Environment And Climate Changes Since 2100cal. A BP During The Holocene Recorded In Dali-Nor Lake

Lake sediment could be used to represent the characteristics of environment and climate change of a catchment. To analyzing the environmental factors of the sediment records could help us to get better understanding in the process of the local environment, climate change and their driving mechanism. Dali Lake is located in the north margin of East Asia Summer Monsoon (EASM) and in mid-eastern of Inner Mongolia, China, which has a sensitive response to the environmental and climate change. An undisturbed sediment core (DL-1) was successfully sampled by a self-made gravity corer from the lake. The environmental factors of the core like particle size, TOC, TN, TP, and organic carbon isotope were analyzed. Meanwhile, the core was dated through AMS 14C,210Pb, and 137Cs. The following results and conclusions were obtained.The contents of TOC, TN, Md, and δ13COrg and their varying characteristics indicated that the lake catchment experienced in three main typical climate periods, i.e. relatively warm and humid periods (1975-1745 cal. a BP,1655-1530 cal. a BP,1420-1280 cal. a BP,1065-855 cal. a BP,595-355 cal. a BP, and 175-0 cal. a BP), relatively cold and dry periods (2100-1975 cal. a BP,1745-1655 cal. a BP,1530-1420 cal. a BP,855-595 cal. a BP, and 355-175 cal. a BP), and relatively warm and dry periods (1280-1065 cal. a BP).The particle size components and the historical records of dust in the north of china indicated that the catchment climate in the periods of 825-750 cal. a BP,385 cal. a BP, and 215-115 cal. a BP were cold and dry. The lake water levels were relatively lower during those periods with high frequent of dust fall. However, the catchment climate in the periods of 1145-1065 cal. a BP was warm and dry. The lake water levels were relatively lower as well during this period with less frequent of dust fall.The results showed that the paleoproductivity (P2) had a significantly positive correlation with TN but few correlations with TP in the sediment of Dali Lake in the period of 2100-200 cal.a BP. This indicated that the productivity in Dali Lake could be mainly controlled by TN, although TP could be one of the important effect factors as well. This may be because that phosphorus was in different phases and the formation circumstance of P2 was too complicated to be identified. However, P2 had a negative correlation with both TP and TN in the period of 200 cal.a BP-50 cal.a BP. This indicated that the vegetation coverage had been decreased meanwhile nomadic intensity had been increased during that period. In the more recent period, more and more human activities affected the formation of P2. As a result of this, P2 represented a positive correlation with both TN and TP since 50 cal.a BP.Compared with stalagmite record in Wanxiang cave, although the variations of EASM obtained from the Dali Lake sediment core were not completely matched, the overall variation tendency was similar. This result also were supported by the references of Daihai sediment, Maowusu Desert, Hunshandake Desert, and Keerqin Desert etc.The periods of 270-360AD and 485-595AD in the Dali Lake catchment represented the Dark Ages Cold Period (DACP), in which the core record clearly corresponded to the cold event of 1.4 cal. ka BP (550AD). The period of 595-1160AD corresponded to the Medieval Warm Period (MWP). The duration of this period was shorter than that in north-west area and longer than that in eastern area of China. The period of 1160-1420AD corresponded to the cold period in the Northern Hemisphere. However, only a short period corresponded to lower the total solar irradiance (ATSI). So, this period might only represent the Little Ice Age (LIA). In the same way, the periods of 1420-1660AD and 1660-1840AD belonged to the LIA as well, in which the climate was a warm and humid in 1420-1660AD, meaning that regional climate warming might be occurred during the period of LIA.The paleoproductivity was higher in the periods of 360-485AD, 595-740AD and 950-1160AD in Dali Lake, which was consistent with the higher sea surface temperature (Alkenone SST) reconstructed through PC6 core in the northwestern area of North Pacific Ocean and the red color intensity units in the Laguna Pallcacocha sediment in the southern area of Ecuador. The results inferred that Dali Lake catchment could be evidently affected by ENSO events during those periods.The significance level of TOC, TN, TP and Md over 5% occurred with a central cycle of 500-year since 2100 cal. a BP. According to coherence spectrum analysis, the Paleoproductivity could be mainly affected by precipitation rather than temperature.Human activity including anomalism, industrialization, and tourism development had increased since 50 cal. a BP. As a result of this, more exogenous organic matter migrated into the lake, meanwhile, the water surface temperature raised and more phytoplankton reproduced due to the greenhouse effect. These might be the main reason that paleoproductivity had increased in Dali lake in this period. This implied that paleoproductivity would continually increase due to the increase of human activity in the Dali Lake catchment.