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| Titel |
Sequential dissipation of a poorly-ventilated water mass upon the last glacial termination - implications for the marine and atmospheric carbon cycles. |
| VerfasserIn |
Samuel L. Jaccard, Eric D. Galbraith, Robert F. Anderson, Roger François |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250039358
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| Zusammenfassung |
| It is believed that no single mechanism can account for the full amplitude of past CO2
variability. But although multiple synergistic processes may be involved, intensified isolation
of deep-water masses from the atmosphere has emerged as a central mechanism for low
glacial CO2. This could have resulted from increased oceanic density stratification,
increased sea ice cover, or a decrease wind-driven vertical mixing. Recent evidence is
consistent with the existence of a poorly ventilated, carbon-rich water mass in a large
portion of the glacial Pacific and Southern Oceans. However, the mechanisms by
which this water mass dissipated upon glacial terminations remains a subject of
debate.
Here we present sedimentary redox-sensitive trace metal records from subarctic Pacific
sites ODP Site 882 & 887 and South Atlantic sites TN057-13/14 to reconstruct changes in
deep ocean oxygenation – and, by inference, respired carbon storage - across the last glacial
termination. These observations are complemented by 230Th-normalized opal measurements,
which we apply as proxies for past organic carbon sedimentation in these diatom-dominated
regions. In combination, these allow us to separate the time-varying influences of deep-water
oxygen concentration and sedimentary organic carbon respiration on the redox state of the
sediment.
Our results suggest that the abyssal Pacific and Southern oceans were depleted in oxygen
during the last glacial maximum, though they were not anoxic. The large and abrupt increase
in sedimentary opal accumulation observed in the Southern Ocean at approx. 18 kyr is
accompanied by a decrease in authigenic uranium concentrations suggesting better
oxygenation at the depth of the core site. Enhanced mixing within the Southern Ocean,
driven by stronger winds and/or changes in the density profile of the water column,
would have invigorated circulation at depth., The increase in the rate of nutrient
supply to the surface would have enhanced the strength of the Southern Ocean High
Nutrient Low Chlorophyll (HNLC) region, and increased the leakage of nutrients into
intermediate and mode waters of the southern hemisphere. Simultaneously, the
decrease in nutrient-poor NADW to the deep sea, caused by the freshwater forcing
associated with Heinrich Event 1, allowed nutrient-rich AABW to dominate the deep
ocean. Both of these mechanisms would have increased global preformed nutrient
concentrations, previously shown to contribute to higher atmospheric pCO2, and could have
explained the large-scale transfer of carbon from the deep ocean to the atmosphere
between 18 and 15 ka. In the subarctic Pacific, the arrival of well-oxygenated abyssal
waters appears to have taken place at the onset of the Bolling/Allerod, 14.7 ka,
accompanying the reinvigoration of North Atlantic Deep Water, which increased the overall
rate of deep ocean ventilation, even as it contributed to an overall decrease in the
preformed nutrient load of the global ocean. The fact that atmospheric pCO2 stopped
increasing at this time is consistent with this interpretation. Our results suggest that this
stepwise reinvigoration of deep water circulation, resulting from the buffeting of
ocean density structure by large inputs of freshwater, was responsible for driving
carbon out of the abyssal ocean during the melting of the large continental ice sheets. |
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