A Guide to the Extraction of Fossil Diatoms from Lithified or Partially Consolidated Sediments

Abstract

Based on experience gained from processing samples from the Cenozoic basins of south-wesin bt Ecuador, a composite procedure for the processing of fossil diatoms from lithified or partially consolidated sediments is presented. The procedure is divided into four stages: (I) initial sample breakdown. (II) cemented rock breakdown and removal of carbonate component. (III) organic matter removal. (IV) clay removal. These are presented in the form of annotated tables linked by an accompanying flow chart. INTRODUCTION Marine Oligo-Miocene sediments from the Manabi and Progreso basins of south-west Ecuador were examined for diatoms. Lithologies encountered varied from partially consolidated clay-rich bentonites of the Portoviejo Formation, to lithified chocolate-brown shades and siltstones of the Tosagua Formation. These units contained a mixed marine fossil assemblage of foraminifera, (Whittaker 1988), radiolaria, diatoms and silicoflagellates. A biostratigraphy for the area has been erected using foraminifera (Whittaker 1988) and radiolaria (Robinson 1992); diatoms and silicoflagellates have also proved essential for palaeo-ecological interpretation and a refined biostratigraphy (Hinchey 1992). Diatomaceous sediments from Ecuador differ considerably in their lithology and therefore react differently to conventional diatom processing techniques (Lohmann 1972, Schrader 1973). Samples from the bentonitic Portoviejo Formation broke down fairly easily, but problems were encountered in the removal of clays from these sediments. Lithified samples from the clay-rich Tosagua Formation, however, proved very problematic; conventional processing techniques were ineffective in most stages of breakdown and the soaking of samples in petroleum spirit (Abelmann 1988, Boden 1991) also failed to disaggregate the samples satisfactorily. OBJ ECTIVES As standard diatom processing techniques failed to breakdown many of the samples collected from Ecuador, a different approach was clearly required. A literature review was conducted on previous techniques and experienced workers were asked for their comments (see acknowledgments). Techniques suggested were applied to material from Ecuador and the most effective methods noted. The result of this work has produced a composite processing procedure which can be generally applied to most types of diatomaceous sediments, including those which are lithified or partially consolidated. The procedure is presented in the form of a flow chart (text-fig. 1) and a series of annotated tables (1-4), each table representing a particular aspect of processing. The procedure is designed to be user friendly and emphasizes the use of wet mounts between certain stages (e.g. removal of clays and organic matter; text-fig. 1, Tables 3 and 4) to evaluate whether or not further processing is required. The tables are designed to work independently of each other and therefore should allow workers to initially assess the lithology of the sediments they are studying and to then determine which of the tables apply to that particular type of sediment. ADDITIONAL NOTES TO TABLES Table 1 Mechanical breakdown of the sediment should be done with as little damage to frustules as possible. Not all stages listed will be necessary. Physico-chemical breakdown (Table 2 (2a) and Table 3 (la-d)) can be used for more resistant samples. Table 2 Warming of sample + HCI (Ib) is not always necessary. Heat only if very little reaction occurs. Table 3 Addition of a few drops of methanol to a hydrogen peroxide solution (lb) prevents strong foaming in organic rich samples. Table 4 In some samples, all clay cannot be removed. As a consequence the worker must assess the residue (using wet mounts) to evaluate what is an acceptable amount of clay in their particular study. Note on the use of wet mounts. Tables 3 and 4 advocate the use of wet mounts at various stages of the cleaning procedure to check whether further processing is required. It must be noted that the removal of material for wet mount preparation will bias any quantitative examination (P. Boden, pers. comm.). Therefore, if high resolution quantitative studies are to be attempted it is suggested that either 1) wet mounts should not be used, or that 2) two splits of the same sample be processed. One split being used to assess whether further processing is necessary and the other used for quantitative work. The wet mount procedure forms an essential stage in the cleaning of diatomaceous lithified sediments from Ecuador. However, it has proved unnecessary for DSDP deep sea muds from the equatorial Pacific. Therefore the use of wet mounts is wholly dependant on the nature of sediment and the purpose of study. SLIDE PREPARATION The method of slide preparation utilized will depend on the type of study proposed. The pipetting method of Schrader and Gersonde (1978) proves suitable for biostratigraphic studies, while the ranmicropaleontology, vol. 40, no. 4, pp. 368-372, text-figure 1, tables 1-4, 1994 368 This content downloaded from 157.55.39.104 on Sun, 19 Jun 2016 06:45:44 UTC All use subject to http://about.jstor.org/terms Micropaleontology, vol. 40, no. 4, 1994 I.INITIAL SAMPLE BREAKDOWN