Silica dissolution: palaeoecological interpretation

Seawater is a medium which is not conducive to the preservation of biogenic silica. This is because marine water in today’s oceans is everywhere undersaturated with respect to silica (Berner 1968, 1970; Kastner 1981). Microscopic siliceous skeletons, including diatoms and radiolaria, are therefore prone to dissolution on exposure to seawater, a situation which leads to considerable preservational bias in assemblages, which only represent a percentage (or thanatocoenosis) of the original population in the water colunrn (or biocoenosis). In the oceans, only 1% to 10% of the diatom frustules produced in the photic zone reach the bottom sediment (Calvert 1974), a

situation compounded by morphological adaptations of diatom skeletons (such as large cell diameters, the production of spines and webs composed of chitinous fibres, and by the formation of chains) which help to sustain buoyancy in the water column during life. Preservation of delicate diatom frustules in diatomites is apparently due to the incorporation of diatom frustules into the faecal pellets of copepods and other zooplankton, which speeds their transport to the seafloor and provides some protection from dissolution in the water column (Schrader 1971).

In both marine and lacustrine environments, a number of studies have addressed the problem of under-representation when interpreting diatom assemblages from surface sediments (e.g. Sancetta 1982; Shemesh et a l 1989; Burckle et a l 1992); all have concluded that sediment trap studies provide the only reliable means of comparison with the biocoenosis in the water column, so that a transfer function can be derived and inferences made about the palaeoecological parameters of a preserved assemblage (Battarbee 1986; Shemesh et a l 1989; Juggins 1992). However, transfer functions can only be conducted on an assemblage where the ecological parameters of its constituent species are known, a particularly uncertain method of reconstruction for pre- Quatemary diatom assemblages. Although a number of taxa have remained morphologically unchanged for millions of years (Actinoptychus senarius, an extant species, is recorded from the Upper Cretaceous in the Volga Basin, see Andrews 1979) there is no guarantee that, for example, temperature tolerances have remained at similar levels for such a prolonged period. For example, Stellarima microtrias, a species encountered throughout the Palaeogene section of the North Sea during the present study (a climatically more equable period than at present, with low temperature gradients, Roberts et a l 1984; Rea et a l 1990) is today restricted to pack- ice offshore Antarctica (Hasle & Sims 1986a). It is therefore extremely difficult to attempt meaningful reconstructions of palaeoceanographic conditions using diatoms alone for pre-Quatemary conditions (and practically impossible where these have been pyritised, see below); such studies should only be attempted in conjunction with evidence from other fossil groups, ideally combined with sedimentology and geochemistry.

Over-representation of particular diatom species is a similar problem besetting palaeoceanographic studies. Burckle et a l (1992) conducted a laboratory time-based

study on Quaternary opaline silica-rich sediments from the Sea of Japan and the North Pacific, largely consisting of the remains of the species Coscinodiscus marginatus',

they concluded that the apparent dominance of that species was largely an artefact of preservation. It is probable that the widespread occurrence of oozes consisting almost entirely of broken valves of Ethmodiscus rex in Plio-Pleistocene sediments in the North Pacific (Mikkelsen 1977; Villareal 1993) may be due to the same phenomenon. Similarly, a mid-Pleistocene neritic diatom assemblage from the Nar Valley, eastern England analysed by the present author (Mitlehner 1992), was found to be dominated by the highly robust, resistant valves of the species Lyrella lyra and Nitzschia

punctata, which together accounted for over 50% of the assemblage at some levels.

A study which is particularly pertinent to the present work was carried out by Pedersen (1981), who observed wide variations in the relative abundance of different diatom taxa within the Fur Formation diatomite of northern Denmark (see Chapters 2, 3 and 5, this study). Three types of laminae were recognised (Fig. 6.1, p. 198):-

Type 1: laminae almost exclusively composed of diatom frustules dominated by a

single diatom species, either Coscinodiscus spp. or Stephanopyxis turris.

Type 2: laminae containing both diatoms and clay minerals, with variable thicknesses.

The diatoms are evenly distributed within the laminae, but with variable density; several species are present.

Type 3: clay-rich laminae with few diatom frustules.

It was suggested that type 1 laminae may have been produced either by large blooms of particular species of diatoms or through selective dissolution of the frustules, with type 2 laminae representing long periods of "background" sedimentation and type 3 laminae produced by extraordinary supplies of clay minerals from rivers. No regular alternation of lamination types was observed, thus refuting the suggestion of Bonde (1974, 1979) that the Fur Formation is a varved (i.e. annually deposited) diatomite, and further illustrating the potential difficulties in interpretation of pre-Quatemary diatomaceous sequences.

1 8 5 p m

16 p m

Fig. 6.1. Lamina types in laminated diatomite. Fur Formation (from Pedersen 1981). NB: Diatom species dominant in types 1 and 2 were found to be prevalent in the Corbisema naviculoidea silicoflagellate zone (Fig. 5.5) during the present study; the pennate-dominated assemblage in type 3 was more characteristic of the underlying Naviculopsis danica zone. (A) Lamina type 1 with abundant large diatom valves (C oscinodiscus m orsian u s).

(B) Lamina type 1, detail, showing a matrix of small or fragmented diatom frustules and clay between the large valves of C oscin odiscu s m orsianus.

(C) Lamina type 1 with abundant small frustules of Stephanopyxis turris.

(D) Lamina type 2. C oscinodiscus m orsianus scattered in a matrix of clay, diatom fragments and small diatom species.

(E) Lamina type 2. Trinacria regina in a matrix of diatom fragments. The density of

C oscinodiscus valves is much lower than in the type 1 lamina.

(F) Possible type 3 lamina with fragments of diatoms, clay and complete specimens of small diatoms, Sceptroneis gem m ata and Stephanopyxis turris.