From Greenhouse to Icehouse: The Marine Eocene-Oligocene Transition

By Donald R. Prothero; Linda C. Ivany et al. | Go to book overview

CHAPTER 25
Deep-Water Benthic Foraminiferal Events
from the Massignano Eocene/Oligocene
Boundary Stratotype, Central Italy

Rodolfo Coccioni and Simone Galeotti


ABSTRACT

We carried out a quantitative and semi-quantitative analysis of late Eocene-earliest Oligocene deep-water benthic foraminifera from the lower bathyal Massignano Global Stratotype Section and Point. Species distribution and faunal parameters (planktonic/total foram ratio, faunal density, diversity, the proportion of agglutinating and infaunal forms, and the dissolved oxygen index) enabled us to do the following:

1. Recognize and calibrate several, discrete bioevents against a well-established bio-, chrono-, and magnetostratigraphy.

2. Characterize the timing, and the paleoecological and paleoceanographic conditions in the western Tethys that accompanied the end of the well-documented, global Eocene-Oligocene deep-water foraminiferal faunal turnover. The timing of this faunal turnover, which is known to be time-transgressive, is here suggested to follow a latitudinal gradient with areas close to the Antarctic continent showing earlier taxonomic turnover.

3. Recognize that Nuttallides truempyi disappeared well before cold bottom waters reached the western Tethys, indicating that this event was not related to a general cooling and the ensuing enhanced bottom-water corrosivity and oxygenation. A mechanism invoking changes in paleoproductivity regimes possibly related to increased seasonality, is a more likely explanation of the observed pattern.


INTRODUCTION

The Eocene-Oligocene transition contains the intertwined history of global cooling and changes in oceanic circulation (see Prothero, 1994, for a review) and represents one of the three major intervals of turnover in Cenozoic deep-sea benthic foraminifera (see Miller et al., 1992). In contrast to other periods of dramatic and sudden extinction in benthic foraminiferal assemblages (i.e., Paleocene/Eocene boundary), a series of first and last appearances occurred over a period of several millions of years from the late middle Eocene into the earliest Oligocene (see Tjalsma and Lohmann, 1983; Berggren and Miller, 1989; Miller et al., 1992; Thomas, 1992a, b). This faunal turnover reflects major oceanographic changes related to a long-term global cooling and resulted in a species-richness gradient between high and low latitudes as a consequence of decreased diversity among high-latitude faunas during the late Eocene (Thomas and Gooday, 1996). An explanation for such a decrease in diversity at high latitudes has taken into account the physiochemical properties of deep-water masses including high CaC03 corrosivity and/or oxygenation (Thomas, 1992a; Kaiho, 1994a).

Most of the studies on benthic foraminiferal paleoecology across the Eocene-Oligocene transition are based on lower bathyal to abyssal assemblages (Miller et al., 1992). However, no quantitative, highresolution study on benthic foraminiferal assemblages across the Eocene-Oligocene transition has been carried out in the western Tethys. The Massignano Global Stratotype Section and Point (GSSP) offers the opportunity to calibrate the lower bathyal benthic foraminiferal events to the calcareous plankton bio-, magneto-, and chronostratigraphy through the late Eocene (ca. 36 Ma)-earliest Oligocene (ca. 33 Ma).

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