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