亚北极太平洋阿拉斯加湾50 kaBP以来放射虫组合指示的古海洋变化

利用SO202-INOPEX航次在亚北极太平洋阿拉斯加湾获取的沉积物岩心,以及基于AMS14C测年数据建立的年龄模式,开展了50 kaBP以来放射虫的古海洋学研究,获得如下认识:(1) Cycladophora davisiana是阿拉斯加湾最具代表性的优势种放射虫,其次是Siphocampe arachnea和Acanthodesmia micropora;(2)对放射虫的统计数据进行Q型因子分析,提取出4个方差极大因子,可以代表不同的水团甚至是水体的混合作用,其中C.davisiana作为中层水团演化的替代性指标,对冰期-间冰期旋回和快速气候变化事件的响应较为敏感,其变化特征表明该区中层水团的演化主要受控于亚北极太平洋NPIW形成源区及其输送机制的变化;(3)基于放射虫标志种Ceratospyris borealis,Actinomma boreale/leptodermum和Rhizoplegma boreale的变化特征及其环境指示意义,认为50 kaBP以来阿拉斯加湾上层海洋环境的变化具有阶段性:LGM之前,该区海洋表层生产力相对稳定,海冰消长与冰融水脉冲明显地响应于冰阶-间冰阶等气候旋回;LGM期间,受大陆冰盖、大气环流和海洋环流变化的综合影响,冰融水事件以及海冰持续增加、生产力明显下降的状况可能较少出现;LGM以来,在B-A和YD等快速气候变化事件的影响下,冰融水脉冲引起表层生产力快速增加,气候快速变冷则造成生产力急剧降低。

PALEOCEANOGRAPHIC CHANGES SINCE 50 kaBP INFERRED FROM RADIOLARIAN ASSEMBLAGES IN GULF OF ALASKA,SUBARCTIC PACIFIC

The paleoceanographic record of radiolarian assemblage for the past 50 ka is investigated using a box core (SO202-27-6) obtained from the Gulf of Alaska, subarctic Pacific during the R/V Sonne cruise SO202-INOPEX. Ten AMS14C data are used for establishing the stratigraphic age model.The study suggests that:(1) Cycladophora davisiana is the most dominant radiolarian specie in the Gulf of Alaska during the past 50 ka, followed by Siphocampe arachnea and Acanthodesmia micropora. (2) The Q-mode factor analysis obtains 4 maximum variance factors, which represent different radiolarian assemblages. Among them, C. davisiana can be used as a proxy for intermediate water of the Gulf of Alaska, which was sensitive to the rapid climate change of glacial-interglacial cycle, and the variable pattern reveals that the evolution of intermediate water was controlled by the formation and transportation of NPIW from subarctic Pacific. (3) Based on the indicative radiolarian species for environmental changes, such as Ceratospyris borealis, Actinomma boreale/leptodermum and Rhizoplegma boreal, we argue that the upper ocean conditions in the Gulf of Alaska underwent periodic changes during the past 50 ka. Before LGM, relatively stable surface productivity, sea-ice expansion and retreat, and melt-water pulse (MWP) responded clearly to the interstadial (GI) to stadial (GS) cycles. During LGM, under the conditions of complex ice sheet effects, atmospheric circulation and oceanic currents, there were rarely WMP,continuous sea-ice extension and limited productivity in the Gulf of Alaska. Since LGM, rapid increase and decrease in surface productivity was caused by rich-nutrient water from MWP and by rapid cooling event, respectively.