Two Modes of Change in Southern Ocean Productivity Over the Past Million Years

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Science  22 Mar 2013:
Vol. 339, Issue 6126, pp. 1419-1423
DOI: 10.1126/science.1227545

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The variability of atmospheric carbon dioxide concentrations over glacial cycles, which are central aspects of the climate cycle, was documented decades ago. However, it has been difficult to identify which mechanisms have driven CO2 variability. Attention has focused on the Southern Ocean, because of its unique combination of hydrology and biology, although it has not been clear how the different behaviors of its Antarctic and Subantarctic zones might be reconciled with the observations of atmospheric CO2 change. Jaccard et al. (p. 1419) present a record of productivity from the Atlantic Antarctic Zone that extends back in time far enough to cover the last 10 glacial cycles. The findings show how the combination of effects in the Antarctic and Subantarctic zones can explain most of the atmospheric CO2 record over the past million years.


Export of organic carbon from surface waters of the Antarctic Zone of the Southern Ocean decreased during the last ice age, coinciding with declining atmospheric carbon dioxide (CO2) concentrations, signaling reduced exchange of CO2 between the ocean interior and the atmosphere. In contrast, in the Subantarctic Zone, export production increased into ice ages coinciding with rising dust fluxes, thus suggesting iron fertilization of subantarctic phytoplankton. Here, a new high-resolution productivity record from the Antarctic Zone is compiled with parallel subantarctic data over the past million years. Together, they fit the view that the combination of these two modes of Southern Ocean change determines the temporal structure of the glacial-interglacial atmospheric CO2 record, including during the interval of “lukewarm” interglacials between 450 and 800 thousand years ago.

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