Search

Abstract: Bai Nuur (41°38.590′N, 114°30.922′E, ∼1346 m a.s.l.) and Ulan Nuur (41°44.214′N, 115°05.630′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 14C 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 palaeoenvironmental 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 reconstructed around the two small lakes (2.3 km2 and 8.9 km2) coincide with a palaeoenvironmental record from Anguli-nuur Lake, a much larger lake (47.6 km2) 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. [Copyright &y& Elsevier]

Abstract: Two cores, one 1141-cm long (An-S) and the other 885-cm long (An-A), were retrieved from Anguli-nuur Lake (41°18′–24′N, 114°20′–27′E, ∼1315masl), one of the largest lakes in the transition zone between a semi-humid and semi-arid climate parallel to the present limit of the southeast monsoon along the southeastern Inner Mongolia Plateau in north China. Mineral-magnetic parameters (χ lf, ARM, IRM300mT, SIRM and IRM−300mT) were measured on An-S and two additional parameters (χ ARM and HIRM) and four inter-parametric ratios (χARM/SIRM, IRM300mT/SIRM, IRM−300mT/SIRM and SIRM/χ lf) were calculated. Potential sources of these lake sediments (catchment soils and dune materials close to the lake and in a distant sand plain) were sampled, and the magnetic properties of the surface-material specimens were measured. A chronological model was developed for An-S by comparing and combining AMS14C dates of An-S with 137Cs, 210Pb and AMS14C dates of An-A. With the help of surface-material magnetism, the magnetic data of An-S in combination with particle size, TOC and C/N and pollen analyses indicate the environmental changes during the last ∼10,000years around this lake. Conditions began to ameliorate at 10,900cal. yr BP (9600 14C yr BP) and thus relatively wet and warm environments prevailed during 10,900–8900cal. yr BP (9600–8000 14C yr BP). The Holocene optimum or the wettest and warmest conditions, was during 8900–7400cal. yr BP (8000–6500 14C yr BP). The environment began to deteriorate from 7400cal. yr BP (6500 14C yr BP) and the driest and coolest conditions occurred during 2200–480cal. yr BP. There may have been a minor amelioration after 480cal. yr BP. The inferred changes in palaeoenvironmental conditions around Anguli-nuur Lake are broadly in agreement with those around most other sites on the Inner Mongolia Plateau. [Copyright &y& Elsevier]