Radiocarbon dating of playa-lake hydrologic changes: Examples from northwestern China and central Australia

Abstract

Accurate radiocarbon dating of arid-zone playas is restricted by shortage of suitable organic matter. Analyses from two large systems in Australia (Lake Frome) and China (Qarhan playa in the Qaidam Basin) demonstrate the problems associated with the use of carbonates for dating. Organic carbon, although requiring treatment of large samples (1–2 kg), provides the most accurate basis for establishing chronologic control. In Qarhan, a deposit of halite averaging about 30 m thick, extends over an area of 5800 km 2. Carbonate in the system is dominantly derived as clastics from loess transported by northwesterly winds. The large component of dead carbon thus involved explains anomalous patterns of dates derived from these materials. Such results are consistently much older than those based on organic carbon. In Lake Frome, subject to a more maritime climate in central Australia, the carbonate component is dominantly authigenic lacustrine in origin contaminated by a small detrital component. It provides ages about one half-life older than those from organic carbon. The sequence from Qarhan, supplemented by evidence of lacustral (pluvial) episodes in two other sub-basins in Qaidam, Kunteyi and Xiao Qaidam, illustrates the presence there of expanded waterbodies persisting from at least 40,000 to about 15,000 yr B.P. in areas where little surface-water persists today. The large evaporite deposits formed between about 25,000 to sometime after 15,000 yr B.P. Holocene climate appears to have remained hyperarid like that of today. At Lake Frome, an early lacustral phase gave way before 18,000 yr B.P. to drying and aeolian activity. Water had returned to the system by 16,000 yr B.P. signalling a decline if not the end of aridity. Lacustrine conditions persisted through early Holocene with drying and development of playa facies about 7000 yr B.P. After a brief return to lacustrine conditions between about 6000-4000 yr B.P. a dry playa environment has continued to the present day. The Frome and Qarhan playas, although in comparable latitudes lie in very different climatic and tectonic settings. Their palaeohydrologic records are neither in phase nor in distinct anti-phase emphasizing the complexity of comparing palaeoclimatic records from different circulation systems.