Arctic closure as a trigger for Atlantic overturning at the Eocene-Oligocene Transition

David K Hutchinson,Helen K Coxall,Matt OʹRegan,Agatha M De Boer,Johan Nilsson,Rodrigo Caballero

doi:10.1038/s41467-019-11828-z

David K Hutchinson, Helen K Coxall + Show 4 more

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https://doi.org/10.1038/s41467-019-11828-z

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Journal: Nature Communications	Publication Date: Aug 22, 2019
Citations: 48	License type: open-access

Affiliation: Stockholm University

Abstract

The Eocene-Oligocene Transition (EOT), approximately 34 Ma ago, marks a period of major global cooling and inception of the Antarctic ice sheet. Proxies of deep circulation suggest a contemporaneous onset or strengthening of the Atlantic meridional overturning circulation (AMOC). Proxy evidence of gradual salinification of the North Atlantic and tectonically driven isolation of the Arctic suggest that closing the Arctic-Atlantic gateway could have triggered the AMOC at the EOT. We demonstrate this trigger of the AMOC using a new paleoclimate model with late Eocene boundary conditions. The control simulation reproduces Eocene observations of low Arctic salinities. Subsequent closure of the Arctic-Atlantic gateway triggers the AMOC by blocking freshwater inflow from the Arctic. Salt advection feedbacks then lead to cessation of overturning in the North Pacific. These circulation changes imply major warming of the North Atlantic Ocean, and simultaneous cooling of the North Pacific, but no interhemispheric change in temperatures.

Highlights

The Eocene-Oligocene Transition (EOT), approximately 34 Ma ago, marks a period of major global cooling and inception of the Antarctic ice sheet
There are two main hypotheses for what triggered the Atlantic meridional overturning circulation (AMOC): deepening of the Southern Ocean gateways led to wind-driven upwelling of deep water in the Southern Ocean, forcing the AMOC13,14; and deepening of the Greenland Scotland Ridge (GSR) led to bi-directional flow over the ridge that allowed deep water to form in the North Atlantic[15]
These analyses suggest that in the late Eocene, northern component water was confined to shallower depths and northern latitudes, whereas around the EOT, northern component water deepened and expanded across the equator, which is supported by Neodymium isotope tracers[20]

Summary

Introduction

The Eocene-Oligocene Transition (EOT), approximately 34 Ma ago, marks a period of major global cooling and inception of the Antarctic ice sheet. There are two main hypotheses for what triggered the AMOC: deepening of the Southern Ocean gateways led to wind-driven upwelling of deep water in the Southern Ocean, forcing the AMOC13,14; and deepening of the Greenland Scotland Ridge (GSR) led to bi-directional flow over the ridge that allowed deep water to form in the North Atlantic[15] These hypotheses have yet to be tested in a climate model that is designed with late Eocene palaeogeography and capable of capturing the ocean circulation at the EOT. Recent proxy data from the Labrador Sea shows that North Atlantic bottom waters were increasing in salinity and density in the three millions years prior to the EOT12 This salinification trend, preceding the EOT, suggests that local forcing may have enabled North Atlantic sinking independently of the Antarctic glaciation.

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