Effect of Disjunct Size Distributions on Foraminiferal Species Abundance Determinations

Ronald E. Martin, W. David Liddell

Studies of foraminiferal distribution and abundance have typically employed a procedure ("standard method") that entails counting approximately 300 specimens from a size range greater than some specified minimum (commonly 63 or 125 µm). This method fails to take into account that foraminifera may be found only within certain size fractions, either because of species specific size ranges or taphonomic processes (sorting, transport, abrasion). Use of a modified counting procedure ("sieve method") takes into account foraminiferal size distributions. The sieve method uses counts of up to 300 specimens in each sand-size fraction (0.125-0.25, 0.25-0.5, 0.5-1, 1-2 mm) of each sample. Counts are then totaled for each sample (up to 1,200 specimens per site) and used in determ nation of species abundances for each site. The sieve method has been of considerable utility in recognition of a foraminiferal bathymetric zonation preserved in sediment assemblages from fringing reef environments at Discovery Bay, north Jamaica. Well-documented reef zones (based on corals and physiography) are clearly defined in Q-mode cluster analysis (UPGMA) of species abundances determined using the sieve method. In contrast, individual fore 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 alternative counting procedure appears to compensate for 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 appear to mask the zonation using the standard procedure. Thus, foraminiferal sediment assemblages appear to contain more information than had previously been suspected.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.