Saline rhythm and climatic change since 20.6 kyr bp from the Qiulinanmu Playa Lake in Tibet

Abstract

A rare lacustrine rhythmite from the Qiulinanmu Playa Lake in Tibet is presented, which was composed of distinct layers of mirabilite, borax, and clay with thickness measuring 1.5–75 cm, 0.5–20 mm, and 0.5–30 mm, respectively. The thickness of each layer was measured in the field and dating was performed using 14C and U data. The lake was previously supplied by river water, groundwater, and hot springs enriched in sodium, sulfur, and boron. Control factors on the rhythmite were climate oscillations and compositional differences of water. The adsorption of boron by clay minerals could be attributed to the alternation between clay and borax layers. Low temperature, high pH, and high evaporation exerted significant influences on the precipitation of original borax and mirabilite. In the evaporative lake, the precipitation of mirabilite (a mineral indicative of a cold-dry environment) was dominant and produced the asymmetric couplets. The lake water was poor in chloride and rich in sodium sulfate, and an abrupt change was observed at 11.4 cal. kyr bp. According to the laminated sediment and the thickness of the minerals, the climate was dry-cold with four sub-warm phases before 11.4 kyr bp (after 20.6–11.4 kyr bp), and wet-warm with four sub-cold pulses after 11.4 kyr bp (11.4–5.2 kyr bp). The two thickest mirabilite layers suggested extremely hard conditions after 20.6 kyr bp, and before 11.4 kyr bp, respectively.