Relation of counting methods to taphonomic gradients and biofacies zonation of foraminiferal sediment assemblages

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Marine Micropaleontology



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The routine procedure for most distributional studies of foraminifera has been to count approximately 300 specimens of all sizes greater than some specified minimum (usually between 63 and 125 μm), and determine percent abundance of each species using total counts. This method fails to take into account that foraminiferal species may be found predominantly within certain size fractions, either because of species-specific size ranges or taphonomic processes (e.g., sorting, transport, abrasion, dissolution). Use of an alternative counting procedure (“sieve method”) takes into account foraminiferal size distributions. The sieve method utilizes counts of up to 300 specimens in each sand size fraction (0.125–0.25, 0.25–0.5, 0.5–1.0, 1–2 mm) of each sample. Counts are then totaled for each sample (up to 1200 specimens per site) and used in determination of species abundances for each site.

The sieve method has been of considerable utility in recognition of foraminiferal bathymetric zonation preserved in sediment assemblages of fringing reef environments at Discovery Bay, Jamaica. Well-documented reef zones (originally based on corals and physiography) are relatively well-defined in Q-mode cluster analysis (UPGMA) of species abundances determined using the sieve method. In contrast, reef zones are not recognized in cluster analysis of foraminiferal species abundances based on the standard method, nor by cluster analysis of species abundances within individual size fractions. The sieve counting procedure compensates for operator bias in specimen counts introduced by large and unusually abundant species (e.g.,Amphistegina gibbosa) and by small but colorful forms (e.g.,Discorbis rosea), which mask the zonation using the standard procedure. The sieve method does not alter overall depth-related trends in species abundance, distribution, or diversity as determined by the standard method. Thus, foraminiferal sediment assemblages contain more paleoenvironmental information than had previously been thought.

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