A profile of the isotopic composition 634S, of elemental sulphur, acid-lablle sulphide, chromous-reducible sulphide and dissolved sulphate was recorded from 30 cm long cores taken from a sandy sedlrnent at the low water mark at Mlll Bay, a beach near the mouth of the Salcombe Estuary SW England The sediment is inhabited by the bivalve mollusc Luanoma boreahs, which has concentrations of elemental sulphur within endosymbiotic sulphur-oxidising bactena in the gills Sulphur from the g~l l s was

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... e depleted in 34S (634S = -38%) than any of the sulphur species within the sedunent The 6% of elemental sulphur in the sediment vaned from -19 5 % at 0 to 5 cm depth to -25 4% at 15 to 20 cm depth The iron sulphides showed similar depth profdes for 634S from +6 to +9% at 0 to 5 cm depth to -26 to -29% at 15 to 20 cm depth Interstitial dissolved sulphate showed an irregular profile, considerably depleted with respect to seawater sulphate, in the upper 3 cm (634S + l 5 8%0) increasing \nth depth to +20 l %O at 18 to 23 cni depth and then decreasing to + l 1 5%0 at 23 to 28 cm Model experiments on water pumping through permeable tubes of smilar diameter to the inhalant tube of the bivalve illustrated the oxidatlve processes taking place around the inhalant tube and demonstrated the mobilisation of reduced sulphur from the iron sulphides in the sand that leads to highly depleted 34S in the gill Results are discussed with respect to the nutritional strategies of lucinid bivalve niolluscs