Benthic foraminifers: proxies or problems? A review of paleocological concepts

In this paper, we review benthic foraminiferal distribution patterns in the context of their use as proxy to reconstruct paleoenvironments, in particu lar against the background of relevant biological data. These data suggest that benthic foraminifera, as most microbiota, are not stenotopic to most e nvironmental variables. A more generalist strategy seems beneficial since t he low degree of specialisation prevents rapid extinction. This immediately suggests that parameters like temperature and salinity are not very import ant in benthic foraminiferal distribution and thus not in proxy studies. Ox ygen and organic flux, on the other hand, are of great ecological importanc e and it is not surprising that all viable proxies today are based on relat ionships with these parameters. Organic flux (food) is important in decidin g on abundance, but is subordinate as soon as oxygen starts to be limiting. This is the case in almost all shallow water systems with muddy substrates , sometimes down to considerable waterdepth. Microhabitat patterns are shap ed through the arrangement of species along a redox gradient, whereby speci es distribution seems to be correlated with specific redox levels. It canno t be excluded that a relationship with specific bacterial suites also plays a role here. However, the coupling to the successive redox stages provides a means for very detailed reconstructions of oxygenation. On the other han d, organic flux reconstructions can be distorted due to the fact that the s ame flux levels, though with different oxygenation, are characterised by di fferent suites of species. Therefore, assemblage characteristics are discus sed as additional means to constrain organic flux reconstructions. It is concluded that species distribution with depth is mainly a function o f organic flux and oxygenation. In this sense, paleobathymetry should not b e based on individual marker species, but preferably on quantitative assemb lage characteristics such as P/B ratios. However, more generalised schemes of assemblage successions with depth are clearly helpful. It is further con cluded that benthic foraminiferal distribution, and microhabitat occupation , are regulated by the interplay of organic flux, oxygen and competition. E ach of these factors is acting in a different way and leads to the complex pattern as found in living associations. (C) 1999 Elsevier Science B.V. All rights reserved.