A revised method for determining the absolute abundance of diatoms

Diatoms and other microfossils are used extensively in reconstructions of past climates and environments, in both terrestrial and marine settings. Both relative and absolute abundances of diatom taxa in sediment are important tools in these reconstructions. However, acquiring these data is a labor-intensive process. Settling-based diatom slide preparation techniques often bias samples through aliquot subsampling and sediment washing. Other techniques involve the use of added markers, which might obscure diatoms on the slide. This paper presents a revision to the widely adopted settling-based diatom slide preparation method presented by Scherer (J Paleolimnol 12:171-179, 1994) and provides a direct comparison to another widely used method. Evenly distributed diatom slides can be created by a settling process, which yields multiple statistically similar diatom slides without needing to clean sediment of salts or do aqueous subsampling, which may impart a bias in the sample when there is a wide range of particle shapes and sizes in the assemblage. Two samples originally utilized by Scherer (J Paleolimnol 12:171-179, 1994) were prepared via the updated method through a series of replicates. These results were compared to the same samples, processed with the method of Schrader and Gersonde (Utrecht Micropaleontol Bull 17:129-176, 1978), utilizing Petri dishes and the original results of Scherer (J Paleolimnol 12:171-179, 1994). The new modification presented here produces smaller standard deviations than the original Scherer method, and order of magnitude better statistics than the Schrader and Gersonde (Utrecht Micropaleontol Bull 17:129-176, 1978) method.