Preparation of Study Material

3.1.4.1 Diatom Recovery

Initially, the standard procedures for diatom preparation (including the use of Hydrogen peroxide, Battarbee 1986) were conducted on DC samples from BP wells 15/28a-3 and 21/9-1. However, it quickly became apparent that the vast majority of diatoms recovered were replaced or infilled with pyrite. As Hydrogen peroxide has an adverse effect on pyrite, an alternative technique was implemented (Table 3.2, p. 65).

WELL NUMBER DC CORE SWC TOTAL 15/28A-3 58 0 0 58 21/9-1 43 16 0 59 16/13A-4 0 16 0 16 16-13A-5 0 16 0 16 *3/30A-4 4 0 0 4 *16/16A-3 2 0 0 2 *22/6-1 0 2 0 2 *22/6A-2 2 0 0 2 *208/15-1 1 0 1 2 *21/10-1 1 0 0 1 *22/6A-6 0 0 1 1 111 50 2 163

* = Samples from BP diatom reference collection

Table 3.1 Offshore sample provenance

Stage 1 Dry sample is boiled in water and sodium hydroxide (NokOM ) to disaggregate the sediment.

Stage 2 Sample plus water is wet-sieved through a 63-micron sieve to remove the finest clay. Care is required to ensure that any heavy particles are retained from the <63 micron fraction, in order to recover any small pyritised diatoms.

Stage 3 Residues are oven-dried, and dry-sieved into different size-fractions. Stage 4 Each size fraction is spread onto a picking tray and picked under a Wild

incident-light microscope, using a fine krusk. A maximum of four trays is picked per sample. Very small diatoms (<45pm) are picked using a pig’s eyelash, affixed to a piece of wooden dowelling with araldite glue (Sims

1989).

Stage 5 Picked diatoms are placed in microfaunal slides, each morphology being placed into a numbered grid square.

Table 3.2 Processing techniques used in this study

Despite early reservations regarding possible corrosion of any residual silica preserved on the pyritised moulds, it was found that this procedure was the most effective for maximum recovery of diatoms. To test whether any corrosion was occurring, it was decided to carry out a control, involving boiling a duplicate sample in water without the Na OH • SEM examination of diatoms from the 2 differently-processed samples revealed no evidence of dissolution or corrosion, and so it was decided to continue the processing technique outlined above.

Samples from BP cores 16/13a-4 and 16/13a-5, Viking Graben (see above) were also processed for diatoms. Containing highly-indurated, well-laminated claystones (with lignite and fish-debris) intercalated with ash-bands and with carbonate concretions, these cores held good potential for recovery of both pyritised and non-pyritised diatoms as a similar lithology from the Norwegian Sector had yielded a well-preserved diatom assemblage (Malm et al. 1984). Initial smear slides taken from the most promising intervals (i.e. concretionary layers) revealed no preserved diatoms, and so the samples were subjected to more prolonged processing.

Due to the highly-indurated nature of the core samples, and the fragility of any non-

pyritised diatoms which may be present, it was decided to break down the rock fragments by freeze-thaw, a technique used successfully by J. Hinchey (pers. comm.

1993). This involved soaking the rock in water for 24 hours, then placing it in a freezer for a further 24 hours, and finally dropping the frozen sample into boiling water. While this proved effective in breaking down the sample, examination of the broken-down residue revealed only a few, poorly-preserved pyritised diatoms. Subsequent processing with hydrogen peroxide produced no further diatoms.

Strewn slides containing well-preserved, non-pyritised diatoms (as well as silicoflagellates, ebridians, radiolaria and archaeomonads) were also examined from the Fur Formation, Denmark (see above). These were sent to the Natural History Museum by Dr. M. Homann, and had been previously processed and cleaned. In any case, the extreme purity of many of the layers of diatomite from which these samples came meant that minimal processing was required (Homann 1991, p. 13). Therefore, it was only necessary to take a pipetted sample from each phial and dry it at on a mica strip at normal room temperature, before picking and/or coating the sample for SEM examination. Strewn slides from each sample provided by Dr. Homann were mounted by K. Childs, and are now housed in the diatom collections at the Natural History Museum, London. A minimum of four hundred specimens were counted from each of these slides, and in addition silicoflagellates and radiolaria were included and logged (Chapter 5).

Electron micrographs of specimens from all localities were taken using the JEOL T-200 Scanning Electron Microscope (SEM) and its successor the Zeiss DSM 940 at U.C.L., as well as the Hitachi S-800 field emission SEM at the Natural History Museum. Some specimens in the strewn slides from Fur, Denmark were also photographed via the transmitted light camera microscope, under phase-contrast light in order to illustrate the differential silicification of some of the diatom taxa. Due to the extreme opacity of both the pyritised and calcified diatoms which formed the majority of specimens examined, it was not decided to photograph these in transmitted light. Identification of diatoms was helped by the publication, during the course of the present study, of a monograph on the diatoms of the Fur Formation by Homann (1991). However, whilst of undeniable value this publication only included light microscope photographs of diatoms, and it often proved difficult to determine the

status of the 3-dimensional, pyritised morphologies from the North Sea using this reference alone. Therefore, it was decided to photograph specimens from strewn slides sent by M. Homann (see above) via the SEM, and this approach proved extremely effective in determining the identity of the North Sea morphotypes, many of which only had fragmentary remains of the original characters used for species level taxonomy. In the course of this work it became necessary to emend some of Homann’s species designations. This was facilitated by referring to the original species descriptions and illustrations (e.g. Heiberg 1863; Grunow 1884; Schmidt 1874-1959) and, where possible, the holotypes. Unfortunately, many of the original specimens housed in the Natural History Museum were mounted in canada balsam, and so SEM observation of these was not possible. Nevertheless, some of Homann’s descriptions were found to be invalid and these are discussed in Chapter 4.

Silicoflagellates, found in the samples from Fur, were identified by reference to Perch-Nielsen (1976 & 1985). These proved of great biostratigraphical use, and allowed the accurate placing of samples within Perch-Nielsen’s (1976) scheme.

3.1.4.2 Differential Preservation: Microprobe Analysis

SEM observation of diatom specimens from different localities around the North Sea showed considerable variations in preservational style, with many specimens replaced and/or infilled with pyrite and other authigenic minerals. However, the degree of pyritic replacement and the identity of the mineral encrusting the specimens were not known, and as it was felt that this would give valuable information about palaeoenvironments and diagenetic history, it was decided to subject selected specimens to energy-dispersive X-ray microprobe analysis (EDX). This was conducted using a Link An-10000 EDX Microanalysis System in the Mineralogy Department of the Natural History Museum, London under the supervision of Dr. T. Williams. Specimens were selected from the central part of the North Sea Basin (BP well 15/28a-3) and from the basin margin (Knokke no.l well), with an unpyritised diatom frustule from the Fur Formation used as a control. The results of this investigation are discussed in Chapter 6.

PART II: RESULTS AND