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 particular against the background of relevant biological data. These data suggest that benthic foraminifera, as most microbiota, are not stenotopic to most environmental variables. A more generalist strategy seems beneficial since the low degree of specialisation prevents rapid extinction. This immediately suggests that parameters like temperature and salinity are not very important in benthic foraminiferal distribution and thus not in proxy studies. Oxygen and organic flux, on the other hand, are of great ecological importance and it is not surprising that all viable proxies today are based on relationships with these parameters. Organic flux (food) is important in deciding 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 shaped through the arrangement of species along a redox gradient, whereby species distribution seems to be correlated with specific redox levels. It cannot 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 hand, organic flux reconstructions can be distorted due to the fact that the same flux levels, though with different oxygenation, are characterised by different suites of species. Therefore, assemblage characteristics are discussed as additional means to constrain organic flux reconstructions. concluded that species distribution with depth is mainly a function of organic flux and oxygenation. In this sense, paleobathymetry should not be based on individual marker species, but preferably on quantitative assemblage characteristics such as P/B ratios. However, more generalised schemes of assemblage successions with depth are clearly helpful. It is further concluded that benthic foraminiferal distribution, and microhabitat occupation, are regulated by the interplay of organic flux, oxygen and competition. Each of these factors is acting in a different way and leads to the complex pattern as found in living associations.