Atlantic-Pacific seesaw and its role in outgassing CO2during Heinrich events

L Menviel,J Yu,K J Meissner,A Mouchet,M H England

doi:10.1002/2013pa002542

Abstract

[1] Paleoproxy records indicate that a marked weakening of the Atlantic Meridional Overturning Circulation (AMOC) during Heinrich events was often accompanied by a notable atmospheric CO2 increase. However, previous modeling studies display conflicting atmospheric CO2 responses to an AMOC shutdown. Here we use model simulations combined with paleoproxy records to show that depending on the deep and bottom water transport in the Northern and Southern Pacific Ocean during an AMOC weakening, the ocean can act either as a sink or a source of carbon. Results from idealized meltwater experiments as well as from a transient experiment covering Heinrich stadial 4 suggest that a shutdown of the AMOC during Heinrich stadials 4 (HS4) and 1 (HS1) led to an enhancement of Antarctic Bottom Water (AABW) and North Pacific Deep Water (NPDW) transport. We show that enhanced deep and bottom water transport in the Pacific Ocean ventilates deep Pacific carbon through the Southern Ocean, thus contributing to a rise in atmospheric CO2. This mechanism yields a good agreement between paleoproxy records and modeling results, thus highlighting the possible establishment of an Atlantic-Pacific seesaw during Heinrich stadials. Enhanced AABW and NPDW transport could account for most of the observed atmospheric CO2 increase during HS4 and for about 30% of the atmospheric CO2 increase during HS1.

Highlights

[2] The last glacial period and the last deglaciation were punctuated by millennial-scale events during which the Atlantic Meridional Overturning Circulation (AMOC) weakened substantially [McManus et al, 2004]
[4] Here we show that knowledge of changes in North Atlantic Deep Water (NADW) and in North Pacific Deep Water (NPDW) and Antarctic Bottom Water (AABW) transport is required to fully understand the responses of the global climate and oceanic carbon cycle during Heinrich events
[28] Our results show that solubility effects as well as changes in the deep and bottom water mass transport in the Pacific Ocean are the primary control of the marine carbon cycle during a shutdown of the Atlantic Meridional Overturning Circulation (AMOC)

Summary

Introduction

[2] The last glacial period and the last deglaciation were punctuated by millennial-scale events during which the Atlantic Meridional Overturning Circulation (AMOC) weakened substantially [McManus et al, 2004] These events, called Heinrich events [Vidal et al, 1997], occurred most likely due to the addition of meltwater in the North Atlantic as a result of ice sheet instabilities [Heinrich, 1988]. Most of the studies [Marchal et al, 1999; Bouttes et al, 2012; Matsumoto and Yokoyama, 2013] which concluded that the oceanic carbon content decreased as a result of an AMOC shutdown, applied global salt compensation in their experimental design. We further suggest that a strengthening of AABW and NPDW formation during HS1 and HS4 caused a carbon release from the deep Pacific Ocean, leading to an atmospheric CO2 increase

Methods

Idealized LGM Experiments

Validation of the Mechanism for Heinrich Stadial 4

Discussion and Conclusions