Heterogeneity in global vegetation and terrestrial climate change during the late Eocene to early Oligocene transition

Matthew J Pound,Ulrich Salzmann

doi:10.1038/srep43386

Abstract

Rapid global cooling at the Eocene – Oligocene Transition (EOT), ~33.9–33.5 Ma, is widely considered to mark the onset of the modern icehouse world. A large and rapid drop in atmospheric pCO2 has been proposed as the driving force behind extinctions in the marine realm and glaciation on Antarctica. However, the global terrestrial response to this cooling is uncertain. Here we present the first global vegetation and terrestrial temperature reconstructions for the EOT. Using an extensive palynological dataset, that has been statistically grouped into palaeo-biomes, we show a more transitional nature of terrestrial climate change by indicating a spatial and temporal heterogeneity of vegetation change at the EOT in both hemispheres. The reconstructed terrestrial temperatures show for many regions a cooling that started well before the EOT and continued into the Early Oligocene. We conclude that the heterogeneous pattern of global vegetation change has been controlled by a combination of multiple forcings, such as tectonics, sea-level fall and long-term decline in greenhouse gas concentrations during the late Eocene to early Oligocene, and does not represent a single response to a rapid decline in atmospheric pCO2 at the EOT.

Highlights

Rapid global cooling at the Eocene – Oligocene Transition (EOT), ~33.9–33.5 Ma, is widely considered to mark the onset of the modern icehouse world
The palaeoecological evidence can be divided into three major groups (Fig. 2) which show either (a) no change across the EOT22,23, (b) a pronounced cooling at the EOT17,24–26, often associated with an increase in seasonality and lower winter temperatures[27,28], or (c) a gradual change that had already started before the EOT during the middle to late Eocene and continued into the Oligocene[12,23,29,30]
Climate change during the EOT has been linked to a decline in atmospheric CO2 of 400–600 ppmv over a 0.5 Ma period[4,5], which should have forced a latitudinal reorganisation of biome distributions[36]

Summary

Introduction

Rapid global cooling at the Eocene – Oligocene Transition (EOT), ~33.9–33.5 Ma, is widely considered to mark the onset of the modern icehouse world. We conclude that the heterogeneous pattern of global vegetation change has been controlled by a combination of multiple forcings, such as tectonics, sea-level fall and long-term decline in greenhouse gas concentrations during the late Eocene to early Oligocene, and does not represent a single response to a rapid decline in atmospheric pCO2 at the EOT. The EOT is represented in the marine realm by a two stepped shift in the bottom water oxygen isotopes associated with glacial advance on Antarctica[2,13], extinctions in the marine realm[1], a deepening of the calcium compensation depth[14] and a cooling of global climate[2,6]. Our study did not find such a strong equatorward shift in palaeo-biomes or terrestrial cooling and challenges previous reconstructions of a large and rapid pCO2 decline at the EOT

Methods

Results

Conclusion