Seasonal thermocline development in the shelf seas of NW Europe results in the formation of pronounced frontal regions which separate stratified from well-mixed water. Numerical M2 palaeotidal modelling of the NW European continental shelf during successive stages of the Holocene transgression indicates that seasonal stratification started to develop in the central Celtic Sea once a critical threshold water depth had been exceeded at around 9000 years BP. This hypothesis has been tested through faunal, isotopic and geochemical analysis of an AMS 14C-dated British Geological Survey vibrocore from the Celtic Deep bathymetric basin; a record which spans the last 12 000 years. Marked positive trends in the stable oxygen isotopic data from the benthic foraminifera Ammonia batavus and Quinqueloculina seminulum between 9000 and about 5000 years BP represent a 4–5°C cooling in summer bottom water temperature caused by the progressive replacement of tidally well-mixed by stratified water. Stable carbon isotopic data from the same species, measured through the same core intervals, show a negative trend consistent with net oxygen utilization and decomposition of organic matter beneath the productive frontal zone. These geological data indicate the progressive evolution of seasonally stratified water in the Celtic Sea during the earlier part of the Holocene. Modern shelf sea frontal and thermocline development are known to influence biological productivity, sedimentation rates, the organic content of shelf sediments and the climate of surrounding landmasses. Hence these palaeodata have significance in defining the role of shelf sea environments in the climate system.