Holocene shelf-sea seasonal stratification dynamics: a dinoflagellate cyst record from the Celtic Sea, NW European shelf

Published records of the Holocene evolution of seasonal strati. cation in the Celtic Sea (NW European shelf) have been based on benthic proxies, notably benthic foraminifera and associated stable isotopic data. We have investigated organic-walled dinoflagellate cyst assemblages from a well-dated Holocene sequence from the central Celtic Sea in order to assess the signal from this planktonic proxy and to reconstruct paired bottom- and surface-water conditions through time. This sequence has, on the basis of the benthic proxies, been interpreted previously as a record of the replacement of tidally mixed water by stratified water associated with tidal-front migration during the early Holocene. Interpretation of the dinocyst record has been facilitated by a parallel study of the distribution of cysts from Celtic Sea surface sediments and their relationship with seasonal water masses. The dinocyst stratigraphy indicates mixed-water conditions during the early Holocene consistent with reduced water depths (hence lowered sea level) over the core site. The first significant change in the dinocyst assemblages is recorded at around 6650 cal. years BP and indicates a transition from mixed-frontal conditions to seasonal strati. cation. This interpretation of frontal migration is consistent with changes in the benthic foraminiferal assemblages and associated stable isotopes at the same core depth. From 6650 to 3600 cal. years BP, the significant occurrence of Bitectatodinium tepikiense accompanied by Spiniferites elongatus is attributed to strong seasonality, with winter sea-surface temperatures possibly below 5degreesC. Another transition at 3600 cal. years BP is attributed to a reduction in seasonality generated by milder winter conditions linked to a stronger influence of the thermohaline circulation over the studied area. This transition is not recorded by the benthic proxies and is attributed to climate forcing rather than to any change in tidal dynamics. It is notable that many mires in western Britain record distinct wet shifts contemporary with this change.