An intermediate depth layer, approximately 1 km thick, in the northwestern Indian Ocean contains essentially no detectable dissolved oxygen. Previous suggestions for primary causes of this feature have been: (a) very slow movement within the layer, allowing a long time for organic decomposition to consume the oxygen; (b) very large local consumption rates, resulting from enormous productivity in the surface layer; or (c) low oxygen concentrations in the waters entering the layer from the south, due to their long transit from their sea-surface sources. Observations reported here of a transient anthropogenic trace gas, trichlorofluoromethane (F-11 or freon 11), however, demonstrate that the residence time for water in the low-oxygen layer is not espciallt long, about 10 years. Concurrent summertime measurements of surface productivity, while high, preclude an exceptional mean consumption rate at depth. An oxygen budget for the layer supports the idea that the near-zero concentration is maintained by moderate consumption applied to waters with initially low oxygen concentration that pass through the layer at moderate speed.
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