The Effect of Mississippi River Delta Outflow on Bathyal Benthic Foraminiferal Assemblages

Richard A. Denne

The Mississippi River outflow exerts a strong influence on benthic foraminifera found not only in neritic depths, but also on those from bathyal waters. Multivariate analyses on relative abundance and presence/absence data from the bathyal northwestern Gulf of Mexico delineated an assemblage best developed in 100-400 m of water from 88.7° to 90° W. This assemblage is composed of outer neritic to upper bathyal species which are much more common proximal to the delta than distal (Bolivina alata, B. barbata, Bulimina marginata, Epistominella vitrea, Hansenisca regularis, Nonionella opima). Qualitative analysis of abundance data from this and previous studies separated species into 5 categories: (1) the aforementioned assemblage, (2) bathyal species more ab ndant proximal to the delta above 800 m (Bulimina aculeata, Cassidulina neocarinata), (3) outer neritic to upper bathyal species less abundant proximal to the delta (Bolivina minima, Uvigerina laevis), (4) bathyal species less abundant near the delta above 800 m (Alabaminella turgida, Bolivina albatrossi, Cassidulina subglobosa, Epistominella exigua), and (5) species showing no change in their distribution (e.g. Bolivina ordinaria, Uvigerina peregrina). The main cause for these patterns in bathyal foraminifera is the increased productivity within the delta's outflow, raising the nutrient content of nearby sediments and lowering dissolved oxygen levels of waters underneath the outflow. No actual "elevation" or "depression" of species' upper and lower de th limits was found for any bathyal species except for very rare forms, suggesting that productivity affects only the abundances, not the occurrences of common bathyal foraminifera.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995