Recrystallization of Holocene calcareous root tubes in the Tengger Desert, Northwest China and its effects on the reliability of paleoenvironmental reconstruction results

Reconstructing paleoenvironmental conditions using geochemical proxies from pedogenic carbonate must ensure that the pedogenic carbonate is not recrystallized. Calcareous root tubes (CRTs) formed by carbonate precipitation along the roots have been widely used for paleoenvironmental reconstruction in arid and semi-arid regions. However, whether further recrystallization occurs after CRT formation and its impact on the reliability of paleoenvironmental reconstruction remains uncertain. In this study, 52 Holocene age CRT samples obtained from 17 sites in the Tengger Desert of northwestern China were described using cathodoluminescence and optical microscopy to clarify this uncertainty. The CRT outer (detrital) layer is composed of detrital material and carbonate, while the inner (precipitated carbonate) layer is composed only of carbonate. The results of this study showed that recrystallized carbonates occurred in the detrital layers of 45 of the samples (~87%), but no recrystallization was observed in the precipitated carbonate layer in any of the CRT samples. CRT recrystallization is caused by the introduction of calcium-bearing meteoric waters after CRT formation. These CRT recrystallization characteristics not only occurred in the Tengger Desert, but also in other regions with seasonal soil moisture deficits. This study showed that carbonate from the CRT detrital layer is not a suitable material for ¹⁴C dating and δ¹³C and δ¹⁸O analyses; however, carbonate from the precipitated carbonate layer could be a suitable material for these analyses. Moreover, paleoenvironmental reconstruction results derived from changes in the Sr/Ca and Mg/Ca ratios of CRTs should not be affected by recrystallization.