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In this Letter, an error in the legend of Fig. 2a was drawn to our attention by G. de Souza (Eidgenössische Technische Hochschule (ETH), Zürich). The correction of Si* for denitrification was erroneously described as ''We correct for this by using N* (ref. 30), to define a corrected Si* 5 Si(OH) 4 2 NO 3 2 2 d(N*) where d is set to one if N* is smaller than 22 mmol kg 21 and to zero otherwise.'' The actual correction used in producing Fig. 2a was done as follows: ''We correct for this by using N* 5 NO 3 2 2 16PO 4 , to define a corrected Si* 5 Si(OH) 4 2 NO 3 2 1 d(N*) where d is set to one if N* is smaller than 23 mmol kg 21 and to zero otherwise.'' This has been corrected in the HTML version online.

We use a large initial condition suite of simulations (30 runs) with an Earth system model to assess the detectability of biogeochemical impacts of ocean acidification (OA) on the marine alkalinity distribution from decadally repeated hydrographic measurements such as those produced by the Global Ship-Based Hydrographic Investigations Program (GO-SHIP). Detection of these impacts is complicated by alkalinity changes from variability and long-term trends in freshwater and organic matter cycling and ocean circulation. In our ensemble simulation, variability in freshwater cycling generates large changes in alkalinity that obscure the changes of interest and prevent the attribution of observed alkalinity redistribution to OA. These complications from freshwater cycling can be mostly avoided through salinity normalization of alkalinity. With the salinity-normalized alkalinity, modeled OA impacts are broadly detectable in the surface of the subtropical gyres by 2030. Discrepancies between this finding and the finding of an earlier analysis suggest that these estimates are strongly sensitive to the patterns of calcium carbonate export simulated by the model. OA impacts are detectable later in the subpolar and equatorial regions due to slower responses of alkalinity to OA in these regions and greater seasonal equatorial alkalinity variability. OA impacts are detectable later at depth despite lower variability due to smaller rates of change and consistent measurement uncertainty.

Abstract. We examine the processes responsible for the distribution of •513C in a global ocean model. The dominant sources of gradients are biological processes and the temperature effect on isotopic fractionation. However, in a model without biology developed to examine the temperature effect of isotopic fracfionation in isolation, we find an almost uniform/513C distribution. Extremely slow/513C air-sea equilibration does not permit the surface ocean to come into equilibrium with the atmosphere and/513C in the ocean thus becomes well mixed. However biological effects, which are interior to the ocean, are strongly expressed and minimally effected by air-sea exchange. Biological fractionation thus dominates the oceanic/513C distribution. An important feature of the model is an extremely large northward transport of isotopic anomaly. The transfer from the ocean to the Northern Hemisphere atmosphere of 120 Pg C %o is equivalent in magnitude to the signal that would be generated by a net terrestrial biospheric uptake of--5 Pg C yr-1 from the Northern Hemisphere atmosphere, or an = 1-2%o disequilibrium between terrestrial respiration and photosynthesis. Improved ocean model simulations and observational analysis are required to test for the possible existence of such a large oceanic transport of isotopic anomaly.

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