Foraminifera as Holocene environmental indicators in the South Alligator River, Northern Australia

Samples of plankton, surface sediments and Holocene deposits from the South Alligator River, an energetic macrotidal river in northern Australia, are examined to evaluate the usefulness of estuarine foraminifers for palaeoenvironmental interpretation of sediments deposited in tropical macrotidal conditions. Although marine and estuarine foraminifers in the estuarine channel are mixed by strong tidal currents, there are significant trends along the 80-km length of the tidal river. The relative proportions of marine and porcellaneous taxa decrease upstream from the mouth, but hyaline taxa and those with brackish affinities increase upstream. Foraminifers in intertidal sediments have distinctly different assemblages in the uppermost intertidal, mid-tidal (mangrove), lower intertidal and subtidal zones. In addition to the effects of tidal mixing, preserved thanatacoenoses are affected by selective dissolution and/or pyritisation within the sediment bodies. Despite the effects of adverse taphonomic factors, foraminiferal assemblages preserved in drill cores provide an accurate guide to original depositional environments (intertidal/subtidal; brackish estuarine/coastal marine), in mid-Holocene sediments in the South Alligator area. Foraminifers are found to improve the precision of previous palaeoenvironmental reconstructions made on the basis of sedimentary facies and pollen analyses.