A NEW SAMPLE PREPARATION TECHNIQUE FOR CALCULATING CALCAREOUS NANNOFOSSIL ABSOLUTE ABUNDANCES AND UNCERTAINTIES FOR PALEOCEANOGRAPHIC STUDIES

Studies of living nannoplankton show that they have limited ecological preferences and thus are very good paleoceanographic indicators. The use of quantitative analyses to determine calcareous nannofossil accumulation rates has proved very useful as a proxy for understanding paleoproductivity and reconstructing paleoceanographic conditions. However, most sample preparation methods in use still do not provide an accurate representation of the accumulation rates of nannofossils or the errors associated with estimates of absolute abundance. Two approaches were used in determining the accumulation rates (AR) of the calcareous nannofossils. The first uses a random settling technique (RST) to determine the absolute abundances of calcareous nannofossils. The basic assumption of the random settling technique is that nannofossils that are allowed to settle out from a suspension onto a cover slip will be more randomly and evenly distributed than those prepared by the smear slide technique. The absolute abundance of the nannofossils, in specimens/gm, is calculated by using the weight of sediment put into the overall suspension and the ratio of the volume of water above the cover slip to the total volume of water in the settling chamber. The second method used is a combination of the spiking method with RST. The spiking method (SM) adds a known quantity of tracer particles to a weighed sample to calculate the unknown amount of a component in the sample. This involves doing the RST with the introduction of an aliquot of microbeads to the settling chamber. This improves the SM by ensuring a random distribution of microbeads and nannofossils. The absolute abundance of nannofossils is calculated from the known value of microbeads added to a sample and the ratio of nannofossils counted to microbeads counted. The addition of the microspheres yields an independent estimate of nannofossil accumulation rate in addition to the original random settling technique and the two can be compared to each other. These techniques can help determine how calcareous nannoplanktons reflect productivity changes in different global climatic settings and they were applied to data from the Eocene of the Western North Atlantic. The absolute abundances were calculated with their error bars and both techniques yielded similar results.