Accurate values of δ(17)O and (17)Oexcess of the international standards NBS19-CO2 and NBS18-CO2 are now available. The new values should be used for normalization of measured oxygen isotope ratios of carbonates to allow meaningful comparison of results among different laboratories.

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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 À20‰. 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 = À24.5‰), the in situ ε app was only À14‰ 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.

The Atlantic Meridional Overturning Circulation (AMOC) in the ocean is a critical component of the global climate. It transports large amounts of heat from the tropics to high latitudes, affects the cycling of carbon, oxygen, and other biologically essential nutrients, and affects the Atlantic Multidecadal Variability and the associated impacts (