• 14C Depth Profiles as Indicators of Trends of Climate and 14C/12C Ratio

      Brown, Robert H. (American Journal of Science, 1986-01-01)
      Composite curvature averages for 14C age depth profiles of deep ocean sediment, continental sediment, and soil each indicate a global trend for 14C age increment per cm depth to increase with 14C age over the range for which a definitive statistical sample is available. The global trend indicated for peat profiles is constant 14C age increment per cm depth over the past 10,000 14C yr. Correlation coefficients between changes in 14C yr/cm and maximum profile thickness contradict compaction as an adequate explanation for the global trend indicated by sediment and soil profiles. This trend must be explained by additional factors such as progressively decreasing contamination from older carbon, increasing cosmic ray intensity, decreasing geomagnetic intensity, diminishing 12C in the active biosphere during profile accumulation, and climate factors affecting the rate of accumulation. The diverse trend of peat profiles may indicate climatic conditions more favorable to peat growth during the earlier portion of the past 10,000 yr.
    • Annual Variations of the 14C Content of Soil CO2

      Dörr, Helmut; Münnich, K. O. (American Journal of Science, 1986-01-01)
      A 6-year and a 2-year record of 14C measurements of soil CO2 in two soils are presented and discussed. The annual 14C variation of soil CO2 is controlled by the seasonally varying contribution of root respiration and of microbial decomposition of organic matter producing soil CO2. The Delta-14C soil CO2 difference between summer and winter is ca 50 per mil in a soil where turnover of organic matter is fast (r = 2.5a) and ca 100 per mil in a soil of slow turnover (r = 60a). A simple model describing the movement and turnover of organic matter is derived, giving the depth distributions of organic carbon and of 14C. The model needs a subdivision of the carbon reservoir into at least two reservoirs with residence times of r1= 1a and r2 = 100a, respectively, and with a vertical transfer velocity in the order of 0.6mm/a.
    • Institut Royal du Patrimoine Artistique Radiocarbon Dates XI

      Dauchot-Dehon, Michele; Van Strydonck, Mark; Heylen, Jon (American Journal of Science, 1986-01-01)
    • The Temporal Distribution of 'Bomb' 14C in a Forest Soil

      Harkness, D. D.; Harrison, A. F.; Bacon, P. J. (American Journal of Science, 1986-01-01)
      Patterns of 14C enrichment in the superficial plant debris and mineral soil horizons of an established woodland have been monitored at regular intervals during the past 15 years. These data are compared with a model evaluation of carbon turnover based on the recorded changes in atmospheric 14C concentration since AD 1900. Leaf litter and decomposing plant debris are characterized by steady-state turnover values of ca 2 and ca 8 years, respectively. A two-component system of `fast' (less than or equal to 20 yr) and `slow' (ca 350 yr) cycling carbon is indicated for the surface (0-5cm) soil humus; below 10cm, the `fast' component is rare (<5%). Selective microbal humification of leaf litter, branch, and root debris is proposed to explain a delay of several years in the peak transfer of `bomb' 14C to the soil carbon pool.
    • The Use of Natural and Anthropogenic 14C to Investigate the Dynamics of Soil Organic Carbon

      O'Brien, Bernard John (American Journal of Science, 1986-01-01)
      Radiocarbon has been measured in two soil profiles, one of which has been covered by a building since 1956. A comparison of the Delta-14C values in horizons of each profile gives an estimate of the total input of atom bomb 14C into the soil profile. From the Delta-14C and carbon density profile data, the carbon input rates, respiration rates, and diffusivity are calculated. The lack of vegetation on one soil affects the mobility and the respiration rate of the soil carbon in that soil. The data from this soil profile are also used to check the assumption, used in previous analyses, that there is a uniform distribution of "old" carbon down the soil profile. The input rate, turnover time, and diffusivity parameters determined from the Delta-14C profiles in these soils are compared with other published data on pasture and forest soils.
    • Thin Layer delta-13C and Delta-14C Monitoring of "Lessive" Soil Profiles

      Becker-Heidmann, Peter; Scharpenseel, Hans-Wilhelm (American Journal of Science, 1986-01-01)
      The natural 14C and 13C content of soil organic matter and their dependence on depth for two Alfisols are presented. This soil type which covers a large area of the earth's surface is characterized by clay migration processes ("Lessivé"). The samples were taken as successive horizontal layers of 2cm depth from an area of ca 1 m2 size as deep as the C content allows 14C analysis. The minima of the D14C distribution decrease with depth, while the maxima increase in the upper, leached horizon (A1) due to bomb 14C and decrease in the lower, clay illuviated (Bt). delta-13C indicates proceeding decomposition in Al and protection of carbon, probably due to the formation of clay humus complexes in Bt. delta-13C values were also used for age correction of the 14C data due to isotopic fractionation. The D14C and delta-3C depth distributions are characterized by sharp peaks at the boundaries of the horizons, probably caused by the influence of textural changes on the transport of C with percolating water.