From Greenhouse to Icehouse: The Marine Eocene-Oligocene Transition

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

CHAPTER 4
Changes in Shallow-Marine Faunas from the
Northeastern Pacific Margin Across the
Eocene/Oligocene Boundary

Elizabeth A. Nesbitt


ABSTRACT

Shallow-marine molluscan faunas of the northeastern Pacific margin change from middle Eocene through the early Oligocene as the global climatic conditions changed from greenhouse to global icehouse. Faunal changes are described in terms of paleocommunity reorganization over four biozones during this critical 10-million-year period. Although there is rapid species turnover at each of the four biozone boundaries under investigation, the molluscan community structure changed at a much slower pace, taking 5 to 6 million years. The middle Eocene/late Eocene boundary is notable for the immediate loss of tropical taxa and drop in species richness. Cool-water gastropod taxa, endemic to the northern Pacific, slowly replace the locally extinct genera. This response to the rapid cooling events, previously characterized in deep-sea studies, is concomitant with a rapid change in tectonic setting and subsequent depositional environment. In contrast to the gastropods, changes in bivalve taxa reflect the replacement of craton-derived feldspathic and arkosic sediments in the shallow-shelf substrate, to magmatic arc-derived tuffaceous sediments. The molluscan paleocommunity analysis reveals two different environmental signatures.


INTRODUCTION

The Terminal Eocene Event was described by Wolfe in 1978, and the intervening twenty years have revealed that the climatic shift from global greenhouse to icehouse is more complex and severe, and extends from late middle Eocene through early Oligocene time (e.g. prothero and Berggren, 1992; prothero, this volume). Four global events mark progressively deteriorating climates over this transition, and the biozonal boundaries based on different taxa illustrate the protracted temperature shifts. The global climate changed from very wide tropical and subtropical belts that prevailed through the Cretaceous and early Tertiary to the narrowed tropics, wide temperate and boreal zones that characterized the Neogene. This change coincides with major marine extinctions depicted in studies of open-ocean, deepsea organisms (e.g. Thomas, 1989; Schellenberg, 1999), but very few studies have documented these events in relation to nearshore organisms. Diverse and abundant molluscan faunas from paleogene outcrops along the margin of the northeastern Pacific Ocean provide a different and independent database for studying the effects of this major global climate change. This study presents a preliminary investigation of the changes in paleocommunity structure from 40 million to 30 million years ago.

The Eocene-Oligocene boundary is placed at the last occurrence of the planktonic Foraminifera genus Hantkenina in the marine section at Massignano, northern Italy, that is dated at 33.7 Ma (Berggren et al., 1995). The epoch boundary is 0.1 million years younger than the worldwide planktonic foraminiferal Zone boundary p17/18 based on the LAD of the Turborotalia cerroazulensis/cunialensis group, and neither boundary coincides with nannoplanktonic zonal boundaries (Berggren and Miller, 1988; Berggren et al., 1995) (figure 4.1). After the early Eocene temperature maxima, global cooling is first recorded at the end of the middle Eocene (37 Ma), when the deepsea, benthic foraminiferal δ18O isotope record shows a 1‱ drop, corresponding to a 4°C temperature

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