Does carbonate-associated sulphate record nutrition in lucinid and thyasirid bivalve shells from modern hydrocarbon seeps?

We test whether chemosymbiotic bivalves with sulphide-oxidizing bacteria record their nutritional strategy in the sulphur isotope composition of the carbonate-associated sulphate (CAS) in their shells, as a possible indicator of thiotrophic chemosymbiosis in the fossil record. The hypothesis rests on the possible incorporation of ³⁴S-depleted sulphate resulting from sulphide oxidation in sufficient quantity to affect the intrashell sulphate-sulphur isotope mass balance and hence the isotopic composition of sulphate, which is incorporated into carbonate with little or no fractionation. We analysed shell material of lucinid (Lucinoma asapheus) and thyasirid (Thyasira vulcolutre) bivalves from active mud volcanoes in the Gulf of Cadiz. Our results show that the CAS-δ³⁴S values of the bivalve shells do not reflect the variety of sulphur sources present at hydrocarbon seeps, but instead only record seawater sulphate values. Low δ³⁴S values were, however, measured in the animals’ soft tissues and shell organic matter (SOM), both displaying a strong influence of the depleted sulphide used as nutrition by the chemosynthethic bacteria. Given its potential for long-term preservation, SOM may therefore represent a more promising record of chemosymbiosis in the fossil record, while CAS from seep bivalves can be used to reconstruct local seawater sulphate.