Mixing processes in the deep water of the Gulf of Elat (Aqaba): Evidence from measurements and modeling of the triple isotopic composition of dissolved oxygen

A time series of the ¹⁷O excess (¹⁷Δ) was measured in the Gulf of Elat (Aqaba) between May 2007 and August 2009.¹⁷Δ is unaffected by respiration; thus it is a unique, conservative tracer that preserves the signature acquired in the photic zone, the source region for deep-water formation. In this study we used ¹⁷Δ to assess the ratio of photosynthetic O₂ to atmospheric O₂ in deep water. We observed an increase of ¹⁷Δ by 20 per meg in the water residing below 300 m over a period of 3 months, followed by a decrease of 60 per meg over the next 7 months. These changes indicated penetration of photosynthetic O₂, followed by penetration of atmospheric O₂ into the deep water. To test whether vertical mixing could explain the observed variations in ¹⁷Δ, we compared our results with simulated values obtained from a one-dimensional hydrodynamic model, which was extended to include dissolved O₂ isotopes. Although successfully reproducing the observed temperatures, salinities, and dissolved O₂ concentrations in the gulf, the model could not reproduce the observed variations in ¹⁷Δ in the deep water. This discrepancy shows that horizontal mixing processes have an important role in the interaction between deep and surface water in the gulf. We suggest that for the most part, these processes occur along the coastal boundaries of the gulf.