Carbonate Rhizoliths in Dune Sands of the Belaya River Valley (Upper Angara Region)

Аннотация

Results of investigating carbonate rhizoliths formed in the Early Holocene dune sands in the Belaya River valley are presented. Carbonate accumulations are characterized by large sizes (2 to 7 cm in diameter, and about 1 m in length), which is associated with a high content of carbonates in the enclosing sediments and long-lasting rhizoliths formation. Morphology, mineralogical and isotopic composition of accumulations attest to their origin related to roots and root microorganisms activity. The redistribution of carbonates from the surrounding sediments and their concentration near the roots increased the carbonate content there to more than 30%. With the distance from the roots, the amount of carbonates decreases to 10–12% causing a concentric structure of rhizoliths. In the cross-section of the accumulations, the central and peripheral parts have distinct differences: in the central parts, carbonates are completely recrystallized and represented by pure calcite with insignificant inclusions of quartz and feldspars; on the periphery, coarse-grain silicate material prevails, and it is weakly cemented by clay-carbonate plasma bridges. The δ¹³C values of carbonates range from –7.4 to –1.5‰ and increase to the periphery owing to recrystallization of primary carbonates. The δ¹³C value of the rhizoliths’ organic residues is –22.15‰ indicating the formation of carbonate accumulations around the roots of C3-vegetation. The carbonate δ¹⁸O values of rhizoliths vary from ‒10.34 to –11.99‰ demonstrating the trend towards ¹⁸O enrichment from the inner to outer layers. Calculation of the annual temperatures using the δ¹⁸O values of the inner layers of rhizoliths consisting only of secondary carbonates, showed significant temperature deviations during the formation of carbonate accumulations from modern ones. The radiocarbon age of rhizoliths carbonate cementations (7160 ± 100 kyr BP) is slightly less than the age of enclosing deposits. Organic residues from the central parts of the rhizoliths are much younger (1770 ± 40 kyr BP) which is explained by the penetration of organic matter of soils and plant biomass through the cavities which apparently did not cause contamination of the carbonate component of the rhizoliths with younger carbon.