Environmental characteristics of Mid-Holocene recorded by lacustrine sediments from Lake Daihai, north environment sensitive zone, China

Climate proxies, such as total organic carbon and nitrogen (TOC, TN), carbonate content (CaCO₃), grain-size and pollen of the sediment core retrieved from enclosed Lake Daihai which lies in the north China environment sensitive zone are analyzed to reconstruct the environment evolution of the area based on high-resolution radiocarbon chronology. The results reveal that the TOC and TN contents of the sediments correlate well with pollen percentage and pollen flux variations during the Holocene, and both reach their peak values simultaneously at 6.7-3.5 ka BP (calendar age, 7.6-3.6 ka BP). Since 6.7 ka BP, both the CaCO₃ and organic matter contents of the core have simultaneous variations, and their high values also occur during 6.7-3.5 ka BP. While during 9.0-6.7 ka BP (calendar age, 10-7.6 ka BP) relatively lower level of organic matter content and pollen flux correspond to the higher level of carbonate content. The above relations suggest that during 6.7-3.5 ka BP, the productivity and effective precipitation were greatly improved in the lake drainage area, and this would probably strengthen the hydrodynamic conditions, enhancing organic matter, pollen and carbonate inputs from terrestrial sources. Such processes would account for the enrichment of both organic matter and carbonate in the sediments. While during 9.0-6.7 ka BP, the lower level of organic matter, pollen flux but high carbonate content show depressed productivity and declined vegetation coverage. The higher carbonate content at this stage would have probably resulted from the higher evaporation ratio of the lake water under relatively drier climate conditions. Therefore, it is inferred that during 6.7-3.5 ka BP, the climate was more humid with abundant rainfalls and vegetation was more flourishing in the Lake Daihai area. This can be seen as Holocene Climate Optimum (HCO). As a result, this has evident discrepancies with the traditional notion that the HCO occurs at Early Holocene or early Mid-Holocene.