Absolute nannofossil abundance estimates: quantifying the pros and cons of different techniques

A quick and inexpensive method to determine absolute nannofossil abundance in deep sea sediments – the “drop” technique (modified dilutionmethod) – was compared to two other available methods – the filtration and random settling techniques. All techniques rely on the same basicprinciple, under which a volume of known concentration (bulk sediment weight/mL) is distributed evenly over a known total area (glass slide orfilter) to then count particles within a set of (randomly) selected fields of view. The three preparation techniques were also calibrated by spiking thesamples with microbeads to approach the “real values” as closely as possible. Significant offsets in abundance estimates between methods mainlyreflect bias due to the uneven distribution and/or loss of particles. We show that the drop technique is most consistent and accurate in estimating“real values” and offers similar or better reproducibility than the other techniques. The drop method also allows detection of the same trends withor without calibration with microbeads. The filtration method holds the risk to drastically underestimate absolute abundances, while the settlingtechnique is demanding in terms of time and may suffer from advection processes. The composition of nannofossil assemblages can be reliablydetermined by any of the three different techniques.