Date of this Version
Ocean Drilling Program Leg 171B Hole 1051B penetrated a continuous sequence of upper middle Eocene pelagic sediment dominated by rich calcareous nannofossil assemblages. Samples were taken at 10 cm spacing from a 20.2 meter section of the upper middle Eocene. This interval of pelagic sediment had previously been interpreted to represent ~500ky. Our revised age model, using additional data, indicates this section represents ~842ky.
The Eocene Epoch was characterized by one of the most dramatic climatic transitions in the last 65 my, changing the greenhouse earth of the early Eocene to the icehouse world of early Oligocene. Here changes in calcareous nannofossils communities are observed in association with climatic variability that accompanied this transition.
The relative abundance of calcareous nannofossil species was determined by conducting a count of 456 individuals for each of 203 samples. These data were analyzed using richness, diversity, and CABFAC factor analysis. This latter method showed 88.5% of the variance within the data was accounted for within two factors. Factor 1 displays a strong correlation with Shannon Diversity. The character and structure of the calcareous nannofossil communities were examined using Shannon diversity, species richness, and evenness. Shannon Diversity is shown to have statistically significant correlations to richness and evenness. Richness and evenness are negatively correlated; Richness increases slightly up-section while evenness decreases slightly.
The calcareous nannofossil species present in each sample were also examined for biostratigraphic significance. It was determined that the upper middle Eocene sediments studied here are within Calcareous Nannofossil Zone CP14b. The extinction of Sphenolithus obtusus is of potential biostratigraphic value. Sphenolithus obtusus shows peak abundance of 12% in the lower portion of the section examined, a rapid decline in population followed. Assuming a constant rate of sediment accumulation (24 m/my), it is estimated to have taken approximately 126ky for S. obtusus to reach peak abundance and ~84ky to fall to 1% abundance levels where it persisted for another 100ky.
Advisor: David K. Watkins