A PROCESS- AND PROVENANCE-BASED ATTEMPT TO UNRAVEL INCONSISTENT RADIOCARBON CHRONOLOGIES IN LAKE SEDIMENTS: AN EXAMPLE FROM LAKE HEIHAI, NORTH TIBETAN PLATEAU (CHINA)

Aquatic macrophytes from a lacustrine environment are highly prone to a reservoir effect, resulting in an overestimation of age. This is often caused by the incorporation of dissolved carbon (CO2 and HCO3-) through photosynthesis from lake waters that have a different C-14 activity than the atmosphere. The atmosphere-water disparity is often produced by a mixing of carbon between the water body and its terrestrial surroundings, a process highly prone to temporal variations. Thus, only a comprehensive understanding of the C-14 budget over time enables a reliable chronology of lacustrine records. We studied lacustrine sediments from Lake Heihai on the northern Tibetan Plateau with a recent reservoir effect of 6465 +/- 75 C-14 yr as estimated from accelerator mass spectrometry (AMS) dating of three living aquatic plants. Age inversions in a well-laminated composite core from the lake suggest that the reservoir effect markedly changed over the depositional period. In the lower part of the core, an excellent correlation was observed between the allochthonous input of dolomite and the inverse C-14 ages, indicating the incorporation of dissolved C-14-dead carbon from a limestone catchment in the plant material. For the upper part of the core, sediment recycling of Holocene high-stand deposits may have further contributed to the reservoir effect. These findings give rise to a reliable process-and provenance-based chronology within a confidence interval supported by Cs-137 measurements and magnetostratigraphic investigations. Our results highlight the need to identify the interactions of lakes with their surroundings to estimate reservoir-corrected ages in lacustrine settings.