Early- and mid-Holocene palaeoenvironments as revealed by mineral magnetic, geochemical and palynological data of sediments from Bai Nuur and Ulan Nuur,southeastern inner Mongolia Plateau, China

摘要

Bai Nuur (41°38.590'N, 114°30.922'E, ~1346 m a.s.l.) and Ulan Nuur (41°44.214'N, 115° 05.63 O'E, ~1246 m a.s.l.) are two small lakes situated in the transition zone between semi-humid and semi-arid climate regimes that runs parallel to the present limit of the southeast monsoon along the southeastern Inner Mongolia Plateau in north China. A 215 cm-deep sediment sequence was recovered from Bai Nuur and a 244 cm-deep one from Ulan Nuur. Soils were sampled from the catchments of the two lakes and dune materials from a sandy land lying further north and northwest. Mineral magnetic measurements were performed on the sediments, soils and dune-materials. AMS ~(14)C dating and analyses of particle-size, TOC and C/N and pollen composition were also made for the sediments. The mineral magnetic properties of the two sediment sequences, in combination with the other proxy-climate data, reveal the palaeo environmental development during early- and mid-Holocene around the two lakes. Climate began to ameliorate at 10,600 or 10,500 cal. BP and maximum humidity and warmth were reached during 10,600 or 10,500-9700 or 9600 cal. BP around Bai Nuur. Environments remained generally wet and/or warm until 7700 or 7600 cal. BP. Climate deterioration commenced at 7700 or 7600 cal. BP around Bai Nuur and at 7200 or 7100 cal. BP around Ulan Nuur. This deterioration intensified from 6900 or 6800 cal. BP onwards around Bai Nuur and from 6200 or 6100 cal. BP onwards around Ulan Nuur. The differences in timing of the reconstructed environmental changes at the two short-distanced sites may be due to the differences across the narrow climatic and vegetative transition zone, errors and uncertainties of dating and/or different chronostratigraphic frames of the two sequences. The palaeoenvironments recon structed around the two small lakes (2.3 km~2 and 8.9 km~2) coincide with a palaeoenvironmental record from Anguli-nuur Lake, a much larger lake (47.6 km~2) in the same region. These data also agree with reconstructions at several other sites in different parts of the Inner Mongolia Plateau, although the Holocene amelioration took place earlier than in northern Mongolia.