Massive Deposition of Carbonate Nodules in the Hyperarid Northwest Qaidam Basin of the Northern Tibetan Plateau

Abstract

Concretionary nodule formation is a subaerial process common to both Earth and Mars. The resulting nodules contain information about particular paleoclimatic cycles. However, their genesis and the mineralogical and geochemical effects of climate change on nodule formation have not well constrained so far, limiting their applications to paleoclimatic reconstruction. Since the late Pleistocene, the Qaidam Basin in the northern Tibetan Plateau has been subjected to extreme drought, resulting in a vast area of playas with a diverse eolian morphology that resembles the surface of Mars. Recently, a massive carbonate nodule field was discovered on an ancient diluvial platform in the northwest Qaidam Basin. Detailed analyses revealed that an early period of aragonite precipitation (251 kyr before present) provided seeding material for the later growth of nodule bodies during a period of ∼100 kyr. The highly unstable hydrologic and geochemical conditions during this period led to the growth of high-Mg calcite zones with varied elemental contents (e.g., Mn), crystal patterns, and porosity levels within the nodule bodies. The δ13C and δ18O values increased by 3‰ and 6‰, respectively, from the cores to the rims of the measured nodules. Rayleigh distillation model suggested that 50% of the H2O and 25% of the dissolved CO2 had been removed during nodule growth in response to the drought. These results show that nodules formed in hyperarid environments can record important hydrologic information, which may provide insights for paleo-environment studies on Mars by examining the formation of nodules in Gale Crater.