Carbonate cementing minerals in rhizoliths from Badain Jaran Desert: Implication for pedo-diagenesis and environment of dune soil

Abstract

Pedogenic carbonates are a good proxy of soil formation and functioning because the morphology of minerals is directly related with the processes of dissolution–precipitation. Carbonate rhizolith is one type of pedogenetic carbonate forming in modern soils of dunes and loess. The calcite micro-morphology with the sedimentary hydrogeochemical conditions prevailing during their formation remains open. Electron microscopy with an X-ray energy dispersive spectrometer and X-ray diffraction were used to investigate in detail the micro-morphology of carbonate cements of the rhizoliths and obtain paleoenvironmental information in the dune soil of Badain Jaran Desert in Northwest China. Two types of carbonates rhizoliths, primary rhizoliths (PR) and reworked rhizoliths (RR), were found. The PRs are present in situ within dune soils. The RRs are being eroded out from soil, potentially dislocated or transferred, and weathered at the soil surface. PRs displayed homogeneous micritic mass of calcite crystals, with pores, cavities and voids among the mass. RRs displayed various morphologies of calcite crystals (polyhedral, euhedral rhombic, tooth-like, prismatic and pillar), rosettes, short rods, and calcified fungal hyphae. The diagenesis differences at soil depth and surface affected the crystal morphologies. The homogeneous micritic mass of calcite crystals of the PRs were formed during the early diagenesis, preferentially in wet soils with semi-closed and semi-redox conditions. In contrast, the cementing minerals of the RRs became bigger and clearer in the late diagenesis than during early diagenesis due to recrystallization in open, dry and oxidative conditions at the soil surface. Therefore, the cement crystal morphologies of the rhizoliths can reflect the local environmental and climatic conditions. These objects can be used as environmental and pedo-diagenesis proxies in dune soil or sediments, recording the associated conditions of moisture, temperature, wind, erosion, weathering, solar radiation at the soil surface or at depth. Therefore, assessing the micromorphology of pedogenic carbonates has important implications for soil ecology and evolution as well as plant root physiology in deserts.