不同粒径深海沉积物中底栖有孔虫的分布特征及其对古海洋环境重建的影响

DISTRIBUTIONPATTERNS OFBENTHICFORAMINIFERAIN DIFFERENT SIZE FRACTIONS OF THE DEEP-SEA SEDIMENTS AND THEIR IMPACT ON THE PALEOCEANOGRAPHIC RECONSTRUCTION

提要通过对位于葡萄牙岸外的IODP U1391站位深海氧同位素(MIS)12–13期不同尺寸沉积物组份(125~250μm、>250μm和>125μm)中底栖有孔虫相对丰度的统计,揭示了底栖有孔虫主要属种在不同组份中的分布差异,并探讨了这种差异对古海洋环境重建的影响。研究发现,Bulimina mexicana,Melonis barleeanum,Cassidulina neoteretis,Nonionella turgida,Bulimina exilis,Brizalina spp.和Trifarina carinata多出现在125~250μm组份中;Globobulimina spp.,Uvigerina proboscidea,Cibicidoides spp.,Cibicides spp.和Gyroidinoides orbicularis多出现在>250μm组份中;Bulimina aculeata和Uvigerina peregrina parva在125~250μm组份和>250μm组份中的比例相当、分布模式相似。125~250μm组份包含了>250μm组份中出现的主要属种,但在125~250μm组份中具有重要古环境指示意义的主要属种,如Cassidulina neoteretis和Nonionella turgida,在>250μm组份却很罕见。125~250μm组份和>125μm组份具有相同的主要属种且在其相对丰度模式上表现出了高度一致性,这表明>125μm组份中底栖有孔虫所记录的古海洋学信号主要表现在细组份上。125~250μm、>250μm和>125μm三种组份中底栖有孔虫群落的因子分析显示了大体一致的趋势,这说明粗组份对低分辨率的古海洋学研究不会带来显著不利影响;但在细节上,由于不同组份中底栖有孔虫的分布模式存在明显差异,>250μm组份的分析很可能影响到千年时间尺度及更高分辨率的古环境解释。因此,与>250μm组份相比,我们认为对>125μm组份的底栖有孔虫分析能得出更可靠的古环境变化信息。

Relative abundance of benthic foraminifera from IODP Site U1391 drilled off the Portuguese margin at a water depth of 1085 m have been investigated to determine the effect of different size fractions(>250μm,125~250μm and>125μm)on the distribution patterns of benthic foraminifera and further explore its implication for the paleoceanographic studies.The result suggests that Bulimina mexicana,Melonis barleeanum,Cassidulina neoteretis,Nonionella turgida,Bulimina exilis,Brizalina spp.and Trifarina carinata mainly occur in the 125~250μm size fraction;Globobulimina spp.,Uvigerina proboscidea,Cibicidoides spp.,Cibicides spp.and Gyroidinoides orbicularis mainly occur in the>250μm size fraction;Bulimina aculeatea and Uvigerina peregrina parva display comparable relative abundance and similar distribution patterns in the 125~250μm and the>250μm size fractions.The 125~250μm size fraction contains the major species identified in the>250μm size fraction,but the important major species such as Cassidulina neoteretis and Nonionella turgida are missing in the>250μm size fraction.The major species recognized in the 125~250μm size fraction show consistency in relative abundance patterns compared to the>125μm size fraction and multivariate faunal analyses of both size fractions give a very similar result,suggesting that the paleoceanographic signal recorded in the>125μm size fraction is mostly expressed in the 125~250μm size fraction and to a small degree in the>250μm size fraction.Factor analyses of the three size fractions reveal comparable general trends,suggesting that the coarse fraction has no negative effect on low-resolution studies,but in detail there exist significant differences because of the inconsistent distribution patterns of benthic foraminifera in different size fraction,implying that the chosen size fraction especially the coarse fraction probably affects the paleoenvironmental interpretations for millennialscale and higher-resolution studies.Therefore,we hold the opinion that benthic foraminiferal analysis from the>125μm size fraction yields more reliable information on paleoenvironmental changes than that from the>250μm size fraction.