Radiocarbon age discrepancies between the carbonate cement and the root relics of rhizoliths from the Badain Jaran and the Tengeri deserts, Northwest China

Carbonate rhizolith is one of mineral encrustations of plant roots. The radiocarbon (C-14) dates and stable carbon isotopes of rhizoliths are widely used to study geochronology, paleoclimate, paleoecology and paleoenvironment changes. However, due to a few available comparisons of C-14 dates of the carbonate cement and the plant root relics within the same rhizoliths, the carbon sources that form rhizolith cements are largely unknown. Here, we present accelerator mass spectrometry (AMS) C-14 dates of carbonate cement and the plant root relics within the same rhizoliths from dune surface soils in the Badain Jaran and the Tengeri deserts, Northwestern China. The modern dead plant roots and bulk soil carbonates nearby were also dated. It is found that the root relics range from 1825 CE to 1983, while the carbonate cements of the rhizoliths have ages from similar to 6,900 to similar to 4,700 calibrated years before present (calBP). The ages of the bulk soil carbonates vary from 18,700 to 35,300 and 39,300 calBP, revealing complicated soil carbonate matrix. The results suggested that the carbon sources for the carbonate cements of the rhizoliths were derived mainly from ancient soil carbonates with a small contribution from dead plant roots and a minor contribution from lithogenic carbonates. Mass balance of stable carbon isotopes was used to calculate the proportional contribution of the carbon from ancient soil carbonates and plant roots relics. After subtracting the modern carbon contribution from the decomposed plant roots in fraction of modern carbon ((FC)-C-14), the mass balance estimated that the ancient soil carbonates were formed within 7,750-5,000 calBP, about 850-300 years older than obtained rhizolith cement ages. It is proposed that dead roots decay triggered the rhizoliths formation by releasing CO2 to form acid media with free HCO31-/CO32- ions, which caused dissolution of the old soil and lithogenic carbonates in soil matrix. Carbonate recrystallization took place to form rhizoliths tubes around the dead roots. Detailed age discrepancies between rhizolith cements and dead root relics provide new insight for better understanding the dynamics of the rhizolith formation and associated implications for paleoenvironment reconstructions.