Processes controlling the 14C content of soil carbon dioxide: Model development

Chemical Geology, 107 (1993) 225-226 Elsevier Science Publishers B.V., Amsterdam Processes controlling the 14C content of soil carbon dioxide: Model development Yang Wang a, Ronald Amundson a and Susan Trumbore b aDepartment of Soil Science, University of California, Berkeley, CA 94720, USA bDepartment of Geosciences, University of California, Irvine, CA 92717, USA (Received March 1, 1993; revised and accepted March 30, 1993 ) Carbon-14 dates from soil carbonates have been considered unreliable estimates of the age ofpedogenesis because of unknown initial 14C/ ~2C ratios in the carbonate and the possibility of subsequent contamination with environ- mental 14C. Comparison of carbonate 14C ages with ~4C ages of coexisting organic matter sug- gests that radiocarbon dates calculated from pedogenic carbonate in arid areas were ~ 500- 7000 radiocarbon years too old. On the other hand, radiocarbon dates of pedogenic carbon- ate from the sub-humid part of southeastern Australia were much younger than either the known age of deposition in which the carbon- ate was segregated, or the likely age of pedoge- nesis. These discrepancies have been attrib- uted to an initial low Jac content of soil carbonate due to the limestone dilution ef- fect a n d / o r secondary contamination by envi- ronmental ~4C. The limestone dilution ef- fect states that soil carbonate derives half of its C from d e a d calcium carbonate and another half from atmospheric CO2, suggesting that ra- diocarbon age of such carbonate would be about one half-life of 14C ( ~ 5570 yr) older than the true age. However, studies on soil CO2 and ~ 3 C of soil CO2 and pedogenic carbon- ates indicate that the CO2 and its isotopic spe- cies in a soil system are in isotopic equilib- rium. This implies that the ~4C content of pedogenic carbonate should be determined by 14C content of soil CO 2 and inherited dead carbon should not affect the age of soil pedo- genie carbonate. We present here a diffusion-reaction model that analyzes the 14CO2 distribution in soils. Carbon dioxide is produced in soils by biolog- ical processes and is transported to the atmo- sphere by diffusion. The diffusion mechanism applies to its isotopic species ~2CO2, 13CO2 and '4CO2 as well. Different isotopic species of CO2 react and diffuse independently of each other according their own concentration gradient and their own sources and sinks. Since the concen- tration of ~4CO2 in a soil profile is controlled by the production and decay of ~4CO2, and by diffusion through the soil to the atmosphere, the concentration of ~4CO2 can be described by a diffusion-reaction equation: OC2 4 /-)14 v ~s Ot - - - s Oz 2 + where C2 4 represents the 14CO2 concentration in the soil air (mol c m - 3 ); D ] 4 is the diffusion coefficient of 14CO2 in the soil (cm 2 s-l ); q~]4 is the production of ~4CO2 in the soil (mol c m - 3 s - l ) ; and 2 the decay constant of 14C (3.84-10 -12 s -l ). To model 14CO2, informa- tion is needed regarding the production of ~4CO 2 (q~4) in a soil. With the simplified as- sumption that soil CO2 is primarily produced (a) by root respiration with practically no dif- 0009-2541/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.