Revising Estimates of Aquatic Gross Oxygen Production by the Triple Oxygen Isotope Method to Incorporate the Local Isotopic Composition of Water Manning, Cara C.; Howard, Evan M.; Nicholson, David P.; Ji, Brenda Y.; Sandwith, Zoe O.; Stanley, Rachel H. R.
Measurement of the triple oxygen isotope (TOI) composition of O₂ is an established method for quantifying gross oxygen production (GOP) in natural waters. A standard assumption to this method is that the isotopic composition of H₂O, the substrate for photosynthetic O₂, is equivalent to Vienna standard mean ocean water (VSMOW). We present and validate a method for estimating the TOI composition of H₂O based on mixing of local meteoric water and seawater H₂O end-members, and incorporating the TOI composition of H₂O into GOP estimates. In the ocean, GOP estimates based on assuming the H₂O is equivalent to VSMOW can have systematic errors of up to 48% and in low-salinity systems, errors can be a factor of 2 or greater. In future TOI-based GOP studies, TOI measurements of O2 and H₂O should be paired when the H₂O isotopic composition is expected to differ from VSMOW.
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