Sclerochronological evidence of pronounced seasonality from the Pliocene of the&#160;southern North Sea Basin, and its implication
Authors: Johnson Andrew , Valentine Annemarie , Leng Melanie , Schöne Bernd , Sloane Hilary , Goolaerts Stijn 
Published: March 2021

&lt;p&gt;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 &amp;#176;C and winter temperatures above 10 &amp;#176;C [1]. However, summer and winter temperature estimates from oxygen-isotope (&amp;#948;&lt;sup&gt;18&lt;/sup&gt;O) sclerochronology of benthic invertebrates are typically in the respective cool temperate range when calculated using a plausible modelled value for water &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O of +0.1&amp;#8240;. For instance, examples of the bivalve mollusc &lt;em&gt;Aequipecten opercularis&lt;/em&gt; from the Ramsholt Member indicate summer maximum temperatures of 11.0&amp;#8211;15.7 &amp;#176;C and winter minimum temperatures of 4.4&amp;#8211;7.1 &amp;#176;C [2]. Amongst other evidence, the pattern of microgrowth-increment variation in Ramsholt-Member &lt;em&gt;A. opercularis&lt;/em&gt; 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 &gt; 15 &amp;#176;C) than in the modern North Sea (&lt; 13 &amp;#176;C) [2]. This conclusion is not changed by adoption of a different (invariant) value for water &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O 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 &lt;em&gt;A. opercularis&lt;/em&gt; from several horizons in the Lillo Formation (Belgium) and the Oosterhout Formation (Netherlands) indicate seasonal ranges in benthic temperature of 10&amp;#8211;14 &amp;#176;C. Seasonal variation in water &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O can only plausibly account for about 1 &amp;#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 &amp;#176;C or more. Other bivalves (&lt;em&gt;Atrina fragilis&lt;/em&gt;, &lt;em&gt;Arctica islandica,&lt;/em&gt; &lt;em&gt;Pygocardia rustica&lt;/em&gt;, &lt;em&gt;Glycymeris radiolyrata&lt;/em&gt;) do not indicate such a high seasonal range in benthic (and hence surface) temperature but this can be attributed to inadequate sampling&amp;#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.&lt;/p&gt;&lt;p&gt;References:&lt;/p&gt;&lt;p&gt;[1] Vignols et al. (2019), Chem. Geol. 526, 62&amp;#8211;83. https://doi.org/10.1016/j.chemgeo.2018.05.034.&lt;/p&gt;&lt;p&gt;[2] Johnson et al. (2020), Palaeogeogr. Palaeoclimatol. Palaeoecol. 561. https://doi.org/10.1016/j.palaeo.2020.110046.&lt;/p&gt;&lt;p&gt;[3] Head (1997), J. Paleontol. 71, 165&amp;#8211;193. https://doi.org/10.1017/S0022336000039123.&lt;/p&gt;&lt;p&gt;[4] Valentine et al. (2011), Palaeogeogr. Palaeoclimatol. Palaeoecol. 309, 9&amp;#8211;16. https://doi.org/10.1016/j.palaeo.2011.05.015.&lt;/p&gt;