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

Abstract

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, δ13C and δ15N 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 δ13C and δ15N 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 δ13C and δ15N values. From 9800 to 7700 cal yr BP, high, stable TOC and TN concentrations and C/N ratios together with low δ13C and δ15N 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 δ13C increased in its value and δ15N 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 δ13C values decreased abruptly, while δ15N 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 δ13C and δ15N 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.