Samuel L. Jaccard (Bern, Switzerland) and Eric D. Galbraith (Montreal, Canada)
Oxygen is supplied to seawater by marine photosynthesis and exchange with the atmosphere, and consumed by the respiration of organic matter in the ocean interior. Both sides of this re-lationship are thus dependent on physical ocean circulation: oxygen supply is determined by the physical transport of oxygenated waters from the mixed layer into the ocean subsurface, while organic matter export relies on the physical circulation to supply the surface ocean with nutrients. Thus, the occurrence of oxygen-depleted (hypoxic) waters is inextricably linked to ocean circulation, both through the supply and demand of oxygen. In addition, the oxy-gen flux is physically dependent on the combined effects of the temperature dependence of oxygen solubility and the degree of saturation at the surface, crucially influenced by wind and sea-ice processes. All of these factors are linked to the sea surface conditions in the high latitude source areas where the thermocline is ventilated. It is expected that as these regions warm, large-scale deoxygenation of the upper oceans will ensue. Recent observations appear to confirm this prediction, though these changes remain subtle in the face of natural decadal variability (Ito and Deutsch 2010). Paleoceanographic records extend beyond the noise of recent decadal variability, to provide an independent perspective on the links between climate and hypoxia.
A number of factors should have conspired to increase the oxygenation of the upper ocean during the last ice age. First, lower temperatures would have increased oxygen solubility, while strong winds at high-latitudes would have helped to equilibrate the surface waters and contributed to vigorous thermocline ventilation. In addition, global cooling would have en-hanced the vertical transport of carbon from the surface to the deep ocean, due to the overall temperature dependence on the remineralization rate of sinking organic matter (Matsumoto 2007). The nutrient inventory of the upper ocean indeed appears to have been lower (Sigman and Haug 2003), which would have decreased the demand for oxygen through remineraliza-tion in much of the upper ocean. Consistent with this, paleoceanographic proxy records from near oxygen minimum zones have been interpreted as showing generally higher oxygen con-centrations during the Last Glacial Maximum (LGM; Galbraith et al. 2004). Importantly, the deep ocean (below ~2.5 km) appears to have been different, with lower oxygen concen-trations during the LGM, consistent with greater respired carbon storage there (Jaccard and Galbraith 2012, Jaccard et al. 2014 and references therein).
In order to elucidate the global evolution of ocean oxygenation throughout the degla-ciation, we compiled available records of the four most commonly measured sedimentary
Samuel L. Jaccard and Eric D. Galbraith
112 Nova Acta Leopoldina NF 121, Nr. 408, 111–115 (2015)
proxies of low-oxygen concentrations (Jaccard and Galbraith 2012, Jaccard et al. 2014).
Three of these (redox-sensitive trace metal concentrations, benthic faunal assemblages and sedimentary laminations) are sensitive to oxygen concentrations at the sediment-water inter-face of the sediment core site. In addition, we include bulk sedimentary δ15N (Galbraith et al. 2013), which is enriched by denitrification in the cores of oxygen minimum zones, from which it spreads by mixing and advection to adjacent regions where it can be recorded in sedimentary organic matter.
The deglacial transition at the end of the last ice age involved a warming of the global surface ocean by approximately 2 ºC (Shakun et al. 2012), similar to the magnitude of anthropogenic warming, but over a period of more than 10,000 years. Chronicling the de-glacial progression between the LGM and Holocene requires a temporal framework. The oxygen-sensitive proxies are therefore analysed according to the sense of change between the four well-recognized intervals of the last deglaciation (Jaccard and Galbraith 2012, Jaccard et al. 2014). First among these, during the LGM to the Heinrich stadial 1 (HS1) transition, a coherent deoxygenation occurs in the upper 1000 m of the eastern tropical Pacific. In contrast, all available proxies show that oxygenation increased throughout the Arabian Sea during the LGM to HS1 transition. Meanwhile, none of the numerous records from north of 20 °N in the Pacific show any sign of change, nor do trace metal records from the SE Pacific. Foraminiferal abundances in the North Atlantic registered a coeval deox-ygenation, though the degree of oxygen depletion there was relatively minor (to near the threshold of hypoxia, but not significantly beyond). Subsequently, the transition from HS1 to the Bølling-Allerød/Antarctic Cold Reversal (BA/ACR) saw an extremely widespread decrease in oxygenation throughout the Indo-Pacific that extended as deep as 3000 m. The only exceptions to this are among the eastern tropical Pacific sites that experienced the early deoxygenation during the LGM to HS1 transition. Globally, this transition represents the lion’s share of the deglacial upper-ocean deoxygenation, given that the northern Pacific and Indian Oceans include the vast majority of the world’s hypoxic waters. In most sed-iment records with high temporal resolution, this transition occurs very quickly. Finally, and perhaps most surprisingly, the BA/ACR to Holocene transition involved a coherent increase in oxygenation throughout most of the low-oxygen waters of the Indo-Pacific, particularly the North Pacific. This late deglacial increase in oxygenation is apparent in all available records, with no apparent dependence on water depth. In fact, the increase of oxygenation between the BA/ACR and the Holocene is more consistent among records than is the overall LGM to Holocene upper-ocean deoxygenation. The dramatic coherency of regional changes in upper-ocean oxygenation that occurred between the deglacial intervals shows that deoxygenation varied more strongly on a millennial timescale than it did on the longer, glacial-interglacial timescale. Ocean oxygenation, therefore, did not respond line-arly to changes in global temperature, particulline-arly given that the LGM to BA/ACR change involved only a fraction of the total glacial-interglacial temperature increase, and the large HS1 to BA/ACR transition involved warming only in the Northern Hemisphere. So what drove these clear, millennial-timescale changes in oxygenation?
As discussed above, the two possible causes are changes in the oxygen supply rate, and changes in subsurface respiration rates. During HS1, oxygen supply might have been expect-ed to decrease in waters ventilatexpect-ed by the Southern Ocean, due to warming at high southern latitudes, while oxygen supply to the North Pacific may have increased due to wind-driv-en changes in circulation (Meissner et al. 2005). Meanwhile, weakwind-driv-ening of the monsoonal
Deglacial Changes in Ocean (De)Oxygenation
Nova Acta Leopoldina NF 121, Nr. 408, 111–115 (2015) 113
upwelling may have reduced export in the Arabian Sea (Altabet et al. 2002), even while increased trade winds enhanced upwelling in the Eastern tropical Pacific (Kienast et al.
2006). Additionally, changes in the density structure of the ocean (Schmittner et al. 2007) or the strength of the Southern westerlies (Anderson et al. 2009) may have increased the supply of nutrients to the eastern tropical Pacific during HS1 (Robinson et al. 2007). The lack of apparent LGM to HS1 change in the North Pacific may indicate either that oxygenation changes there were indeed minimal, or that hypoxia was entirely absent during the LGM and therefore could not become any less extensive during HS1, so that increased oxygenation was not recorded by the proxies. During the transition to the BA/ACR, when the Northern Hemisphere warmed abruptly, decreased oxygen solubility would have been accompanied by an invigoration of the global overturning circulation that increased the nutrient supply to the surface ocean (Schmittner et al. 2007). Thus, during the deglacial transitions, it seems likely that changes in both oxygen supply and oxygen consumption would have contributed to the observed patterns, due to coupled changes in oceanic and atmospheric circulation, intimately linked to the large-scale overturning of the ocean.
Although the overall deoxygenation of the upper Indo-Pacific during the first half of the deglaciation is in keeping with the expectation of less oxygen in a warmer ocean, the sense of change between the BA/ACR and the Holocene is not. Despite the persistence of rela-tively large ice sheets and global average temperatures that were still well below modern, the northern Indo-Pacific hypoxic zone appears to have contracted following the BA/ACR.
Given that the colder subducting waters would have had higher saturation oxygen concen-trations than today, the consumption of oxygen must have been higher. Two mechanisms are proposed for this.
First, a deglacial trend from low preformed nutrients to high preformed nutrients could have coincided with rapid oxygen supply to the deep ocean and/or the Southern Hemisphere, so that a large burden of remineralization was focused in the upper portion of the northern Indo-Pacific; all else being equal, this would imply that the preformed nutrient inventory of the ocean increased following the BA/ACR, which is consistent with nitrogen isotope records from the Southern Ocean and subarctic Pacific (Galbraith et al. 2013 for a review). Sec-ond, it is possible that the consumption rate of oxygen decreased from the BA/ACR to the Holocene due to a deglacial decrease of global nutrient inventories. It has been argued that the nitrogen and phosphorus inventories of the global ocean were 10 –25 % higher during the glacial period, due to reduced removal of nitrogen and phosphorus through denitrification and apatite burial, respectively (Deutsch et al. 2004, Ganeshram et al. 2002). It has been reasoned that the reduction of these nutrient inventories was a response to the warmer condi-tions of the Holocene, limiting the global export production to that which could be respired given the global oxygen supply, thereby preventing widespread anoxia (Lenton and Watson 2000). This suggestion is consistent with proxy evidence that export flux in the North Pacific was particularly high during the BA/ACR (Kohfeld and Chase 2011) though testing the idea with export proxies is problematic, since the regional demand for oxygen is integrated over large areas and cannot be reliably inferred from a few points.
Forecasting the evolution of oceanic oxygen concentrations over the upcoming centu-ries will require a thorough understanding of the physical ocean response to anthropogenic forcing, as well as the changes in respiration rates that will result. Although the geological archive from the last deglaciation cannot directly help with these predictions, it presents em-phatic evidence that the evolution of oxygenation will not be a straightforward function of
Samuel L. Jaccard and Eric D. Galbraith
114 Nova Acta Leopoldina NF 121, Nr. 408, 111–115 (2015)
temperature, and that we may be in for surprises. Improved constraints on the deglaciation will require new palaeo-proxies that can quantify changes in dissolved oxygen and respired carbon concentrations, as well as greater spatial coverage.
References
Altabet, M. A., Francois, R., Murray, D. E., and Prell, W. L.: The effect of millennial-scale changes in Arabian Sea denitrification on atmospheric CO2. Nature 415, 159 –162 (2002)
Anderson, B. A., Ali, S., Bradtmiller, L. I., Nielsen, S. H. H., Fleisher, M. Q., Anderson, B. E., and Burck-le, L. H.: Wind-driven upwelling in the Southern Ocean and the deglacial rise in atmospheric CO2. Science 323, 1443 –1448 (2009)
Deutsch, C., Sigman, D. M., Thunell, R. C., Meckler, A. N., and Haug, G. H.: Isotopic constraints on glacial/
interglacial changes in the oceanic nitrogen budget. Global Biogeochem. Cycles 18, doi:10.1029/2003GB002189 (2004)
Galbraith, E. D., Kienast, M., Pedersen, T. F., and Calvert, S. E.: Glacial-interglacial modulation of the marine nitrogen cycle by oxygen supply to intermediate waters. Paleoceanography 19, doi:1029/2003PA001000 (2004) Galbraith, E. D., Kienast, M., Albuquerque, A. L., Altabet, M. A., Batista, F., Bianchi, D., Calvert, S.
E., Contreras, S., Crosta, X., De Pol-Holz, R., Dubois, N., Etourneau, J., Francois, R., Hsu, T.-C., Iva-nochko, T., Jaccard, S. L., Kao, S.-J., Kiefer, T., Kienast, S., Lehmann, M. F., Martinez, P., McCarthy, M., Meckler, A. N., Mix, A., Möbius, J., Pedersen, T. F., Pichevin, L., Quan, T. M., Robinson, R. S., Rya-benko, E., Schmittner, A., Schneider, R., Schneider-Mor, A., Shigemitsu, M., Sinclair, D., Somes, C., Studer, A. S., Tesdal, J.-E., Thunell, R., and Terence Yang, J.-Y.: The acceleration of oceanic denitrification during deglacial warming. Nature Geosci. 6, 579 –584 (2013)
Ganeshram, R. S., Pedersen, T. F., Calvert, S. E., and Francois, R.: Reduced nitrogen fixation in the glacial ocean inferred from changes in marine nitrogen and phosphorus inventories. Nature 415, 156 –159 (2002) Ito, T., and Deutsch, C.: A conceptual model for the temporal spectrum of oceanic oxygen variability. Geophys.
Res. Lett. 37, doi:10.1029/2009GL041595 (2010)
Jaccard, S. L., Galbraith, E. D., Frölicher, T. L., and Gruber, N.: Ocean (de)oxygenation across the last degla-ciation: Insights for the future. Oceanography 27/1, 26 –35 (2014)
Jaccard, S. L., and Galbraith, E. D.: Large climate-driven changes of oceanic oxygen concentrations during the last deglaciation. Nature Geosci. 5, 151–156 (2012)
Kienast, M., Kienast, S. S., Calvert, S. E., Eglinton, T. I., Mollenhauer, G., Francois, R., and Mix, A. C.:
Eastern Pacific cooling and Atlantic overturning circulation during the last deglaciation. Nature 443, 846 – 849 (2006)
Kohfeld, K. E., and Chase, Z.: Controls on deglacial changes in biogenic fluxes in the north Pacific Ocean. Quat.
Sci. Rev. 30, 3350 –3363 (2011)
Lenton, T. M., and Watson, A. J.: Redfield revisited: 1. Regulation of nitrate, phosphate and oxygen in the ocean.
Global Biogeochem. Cycles 14, 225 –248 (2000)
Matsumoto, K.: Biology-mediated temperature control on atmospheric pCO2 and ocean biogeochemistry. Geophys.
Res. Lett. 34, doi:10.1029/2007GL031301 (2007)
Meissner, K. J., Galbraith, E. D., and Völker, C.: Denitrification under glacial and interglacial conditions: A physical approach. Paleoceanography 20, doi:10.1029/2004PA001083 (2005)
Robinson, R. S., Mix, A., and Martinez, P.: Southern Ocean control on the extent of denitrification in the southeast Pacific over the last 70 ka. Quat. Sci. Rev. 26, 201–212 (2007)
Deglacial Changes in Ocean (De)Oxygenation
Nova Acta Leopoldina NF 121, Nr. 408, 111–115 (2015) 115
Sigman, D. M., and Haug, G. H.: Biological pump in the past. In: Elderfield, H. (Ed.): The Oceans and Marine Geochemistry. Treatise on Geochemistry. Vol. 6, pp. 491–528. Oxford: Elsevier Pergamon 2003
Shakun, J. D., Clark, P. U., He, F., Marcott, S. A., Mix, A. C., Liu, Z., Otto-Bliesner, B., Schmittner, A., and Bard, E.: Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation.
Nature 484, 49 –55 (2012)
Schmittner, A., Galbraith, E. D., Hostetler, S. W., Pedersen, T. F., and Zhang, R.: Large fluctuations of dissolved oxygen in the Indian and Pacific oceans during Dansgaard-Oeschger oscillations caused by variations of North Atlantic Deep Water subduction. Paleoceanography 22, doi:10.1029/2006PA001384 (2007)
Prof. Samuel L. Jaccard, Ph.D.
University of Bern
Institute of Geological Sciences and
Oeschger Center for Climate Change Research Baltzerstrasse 1+3
CH-3012 Bern Switzerland
Phone: +41 31 6314568
E-Mail: samuel.jaccard@geo.unibe.ch
Prof. Eric D. Galbraith, Ph.D.
McGill University
Department of Earth and Planetary Science 3450 University St.
Montreal, Quebec Canada H3A 2A7 Phone: +1 514 3983677 E-Mail: eric.galbraith@mcgill.ca
Nova Acta Leopoldina NF 121, Nr. 408, 117–121 (2015)
117