Benthic foraminiferal counts from the >250 μm and 125-250 μm size fractions of IODP Site U1391 during 490-424 ka

Relative abundance of benthic foraminifera from IODP Site U1391 drilled off the western Iberian margin at a water depth of 1085 m have been investigated to determine the effect of different size fractions (>250 μm, 125-250 μm and >125 μm) on the distribution patterns of benthic foraminifera and further explore its implication for the paleoceanographic studies. The 125-250 μm size fraction contains the major species identified in the >250 μm size fraction and also the important smaller species such as Cassidulina neoteretis, Nonionella turgida and Brizalina spp. that are missing in the coarse fraction. The major species recognized in the 125-250 μm size fraction show consistency in relative abundance patterns compared to the >125 μm size fraction and multivariate faunal analyses of both size fractions give a very similar result, suggesting that the paleoceanographic signal recorded in the >125 μm size fraction is mostly expressed in the 125-250 μm size fraction and to a small degree in the >250 μm size fraction. However, it does not mean that the benthic foraminiferal analysis of the >250 μm size fraction is useless. Bulimina aculeata and Uvigerina peregrina parva are dominant species and show similar distribution patterns in both coarse and fine fractions together with Planulina ariminensis, suggesting that their relative abundance in the >250 μm size fraction can be used to reliably reconstruct the paleoenvironmental changes. Miliolids are regarded as oxic indictors with more large tests in better-oxygenated bottom water conditions, which makes the relative abundance of miliolids in the >250 μm size fraction a more suitable proxy of dissolved oxygen levels compared to the fine fractions. 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 studies that require high resolution.本研究针对伊比利亚西部陆缘海域水深1085米处钻探获取的综合大洋钻探计划（Integrated Ocean Drilling Program，简称IODP）U1391站位沉积物，对其底栖有孔虫（benthic foraminifera）的相对丰度展开系统调查，旨在明确不同粒级组分（>250 μm、125~250 μm及>125 μm）对底栖有孔虫分布格局的影响，并进一步探讨其在古海洋学研究中的指示意义。125~250 μm粒级组分不仅涵盖>250 μm粒级中已鉴定的主要优势类群，还包含粗粒级组分中缺失的重要小型种，如*Cassidulina neoteretis*、*Nonionella turgida*及*Brizalina* spp.。相较于>125 μm粒级组分，125~250 μm粒级中识别出的主要优势种的相对丰度分布格局具有高度一致性，且两种粒级的多变量群落分析结果极为相似，这表明>250 μm粒级所记录的古海洋学信号，主要体现在125~250 μm粒级中，仅在极小程度上反映于>250 μm粒级组分内。但这并不意味着针对>250 μm粒级的底栖有孔虫分析毫无价值。*Bulimina aculeata*与*Uvigerina peregrina parva*为该套样品中的优势种，它们与*Planulina ariminensis*一样，在粗、细粒级中均呈现相似的分布格局，这说明>250 μm粒级中这些物种的相对丰度可用于可靠重建古环境变化。粟虫类（Miliolids）被视为底层水富氧的指示类群，其壳体在氧化条件更佳的底层水环境中个体更大，因此相较于细粒级组分，>250 μm粒级中粟虫类的相对丰度是更合适的溶解氧水平代用指标。对三个粒级组分的因子分析显示出相似的总体演化趋势，表明粗粒级组分对低分辨率古环境研究无负面影响；但由于不同粒级中底栖有孔虫的分布格局存在差异，细节层面仍存在显著区别，这意味着在需要高分辨率的古环境研究中，所选粒级（尤其是粗粒级）可能会对最终的古环境解释产生影响。