History of Environmental Change during Early-Middle Holocene Recorded by Beihuqiao Core in Zhejiang China

Key words: 
• BHQ Core /
• total organic carbon /
• stable carbon isotope in organic matter /
• Early-Middle Holocene /
• Holocene Megathermal

Abstract: The study area is situated in the southwest of Hangjiahu plain, where the surface water system is developed and most rivers originate from Tianmu mountain and Mogan mountain in the northwest of Zhejiang. The sediment samples were collected from a core drilled at the Beihuqiao (BHQ) of Yuhang town, where the location is 30°22.443' N, 119°56.237' E. The BHQ core is 19.0 m in length, and core sediments are composed of homogeneous gray, clayey silt and silt with a little carbonate nodules except for an intercalation of silty sand in the lower part. No disturbance was observed in the whole core. The part (4.775~19.0 m) of the core was sampled at 2.5 cm increments. Four bulk samples were collected for AMS radiocarbon dating from the organic-rich sediments. And 136 samples were chosen for TOC and δ¹³C_org analysis at about 10 cm intervals.
Combined with the proxies of grain size parameters, pollen data, C/N radio and age data, and contrasted with researches of surrounding areas, TOC and δ¹³C_org in organic matter, BHQ core in Zhejiang province are analyzed to interpret the basic environmental information and climate changes during the Early-Middle Holocene. Results reveal that the paleoclimate changes in Hangjiahu plain during the Early-Middle Holocene which can be divided into three periods: (1)11.4~8.7 ka B.P.. Organic carbon isotope increased in the range of -27.24‰ to -23.4‰ and TOC content increased obviously in the range of 0.19% to 0.69%, indicating that the cold-dry climate turned gradually to warm and humid in this period. (2)8.7~8.0 ka B.P.. Compared with the last stage, TOC turned to be negative and fluctuated between 0.2%~1.33%, δ¹³C_org fluctuated slightly between -24.91‰~-22.93‰ and tended to be positive. As a whole, the climate was dry, and presented cold-dry to humid and warm-dry trend. (3)8.0~4.2 ka B.P.. The major characteristic of the climate in the northern of Zhejiang province was warm and humid, however, some cold-dry events also happened at different time scales in this period. And during the period, TOC and δ¹³C_org fluctuated in 0.18%~2.18% and -26.33‰~-19.09‰ ranges, respectively. Especially during 8.0~5.7 ka B.P., TOC content improved obviously in 0.43%~2.18% and δ¹³C_org tended to be negative in -25.79‰~-23.15‰, which indicated the Holocene Megathermal emerged in this region. Meanwhile, there were two cold events occurred in about 5.5 ka B.P. and 4.2 ka B.P.. During these two periods, TOC tended to down and δ¹³C_org had a positive fluctuation, and they also corresponded with the contemporaneous pollen data and grain sizes parameters.
Combined with multiple climatic proxies of BHQ core and the results of correlation analysis between these proxies, the BHQ core sediments provide a reliable basis interpretation of palaeoclimate. TOC and δ¹³C_org of BHQ core can be good proxies of paleoclimate in this research zone and indicate the changes of precipitation and temperature respectively. At the same time, this study also effectively suggests the evolution law of paleoenvironment in the northern of Zhejiang province during the Early-Middle Holocene.