MAINTENANCE OF THE LOW-OXYGEN LAYER IN THE CENTRAL ARABIAN SEA

An intermediate depth layer, approximately 1 km thick, in the northwes tern 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 organi c decomposition to consume the oxygen; (b) very large local consumptio n rates, resulting from enormous productivity in the surface layer; or (c) low oxygen concentrations in the waters entering the layer from t he south, due to their long transit from their sea-surface sources. Ob servations reported here of a transient anthropogenic trace gas, trich lorofluoromethane (F-11 or freon 11), however, demonstrate that the re sidence time for water in the low-oxygen layer is not especially long, about 10 years. Concurrent summertime measurements of surface product ivity, while high, preclude an exceptional mean consumption rate at de pth. An oxygen budget for the layer supports the idea that the near-ze ro concentration is maintained by moderate consumption applied to wate rs with initially low oxygen concentration that pass through the layer at moderate speed.