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Deglacial pulses of deep-ocean silicate into the subtropical North Atlantic Ocean

A Corrigendum to this article was published on 11 December 2013


Growing evidence suggests that the low atmospheric CO2 concentration of the ice ages resulted from enhanced storage of CO2 in the ocean interior, largely as a result of changes in the Southern Ocean1. Early in the most recent deglaciation, a reduction in North Atlantic overturning circulation seems to have driven CO2 release from the Southern Ocean2,3,4,5, but the mechanism connecting the North Atlantic and the Southern Ocean remains unclear. Biogenic opal export in the low-latitude ocean relies on silicate from the underlying thermocline, the concentration of which is affected by the circulation of the ocean interior. Here we report a record of biogenic opal export from a coastal upwelling system off the coast of northwest Africa that shows pronounced opal maxima during each glacial termination over the past 550,000 years. These opal peaks are consistent with a strong deglacial reduction in the formation of silicate-poor glacial North Atlantic intermediate water2 (GNAIW). The loss of GNAIW allowed mixing with underlying silicate-rich deep water to increase the silicate supply to the surface ocean. An increase in westerly-wind-driven upwelling in the Southern Ocean in response to the North Atlantic change has been proposed to drive the deglacial rise in atmospheric CO2 (refs 3, 4). However, such a circulation change would have accelerated the formation of Antarctic intermediate water and sub-Antarctic mode water, which today have as little silicate as North Atlantic Deep Water and would have thus maintained low silicate concentrations in the Atlantic thermocline. The deglacial opal maxima reported here suggest an alternative mechanism for the deglacial CO2 release5,6. Just as the reduction in GNAIW led to upward silicate transport, it should also have allowed the downward mixing of warm, low-density surface water to reach into the deep ocean. The resulting decrease in the density of the deep Atlantic relative to the Southern Ocean surface promoted Antarctic overturning, which released CO2 to the atmosphere.

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Figure 1: Silicate concentration and locations of study sites.
Figure 2: Opal record from ODP Site 658 compared with atmospheric CO2 concentration and productivity and carbonate preservation in the Southern Ocean.
Figure 3: Comparison of opal records for the last glacial termination.
Figure 4: Schematic north–south section of the Atlantic Ocean at different stages of the most recent glacial termination.

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This research used samples provided by the ODP, which is sponsored by the US NSF and participating countries under the management of the Joint Oceanographic Institutions. XRF data were acquired at the XRF Core Scanner Lab at MARUM – Center for Marine Environmental Sciences, University of Bremen, with support from the DFG-Leibniz Center for Surface Process and Climate Studies at the University of Potsdam. Further support was provided by the US NSF through grant OCE-1060947 to D.M.S. and by NSERC and CFCAS to R.F. We thank V. Lukies for XRF scanning technical support and T. Westerhold for help in deriving the new composite depth. M. Soon and T. Kane are acknowledged for opal and excess 230Th measurements, respectively, in core MD03-2705.

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Authors and Affiliations



A.N.M. and G.H.H. designed the study; A.N.M. collected the XRF data at ODP Site 658; U.R. facilitated and oversaw the XRF scanning; R.F. contributed the 230Th-normalized opal flux data; R.T. provided data and background knowledge on Site 658; A.N.M., S.L.J. and A.M.G. undertook the comparisons with Southern Ocean data; A.N.M., D.M.S. and G.H.H. wrote the first draft of the manuscript; and K.A.G. and L.C.P. provided input regarding tropical Atlantic observations. All authors contributed to the interpretation and the preparation of the final manuscript.

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Correspondence to A. N. Meckler.

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The authors declare no competing financial interests.

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Supplementary Information

This file contains Supplementary Figures 1-3, which includes a correction of the ln (Si/Al)-based opal record from ODP Site 658 for changes in grain size using ln (Zr/Al) data, a comparison of the ln (Si/Al) record to other data from Site 658 including close-ups of the last five glacial terminations, and information on how the new stratigraphy for Site 658 was derived. This file also contains Supplementary Discussions 1 and 2, which provide information about (1) the potential effects of preservation on opal concentration and (2) previously published evidence for intermediate water mass structure, and Supplementary References. (PDF 1055 kb)

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Meckler, A., Sigman, D., Gibson, K. et al. Deglacial pulses of deep-ocean silicate into the subtropical North Atlantic Ocean. Nature 495, 495–498 (2013).

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