Distinct roles of the Southern Ocean and North Atlantic in the deglacial atmospheric radiocarbon decline

doi:10.1016/j.epsl.2014.03.020

Earth and Planetary Science Letters

Volume 394, 15 May 2014, Pages 198-208

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https://doi.org/10.1016/j.epsl.2014.03.020 Get rights and content

Highlights

•
Complete simulation of deglacial atmospheric $Δ {}^{14}C$ decline.
•
New estimate of ¹⁴C production history consistent with preindustrial ¹⁴C activity.
•
North Atlantic circulation changes and Southern Ocean CO₂ release both required.
•
NADW onsets account for rapid $Δ {}^{14}C_{atm}$ declines during late HS1 and YD.
•
“Early” $Δ {}^{14}C_{atm}$ declines implicate westerly winds as drivers of NADW onset.

Abstract

In the context of the atmospheric CO₂ ¹⁴C/C ( $Δ {}^{14}C_{atm}$ ) changes since the last ice age, two episodes of sharp $Δ {}^{14}C_{atm}$ decline have been related to either the venting of deeply sequestered low-¹⁴C CO₂ through the Southern Ocean surface or the abrupt onset of North Atlantic Deep Water (NADW) formation. In model simulations using an improved reconstruction of ¹⁴C production, Atlantic circulation change and Southern Ocean CO₂ release both contribute to the overall deglacial $Δ {}^{14}C_{atm}$ decline, but only the onset of NADW can reproduce the sharp $Δ {}^{14}C_{atm}$ declines. To fully simulate $Δ {}^{14}C_{atm}$ data requires an additional process that immediately precedes the onsets of NADW. We hypothesize that these “early” $Δ {}^{14}C_{atm}$ declines record the thickening of the ocean's thermocline in response to reconstructed transient shutdown of NADW and/or changes in the southern hemisphere westerly winds. Such thermocline thickening may have played a role in triggering the NADW onsets.

Graphical abstract

Introduction

Atmospheric ¹⁴C/C has declined from the Last Glacial Maximum (LGM) to the preindustrial modern, with two main episodes of rapid $Δ {}^{14}C_{atm}$ decline during deglaciation (e.g., Hughen et al., 2004, Bronk Ramsey et al., 2012, Southon et al., 2012) (Fig. 1). The significance of this record is vigorously debated. Two explanations have been proposed for the sharpest $Δ {}^{14}C_{atm}$ declines: (a) the Southern Ocean ventilation of an hypothesized isolated volume of carbon dioxide-rich abyssal water, yielding synchronous atmospheric CO₂ rise and $Δ {}^{14}C_{atm}$ decline (Broecker and Barker, 2007, Marchitto et al., 2007, Skinner et al., 2010), or (b) the resumption of NADW formation transferring ¹⁴C from the atmosphere into the deep ocean (Keir, 1983, Hughen et al., 1998, Hughen et al., 2004, Köhler et al., 2006, Laj et al., 2004, Muscheler et al., 2008), in which case $Δ {}^{14}C_{atm}$ is expected to decline most steeply only after most of the atmospheric CO₂ rise. In general, however, skepticism has been expressed that any oceanic mechanism can explain the observed $Δ {}^{14}C_{atm}$ changes (Broecker, 2009).

Here, supported by an improved estimate of ¹⁴C production rate change, we attempt a complete simulation of the deglacial $Δ {}^{14}C_{atm}$ history. We find that the $Δ {}^{14}C_{atm}$ history is surprisingly consistent with the consensus view of deglacial ocean changes, with alternating increases in North Atlantic deep ventilation and Southern Ocean CO₂ release. Two subtle but coherent deviations between model and observations that immediately precede the onsets of NADW formation at ∼12 and ∼15 thousand years before present (kyr BP) can be taken as evidence for one further ingredient, and we will argue that this as-of-yet unrecognized dynamic may be important in the mechanism of the South-to-North teleconnection.

Section snippets

The new CYCLOPS model

All carbon cycle model simulations in this study were generated using a new high-performance implementation of the legacy CYCLOPS global carbon cycle box model (e.g., Hain et al., 2010, Keir, 1988, Sigman et al., 1998, Sigman et al., 2003, Robinson et al., 2005b). We use the same model configuration as in (Hain et al., 2010, Hain et al., 2011), with 18 ocean reservoirs, one atmospheric carbon reservoir, and one fixed size terrestrial carbon reservoir (3000 PgC). The operation of the biological

CO₂ versus $Δ {}^{14}C_{atm}$ sensitivities

Our knowledge of the global ocean circulation and air/sea carbon exchange predicts distinct radiocarbon and CO₂ effects of deep ocean ventilation by the North Atlantic and the Southern Ocean (Fig. 4). To clarify these expectations and the principle dynamics, we focus here on the results of five 1000-yr sensitivity experiments: (1) transition of North Atlantic-sourced overturning from GNAIW- to NADW-based, (2) stalling of GNAIW-based overturning, (3) demise of glacial Subantarctic Zone iron

Deglacial $Δ {}^{14}C_{atm}$ explained?

Given the vigor of the debate regarding the drivers of deglacial $Δ {}^{14}C_{atm}$ changes, the model-data fit that we achieve using an idealized deglacial scenario comes as a surprise. It has been argued that only the deglacial release of a hypothesized large and severely ¹⁴C-deplete deep ocean carbon reservoir can explain the magnitude and pace of $Δ {}^{14}C_{atm}$ decline associated with HS1 (e.g., Broecker and Barker, 2007). Some workers have taken observed $Δ {}^{14}C$ anomalies in some mid-depth ocean sites (e.g.,

Conclusion

We present the most rigorous attempt to date to simulate the deglacial history of atmospheric $Δ {}^{14}C$ changes, accounting for the effect of changes in Earth's magnetic field strength (and its uncertainty) on cosmogenic ¹⁴C production and separating the contributions of changes in the North Atlantic and Southern Oceans. Our simulations suggest that the repeated stalling and resumption of NADW formation was the principle driver for the two main deglacial episodes of rapid $Δ {}^{14}C_{atm}$ decline, with much

Acknowledgments

The authors thank C. Laj and I. Usoskin for sharing the GLOPIS dataset and the ¹⁴C production model, and E. Bard, M. Bender, W. Broecker, J. Sarmiento and S. Thornalley for discussion. Reviews by the editor J. Lynch-Stieglitz, J. Adkins, and an anonymous reviewer greatly improved the manuscript. Support was provided by the Walbridge Fund Graduate Award of the Princeton Environmental Institute, the Charlotte Elizabeth Procter Honorific Fellowship of Princeton University (to M.P.H.), the UK NERC

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Simulations were carried out with annual-mean circulation fields derived from the equilibrium state of the model, and thus do not include seasonal variability. In addition to the OGCM simulations, we also conducted a suite of sensitivity tests in the 18-box ocean model CYCLOPS (Hain et al., 2014; after Keir, 1988) in which Zn cycling is linked to that of PO4 exactly as in the OGCM. The simplicity of the box-model formulation allows us to conduct a large ensemble of 10,000 simulations with varying Zn:PO4 uptake behaviours.

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Distinct roles of the Southern Ocean and North Atlantic in the deglacial atmospheric radiocarbon decline

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

The new CYCLOPS model

CO2 versus Δ C atm 14 sensitivities

Deglacial Δ C atm 14 explained?

Conclusion

Acknowledgments

Quat. Sci. Rev.

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Quat. Sci. Rev.

Earth Planet. Sci. Lett.

Quat. Sci. Rev.

Quat. Sci. Rev.

Deep-Sea Res., Part 1, Oceanogr. Res. Pap.

Wind-driven upwelling in the Southern Ocean and the deglacial rise in atmospheric CO2

Science

What do Δ C 14 changes across the Gerzensee oscillation/GI-1b event imply for deglacial oscillations?

J. Quat. Sci.

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J. Geophys. Res., Oceans

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Radiocarbon

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On the driving processes of the Atlantic meridional overturning circulation

Rev. Geophys.

Increased biological productivity and export production in the glacial Southern-Ocean

CO₂ versus $Δ {}^{14}C_{atm}$ sensitivities

Deglacial $Δ {}^{14}C_{atm}$ explained?

Wind-driven upwelling in the Southern Ocean and the deglacial rise in atmospheric CO₂

What do $Δ {}^{14}C$ changes across the Gerzensee oscillation/GI-1b event imply for deglacial oscillations?

The mysterious ¹⁴C decline

Glacial water mass geometry and the distribution of $δ {}^{13}C$ of $\sum {CO}_{2}$ in the western Atlantic Ocean

Contribution of Southern Ocean surface-water stratification to low atmospheric CO₂ concentrations during the last glacial period

¹⁴C activity and global carbon cycle changes over the past 50,000 years

Preformed phosphate, soft tissue pump and atmospheric CO₂

Role of marine biology in glacial–interglacial CO₂ cycles

A model-based interpretation of low-frequency changes in the carbon cycle during the last 120,000 years and its implications for the reconstruction of atmospheric $Δ {}^{14}C$

The control of atmospheric pCO₂ by ocean ventilation change: the effect of the oceanic storage of biogenic carbon