Reply to Nicholson's comment on "Consistent calculation ofaquatic gross production from oxygen triple isotope measurements" by Kaiser (2011)

[摘要] 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. "Biologicalend-member" refers to the relative oxygen isotope ratio difference betweenphotosynthetic oxygen and Air-O₂ (abbreviated ¹⁷δ_Pand ¹⁸δ_P for ¹⁷O/¹⁶O and ¹⁸O/¹⁶O,respectively). The comment claims that this leads to an overestimate of thediscrepancy between previous studies and that the resulting gross productionrates are "30% too high".Nicholson recognises the improved accuracy of Kaiser's direct calculation("dual-delta") method compared to previous approximate approaches based on¹⁷O excess (¹⁷Δ) and its simplicity compared to previous iterativecalculation methods. Although he correctly points out that differences inthe normalised gross production rate (g) are largely due to different inputparameters used in Kaiser's "base case" and previous studies, he does notacknowledge Kaiser's observation that iterative and dual-delta calculationmethods give exactly the same g for the same input parameters (disregardingkinetic isotope fractionation during air-sea exchange). The comment is basedon misunderstandings with respect to the "base case" ¹⁷δ_Pand ¹⁸δ_P values. Since direct measurements of¹⁷δ_P and ¹⁸δ_Pdo not exist or have beenlost, Kaiser constructed the "base case" in a way that was consistent andcompatible with literature data. Nicholson showed that an alternativereconstruction of ¹⁷δ_P gives g values closer to previousstudies. However, unlike Nicholson, we refrain from interpretingeither reconstruction as a benchmark for the accuracy of g.A number of publications over the last 12 months have tried to establishwhich of these two reconstructions is more accurate. Nicholson draws onrecently revised measurements of the relative ¹⁷O/¹⁶O differencebetween VSMOW and Air-O₂ (¹⁷δ_VSMOW; Barkan and Luz,2011), together with new measurements of photosynthetic isotopefractionation, to support his comment. However, our own measurementsdisagree with these revised ¹⁷δ_VSMOW values. If scaled fordifferences in ¹⁸δ_VSMOW, they are actually in goodagreement with the original data (Barkan and Luz, 2005) and supportKaiser's "base case" g values. The statement that Kaiser's g values are "30% too high" can therefore not be accepted, pending future work toreconcile different ¹⁷δ_VSMOW measurements.Nicholson also suggests that approximated calculations of gross productionshould be performed with a triple isotope excess defined as ¹⁷Δ^#≡ln (1+¹⁷δ)–λln(1+¹⁸δ), with λ=θ_R=ln(1+¹⁷ϵ_R ) / ln(1+¹⁸ϵ_R).However, this only improves the approximation for certain ¹⁷δ_P and ¹⁸δ_P values, for certain net to grossproduction ratios (f) and for certain ratios of gross production to grossAir-O₂ invasion (g). In other cases, the approximated calculation basedon ¹⁷Δ^†≡¹⁷δ – κ ¹⁸δ with κ=γ_R=¹⁷ϵ_R/¹⁸ϵ_R (Kaiser, 2011a)gives more accurate results.

[学科分类] 地球化学与岩石