The distribution of agglutinated foraminifera in NW European seas:
Baseline data for the interpretation of fossil assemblages
Assemblages composed entirely of agglutinated foraminifera occur in a restricted range of modern environments: where carbonate dissolution is prevalent, e.g., marshes, deep shelves or ocean depths below the carbonate compensation depth. Fossil agglutinated assemblages undoubtedly come from a wider range of environments than these modern analogues. Natural post-depositional or diagenetic dissolution of carbonate foraminiferal tests in original dead assemblages (ODAs) leads to the formation of residual agglutinated assemblages. Simulating this process experimentally by treating modern ODAs with dilute acid gives rise to acid-treated assemblages (ATAs). This paper provides a synthesis of previous work on 261 samples and provides new data on a further 87 samples.
Investigated environments range from marginal marine marshes and estuaries through fjord and shelf seas to deep sea. The pattern of species diversity in ATAs follows the same pattern as that for ODAs and living assemblages: there is a progressive increase in diversity from marginal marine to deep sea. Because agglutinated foraminifera are normally only a small component of ODAs, the ATAs provide a much more reliable guide to species and generic distributions. Although the faunal distribution patterns are well defined in terms of broad environment, it has proved difficult to account for individual distributions using the available data on abiotic environmental factors. In NW European seas, Miliammina is confined to intertidal to shallow subtidal marginal marine environments but elsewhere it is common on deep shelves (e.g., off Antarctica).
In summary where evolution can not explain the absence of calcareous taxa, ATAs fill the no-analogue gap and provide baseline data that can be used to interpret the ecology of fossil agglutinated assemblages.
KEY WORDS: carbonate dissolution; taphonomy; experiments; palaeoecological interpretation; species diversity
PE Article Number: 14.2.14A
Copyright: Palaeontological Association July 2011
Submission: 29 July 2010. Acceptance: 8 April 2011