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A time series of the 17O excess (17D) was measured in the Gulf of Elat (Aqaba) between May 2007 and August 2009. 17D 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 17D to assess the ratio of photosynthetic O2 to atmospheric O2 in deep water. We observed an increase of 17D 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 17D, 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 17D 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. Mixing processes between deep and surface water play a major role in determining the physical and biogeochemical properties of marine environments. For example, mixing processes affect the distribution of heat and salt, which controls the physical properties of the water column in confined water bodies as well as in the ocean. From the biogeochemical perspective, the supply of nutrient-rich water to the photic zone enables primary producers to fix CO2 into organic matter. At the same time, the penetration of surface water with high O2 concentration into the ocean’s interior enables the re-mineralization of the organic matter. In this manner, mixing processes control the oceanic CO2 and O2 cycles, and therefore characterizing the patterns of water circulation and mixing is an important aspect of marine biogeochemistry. Studies of mixing processes are often conducted using natural tracers such as spatial distributions of conservative properties such as salinity and potential temperature (Stern 1967), anthro |
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