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Microfossil-based palaeoenvironmental records in intertidal environments: a review of the ecological and taphonomic controls on foraminiferal assemblage development

Berkeley, A., Perry, C.T., Smithers, S., Horton, B.P., Taylor, K.G.
2 007
Earth Science Reviews
Intertidal foraminifera from surface sediments are commonly used as a high-resolution indicator of sea-level change. The integrity of this approach is based upon the assumption that surface assemblages are similar in composition to their buried, fossil counterparts. This assumption may, in some cases, be inappropriate due to subsurface (or infaunal) production and taphonomic alteration. Here, we review the current understanding of foraminiferal production and taphonomic loss in intertidal environments, and examine the extent to which these processes can affect the development of foraminiferal assemblages in intertidal environments.As a sediment layer undergoes burial, dead tests are added at a rate proportionate to the standing crop at that depth. As a consequence, test accumulation within the layer is proportionate to the depth-integrated standing crop. Depending on the nature of its vertical living distribution, the upper 1 cm of sediment may therefore not adequately characterise a given species total input. Instead, a comparison of each species contribution to the entire, depth-integrated living community may offer a better a priori estimate of relative input to the sediment column. This provides an improved base-line for identifying ‘true’ taphonomic trends downcore.The potential for infauna to create discrepancies between surface and subsurface assemblages may be minimised however, where: (1) reproduction is concentrated near to the sediment surface, transforming an apparent infaunal standing crop into an effective epifaunal production; (2) the living fauna is composed similarly at all depths of production; and/or (3) intense bioturbation homogenises the upper sediment layers. The likely decline in the intensity of taphonomic processes with depth may mean that the foraminiferal production from different depths may not experience taphonomic processes uniformly. Consequently, in addition to their possible gradual enrichment due to subsurface production, infaunal species may be preferentially preserved by virtue of their production closer to the base of the taphonomically active zone. Additionally, homogenisation of the sediment column by reworking may mix temporally and perhaps environmentally distinct faunas, affecting the stratigraphic integrity of the sediment deposit.The most important taphonomic processes in intertidal environments are those associated with early diagenesis. While the organic cements of agglutinated taxa may be directly oxidised, the associated changes in the saturation state of porewaters with respect to calcium carbonate may affect calcareous species. The type of diagenetic reaction operating within the sediment provides a quantitative control on agglutinated test degradation but a largely qualitative control on calcareous preservation. In addition, calcareous test preservation is dependent upon the rate with which organic matter is supplied, while agglutinated test preservation is independent of organic matter concentrations. Although the specific morphological pathways of test degradation for calcareous species are well understood, the stages under which agglutinated tests are lost are less clear.The combined but varying influences of production (infaunal and species composition), taphonomic processes and bioturbation may create complex pathways in the transition from live to dead to fossil assemblages. Understanding the extent to which infauna, taphonomic loss, and sediment reworking affect assemblage development in different intertidal environments is essential in order to interpret the fossil record.
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Department of Earth and Environmental Science / University of Pennsylvania, 251 Hayden Hall, 240 South 33rd Street, Philadelphia, PA 19104-6316