THE PALAEOENVIRONMENTAL SIGNIFICANCE OF CARBON ISOTOPES IN ORGANIC LACUSTRINE SEDIMENTS IN ARID CHINA

The carbon isotope composition of organic lacustrine sediments is a convincing palaeo- environmental proxy used to investigate the vegetation changes in arid China by means of isotope geochemistry. Sources of organic matter in lacustrine sediments are terrestrial plants (C3, C4, CAM) and aquatic higher plants and algae, displaying a characteristic carbon isotope composition. For this study, Holocene organic lacustrine sediments of Bosten Lake, were analyzed with respect to the carbon isotope composition. The results of carbon isotope analysis all fall into the range of C3 and C4 plants (-10°ë to -32°ë). An accompanying analysis of modern terrestrial and aquatic plants indicates the organic matter and organic components in the lacustrine sediments marked by heavier carbon isotope values originates primarily from submerged macrophytes rather than C4 plants. The carbon isotope values of submerged macrophytes in lakes of the Badan Jilin Desert, in Bosten Lake, Qinghai Lake, and others range from -12.9°ë to -18.8°ë. Carbon isotope values of terrestrial plants collected on the Tibetan Plateau and in the Qilian Mountains at altitudes exceeding 4000m amount to ¨C25°ë to ¨C28°ë, demonstrating that all collected plants belong to the C3 type. In the arid Badan Jilin Desert and the Chaidamu Basin plant carbon isotope values reach ¨C27°ë to¨C28°ë, but a few C4 type plants were found in addition. In the warm and wet region of China (Chengdu, Guangzhou, Nanjing, etc.) the plant carbon isotope values are lighter (-28.8°ë to -32°ë) than in the cold and arid region. Because C4 plants are less abundant in the cold and arid region of China, there is a close relationship between carbon isotope values of organic matter and rainfall. We conclude that the carbon isotope composition of organic lacustrine sediments in arid China mainly originates from organic matter of terrestrial C3 plants, submerged macrophytes and algae. Heavier carbon isotope values of organic lacustrine sediments result from organic matter of submerged macrophytes and algae with a high aquatic productivity. Lighter carbon isotope values of organic lacustrine sediments mainly derive from C3 plants and may be regarded to reflect more favourable conditions for the terrestrial vegetation, thus pointing to warmer and wetter climatic conditions.