Distinct roles of the Southern Ocean and North Atlantic in the deglacial atmospheric radiocarbon decline
Graphical abstract
Introduction
Atmospheric 14C/C has declined from the Last Glacial Maximum (LGM) to the preindustrial modern, with two main episodes of rapid 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 declines: (a) the Southern Ocean ventilation of an hypothesized isolated volume of carbon dioxide-rich abyssal water, yielding synchronous atmospheric CO2 rise and 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 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 changes (Broecker, 2009).
Here, supported by an improved estimate of 14C production rate change, we attempt a complete simulation of the deglacial history. We find that the 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 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 explained?
Given the vigor of the debate regarding the drivers of deglacial 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 decline associated with HS1 (e.g., Broecker and Barker, 2007). Some workers have taken observed 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 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 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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