Microfacies analysis and varve-based chronology reveal a variable ¹⁴C reservoir effect in Lake Shira, northern Inner Asia, over the past 2500 years

Here, we present the first fully varve-based chronology for the deposits of the deep-water Lake Shira (Chulym-Yenisei Basin, South Siberia), derived from a new sediment core. The very well-preserved varves show typical properties of the clastic-biogenic and endogenic types that can be subdivided into four sublayers representing winter–early spring, late spring, early summer and late summer–autumn. The analysed sediment section of 147 cm length comprises 2491 varve years with a total counting error of 1.6 % (i.e. ±40 years), making the new sediment core from Lake Shira a unique high-resolution archive for multi-proxy studies of past climate/environmental change and human-environment interactions. Direct comparison of nine AMS radiocarbon (¹⁴C) dates based on sediment bulk organic fractions with an age-depth model based on varve counting made it possible to examine the ¹⁴C reservoir effect in the lake. The reservoir effect is a common issue when estimating the age of environmental proxies from lacustrine sedimentary archives in Inner Asia. Although a constant reservoir effect is commonly used to correct the ¹⁴C dates from a single core or lake basin, our results from Lake Shira demonstrate that it varies significantly over the last 2500 years, ranging from 240 ± 30 to 1045 ± 30 years. The spatiotemporal variability of the reservoir effect can considerably reduce the accuracy of age-depth models based solely on the bulk organic sediment fraction. Where varved sediment is unavailable, as is usually the case, lignin phenols, terrestrial plant remains and purified pollen concentrates should be considered as alternative dating materials.