The sulphur cycle has evolved over the course of the Earth’s hisroty(1,2). The early Earth’s surface environment was reducing, containing little atmospheric oxygen(3), and with seawater sulphate concentrations estimated at less than a few per cent of those found today, The accumulation of sulphate in the ocean to much higher concentrations was probably coincident with the initial accumulation of oxygen in the atmosphere and the consequent oxidative weathering of sulphide minerals on land(4,5). Past changes in sulphate concentrations in ancient oceans have previously been assessed by comparing the systematics of sulphur Isotope fractionation by sulphate-reducing bacteria(6-9) with the isotopic composition of sedimentary sulphides(1,2,5,10,11). But such interpretations have proven equivocal : the generally small S-34 depletions in Archaean sulphides (deposited similar to 2.5-3.8 billion years ago) have been separately argued to result both from rapid sulphate reduction in a sulphate-rich ocean(5,12), and from sulphide formation in a sulphate-poor ocean(1,2,11). Here we report large S-34 depletions of 20-25%, observed during rapid sulphate reduction by sulphate-reducing bacteria in modern photosynthetic cyanobacterial mats from Solar Lake, Sinai. We conclude that high sulphate concentrations give rise to highly S-34-depleted sulphides, and thus that appreciable concentrations of seawater sulphate did not accumulate until the initial accumulation of oxygen into the atmosphere in post-Archaean times.