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We use a 27 year long time series of repeated transient tracer observations to investigate the evolution of the ventilation timescales and the related content of anthropogenic carbon(Cant) in deep and bottom water in the Weddell Sea. This time series consists of chlorofluorocarbon (CFC) observations from 1984 to 2008 together with first combined CFC and sulphurhexafluoride (SF6) measurements from 2010/2011 along the Prime Meridian in the Antarctic Ocean and across the Weddell Sea. Applying the Transit Time Distribution (TTD) method we find that all deep water masses in the Weddell Sea have been continually growing older and getting less ventilated during the last 27years. The decline of the ventilation rate of Weddell Sea Bottom Water (WSBW) and Weddell Sea Deep Water (WSDW) along the Prime Meridian is in the order of 15–21%; the Warm Deep Water (WDW) ventilation rate declined much faster by 33%. About 88–94% of the age increase in WSBW near its source regions (1.8–2.4 years per year) is explained by the age increase of WDW (4.5 years per year). As a consequence of the aging, the Cant increase in the deep and bottom water formed in the Weddell Sea slowed down by 14–21% over the period of observations.

The World Ocean takes up a large portion of the anthropogenic CO2 (Cant) emitted into the atmosphere. Determining the resulting increase in dissolved inorganic carbon (CT, expressed in µmol kg-1) is challenging, particularly in the subsurface and deep Southern Ocean where the time rate of change of CT (in µmol kg−1 decade−1) is often expected to be low. We present a determination of this time trend of CT in a dataset of measurements that spans 35 years, comprising 10 cruises in the 1973-2008 period along the 0º-meridian in the Weddell Gyre. The inclusion of many cruises aims to generate results that are more robust than may be obtained by taking the difference between only one pair of cruises, each of which may suffer from errors in accuracy. To further improve consistency between cruises, data were adjusted in order to obtain time-invariant values of CT (and other relevant parameters) over the 35 years in the least ventilated local water body, this comprising the deeper Warm Deep Water (WDW) and upper Weddell Sea Deep Water (WSDW). It is assumed that this normalization procedure will allow trends in CT in the more intensely ventilated water masses to be more clearly observed. Time trends were determined directly in measurements of CT, and alternatively in back-calculated values of preformed CT (CT0; i.e., the CT of the water at the time that it lost contact with the atmosphere). The determined time trends may be attributed to a combination of natural variability (in hydrography or biogeochemistry) and increased uptake of anthropogenic CO2 from the atmosphere. In order to separate these natural and anthropogenic components, an analysis of the residuals of a multivariate linear regression (MLR), involving the complete time series of all 10 cruises, was additionally performed. This approach is referred to as the Time Series Residuals (TSR) approach. Using the direct method, the time trends of CT in the WSDW are quite small and non-significant at +0.176±0.321 µmol kg−1