The deep-sea climate record of the Eocene-Oligocene Transition

Abstract

Approximately 34 million years ago, at the Eocene-Oligocene Transition (EOT), Earth abruptly transitioned to a climate state in which Antarctica became cool enough to sustain large ice sheets for the first time in tens to hundreds of millions of years. The introduction of this new component in the Earth system fundamentally changed the predictability of the climate response to astronomical forcing. Our understanding of EOT climate change and the feedback processes that drove the inception of sustained large-scale Antarctic glaciation, however, remains limited by the paucity of highly resolved and well-dated deep-sea records. Here I present records that address this problem and reappraise the sequence of events leading to the development of sustained large-scale Antarctic glaciation. In Chapter 3, I present bulk sediment stable isotope and carbonate content records from a depth transect of sites in the equatorial Pacific Ocean and one site from the subpolar South Atlantic Ocean, together with a new benthic foraminiferal stable isotope record from the shallowest and most expanded Pacific site. These records are used to reconstruct, in detail, changes in the calcite compensation depth (CCD) at the EOT. My analysis reveals (i) a global carbon cycle perturbation defined by a CCD shoaling and negative carbon isotope excursion at the onset of the EOT and leading directly into the first EOT CCD deepening step, (ii) a distinctive CCD over-deepening and settling pattern in the earliest Oligocene, and (iii) comparable results in both the eastern equatorial Pacific and subpolar South Atlantic suggesting that these CCD changes are likely driven by changes in global deep ocean chemistry. These findings show that the carbon cycle was perturbed prior to the inception of Antarctic glaciation. Once large-scale glaciation was initiated, however, rapid changes in global seawater chemistry forced a transient increase in carbon burial flux in the deep ocean far exceeding the new steady state values. In Chapter 4, I present the first benthic foraminiferal multiply substituted ‘clumped’ isotope deep-sea temperature records for the EOT. These records come from Sites U1334, U1333, and 1218 in the eastern equatorial Pacific and provide the first direct and robust evidence for deep-sea cooling across the earliest Oligocene oxygen isotope step. The onset of this deep-sea cooling, however, precedes the abrupt d18Ob increase that marks the rapid expansion of the Antarctic ice sheet by ~100 kyrs and is short-lived. Within 300 kyrs of the onset of sustained Antarctic glaciation, deep-sea temperatures in the eastern equatorial Pacific return to values similar to those of the late Eocene. The structure of deep-sea temperature change in the eastern equatorial Pacific takes a similar form to reconstructions of atmospheric carbon dioxide concentrations. The early onset cooling of the deep ocean may have been an important preconditioning mechanism for the rapid onset of large-scale Antarctic glaciation. In Chapter 5, I present a high resolution benthic foraminiferal stable isotope stratigraphy and a multiply substituted ‘clumped’ isotope deep-sea temperature record for Site U1406 in the northwest North Atlantic Ocean, spanning the late Eocene to the early Oligocene. These records are consistent with a high nutrient, low salinity, Arctic-imprinted northern component water in the deep North Atlantic during the late Eocene. Deep-sea cooling in the northwest North Atlantic Ocean approximately ~200 kyrs after the initiation of large-scale Antarctic glaciation is not seen in temperature records from the surface ocean and the deep eastern equatorial Pacific. Early Oligocene cooling in the deep northwest North Atlantic Ocean may, therefore, have been driven by regional changes in deep-sea hydrography, possibly related to the onset of Antarctic glaciation and/or tectonic-induced changes in the connectivity of the Arctic Ocean, Nordic Seas, and North Atlantic Ocean. Lastly, in Chapter 6, new high resolution benthic foraminiferal stable isotope records from Sites U1331–U1334 are presented in a data report to further refine the sequences of events in the late Eocene and in the run up to the onset of Antarctic glaciation.

More information

Identifiers

ORCID for Victoria Emma Taylor: orcid.org/0000-0002-6909-1575

ORCID for Paul Wilson: orcid.org/0000-0001-6425-8906

Catalogue record

Export record