Impact of oceanic processes on the carbon cycle during the last termination

N Bouttes,D M Roche,D Paillard,L Bopp,E Michel,C Waelbroeck,M Kageyama,A Lourantou

doi:10.5194/cp-8-149-2012

Abstract

Abstract. During the last termination (from ~18 000 years ago to ~9000 years ago), the climate significantly warmed and the ice sheets melted. Simultaneously, atmospheric CO2 increased from ~190 ppm to ~260 ppm. Although this CO2 rise plays an important role in the deglacial warming, the reasons for its evolution are difficult to explain. Only box models have been used to run transient simulations of this carbon cycle transition, but by forcing the model with data constrained scenarios of the evolution of temperature, sea level, sea ice, NADW formation, Southern Ocean vertical mixing and biological carbon pump. More complex models (including GCMs) have investigated some of these mechanisms but they have only been used to try and explain LGM versus present day steady-state climates. In this study we use a coupled climate-carbon model of intermediate complexity to explore the role of three oceanic processes in transient simulations: the sinking of brines, stratification-dependent diffusion and iron fertilization. Carbonate compensation is accounted for in these simulations. We show that neither iron fertilization nor the sinking of brines alone can account for the evolution of CO2, and that only the combination of the sinking of brines and interactive diffusion can simultaneously simulate the increase in deep Southern Ocean δ13C. The scenario that agrees best with the data takes into account all mechanisms and favours a rapid cessation of the sinking of brines around 18 000 years ago, when the Antarctic ice sheet extent was at its maximum. In this scenario, we make the hypothesis that sea ice formation was then shifted to the open ocean where the salty water is quickly mixed with fresher water, which prevents deep sinking of salty water and therefore breaks down the deep stratification and releases carbon from the abyss. Based on this scenario, it is possible to simulate both the amplitude and timing of the long-term CO2 increase during the last termination in agreement with ice core data. The atmospheric δ13C appears to be highly sensitive to changes in the terrestrial biosphere, underlining the need to better constrain the vegetation evolution during the termination.

Highlights

The last termination, which took place between ∼18 000 and ∼9000 years ago, is characterized by a global warming (Visser et al, 2003; North Greenland Ice Core Project members, 2004; EPICA community members, 2004; Barker et al, 2009) associated to a shrinking of the ice sheets (Peltier, 1994, 2004; Svendsen et al, 2004)
We use the intermediate complexity model CLIMBER-2 to explore the impact of three oceanic mechanisms on the evolution of the carbon cycle during the last deglaciation: iron fertilization, sinking of brines and stratification-dependent diffusion
The carbonate compensation mechanism is included in the CLIMBER-2 model, which has already been used to study the LGM carbon cycle (Brovkin et al, 2002b, 2007; Bouttes et al, 2009, 2010, 2011)

Summary

Introduction

The last termination, which took place between ∼18 000 and ∼9000 years ago, is characterized by a global warming (Visser et al, 2003; North Greenland Ice Core Project members, 2004; EPICA community members, 2004; Barker et al, 2009) associated to a shrinking of the ice sheets (Peltier, 1994, 2004; Svendsen et al, 2004). N. Bouttes et al.: Ocean processes during the last termination explain the warming and shrinking of ice sheets (Berger et al, 1998; Charbit et al, 2005; Ganopolski et al, 2010), in association with the change of insolation. Bouttes et al.: Ocean processes during the last termination explain the warming and shrinking of ice sheets (Berger et al, 1998; Charbit et al, 2005; Ganopolski et al, 2010), in association with the change of insolation Explaining such an increase remains a challenge

Methods

Results

Conclusion