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The comment by Nicholson (2011a) questions the "consistency" of the "definition" of the "biological end-member" used by Kaiser (2011a) in the calculation of oxygen gross production. "Biological end-member" refers to the relative oxygen isotope ratio difference between photosynthetic oxygen and Air-O<sub>2</sub> (abbreviated <sup>17</sup>δ<sub>P</sub> and <sup>18</sup>δ<sub>P</sub> for <sup>17</sup>O/<sup>16</sup>O and <sup>18</sup>O/<sup>16</sup>O, respectively). The comment claims that this leads to an overestimate of the discrepancy between previous studies and that the resulting gross production rates are "30% too high". Nicholson recognises the improved accuracy of Kaiser's direct calculation ("dual-delta") method compared to previous approximate approaches based on <sup>17</sup>O excess (<sup>17</sup>Δ) and its simplicity compared to previous iterative calculation methods. Although he correctly points out that differences in the normalised gross production rate (<i>g</i>) are largely due to different input parameters used in Kaiser's "base case" and previous studies, he does not acknowledge Kaiser's observation that iterative and dual-delta calculation methods give exactly the same <i>g</i> for the same input parameters (disregarding kinetic isotope fractionation during air-sea exchange). The comment is based on misunderstandings with respect to the "base case" <sup>17</sup>δ<sub>P</sub> and <sup>18</sup>δ<sub>P</sub> values. Since direct measurements of <sup>17</sup>δ<sub>P</sub> and <sup>18</sup>δ<sub>P</sub>do not exist or have been lost, Kaiser constructed the "base case" in a way that was consistent and compatible with literature data. Nicholson showed that an alternative reconstruction of <sup>17</sup>δ<sub>P</sub> gives <i>g</i> values closer to previous studies. However, unlike Nicholson, we refrain from interpreting either reconstruction as a benchmark for the accuracy of <i>g</i>. A number of publications over the last 12 months have tried to establish which of these two reconstructions is more accurate. Nicholson draws on recently revised measurements of the relative <sup>17</sup>O/<sup>16</sup>O difference between VSMOW and Air-O<sub>2</sub> (<sup>17</sup>δ<sub>VSMOW</sub>; Barkan and Luz, 2011), together with new measurements of photosynthetic isotope fractionation, to support his comment. However, our own measurements disagree with these revised <sup>17</sup>δ<sub>VSMOW</sub> values. If scaled for differences in <sup>18</sup>δ<sub>VSMOW</sub>, they are actually in good agreement with the original data (Barkan and Luz, 2005) and support Kaiser's "base case" <i>g</i> values. The statement that Kaiser's <i>g</i> values are "30% too high" can therefore not be accepted, pending future work to reconcile different <sup>17</sup>δ<sub>VSMOW</sub> measurements. Nicholson also suggests that approximated calculations of gross production should be performed with a triple isotope excess defined as <sup>17</sup>Δ<sup>#</sup>≡ ln (1+<sup>17</sup>δ)–λ ln(1+<sup>18</sup>δ), with λ = θ<sub>R</sub> = ln(1+<sup>17</sup>ϵ<sub>R</sub> ) / ln(1+<sup>18</sup>ϵ<sub>R</sub>). However, this only improves the approximation for certain <sup>17</sup>δ<sub>P</sub> and <sup>18</sup>δ<sub>P</sub> values, for certain net to gross production ratios (<i>f</i>) and for certain ratios of gross production to gross Air-O<sub>2</sub> invasion (<i>g</i>). In other cases, the approximated calculation based on <sup>17</sup>Δ<sup>†</sup> ≡<sup>17</sup>δ – κ <sup>18</sup>δ with κ = γ<sub>R</sub> = <sup>17</sup>ϵ<sub>R</sub>/<sup>18</sup>ϵ<sub>R</sub> (Kaiser, 2011a) gives more accurate results.