Carbon and nitrogen signatures of sedimentary organic matter from Dali Lake in Inner Mongolia: Implications for Holocene hydrological and ecological variations in the East Asian summer monsoon margin

摘要

Environmental changes in the East Asian summer monsoon (EASM) margin have an important impact on the global climate system. This study presents the results of high-resolution analyses of TOC/TN (C/N) ratio, delta C-13 and delta N-15 of sedimentary organic matter extracted from a sediment core from Dali Lake, Inner Mongolia, in order to monitor Holocene hydrological and ecological variations in the EASM margin. Concurrent increases in the values of these proxies are generally interpreted to reflect intensified surface runoff and vegetation development in the lake catchment, elevated lake levels and enhanced lake productivity; however, changes in delta C-13 and delta N-15 may also result from within-lake processes due to changes in lake level. These data indicate that Dali Lake experienced gradual rises in water level and primary productivity from 11,500 to 9800 cal yr BP, as documented by increases in TOC and TN concentrations, C/N ratios and delta C-13 and delta N-15 values. From 9800 to 7700 cal yr BP, high, stable TOC and TN concentrations and C/N ratios together with low delta C-13 and delta N-15 values suggest a status of high stands and high productivity for the lake and a development of terrestrial vegetation in the catchment. Between 7700 and 5900 cal yr BP, TOC and TN were fluctuating at higher concentrations and C/N maintained high ratios, while delta C-13 increased in its value and delta N-15 remained at low values, denoting a further rise in lake level and a notable improvement of aquatic and terrestrial ecosystems. Around 5900 cal yr BP, TOC and TN concentrations, C/N ratios and delta C-13 values decreased abruptly, while delta N-15 value increased rapidly, implying dramatic drops in lake level and water temperature and drastic declines of aquatic and terrestrial ecosystems. Subsequently all geochemical proxies increased in their values until 4850 cal yr BP, indicating a gradual hydrological and ecological recovery. From 4850 to 750 cal yr BP, decreasing trends of TOC and TN concentrations and C/N ratios and increasing trends of delta C-13 and delta N-15 values denote a general decline in the hydrological and ecological conditions. The last 750 cal yrs witnessed the pattern of hydrological and ecological changes occurring from 5900 to 4850 cal yr BP. We suggest that hydrological and ecological changes in the EASM margin during the Holocene were closely related to the combined effects of regional precipitation and temperature which were ultimately controlled by the Northern Hemisphere summer insolation, the boundary conditions and the physical environment of ocean current. (C) 2016 Elsevier Ltd and INQUA. All rights reserved.