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

Abstract

Abstract. The comment by Nicholson (2011a) questions the "consistency" of the "definition" of the "biological endmember" 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 2 (abbreviated 17 δ P and 18 δ P for 17 O / 16 O and 18 O / 16 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 17 O excess ( 17 ∆) and its simplicity compared to previous iterative calculation methods. Although he correctly points out that differences in the normalised gross production rate (g) 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 g 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" 17 δ P and 18 δ P values. Since direct measurements of 17 δ P and 18 δ P 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 17 δ P gives g values closer to previous studies. However, unlike Nicholson, we refrain from interpreting either reconstruction as a benchmark for the accuracy of g. Barkan and Luz, 2011), together with new measurements of photosynthetic isotope fractionation, to support his comment. However, our own measurements disagree with these revised 17 δ VSMOW values. If scaled for differences in 18 δ VSMOW , they are actually in good agreement with the original data and support Kaiser's "base case" g values. The statement that Kaiser's g values are "30 % too high" can therefore not be accepted, pending future work to reconcile different 17 δ VSMOW measurements.Nicholson also suggests that approximated calculations of gross production should be performed with a triple isotope excess defined as 17 ∆ # ≡ ln(1 + 17 δ)-λ ln(1+ 18 δ), with λ = θ R = ln(1+ 17 ε R ) / ln(1+ 18 ε R ). However, this only improves the approximation for certain 17 δ P and 18 δ P values, for certain net to gross production ratios (f ) and for certain ratios of gross production to gross Air-O 2 invasion (g). In other cases, the approximated calculation based on 17 ∆ † ≡ 17 δ-κ 18 δ with κ = γ R = 17 ε R / 18 ε R (Kaiser, 2011a) gives more accurate results.

Ecosystem metabolism in salt marsh tidal creeks and ponds : applying triple oxygen isotopes and other gas tracers to novel environments

“…Xsat the saturated equilibrium ratio for each isotope (Table S1; this and all other supplementary tables are available electronically as described in the data availability statement). Alternate choices for i XP and i Xsat and λ may be appropriate under some circumstances , Kaiser and Abe 2012. The above equation is identical to Equation S8 in the Supplemental Information of , and neglects fractionation effects during gas exchange and bubble fluxes .…”

Biological production in the Bellingshausen Sea from oxygen-to-argon ratios and oxygen triple isotopes

“…G is calculated from oxygen triple isotope measurements. From the two G values (G 1 to G 2 given in Table 2) calculated using two different pairs of 17 δ P and 18 δ P (Kaiser and Abe, 2012), those based on Kaiser and Abe (2012) are about 40 % higher than the ones based on Barkan and Luz (2011) (Fig. 9).…”

“…A current disagreement in the literature regarding the isotope ratio difference between seawater and air O 2 shows values between 17 δ w = −11.888 ‰/ 18 δ w = −23.324 ‰ (Barkan and Luz, 2011) and 17 δ w = -12.107 ‰/ 18 δ w = −23.647 ‰ (Kaiser and Abe, 2012). Thus, we calculated two G values (G 1 and G 2 ) based on the two different sets of oxygen isotopic signatures resulting from photosynthetic activity ( 17 δ P and 18 δ P values) taken from Table 3 rows 6 m ( 17 δ P = −9.761 ‰/ 18 δ P = −19.301 ‰) and 7 m ( 17 δ P = −9.980 ‰/ 18 δ P = −19.625 ‰) in Kaiser and Abe (2012).…”

“…Temperature dependent δ eq values are calculated as 18 δ eq = e −0.00072951+0.42696T /K −1 (Benson et al, 1979) and 17 δ eq = (1 + 18 δ eq ) 0.518 e (0.6θ/ • C + 1.8) ppm − 1 ). The isotopic composition of photosynthetic O 2 is based on that of Vienna Standard Mean Ocean Water (VSMOW) (Barkan and Luz, 2011;Kaiser and Abe, 2012), the relative 17 O/ 16 O difference between seawater and VS-MOW of −5 ppm and measurements of the mean isotopic fractionation during photosynthesis by marine species of 17 ε P = 1.773 ‰ and 18 ε P = 3.389 ‰ (Eisenstadt et al, 2010).…”

Biological production, export efficiency, and phytoplankton communities across 8000 km of the South Atlantic

“…(2) where i X P is the photosynthetic production ratio and i X sat the saturated equilibrium ratio for each isotope (Table S2). Alternate choices for i X P and i X sat and λ may be appropriate under some circumstances [Kaiser, 2011;Nicholson, 2011;Kaiser and Abe, 2012]. The above equation is identical to equation (S8) in the supporting information to Prokopenko et al [2011] and neglects fractionation effects during gas exchange and bubble fluxes [Kaiser, 2011].…”