Chemical Geology (Isotope Geoscience Section), 59 (1986) 283-292. Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 283 DATING PEAT WITH U/Th DISEQUILIBRIUM: SOME GEOCHEMICAL CONSIDERATIONS ALBERT VAN DER WIJK, FARID EL-DAOUSHY*, AREND R. ARENDS and WILLEM G. MOOK Isotope Physics Laboratory, University of Groningen, 9718 CM Groningen (The Netherlands) (Received November 12,1986; revised and acceptedJune 3,1986) Abstract Van der Wijk, A., El-Daoushy, F., Arends, A.R. and Mook, W.G., 1986. Dating peat with U/Th disequilibri- um: some geochemical considerations. Chem.Geol. (Isot. Geosci. Sect.), 59: 283-292 The applicability of the U/Th disequilibrium dating method (UTD) for peat is investigated. Dating is performed by measurements of the U and Th isotopesin the bulk material of the sampleaswell asin different fractions. Due to the complex geochemistry of U and Th in organic matter, dating peat by UTD is not a simple and straightforward method. It may appear from our resultsthat the number of cases in which the method is applicable is limited because post-formation contamination with Th may occur. How- ever, one interesting case, the Tenaghi Phillippon core from Greece, seems to fulfii the requirement that little or no environmental Th has entered the system after formation. Results obtained from this core agree well with “C data. Evidence is found for the theory that complexation of actinides with humic and fulvic acids limits the mobility of U and Th. The data can be explained assuming that ion exchange is the dominant process in the accumulation of U and Th by peats. 1. Introduction In studies of organic sediments such as peats, the 14C dating method is routinely in- corporated. However, it only applies to depos- its younger than 70 ka B.P. so that there is still need for a method that extends beyond this limit. Vogel and Kronfeld (1980) suggest- ed a new method for dating peat based on preferential uptake of U by peats due to its high solubility in groundwater relative to Th. The amount of 230Th in the sample, origi- nating from radioactive decay of 234U (radio- genie Th), is a measure of the sample age. In principle, ages up to 350 ka can be measured *Present address: Institute of Physics, University of Uppsala, Box 530, S-751 21 Uppsala, Sweden. adequately because of the long half-lives of 23@Th (75.2 ka) and 234U (247 ka). Vogel and Kronfeld (1980) assumed zero initial Th activity in peat. The presence of 232Th in their samples was attributed to the presence of detrital inorganic material in the sample. The inorganic 23?l?hcontribution was calculated from the average of detrital 230Th/ 232Th activity ratios measured at several local- ities in South Africa. Ages obtained were shown to be in good agreement with 14Cages. They appeared not to be very sensitive to the magnitude of the detital 230Th/232Th activity ratio f, due to relatively low 232Th activities. However, inorganic 230Th/232Th activity ratios vary strongly in space and time as shown by Szabo (1982). Therefore in peat with high concentrations of inorganic detri- 0168~9622/86/$03.50 @ 1986 ElsevierSciencePublishers B.V.