Deglacial pulses of deep-ocean silicate into the subtropical North Atlantic Ocean
Records of biogenic opal export in the North Atlantic Ocean show pronounced maxima during each glacial termination over the past 550,000 years, consistent with a strong deglacial reduction in the formation of silicate-poor glacial North Atlantic intermediate water and a consequent increase in upward...
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| Published in: | Nature (London) Vol. 495; no. 7442; pp. 495 - 498 |
|---|---|
| Main Authors: | , , , , , , , , , |
| Format: | Journal Article |
| Language: | English |
| Published: |
London
Nature Publishing Group UK
28-03-2013
Nature Publishing Group |
| Subjects: | |
| Online Access: | Get full text |
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| Summary: | Records of biogenic opal export in the North Atlantic Ocean show pronounced maxima during each glacial termination over the past 550,000 years, consistent with a strong deglacial reduction in the formation of silicate-poor glacial North Atlantic intermediate water and a consequent increase in upward silicate transport.
Deep-ocean opal and carbon dioxide release
The Southern Ocean is thought to have an important role in driving glacial/interglacial changes in atmospheric carbon dioxide concentrations, but the mechanism responsible for carbon dioxide release upon deglaciation remains unclear. Based on a record of biogenic opal export that exhibits opal maxima during each glacial termination of the past 550,000 years, Nele Meckler
et al
. propose that deglacial reduction in glacial North Atlantic intermediate water led to downward mixing of warm, low-density surface water ultimately triggering Antarctic overturning and carbon dioxide release to the atmosphere.
Growing evidence suggests that the low atmospheric CO
2
concentration of the ice ages resulted from enhanced storage of CO
2
in the ocean interior, largely as a result of changes in the Southern Ocean
1
. Early in the most recent deglaciation, a reduction in North Atlantic overturning circulation seems to have driven CO
2
release from the Southern Ocean
2
,
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 water
2
(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 CO
2
(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 CO
2
release
5
,
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 CO
2
to the atmosphere. |
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| ISSN: | 0028-0836 1476-4687 |
| DOI: | 10.1038/nature12006 |