Technical note: Consistent calculation of aquatic gross production from oxygen triple isotope measurements

Abstract

Abstract. Oxygen triple isotope measurements can be used to calculate aquatic gross oxygen production rates. Past studies have emphasised the appropriate definition of the 17 O excess and often used an approximation to derive production rates from the 17 O excess. Here, I show that the calculation can be phrased more consistently and without any approximations using the relative 17 O/ 16 O and 18 O/ 16 O isotope ratio differences (delta values) directly. I call this the "dual delta method". The 17 O excess is merely a mathematical construct and the derived production rate is independent of its definition, provided all calculations are performed with a consistent definition. I focus on the mixed layer, but also show how time series of triple isotope measurements below the mixed layer can be used to derive gross production.In the calculation of mixed layer productivity, I explicitly include isotopic fractionation during gas invasion and evasion, which requires the oxygen supersaturation s to be measured as well. I also suggest how bubble injection could be considered in the same mathematical framework. I distinguish between concentration steady state and isotopic steady state and show that only the latter needs to be assumed in the calculation. It is even possible to derive an estimate of the net production rate in the mixed layer that is independent of the assumption of concentration steady state.I review measurements of the parameters required for the calculation of gross production rates and show how their systematic uncertainties as well as the use of different published calculation methods can cause large variations in the production rates for the same underlying isotope ratios. In particular, the 17 O excess of dissolved O 2 in equilibrium with atmospheric O 2 and the 17 O excess of photosynthetic O 2 need to be re-measured. Because of these uncertainties, all calculaCorrespondence to: J. Kaiser (j.kaiser@uea.ac.uk) tion parameters should always be fully documented and the measured relative isotope ratio differences as well as the oxygen supersaturation should be permanently archived, so that improved measurements of the calculation parameters can be used to retrospectively improve production rates.

Comment on: "Technical note: Consistent calculation of aquatic gross production from oxygen triple isotope measurements" by Kaiser (2011)

“…3a and b from Kaiser (2011). In addition to the results presented by Kaiser, I have added two green lines to the plot that show the error induced by the choice of equation form alone (Fig.…”

“…3). Since differing definitions of 17 O excess are used, I repeat here definitions 4 and 7 from Kaiser (2011) 17 † = 17 δ − κ 18 δ…”

“…I will argue that the 249 must be applied instead to oxygen in biological steady state with seawater ( 17 S0 ) which is influenced by photosynthesis and respiration, rather than 17 P in Published by Copernicus Publications on behalf of the European Geosciences Union. 2994 D. P. Nicholson: Comment on Kaiser (2011) order for consistent comparison between calculation methods. I introduce the notation 17 S0 to refer specifically to the biological steady-state condition in which P = R (and thus f = 0, where f is the net to gross production ratio).…”

“…Kaiser (2011) has introduced an improved method for calculating gross productivity from the triple isotopic composition of dissolved oxygen in aquatic systems. His equation avoids approximations of previous methodologies, and also accounts for additional physical processes such as kinetic fractionation during invasion and evasion at the airsea interface.…”

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

“…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).…”