| Summary: | Various elements of the biota of the early Pliocene Coralline Crag Formation (southern North Sea Basin, eastern England) have been taken to indicate a warm temperate marine climate, with summer surface temperatures above 20 °C and winter temperatures above 10 °C [1]. However, summer and winter temperature estimates from oxygen-isotope (δ18O) sclerochronology of benthic invertebrates are typically in the respective cool temperate range when calculated using a plausible modelled value for water δ18O of +0.1‰. For instance, examples of the bivalve mollusc Aequipecten opercularis from the Ramsholt Member indicate summer maximum temperatures of 11.0–15.7 °C and winter minimum temperatures of 4.4–7.1 °C [2]. Amongst other evidence, the pattern of microgrowth-increment variation in Ramsholt-Member A. opercularis points to a depth below the summer thermocline, hence the temperatures recorded for that season provide an underestimate of surface temperature; this may well have been in the warm temperate summer range [2], as suggested by the pelagic dinoflagellate biota [3]. However, the cool temperate benthic winter temperatures indicated by isotopic data are likely also to have obtained at the surface, pointing to a greater seasonal range in surface temperature (perhaps > 15 °C) than in the modern North Sea (< 13 °C) [2]. This conclusion is not changed by adoption of a different (invariant) value for water δ18O and also follows from data for a specific late Pliocene interval (Mid-Piacenzian Warm Period) elsewhere in the southern North Sea Basin (Belgium, Netherlands [4]). Here we present isotopic evidence of a seasonal range in surface temperature higher than now at other times in the late Pliocene. Examples of A. opercularis from several horizons in the Lillo Formation (Belgium) and the Oosterhout Formation (Netherlands) indicate seasonal ranges in benthic temperature of 10–14 °C. Seasonal variation in water δ18O can only plausibly account for about 1 °C of these ranges. Taking into consideration ...
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