radiocarbon constraints on southern ocean...

Nova Acta Leopoldina NF 121, Nr. 408, 49 –54 (2015)

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Radiocarbon Constraints on Southern Ocean Circulation

Andrea Burke,1 Andrew L. Stewart,2 Jess F. Adkins,3 Raffaele Ferrari,4

Malte F. Jansen,5 Andrew F. Thompson,3 and Laura F. Robinson6

With 2 Figures

The Southern Ocean is thought to play a fundamental role in driving glacial-interglacial car-bon cycle changes. The reason for this is two-fold: the ice core records of atmospheric CO2 and Antarctic temperature are strongly correlated, and the Southern Ocean is a region that connects the deep ocean to the surface ocean through deep water formation and sloping iso-pycnals in the Antarctic Circumpolar Current (ACC). Since the deep ocean is the major res-ervoir of carbon at the earth surface, it is likely that the change in atmospheric CO2 over the deglaciation is linked to processes involving the deep ocean.

Ocean radiocarbon reconstructions provide an important record of changes in ocean cir-culation and CO2 exchange, and are thus keys for examining the processes that led to the rapid climate and CO2 shifts that characterize the last deglaciation. In this talk I will present a compilation of new and published radiocarbon data from the Southern Ocean, interpreted with help from a two-dimensional dynamical model, to examine the circulation history in this important region. The radiocarbon records are reconstructed from benthic foraminifera from sediment cores in the high latitude South Atlantic (Barker et al. 2010, Skinner et al. 2010) and deep-sea corals from the Drake Passage (Burke and Robinson 2012). In contrast to radiocarbon depth profiles in the modern Southern Ocean, these data show a significant mid-depth radiocarbon minimum during the Last Glacial Maximum (LGM) (Fig. 1). The large vertical radiocarbon gradients that characterized the LGM then disappeared during the early deglaciation, resulting in a much more homogenous radiocarbon depth profile. To first order, these results can be interpreted as increased isolation of waters at mid-depths at the LGM, and then ventilation and mixing during the early deglaciation. But what were the mechanisms that led to this glacial mid-depth radiocarbon minimum?

There are several hypotheses that invoke changes in the Southern Ocean to explain gla-cial-interglacial carbon cycling, and these can be roughly divided into physical and biolog-ical processes (for an extensive discussion of these see Fischer et al. [2009] and references

1 Department of Earth and Environmental Sciences, Irvine Building, University of St Andrews, St Andrews, KY16 8YG, United Kingdom; [email protected].

2 Department of Atmospheric and Oceanic Sciences, UCLA, Los Angeles, CA, USA.3 Division of Geological and Planetary Sciences, Caltech, Pasadena, CA, USA.4 Department of Earth, Atmospheric and Planetary Sciences, MIT, Cambridge, MA, USA.5 Department of the Geophysical Sciences, University of Chicago, Chicago, IL, USA.6 School of Earth Sciences, University of Bristol, Bristol, United Kingdom.

A. Burke, A. L. Stewart, J. F. Adkins, R. Ferrari, M. F. Jansen, A. F. Thompson, and L. F. Robinson

50 Nova Acta Leopoldina NF 121, Nr. 408, 49 –54 (2015)

Fig. 1 From Burke et al., submitted. (top) Radiocarbon data plotted as atmosphere normalized Δ14C from the At-lantic sector of the Southern Ocean. Modern water column data are plotted as lines and come from GLODAP (Key et al. 2004). Data reconstructed from the LGM are plotted as stars (Barker et al. 2010, Skinner et al. 2010, Burke and Robinson 2012) (bottom). Map showing location of sediment cores or corals (stars) and water column stations (dots).

therein). In this talk we will focus on the physical processes, namely wind and sea ice. The wind hypotheses (e.g. Toggweiler et al. 2006, Anderson et al. 2009) invoke a change in the strength or position of the westerly winds over the Southern Ocean. These hypotheses suggest


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that weaker or equatorward-shifted winds during the Last Glacial Maximum would reduce Southern Ocean upwelling, thus limiting the amount of carbon that is brought to the surface ocean from the deep. Over the deglaciation, the winds either strengthened or shifted poleward with warming Antarctic temperatures, which would increase Southern Ocean upwelling and would release CO2 to the atmosphere. The sea ice hypotheses (e.g. Stephens and Keeling 2000) suggests that a greater extent of sea ice in the glacial period acted to ‘cap’ the Southern Ocean and reduced air-sea gas exchange, which would result in lower glacial atmospheric CO2 concentrations. Over the deglaciation, as Antarctic temperatures warmed and there was a reduction in sea ice extent, the Southern Ocean became ‘uncapped’, providing a source of CO2 to the atmosphere.

Last year we put forth a hypothesis that highlighted another potential role for sea ice in glacial-interglacial climate change: it is intimately linked to the geometry of the overturning circulation (Ferrari et al. 2014) (Fig. 2). The quasi-permanent sea ice edge in the South-ern Ocean is the boundary between positive and negative buoyancy forcing in the surface ocean. North of the sea ice edge, waters become less dense and flow equatorward forming the upper overturning branch, and south of the sea ice edge waters become more dense and flow poleward forming the lower overturning branch. Today the overturning circulation forms one continuous figure-eight loop. North Atlantic Deep Water flows southward in the Atlantic and upwells south of the sea ice edge, becomes more dense, and returns north in the Atlantic and Pacific basins in the lower overturning branch. Diapycnal diffusion in these ocean basins transforms water from the lower overturning branch to a density (corresponding to a depth of ~2 km) that outcrops in the ACC north of the sea ice boundary, thus closing the overturning circulation loop in the upper branch (Fig. 2). We hypothesized that the glacial expansion of sea ice (Gersonde et al. 2005) would result in a shoaling of the boundary between the upper and lower overturning branches in basins north of the ACC, away from enhanced diapycnal mixing associated with mid ocean ridges. A shoaling of the boundary above 2 km water depth would mean that the overturning circulation could not be closed by diapycnal diffusion in a single figure eight loop, and thus there would have to be two separate overturning cells, and NADW would be confined to the upper cell (Fig. 2). This geometry is consistent with δ18O and δ13C data from the Atlantic (e.g. Sarnthein et al. 2000, Curry and Oppo 2005, Lund et al. 2011) which show a shoaling of the boundary between northern- and southern-sourced water during the LGM. The restriction of northern-sourced water to the upper overturning branch would result in a deep southern-sourced water mass that was more isolated from the atmosphere com-pared to today, potentially providing a means to store more carbon in the deep.

To test our hypothesis and determine the effect of this process on glacial radiocarbon distributions, we created an idealized, 2D, residual-mean dynamical model of the global over-turning circulation with a decaying tracer that is advected by the circulation to simulate radio-carbon for a comparison to published radiocarbon data (Burke et al. submitted). The model is simple and efficient, but it includes the physical and dynamical elements necessary to test our hypothesized changes in the circulation. We find that an expansion of sea ice under glacial conditions leads to a shoaled boundary between the upper and lower branches of the over-turning circulation as we hypothesized, as well as a minimum in radiocarbon at mid-depths, as seen in the radiocarbon data. Thus sea ice provides a potential means by which to isolate the deep ocean and store excess carbon, without relying solely on a large reduction in air-sea gas exchange. We note that although the westerly winds hypothesis is a popular paradigm, eddy resolving models of the Southern Ocean do not show a strong sensitivity of overturning



Fig. 2 (Upper) Schematic of the modern overturning circulation. The coloured bands are a zonally-averaged view of the major water masses, whereby green is North Atlantic Deep Water (NADW), blue is Antarctic Bottom Water (AABW), red is Pacific/Indian Deep Water (PDW/IDW), and orange is Antarctic Intermediate Water (AAIW). The dashed vertical lines represent diapycnal mixing of AABW into NADW and PDW/IDW. The dashed black line rep-resents the boundary between the upper and lower overturning branches. The jagged gray line represents the top of major bathymetric features showing the depth beneath which mixing is enhanced. (Lower) Schematic of the LGM overturning circulation. The extent of sea ice is further equatorward compared to the modern. Mixing-driven up-welling of abyssal waters is confined below 2 km and it cannot lift waters high enough to upwell north of the ice line. As a result the abyssal overturning circulation closes on itself, forming two distinct cells. From Ferrari et al. 2014.

circulation to changes in winds (Munday et al. 2013), a process known as eddy saturation. Thus winds alone are not likely to cause a change the overturning circulation on glacial-in-terglacial timescales; a change in the surface buoyancy forcing is needed. Furthermore, our model shows that the mid-depth radiocarbon minimum is more sensitive to sea ice extent and circulation geometry than changes in wind strength.

Following our thought process forward in time from the LGM, we suggest that a con-sequence of a reduction in the sea ice during deglaciation would be to deepen the boundary between overturning branches, thus ending the isolation of the deep cell and transitioning


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back into a circulation geometry of a figure-eight cell. This circulation restructuring due to the reduced extent of sea ice and coupled with increased gas exchange in the surface Southern Ocean, could potentially drive the changes in atmospheric CO2

that have been reconstructed over this time period. A final point is that the deglaciation is not characterized by a monotonic increase in temperature and CO2. Notably, the deglacial CO2 rise is punctuated by a pause during the Antarctic Cold Reversal (ACR). We capture an intriguing signal in our shallow deep-sea coral radiocarbon records during this time period: a sharp decrease in radiocarbon that lasts ~ 1,000 years. We suggest that this is a result of either an increase in surface strati-fication or a northward shift of fronts, potentially due to a readvance of sea ice, and that this may help to explain the pause in the atmospheric deglacial CO2 rise.

References

Anderson, R. F., Ali, S., Bradtmiller, L. I., Nielsen, S. H. H., Fleisher, M. Q., Anderson, B. E., and Burckle, L. H.: Wind-driven upwelling in the Southern Ocean and the deglacial rise in atmospheric CO2. Science 323/5920, 1443 –1448; doi:10.1126/science.1167441 (2009)

Barker, S., Knorr, G., Vautravers, M. J., Diz, P., and Skinner, L. C.: Extreme deepening of the Atlantic over-turning circulation during deglaciation. Nature Geosci. 3/8, 567–571; doi:10.1038/ngeo921 (2010)

Burke, A., and Robinson, L. F.: The Southern Ocean’s role in carbon exchange during the last deglaciation. Science 335/6068, 557–561; doi: 10.1126/science.1208163 (2012)

Curry, W. B., and Oppo, D. W.: Glacial water mass geometry and the distribution of δ13C of ΣCO2 in the western Atlantic Ocean. Paleoceanography 20/1; doi: 10.1029/2004PA001021 (2005)

Ferrari, R., Jansen, M. F., Adkins, J. F., Burke, A., Stewart, A. L., and Thompson, A. F.: Antarctic sea ice control on ocean circulation in present and glacial climates. Proc. Natl. Acad. Sci. USA 111/24, 8753 – 8758; doi: 10.1073/pnas.1323922111 (2014)

Fischer, H., Schmitt, J., Lüthi, D., Stocker, T. F., Tschumi, T., Parekh, P., Joos, F., Köhler, P., Völker, C., Gersonde, R., Barbante, C., Le Floch, M., Raynaud, D., and Wolff, E.: The role of Southern Ocean processes in orbital and millennial CO2 variations – A synthesis. Quat. Sci. Rev. 1/13; doi: 10.1016/j.quasci-rev.2009.06.007 (2009)

Gersonde, R., Crosta, X., Abelmann, A., and Armand, L.: Sea-surface temperature and sea ice distribution of the Southern Ocean at the EPILOG Last Glacial Maximum-a circum-Antarctic view based on siliceous microfossil records. Quat. Sci. Rev. 24/7, 9, 869 – 896; doi: 10.1016/j.quascirev.2004.07.015 (2005)

Key, R. M., Kozyr, A., Sabine, C. L., Lee, K., Wanninkhof, R., Bullister, J. L., Feely, R. A., Millero, F. J., Mordy, C., and Peng, T. H.: A global ocean carbon climatology: Results from Global Data Analysis Project (GLODAP). Global Biogeochem. Cycles 18/4; doi: 10.1029/2004GB002247 (2004)

Lund, D. C., Adkins, J. F., and Ferrari, R.: Abyssal Atlantic circulation during the Last Glacial Maximum: Con-straining the ratio between transport and vertical mixing. Paleoceanography 26/1; doi: 10.1029/2010PA001938 (2011)

Munday, D. R., Johnson, H. L., and Marshall, D. P.: Eddy saturation of equilibrated circumpolar currents. J. Phys. Oceanogr. 43/3, 507–532 (2013)

Sarnthein, M., Stattegger, K., Dreger, D., Erlenkeuser, H., Grootes, P., Haupt, B., Jung, S., Kiefer, T., Kuhnt, W., and Pflaumann, U.: Fundamental modes and abrupt changes in North Atlantic circulation and climate over the last 60 ky – Concepts, reconstruction and numerical modeling. In: Schafer, P., Ritzrau, W., Schluter, M., and Thiede, J. (Eds.): The Northern North Atlantic: A Changing Environment. Vol. 16; pp. 365 – 410. Berlin: Springer 2000

Skinner, L. C., Fallon, S., Waelbroeck, C., Michel, E., and Barker, S.: Ventilation of the deep Southern Ocean and deglacial CO2 rise. Science 328/5982, 1147–1151; doi: 10.1126/science.1183627 (2010)



Stephens, B. B., and Keeling, R. F.: The influence of Antarctic sea ice on glacial-interglacial CO2 variations. Na-ture 404/6774, 171–174 (2000)

Toggweiler, J. R., Russell, J. L., and Carson, S. R.: Midlatitude westerlies, atmospheric CO2, and climate change during the ice ages. Paleoceanography 21/2, doi: 10.1029/2005PA001154 (2006)

Dr. Andrea Burke Department of Earth and Environmental Sciences Irvine Building, University of St Andrews St Andrews, KY16 8YG United Kingdom Phone: +44 1334 463910 Fax: +44 1334 463949 E-Mail: [email protected]

radiocarbon constraints on southern ocean...

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