The ratio of agglutinated : hyaline : porcellaneous foram tests has been used extensively to differentiate among a range of modern environments. Ternary plots of the relative frequencies of test type distinguish fields for hypersaline and marine lagoons, estuaries and open shelf seas (Fig. 9.7). Fossil faunas may be plotted on these templates, and these allow paleontologists to estimate the salinity of ancient environments.
The ratio of infaunal : epifaunal benthic foraminiferans has also been widely used to determine the relative content of dissolved oxygen and/or organic carbon on the seafloor. Epifaunal and infaunal foraminiferans can be distinguished by their test morphologies, where epifaunal forms occur mainly in aerobic conditions with low amounts of organic carbon, and infaunal forms occur in more oxygen-deficient conditions with higher organic carbon content.
Measures of the ratio of benthic : planktonic foraminiferans are also useful in environmental studies. In general terms, the percentage of benthic taxa declines rapidly below depths of about 500 m in modern seas and oceans. Data from living assemblages have been used to interpret paleoen-vironments with diverse fossil foraminiferan faunas. For example, microfossil analysis of the upper part of the Late Cretaceous chalk of the Anglo-Paris basin has suggested water depths of between 600 and 800 m during the Turonian on the basis of the high proportions of planktonic foraminifer-ans; however, by the Campanian, water depths of about 100 m are suggested by the rich benthic fauna.
Allogromia rAmmonia freshwater normal marine lagoons and Discoris irbonate platforms
Rotaliina Textularüna brackish lagoons " and estuaries most shelf seas spinose planktonics
Globigerinoides keeled planktonics
Globorotalia maximum diversity of calcareous benthic forms maximum diversity of agglutinated forms
Figure 9.7 Foram test and environments: distribution of test types and genera of Foraminifera against environmental gradients. (From Armstrong & Brasier 2005.)
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