| High resolution environmental record, which is a more accurate environmental record on the time scale and extraction of regional climate information, can more accurately indicate the regional ancient environmental changes. Lake sediment, one kind of high resolution environmental records, has the advantages of continuous deposition, large amount of storage information, high deposition rate and high time resolution.What’s more, as an important material for studying the evolution of the ancient environment, lake sediment is an important foundation for the reconstruction of regional ancient climate. The total organic carbon (TOC) and its carbon isotope (δ13Corg) indexes are sensitive to the environmental and other aspects, such as the source of organic matter, the vegetation change, and so on. Carbonate deposition is a common feature of lacustrine deposits, and its carbon and oxygen isotopes (δ13Ccarb and δ180Carb) have special significance in indicating the area temperature, precipitation and evaporation around the lake. Therefore, the stable carbon and oxygen isotope in lake sediments have a quite extensive using in the field of paleoclimate reconstruction. In our paper, based on the North Lake Bridge in Northern Zhejiang (BHQ) drilling-this continuity is better and higher resolution lacustrine sediment extraction of total organic carbon (TOC) and carbon isotope (δ13Corg) and carbonate,δ13Ccarb and δ18Ocarb proxies, combined with AMS14C dating, grain size parameters and earth chemical index Mg/Sr and sporopollen assemblage and other existing research results of BHQ drilling during the early Holocene in the Northern Zhejiang Province, we can draw the preliminary conclusions of paleoclimate evolution characteristics by comprehensive analysis as follow:(1)11.4-8.7 cal ka BP (19-17 m). During this period. TOC and δ13COrg contents from the bottom to top present an increasing trend, then the TOC content decreases slightly, while the contents of carbonate,δ18Ocarb and δ13Carb are significantly lower than the average content of general core, but then they tend to rise. The reverse fluctuations between TOC and carbonate content in the study area show that the precipitation was gradually improved, and the climatic conditions changed from the earlier drought to the later humid stage. Synchronous changes of carbon and oxygen isotopes show that temperature in the lake area had experienced a gradual recovery from early to later period. Early Holocene appeared a relatively short and obvious warm period. Moreover, Mg/Sr index within the section curve presents a bottom-up increasing trend and the pollen data shows that the gross had a tendency of increase and tree was dominant, which indicated the climatic conditions improved and the regional temperature rose again. The content of S13Corg in this section is low as well as TOC and has a positive correlation with TOC, we think δ13Corg in this time was not sensitive to indicate the increase of temperature because it only reflected the change of organic matter content as during the early drought the level of the lake in Beihu Caodang was low and the emerged plant was dominant. Besides, the C/N ratio is lower than 10, which indicated that the sedimentary environment was dominated by shallow lacustrine facies.(2)8.7-8.0 cal ka BP (17-8.9 m). Cold-dry stage. The curves of the five indicators of TOC,δ13COrg, carbonate,δ18Ocarb and δ13Ccarb in this period fluctuate frequently, but the amplitude is relatively uniform. Specifically, the content of TOC is relatively low, while the content of organic carbon isotope is relatively positive, and the contents of carbonate,δ18Ocarb and δ13CCarb in this section tend to be significantly increased. That indicated the research area was in a relatively stable environment, the precipitation and temperature were relatively lower than the last stage. Thus, during this period the climate was relatively cold-dry. Compared with the last stage, C/N ratio increases slightly, which indicated the terrestrial C3 plants had developed. Correspondingly, the pollen quantity and Mg/Sr decrease significantly, which also reflected the climate became colder and arid. Moreover, the climate characteristic in this stage is more close to the North Atlantic 8.2 cold events.(3)8.0-5.7 cal ka BP (8.9-6.7 m). The Holocene Optimum. In this period, the content of TOC increases significantly and reached the maximum, while the content of organic carbon isotope is negative. At the same time, the contents of carbonate, δ18OCarb and δ13CCarb tend to be negative. Nevertheless, the above five indicators appear fluctuations. From bottom to top, the content of TOC decreases, while the carbonate, δ18Ocarb and δ13CCarb tend to increase. The correlation analysis shows that TOC has obviously negative correlations with δ13Corg, carbonate,δ180Carb and δ13Ccarb respectively. There are also significantly positive correlations among carbonate, δ180Carb and δ13CCarb. It indicated in this time the climatic condition had changed that the precipitation and temperature increased remarkably. Generally, climate showed more obvious features of warm and wet. But there were also some fluctuation of the temperature and humidity during this phase. Moreover, at the late stage the precipitation and temperature tended to fall. The C/N ratio is in the trend of increasing slightly, which indicated that the terrestrial C3 plants developed rapidly. The palynological assemblages attain to the optimal allocation which indicated the ratio of broad-leaved and coniferous trees increased significantly and woody plant had been in a dominant position in this stage. Mg/Sr ratio also reaches the highest value of the whole hole.(4)5.7-4.2 cal ka BP (6.7-4.775 m). The content of TOC decreases and is lower than the average level of the whole hole, but the organic carbon isotope shows a fluctuating upward trend. And the contents of carbonate,δ180Carb and δ13CCarb gradually decrease to zero. The C/N ratio in this section gradually reaches the maximum, but not more than 20. Mg/Sr ratio curve tends to decline. Therefore, compared to the last stage, the climatic conditions of this period gradually turned to cold and dry. Lake algae and terrestrial plants were common breeding. Meanwhile, there were a large number of Oryza pollen found in palynological assemblages, which also means that human activities in this period became active, and had a relatively strong effect on the regional sedimentary environment. Also this period records the North Atlantic 5.5 cold events and 4.2 cold events, which are closely related to the rise and the decline of Liangzhu culture. |
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