Abstract
The isotopic composition of dissolved O 2 (δ 18 O) in aquatic environments is strongly affected by the preferential uptake of the lighter isotopologue during biological consumption processes. Numerous studies have shown that during incubation experiments, the isotopic effect of microorganism respiration ( ε organism ) is on the order of −20‰. However, studies of the co‐variations of O 2 and δ 18 O in natural environments show considerably weaker in situ fractionation ( ε app ). A possible explanation for this discrepancy is that a significant fraction of the O 2 consumption is diffusion‐limited. Although this is a generally accepted mechanism in sediments, it cannot explain the weak fractionations observed in mid‐ocean sites. Here, we analyze a time series of O 2 , δ 18 O, and auxiliary data from the northern Gulf of Aqaba (Red Sea). Although an incubation experiment showed strong fractionation against the heavy isotopologue ( ε organism = −24.5‰), the in situ ε app was only −14‰ in deep water isolated from the photic zone. We show that this result requires an additional O 2 consumption mechanism with weak fractionation, rather than mixing, and suggest that this mechanism is diffusion‐limited respiration into aggregates of organic material. We estimate that this mechanism could be responsible for 30% of the O 2 consumption in the Gulf and suggest that it may also constitute a major O 2 consumption pathway in the world's oceans.