Formation and paleoenvironment of rhizoliths of Shiyang River Basin, Tengeri Desert, NW China

摘要

Rhizoliths found in bed of the late Quaternary paleolake Zhuyezhe, Minqin Basin, central Tengeri Desert, NW China were studied. Vegetation coverage is present at some locations in the paleolake area although, much of the bed area is covered with moving dunes. Rhizoliths are occasionally eroded out and exposed to the air. The morphology of the rhizoliths resembles singular or branching carbonated tubules which are hollow in their central part. The rhizoliths are black to grey, and are broken and scattered randomly on the sand surface of the lake bed. The lacustrine deposits are black greenish silt or silty clay with a large quantity of white petite lake snail shells. The possible plant types, sources and formation conditions of carbon dioxide and calcium, the sedimentary and diagenetic environments, and the process of rhizolith formation were discussed via examining rhizoliths macromorphology, studying the micromorphology and mineralogy by microscopy, cathodoluminescence and scanning electron microscope, and studying the chemical compositions of cementing minerals and fragments by energy dispersive X-ray spectra. The original roots of the rhizoliths belong to hydrophytes, such as Typha latifolia, Scirpus maritimus and Carex stenophylla. Lake snails and previous pollen data indicate that the rhizoliths formed in a sedimentary environment of shallow fresh water lake like marginal or palustrine areas during the Holocene. Shallow lake water disturbance by desert wind above the loose sandy sediment or soil was favoring the rhizoliths formation. A continuous supply of oxygen through water disturbance led to complete oxidation of roots and producing carbon dioxide. Dissolution of CO2 in water and so, carbonic acid production resulted in minerals weathering such as feldspar and primary carbonate particles and the release of K+ and Ca2+ ions. At presence of CO2 and Ca2+ saturated in the pore water around the roots of hydrophytes within the sediments or soil, carbonate precipitation occurred around the root channel and led to rhizoliths formation as tubules. The petrifaction process therefore, has happened in margin of the paleolake with shallow water and weak redox condition, which implies the suitability of rhizolith for reconstructing paleoenvironment.