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| Direct observation of increasing CO2 in the Weddell Gyre along the Prime Meridian during 1973-2008 van Heuven, S.M.A.C.; Hoppema, M.; Huhn, O.; Slagter, H.A.; de Baar, H.J.W. (2011). Direct observation of increasing CO2 in the Weddell Gyre along the Prime Meridian during 1973-2008. Deep-Sea Res., Part 2, Top. Stud. Oceanogr. 58(25-26): 2613-2635. dx.doi.org/10.1016/j.dsr2.2011.08.007
In: Deep-Sea Research, Part II. Topical Studies in Oceanography. Pergamon: Oxford. ISSN 0967-0645; e-ISSN 1879-0100, meer
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Time trends were determined directly in measurements of C-T, and alternatively in back-calculated values of preformed C-T (C-T(0); i.e., the C-T 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 C-T in the WSDW are quite small and non-significant at +0.176 +/- 0.321 mu mol kg(-1) decade(-1). On the other hand, the measured concentration of C-T 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 mu mol kg(-1) decade(-1). The spatial distribution of these determined increases of C-T 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 C-T. Time trends in back-calculated values of C-T(0) appear to be obscured due to uncertainties in the measurements of O-2. Finally, the shallow waters ( < 200 m 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 C-T in the WSBW. The residuals of the TSR method do exhibit a time trend (considered representative of the time trend of C-ant) of +0.445 +/- 0.405 mu mol kg(-1) decade(-1) (i.e., only 38% of the direct observed time trend in C-T) thus only partly supporting the attribution of the measured time trend of C-T 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 C-T. After all, the time trend in measured C-T is most straightforwardly ascribed entirely to uptake of C-ant, and assuming an exponentially growing history of storage, the observed increase of C-T in the WSBW suggests that a total amount of C-ant of 6 +/- 3 mu mol kg(-1) has accumulated in this water mass between the onset of the Industrial Revolution and 1995. Extrapolating the determined time trend, the rate of storage of C-an |
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