Paper No. 12
Presentation Time: 9:00 AM-6:00 PM
LATE EOCENE–EARLY OLIGOCENE OSTRACODE TURNOVERS IN THE MOSSY GROVE CORE, HINDS COUNTY, MISSISSIPPI
YAMAGUCHI, Tatsuhiko, Division of Geosciece Research, Scripps Institution of Oceanography, 206 Vaughan Hall, La Jolla Shore Dr. MS-0244, La Jolla, CA 92093, NORRIS, Richard D., Scripps Institution of Oceanography/UCSD, MS-0244, 427 Vaughan Hall, La Jolla, CA 92093-0244 and DOCKERY III, David T., Mississippi Office of Geology, Mississippi Department of Enviromental Quality, P.O. Box 2279, Jackson, MS 39225-2279, tyamaguchi@mail.ucsd.edu
The Late Eocene–Early Oligocene global climatic cooling forced biotic turnovers. Pelagic plankton underwent turnovers during the Oi-1 oxygen isotope event with increasing cold-water tolerant taxa and reducing tropical-subtropical taxa. However there are a few discussions on shallow-marine biota, correlating with isotope events. Although more than 400 shallow-marine ostracode taxa were reported from the Paleogene strata in the Gulf Coast of the US, the ostracode species diversity and composition have seldom been discussed. Here we present ostracode faunal changes during the Late Eocene–Oligocene in the Mossy Grove Core, that was drilled in northwestern Hinds County, Mississippi. The core constitutes mudstone of the Moodys Branch, Yazoo, and Forest Hill Formations. Previously the geologic age of the core was assigned with planktonic foraminifer biostratigraphy and the 40Ar/39Ar radiometric ages of bentonite layers.
We obtain 33 ostracode species from 118 samples of the Yazoo sediments of the core. Using the cluster and factor analyses, we identify four assemblages. The boundaries between the assemblages are dated to be ca. 35, ca. 33.96, and ca. 33.63 Ma. In species diversity, the assemblage during 35–33.96 Ma interval is the highest, having 7–16 species. The assemblage before 35 Ma consists 7–9 species. The assemblage during 33.96–33.63 Ma shows 8–15 in species richness, whereas the assemblage after 33.63 Ma indicates 5–10. Species diversity decreased after ca. 33.96 Ma. These turnovers correlate with the late Eocene–early Oligocene stepwise climatic cooling, that is shown in foraminiferal oxygen isotope records.
The sediments correlate with the Actinocythereis montgomeryensis–Acanthocythereis florienensis ostracode Zones, because the ostracodes contain A. montgomeryensis, Actinocythereis gibsonensis, and Actinocythereis boldi. A previous study identified a turnover at the end of the Acan. florienensis Zone. However we recognize the ostracode faunal change with diversity loss in the A. florienensis Zone and suggest that the change response to global sea level fall and climatic cooling.
