Revised Technique for Calculation of Calcareous Nannofossil Accumulation Rates

Abstract

A rapid and easy settling technique designed to estimate quantitative abundances of calcareous nannofossils is described. Compared with other methods, the time spent in weighing and other time-consuming steps are compensated by the possibility of standardizing and producing large series of samples at the same time. Formulas for the calculation of absolute abundances and accumulation rates are presented. A test in which the examination of several samples where the amount of sediment as well as other parameters have been changed demonstrates a random distribution of nannoliths across the slide. Variations in the calculated accumulation rates were minimal, suggesting high reliability. INTRODUCTION Coccolithophores, and probably most of the calcareous nannoplankton, are primary producers whose distribution in the oceans is controlled by several parameters. Sea surface temperature (SST), related to climatic zoning, is one of the most important limiting parameters e.g.: different species in varying abundances characterize equatorial to subpolar regions (McIntyre et al. 1970; Haq 1980; Winter et al 1994). At both local and global level, biolimiting nutrient distribution is another important factor related to coccolithophore abundance (e.g. Molfino and McIntyre 1990; Young 1994). Changes in SST or in nutrient concentrations in the oceans are related to paleoceanographic changes and knowledge of coccolithophore (nannofossil) abundance in sediments could therefore help to understand the evolution of the world's oceans. The aim of the present study was to test an easy and rapid technique for estimating nannolith abundance per gram of sediment. This technique allows one to convert the amount of nannofossils into nannofossil fluxes when precise age models and sediment densities are available. Several techniques have been proposed to estimate the absolute abundances of nannofossils (Wei 1988; Beaufort 1991; Okada 1992; Henriksson 1995). Some of these techniques are very time-consuming, or involve instrumental problems in the preparation of well spread surfaces for nannofossil counting, often meaning than many visual fields (nannofossils) must be counted. In other cases, the proposed techniques allow only afford an estimation of the true abundance, as for example in Backman and Shackleton (1983). PROCEDURE The aim of the technique proposed here is to obtain slides on which the nannoliths are distributed homogeneously, and to achieve the possibility of standardizing the procedure in order to save time. Our experience is that with this procedure it is possible to prepare sets of up to one hundred samples at one time; i.e., a rate of around 50 samples per day. This technique was tested using both deep-sea sediments, where nannofossils were the most important component, and sediments, in which the calcareous nannofossil component was diluted by organic or inorganic material. The steps below describe the procedure, and should be followed carefully. Step 1. Sample drying. A small amount of sediment is dried in a heater at a temperature of about 50?C. Step 2. Weighing of an amount of dried sediment. This amount depends on the richness of the samples in nannofossils, as well as on the parameters discussed below. For routine analysis we used mass s between 0.1 to 0.3 g. Step 3. The weighed sediment is placed in a bottle, and a varying volume of distilled water is added (we used 10 to 30 ml). The mixture is shaken and, if necessary, placed in an ultrasound apparatus for a few seconds. For samples rich in organic matter, the distilled water can be substituted with H202. It is advisable to leave the mixture for a few hours under conditions of regular