Sclerochronological evidence of pronounced seasonality from the Pliocene of the southern North Sea Basin, and its implication

<p>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 &#176;C and winter temperatures above 10 &#176;C [1]. However, summer and winter temperature estimates from oxygen-isotope (&#948;¹⁸O) sclerochronology of benthic invertebrates are typically in the respective cool temperate range when calculated using a plausible modelled value for water &#948;¹⁸O of +0.1&#8240;. For instance, examples of the bivalve mollusc <em>Aequipecten opercularis</em> from the Ramsholt Member indicate summer maximum temperatures of 11.0&#8211;15.7 &#176;C and winter minimum temperatures of 4.4&#8211;7.1 &#176;C [2]. Amongst other evidence, the pattern of microgrowth-increment variation in Ramsholt-Member <em>A. opercularis</em> 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 &#176;C) than in the modern North Sea (< 13 &#176;C) [2]. This conclusion is not changed by adoption of a different (invariant) value for water &#948;¹⁸O 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 <em>A. opercularis</em> from several horizons in the Lillo Formation (Belgium) and the Oosterhout Formation (Netherlands) indicate seasonal ranges in benthic temperature of 10&#8211;14 &#176;C. Seasonal variation in water &#948;¹⁸O can only plausibly account for about 1 &#176;C of these ranges. Taking into consideration microgrowth-increment evidence of a setting below the summer thermocline, the seafloor ranges imply that the surface seasonal range was sometimes 17 &#176;C or more. Other bivalves (<em>Atrina fragilis</em>, <em>Arctica islandica,</em> <em>Pygocardia rustica</em>, <em>Glycymeris radiolyrata</em>) do not indicate such a high seasonal range in benthic (and hence surface) temperature but this can be attributed to inadequate sampling&#8212;time-averaging or a failure to recover evidence of seasonal extremes because of growth breaks. The high surface temperature range could reflect a reduction in vigour of the North Atlantic Current and hence diminished oceanic supply of heat in winter.</p><p>References:</p><p>[1] Vignols et al. (2019), Chem. Geol. 526, 62&#8211;83. https://doi.org/10.1016/j.chemgeo.2018.05.034.</p><p>[2] Johnson et al. (2020), Palaeogeogr. Palaeoclimatol. Palaeoecol. 561. https://doi.org/10.1016/j.palaeo.2020.110046.</p><p>[3] Head (1997), J. Paleontol. 71, 165&#8211;193. https://doi.org/10.1017/S0022336000039123.</p><p>[4] Valentine et al. (2011), Palaeogeogr. Palaeoclimatol. Palaeoecol. 309, 9&#8211;16. https://doi.org/10.1016/j.palaeo.2011.05.015.</p>