DUGi

Analysis of Fossil Planktonic Foraminifera: The Sieve Mesh Effect

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The choice of the sediment size fraction in the analysis of fossil planktonic foraminifera is ofgreat importance in determining the composition of assemblages. In past studies several sizefractions have been utilised. Imbrie and Kipp (1971) stated that “smaller fractions give rise to toomany uncertainties in the identification of small specimens and require too long to process. Largemesh size yield undesiderable loss of small species, and small specimens of larger species”. Theadoption of coarser sieve meshes has the effect to reduce the percentages of small sized (usuallyliving in cold waters) species (Bé and Hutson, 1977). In a comparison between >63 μm and >150μm size-fraction of planktonic foraminifera assemblages from NW Atlantic Ocean, Smart (2002),stated that because particular smaller species are either under-represented or even absent from thelarger (>150 μm) size-fraction, the smaller (>63 μm) size-fraction must be included in studies ofplanktonic foraminifera. Di Donato et al. (2008) highlighted in the >150 μm size fraction of a corefrom the Tyrrhenian Sea, a great loss of the small-sized species Turborotalita quinqueloba in glacialsamples, where this species is very abundant. This caused an increase in warm water species such asGlobigerinoides ruber, apparently reaching typical Holocene values in some full glacial levels.From the above mentioned statements, it is clear that treatment changes can strongly influence theresults of palaeoclimatic reconstructions based on planktonic foraminiferal assemblages.As stated by Aitchison (1986, 1992) scale invariance and subcompositional coherence arefundamental properties of the compositional data analysis (CODA). The main goal of this paper isto verify if the variable relationships pointing out from CODA of foraminiferal assemblages are ornot influenced by the size fraction and, more in general, to test the robustness of CODA respect totreatment changes related to different preparation techniques for the analysis of planktonicforaminifera

Universitat de Girona. Departament d’Informàtica i Matemàtica Aplicada