Carbonate crusts of Paleolake Zhuyeze, Tengeri Desert, China: Formation mechanism and paleoenvironmental implications

摘要

Carbonate crusts with unusual morphologies are scattered on the Holocene sand surface, i.e., dunes, which cover the Paleolake Zhuyeze bed in the Tengeri Desert, China. Two types of the crusts were identified. The first type contained fossilized wrinkles of plant root cortex, while the second type consisted of hollow chambers without the wrinkle features. Optical microscopy, scanning electron microscopy and cathodoluminescence analyses were performed to investigate the mineralogy and geochemistry of the crusts. Fossilized cortex texture imprints on the first type of crusts suggested that the crusts formed around the deceased rhizomes of reeds (Phragmites communis) as the nuclei of encrustation. Weakly oxidizing soil-sediment with rhizome was a prerequisite, which was favorable for rhizome decomposition to produce sufficient CO2 and HCO3- and generate carbonate minerals of encrustation on rhizome surfaces, with lake water as the main Ca2+ supply. A conceptual formation mechanism of the encrustation was conjectured. Similar to the formation mechanism of the first type, another type of crusts with chambers and without fossilized imprints of the cortex textures formed around the spherical rhizomes of Scirpus maritimus. The crust characteristics and other evidence suggested that the lake water was fresh; water plants, such as P. communis and S. maritimus, and snails previously inhabited the water; and encrustation reflected a stable, slow-oxidizing soil-sediment environment. These desert crusts provided insights to study paleoecology, paleohydrology and water-soil/sediment interface environment in the paleolake.