Mathis P. Hain (Southampton, UK), Daniel M. Sigman (Princeton, NJ, USA), and Gerald H. Haug ML (Zürich, Switzerland)
The radiocarbon (14C) isotopic composition of atmosphere and ocean have long been a tar-get of climate research because they record changes in ocean circulation and carbon cycle across the end of the last ice age, the largest climate transition in recent geologic times. The general notion is that during glacial times CO2 was sequestered in the deep ocean, where it was isolated from 14C resupply from cosmogenic 14C production and has to loose much of its radiocarbon content to decay. During deglaciation this sequestration of carbon was upended by changes in the carbon cycle and ocean circulation, thereby releasing the previously iso-lated low-14C CO2 back to the atmosphere. Changes in the Southern Ocean utilization and transport of nutrients are thought to be central to the glacial sequestration of carbon at depth and the deglacial release of CO2 from the ocean (e.g., Sigman et al. 2010, Hain et al. 2010, 2013), but sensitivity experiments with a carbon cycle box model show that these processes would have surprisingly little effect on the 14C/C (i.e., ∆14Catm) ratio of CO2 in the atmosphere (Hain et al. 2014).
In the context of the atmospheric CO2 ∆14Catm changes since the last ice age, two episodes of sharp ∆14Catm 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 cosmogenic
14C production, Atlantic circulation change and Southern Ocean CO2 release both contribute to the overall deglacial ∆14Catm decline, but only the onset of NADW can reproduce the sharp
∆14Catm declines (Hain et al. 2014). This finding suggests that millennial-timescale variations of ∆14Catm during the deglaciation are a sensitive recorder of changes in global circulation patterns, with only a modest imprint arising from changes in the carbon cycle that are respon-sible for the deglacial ocean release of CO2 to the atmosphere.
If ocean circulation is indeed the primary driver of millennial-timescale ∆14Catm variations, the exact timing of the changes in ∆14Catm may hold important information on the climate dynamics that are operating during glacial terminations. Along these lines, the onset of both episodes sharp ∆14Catm decline precedes the canonical timing of abrupt Northern Hemisphere warming by about 500 years. Thus, to fully simulate ∆14Catm data requires an additional pro-cess that immediately precedes the onsets of NADW formation. We hypothesize that these
“early” ∆14Catm declines record the thickening of the ocean’s thermocline, giving rise to an expansion of buoyant body of water that constitutes the subtropical gyres at the expense of the volume of deep water. Due to the large differential in 14C/C between upper ocean and
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90 Nova Acta Leopoldina NF 121, Nr. 408, 89 –91 (2015)
deep waters we estimate that ∆14Catm declines 12 ‰ per 100 m of thermocline thickening/
pyc nocline deepening. This implied change in the density structure and circulation of the ocean is consistent with simple physical models (e.g. Gnanadesikan 1999) and it arises in general circulation models in response to forced shutdown of NADW and/or changes in the Southern Hemisphere westerly winds (e.g. Zhang 2007, Chang et al. 2008, Mignone et al.
2006, Lauderdale et al. 2013) – such as reconstructed for Heinrich stadial 1 and the Young-er Dryas preceding the onset of NADW formation. Based on these findings, we suggest that the onset of the sharp ∆14Catm decline records an imbalance in the ocean’s buoyancy budget that progressively modifies the ocean’s density structure so as to set up the subsequent abrupt re-initiation of NADW formation (Hain et al. 2014).
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Hain, M. P., Sigman, D. M., and Haug, G. H.: The biological pump in the past. In: Mottl, M. J. (Ed.): Treatise in Geochemistry. 2nd ed. doi:10.1016/B978-0-08-095975-7.00618-5. Amsterdam (etc.): Elsevier 2013
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Simulating Atmospheric Radiocarbon through Deglaciation
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Prof. Daniel M. Sigman, Ph.D.
Princeton University Department of Geosciences M52 Guyot Hall
Princeton, NJ 08544 USA
Phone: +1 609 2582194 E-Mail: sigman@princeton.edu
Prof. Dr. Gerald H. Haug ETH Zürich
Department of Earth Sciences Geological Institute NO G 51.1 Sonneggstrasse 5 8092 Zürich Schweiz
Phone: +41 44 6328610 Fax: +41 44 6321080 E-Mail: gerald.haug@erdw.ethz.ch
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