From Greenhouse to Icehouse: The Marine Eocene-Oligocene Transition

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

CHAPTER 10
Terrestrial Eocene-Oligocene Vegetation and
Climate in the Pacific Northwest

Jeffrey A. Myers


ABSTRACT

Stratigraphically dense and well-dated paleofloras from sequences throughout the U.S. West Coast permit recalibration of the timing and magnitude of vegetational and paleoclimate events during the EoceneOligocene transition in western North America. Paleoclimate data from these assemblages indicate that a major cooling step of ˜3–4°C in mean annual temperature (MAT) took place between 39 and 38.5 Ma. The vegetational response to cooling was the geologically rapid replacement of evergreen broad-leaved forest by deciduous broad-leaved forest in many areas of the interior Pacific Northwest, and the entry of diverse temperate lineages into near-sea level vegetation. A latest Eocene warming event produced a ˜3–4°C increase in mean annual temperature (MAT) and returned much of the Pacific Northwest to warm temperate conditions by ˜37 Ma, permitting the reentry of thermophylic Goshen-type indicator species to low and moderate elevations. A second major pulse of climatic cooling took place between 34.0 and > 33.6 Ma, and produced no more than a ˜3–6°C decline in MAT, a moderate increase in seasonality, and a pronounced decrease in dry month precipitation. This “boundary event” resulted in the widespread replacement of subtropical evergreen broad-leaved vegetation by deciduous broad-leaved vegetation dominated by diverse cool temperate lineages, although many thermophylic lineages persisted well into the Oligocene near sea level.

Revised estimates of the pattern, magnitude, and timing of late middle Eocene to early Oligocene climate change agree more closely with evidence from marine isotopes and with extinction and radiation events of mammals and shallow marine invertebrates than previous estimates. It is now clear that cooling near the Eocene/Oligocene boundary was only the last of a series of cooling and warming steps that impacted continental western North America, and that middle-late Eocene climate and vegetational change was at least as severe those of the more famous boundary event. However, the boundary cooling event crossed the minimum temperature tolerance of most subtropical lineages and resulted in the most catastrophic and permanent continental biotic crisis of the Cenozoic.


INTRODUCTION

Paleoclimate estimates from continental paleofloras indicate that continental climate cooled in a series of steps from the late Paleocene thermal maximum through the early Oligocene. The record is punctuated by shorter term warming and cooling steps (Wolfe, 1971, 1992, 1994), by far the most frequently cited and most catastrophic of which was interpreted by Wolfe (1992, 1994) to have taken place over a span of ˜1 million years near 33 Ma, just after the Eocene/ Oligocene boundary. The continental event is mirrored by an approximately 1‰ positive marine oxygen isotopic shift centered between 33.6 and 33.5 Ma (Zachos et al., 1996; Alroy et al., 2000). This event, termed the Terminal Eocene Event (TEE) by Wolfe (1978), had profound effects on vegetation, particularly at high and mid latitudes, resulting in the replacement of warm subtropical, predominantly evergreen, vegetation with predominantly deciduous temperate vegetation. In many areas of the interior Pacific Northwest and Alaska, cold-month mean tem-

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