Lake Huguangyan (Huguang-Maar) - Monsoon variability in Southeast China
The principal objective of the study is to gain palaeoenvironmental and palaeoclimatological information about sub-tropical Southeast Asia by means of multiproxy analyses of a continental high-resolution sedimentary record. Climatic variations in South China, one of the most densely populated and economically fast growing areas in the world, will have important social and economic impacts. Therefore, studying past climatic variability will give the necessary information to understand future climate change. High resolution studies of sediment sequences from Lake Huguang Maar, a site in a climatologically sensitive setting near the South China Sea, allowed reconstructing palaeoenvironmental and palaeoclimatic variations of the last 75,000 years. A co-operative agreement between GFZ-Potsdam and IGCAS-Beijing has enabled us to complete site surveys and a drilling campaign at Lake Huguangyan, yielding in a total recovery of more than 220 m of lake sediments from different profiles.
Lake Huguangyan is mainly under the influence of the East Asian summer monsoon and related atmospheric circulations. However, during winter and spring frequent cold spells with drastic temperature drop and strong northeasterly winds reach the northern South China Sea coast. Especially these winter monsoon spells can have a strong impact on sedimentation – on sediment composition with their local and remote dust load, and on redox conditions due to enhanced lake mixing.
Sediment composition has been investigated by bulk geochemical proxies as total organic carbon, carbonate and nitrogen content, and high-resolution scanning of selected elements like Titanium. The main magnetic carrier in the sediment samples from Lake Huguang Maar is magnetite with a small amount of titanium. The preservation/dissolution of the magnetite and other Fe-bearing minerals in the sediment was mainly controlled by the limnological conditions in the lake. In parts of the sediment corresponding to partly oxygenated lake bottom (related to periods with stronger lake mixing due to more intense winter monsoon) all over magnetic properties are dominated by the well preserved primer magnetic mineralogy. When anoxic conditions developed in the sediment due to a stratification of the water body, magnetite particles have been largely affected by reductive dissolution. In principle the dissolution of magnetite under anoxic conditions has preceded that of the antiferromagnetic minerals like hematite or goethite. Indeed in Huguang Maar the relative abundance of haematite (S-ratio) turns out to be an indicator of the lake bottom conditions in terms of oxygen availability. Climatic controls appear to modulate the sedimentary magnetic signal.
During the last Glacial, between 40,000 and 15,000 years BP, climatic conditions at the Huguangyan site were cool, dry, and rather stable, with no hints for strong millennial-scale cyclicity, at least in the presently available data set. During the Late Glacial, however, climatic variations recorded in the Huguangyan sediments show strong coincidence with other regional and northern hemispheric records like Hulu cave, marine records from e.g. the Cariaco Basin, and Greenland ice cores. For the last 4500 years high-resolution µ-XRF element scans as well as redox-parameters, explained as winter monsoon proxy for Lake Huguangyan, exhibit surprisingly large coincidence with precipitation proxy (Titanium content) from Cariaco Basin. Moreover, some dynastic changes of the Chinese history seem to be in time with abrupt changes of classical Maya cultures in Central America. Mechanistically this could be explained by migrations of the intertropical convergence zone (ITCZ) at both sides of the Pacific. Ongoing studies of deeper parts of the record will evaluate if this relation holds for the last Glacial period, too.
Backer: GFZ, CAS Beijing
Partner: G. Haug (ETH Zürich), J. Liu, J. Han, H. Lü, G. Chu, L. Wang, Q. Liu (CAS Beijing)