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