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INTRODUCTIONThe palaeoecological interpretation of fossil assemblages of agglutinated foraminifera is dependent on comparison with modern analogues. However, modern living and dead assemblages with exclusively organo- and ferro-agglutinated walls (Banner et al. 1991) are found only in environments where the water is under-saturated in carbonate. In the modern world such conditions occur especially on high intertidal marshes and in the deep sea below the calcite compensation depth but also occasionally in fjords (Loch Etive, Scotland, Murray et al. 2003) and on deep continental shelves (Larsen shelf, Antarctica, Murray and Pudsey 2004). However, it is unlikely that all fossil agglutinated assemblages come from this limited range of modern analogues. This raises two possibilities regarding fossil agglutinated assemblages. Alternative 1: they lived in or were predominant in a wider range of environments than at present (i.e., discrepancy induced by evolution and changing climate and palaeogeography, e.g., Nagy et al. 2010). Alternative 2: that some of them are secondary assemblages derived from original assemblages that included calcareous forms (as suggested by Scott et al. 1983). The primary aim of this paper is to fill the no-analogue gap in modern data by exploring the second alternative. This, in turn, should shed light on the first alternative. A simple way to approach this problem is to simulate the natural processes of carbonate dissolution by experimentally dissolving original dead assemblages (ODAs) to obtain residual acid-treated assemblages (ATAs) from a wide range of modern environments. Our studies to date have shown that ATAs retain a considerable amount of ecological information (see Table 1 for source references and references with ecological information). A further advantage of studying ATAs is to provide much more reliable data on the distribution of agglutinated species; the latter are commonly heavily outnumbered by calcareous forms in ODAs, and consequently it is difficult to obtain a statistically reliable sample. We use dead rather than living assemblages because the present study is not ecologically process-oriented. The assemblages living at the site at the time of collection are influenced by patchiness and seasonality whereas the time-averaged dead assemblages, even though they may include exotic forms, reflect longer-term conditions and the potential fossil assemblages. The data for agglutinated foraminifera in this paper are based on >63 ?m size fraction for all environments except deep sea where >125 ?m was used. These cutoffs should be borne in mind if the size fraction used for fossil material is greater than the reference information presented here especially for species diversity and species abundance. This study is the first to synthesise the distribution of agglutinated taxa in environments ranging from intertidal, through shelf seas, to deep sea for the NE Atlantic margin. It includes data on 87 new samples together with data from work previously completed. In addition, the aims of this study are: • To establish patterns of species diversity for ATAs with respect to major environments. • To compare these ATA species diversity patterns with the ODAs and also with live patterns (summarised in Murray 2006). • To determine the distribution of taxa with respect to broad environments; to determine ecological controls. • To determine the role of local transport in modifying agglutinated assemblages. |
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