POSSIBLE FACTORS CAUSING OLDER RADIOCARBON AGE FOR BULK ORGANIC MATTER IN SEDIMENT FROM DAIHAI LAKE, NORTH CHINA

Many factors may influence the radiocarbon age results of lacustrine sediments, among which the hardwater effect is particularly important. Daihai Lake is a closed lake located in the semi-arid region of Inner Mongolia, China. High concentrations of HCO3- and CO32- and high pH values in the lake water imply that there is a hardwater effect when using bulk lacustrine sediment samples for C-14 dating. To correct the apparent C-14 age, we present a pilot study based on a series of C-14 ages of lake surface sediment, lake water, submerged aquatic plant (Myriophyllum), fish bone (Cyprinus carpio), and surface soil samples from and around Daihai Lake. Assuming that the relationship between the C-14/C-12 ratio of DIC and of atmospheric CO2 was constant (at 0.816), the hardwater effect ages calculated for the past 8000 yr would have varied from 949 to 1788 yr. Together with the reservoir effect and soil organic matter input, the hardwater effect is a major factor causing changes in apparent age when using bulk organic matter for C-14 dating.