Reply to Nicholson's comment on "Consistent calculation of aquatic gross production from oxygen triple isotope measurements" by Kaiser (2011)
1School of Environmental Sciences, University of East Anglia, Norwich, UK
2Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan
Abstract. 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-O2 (abbreviated 17δP and 18δP for 17O/16O and 18O/16O, respectively).
This comment has no merit for the following reasons: (a) the isotopic composition of photosynthetic oxygen cannot be "defined", it can only be measured, modelled or calculated based on other data; (b) the isotopic composition of photosynthetic oxygen was not "defined" in Kaiser (2011a), but derived from published measurements; (c) the published measurements themselves were inconsistent and no single result could be identified as best; (d) since no best value could be identified, a hypothetical base case was constructed in a way that was consistent with previous publications; (e) the values of 17δP=−11.646‰ and 18δP=−22.835‰ assumed for the base case are compatible with the experimental evidence published before the paper of Kaiser (2011a); (f) even if the "biological end-member" was based on a definition, there could be no argument about the "consistency" of this definition – as per its nature, a definition is arbitrary.
The qualification of base case gross production values as being "30 % too high" must therefore also be rejected. Even though recently revised measurements of the relative 17O/16O isotope ratio difference between VSMOW and Air-O2, 17δVSMOW (Barkan and Luz, 2011), do support lower estimates of gross production, our own measurements disagree with these revised 17δVSMOW values. If scaled for differences in 18δVSMOW, they are actually in good agreement with the original data (Barkan and Luz, 2005). Moreover, species-dependent differences in photosynthetic isotope fractionation (Eisenstadt et al., 2010) correspond to an uncertainty of at least 15 % around the central estimate for the inferred gross production.
Nicholson (2011a) also suggests that approximated calculations of gross production should be performed with a triple isotope excess defined as 17Δ#≡ln(1+17δ)−λln(1+18δ), with λ=θR=ln(1+17εR)/ln(1+18εR). However, this only improves the approximation for certain 17δP and 18δP values, for certain net to gross production ratios (f) and for certain ratios of gross production to gross Air-O2 invasion (g). In other cases, the approximated calculation based on 17Δ†≡17δ−κ18δ with κ=γR=17εR/18εR gives better results.