Freshwater Reservoir Effects in the Eurasian Steppe Zone and Their Influence on the Radiocarbon Ages of Bone Samples

Reservoir effects often hinder the accuracy of the radiocarbon dates from human and animal bone samples. The freshwater reservoir effect (FRE) refers to the difference between the radiocarbon age of the purely terrestrial sample (the 14C content of which is comparable to the atmospheric 14C level) and contemporary freshwater sample or any sample, in which part of 14C comes from the reservoir with a lower level of 14C compared to atmosphere. The latter samples include not only aquatic fauna, but also terrestrial animals, whose diet includes aquatic sources. The aim of this work is to demonstrate the widespread and variability of FREs on the territory of Russia and Kazakhstan, as well as to draw the attention of specialists working in the area to the necessity of taking into account this important and still not fully understood factor while dating the bones. To identify modern FREs, nine samples of modern fish of various species from six regions of Siberia and Kazakhstan have been radiocarbon dated. The results were compared with data from previous studies of FRE in the three regions. The following observations can be made: 1) the FRE is clearly present in all analysed regions, except of the Kharga Lake. The Carassius sample from the Kharga Lake showed only minor influence of FRE, however, to confirm the absence of the effect in this reservoir, further analysis of modern aquatic fauna is required; 2) the extent of the FRE in the area varies significantly between different reservoirs; 3) the FRE results within the same reservoir appear to be distinct for different species of fish. The four samples of fish from the middle Yenisei River (and its tributary Karasuk River, Minusinsk basin) revealed ca. 600 14C year difference in the FRE values between the samples. The difference between the two pike specimens from the Karasuk River is more than 100 14C years; 4) at the moment, the most significant FRE offsets (nearly 2,00014C years) have been recorded in the upper Lena River (Irkutsk region).