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IEA-04 The geoarchaeological perspective: Human interactions with the geosphere

Climate and human-induced land cover change in the recharge area of Xiangshui River, SW China during the Holocene: Insights from the presence and absence of the riverine tufa

Zaihua Liu, Institute of Geochemistry, Chinese Academy of Sciences (China)
Hongchun Li, National Cheng Kung University (China)
Naijung Wan, National Cheng Kung University (China)
 

Measurements of δ¹⁸O and δ¹³C in the tufa samples dated by AMS-¹⁴C method, and deposition rates have been used to reconstruct the land cover (vegetation/soil) change in the recharge area of Xiangshui River, Libo, SW China during the Holocene. Stable carbon isotope δ¹³C and deposition rate time record in the calcareous tufa profile can be interpreted as land cover record if dependence between these environmental proxies and vegetation/soil for the specific climatic region is known. Results show that there is no tufa deposits at all at the site before ∼5520 a. BP and after ∼1410 a. BP. Tufa deposited mainly during ∼5520 a. BP-∼1410 a. BP. Based on the characteristics of δ¹³C, δ¹⁸O and deposition rates of the calcareous tufa profile, three stages (I: 5520-3381 a. BP; I-II: 3381-2330 a. BP; II: 2330-1410 a. BP) of land cover could be distinguished, indicating possibly the largest land cover with vegetation mainly C3 plants in the stage I (with lowest δ¹³C and deposition rate), the smallest land cover (or more karst rocky desertification with vegetation mainly C4 plants) in the stage II (with highest δ¹³C and deposition rate), and the intermediate land cover in the stage I-II. The causes for the absence of tufa deposits before ∼5520 a. BP and after ∼1410 a. BP are possibly totally different. From ∼1410 a. BP till now, the absence of tufa deposits was mainly related to the human activity-induced karst rocky desertification in the recharge area of the river, which resulted in the decrease in soil CO₂ production in the area, and thus in the supply of Ca²⁺ and HCO₃^- for tufa deposition. However, the area was under the climatic optimum (warm and wet) of the Holocene before ∼5520 a. BP., which provided optimal condition for vegetation/soil development, the latter resulting in the lower saturation environment with respect to calcite due to production of more soil CO₂. Therefore, it was also not possible for tufa deposition at the site in the early and middle Holocene before ∼5520 a. BP. Therefore, we suggest that the absence of tufa deposits may be indication of the two extremes of land cover.