Gerald H. Haug ML,1 Anja Studer,1,2 Abby Ren,2 Sascha Serno,3 Samuel L. Jaccard,4 Alfredo Martínez-García,1 Robert F. Anderson,3 Gisela Winckler, Rainer Gersonde,5 Ralf Tiedemann,5 and
Daniel M. Sigman2
With 2 Figures
We argue for a pervasive link between cold climates and polar ocean stratification (Haug et al. 1999, Sigman et al. 2004, 2007). In both the Subarctic North Pacific and the Antarctic Zone of the Southern Ocean, ice ages were marked by low productivity (Fig. 1 and 2, Jac-card et al. 2010, 2013). The accumulated evidence from sediment cores points to an increase in density stratification that reduced the supply of nutrients from the ocean interior into the sunlit surface in both of these regions. The last ice age was associated with stratification of the Antarctic and the subarctic North Pacific, and it can be argued that the well-known glacial decrease in North Atlantic Deep Water indicates a similar stratification of the North Atlantic. This link also applies to longer timescales, including the onset of extensive Northern Hemisphere glaciation 2.7 million years ago, which was concurrent with stratification of the Subarctic North Pacific and the Southern Ocean. The generality of the cooling/stratification connection calls for a general mechanism. Such a mechanism is provided by the non-linear relationship between the temperature of seawater and its density: Cooling of the ocean will decrease the role that temperature plays in the density structure of the polar water column, allowing the freshwater cap that is always present in the polar regions to cause greater den-sity stratification, allowing the freshwater cap to intensify further. Nutrient-rich polar ocean regions such as the Antarctic and the Subarctic Pacific represent a “leak” in the biological pump, allowing deeply sequestered carbon dioxide to escape back into the atmosphere, and stratification of these regions largely stops that leak. Thus, the link between climate cooling and the stratification of nutrient-rich polar regions represents a positive feedback in the cli-mate system, raising atmospheric carbon dioxide during warm periods and reducing it during cold periods.
1 Department of Earth Sciences, ETH Zürich, Zürich, Switzerland.
2 Department of Geosciences, Princeton University, Princeton, USA.
3 Lamont-Doherty Earth Observatory, Columbia University, Palisades, USA.
4 Institute of Geological Sciences, University of Bern, Bern, Switzerland.
5 Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany.
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Fig. 1 Records of (A) atmospheric pCO2 (Lüthi et al. 2008), (B) ODP 1094 planktic foraminifera δ18O, (C) ODP 1094 Ba/Fe (data smoothed by a five-point running mean), (D) ODP 1094 Ca/Fe (data smoothed by a five-point running mean), (E) Fe flux to subantarctic core ODP1090, and (F) ODP 1090 sedimentary alkenone concentration covering the past 1 Ma. Red/grey shadings highlight intervals were Antarctic (AZ)/subantarctic (SAZ) processes, respectively, are dominantly controlling the partitioning of CO2 between the ocean interior and the atmosphere (from Jaccard et al. 2013).
During the last ice age, opal accumulation (and Ba/Al) in the Antarctic (Fig. 1) and the Sub-arctic North Pacific (Fig. 2) has been much slower than it is during the current interglacial period, suggesting less biological production and a reduced rain of biogenic material to the seafloor during glacial times. In the same glacial-age Antarctic and Subarctic North-Pacif-ic sediments, the 15N/14N ratio of diatom-bound organic matter is markedly higher, which suggests that nutrient consumption (the ratio nutrient uptake to nutrient supply) was greater during ice ages. Taking these observations together, it can be argued that the glacial Antarctic and Subarctic North-Pacific surface was more isolated from the nutrients and carbon of deep Antarctic water, or, more physically, that the surface of the “polar twins”, the Antarctic and Subarctic North-Pacific, was stratified during the last ice age.
The Polar Oceans during the Deglaciation
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Over the last deglaciation, the Southern Ocean is considered the dominant source of the at-mospheric CO2 rise with the Southern Hemisphere warming driving physical and biogeo-chemical changes in the Southern Ocean that vented CO2 to the atmosphere starting at 17.6 ka.
However, cooling or stalled warming characterized the Southern Ocean during the Bølling/
Allerød period and the post-Younger Dryas period. Detailed reconstructions of CO2 over the deglaciation show ~15 ppm maxima corresponding with the Bølling/Allerød period and the post-Younger Dryas period. Both periods were characterized by Northern Hemisphere warming, including the subarctic North Pacific. Biogenic productivity proxies and nitrogen isotopic evidence indicate that, beginning at the Bølling/Allerød and after the Yougner Dryas, the Subarctic North Pacific underwent a marked increase in vertical mixing within the upper ocean, leading to the modern condition of incomplete consumption of the nutrient supply. Bio-genic opal flux and calcium carbonate data indicate that, during the Bølling/Allerød period and possibly in the post-Younger Dryas interval as well, vertical mixing reached so deeply in the Subarctic North Pacific that deeply sequestered CO2 would have been vented to the atmosphere. Thus, overturning the Subarctic North Pacific may explain the atmospheric CO2 peaks corresponding to Northern Hemisphere warming events, just as the Southern Ocean has vented CO2 to the atmosphere during its own periods of warming.
Fig. 2 Ca/Al (A) and Ba/Al (B) records from subarctic North Pacific ODP Site 882 compared to the EDC deuterium (dD) (C) (Jouzel et al. 2007) and CO2 (Lüthi et al. 2008) (D) records during the past 800 ka. Glacial terminations are indicated using Roman numerals in subscript (from Jaccard et al. 2010).
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Jaccard, S. L., Hayes, C. T., Martínez-García, A., Hodell, D. A., Anderson, R. F., Sigman, D. M., and Haug, G. H.: The roles of the Antaractic and Subantarctic zones in ocean productivity and atmospheric CO2 over the million years. Science 339, 1419 –1423 (2013)
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Lüthi, D., Le Floch, M., Bereiter, B., Blunier, T., Barnola, J.-M., Siegenthaler, U., Raynaud, D., Jou-zel, J., Fischer, H., Kawamura, K., and Stocker, T. F.: High-resolution carbon dioxide concentration record 650,000 – 800,000 year before present. Nature 453, 379 –382 (2008)
Sigman, D. M., Boer, A. M. de, and Haug, G. H.: Antarctic stratification, atmospheric water vapor, and Heinrich events: A hypothesis for late Pleistocene deglaciations. In: Schmittner, A., Chiangm, J. H. C., and Hemming, S.
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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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