Changes of the oceanic circulation, as supposedly occurred, are considered the most important factor, besides variations of the production rate, affecting the global distribution of 19C. 14C variations due to changes of the atmospheric CO2 level or the air-seaexchange probably did not exceed one to a few percent. Fluctuations of the forest biomass, which may have occurred between Glacial and Postglacial, hardly affected the 14C concentration over a long term. Responses of the atmospheric 14C concentration are also calculated for variations of the 14C production rate by cosmic radiation. The following cases are considered: a step change, square-wave changes producing "wiggles", and sinusoidal variations.

Low-level gas proportional counting in an underground laboratory

Abstract: Results of measurements in an underground laboratory, 70m of water equivalent. below surface, are given. Background values of proportional gas counters with volumes between 16cc and 1.5L are lower by a factor of 2-4.5 compared to the values in the previously used laboratory (7m w.e.). High counting gas pressures, up to 5 at CHand up to 36 at P-10, enable the use of relatively small counters with correspondingly small background contributions from the walls. A separation of the residual background into different components is attempted, distinguishing a pressure dependent volume effect and surface correlated contributions. It can be shown that the selection of radioactively pure counter construction material is very important for a good low :level counting system.

The carbon cycle and its perturbation by man

Abstract: Calibration and validation of carbon cycle models is mainly based on isotope data of the steady state of the system as well as its responses to natural and anthropogenic disturbances. The progress in the use of carbon isotopes is reviewed. The discussion covers the following phenomena: Response of the system to the anthropogenic CO2 input: rise of atmospheric CO2, dilution of the atmospheric 14C concentration Prenuclear 14C distribution in the ocean 14C rise in atmosphere and ocean due to bomb produced 14C Modulation of natural 14C production and its atmospheric response, as reflected in 14C/C ratios recorded in tree-rings Past changes of the atmospheric CO2 concentration inferred from ice core studies. The information is used to derive the probable range of the CO2 release, resp. uptake by the biosphere, and to check the general validity of carbon cycle models.