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Direct observation of increasing CO2 in the Weddell Gyre along the Prime Meridian during 1973-2008

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Van Heuven, S. M. A. C. , Hoppema, M. , Huhn, O. , Slagter, H. A. and De Baar, H. J. W. (2011): Direct observation of increasing CO2 in the Weddell Gyre along the Prime Meridian during 1973-2008 , Deep-Sea Research II, 58 , pp. 2613-2635 . doi: 10.1016/j.dsr2.2011.08.007
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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 decade−1 . On the other hand, the measured concentration of CT in the Weddell Sea Bottom Water (WSBW) is shown to rise slowly but significantly over the period from 1973 to 2008 at a rate of +1.151±0.563 µmol kg−1 decade−1. The spatial distribution of these determined increases of CT in the deep Weddell Gyre closely resembles that of the increase of the anthropogenic tracer CFC-12, this strong similarity supporting a mostly anthropogenic cause for the increasing trend of CT. Time trends in back-calculated values of CT0 appear to be obscured due to uncertainties in the measurements of O2. Finally, the shallow waters (<200m depth) do not allow for interpretation since these are strongly affected by seasonality. Due to the small time trend signal in the WSBW, the TSR approach does not allow for unambiguous attribution of the observed trend in CT in the WSBW. The residuals of the TSR method do exhibit a time trend (considered representative of the time trend of Cant) of +0.445±0.405 µmol kg−1 decade−1 (i.e., only 38% of the direct observed time trend in CT¬) thus only partly supporting the attribution of the measured time trend of CT to uptake of anthropogenic CO2. Another TSR-derived result suggests that there is no significant time trend of biogeochemical changes. A time trend in hydrography of mixing between two deep water masses does exist, as evidenced by a slight positive time trend in the temperature of the WSBW, but is inadequate to explain the time trend of CT. After all, the time trend in measured CT is most straightforwardly ascribed entirely to uptake of Cant, and assuming an exponentially growing history of storage, the observed increase of CT in the WSBW suggests that a total amount of Cant of 6±3 µmol kg−1 has accumulated in this water mass between the onset of the industrial revolution and 1995. Extrapolating this result, the rate of storage of Cant in the deep Weddell Gyre (>3000m, west of 20ºE) is calculated to be about 12±6 TgC year−1 over the 1973-2008 period. This rate of storage is likely somewhat lower than the rate of export of Cant from the surface water into the deep Weddell Gyre, this due to continuous loss of Cant with WSDW flowing out of the Weddell Gyre into the deep basins of the other oceans as AABW.

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