The δ¹⁸O of dissolved O₂ in the surface waters of the subarctic Pacific: A tracer of biological productivity

Abstract

The 18O/16O of dissolved oxygen was measured in the upper ocean of the subarctic Pacific in 1988. In May and August, at stations Papa (50°N, 145°W) and R (53°N, 145°W), the mean δ18O in the mixed layer was 23.84±0.20 and 24.00±0.24 ‰ (versus standard mean ocean water) consistently more depleted than atmospheric saturation levels by about 0.4 ‰. This relative depletion is caused by input of photosynthetically produced O2. A value for the isotopic fractionation effect during respiration (αT) of 0.978±0.006 was determined from the time rate of change of the concentration and δ18O of O2 in the mixed layer measured during August 1988. Below the mixed layer (100–280 m) the O2 concentration decreased with a corresponding increase in δ18O. Model derived values for αT over this depth region ranged from 0.980 to 0.988 and depended on the mixing model. The difference between αT determinations for the surface layer versus upper thermocline likely results from mixing model inaccuracies or different isotope fractionation effects during plankton and bacterial respiration. If the calculated mixed layer αT values apply oceanwide, then photosynthesis and respiration by the marine biota have a similar effect to land plants in maintaining the δ18O of atmospheric O2 at 23.5 ‰.

Aerobic respiration and hypoxia in the Lower St. Lawrence Estuary: Stable isotope ratios of dissolved oxygen constrain oxygen sink partitioning

“…Openness of the system-Previous studies have demonstrated that open-system aspects and diffusion have a firstorder influence on the distribution of isotope tracers in the ocean or other aquatic environments (Bender 1990;Quay et al 1993;Lehmann et al 2003). The suitability of the Rayleigh approach to calculating e O-app (i.e., the basis for the estimation of R WC : R Sed ) depends on rates of O 2 replenishment by O 2 production through photosynthesis or vertical mixing.…”

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

“…For the respiration fractionation 18 ε R , a large range from −6 ‰ for fish and human respiration to −24 ‰ for the alternative respiratory pathway (AOX) has been reported Helman et al, 2005). For the purposes of the present study, I use 18 ε R = (−20 ± 4) ‰, which is more representative of values reported for marine communities: (−22 ± 3) ‰ in the Southern Ocean (Hendricks et al, 2004), (−21 ± 2) ‰ in the Equatorial Pacific (Hendricks et al, 2005), (−22 ± 6) ‰ in the subarctic Pacific (Quay et al, 1993) and (−20 ± 3) ‰ for different marine organisms (Kiddon et al, 1993). .…”

“…dc /dt = 0, is not required. Most parameters (ε E , ε I , δ P ) and variables (s, δ) can be measured with sufficient precision to calculate g. However, ε R is often not sufficiently well-constrained to give precise results for g (Quay et al, 1993). Moreover, f has to be estimated from s and g via f = s/g (Hendricks et al, 2004), which can introduce additional uncertainties, because the relationship f = s/g is derived from assuming dc/dt = 0 and therefore P -R = sν mix c sat , cf.…”

“…The advantage of the oxygen triple isotope technique over 18 O/ 16 O isotope ratio measurements in determining production is that the calculated rates are independent of the respiratory isotope effect that is not well known and that would otherwise lead to significant uncertainties (Hendricks et al, 2004;Quay et al, 1993;Venkiteswaran et al, 2008). If implemented correctly, the additional information from the 17 O/ 16 O isotope ratio measurement allows elimination of the respiratory isotope effect from the production calculation.…”

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

“…Moreover, θ R can only be computed if 18 ε R is also known. Even though the influence of the uncertainty in 18 ε R is not as severe as when 18 δ were used for the calculation directly (Quay et al, 1993), this goes against the rationale behind the triple oxygen isotope technique (i.e., the absence of the need to know 18 ε R ). Finally, the suggested approximations are mathematically inconsistent with Eqs.…”