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The spatial and vertical distribution of living (stained) benthic foraminifera from a tropical, intertidal environment, north Queensland, Australia

Berkeley, A., Perry, C.T., Smithers, S.G., Horton, B.P.
2 008
Marine Micropaleontology
We investigated the distribution of living (stained) benthic foraminifera across a tropical, intertidal shoreline adjacent to Cocoa Creek, Queensland, Australia for the purpose of better understanding the nature of test production and ultimately fossil assemblage development within such environments. Short cores (up to 1 m) were collected during the wet and dry season, along an elevational gradient comprising non-vegetated intertidal mudflat and higherintertidal mangrove forest environments. The distribution of stained specimens can be broadly delineated into assemblages characterising 'upper mangrove' (2.64-2.91 m above Lowest Astronomical Tide (LAT)) and 'low mangrove-mudflat' (1.62-2.18 m above LAT) environments. Agglutinated species were generally limited to upper mangrove stations. Calcareous species occurred within all of the intertidal environments examined but differ in their composition between upper and lower intertidal settings. Upper mangrove faunas were characterised by the agglutinated species Arenoparrella mexicana, Haplophragmoides wilberti, Miliammina fusca, Miliammina obliqua and Trochammina inflata and the calcareous species Helenina anderseni. Live (stained) assemblages at lower intertidal elevations were dominated by the calcareous species Ammonia aoteana, as well as Rosalina spp., Elphidium oceanicum, Triloculina oblonga, Ammonia pustulosa and Shackoinella globosa. Absolute densities of stained specimens were lowest within the low mangrove sediments (6-8 specimens per cm(2)) but higher within the upper mangrove (25-249 specimens per cm(2)) and mudflat (8-77 specimens per cm(2)). Abundances within mudflat sediments varied seasonally (being significantly larger in the wet season) whereas those within mangrove zone sediments changed little through the year. Stained foraminifera were found to a maximum depth of 50 cm at Cocoa Creek, extending to the greatest depth at the upper mangrove stations, and were generally shallowest beneath the lower mangroves. All stations had over 50% of their foraminiferal standing crop below the upper I cm, although at most stations <10% of living specimens occurred within this layer. Comparison of this data with dead assemblages suggests that: (1) abundances of stained specimens, averaged over two sampling events, reasonably approximate aggregate in situ test production; and (2) a clear enrichment of infaunal species occurs within the mangrove. The latter effect was less conspicuous on the mudflat due to high rates of mixing and assemblage time-averaging, which obscure the signal of test production within the dead assemblages. In both cases, the validity and stratigraphic resolution of surface assemblages for sea-level and other palaeoenvironmental reconstructions is questionable. (C) 2008 Elsevier BY. All rights reserved.
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