The 3He distribution in deep water over the Mid-Atlantic Ridge

³He/⁴He ratios in dissolved helium at GEOSECS stations 115, 117, 30, and 120 provide an east-west section across the Mid-Atlantic Ridge at 30°N. Below 1500 m depth, the³He/⁴He profiles show little structure and have values within a few percent of the atmospheric ratio, indicating that the Mid-Atlantic Ridge is not a significant source of injected³He at this latitude. Mean³He/⁴He ratios calculated for the deep water at each station show that the³He/⁴He ratio in the western Atlantic at 30°N is 2–3% higher than in the eastern basin, probably due to mixing between a³He-rich boundary current in the western basin and low-³He deep water to the east.