Abstract
Recent changes in the Southern Ocean overturning circulation have been attributed either to natural variability or to anthropogenic forcing. The 14C analyses of mollusk shells from Kerguelen Islands evidence a high decadal variability of the sea surface radiocarbon reservoir ages in the southern South Indian ranging from 832 to 1005 (±30) years between 1910 and 1930. Such high variability is common to the different sectors of the Southern Ocean (southern South Pacific and Atlantic), at inter-annual and decadal timescales between 1901 and 1950. Notably, a decrease of Δ14C of ∼40‰ (increased R of ∼290 years) occurred between 1901 and 1940, followed by an increase of ∼30‰ between 1940 and 1958. Using a simple box model of ocean–atmosphere 14CO2 exchange, this aging is attributed to increased upwelling of cold and 14C-depleted subsurface seawaters due to intensified poleward Southern Westerlies. The sea surface 14C aging is indeed well correlated to a positive phase of the Southern Hemisphere Annular Mode and to a cooling of the sea surface temperatures in the Southern Ocean between 1901 and 1940. These pre-1950 sea surface 14C changes highlight the importance of natural changes in wind intensities and ocean dynamics.
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