On the use of dissolved oxygen isotopologues as biogeochemical tracers
in the Pacific Ocean
Abstract
The isotopic composition of dissolved oxygen offers a family of
potentially unique tracers of respiration and transport in the
subsurface ocean. Uncertainties in transport parameters and isotopic
fractionation factors, however, have limited the strength of the
constraints offered by 18O/16O and 17O/16O ratios in dissolved oxygen.
In particular, puzzlingly low 17O/16O ratios observed for some
low-oxygen samples have been difficult to explain. To improve our
understanding of oxygen cycling in the ocean’s interior, we investigated
the systematics of oxygen isotopologues in the subsurface Pacific using
new data and a 2-D isotopologue-enabled isopycnal reaction-transport
model. We measured 18O/16O and 17O/16O ratios, as well as the
“clumped” 18O18O isotopologue in the northeast Pacific, and compared
the results to previously published data. We find that transport and
respiration rates constrained by O2 concentrations in the oligotrophic
Pacific yield good measurement-model agreement across all O2
isotopologues only when using a recently reported set of respiratory
isotopologue fractionation factors that differ from those most often
used for oxygen cycling in the ocean. These fractionation factors imply
that an elevated proportion of 17O compared to 18O in dissolved
oxygen―i.e., its triple-oxygen isotope composition―does not uniquely
reflect gross primary productivity and mixing. For all oxygen
isotopologues, transport, respiration, and photosynthesis comprise
important parts of their respective budgets. Mechanisms of oxygen
removal in the subsurface ocean are discussed.