Freshwater reservoir effect on re-dating of Eurasian Steppe cultures

Siberia and the Eurasian steppe is an unmatched economical, political and cultural interface between the East and West from early prehistory to the present. The turbulent history of migrations and collisions of tribes and nations, rise of empires and fall of civilizations in the region has forged its unique heritage drawing enormous interest of the global academic community (Mair 2006; Kuzmina 2008; Beckwith 2009). A growing number of 14C ages from archaeological human bone samples appear considerably older than expected from established archaeological chronologies based on associated wood and/or charcoal 14C ages and on traditional archaeological chronology for the sites (Epimahov et al. 2005; Görsdorf et al. 2001; Alekseev et al. 2005), possibly due to the freshwater reservoir effect (FRE). The refinement of radiocarbon dates in the region is crucial for understanding the chronology of cultural transitions and development of these interactions. There is a strong demand to document FRE in various regions of Siberia and the Eurasian steppe. –It is often particular human burials that are of interest but they do not always have associated terrestrial material for dating. Furthermore, there is a large number of human bone 14C dates from past research, and it is important to assess their accuracy, where a FRE correction is required. Archaeologists often sample human bone to accurately radiocarbon (14C) date past societies, but the reservoir effect is a major source of dating uncertainty when a portion of the carbon in the diet comes from a non-atmospheric reservoir with a lower 14C level (e.g. marine/freshwater). The difference between 14C age of such a bone sample and that of a contemporaneous, purely terrestrial sample is termed the “reservoir offset”. The extent of the marine reservoir effect has been widely explored in various regions of the world. Stuiver&Braziunas (1993) estimated the average global marine reservoir correction. Regional differences from this correction (ΔR) based on 14C measurements of known age, pre-bomb marine samples (i.e. collected before atmospheric nuclear testing in the 1950’s and 1960’s) have been compiled in the marine reservoir correction database (www.calib.org/marine). This data is used to correct 14C ages from human/animal bone samples with an estimated percent contribution of marine resources in their diet. Marine ΔR is assumed to be constant for a given region although it may vary with time (Ascough et al. 2007). In contrast to the marine reservoir effect, research on the extent of freshwater reservoir effect is rather scarce. Most FRE studies have been focussed on Europe (Cook et al. 2001, 2002; Fischer&Heinmeier 2003; Olsen et al. 2010; Keaveney&Reimer 2012; Wood et al. 2013; Lougheed et al. 2013) and North America (e.g. Ingram&Southon 1996; Goodfriend&Flessa 1997; Culleton 2006), and only a few in Siberia and theEurasian Steppe (Lillie et al. 2009; Nomokonova et al. 2013; Shishlina et al. 2007, 2009, 2012). The main conclusions from this research are that FRE is highly variable through space, may be offset from true radiocarbon ages by several thousand years, and that “each population thought to be affected by a FRE must be examined individually” (Wood et al. 2013). The isotopic baseline can also be sufficiently variable, with freshwater resources demonstrating elevated “marine” δ13C values (Katzenberg&Weber 1999).

Archaeologists often sample human or animal bones to accurately radiocarbon (14C) date past societies, but there can be a major error when a portion of the carbon in the diet comes from a lake or river with an old carbon source (e.g. limestone). This resulting offset between the human or animal bone and a terrestrial samples such as a plant is called the freshwater reservoir offset. This project investigated the magnitude and variability of this offset in the Eurasian steppe by radiocarbon dating paired human or animal bone and terrestrial material as well as modern fish.

a) freshwater reservoir effects are widely dispersed and highly variable in the Eurasian Steppe in both modern and archaeological samples, including humans (the latter is particularly important for archaeologists as human remains are very often sampled for 14C dating and the resulting dates might be misleading). FRE values vary between 2 and 752 14C years in modern fish, 0 and 1071 14C years in archaeological humans, and between 15 and 1421 14C years in archaeological fish;
b) there is an inconsistency in freshwater reservoir offset values between modern and archaeological fish within single sites; for example, modern fish from the Kharga Lake (Republic of Buryaia) show an offset of 250 14C years while archaeological fish show virtually no offset; conversely, in the Keksen Kuyuk Kala (Juvara) site in Kazakhstan (Aral sea region) modern fish demonstrate an FRE offset of 57 14C years while archaeological fish have an offset value of 263 14C years.
c) there is also variability of freshwater reservoir offsets in modern fish within single reservoirs between different species and fish of different size/age probably due to differing diets of the fish but also potentially to modern agriculture practices; as a clear example – offsets in modern fish from the Minusinsk Basin (Middle Yenisei River flow) vary between 165 and 752 14C years.
d) there is no relationship found between freshwater reservoir offsets and δ13C or δ15N values of either humans or fish;
d) δ34S tests for the samples from the Minusinsk Basin (Southern Siberia) were inconclusive. The results between archaeological humans and herbivores, and modern fish are overlapping and as such do not contribute to the palaeodietary picture for the region;
e) for modern and archaeological fish samples, both δ13C and δ15N values are overlapping and very variable even within single sites (though mostly reflect C3 ecosystems);
f) there is a strong influence of climate on isotope values of terrestrial inhabitants – archaeological humans and fauna from various sites in Kazakhstan (with a generally more arid climate than in South and West Siberia) are generally elevated in nitrogen isotopic values.