渤海湾西岸CZ01钻孔沉积物粒度端元分析及其气候—海平面变化响应*

沧州地区位于海陆交互的渤海湾西岸,易受到海平面变化和极端气候事件的影响,对于全球气候变化的响应十分敏感。应用非参数化端元分析模型将沧州地区CZ01钻孔中更新世晚期以来的沉积物粒度划分出6个端元并分析其物源,结合已有地质记录,揭示不同时间尺度下各端元对气候—海平面变化的响应。结果表明: (1)EM1(5.01 μm)主要为远源的风尘输入,EM2(13.18 μm)和EM3(39.81 μm)为古黄河所携带的沉积物,EM4(69.18 μm)和EM5(138.04 μm)为海相沉积物,EM6(275.42 μm)可能指示古洪水等极端气候事件。(2)深海氧同位素(MIS)Ⅰ 阶段,气候温暖湿润,EM4+5含量指示渤海海平面整体呈波动上升的趋势并逐渐接近现代海平面。该阶段内由于11.5 ka BP左右的新仙女木事件以及5.1 ka BP左右冷干事件的发生,渤海海平面在稳定上升状态后出现停滞或小幅下降的现象; 而在9.5 ka BP、7.5 ka BP、5.8 ka BP和1.7 ka BP左右,东亚夏季风增强导致降水增加,渤海海平面升高。(3)MIS Ⅵ 阶段北半球气候冷干,150~132 ka BP左右因喜马拉雅运动减弱造成的区域沉降中心转移致使渤海海面升高。MIS Ⅴ 阶段气候波动剧烈: 在间冰期暖期(5a、5c和5e)气候暖湿,渤海海平面上升; 而MIS5b和5d时期渤海海面高度较低。MIS Ⅳ 阶段较MIS5a末期海平面突然下降后趋于稳定,期间出现若干次小规模海侵事件,可能与东亚夏季风频繁变化有关。MIS Ⅲ 阶段至末次冰盛期海平面大幅度下降且存在周期性升降变化,并在46 ka BP左右出现大规模海侵事件。MIS Ⅱ 阶段较MIS Ⅲ 阶段海平面出现小幅度下降,为低海平面时期; 伴随15 ka BP左右冰盛期的结束,东亚夏季风增强,海平面开始上升。渤海海平面180 ka BP以来的变化记录与北半球乃至全球范围内的地质记录存在一致性,与太阳辐射波动引起的冰川消融及东亚夏季风变化密切相关。

Sediment grain size end-member analysis and its response to climate and sea-level changes in CZ01 borehole on west coast of Bohai Bay

Located on the west coast of the Bohai Bay where land and sea interact,the Cangzhou region is vulnerable to sea level changes and extreme weather events and is very sensitive to the response to global climate change. A non-parametric end-member analysis model was applied to classify the sediment grain size of CZ01 borehole in Cangzhou region into six end-members since the late Mid-Pleistocene and analyze their sources,and to reveal the response of each end-members to climate and sea-level changes at different time scales in conjunction with the existing geological record. The results show that: (1)EM1(5.01 μm)is mainly a remote wind-dust input,EM2(13.18 μm)and EM3(39.81 μm)are sediments carried by the ancient Yellow River,EM4(69.18 μm)and EM5(138.04 μm)are marine sediments,EM6(275.42 μm)may indicate extreme climate events such as paleofloods. (2)During the Marine Oxygen Isotope Stages(MIS)Ⅰ,the climate is warm and humid,and the EM4+5 indicates that the sea-level of the Bohai Sea is wavelike rising and gradually reaches the modern sea level. During this period,the Bohai Sea level stagnates or decreases slightly after a steady state of rise due to the Younger Dryas impact hypothesis around 11.5 ka BP and a dry-cold climate event around 5.1 ka BP,while around 9.5 ka BP,7.5 ka BP,5.8 ka BP and 1.7 ka BP,the Bohai Sea level rises due to increased precipitation caused by strengthening East Asian summer monsoon. (3)The cold and dry climate in the Northern Hemisphere in MIS Ⅵ stage,and the regional subsidence center transfer caused by the weakening of Himalayaorogeny around 150~132 ka BP resulted in the increase of Bohai Sea surface. In MIS Ⅴ stage,the climate fluctuated dramatically: during the warm period of interglacial period(5a,5c and 5e),the climate was warm and humid,and the sea-level of Bohai Sea rose, and the sea level height of the Bohai Sea during MIS5b and 5d is lower. Compared with the end of MIS5a,the sea-level in MIS Ⅳ decreased suddenly and then tended to be stable. Several small-scale transgression events occurred during the period,which may be related to the frequent changes of East Asian summer monsoon. From the MIS Ⅲ stage to the Last Glacial Maximum,the sea-level decreased significantly with periodic fluctuations,and a large-scale transgression occurred at about 46 ka BP. MIS Ⅱ stage showed a slight decreased sea-level compared with MIS Ⅲ stage,which was a low sea-level period. Due the termination of the glacier period around 15 ka BP,and the strengthened East Asian summer monsoon, sea-level began to rise again. The sedimentary records of the Bohai Sea since 180 ka BP are consistent with the geological records in the northern hemisphere and even worldwide,which are closely related to the glacier melts caused by solar radiation fluctuation and the change of East Asian summer monsoon.