Benthic Foraminiferal Distribution in Deep-Water Periplatform Carbonate Environments
Ronald E. Martin
In contrast to clastic depositional environments, bathymetric distribution of benthic foraminifera in deep-water carbonate environments has been largely neglected. Approximately 260 species and morphotypes of benthic foraminifera were identified from 12 sediment samples (piston core top and grab) collected along two traverses approximately 25 km apart across the northern (windward) margin of Little Bahama Bank at depths of 275 to 1,135 m. Most species exhibit great variation in abundance with depth. However, Globocassidulina subglobosa, Cibicides rugosus, and Cibicides wuellerstorfi are all reliable depth indicators (Spearman's r > 0.91; p < 0.005), being most abundant at depths > 1,000 m, and correspond to lower slope (> 900 m) periplatform aprons.
Individual foraminiferal suborders (Miliolina, Rotaliina, Textulariina) show no consistent depth-related trends. However, certain operational taxonomic groups, such as reef-dwelling peneroplids and soritids (suborder Miliolina) and rotaliines (suborder Rotaliina) are significantly more abundant at depths < 300 m (95% C.I.: 2.6 ± 2.2% and 6.9 ± 2.7%, respectively) than at greater depths (95% C.I.: 0.3 ± 0.2% and 2.0 ± 0.8%; Mann-Whitney U, p < 0.01), reflecting downslope bottom transport in proximity to bank-margin reefs. Small miliolines (i.e., suborder Miliolina minus peneroplids and soritids) and rosalinids and discorbids (suborder Rotaliina) are also more abundant at depths < 300 m (95% C.I.: 27.5 ± 7.4% and 32.6 ± 8.5%, respectively) than at greater depths (95% C.I.: 10.0 ± 3.9% and 1.5 ± 1.6%; Mann-Whitney U, p < 0.01) and are winnowed from the carbonate platform.
Assemblages exhibit greatest variation in diversity (species number, s; Shannon-Weaver, H^prime; evenness, J^prime) at depths > 900 m; indices for shallower assemblages tend to be grouped more tightly at relatively high values. Increased variation in diversity with depth reflects the influence of sediment gravity flows below 900 m, which mix shallow and deep-water species.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.