Abstract:
In an earlier study (Henderiks & Torner, 2006, Marine Micropaleontology, 58: 207-218), nannofossil proportion estimates between replicate smear-slides showed substantially higher variance than those obtained from replicate sprayed slides. This study revisits this potentially disconcerting issue, detailing the internal accuracy and reproducibility of relative nannofossil species abundances obtained with the same two preparation techniques: the simple smear-slide and the spray method. In addition, accuracy and reproducibility were tested for a semi-quantitative method, in which the number of specimens per mm (super 2) is determined from smear-slides. To test the overall reproducibility of these two preparation methods, replicate slides were prepared and analysed with each technique for a set of six samples selected from Oligocene deep-sea sediments from the equatorial Pacific (ODP Leg 199). Both smear and spray replicates indicate a good reproducibility of proportion estimates within the same method, since no statistically significant differences between the replicate slides were observed. However, when comparing the smear with spray replicates, the dataset reveals significantly different species proportions between slides, indicating that the two preparation techniques are not statistically comparable. It appears that the dominant taxon, Cyclicargolithus floridanus. is enriched in the smear-slides, whereas the abundance of the subordinate taxa is about the same for either method. A comparative test of abundance counts between dense and thin areas on the same smear-slide indicates nannofossil size sorting in the smearing technique, with proportionally more large specimens in the dense ripples. This study confirms an earlier observation, that the smear method results in lower proportion estimates of larger nannofossil taxa in comparison to the spray method.