腾格里沙漠植物钙质根管Sr／Ca比值揭示的全新世千年尺度有效湿度变化

HOLOCENE MILLENNIAL-SCALE EFFECTIVE MOISTURE CHANGES REVEALED BY Sr/Ca RATIOS FROM CALCAREOUS ROOT TUBES IN THE TENGGER DESERT

在阿拉善高原广泛分布不同地质历史时期的植物钙质根管，探索从这些植物钙质根管中提取可靠的环境信号并进行古环境重建，是一项具有一定指示意义的工作。本文共采集了腾格里沙漠14组全新世植物钙质根管样品，在14C测年结果的基础上，通过x射线荧光光谱法对其进行元素测定，结果显示：8～5cal．kaB．P．阶段植物钙质根管中Sr／Ca比值较高；5～2cal．kaB．P．阶段时Sr／Ca比值较小。本文尝试用植物钙质根管中Sr／Ca比值的变化和植物钙质根管时间分布，重建该区域全新世千年尺度上的有效湿度变化：早全新世时期（8cal．kaB．P．之前）气候干旱，中全新世时期（8～5cal．kaB．P．）气候湿润，晚全新世时期（5～2cal．kaB．P．）气候由湿润转向干旱。上述重建结果与腾格里沙漠猪野泽地区以及毗邻的石羊河中游红水河地区的全新世千年尺度有效湿度变化重建结果基本一致，说明植物钙质根管中的Sr／Ca比值可以指示腾格里沙漠地区全新世千年尺度的有效湿度变化。因此，可以从植物钙质根管中提取可靠的古环境信号并将其应用于古环境重建研究中。

Calcareous root tubes, also called rhizoliths, are defined as secondary carbonate terrestrial plants. Calcareous root tubes frequently occur in calcareous soils with a crusts produced by the root of pronounced seasonal moisture regime. Paleo calcareous root tubes are widely distributed across the Alashan Plateau, Northwest China. Previous studies reveal that calcareous root tubes have potential to reconstruct the paleo-environment and paleo-vegetation. However, it remains unclear whether any environmental signals could be extract from the calcareous root tubes and used for reconstructing paleo-environmental conditions. The Tengger Desert, with area of approximately 42700km2 , is the fourth-largest desert in China. The annual average temperature of the Tengger Desert is 7.0- 9.7℃, the regional annual average precipitation is 125 - 160mm and the annual average potential evaporation is 2400 - 3000mm. The effective moisture level affects the formation of calcareous root tubes, and different geochemical elements has different features of transport and enrichment in various controlling factors. Therefore, influenced by the soil water changes and the special nutrient-acquiring mechanism of terrestrial plants that live in calcareous soils with deficit nutrition, Sr and Ca was enriched in the calcareous root tubes and the Sr/Ca ratios will changes in different effective moisture level. In this study, based on previous conventional 14C dating results of 14 Holocene calcareous root tube samples （37°54＇59″- 38°5′31″N, 104°10′34″- 105°22′59″E） collected from the Tengger Desert （37°26＇28″- 39°58′59″N, 102048′25″- 105°38′13″E）, we used the X-ray fluorescence to analyzed the elemental composition of the calcareous root tubes samples. The results show that the highest value of Sr/Ca ratios （2332.02μmol/mol） was occurred in 8-5cal.ka B.P. with the mean value of 1215.26μmol/mol, and the Sr/Ca ratios of calcareous root tubes was decreased during 5-2cal.ka B.P. with mean value of 891.81μmol/mol, but not in the Early Holocene. Based on the Sr/Ca ratios＇ variation and temporal distributions of the calcareous root tubes in the Tengger Desert, we reconstructed the effective moisture level changes in this area during the Holocene. The results reveal that the climate was dry in Early-Holocene （ca.8cal.ka B.P.）, humidity in Mid-Holocene （8 5cal.ka B.P.）, and from humidity to dry in Late-Holocene （5- 2cal.ka B.P.）. The reconstruction results were consistent with other previous reconstruction results from lake sediments of Zhuyeze terminal lake section and Hongshui River section, and consistent with the Holocene moisture pattern of the Asian monsoon margin of Northwestern China, which demonstrated that the Sr/Ca ratios of paleo calcareous root tubes can reflect millennial- scale effective moisture changes in this area. These studies reveal that the environmental signals can be extract from the calcareous root tubes and can be applied to paleo environment reconstruction.