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The chemical composition of the Bannock basin has been studied in some detail(1,2). We recently showed that unusual microbial populations, including a new division of Archaea (MSBL1)(3), inhabit the NaCl-rich hypersaline brine. High salinities tend to reduce biodiversity(4), but when brines come into contact with fresher water the natural haloclines formed frequently contain gradients of other chemicals, including permutations of electron donors and acceptors, that may enhance microbial diversity, activity and biogeochemical cycling(5,6). Here we report a 2.5-m-thick chemocline with a steep NaCl gradient at 3.3 km within the water column between Bannock anoxic hypersaline brine(7) and overlying sea water. The chemocline supports some of the most biomass-rich and active microbial communities in the deep sea, dominated by Bacteria rather than Archaea, and including four major new divisions of Bacteria. Significantly higher metabolic activities were measured in the chemocline than in the overlying sea water and underlying brine; functional analyses indicate that a range of biological processes is likely to occur in the chemocline. Many prokaryotic taxa, including the phylogenetically new groups, were confined to defined salinities, and collectively formed a diverse, sharply stratified, deep-sea ecosystem with sufficient biomass to potentially contribute to organic geological deposits.