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Benthic foraminifera are one of the principal means of interpreting
the bathymetry of ancient depositional environments. With the advent of geohistory methods that require quantitative paleodepth data, greater importance has been placed on benthic foraminifera as paleodepth indicators. The purpose of this symposium is to review some of the problems and prospects offered by our current state-of-knowledge.
Widely used foraminiferal paleodepth indicators include faunal trends (species abundance, species diversity, test composition, planktonic to benthic ratios) and species depth limit (e.g., UDL) based upon distributional studies of modern benthic microfaunas. The application of such ecologic approaches in the interpretation of fossil data relies on three assumptions that have received little attention: that modern distributional patterns are analogous to those of the past; that homeomorphs of modern taxa had similar ecological adaptation; and that the depth habitats of modern species were the same in the past. The first appears sounds, but growing evidence suggests that the latter two require revision.
From the study of foraminiferal ecology it is clear that species distribution is unrelated to bathymetry per se but coincides with water-mass, sediment, and nutrient boundaries in the ocean. Because such boundaries vary within the oceans, few, if any, modern species are actually isobathyal (same upper depth limit) outside of limited regions. Paleobathymetric models calibrated in terms of modern species are probably less precise than originally presumed. During the Cenozoic, climatic fluctuations produced major changes in water-mass distribution, sediment patterns and oceanic productivity that, in turn, caused shifts in the depth distribution of many deep sea species. How bathyal assemblages in continental margins responded to these environmental changes remains an unanswered question n foraminiferal paleobathymetry. Also unclear is the paleobathymetry of fossil assemblages, for which there are no direct modern analogs such as the agglutinated assemblages found in Cretaceous and early Tertiary time.
Despite current problems, benthic foraminiferal paleobathymetric approaches are basically sound and as benthic faunal responses to climatic-oceanographic changes are better understood, benthic foraminifera will be even more useful indicators of environment.
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