A Caveat on Radiocarbon Dating of Organic-Poor Bulk Lacustrine Sediments in Arid China

Abstract

Characterized by a dry climate, the arid area of China represents a unique landscape. A proper understanding of the driving mechanisms behind the changes of this ecologically vulnerable landscape requires placing the instrumental records within a geological context. Lakes in this area bear rich information about past climatic and environmental changes presumably regulated by the westerlies at various timescales. The lacustrine records obtained in this area heavily rely on radiocarbon ages, which are usually subject to the temporal and spatial variability of the 14C reservoir effect. Yet, little is known about the 14C reservoir age of lacustrine systems in this area. This study reports an anomalously large 14C reservoir age of about 11,000 ± 2000 yr from a saline lake in NW China by comparing 14C and OSL chronologies. The modeling study suggests that this age offset appears to be an inherent phenomenon in lacustrine systems, which mainly arises from the introduction of pre-aged organic matter from the catchment and the conversion of 14C-depleted dissolved inorganic carbon to organic matter by photosynthesis. Compared to the large age offset induced by the 14C-dcficient exogenous carbon, the reservoir effect due to retention of organic matter in the lake water appears to be inconsequential. The results reveal the pitfall of 14C dating on organic-poor bulk lacustrine sediments in this barren landscape, and thus highlight the need for alternate dating methods to constrain the chronology of lacustrine records.