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1 Marine climate and hydrography of the Coralline Crag (early Pliocene, UK): 1 2 2 isotopic evidence from 16 benthic invertebrate taxa 3 4 5 3 6 7 4 Rebecca M. Vignols a,1, Annemarie M. Valentine b,2, Alana G. Finlayson a,3, 8 9 a c d 10 5 Elizabeth M. Harper , Bernd R. Schöne , Melanie J. Leng , 11 12 6 Hilary J. Sloane d, Andrew L.A. Johnson b,* 13 14 15 7 16 a 17 8 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 18 19 9 3EQ, UK 20 21 b 22 10 Department of Natural Sciences, College of Life and Natural Sciences, University of Derby, 23 24 11 Kedleston Road, Derby DE22 1GB, UK 25 26 c 27 12 Institute of Geosciences, University of Mainz, Johann-Joachim-Becher-Weg 21, 55128 28 29 13 Mainz, Germany 30 31 d 32 14 NERC Isotope Geoscience Facilities, British Geological Survey, Nicker Hill, Keyworth 33 34 15 NG12 5GG, UK 35 36 16 Current addresses. 37 38 1 39 17 British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK 40 41 18 2 Student Advice and Support Services, Loughborough University, Loughborough LE11 3TU, 42 43 44 19 UK 45 46 20 3 Oil & Gas Authority, 48 Huntly Street, Aberdeen AB10 1SH, UK 47 48 * 49 21 Corresponding author. 50 51 22 Email address: [email protected] (A.L.A. Johnson) 52 53 54 23 55 56 24 Keywords: Coralline Crag, Pliocene, Paleotemperature, Oxygen isotope thermometry, 57 58 25 Uniformitarianism 59 60 1 61 62 63 64 65 26 1 2 27 ABSTRACT 3 4 5 28 The taxonomic composition of the biota of the Coralline Crag Formation (early Pliocene, 6 7 29 eastern England) provides conflicting evidence of seawater temperature during deposition, 8 9 10 30 some taxa indicating cool temperate conditions by analogy with modern representatives or 11 12 31 relatives, others warm temperate to subtropical/tropical conditions. Previous isotopic (δ18O) 13 14 15 32 evidence of seasonal seafloor temperatures from serial ontogenetic sampling of bivalve 16 17 33 mollusk shells indicated cool temperate winter (< 10 °C) and/or summer (< 20 °C) conditions 18 19 34 but was limited to nine profiles from two species, one ranging into and one occurring 20 21 22 35 exclusively in cool temperate settings at present. We supplement these results with six further 23 24 36 profiles from the species concerned and supply seven more from three other taxa (two 25 26 27 37 supposedly indicative of warm waters) to provide an expanded and more balanced database. 28 29 38 We also supply isotopic temperature estimates from 81 spot and whole-shell samples from 30 31 32 39 these five taxa and 11 others, encompassing ‘warm’, ‘cool’ and ‘eurythermal’ forms by 33 34 40 analogy with modern representatives or relatives. Preservation tests show no shell alteration. 35 36 41 Subject to reasonable assumptions about water δ18O, the shell δ18O data either strongly 37 38 39 42 indicate or are at least consistent with cool temperate seafloor conditions. The 40 41 43 subtropical/tropical conditions suggested by the presence of the bryozoan Metrarabdotos did 42 43 13 44 44 not exist. Microgrowth-increment and δ C evidence indicate summer water-column 45 46 45 stratification during deposition of the Ramsholt Member, unlike in the adjacent southern 47 48 49 46 North Sea at present (well mixed due to shallow depth and strong tidal currents). Summer 50 51 47 maximum surface temperature was probably about 5 °C above seafloor temperature and thus 52 53 often slightly higher than now (17–19 °C rather than 16–17 °C), but only sometimes in the 54 48 55 56 49 warm temperate range. Winter minimum surface temperature was below 10 °C and possibly 57 58 59 60 2 61 62 63 64 65 50 the same as at present (6–7 °C). An expanded surface temperature range compared to now 1 2 51 may reflect withdrawal of oceanic heat supply in conjunction with higher global temperature. 3 4 5 52 6 7 53 1. Introduction 8 9 10 54 11 12 55 The Coralline Crag Formation is an early Pliocene marine unit up to 20 m thick occurring 13 14 15 56 in Suffolk, East Anglia, eastern England. Although actual exposures are fairly limited in 16 17 57 number, it has an almost continuous onshore outcrop stretching some 25 km south-west from 18 19 58 the coastal town of Aldeburgh (Fig. 1), and extends 14 km north-east from there under the 20 21 22 59 southern North Sea (Balson, 1992). Outliers at Ramsholt, Sutton Knoll (also known as 23 24 60 Rockhall Wood) and Tattingstone (now submerged beneath a reservoir) extend the area of 25 26 27 61 occurrence of the formation 27 km west-south-west of the main onshore outcrop. The unit is 28 29 62 typically a fine–medium sand, strongly bioturbated and with a mud admixture of up to 24% 30 31 32 63 in the Ramsholt Member, but generally less bioturbated and with a lower mud content in the 33 34 64 overlying Sudbourne and Aldeburgh members (Balson et al., 1993); the calcium carbonate 35 36 65 content is high throughout (usually 60–70%; Balson et al., 1993). Unconformably below and 37 38 39 66 above the Coralline Crag are more extensive marine units with a much lower calcium 40 41 67 carbonate content: respectively, the Eocene London Clay Formation and the late Pliocene– 42 43 44 68 early Pleistocene Red Crag Formation. 45 46 69 Following reassignment of the thin (c. 4 m) deposit occupying a small area at St Erth, 47 48 49 70 Cornwall, to the Pleistocene (see Fig. 1), the Coralline Crag is the only onshore early 50 51 71 Pliocene (Zanclean) deposit in the British Isles. On the opposite side of the southern North 52 53 Sea, in Belgium and The Netherlands (Fig. 1), there is an extensive, thick and fairly complete 54 72 55 56 73 sequence of Pliocene marine deposits (Marquet and Herman, 2009), but in Belgium there 57 58 74 appears to be a gap corresponding to the time of deposition of the Coralline Crag (De 59 60 3 61 62 63 64 65 75 Schepper et al., 2009; Fig. 2). In The Netherlands, equivalent-aged strata are known only 1 2 76 from boreholes so, as an outcropping, moderately well-exposed unit, the Coralline Crag is the 3 4 5 77 best source of information on marine environmental conditions in the southern North Sea 6 7 78 Basin for part of the Pliocene. The interval concerned is not represented in the stratigraphic 8 9 10 79 sequence of the northern part of the Atlantic Coastal Plain of the USA (Fig. 2), or farther 11 12 80 south (Ward et al., 1991), so the Coralline Crag is important for interpreting early Pliocene 13 14 15 81 environmental history in the wider North Atlantic area. It figures in global reconstructions of 16 17 82 sea-surface temperature for part of the late Pliocene (Mid-Piacenzian Warm Period) and in 18 19 83 tests of paleoclimate models (including ones used in IPCC projections) tuned to this interval, 20 21 22 84 the data involved (from ostracod-assemblage analysis; see below) being assigned a medium 23 24 85 ‘confidence level’ in recognition of the age discrepancy (Dowsett et al., 2012, 2013). 25 26 27 86 The Coralline Crag has long been of interest to geologists, partly because of its unusual 28 29 87 lithofacies for the Cenozoic of the immediate area, but mainly because of its abundant and 30 31 32 88 diverse biota. Volumetrically, the dominant elements are mollusks and bryozoans (originally 33 34 89 thought to be corals, hence the name of the unit; Charlesworth, 1835). The detailed 35 36 90 taxonomic composition of the biota was the subject of many studies in the 19th and early 20th 37 38 39 91 centuries (Charlesworth, 1835; Wood, 1848, 1851–61, 1872–74, 1879, 1882; Busk, 1859; 40 41 92 Prestwich, 1871; Harmer, 1896, 1914–18, 1920–25), and attention has continued into recent 42 43 44 93 times (e.g., Cadée, 1982; Bishop, 1987, 1994; Balson and Taylor, 1982; Bishop and 45 46 94 Hayward, 1989; Wood et al., 1993; Head, 1997, 1998; Long and Zalasiewicz, 2011; Taylor 47 48 49 95 and Taylor, 2012). From the time of the earliest studies there has existed a view that the 50 51 96 Coralline Crag represents warm conditions, based on the modern temperature associations of 52 53 certain taxa (‘ecological uniformitarianism’). For example, from the common occurrence of 54 97 55 56 98 the bryozoans Cupuladria and Metrarabdotos, restricted today to areas where seafloor 57 58 99 temperature does not fall below 12 °C and 16 °C, respectively (Lagaaij, 1963; Cheetham, 59 60 4 61 62 63 64 65 100 1967), a warm temperate to subtropical/tropical marine climate has been inferred (e.g., Taylor 1 2 101 and Taylor, 2012). This is in marked contrast to the cool temperate marine climate of the 3 4 5 102 southern North Sea at present (typical winter minimum temperature: 6–7 °C). Temperatures 6 7 103 in the cool temperate winter range (< 10 °C) have in fact been inferred from Coralline Crag 8 9 10 104 benthic foraminifers (Murray, 1987), and only just into the warm temperate range from 11 12 105 benthic ostracods (Wood et al., 1993). Different groups therefore give different indications. 13 14 15 106 Varying indications are supplied by mollusks, ‘Mediterranean’ (warm temperate) and 16 17 107 ‘Arctic’ taxa having been recorded in some of the earliest work (Prestwich, 1871) and a 18 19 20 108 similar diversity acknowledged in recent studies (Long and Zalasiewicz, 2011, 2012). 21 22 109 Investigations of temperature by biotic-assemblage analysis have now been supplemented 23 24 18 25 110 by studies using the isotopic (δ O) composition of Coralline Crag mollusks. Following initial 26 27 111 work on single calcium carbonate samples from shells (e.g., Gladenkov and Pokrovskiy, 28 29 112 1980), research involving serial ontogenetic sampling (isotope sclerochronology) has been 30 31 32 113 undertaken to provide seasonally resolved data. The first results, indicating winter 33 34 114 temperatures in the cool temperate range and similar to those at present in the southern North 35 36 37 115 Sea, were thought possibly to be an artefact of alteration in view of the suggestions of higher 38 39 116 temperature provided by the large size of microgrowth increments in the bivalve-mollusk 40 41 42 117 subject, Aequipecten opercularis (Johnson et al., 2000). The preservation of shell calcite was 43 44 118 subsequently determined to be pristine and microgrowth-increment size in this species shown 45 46 119 probably to relate to food availability and oxygenation rather than temperature; similar winter 47 48 49 120 temperatures to those previously derived were obtained from further examples of A. 50 51 121 opercularis (making a total of nine) and from an example of the aragonitic bivalve, Arctica 52 53 54 122 islandica (Johnson et al., 2009). Summer temperature estimates from these species were in 55 56 123 the cool temperate range for this season (below 20 °C, according to the definition of Hall, 57 58 59 124 1964) and several degrees lower than the present summer maximum in the southern North 60 5 61 62 63 64 65 125 Sea (typically 16–17 °C). For the eight Ramsholt-Member shells, this was thought probably 1 2 126 to reflect greater depth (c. 50 m) and much weaker tidal currents, enabling water-column 3 4 5 127 stratification in summer—i.e., restriction of summer warming largely to surface waters. This 6 7 128 provides a means of reconciling cool temperate summer seafloor temperatures from shell 8 9 18 10 129 δ O with warm temperate (or even subtropical) surface temperatures from assemblages of 11 12 130 pelagic dinoflagellates (Head, 1997, 1998). Since dinoflagellates typically encyst in winter 13 14 15 131 they are not sensitive to temperature in that season (Valentine et al., 2011), so their warm 16 17 132 temperate signature in this instance is restricted not only to surface temperature but to 18 19 133 summer, and hence in no conflict with isotopic evidence of cool temperate winter conditions 20 21 22 134 on the seafloor, and very probably at the surface (Johnson et al., 2009). 23 24 135 The δ18O-based temperatures from A. opercularis and A. islandica are entirely consistent 25 26 27 136 with the tolerances of modern representatives, the former species extending into and the latter 28 29 137 being confined to cool temperate settings. We cannot, however, be sure that they are correct 30 31 32 138 because they were calculated using one of several possible equations, each of which supplies 33 34 139 a slightly different result and all of which involve a second ‘unknown’: water δ18O. Our 35 36 140 preferred value for this (+0.1‰), giving the above results, is based on modelling for the 37 38 39 141 specific location and time, but another modelled value (+0.5‰) gives higher temperatures 40 41 142 (some just into the warm temperate range for winter but well below the minimum 42 43 44 143 temperature suggested by the presence of the bryozoan Metrarabdotos), while the lowest 45 46 144 estimate for global average water δ18O (–0.5‰) gives much lower temperatures (Johnson et 47 48 18 49 145 al., 2009). The influence of the value chosen for water δ O is seen in the preliminary results 50 51 146 (temperatures as low as 3.6 °C for a value of –0.35‰) reported by Williams et al. (2009, p. 52 53 98) from the δ18O of additional specimens of A. islandica (data considered in full below: A. 54 147 55 56 148 islandica specimens 1, 2 and 5). 57 58 59 60 6 61 62 63 64 65 149 While absolute temperature cannot be specified with complete confidence from the δ18O 1 2 150 of skeletal calcium carbonate, this form of data does provide a means of testing whether 3 4 5 151 Coralline Crag taxa lived at different temperatures, as implied by the different temperature 6 7 152 associations of modern representatives and close relatives. In order to explain the varying 8 9 10 153 temperature indications from the Coralline Crag, Johnson et al. (2009) adopted the view that 11 12 154 the unit must represent a significantly fluctuating marine climate. Although deposition may 13 14 15 155 have taken as little as 200,000 years, this is sufficient to span glacial and inter-glacial stages, 16 17 156 and the close juxtaposition of ‘warm’ and ‘cool’ taxa could reflect reworking (Williams et al., 18 19 157 2009). Even so, such studies of multiple discrete horizons as have been undertaken (Head, 20 21 22 158 1997; Long and Zalasiewicz, 2011) have provided little evidence of temporal change in 23 24 159 marine climate, and climate change is scarcely credible as an explanation for occurrences in 25 26 27 160 the same bed of articulated examples of both the ‘warm’ bivalve species Panopea glycymeris 28 29 161 and the ‘cool’ species A. islandica (Long and Zalasiewicz, 2012). It is conceivable that higher 30 31 32 162 summer surface temperatures than now enabled the coexistence of ‘warm’ with ‘cool’ bivalve 33 34 163 taxa by providing suitable conditions for the pelagic larvae (cf. Raffi et al., 1985; Long and 35 36 164 Zalasiewicz, 2011). However, this is essentially speculative, requiring confirmation that the 37 38 39 165 adults actually lived at the same temperature as those of ‘cool’ taxa, and that the water 40 41 166 column was stratified (i.e., much warmer at the surface) in summer. 42 43 44 167 In the following, we supplement the existing isotopic temperature data for A. opercularis 45 46 168 and A. islandica, and supply data for 11 further bivalve species, two gastropod mollusk 47 48 49 169 species and a brachiopod species. This set spans ‘cool’, ‘warm’ and ‘eurythermal’ species on 50 51 170 the evidence of modern representatives and close relatives, thus allowing us not only to test 52 53 for predicted temperature differences but also to obtain a representative overview (as far as it 54 171 55 56 172 was possible to preordain) of Coralline Crag conditions. Limitations in the availability of 57 58 173 analytical resources, and the variable or unknown growth rate of species, led us to acquire a 59 60 7 61 62 63 64 65 174 heterogeneous dataset in terms of the temporal resolution of sampling. However, this is 1 2 175 sufficient, if used with care, to determine whether species supposedly indicative of different 3 4 5 176 temperature regimes did indeed occupy them, and, with appropriate qualifications, to supply 6 7 177 estimates of absolute seafloor temperature. We use δ13C data (obtained from shell samples 8 9 18 10 178 alongside δ O), together with microgrowth-increment data from A. opercularis, to test for 11 12 179 summer water-column stratification (Johnson et al., 2009, 2017). From the outcome of this 13 14 15 180 test we are able to project summer surface from seafloor temperatures, again with appropriate 16 18 17 181 qualifications. We supply evidence of excellent shell preservation to confirm that the δ O 18 19 182 and δ13C values can be accepted as original. 20 21 22 183 23 24 184 2. Material and methods 25 26 27 185 28 29 186 2.1. Taxa chosen 30 31 32 187 33 34 188 The generic and specific taxonomy used in the following, together with information on 35 36 189 the geographic ranges of modern representatives and close relatives, derives from a variety of 37 38 39 190 authoritative on-line sources (Encyclopaedia of Life, Marine Bivalve Shells of the British 40 41 191 Isles, Marine Life Information Network, Ocean Biogeographic Information System [OBIS], 42 43 44 192 World Register of Marine Species), supplemented by Huber (2010). The taxonomy differs in 45 46 193 some respects from that in other recent work on the Pliocene biota of the southern North Sea 47 48 49 194 Basin (Marquet, 2002, 2005; Long and Zalasiewicz, 2011, 2012). However, the identity of 50 51 195 the taxa referred to should be clear. Illustrations of some, together with isotopic-sampling 52 53 traces, are provided in Figure 3. 54 196 55 56 197 As ‘warm’ taxa from the Coralline Crag we chose the bivalves Cardites squamulosa 57 58 198 ampla (Chavan and Coatman 1943), Coripia corbis (Philippi 1836) and Talochlamys 59 60 8 61 62 63 64 65 199 multistriata (Poli 1795), the gastropod mollusk Ringicula buccinea (Brocchi 1814), and the 1 2 200 unusually large brachiopod Pliothyrina maxima (Charlesworth 1837). C. squamulosa ampla 3 4 5 201 (Fig. 3C) and C. corbis are both carditids, a family not represented in the modern North Sea. 6 7 202 C. squamulosa ampla appears to be extinct, but has a very close modern relative (probably a 8 9 10 203 descendant) in Cardites antiquatus (Linnaeus 1758), which is restricted to the Mediterranean 11 12 204 (Fig. 3D). C. corbis is extant and likewise restricted today to the Mediterranean, as is the 13 14 15 205 pectinid T. multistriata (Fig. 3B). Records of the latter from southern Africa are very distant 16 17 206 from the main area of occurrence and probably represent a different species. The ringiculid R. 18 19 207 buccinea (Fig. 3H) and terebratulid P. maxima are both extinct. Recent records of close 20 21 22 208 modern relatives of the former are restricted to the Mediterranean, although Jeffreys (1871) 23 24 209 did record Ringicula auriculata (Ménard de la Groye 1811) from the North Sea. Being 25 26 27 210 representative of an extinct genus, P. maxima has no close modern relatives. Other 28 29 211 Pliothyrina species occur in the Plio-Pleistocene of Italy, suggesting an affinity for warm 30 31 32 212 conditions. No such large brachiopods occur in the modern North Sea. 33 34 213 As ‘cool’ taxa we chose the bivalves Arctica islandica (Linnaeus 1767), Mya truncata 35 36 214 (Linnaeus 1758), Glycymeris obovata (Lamarck 1819), Astarte gracilis Goldfuss 1837 and 37 38 39 215 Astarte omalii (de la Jonkaire 1823), and the gastropod Retusa conuloidea (Wood 1851). 40 41 216 Like the arcticid A. islandica, the myid M. truncata is extant and occurs at present in the 42 43 44 217 North Sea but extends no farther south in the eastern North Atlantic than northern France 45 46 218 (northern boundary of warm temperate marine climate; Hall, 1964). The glycymerid G. 47 48 49 219 obovata is extinct. The close modern relative G. glycymeris (Linnaeus 1758) is absent from 50 51 220 the North Sea at present but occurs as far south as northern France. The astartids A. gracilis 52 53 (Fig. 3E) and A. omalii are likewise extinct. The close modern relatives Astarte elliptica 54 221 55 56 222 (Brown 1827) and A. montagui (Dillwyn 1817) occur essentially around the UK (although 57 58 223 not in the southern North Sea) at present, but A. acuticostata Friele 1877 is found only in the 59 60 9 61 62 63 64 65 224 northern UK and farther north. The retusid R. conuloidea is also extinct. The close modern 1 2 225 relatives R. obtusa (Montagu 1803) and R. truncatula (Bruguière 1792) occur at present in 3 4 5 226 the North Sea, and the former is found elsewhere around the UK and northern France. 6 7 227 As ‘eurythermal’ taxa we chose the bivalves Aequipecten opercularis (Linnaeus 1758), 8 9 10 228 Palliolum tigerinum (Müller 1776), Pecten aff. grandis J. de C. Sowerby 1828, Lucinoma 11 12 229 borealis (Linnaeus 1767) and Spisula arcuata (J. Sowerby 1817). The pectinids A. 13 14 15 230 opercularis (Fig. 3A) and P. tigerinum are both extant and occur today from Norway (cool 16 17 231 temperate marine climate) to the Mediterranean, although the latter is absent or scarce in the 18 19 232 southern North Sea. The pectinid P. aff. grandis is extinct. The close modern relative Pecten 20 21 22 233 maximus (Linnaeus 1758) occurs at present from Norway to the Iberian Peninsula but is 23 24 234 relatively uncommon in the North Sea. The lucinid L. borealis (Fig. 3F) is extant and occurs 25 26 27 235 today from the northern UK to the Mediterranean. The mactrid S. arcuata (Fig. 3G) is 28 29 236 extinct. The close modern relatives S. elliptica (Brown 1827), S. solida (Linnaeus 1758) and 30 31 32 237 S. subtruncata (da Costa 1778) are mainly known at present from the North Sea, but S. solida 33 34 238 is recorded from the Mediterranean. 35 36 239 37 38 39 240 2.2. Specimens used 40 41 241 42 43 44 242 Specimens (total 83) were selected from pre-existing collections in the Sedgwick 45 46 243 Museum, University of Cambridge, UK; Geological Collections, Department of Natural 47 48 49 244 Sciences, University of Derby, UK; Institute of Geosciences, University of Mainz, Germany; 50 51 245 or they were collected from the Sutton Knoll site. Wherever possible, we utilised material 52 53 known to be from the Ramsholt Member, where preservation is generally better (Balson, 54 246 55 56 247 1983), but no finer stratigraphic determination was attempted. None of the specimens were 57 58 248 definitely from the overlying Sudbourne or Aldeburgh members. Individuals were selected 59 60 10 61 62 63 64 65 249 that visually appeared well preserved, without obvious bioerosion and as whole as possible. 1 2 250 Taxon, repository and provenance of specimens, mineralogy of the sampled layer, type of 3 4 5 251 sampling undertaken, sample positions (ontogenetic sampling), laboratory where analysis 6 7 252 was conducted, raw isotope data (including δ13C values from spot and whole-shell samples 8 9 10 253 not plotted herein), and raw microgrowth-increment data from A. opercularis, are given in 11 12 254 Supplementary Material A. 13 14 15 255 16 17 256 2.3. Isotopic sampling strategy 18 19 257 20 21 22 258 Ideally we would have sampled all species ontogenetically, and done so at high temporal 23 24 259 resolution through the growth of all individuals selected—this would have enabled detailed 25 26 27 260 comparisons between species and the most precise reconstructions of seasonal temperatures. 28 29 261 Quite apart from the large analytical resource required, it would have been technically 30 31 32 262 difficult to achieve high resolution for years of slow growth (e.g., in late ontogeny) and very 33 34 263 time-consuming in those species (the majority) without proven annual growth lines, these 35 36 264 requiring preliminary sampling in order to determine annual increments from δ18O profiles. 37 38 39 265 We therefore took a more diverse approach, nevertheless obtaining some ontogenetic data 40 41 266 from each of two ‘warm’ (C. squamulosa ampla, P. maxima; four and two specimens, 42 43 44 267 respectively), one ‘cool’ (A. islandica; four specimens) and two ‘eurythermal’ (A. 45 46 268 opercularis, L. borealis; two specimens and one, respectively) taxa. 47 48 49 269 A. islandica shells exhibit internal growth lines (Fig. 4A, B) which have been proved to 50 51 270 represent annual growth breaks (Witbaard, 1997; Witbaard et al., 1997; Schöne et al., 2005a). 52 53 We could therefore determine at the outset a sample-spacing necessary to achieve a given 54 271 55 56 272 temporal resolution. Since similar structures in other ecologically equivalent (infaunal) 57 58 273 bivalves are known to be annual (Richardson, 2001), we took those in C. squamulosa ampla 59 60 11 61 62 63 64 65 274 (Fig. 4C) to have this periodicity. The annual nature of growth lines has been less widely 1 2 275 demonstrated in brachiopods so we could not be sure of the periodicity of those seen 3 4 5 276 externally in P. maxima. We therefore took samples from the two shells at quite close spatial 6 7 277 separation (mean spacings: 1.2 and 1.4 mm) in the hope that this would provide high 8 9 18 10 278 temporal resolution. In that the profile from one shell showed two full, well-defined δ O 11 12 279 cycles (13 samples in each; see Fig. 10A), which can only reasonably be ascribed to seasonal 13 14 15 280 temperature variation, this proved to be the case. The formation of external growth lines 16 17 281 (‘steps’ in the shell profile) in A. opercularis does not follow a regular periodicity, but growth 18 19 282 in this species is known to be quite rapid (Johnson et al., 2009). By taking samples from the 20 21 22 283 two shells at a similar separation (mean spacings: 1.0 and 1.3 mm) to those from P. maxima 23 24 284 we could therefore anticipate high temporal resolution. This strategy produced profiles 25 26 18 27 285 showing between one and two well-defined δ O cycles (more than 15 samples per cycle in 28 29 286 each shell; see Fig. 9A, B), confirming high temporal resolution. Although samples from the 30 31 32 287 single L. borealis shell were taken at a comparable separation (mean spacing: 1.1 mm), the 33 34 288 interval sampled (12.4–19.7 mm from the origin of growth) did not provide clear evidence of 35 36 289 cyclicity (see Fig. 11A). Equivalent growth in the ecologically similar (infaunal) bivalve A. 37 38 39 290 islandica occupies 1–2 years (Witbaard et al., 1997) so the temporal resolution of sampling in 40 41 291 L. borealis was almost certainly lower than for P. maxima and A. opercularis. The strategy 42 43 44 292 for A. opercularis did, however, provide at least some high resolution data from a 45 46 293 ‘eurythermal’ species, and for P. maxima from a ‘warm’ species. We obtained equivalent 47 48 49 294 data from one specimen of the ‘cool’ species, A. islandica, by sampling years 17–25 at 9–17 50 51 295 samples per year, supplementing existing data (Johnson et al., 2009) for year 3 (mean 52 53 samples per year: 12.5 ± 2.5, ± 1σ). Single samples spanning each annual increment were 54 296 55 56 297 taken over years 4–16, adding a low-resolution record to the data from this shell. Other 57 58 298 specimens of A. islandica were sampled at intermediate resolution (total of 12 full years; 2–7 59 60 12 61 62 63 64 65 299 samples per year, mean 4.2 ± 1.8), as were specimens of the second ‘warm’ taxon, C. 1 2 300 squamulosa ampla (total of 18 full years, 3–8 samples per year, mean 5.1 ± 1.5), assuming 3 4 5 301 that the growth lines used to define years were indeed annual. 6 7 302 The temporal resolution of sampling is likely to impact on the seasonal temperatures 8 9 10 303 determined so, in general, comparisons should only be made between data of the same 11 12 304 original resolution, or ‘resampled’ to provide the same resolution (Wanamaker et al., 2011). 13 14 15 305 However, comparisons between data of different resolution are meaningful where the lower- 16 17 306 resolution data provides a more extreme seasonal temperature. Spot samples representing 18 19 307 fairly short intervals (no more than a year) are potentially able to yield temperatures outside 20 21 22 308 the seasonal range determined from ontogenetic samples, thus providing a fuller picture of 23 24 309 variability. They are also valuable as a check, albeit crude, on the representativeness of the 25 26 27 310 temperatures from ontogenetic samples. Accordingly, we supplemented the ontogenetic 28 29 311 profiles (13, from five taxa) with 23 spot samples (similar in size to individual ontogenetic 30 31 32 312 samples) from 19 specimens of the same taxa (eight specimens of A. opercularis, four of L. 33 34 313 borealis, three of A. islandica, two each of C. squamulosa ampla and P. maxima). As a check 35 36 314 on whether the ontogenetically sampled species are representative of the Coralline Crag biota 37 38 39 315 as a whole we took a further 30 spot samples from 24 specimens of other ‘warm’, ‘cool’ and 40 41 316 ‘eurythermal’ taxa (six specimens of A. gracilis, five of G. obovata, three each of A. omalii 42 43 44 317 and S. arcuata; two each of T. multistriata, P. tigerinum and M. truncata, one of P. aff. 45 46 318 grandis), and nine ‘whole-shell’ (see section 2.7.2) samples of each of C. corbis (‘warm’), R. 47 48 49 319 buccinea (‘warm’) and R. conuloidea (‘cool’), the specimens of these three species being too 50 51 320 small (< 3 mm) to manipulate for spot sampling. Whole-shell samples have the disadvantage 52 53 that they incorporate inner-layer material, which may have a non-equilibrium isotopic 54 321 55 56 322 composition (Hickson et al., 1999), and in our case they were larger than spot samples, 57 58 323 possibly representing up to a year given the size of the specimens used. 59 60 13 61 62 63 64 65 324 1 2 325 2.4. Preparation of material for investigation 3 4 5 326 6 7 327 Material was prepared for investigation (assessment of preservation, detection of annual 8 9 10 328 growth lines, measurement of microgrowth increments, isotopic sampling and analysis) in 11 12 329 various ways according to general convention, institutional preference, and the specific focus 13 14 15 330 of the study. 16 17 331 Specimens were initially scrubbed with a nylon brush in water and (where necessary) 18 19 332 agitated in an ultrasonic bath to detach sediment and encrusting organisms from the outer 20 21 22 333 surface. Those to be photographed for illustrative purposes or microgrowth-increment 23 24 334 measurement (A. opercularis) were coated with a sublimate of ammonium chloride to 25 26 27 335 improve the visibility of surface details. All were subsequently rinsed in tap-water and air- 28 29 336 dried. Specimens of C. squamulosa ampla and A. islandica intended for ontogenetic isotopic 30 31 32 337 sampling, together with two more A. islandica specimens intended for preservational 33 34 338 assessment alone, were encased partly or fully in resin to permit sectioning without risk of 35 36 339 shell disintegration. Specimens were then cut with a circular saw perpendicular to the shell 37 38 39 340 surface along the axis of maximum growth. The cross-sections of encased shells (both 40 41 341 ‘halves’) were ground and polished with increasingly fine abrasive papers or powders (down 42 43 44 342 to 1 μm) to remove scratches, followed by rinsing in deionised water and air-drying. 45 46 343 Specimens of A. opercularis, L. borealis and P. maxima intended for ontogenetic isotope 47 48 49 344 sampling were not encased and sectioned but those of A. opercularis were subjected to an 50 51 345 additional procedure (involving bleach and ethanol) for cleaning the exterior (details in 52 53 Valentine et al., 2011). 54 346 55 56 347 Fully encased and sectioned shells were etched in 1‰ hydrochloric acid and used to make 57 58 348 acetate-peel replicas of the cut surface for identification of annual growth lines under the 59 60 14 61 62 63 64 65 349 optical microscope (Richardson et al., 1979). In the case of A. islandica specimen BRS-AJ1- 1 2 350 CC-D1R, which was only encased in resin adjacent to the axis of maximum growth, an 3 4 5 351 acetate peel was not made but the cut surface of one ‘half’ was stained with Mutvei’s solution 6 7 352 (Schöne et al., 2005b) to enhance the visibility of internal growth lines under the optical 8 9 10 353 microscope. 11 12 354 The ‘half’ of fully encased specimens of C. squamulosa ampla that had not been used for 13 14 15 355 acetate-peel production was employed to make a petrological thin-section for use in 16 17 356 assessment of preservational status by cathodoluminescence (CL). Two specimens of A. 18 19 357 islandica not intended for ontogenetic isotopic sampling were also treated in this way. Thin- 20 21 22 358 sections were carbon-coated and investigated with a ‘cold’ cathode instrument built in-house 23 24 359 at the University of Cambridge (accelerating potential = 26kV; gun current = 500/700 mA; 25 26 27 360 vacuum = 0.01–0.05 Torr). The ‘half’ of one specimen of C. squamulosa ampla used for peel 28 29 361 production, together with an equivalent ‘half’ of one specimen of A. islandica, was re- 30 31 32 362 polished, re-etched and gold-coated for assessment of preservational status in the scanning 33 34 363 electron microscope (SEM). SEM investigations were also made of fractured, ultrasonically 35 36 364 cleaned and similarly coated specimens of 12 of the 16 species (no specimens of G. obovata, 37 38 39 365 P. tigerinum, C. squamulosa ampla and M. truncata were investigated in this way). Two 40 41 366 specimens of C. squamulosa ampla and one of S. arcuata that had undergone nothing more 42 43 44 367 than initial cleaning were used for assessment of preservational status by x-ray diffraction 45 46 368 (XRD) analysis. For C. squamulosa ampla the fairly large amount of powder required (c. 5 47 48 49 369 mg) was obtained by a combination of drilling and scraping; for the small species S. arcuata 50 51 370 the entire shell was crushed using a pestle and mortar. 52 53 54 371 55 56 372 2.5. Assessment of preservation 57 58 373 59 60 15 61 62 63 64 65 374 Shelled organisms exhibit taxon-specific microstructures (e.g., Bieler et al., 2014) which 1 2 375 are unlike abiogenic occurrences of the minerals concerned. The appearance of shell material 3 4 5 376 under the SEM can therefore be used to judge whether it is in the original state or altered. 6 7 377 Cathodoluminescence is blue in the case of aragonite and orange in calcite; hence, the 8 9 10 378 occurrence of orange luminescence in originally aragonitic material is evidence of alteration. 11 12 379 XRD spectra can be used likewise to detect alteration by comparison with calculated spectra 13 14 15 380 for aragonite and calcite. 16 17 381 18 19 382 2.6. Use of growth lines and microgrowth increments 20 21 22 383 23 24 384 The internal growth lines revealed in A. islandica and C. squamulosa ampla by sectioning, 25 26 27 385 and taken to be annual, were used to define annual increments, thus allowing samples from 28 29 386 cross-sections to be referred to a particular ontogenetic year (numbered by counting from the 30 31 32 387 origin of growth) and for the temporal resolution of sampling to be determined. 33 34 388 The microgrowth increments observable on the outer surface of A. opercularis (Fig. 4D) 35 36 389 represent intervals in the order of a day (Broom and Mason, 1978). Their size (anatomical 37 38 39 390 height as seen in 2D digital photographs) through ontogeny was measured using the bespoke 40 41 391 software Panopea© (2004, Peinl and Schöne) for comparison with the pattern of variation in 42 43 18 44 392 δ O. Specimens of A. opercularis from well-mixed settings typically show only high- 45 46 393 frequency (c. monthly), low-amplitude variation in microgrowth-increment size over the span 47 48 18 49 394 of an annual δ O cycle, but superimposed on this in specimens from seasonally stratified 50 51 395 settings is typically a low-frequency (c. annual), high-amplitude pattern (Johnson et al., 2009, 52 53 2017). The microgrowth-increment approach to interpreting hydrographic setting is a useful 54 396 55 13 56 397 supplement to the more established technique involving δ C (e.g., Arthur et al. 1983; Krantz 57 58 398 et al., 1987; Krantz, 1990), which is not infallible (Johnson et al., 2017). 59 60 16 61 62 63 64 65 399 1 2 400 2.7. Isotopic sampling 3 4 5 401 6 7 402 2.7.1. Ontogenetic sampling 8 9 10 403 With the exception of the brachiopod P. maxima, from which material was extracted from 11 12 404 the secondary (inner) layer because of concerns over possible ‘vital’ effects on isotopic 13 14 15 405 composition in the primary (outer) layer (Carpenter and Lohmann, 1995), ontogenetic 16 17 406 isotopic sampling was focused on the outer layer. However, this was either impossible 18 19 407 (because it had flaked away before resin-encasement) or inadvisable (because it had a spongy 20 21 22 408 appearance possibly reflecting chemical alteration) in specimens of C. squamulosa ampla, so 23 24 409 material was taken from the m+1 layer (middle layer outside the pallial myostracum; Bieler et 25 26 27 410 al., 2014). In A. islandica specimen 5 it was discovered retrospectively by examining the 28 29 411 sampling path that some samples had included inner-layer material (Fig. 4B), and a few were 30 31 32 412 largely or entirely from the inner layer. As the latter were in a sequence parallel, rather than 33 34 413 transverse, to annual growth lines they did not form a time series and the data from them has 35 36 414 consequently been discarded. 37 38 39 415 Sampling of shell cross-sections (A. islandica, C. squamulosa ampla) was conducted by a 40 41 416 mixture of drilling and milling (Schöne et al., 2005a). The former technique (employing a 42 43 44 417 vice-mounted hand drill) was used for annual increment 3 of A. islandica specimen BRS- 45 46 418 AJ1-CC-D1R and for sampling of all other sectioned shells (employing a computer- 47 48 49 419 controlled drill); sample spacing was about 1 mm. The latter technique (again employing a 50 51 420 vice-mounted drill, but equipped with a cylindrical bit) was used for whole-increment 52 53 sampling of annual increments 4–16 in BRS-AJ1-CC-D1R and for sub-sampling (9–17 steps 54 421 55 56 422 per increment) of increments 17–25. Sampling of the external surface (A. opercularis, L. 57 58 423 borealis, P. maxima) was by hand-held drill, used to cut shallow pits or grooves parallel to 59 60 17 61 62 63 64 65 424 commarginal striae (A. opercularis). In the case of P. maxima the thin primary layer was 1 2 425 drilled away before sampling the secondary layer. Sample weights were 50–200 μg. 3 4 5 426 6 7 427 2.7.2. Spot and whole-shell sampling 8 9 10 428 Spot samples were taken by drilling the external surface (outer layer) using a computer- 11 12 429 controlled drill. In most cases a single, randomly positioned sample was taken from a shell 13 14 15 430 (e.g., Fig. 3A, E, F). Where more were extracted, these were taken from points representing 16 17 431 different times in ontogeny (e.g., Fig. 3B). Sample weights were 50–200 μg. 18 19 432 Specimens for whole-shell sampling were less than 3 mm in size but in some cases 20 21 22 433 constituted fragments of somewhat larger individuals (e.g., Fig. 3H). Specimens were crushed 23 24 434 using a pestle and mortar. A 50–200 μg aliquot of the resultant powder was taken and the rest 25 26 27 435 discarded. 28 29 436 30 31 32 437 2.8. Isotopic analysis 33 34 438 35 36 439 Measurement of oxygen and carbon isotope ratios (reported as δ18O and δ13C; ‰) was 37 38 39 440 carried out in laboratories at Cambridge, Keyworth, Mainz and Frankfurt (data spanning 40 41 441 ontogenetic year 3 of A. islandica specimen BRS-CC-AJ1-D1R, previously reported in 42 43 44 442 Johnson et al., 2009). Analysis in Cambridge, Mainz and Frankfurt involved a Thermo 45 46 443 Finnigan MAT 253 continuous flow, isotope ratio mass spectrometer coupled to a Gasbench 47 48 49 444 II. Powder samples were dissolved with concentrated phosphoric acid in helium-flushed 50 51 445 borosilicate exetainers at 70 °C (Cambridge) or 72 °C (Mainz, Frankfurt). Analysis at 52 53 Keyworth involved an Isoprime dual inlet mass spectrometer coupled to a Multiprep system. 54 446 55 56 447 Powder samples were dissolved with concentrated phosphoric acid in borosilicate wheeton 57 58 448 vials at 90 °C. All laboratories calibrated their δ18O and δ13C data against NBS-19 and their 59 60 18 61 62 63 64 65 449 own Carrara marble standard. Internal precision (1σ) was < 0.1‰ for δ18O and δ13C. Isotope 1 2 450 values were calculated against the Vienna Pee Dee Belemnite (VPDB) and Craig-corrected 3 4 5 451 (McKinney et al., 1950). 6 7 452 8 9 10 453 2.9. Temperature calculation 11 12 454 13 14 15 455 To conform to previous work on modern, subfossil and Pliocene A. opercularis (Hickson 16 17 456 et al. 1999, 2000; Johnson et al., 2000, 2009; Valentine et al. 2011), temperatures were 18 19 457 derived from the δ18O of calcite from this and other species using the calcite-water 20 21 22 458 fractionation expression of O’Neil et al. (1969), as expanded about 16.9 °C by Shackleton 23 24 459 (1974) into the following equation: 25 26 27 460 28 29 18 18 18 18 2 461 T (°C) = 16.9 – 4.38 (δ Ocalcite – δ Owater) + 0.1 (δ Ocalcite – δ Owater) (1) 30 31 32 462 33 34 463 A linear regression fitted to the temperature and ‘1000 ln(alpha)’ data of O’Neil et al. 35 36 37 464 (1969) for values in the environmental range (temperatures ≤ 25 °C) has an intercept 38 39 465 (temperature = 142.2 °C) with a 2σ-error of 9.3 °C. Although this is large, the δ18O of 40 41 466 modern A. opercularis is close to that predicted from an alternative formulation of equation 42 43 18 44 467 (1), using measured values of temperature and water δ O for the area and time of shell 45 46 468 formation (Hickson et al., 1999). Negative offsets of c. 0.5–1.0‰ (c. 2–4 °C) in winter are 47 48 49 469 almost certainly due largely to ‘missing’ shell as a result of growth interruptions. Similar or 50 51 470 smaller negative summer offsets may reflect a tendency for temperatures to be slightly 52 53 54 471 overestimated using the O’Neil/Shackleton relationship. The more recent equation of Kim 55 56 472 and O’Neil (1997), similarly based on synthetic calcite but giving temperatures 1–2 °C lower 57 58 59 60 19 61 62 63 64 65 473 (Bemis et al., 1998, table 1), may therefore be a little more accurate for A. opercularis, and 1 2 474 by implication other calcitic species. 3 4 18 5 475 We subtracted 0.27‰ from our water δ O values (measured against SMOW) in order to 6 7 476 adjust them to the VPDB scale used for shell carbonate (Gonfiantini et al., 1995). This 8 9 10 477 adjustment was incorporated in a modified version of the aragonite equation of Grossman and 11 12 478 Ku (1986), based on data from foraminifers and mollusks, for calculation of temperatures 13 14 15 479 from aragonitic species: 16 17 480 18 19 481 T (°C) = 20.60 – 4.34 (δ18O – (δ18O – 0.27)) (2) 20 aragonite water 21 22 482 23 24 483 A linear regression fitted to the temperature and ‘shell δ18O – water δ18O’ data of 25 26 27 484 Grossman and Ku (1986) has an intercept (temperature = 20.3 °C) with a 2σ-error of 1.2 °C. 28 18 29 485 We calculated temperatures using the four initial values for water δ O employed by 30 31 32 486 Johnson et al. (2009): the most divergent global average estimates for the Pliocene from 33 34 487 deep-sea foraminiferal data (–0.5‰, –0.2‰; Buchardt and Símonarson, 2003) and the most 35 36 488 divergent site-specific estimates for the Coralline Crag from modelling (+0.1‰, +0.5‰; 37 38 39 489 Johnson et al., 2009, p. 172). Equation (1) is non-linear and with the highest value for water 40 41 490 δ18O (+0.5‰) yields a temperature 3.8 °C higher than with the lowest value for water δ18O (– 42 43 18 44 491 0.5‰) for the highest shell (calcite) δ O measured (+2.69‰), and 4.5 °C higher for the 45 46 492 lowest shell δ18O measured (–0.69‰). Equation (2) is linear and with the highest value for 47 48 18 18 49 493 water δ O yields a temperature 4.3 °C higher than with the lowest value for water δ O for 50 51 494 any value of shell (aragonite) δ18O. 52 53 Note that δ18O of shell aragonite was not corrected for different acid-fractionation factors 54 495 55 56 496 of aragonite and calcite (for further explanation, see Füllenbach et al., 2015). 57 58 497 59 60 20 61 62 63 64 65 498 3. Results 1 2 499 3 4 5 500 3.1. Outcomes of preservation tests 6 7 501 8 9 10 502 In all 13 taxa investigated by SEM there was revealed excellent preservation of both 11 12 503 calcite (foliated and fibrous) and aragonite (homogeneous, crossed lamellar, complex crossed 13 14 15 504 lamellar, composite prisms) microstructures (e.g., Fig. 5), which were the same as those from 16 17 505 related modern taxa (e.g., Williams, 1990; Bieler et al., 2014). A. islandica did not show 18 19 506 changes observed in the experimental diagenesis of that species (Casella et al., 2017). 20 21 22 507 Blocky, diagenetic calcite was rarely seen in any of the shells, or in microborings or 23 24 508 brachiopod punctae (Fig. 5B). These observations confirm the findings of earlier SEM work 25 26 27 509 on A. opercularis (Johnson et al., 2009). 28 29 510 One specimen of the aragonitic species C. squamulosa ampla (specimen 7) showed 30 31 32 511 patches of dull orange cathodoluminescence (Fig. 6B, C) amongst more widespread blue 33 34 512 luminescence, which reflects the original aragonite and was seen ubiquitously in the three 35 36 513 other specimens of C. squamulosa ampla investigated by CL (e.g., 8; Fig. 6A) and in the two 37 38 39 514 of A. islandica. The crossed-lamellar structure (developed only in aragonite) evident within 40 41 515 the patches (Fig. 6C) and their gradational boundaries (contrary to what would be expected of 42 43 44 516 areas containing diagenetic calcite crystals; J.A.D. Dickson, personal communication, 2017), 45 46 517 make it doubtful that the patches mark alteration (see also section 3.2.2). Manganese (the 47 48 49 518 cause of orange cathodoluminescence) is incorporated into the shell of aragonitic bivalves in 50 51 519 varying amounts during growth (e.g., Zhao et al., 2017), and it is conceivable that the patches 52 53 of orange luminescence seen in C. squamulosa ampla specimen 7 relate to times of 54 520 55 56 521 particularly high uptake. Certainly, cathodoluminescence in bivalves is affected by 57 58 522 environmental factors during life and must be interpreted with care (Barbin et al., 1991). 59 60 21 61 62 63 64 65 523 The mineralogy of C. squamulosa ampla specimen 7, as revealed by XRD, was 97% 1 2 524 aragonite (± 1–2%), the same as in the other C. squamulosa ampla specimen (specimen 1; 3 4 5 525 showing only blue luminescence) investigated by XRD. The investigated specimen of S. 6 7 526 arcuata was c. 99% aragonite. These results indicate minimal alteration. All the XRD spectra 8 9 10 527 are available as Supplementary Material B. 11 12 528 13 14 15 529 3.2. Ontogenetic isotope data 16 17 530 18 19 531 Where annual growth lines were identified (A. islandica, C. squamulosa ampla), isotope 20 21 22 532 values have been plotted against ontogenetic year, with numbered year-markers 23 24 533 corresponding to the positions of annual growth lines and representing the end of the year 25 26 27 534 concerned (Figs. 7, 8). Where independent evidence of age was unavailable (A. opercularis, 28 29 535 P. maxima, L. borealis), values have been plotted against linear distance from the origin of 30 31 32 536 growth, measured along the axis of maximum growth (Figs. 9–11). The range, mean and 33 34 537 standard deviation for summer δ18O minima and winter δ18O maxima in each specimen and 35 36 538 taxon are shown diagrammatically in Fig. 12A. Range, mean and standard deviation for δ13C 37 38 39 539 values are given in Fig. 12B. 40 41 540 42 43 44 541 3.2.1. A. islandica 45 46 542 In A. islandica specimen BRS-AJ1-CC-D1R (Fig. 7A), those years (3, 17–25) sampled at 47 48 18 49 543 high resolution show a well-defined annual cyclicity in δ O, with minima nearly always a 50 51 544 little after annual growth lines and maxima roughly half-way between. δ13C parallels δ18O in 52 53 some years (r up to 0.89) but at other times (e.g., over the end of year 21 and start of year 22) 54 545 55 18 56 546 is markedly out of phase (r = –0.58 for 15 samples between the δ O maxima for years 21 57 58 547 and 22). The low-resolution data for years 4–16 plot within the range of the high-resolution 59 60 22 61 62 63 64 65 548 data with the exception of δ13C values for years 6 and 8, which slightly exceed the maximum 1 2 549 in the high-resolution data. 3 4 5 550 A. islandica specimens 1, 2 and 5 (Fig. 7B–D), sampled at intermediate resolution, do not 6 7 551 show such a well-defined cyclicity in δ18O; nevertheless, the number of peaks and troughs 8 9 10 552 approximately corresponds to the number of years indicated by growth lines, at least for 11 12 553 specimens 1 and 2. Given the ‘noise’ in the δ18O profile from specimen 5 (possibly 13 14 15 554 accountable to inclusion of some inner-layer material; see section 2.7.1), it was considered 16 18 13 17 555 appropriate to recognise only one maximum and one minimum δ O value (both year 3). δ C 18 19 556 shows some parallels with δ18O in specimen 5 (r = 0.58 overall) but no real parallels in 20 21 22 557 specimens 1 and 2. 23 24 558 In BRS-AJ1-CC-D1R the mean minimum and maximum δ18O values are +1.76 ± 0.27‰ 25 26 27 559 (± 1σ) and +2.52 ± 0.17‰, respectively (Fig. 12A). The mean (or singleton) minima and 28 29 560 maxima for specimens 1, 2 and 5 range from +1.96 to +2.34‰ (mean +2.09 ± 0.17‰) and 30 31 32 561 +2.63 to +2.94‰ (mean +2.78 ± 0.13‰), respectively. There is a strong positive correlation 33 34 562 (r = 0.95) between mean individual minima and maxima. It is unlikely that the lower mean 35 36 563 minimum and maximum of BRS-AJ1-CC-D1R compared to the other specimens is due to 37 38 39 564 higher resolution sampling; this should yield a higher mean maximum through inclusion of 40 41 565 shell material with the most positive δ18O. 42 43 13 44 566 Mean individual values for δ C span a range of 0.73‰ (Fig. 12B). The species mean is 45 46 567 +2.91 ± 0.31‰. 47 48 49 568 50 51 569 3.2.2. C. squamulosa ampla 52 53 Although only sampled at intermediate resolution, all four specimens show a fairly well- 54 570 55 18 56 571 defined cyclicity in δ O, with maxima nearly always at annual growth lines (Fig. 8A-D). 57 58 572 Mean individual values for summer δ18O minima and winter δ18O maxima range from +1.35 59 60 23 61 62 63 64 65 573 to +2.08 ‰ (mean +1.82 ± 0.28‰) and +2.04 to +2.42‰ (mean +2.28 ± 0.15‰), 1 2 574 respectively (Fig. 12A). There is a strong positive correlation (r = 0.95) between mean 3 4 5 575 individual minima and maxima. The taxon mean for summer minima is slightly lower than 6 7 576 that from A. islandica specimens 1, 2 and 5 (likewise sampled at intermediate resolution) and 8 9 10 577 the taxon mean for winter maxima is more markedly lower than that from A. islandica 11 12 578 specimens 1, 2 and 5. The winter discrepancy may be accountable to the growth breaks 13 14 18 15 579 represented by growth lines at the positions of maximum δ O (i.e., failure to deposit material 16 18 17 580 with higher δ O). 18 19 581 δ13C shows weak parallels with δ18O in specimen 8 (r = 0.39 overall) but no real parallels 20 21 13 22 582 in specimens 1, 7 and 10. Mean individual values for δ C span a range of 0.71‰ (Fig. 12B). 23 24 583 The taxon mean for δ13C (+2.34 ± 0.28‰) is lower by 0.57‰ than that from A. islandica. 25 26 18 13 27 584 Both the δ O and δ C means from specimen 7 occupy a central position in the respective 28 29 585 ranges for this taxon, suggesting that the shell is unaltered, despite some indications from 30 31 32 586 cathodoluminescence to the contrary (section 3.1). 33 34 587 35 36 588 3.2.3. A. opercularis 37 38 18 39 589 Both specimens show one well-defined cycle in δ O incorporating a clear summer and 40 41 590 winter inflection (Fig. 9A, B). It is possible to recognise a second (later) summer minimum in 42 43 44 591 UD 53361 but ‘noise’ makes it difficult to define a second (earlier) winter maximum. 45 46 592 Although not associated with an inflection it is reasonable to take the high value at the start of 47 48 49 593 the profile from UD 53360 as a winter ‘maximum’ because it is substantially greater than the 50 51 594 later winter maximum and clearly not representative of noise. The species means for summer 52 53 minima (+0.44 ± 0.30‰) and winter maxima (+1.86 ± 0.02‰) are lower than the equivalents 54 595 55 56 596 from A. islandica and C. squamulosa ampla (Fig. 12A), and also lower than the more strictly 57 58 597 comparable (high resolution) values from A. islandica specimen BRS-AJ1-CC-D1R (section 59 60 24 61 62 63 64 65 598 3.2.1). A difference of 0.8‰ is to be expected because for a given temperature and water 1 2 599 δ18O the equilibrium calcite value is lower than the aragonite value by that amount (Kim et 3 4 5 600 al., 2007). However, the discrepancy for mean summer minimum is 1.32‰ with A. islandica 6 7 601 specimen BRS-AJ1-CC-D1R. This larger difference than expected may be accountable to 8 9 10 602 rapid growth in A. opercularis over summer intervals and sampling at exceptionally high 11 12 603 temporal resolution, resulting in inclusion of material with the most negative δ18O (note the 13 14 15 604 relatively broad summer sectors containing numerous sample points in Fig. 9A, B). 16 13 18 17 605 δ C shows some parallels with δ O in both UD 53360 and 53361 (r = 0.53 overall in 18 19 606 each) and mean individual values for δ13C are very similar (+0.39 ± 0.16‰ and +0.41 ± 20 21 22 607 0.37‰, respectively). The species mean (+0.40 ± 0.01‰; Fig. 12B) is lower than from A. 23 24 608 islandica and C. squamulosa ampla by 2.51‰ and 1.94‰, respectively. This is largely 25 26 13 27 609 accountable to mineralogical differences in that δ C in calcite is generally 1.4–2.0‰ lower 28 29 610 than in co-precipitated aragonite (Krantz et al., 1987). 30 31 32 611 33 34 612 3.2.4. P. maxima 35 36 613 Specimen 1 shows three well-defined cycles in δ18O (Fig. 10A). The mean values for 37 38 18 18 39 614 summer δ O minima and winter δ O maxima are +0.25 ± 0.49‰ and +1.46 ± 0.22‰, 40 41 615 respectively (Fig. 12A). Specimen 2 shows only an incomplete δ18O cycle but it is possible to 42 43 44 616 recognise a summer minimum and winter maximum. The species means for summer minima 45 46 617 (+0.58 ± 0.33‰) and winter maxima (+1.90 ± 0.44‰) are close to those from A. opercularis 47 48 49 618 (Fig. 12A) and similarly accountable to calcitic mineralogy and sampling at exceptionally 50 51 619 high temporal resolution over summer intervals (note the relatively broad summer sectors 52 53 containing numerous sample points in Fig. 10A). 54 620 55 13 18 56 621 δ C shows parallels with δ O in both specimen 1 (r = 0.83) and specimen 2 (r = 0.96) 57 58 622 but mean individual values for δ13C differ by 1.00‰ (Fig. 12B). The species mean (–0.34 ± 59 60 25 61 62 63 64 65 623 0.45‰) is substantially lower than that from the similarly calcitic A. opercularis, and 1 2 624 probably too far below the equivalent values from the aragonitic taxa A. islandica and C. 3 4 5 625 squamulosa ampla (differences of 3.25‰ and 2.68‰, respectively) to be accountable entirely 6 7 626 to mineralogy. 8 9 10 627 11 12 628 3.2.5. L. borealis 13 14 18 15 629 The δ O profile from L. borealis specimen 3 (Fig. 11A) may depict two annual cycles, 16 17 630 but the amount of variation is small and might reflect ‘noise’. In view of the likely growth 18 19 631 rate (see section 3.2), the two minima and maxima have been taken to represent summers and 20 21 22 632 winters. The ‘summer’ mean (+1.79 ± 0.07‰) is within the ranges of summer means from 23 24 633 specimens of the similarly aragonitic A. islandica and C. squamulosa ampla sampled at 25 26 27 634 intermediate resolution (Fig. 12A). The ‘winter’ mean (+2.16 ± 0.00‰) is likewise within the 28 29 635 winter range from C. squamulosa ampla but substantially below the lowest mean from A. 30 31 32 636 islandica. 33 34 637 δ13C shows no parallels with δ18O. The mean value for L. borealis (–0.01 ± 0.45‰) is far 35 36 638 below the ranges of individual means for the similarly aragonitic A. islandica and C. 37 38 39 639 squamulosa ampla (Fig. 12B). This may reflect reducing conditions in the sediment at the 40 41 640 sites occupied by this chemosymbiotic species; these would have caused porewater δ13C to be 42 43 44 641 particularly low (Presley and Kaplan, 1970; Claypool and Threlkeld, 1983). 45 46 642 47 48 49 643 3.3. Microgrowth-increment data 50 51 644 52 53 The microgrowth increment profiles from A. opercularis specimens UD 53360 and 53361 54 645 55 18 56 646 quite closely parallel the respective profiles for δ O and thus show an approximately annual 57 58 59 60 26 61 62 63 64 65 647 fluctuation (Fig. 9A, B). The amplitude of this variation is fairly high in the former: over 0.3 1 2 648 mm between maximum and minimum of the smoothed profile. 3 4 5 649 6 7 650 3.4. Temperatures from ontogenetic δ18O data 8 9 10 651 11 12 652 The four values for water δ18O given and explained in section 2.9 have been used with 13 14 15 653 equations (1) and (2), as appropriate, to generate temperature profiles (Figs. 7E–H; 8E–H; 16 18 17 654 9C, D; 10C, D; 11B) from the shell δ O data. As can be seen (e.g., Figs. 7F, G; 9D; 10D), 18 19 655 the lowest value for water δ18O (–0.5‰; a global average estimate) yields a number of 20 21 22 656 temperatures under 5 °C, which is at odds with benthic ostracod (Wood et al., 1993) and 23 24 657 bryozoan (Taylor and Taylor, 2012) assemblage evidence that winter temperatures in the 25 26 27 658 Coralline Crag sea were at least as high as in the modern North Sea adjacent to the collection 28 29 659 locations of the specimens (typical winter minimum, 6–7 °C). For this reason, temperatures 30 31 18 32 660 generated using –0.5‰ for the δ O of ambient water are not considered credible (the same 33 34 661 applies to temperatures down to 3.6 °C from A. islandica specimens 1, 2 and 5, calculated 35 36 18 18 37 662 using a water δ O value of –0.35‰; see section 1). Amongst the other water δ O values, we 38 39 663 favour the two highest values because these were generated by modelling for the specific area 40 41 42 664 concerned, and of these we prefer +0.1‰ because it is intermediate between the other 43 44 665 modelled local estimate (+0.5‰) and an alternative global estimate (–0.2‰). Temperature 45 46 666 profiles calculated using a water value of +0.1‰ are indicated by use of a thicker (orange) 47 48 49 667 line in Figures 7–11 to signify our preference. Temperatures discussed in the following are 50 51 668 from these data, except where indicated otherwise. The range, mean and standard deviation 52 53 54 669 for summer temperature maxima and winter temperature minima in each specimen and taxon, 55 56 670 based on a water δ18O value of +0.1‰, are shown diagrammatically in Fig. 13. 57 58 59 671 60 27 61 62 63 64 65 672 3.4.1. A. islandica 1 2 673 The temperature profiles from all four specimens (Fig. 7E–H) show large sectors below 10 3 4 5 674 °C (the upper boundary for winter temperature in a cool temperate marine climate), and the 6 7 675 individual means for winter temperature minima (Fig. 13) are well below this figure. None of 8 9 10 676 the specimens show temperatures above 20 °C (the upper boundary for summer temperature 11 12 677 in a cool temperate marine climate) and the individual means for summer temperature 13 14 15 678 maxima are far below this figure (below 15 °C). The overall mean temperature (mean of all 16 17 679 the temperatures from the four profiles combined) is 10.2 ± 1.5 °C (± 1σ). The positive 18 19 18 20 680 correlation noted between individual mean δ O minima and maxima (section 3.2.1) indicates 21 22 681 that individuals lived under differing overall temperature regimes (slightly warmer/cooler). 23 24 25 682 For years sampled at high resolution in BRS-AJ1-CC-D1R the mean temperature (average of 26 27 683 all data) is 10.4 ± 1.3 °C, close to the figure of 11.1 ± 0.8 °C for the mean of temperatures 28 29 30 684 derived from low-resolution (annual) sampling of other years. 31 32 685 Using a water δ18O value of +0.5‰ still yields temperatures below 10 °C from all shells 33 34 35 686 except specimen 5, and no shell yields a temperature above 20 °C. 36 37 687 The temperatures obtained from Coralline Crag A. islandica are very largely consistent 38 39 40 688 with the cool temperate occurrence of modern representatives (section 2.1). 41 42 689 43 44 3.4.2. C. squamulosa ampla 45 690 46 47 691 The temperature profiles from all four specimens show sectors below 10 °C (Fig. 8E–H), 48 49 50 692 but the proportions below are smaller than in A. islandica. Two of the individual means for 51 52 693 winter temperature minima (Fig. 13) are below 10 °C, but the amounts below are smaller than 53 54 55 694 in A. islandica sampled at the same (intermediate) resolution. None of the specimens show 56 57 695 temperatures above 20 °C and the individual means for summer temperature maxima are far 58 59 60 28 61 62 63 64 65 696 below this figure (below 15 °C). The overall mean temperature is 11.0 ± 1.4 °C. The positive 1 2 18 697 correlation noted between individual mean δ O minima and maxima (section 3.2.2) indicates 3 4 5 698 that individuals lived under differing overall temperature regimes (slightly warmer/cooler). 6 7 699 Using a water δ18O value of +0.5‰, only specimen 1 yields a temperature below 10 °C 8 9 10 700 but none of the shells yields a temperature above 20 °C. 11 12 701 The divergence in winter temperature data from those supplied by A. islandica may be 13 14 15 702 accountable to winter growth breaks signified by growth lines (section 3.2.2), i.e., the C. 16 17 703 squamulosa ampla specimens may have experienced (but not recorded) much the same 18 19 20 704 winter minimum temperatures as the A. islandica specimens. Even if the winter minimum 21 22 705 temperatures experienced by the C. squamulosa ampla specimens were slightly higher, at 23 24 18 25 706 least some of those calculated with a water δ O of +0.1‰ are in the cool temperate range. 26 27 707 The occurrence of winter rather than summer growth breaks is consistent with a cool 28 29 708 temperate interpretation (Jones and Quitmyer, 1996). This contrasts with the warm temperate 30 31 32 709 marine climate associated with the close modern relative C. antiquatus (Fig. 3D; section 2.1). 33 34 710 35 36 37 711 3.4.3. A. opercularis 38 39 712 The temperature profiles from both specimens (Fig. 9C, D) show sectors below 10 °C, 40 41 42 713 and the individual mean/singleton values for winter temperature minima (Fig. 13) are well 43 44 714 below this figure and similar to values from A. islandica sampled at both high and 45 46 47 715 intermediate resolution. Neither of the specimens shows temperatures above 20 °C and the 48 49 716 individual mean/singleton values for summer temperature maxima are far below this figure. 50 51 52 717 The overall mean temperature is 11.3 ± 2.2 °C. 53 54 718 Johnson et al. (2009, table 3) provided figures for the extreme minimum and maximum 55 56 18 57 719 temperatures from δ O profiles of eight other A. opercularis shells from the Ramsholt 58 59 720 Member of the Coralline Crag Formation. For a water δ18O of +0.1‰ the respective means 60 29 61 62 63 64 65 721 are 7.7 ± 0.9 °C and 14.4 ± 1.4 °C. These figures are indistinguishable from the equivalent 1 2 3 722 means of 8.1 ± 0.3 °C and 14.4 ± 1.1 °C from the two ontogenetically sampled Ramsholt 4 5 723 Member shells of the present study. 6 7 18 724 Using a water δ O value of +0.5‰ still yields temperatures below 10 °C from the two 8 9 10 725 shells analysed in the present study, and neither yields a temperature above 20 °C. 11 12 13 726 The temperatures obtained from Coralline Crag A. opercularis are in the cool temperate 14 15 727 range and entirely consistent with the cool (and warm) temperate occurrence of modern 16 17 728 representatives (section 2.1). 18 19 20 729 21 22 730 3.4.4. P. maxima 23 24 25 731 The temperature profiles from both specimens (Fig. 10C, D) show sectors below 10 °C. 26 27 732 The individual mean for winter temperature minima from specimen 1 is 10 ± 0.9°C and the 28 29 30 733 singleton value from specimen 2 is 6.5 °C, the former value being a little higher than the 31 32 734 mean minima from other specimens sampled at high resolution (A. islandica BRS-AJ1-CC- 33 34 35 735 D1R and both specimens of A. opercularis; Fig. 13). Neither of the specimens shows 36 37 736 temperatures above 20 °C and the individual mean/singleton values for summer temperature 38 39 40 737 maxima are far below this figure. The overall mean temperature is 12.4 ± 2.8 °C. 41 42 18 43 738 Using a water δ O value of +0.5‰ still yields temperatures below 10 °C from specimen 2 44 45 739 and neither specimen yields a temperature above 20 °C. 46 47 48 740 The temperatures obtained from P. maxima are lower than expected from the occurrence 49 50 741 of fossil relatives. 51 52 742 53 54 55 743 3.4.5. L. borealis 56 57 58 59 60 30 61 62 63 64 65 744 The temperature profile from the specimen (Fig. 11B) does not show any sectors below 10 1 2 745 °C. The individual means for winter temperature minima and summer temperature maxima 3 4 5 746 Fig. 13) are 10.5 °C and 12.1 °C, respectively. Both are within the ranges of individual means 6 7 747 from C. squamulosa ampla (similarly sampled at intermediate resolution). The overall mean 8 9 10 748 temperature is 11.4 ± 0.7 °C. 11 12 18 13 749 Using a water δ O value of +0.5‰ does not yield any temperatures above 20 °C. 14 15 750 The mean for winter temperature minima is slightly above the cool temperate range but 16 17 751 this could easily reflect insufficiently close sampling to detect cooler temperatures. The 18 19 20 752 temperatures obtained are entirely consistent with the warm and cool temperate occurrence of 21 22 753 modern L. borealis (section 2.1). 23 24 25 754 26 27 755 3.5. Temperatures from spot and whole-shell δ18O data 28 29 30 756 31 32 757 Since, unlike the ontogenetic data, the spot and whole-shell δ18O data cannot be 33 34 758 meaningfully and usefully compared with ontogenetic δ13C and microgrowth-increment data, 35 36 37 759 there is no merit in presenting them in their own right. Instead, they are given translated into 38 39 760 temperatures by the methods employed for ontogenetic data (see section 2.9), using the same 40 41 18 42 761 four water δ O values. Figure 14 shows the 81 sets of four temperatures obtained, arranged 43 44 762 in order of increasing temperature to the right within taxa, with the number of specimens 45 46 47 763 indicated in each case. Included are markers of mean spot/whole-shell temperature for each 48 49 764 taxon based on a water δ18O of +0.1‰, markers of overall mean, mean summer maximum 50 51 and mean winter minimum temperature from ontogenetic data (using a water δ18O value of 52 765 53 54 766 +0.1‰), and markers of average summer maximum and average winter minimum 55 56 767 temperature (surface and seafloor) in the well-mixed southern North Sea at present. 57 58 59 60 31 61 62 63 64 65 768 Temperatures quoted in the following subsections are based on a water δ18O of +0.1‰, 1 2 769 except where stated otherwise. 3 4 5 770 6 7 771 3.5.1. Temperatures from spot data 8 9 10 772 For taxa that were also investigated ontogenetically (A. opercularis, C. squamulosa ampla, 11 12 773 L. borealis, A. islandica, P. maxima), ‘spot’ temperatures fall between the relevant mean 13 14 15 774 summer maximum and mean winter minimum from ontogenetic data in 14 of the 23 cases. 16 17 775 These instances provide confirmation that the ontogenetically analysed specimens of the taxa 18 19 776 concerned (all but A. islandica) are representative of those taxa in terms of the temperatures 20 21 22 777 at which they lived. Of the remaining nine spot temperatures, one from A. opercularis is 23 24 778 above the mean summer maximum from ontogenetic data, but so far above this value (which 25 26 27 779 is corroborated by previously published results; see section 3.4.3), and also the other nine 28 29 780 spot temperatures from A. opercularis, that it must be considered a suspect datum, possibly a 30 31 32 781 product of contamination in sampling. The eight spot temperatures below the relevant mean 33 34 782 winter minimum are constituted by single cases in each of A. opercularis, C. squamulosa 35 36 783 ampla and L. borealis, and all of the five cases in A. islandica. Four of the A. islandica 37 38 39 784 instances are below the lowest mean individual winter minimum (7.1 °C; specimen 1) and 40 41 42 785 two (5.4, 6.0 °C) are below the lowest single winter minimum (6.4 °C; specimen 1) recorded 43 44 786 amongst the ontogenetically analysed specimens. This suggests that the latter may be 45 46 787 unrepresentative—i.e., they lived under slightly warmer conditions than was typical for 47 48 49 788 Coralline Crag A. islandica. The same conclusion would apply for comparisons of 50 51 789 temperatures derived using other water δ18O values. 52 53 54 790 The taxon means for spot temperatures from the other eight species investigated in this 55 56 791 way range from 5.8 °C to 12.5 °C, with a grand mean (mean of means) of 9.7 ± 2.4 °C. These 57 58 59 792 figures compare closely with the equivalents (6.5–11.9 °C; 9.6 ± 1.8 °C) from the five species 60 32 61 62 63 64 65 793 that were also investigated ontogenetically, suggesting that these are representative of the 1 2 794 Coralline Crag biota as a whole in terms of the range of temperatures at which taxa lived. 3 4 5 795 Of the eight species investigated through spot sampling alone, only the ‘warm’ species T. 6 7 796 multistriata yields temperatures at odds with the relevant ‘warm’, ‘cool’ or ‘eurythermal’ 8 9 10 797 designation. Of the five values from T. multistriata, one from each of the two specimens 11 12 798 investigated (5.5, 6.1 °C) is well below the lower threshold of 10 °C for the warm temperate 13 14 15 799 marine climate with which T. multistriata is now associated (section 2.1), and indeed below 16 17 800 the average winter minimum temperature in the southern North Sea at present (Fig. 14). The 18 19 801 equivalent temperatures calculated using a water δ18O value of +0.5‰ (respectively, 7.0, 7.6 20 21 22 802 °C) are still substantially below the 10 °C threshold. 23 24 803 25 26 27 804 3.5.2. Temperatures from whole-shell data 28 29 805 The temperature ranges from whole-shell sampling of C. corbis, R. conuloidea and R. 30 31 32 806 buccinea are, respectively, 8.8–12.7 °C, 7.2–10.1 °C and 10.5–13.4 °C. The combined range 33 34 807 (7.2–13.4 °C) extends to values a little above the grand mean (mean of taxon means) for 35 36 808 summer maximum temperature from ontogenetic sampling (12.6 ± 1.2 °C) and extends rather 37 38 39 809 more below the grand mean for winter minimum temperature from ontogenetic sampling (9.1 40 41 810 ± 1.0 °C). However, the range of temperatures from whole-shell sampling does not exceed 42 43 44 811 the upper limit of taxon means for summer maximum temperature from ontogenetic sampling 45 46 812 (highest value 14.3 °C, from A. opercularis), although it extends below the lower limit of 47 48 49 813 taxon means for winter minimum temperature from ontogenetic sampling (lowest value 8.1 50 51 814 °C, from A. islandica). 52 53 54 815 The mean temperatures from whole-shell sampling of C. corbis, R. conuloidea and R. 55 56 816 buccinea are, respectively, 10.5 ± 1.4 °C, 8.6 ± 1.1 °C and 11.6 ± 0.9 °C. These mean 57 58 817 temperatures do not exceed the upper limit of taxon means (using all data points) from 59 60 33 61 62 63 64 65 818 ontogenetic sampling (highest value 12.4 °C, from P. maxima) but extend below the lower 1 2 819 limit (10.2 °C, from A. islandica). 3 4 5 820 The whole-shell data in part support the conclusion from spot data that the ontogenetically 6 7 821 sampled taxa are representative of the Coralline Crag biota as a whole, but conflicting 8 9 10 822 evidence is provided by individual and taxon mean temperatures from whole-shell sampling 11 12 823 that are below the taxon means for winter minimum (and overall mean) temperature from 13 14 15 824 ontogenetic sampling. This could partly reflect the nature of the data (the taxon means for 16 17 825 winter minimum temperature are averages of averages and hence conservative estimates) or 18 19 826 be a consequence of growth only during winter in the taxa used for whole-shell sampling (the 20 21 22 827 individuals sampled were small enough to have lived only a few months—complete shells < 23 24 828 5 mm; Fig. 3H) 25 26 27 829 Of the three species investigated through whole-shell sampling, R. conuloidea and R. 28 29 830 buccinea give temperatures consistent with their designation as ‘cool’ and ‘warm’ species, 30 31 32 831 respectively. However, C. corbis gives temperatures at odds with its designation as a ‘warm’ 33 34 832 species. Of the nine temperatures from C. corbis, four (8.8, 9.1, 9.2, 9.6 °C) are below the 35 36 833 lower threshold of 10 °C for the warm temperate marine climate with which this species is 37 38 18 39 834 now associated (section 2.1). The equivalent temperatures calculated using a water δ O 40 41 835 value of +0.5‰ (10.5, 10.8, 11.0, 11.3 °C, respectively) are, however, above the 10 °C 42 43 44 836 threshold. 45 46 837 47 48 49 838 4. Discussion 50 51 839 52 53 4.1. Relative temperatures 54 840 55 56 841 57 58 59 60 34 61 62 63 64 65 842 Although resolution was not uniform amongst the five taxa sampled ontogenetically, this 1 2 843 is unlikely to have significantly biased overall taxon mean temperatures, making comparisons 3 4 18 5 844 involving this parameter valid. Using, as previously, a common value for water δ O 6 7 845 (+0.1‰), the overall taxon mean for A. islandica (10.2 ± 1.5 °C) is lower than for C. 8 9 10 846 squamulosa ampla (11.0 ± 1.4 °C) and P. maxima (12.4 ± 2.8 °C). This is in accordance with 11 12 847 the notion of the first as a ‘cool’ taxon and the last two as ‘warm’ taxa, and with the idea of 13 14 15 848 fluctuating marine climate during deposition of the Coralline Crag. The means of spot data 16 17 849 show a similar pattern (i.e., lower for A. islandica than for C. squamulosa ampla and P. 18 19 20 850 maxima; Fig. 14), as do the taxon means for seasonal temperatures (Figs. 13, 14). However, 21 22 851 in the latter case sampling resolution is a potential biasing factor so comparisons must be 23 24 25 852 made more carefully. The (restricted) taxon means for winter minimum and summer 26 27 853 maximum temperature from the three specimens of A. islandica sampled at intermediate 28 29 854 resolution (7.8 ± 0.5 C and 10.8 ± 0.8 C, respectively) are lower than the equivalent means 30 ° ° 31 32 855 from C. squamulosa ampla (9.9 ± 0.6 °C and 12.0 ± 1.2 °C, respectively), likewise sampled at 33 34 35 856 intermediate resolution. Similarly, the individual mean for summer maximum temperature 36 37 857 from the A. islandica specimen sampled at high resolution (12.2 ± 1.2 °C) is lower than the 38 39 40 858 taxon mean from P. maxima (13.7 ± 1.4 °C), likewise sampled at high resolution. While the 41 42 859 individual mean for winter minimum temperature from the A. islandica specimen (8.9 ± 0.7 43 44 45 860 °C) is higher than the taxon mean from P. maxima (8.3 ± 1.8 °C), it is lower than the 46 47 861 individual mean from P. maxima specimen 1 (10.0 ± 0.9 °C). Thus the pattern in the seasonal 48 49 50 862 temperature data largely matches, and therefore to the same extent corroborates, the pattern 51 52 863 seen within the data for overall taxon mean temperature (and amongst spot-temperature 53 54 55 864 means). 56 57 58 59 60 35 61 62 63 64 65 865 Irrespective of their statistical significance, there are only small differences between the 1 2 866 values from ‘cool’ and ‘warm’ species for overall taxon mean temperature from ontogenetic 3 4 5 867 data. Unsurprisingly, the range of variation (2.2 °C) is not expanded by inclusion of values 6 7 868 from the ‘eurythermal’ species A. opercularis (11.3 ± 2.2 °C) and L. borealis (11.4 ± 0.7 °C). 8 9 10 869 At most, therefore, we can only infer small fluctuations in marine climate from these data. As 11 12 870 we have noted, within both A. islandica and C. squamulosa ampla there is covariation 13 14 18 15 871 between individual mean δ O minima and maxima, indicating that individuals of each of 16 17 872 these taxa lived under slightly different overall temperature regimes. All species can, of 18 19 20 873 course, tolerate some variation in mean conditions so such individual differences are entirely 21 22 874 to be expected. We can use the most extreme individual data to gain a broader picture of 23 24 25 875 temperature variation during Coralline Crag deposition. The lowest mean temperature from 26 27 876 an A. islandica individual (average of all data) is from specimen 1 (8.3 ± 1.0 °C) and the 28 29 30 877 highest from C. squamulosa ampla from specimen 8 (12.4 ± 1.4 °C). However, the value 31 32 878 from the latter is exceeded by that from P. maxima specimen 1 (13.3 ± 2.3 °C). We might 33 34 35 879 therefore infer that over intervals similar to individual lifespans average annual seafloor 36 37 880 temperature was sometimes as much as 5 °C higher than over other such intervals during 38 39 40 881 Coralline Crag deposition. Since we do not know the precise stratigraphic horizon of 41 42 882 specimens we can say little about the separation in time of these inferred warmer and cooler 43 44 45 883 phases. It might have been only slightly more than the lifespan of individuals or approaching 46 47 884 the total time taken for deposition of the Coralline Crag. This period, as already noted, was 48 49 50 885 probably long enough to include glacial and interglacial stages, and over a comparable 51 52 886 timespan water-temperature changes at shallow–intermediate depths in the order of 5 °C have 53 54 55 887 been documented during the Pliocene from mid-latitude sites in the North Atlantic (De 56 57 888 Schepper et al., 2013; Bachem et al., 2016), while similar changes in continental air 58 59 60 36 61 62 63 64 65 889 temperature have been determined from the Netherlands (Dearing Crampton-Flood et al., 1 2 890 2018). It is therefore not unreasonable to explain the variation in individual mean 3 4 5 891 temperatures from ontogenetically analysed specimens in terms of fluctuation in marine 6 7 892 climate. The range in these data (8.3–13.3 °C) spans most of the equivalent spot and whole- 8 9 10 893 shell temperatures (Fig. 14), and at least some of those temperatures outside it can be 11 12 894 interpreted as representing seasonal extremes (or possibly sample contamination; see section 13 14 15 895 3.5.1). However, the preponderance of values below the lower bound, including some from 16 17 896 whole-shell samples (R. conuloidea), suggests that the range of variation in annual average 18 19 20 897 temperature extended to values somewhat lower than indicated by the ontogenetically 21 22 898 analysed specimens, although probably not to values so low as to be beyond interpretation in 23 24 899 terms of changes in marine climate between interglacial and glacial stages. 25 26 27 900 To this point we have focussed on the differences in temperature indicated by species, and 28 29 901 individuals within species. However, it is important to recognise and explain similarities. For 30 31 32 902 instance, while above we have identified ontogenetically analysed specimens of A. islandica 33 34 903 and C. squamulosa ampla giving mean temperatures which differ substantially (by 4.1 °C), 35 36 37 904 all three of the other ontogenetically analysed specimens of C. squamulosa ampla give mean 38 39 905 temperatures (1: 10.3 ± 0.9 °C; 7: 10.4 ± 1.0 °C; 10: 10.7 ± 0.9 °C) which are 40 41 42 906 indistinguishable from the mean temperature (10.4 ± 1.3 °C) given by A. islandica BRS-AJ1- 43 44 907 CC-D1R. As noted previously (section 3.4.2), the association of growth lines/breaks with 45 46 18 47 908 δ O maxima in C. squamulosa ampla suggests that individuals experienced winter 48 49 909 temperatures lower than those recorded isotopically. The mean temperatures experienced 50 51 52 910 may therefore likewise have been lower than those determined from the isotopic data and 53 54 911 thus similar to the taxon mean for A. islandica. This would be contrary to the indication of 55 56 57 912 temperature tolerance supplied by the restriction of the close modern relative C. antiquatus to 58 59 913 the warm temperate setting of the Mediterranean, where A. islandica is absent. However, it is 60 37 61 62 63 64 65 914 entirely consistent with the observation of Long and Zalasiewicz (2011, p. 59), which we can 1 2 915 confirm from our own fieldwork, that examples of C. squamulosa ampla and A. islandica 3 4 5 916 occur abundantly alongside each other at certain horizons in the Coralline Crag. 6 7 917 The spot and whole-shell data provide evidence that other Coralline Crag taxa were living 8 9 10 918 at temperatures different from modern representatives or close relatives. We have already 11 12 919 noted that temperatures calculated with a water δ18O of +0.1‰ from T. multistriata and C. 13 14 15 920 corbis are in some cases below the lower limit for the warm temperate marine climate with 16 17 921 which modern representatives of these species are associated (see, respectively, sections 18 19 922 3.5.1, 3.5.2). Continuing for the present with consideration of relative temperatures, we find 20 21 22 923 that amongst the 11 taxa investigated only through spot and whole-shell sampling the mean 23 24 924 of taxon means (grand mean) from the three ‘warm’ taxa (T. multistriata, C. corbis, R. 25 26 27 925 buccinea) is, as expected, higher (10.6 ± 0.7 °C) than that (9.2 ± 1.8 °C ) from the five ‘cool’ 28 29 926 taxa (G. obovata, A. gracilis, A. omalii, M. truncata, R. conuloidea), However, the mean 30 31 32 927 from G. obovata (11.9 ± 2.8 °C) is higher than the grand mean from warm taxa, and remains 33 34 928 higher (10.7 ± 0.9 °C) even if a suspiciously high temperature value (17.8 °C; see Fig. 14) is 35 36 37 929 excluded. Therefore, this appears to be another case where the temperature tolerance of a 38 39 930 close modern relative (G. glycymeris) provides an inaccurate indication of the temperature at 40 41 42 931 which a Coralline Crag species lived. However, it is possible that the equation of Grossman 43 44 932 and Ku (1986) provides an overestimate of temperature from G. obovata (Royer et al., 2013). 45 46 47 933 48 49 934 4.2. Absolute temperatures 50 51 52 935 53 54 936 In the previous subsection we discussed whether temperatures calculated using a water 55 56 937 δ18O value of +0.1‰ conform with expectation in a relative sense—e.g., whether ‘warm’ taxa 57 58 59 938 give higher temperatures than ‘cool’. We can investigate the values themselves by checking 60 38 61 62 63 64 65 939 whether those from Coralline Crag A. islandica are as expected from the detailed information 1 2 940 available on modern representatives of the species, specifically the fact that in the North Sea 3 4 5 941 A. islandica is limited to areas where seafloor temperature does not exceed 16 °C (Witbaard 6 7 942 and Bergman, 2003). Of the 22 summer temperatures recorded from Coralline Crag A. 8 9 10 943 islandica the highest (from ontogenetic year 22 of BRS-AJ1-CC-D1R, which the individual 11 12 944 lived two years beyond) is 15.1 °C. The equivalent temperature calculated using a water δ18O 13 14 15 945 value of +0.5‰ is 16.8 °C—i.e., above what appears to be tolerable by A. islandica. This 16 17 946 strongly suggests that the water δ18O value of +0.1‰ identified at the outset as the most 18 19 20 947 reasonable is indeed a good approximation for Coralline Crag seawater. We can therefore 21 22 948 regard all temperatures calculated using this value as ‘correct’, subject to the accuracy of the 23 24 949 transfer functions used. As indicated in section 2.9, the errors on these are 1-2 °C for the 25 26 27 950 temperature range of interest, with a tendency for the calcite function to overestimate 28 29 951 temperature. The temperatures well below 10 C recorded from numerous examples of the 30 ° 31 32 952 calcitic species A. opercularis (see section 3.4.3), together with specimens of P. maxima, P. 33 34 953 tigerinum and T. multistriata (Figs. 13, 14), therefore seem entirely plausible, and a 35 36 37 954 difference in temperature tolerance between modern and Coralline Crag examples of T. 38 39 955 multistriata can hardly be denied. It remains possible that the relatively small ‘discrepancies’ 40 41 42 956 in the temperature data from the aragonitic taxa C. squamulosa ampla, C. corbis and G. 43 44 957 obovata are a reflection of error in the transfer function. One other firm and important point 45 46 47 958 can, however, be made. This relates to the temperature supposedly indicated by the Coralline 48 49 959 Crag bryozoan Metrarabdotos moniliferum on the basis of the restriction of modern 50 51 52 960 congeners to areas where seafloor temperature never falls below 16 °C (Cheetham, 1967; 53 54 961 Taylor and Taylor, 2012). All 16 of the taxa investigated isotopically give temperatures 55 56 18 57 962 below this for a water δ O value of not only +0.1‰ but also +0.5‰ (Figs. 13, 14). To 58 59 963 achieve a temperature of 16 °C from the high values of shell δ18O recorded by many species 60 39 61 62 63 64 65 964 (e.g., values above +1.75‰ from the calcite of all 10 Ramsholt-Member A. opercularis 1 2 965 specimens ontogenetically sampled to date) would require a water value in excess of +2‰. 3 4 18 5 966 This is implausible given that global average water δ O is probably higher now than in the 6 7 967 Pliocene (due to larger ice volume), yet even in the strongly evaporated setting of the 8 9 18 10 968 Mediterranean Sea maximum water δ O is only +1.68‰ (Pierre, 1999). M. monoliferum 11 12 969 occurs throughout the Coralline Crag (Taylor and Taylor, 2012) and it is inconceivable that it 13 14 15 970 did not co-exist with any of the isotopically analysed species, some of which (e.g., A. 16 17 971 opercularis) are similarly ubiquitous. It therefore seems clear that the tropical/subtropical 18 19 972 present-day occurrence of Metrarabdotos is a false guide to the temperature tolerance of 20 21 22 973 Pliocene M. monoliferum, perhaps reflecting a cold-induced contraction of range in the 23 24 974 Pleistocene from which the genus has not yet recovered (cf. Taylor and Taylor, 2012). 25 26 27 975 28 29 976 4.3. Sea-surface temperatures 30 31 32 977 33 34 978 In the previous subsection we noted that all the investigated taxa provide isotopic 35 36 979 temperatures below 16 °C with a water δ18O value of +0.1‰. It is equally important to note 37 38 39 980 that on the same basis very few provide temperatures above 16 °C, and amongst those that 40 41 981 do, the values attained are never more than 20 °C. Amongst the 13 ontogenetically sampled 42 43 44 982 specimens only P. maxima 1 provides temperatures (maximum 17.6 °C) above 16 °C, while 45 46 983 amongst the 70 specimens investigated through spot and whole-shell samples just one of A. 47 48 49 984 opercularis and one of G. obovata provide similar values (respectively, 19.2 and 17.8 °C; 50 51 985 Fig. 14). The G. obovata datum is as aberrant as the A. opercularis one (see section 3.5.1), 52 53 54 986 and so may likewise reflect contamination. 55 56 987 Of the eight Ramsholt-Member A. opercularis ontogenetically sampled by Johnson et al. 57 58 59 988 (2009), one provided a temperature (16.2 °C) fractionally above 16 °C. On the basis of the 60 40 61 62 63 64 65 989 microgrowth-increment patterns from these A. opercularis specimens, together with biotic 1 2 990 and sedimentological evidence, Johnson et al. (2009) concluded that the Ramsholt-Member 3 4 5 991 sea was fairly deep (c. 50 m) and incompletely mixed in summer (i.e., thermally stratified). 6 7 992 This interpretation would account for the modest summer seafloor temperatures in 8 9 10 993 comparison to the surface temperatures of more than 20 °C indicated by pelagic 11 12 994 dinoflagellate assemblages (Head, 1997, 1998). The A. opercularis specimens used in the 13 14 15 995 present study show an approximately annual pattern of increment-size variation. similar to 16 17 996 the specimens investigated previously, with the range of variation being over 0.3 mm in one 18 19 20 997 (see section 3.3). Comparable ranges were recorded by Johnson et al. (2009, table 2) in three 21 22 998 of five modern specimens from a 50 m-deep, seasonally stratified setting in the Gulf of Tunis 23 24 999 (Mediterranean Sea) but in none of 14 specimens from well-mixed settings, as judged by 25 26 27 1000 being less than 25 m deep and/or meso- to macrotidal (hence subject to significant tidal 28 29 1001 currents). As well as the microgrowth-increment evidence of stratification from the present A. 30 31 32 1002 opercularis specimens, these and ontogenetically analysed specimens of A. islandica and P. 33 34 1003 maxima provide supporting evidence in the form of parallel variation of δ13C with δ18O (see 35 36 37 1004 sections 3.2.1, 3.2.3–4). This can be interpreted as a consequence of high seafloor respiration 38 39 1005 in summer, leading to release of isotopically light carbon for incorporation into shell 40 41 1006 carbonate alongside isotopically light oxygen, followed by autumn mixing down of surface 42 43 44 1007 waters containing isotopically heavy dissolved inorganic carbon (as a consequence of 45 46 1008 summer phytoplankton production in the surface layer) for incorporation into shell carbonate 47 48 49 1009 alongside isotopically heavy oxygen (e.g., Arthur et al., 1983; Krantz, 1990; Johnson et al., 50 51 1010 2017). The critical element in this scenario is the separation of surface from deep waters in 52 53 54 1011 summer (i.e., enough warming to reduce density but insufficient agitation to cause mixing) 55 56 1012 such that a seafloor-surface gradient develops in the δ13C of dissolved inorganic carbon. 57 58 59 1013 Johnson et al. (2009) considered it likely that the summer surface temperature of the 60 41 61 62 63 64 65 1014 Ramsholt-Member sea was about 9 °C higher than seafloor temperature, and thus well into 1 2 1015 the warm temperate range. However, this view was based on a misreading of temperature 3 4 5 1016 data from the modern seasonally stratified setting of the Gulf of Tunis. While summer 6 7 1017 (August) temperature at 50 m (comparable to the depth of the Ramsholt Member sea) is 8 9 10 1018 indeed 9 °C below surface temperature in this area (17 °C, compared to 26 °C at the surface), 11 12 1019 autumn overturn of the water column results in seafloor temperatures of 21 °C in 13 14 15 1020 October/November (NOAA, 1994). Thus a more reasonable ‘stratification factor’ to add to 16 17 1021 maximum isotopic temperatures (9.7–15.5 °C for mean individual/singleton maxima from 18 19 20 1022 ontogenetically sampled specimens; Fig. 13) from the Ramsholt Member would be 5 °C. 21 22 1023 Even making an allowance of 2 °C for a possible underestimate of seafloor temperatures from 23 24 25 1024 aragonitic species, this only yields temperatures as high as 20 °C from three of the 13 26 27 1025 ontogenetically sampled specimens, and two of the five sampled at high resolution. Hence, 28 29 1026 while summer maximum surface temperature was probably often 1–2 °C higher than at 30 31 32 1027 present (i.e., 17–19 °C rather than 16–17 °C), it was only sometimes in the warm temperate 33 34 1028 range. This deduction is supported by two other pieces of evidence. Firstly, as noted in 35 36 37 1029 section 3.2.1, around the end of ontogenetic year 21 and start of year 22 in A. islandica BRS- 38 39 1030 AJ1-CC-D1R the in-phase pattern of variation in δ13C and δ18O switches briefly to an 40 41 42 1031 antiphase pattern. This can be interpreted as a consequence of exceptional storminess, leading 43 44 1032 to mixing down of warm, high-δ13C surface waters earlier than the normal, cooling-induced 45 46 47 1033 overturn in autumn. While the summer temperature in year 22 (15.1 °C) is higher than in any 48 49 1034 other, as would be expected with (atypical) summer circulation of surface waters to the 50 51 52 1035 seafloor, it is well short of 20 °C. Secondly, the single specimen of A. opercularis 53 54 1036 investigated by Johnson et al. (2009) from the current-swept, probably well-mixed setting of 55 56 57 58 59 60 42 61 62 63 64 65 1037 the Sudbourne Member, yielded, as expected, a higher summer temperature than Ramsholt- 1 2 1038 Member forms (18.3 °C; using a water δ18O value of +0.1‰), but again below 20 °C. 3 4 5 1039 It is possible that the depth of the Ramsholt-Member sea insulated the seafloor somewhat 6 7 1040 from the effects of winter atmospheric cooling, as on the US Middle Atlantic shelf, where 8 9 10 1041 winter temperature is 2 °C higher at 30 m than at the surface (Winkelstern et al., 2013). The 11 12 1042 best estimates of winter minimum seafloor temperature, based on high-resolution sampling of 13 14 15 1043 A.islandica specimen BRS-AJ1-CC-D1R, A. opercularis and P. maxima, are 8–9 °C (Fig. 16 17 1044 13). Taking a 2 °C insulating effect into account yields from these estimates a winter surface 18 19 20 1045 temperature of 6–7 °C, the same as in the southern North Sea at present (Fig. 14). 21 22 1046 23 24 25 1047 4.4. Early Pliocene climate and oceanography in the North Atlantic region 26 27 1048 28 29 30 1049 The isotopic evidence of cool early Pliocene conditions provided by the Ramsholt 31 32 1050 Member is matched by isotopic evidence from the basal part of the Sunken Meadow Member 33 34 35 1051 (Yorktown Formation) on the western side of the North Atlantic (Fig. 1; Johnson et al., 36 37 1052 2017). While conditions were warmer during deposition of the upper part of the Sunken 38 39 1053 Meadow Member, they were not as warm as existed during deposition of higher members of 40 41 42 1054 the Yorktown Formation, much later in the Pliocene, as a consequence of a global rise in 43 44 1055 temperature (Johnson et al., 2017). The cool early Pliocene conditions evidenced on the 45 46 47 1056 eastern and western sides of the North Atlantic have also been revealed isotopically in 48 49 1057 Iceland (Gladenkov and Pokrovskiy, 1980; Buchardt and Símonarson, 2003), the cool 50 51 52 1058 temperatures deriving from the Serripes Zone. This has recently been dated by Verhoeven et 53 54 1059 al. (2011) to the mid-Zanclean (c. 4.0–4.5 Ma), overlapping the time of deposition of both the 55 56 57 1060 Ramsholt Member and Sunken Meadow Member. It is possible that the cool conditions 58 59 1061 represented in the three North Atlantic locations relate to a globally recognisable glacial 60 43 61 62 63 64 65 1062 event at c. 4 Ma, whose cause is unclear (De Schepper et al., 2014). On the basis of a low 1 2 1063 mean δ13C from Coralline Crag A. opercularis shells (+0.37‰) compared to pre-industrial 3 4 5 1064 Holocene A. opercularis shells (+0.79‰), Johnson et al. (2009) inferred relatively high 6 13 7 1065 atmospheric CO2. The mean δ C from the further Coralline Crag A. opercularis shells 8 9 10 1066 analysed in this investigation (+0.43 ± 0.29‰; combined results from ontogenetic and spot 11 12 1067 sampling) corroborates the earlier result. If the interpretation in terms of atmospheric CO2 is 13 14 15 1068 correct, one must look to other potential controls for the cool temperatures in the North 16 17 1069 Atlantic, and globally. Southward-flowing cool currents were influential in the western North 18 19 1070 Atlantic (Johnson et al., 2017). Large-scale cross-stratification, dipping to the south-west, is 20 21 22 1071 evidence of a strong, broadly southward flow during deposition of the Sudbourne Member of 23 24 1072 the Coralline Crag Formation. However, this undoubtedly reflects the dominant flow of tidal 25 26 27 1073 currents (Balson, 1999), and the southward direction of this was probably local, like the 28 29 1074 dominant southward flow on the western side of the southern North Sea at present (Wright et 30 31 32 1075 al., 1999, fig. 8.3a). Those depositional sedimentary structures not destroyed by bioturbation 33 34 1076 in the Ramsholt Member are of a smaller scale than in the Sudbourne Member. Given the 35 36 1077 mud content of the sediment, they are probably also a reflection of local (weaker) tidal 37 38 39 1078 currents. A remaining explanation for the temperatures measured or inferred from this unit is 40 41 1079 that they reflect global (atmospheric) warmth and withdrawal of oceanic heat supply by the 42 43 44 1080 Gulf Stream/North Atlantic Drift (Johnson et al., 2009). In that oceanic heat supply raises 45 46 1081 winter sea-surface temperature around Britain but has little influence on summer temperature, 47 48 49 1082 its withdrawal in the context of global warmth might be expected to yield winter temperatures 50 51 1083 much like those at present in the area of the southern North Sea and summer surface 52 53 temperatures somewhat higher, as deduced herein for the early Pliocene of East Anglia. 54 1084 55 56 1085 57 58 1086 5. Conclusions and further work 59 60 44 61 62 63 64 65 1087 1 2 1088 Our isotopic data in part confirm the ‘mixed’ temperature signal of the Coralline Crag 3 4 5 1089 (Williams et al., 2009), which we see as representing fluctuations in marine climate, possibly 6 7 1090 caused by changes in oceanic heat supply. However, marine climate appears to have been 8 9 10 1091 usually in the cool temperate range and never subtropical/tropical, as suggested by the 11 12 1092 bryozoan assemblage. The misleading indication given by this, and to a smaller extent by 13 14 15 1093 other elements of the biota (e.g., ostracods), is a caution to paleoenvironmental interpretation 16 17 1094 solely by analogy with modern forms: ‘ecological uniformitarianism’ is essentially a sound 18 19 1095 methodology, but not infallible. 20 21 22 1096 We have suggested one explanation (slow ‘rebound’ from the effects of severe Pleistocene 23 24 1097 cooling) for the current subtropical/tropical distribution of the bryozoan Metrarabdotos. 25 26 27 1098 Change in temperature tolerance/preference over time is another potential explanation for the 28 29 1099 presence of this and other ‘warm’ taxa in the cool seafloor setting of the Coralline Crag. A 30 31 32 1100 further possibility is that their occurrence there was enabled by the existence of relatively 33 34 1101 warm surface waters—i.e., conditions during the pelagic larval stage, rather than in the 35 36 1102 benthic adult stage, are the real determinant of distribution. This idea, whose germ is in the 37 38 39 1103 thoughts of Raffi et al. (1985) and Long and Zalasiewicz (2011), is open to experimental test 40 41 1104 on modern forms. The complex behavioural responses of A. islandica larvae to changes in 42 43 44 1105 temperature and pressure certainly indicate great sensitivity to environmental conditions at 45 46 1106 this developmental stage (Mann and Wolf, 1983). 47 48 18 49 1107 As well as the suggested experimental work it would be worth expanding δ O-based 50 51 1108 investigations of Coralline Crag temperature to include other benthic taxa, in particular the 52 53 supposedly warm-water bryozoans. While measurement of the δ18O of skeletal carbonate 54 1109 55 56 1110 would be sufficient to determine whether any taxa lived under markedly warmer conditions, 57 58 1111 it would be gratifying to ‘fix’ temperatures by allying this with techniques for accurately 59 60 45 61 62 63 64 65 18 1112 estimating water δ O. ‘Clumped isotope’ (Δ47) analysis (e.g., Winkelstern et al., 2017) and 1 2 1113 measurement of the δ18O of homeotherm phosphate (e.g., Walliser et al., 2015) are propitious 3 4 5 1114 methods. Finally, it would be worth applying geochemical approaches (e.g., foraminiferal 6 7 1115 Mg/Ca, alkenone unsaturation; Dowsett and Robinson, 2009) to material of pelagic origin to 8 9 10 1116 test the accuracy of the sea-surface temperatures deduced from seafloor data herein. 11 12 13 1117 14 15 1118 Acknowledgements 16 17 18 1119 19 20 1120 We thank James Rolfe and Michael Maus for assistance with isotopic analysis; the 21 22 23 1121 Sedgwick Museum for providing Coralline Crag specimens; Sharon Richardson for drafting 24 25 1122 Fig. 1; Peter Balson, Tony Dickson, Robert Marquet and Adrian Wood for helpful 26 27 1123 discussions during the course of the research; and the reviewers for detailed and constructive 28 29 30 1124 comments which greatly improved the paper. 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Multiproxy sclerochronological evidence for 23 24 1416 Plio-Pleistocene regional warmth: United States Mid-Atlantic Coastal Plain. 25 26 27 1417 PALAIOS, 28, 649–660, doi: 10.2110/palo.2013.p13-010r. 28 29 1418 Winkelstern, I. Z., Rowe, M.P., Lohmann, K.C., Defliese, W.F., Petersen, S.V., Brewer , 30 31 32 1419 A.W., 2017. Meltwater pulse recorded in Last Interglacial mollusk shells from 33 34 1420 Bermuda, Paleoceanogr. 32, doi: 10.1002/2016PA003014. 35 36 1421 Witbaard, R., 1997. Tree of the sea: the use of the internal growth lines in the shell of 37 38 39 1422 "Arctica islandica" (Bivalvia, Mollusca) for the retrospective assessment of marine 40 41 1423 environmental change. PhD thesis, University of Groningen, The Netherlands, 149 42 43 44 1424 pp. 45 46 1425 Witbaard, R., Bergman, M.J.N., 2003. The distribution and population structure of the 47 48 49 1426 bivalve Arctica islandica L. in the North Sea: what possible factors are involved? 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A monograph of the Crag Mollusca, or, descriptions of shells from the 20 21 22 1439 Middle and Upper Tertiaries of the east of England. Part 1. Univalves. Palaeontogr. 23 24 1440 Soc. Monogr., pp. i–xi + 1–208, pls. 1–21. 25 26 27 1441 Wood, S.V., 1851–61. A monograph of the Crag Mollusca, or, descriptions of shells from the 28 29 1442 Middle and Upper Tertiaries of the east of England. Part 2. Bivalves. Palaeontogr. 30 31 32 1443 Soc. Monogr., pp. 1–150, pls. 1–12 (1851), pp. 151–216, pls. 13–20 (1853), pp. 217– 33 34 1444 342, pls. 21–31(1857), note 1–2 (1861). 35 36 1445 Wood, S.V., 1872–74. Supplement to the monograph of the Crag Mollusca, with descriptions 37 38 39 1446 of shells from the Upper Tertiaries of the east of England. Univalves and Bivalves. 40 41 1447 Palaeontogr. Soc. Monogr., pp. i–xxxi + 1–231. 42 43 44 1448 Wood, S.V., 1879. Second supplement to the monograph of the Crag Mollusca, with 45 46 1449 descriptions of shells from the Upper Tertiaries of the east of England. Palaeontogr. 47 48 49 1450 Soc. Monogr., pp. i–ii + 1–58. 50 51 1451 Wood, S.V., 1882. Third supplement to the Crag Mollusca comprising Testacea from the 52 53 Upper Tertiaries of the East of England. Univalves and bivalves. Palaeontogr. Soc. 54 1452 55 56 1453 Monogr., 1–24. 57 58 59 60 59 61 62 63 64 65 1454 Wood, S.V., 1882. Third supplement to the Crag Mollusca comprising Testacea from the 1 2 1455 Upper Tertiaries of the East of England. Univalves and bivalves. Palaeontogr. Soc. 3 4 5 1456 Monogr. 1–24. 6 7 1457 World Register of Marine Species. http://marinespecies.org/ (last accessed 29th July 2017). 8 9 10 1458 Wright, J., Colling, A., Park, A., 1999. Waves, Tides and Shallow-Water Processes. 11 12 1459 Butterworth Heinemann/The Open University, Milton Keynes, 227 pp. 13 14 15 1460 Zhao, L., Walliser, E.O., Mertz-Kraus, R., Schöne, B.R., 2017. Unionid shells (Hyriopsis 16 17 1461 cumingii) record manganese cycling at the sediment-water interface in a shallow 18 19 1462 eutrophic lake in China (Lake Taihu). Palaeogeogr. Palaeoclimatol. Palaeoecol. 484, 20 21 22 1463 97–108, doi: 10.1016/j.palaeo.2017.03.010. 23 24 1464 25 26 27 1465 FIGURE CAPTIONS 28 29 1466 30 31 32 1467 Fig. 1. Position of main onshore outcrop of the Coralline Crag Formation in Suffolk, East 33 34 1468 Anglia, eastern England (1 and inset), locations of other sequences in north-west Europe 35 36 1469 mentioned in the text (2–4), and Pliocene paleogeography of the area (adapted from Murray, 37 38 39 1470 1992, map NG1). Sea is blue (beige over current land); brown is land (pink over current sea). 40 41 1471 The St Erth sequence, England (2), was dated by Head (1993) to within the Gelasian, 42 43 44 1472 formerly the uppermost stage of the Pliocene but now placed in the Pleistocene following 45 46 1473 redefinition of the Pliocene/Pleistocene boundary (Gibbard et al., 2010). The borehole 47 48 49 1474 sequence at Ouwerkerk, The Netherlands (3), includes Pliocene material (e.g., Valentine et 50 51 1475 al., 2011) but is only one of many subsurface occurrences of Pliocene deposits throughout 52 53 The Netherlands (Marquet and Herman, 2009). Dock construction at Antwerp, Belgium (4), 54 1476 55 56 1477 provided temporary exposures of the Pliocene sequence there (Marquet and Herman, 2009). 57 58 1478 The outliers of Coralline Crag mentioned in the text are not shown; they extend the area of 59 60 60 61 62 63 64 65 1479 occurrence of the formation less far (27 km west-south-west of the main onshore outcrop) 1 2 1480 than stated by Johnson et al. (2009). 3 4 5 1481 6 7 1482 Fig. 2. Stratigraphy of the Coralline Crag Formation in relation to sequences on the eastern 8 9 10 1483 side of the southern North Sea (Belgium) and western side of the North Atlantic (Virginia 11 12 1484 and N. Carolina, USA). Amended from Johnson et al. (2017, fig. 1) to take account of the 13 14 15 1485 evidence in De Schepper et al. (2009) that the Coralline Crag Formation has no equivalent in 16 17 1486 the Belgian sequence, being younger than the Kattendijk Formation and older than the Lillo 18 19 1487 Formation. Wood et al. (2009), however, consider that the Luchtbal Sands Member of the 20 21 22 1488 Lillo Formation is age-equivalent to the Coralline Crag Formation. Ages in Ma. MPWP = 23 24 1489 Mid-Piacenzian Warm Period. Asterisks indicate units for which there are existing isotopic 25 26 27 1490 paleotemperature determinations (Krantz 1990; Goewert and Surge, 2008; Johnson et al., 28 29 1491 2009, 2017; Williams et al., 2009; Valentine et al., 2011; Winkelstern et al., 2013). 30 31 32 1492 33 34 1493 Fig. 3. Examples of Aequipecten opercularis (A, SM XXXXXX; AeO3), Talochlamys 35 36 1494 multistriata (B, SM XXXXXX; CV1), Cardites squamulosa ampla (C, SM TN552.5.1.2), 37 38 39 1495 Astarte gracilis (E, SM XXXXXX; AG5), Lucinoma borealis (F, SM XXXXXX; PB4), 40 41 1496 Spisula arcuata (G, SM XXXXXX; SA) and Ringicula buccinea (H, SM TN4187.1) from the 42 43 44 1497 Ramsholt Member, Coralline Crag Formation, and of Cardites antiquatus (D, SM 45 46 1498 XXXXXX; a close modern relative of C. squamulosa ampla) from the Mediterranean Sea off 47 48 49 1499 Almería, Spain. Note that the scales differ either side of the dashed line; scale bar = 10 mm in 50 51 1500 each portion. Pits from spot sampling are indicated by arrows in A, B, E, F. The greater 52 53 prominence of commarginal lamellae in D compared to C is partly a reflection of preserved 54 1501 55 56 1502 colour variation. Only the apertural region, representing 60-70% of the total shell height, is 57 58 1503 present in H. 59 60 61 61 62 63 64 65 1504 1 2 1505 Fig. 4. Annual growth increments in Coralline Crag specimens of Arctica islandica (A, B) 3 4 5 1506 and Cardites squamulosa ampla (C), and microgrowth (c. daily) increments in a modern 6 7 1507 specimen of Aequipecten opercularis from the Mediterranean Sea (D). A (54–61 mm from 8 9 10 1508 the origin of growth in A. islandica specimen 1) shows annual growth lines bounding annual 11 12 1509 increments in the outer (OL) and inner (IL) shell layers, together with the last four sample 13 14 15 1510 holes in the outer layer. B (17–21 mm from the origin of growth in A. islandica specimen 5) 16 17 1511 also shows annual growth lines in the outer and inner layers, together with the last four 18 19 1512 sample holes, but note that in this series from early ontogeny some of the sample holes 20 21 22 1513 transgress the boundary between the outer and inner layer. C (31–34 mm from the origin of 23 24 1514 growth, including the ventral edge of the shell, in C. squamulosa ampla specimen 7) shows 25 26 27 1515 annual growth lines in the outer and m+1 (outer part of the middle) layer, closely spaced in 28 29 1516 this late ontogenetic sequence (note that the outer layer, marked by reflexed growth lines, is 30 31 32 1517 often poorly preserved or lost nearer the origin of growth). D (18–26 mm from the origin of 33 34 1518 growth in a modern A. opercularis specimen from near Málaga, Spain; Muséum national 35 36 1519 d’Histoire naturelle, Paris, 21996) shows commarginal striae bounding microgrowth 37 38 39 1520 increments on the outer shell surface. A–C are images of acetate peels from sectioned shells 40 41 1521 (note that the white spots in C are bubbles on the acetate peel). Scale bars represent 1 mm in 42 43 44 1522 each part. dog = direction of extensional growth. 45 46 1523 47 48 49 1524 Fig. 5. Unaltered preservation of calcite (A, B) and aragonite (C-F) shell structures amongst 50 51 1525 isotopically investigated Coralline Crag species. A: foliated structure in Talochlamys 52 53 multistriata; B: fibrous structure adjacent to a puncta in Pliothyrina maxima; C: 54 1526 55 56 1527 homogeneous structure in Arctica islandica; D: crossed-lamellar structure in Cardites 57 58 1528 squamulosa ampla; E: complex crossed-lamellar structure in Coripia corbis; F: complex 59 60 62 61 62 63 64 65 1529 crossed-lamellar structure in Lucinoma borealis. Note that the puncta in B is void of 1 2 1530 diagenetic calcite. A–C, E, F are fracture surfaces; D is a polished and etched surface. Scale 3 4 5 1531 bar = 10 μm for each part. 6 7 1532 8 9 10 1533 Fig. 6. Cathodoluminescence in specimens of Cardites squamulosa ampla. A: specimen 8, 11 12 1534 showing blue luminescence (indicating aragonite) in the inner and outer shell layers (seen in 13 14 15 1535 centre of image, with growth lines visible); sediment containing calcite (specks of orange 16 17 1536 luminescence) is seen to the left and resin to the right. B: specimen 7, showing blue 18 19 1537 luminescence in the inner and outer layers, and patches of dull orange luminescence in the 20 21 22 1538 middle layer; sediment containing large calcite grains (bright orange luminescence) is seen to 23 24 1539 the left and resin to the right (also infilling a cavity in the outer part of the outer layer). C: 25 26 27 1540 enlargement of the boxed area in B, showing faint transverse lines, representing the original 28 29 1541 crossed-lamellar shell structure, in an area of orange luminescence (colour brighter than in B 30 31 32 1542 due to enhancement of image to show shell structure). Scale bar = 1 mm for each part. 33 34 1543 35 36 1544 Fig. 7. A–D: Oxygen isotope (red line) and carbon isotope (black line) data from Coralline 37 38 18 39 1545 Crag specimens of Arctica islandica. Isotopic axis reversed so that lower values of δ O 40 41 1546 (representative of higher temperatures) plot towards the top. Dotted lines in A are for values 42 43 44 1547 from whole-year samples. E–H: Temperature profiles calculated using the oxygen isotope 45 18 46 1548 data in A–D, Equation 2 and values for δ Owater of –0.5‰ (blue lines), –0.2‰ (brown lines), 47 48 18 49 1549 +0.1‰ (orange lines) and +0.5‰ (yellow lines). The profiles for δ Owater of +0.1‰ (thicker 50 51 1550 lines) are preferred. 52 53 54 1551 55 56 1552 Fig. 8. A–D: Oxygen isotope (red line) and carbon isotope (black line) data from Coralline 57 58 1553 Crag specimens of Cardites squamulosa ampla. Isotopic axis reversed so that lower values of 59 60 63 61 62 63 64 65 1554 δ18O (representative of higher temperatures) plot towards the top. E–H: Temperature profiles 1 2 18 1555 calculated using the oxygen isotope data in A–D, Equation 2 and values for δ Owater of – 3 4 5 1556 0.5‰ (blue lines), –0.2‰ (brown lines), +0.1‰ (orange lines) and +0.5‰ (yellow lines). The 6 18 7 1557 profiles for δ Owater of +0.1‰ (thicker lines) are preferred. 8 9 10 1558 11 12 1559 Fig. 9. A, B: Oxygen isotope (red line), carbon isotope (black line) and microgrowth 13 14 15 1560 increment (blue line) data from Coralline Crag specimens of Aequipecten opercularis. 16 18 17 1561 Isotopic axis reversed so that lower values of δ O (representative of higher temperatures) 18 19 1562 plot towards the top. Distance from origin of growth measured as a straight line (not around 20 21 22 1563 the shell periphery) along the axis of maximum growth; increment size measured in the same 23 24 1564 direction. Thin, dashed blue lines = raw increment data; thicker, continuous blue lines = five- 25 26 27 1565 point averages. C, D: Temperature profiles calculated using the oxygen isotope data in A and 28 18 29 1566 B, Equation 1 and values for δ Owater of –0.5‰ (blue lines), –0.2‰ (brown lines), +0.1‰ 30 31 18 32 1567 (orange lines) and +0.5‰ (yellow lines). The profiles for δ Owater of +0.1‰ (thicker lines) 33 34 1568 are preferred. 35 36 1569 37 38 39 1570 Fig. 10. A, B: Oxygen isotope (red line) and carbon isotope (black line) data from Coralline 40 41 1571 Crag specimens of Pliothyrina maxima. Isotopic axis reversed so that lower values of δ18O 42 43 44 1572 (representative of higher temperatures) plot towards the top. Distance from origin of growth 45 46 1573 measured as a straight line (not around the shell periphery) along the axis of maximum 47 48 49 1574 growth. C, D: Temperature profiles calculated using the oxygen isotope data in A and B, 50 18 51 1575 Equation 1 and values for δ Owater of –0.5‰ (blue lines), –0.2‰ (brown lines), +0.1‰ 52 53 (orange lines) and +0.5‰ (yellow lines). The profiles for δ18O of +0.1‰ (thicker lines) 54 1576 water 55 56 1577 are preferred. 57 58 1578 59 60 64 61 62 63 64 65 1579 Fig. 11. A: Oxygen isotope (red line) and carbon isotope (black line) data from a Coralline 1 2 1580 Crag specimen of Lucinoma borealis. Isotopic axis reversed so that lower values of δ18O 3 4 5 1581 (representative of higher temperatures) plot towards the top. Distance from origin of growth 6 7 1582 measured as a straight line (not around the shell periphery) along the axis of maximum 8 9 10 1583 growth. B: Temperature profile calculated using the oxygen isotope data in A, Equation 2 and 11 18 12 1584 values for δ Owater of –0.5‰ (blue line), –0.2‰ (brown line), +0.1‰ (orange line) and 13 14 18 15 1585 +0.5‰ (yellow line). The profile for δ Owater of +0.1‰ (thicker line) is preferred. 16 17 1586 18 19 1587 Fig. 12. Summary statistics for (A) summer δ18O minima (pink) and winter δ18O maxima 20 21 13 22 1588 (sea-green), and (B) all δ C values, from each of the ontogenetically sampled individuals and 23 24 1589 the taxa concerned as a whole. The plots show the range of values (thin lines with caps), 25 26 27 1590 mean (squares; sample number adjacent) and standard deviation (± 1σ; thick lines), with the 28 29 1591 taxic ('mean’) data being the range, mean and standard deviation of individual means. Names 30 31 32 1592 of ‘warm’ taxa in red, ‘cool’ in blue, ‘eurythermal’ in mauve; taxa ordered as in Figure 14 33 34 1593 (see caption for explanation); isotopic axes reversed to match Figures 7–11. 35 36 1594 37 38 39 1595 Fig. 13. Summary statistics for summer temperature maxima (pale red) and winter 40 41 1596 temperature minima (pale blue) from each of the ontogenetically sampled individuals and the 42 43 44 1597 taxa concerned as a whole, calculated using equation (1) or (2), as appropriate, and a water 45 46 1598 δ18O value of +0.1‰. The plot shows the range of values (thin lines with caps), mean 47 48 49 1599 (squares; sample number adjacent) and standard deviation (± 1σ; thick lines) for maxima and 50 51 1600 minima, with the taxic ('mean’) data being the range, mean and standard deviation of 52 53 individual means. Names of ‘warm’ taxa in red, ‘cool’ in blue, ‘eurythermal’ in mauve. 54 1601 55 56 1602 Winter and summer boundary temperatures between cool (C) and warm (W) temperate 57 58 1603 marine climate indicated. 59 60 65 61 62 63 64 65 1604 1 2 1605 Fig. 14. Temperatures (filled circles) calculated from spot and whole-shell δ18O values, 3 4 18 5 1606 Equation (1) or (2), as appropriate, and values for water δ O of –0.5‰ (mid-blue), –0.2‰ 6 18 7 1607 (brown), +0.1‰ (orange) and +0.5‰ (yellow). Temperatures calculated using a water δ O of 8 9 10 1608 +0.1‰ (larger, orange filled circles) are preferred. Names of ‘warm’ taxa in red, ‘cool’ in 11 12 1609 blue, ‘eurythermal’ in mauve; number of specimens in parentheses. Bivalve taxa arranged in 13 14 15 1610 conventional order of families from left (Glycymeridae) to right (Arcticidae). Data arranged 16 17 1611 within taxa in order of increasing temperature to the right. Orange dotted lines signify mean 18 19 1612 of temperatures calculated using a water δ18O of +0.1‰ for each taxon. Green dotted lines, 20 21 22 1613 and pale red and pale blue dashed lines, signify, respectively, overall mean, mean summer 23 24 1614 maximum and mean winter minimum temperatures from ontogenetic data (see sections 25 26 27 1615 3.4.1–3.4.5 and Fig. 13). The grey dashed lines signify average summer maximum and 28 29 1616 average winter minimum temperatures (for both surface and seafloor) in the southern North 30 31 32 1617 Sea at present. Winter and summer boundary temperatures between cool (C) and warm (W) 33 34 1618 temperate marine climate indicated. 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 66 61 62 63 64 65 Figure 1

[Fig. 1 – for 1.5-column reproduction; colour required]

Figure 2

[Fig. 2 – for 1-column reproduction; colour required]

Virginia / N. Carolina Suffolk, UK Antwerp, Belgium

3.0 2.97 Oorderen

Moore House Member * Sands

Member *

Morgarts Beach . 3.21 MPWP

Member * M F 3.29 Rushmere

Member *

Piacenzian LILLO LILLO Luchtbal 3.5 Sands Member 3.60

3.71 Aldeburgh

Member PLIOCENE

M. and F Sudbourne

Member *

4.0 YORKTOWN FORMATION

Ramsholt Zanclean

Member * CORALLINE CRAG CRAG CORALLINE 4.40 Sunken Meadow KATTENDIJK Member * 4.5 FORMATION mmmmm

mmmm Figure 3

[Fig. 3 – for 1.5- or 2-column reproduction]

E A B

F

H C D G

Figure 4

[Fig. 4 – for 1.5- or 2-column reproduction]

dog A Annual growth lines (OL) B OL/IL boundary ((OL)

dog OL IL Annual growth lines (OL)

((OL)

C OL D

m+1 layer

dog dog

Figure 5

[Fig. 5 – for 1.5 or 2-column reproduction]

A B C

D E F

Figure 6

[Fig. 6 – for 2-column reproduction; colour required]

A B C

Figure 7

[Fig. 7 – for 2-column reproduction; colour required] A A. islandica BRS-AJ1-CC-D1R E A. islandica BRS-AJ1-CC-D1R

B A. islandica 1 F A. islandica 1

C A. islandica 2 G A. islandica 2

D A. islandica 5 H A. islandica 5

Figure 8

[Fig. 8 – for 2-column reproduction; colour required] A C. squamulosa ampla 1 E C. squamulosa ampla 1

B C. squamulosa ampla 7 F C. squamulosa ampla 7

C C. squamulosa ampla 8 G C. squamulosa ampla 8

D C. squamulosa ampla 10 H C. squamulosa ampla 10

Figure 9

[Fig. 9 – for 2-column reproduction; colour required]

A A. opercularis UD 53360 C A. opercularis UD 53360

B A. opercularis UD 53361 D A. opercularis UD 53361

Figure 10

[Fig. 10 – for 2-column reproduction; colour required]

A P. maxima 1 C P. maxima 1

B P. maxima 2 D P. maxima 2

Figure 11

[Fig. 11 – for 2-column reproduction; colour required]

A L. borealis 3 B L. borealis 3

Figure 12

[Fig. 12 – for 1.5 or 2-column reproduction; colour required]

A

1 3 2 2 2 1

4 2

3 11 2 1 2 3 4 2 7 6 1 4 4 5 2 4 4 1 4 8 4 10 1 7 4 4

B

30 2 7 8 34 32 2

29 20 39 4 20 143 10 18 4 29

Figure 13

[Fig. 13 – for 1.5- or 2-column reproduction; colour required]

W C

Summer

1 3 2 4 2 2 1 4 3 11

3 7 6 1 4 4 5 2 W 4 4 2 4 8 4 C

10 Winter 2 1 2 1 2 7 4 4 1

Figure 14

[Fig. 14 – for 2-column reproduction; colour required]

(2)

(9)

(3)

(2)

(4)

(6)

(5) (2)

(3)

(9)

(8) (1)

(2)

(9)

(3)

(2)

a

corbis

omalii

gracilis W

grandis

borealis

arcuat

maxima

obovata

C.

truncata

A. A. islandica

buccinea

. .

A. A. tigerinum

L. C

S

P.

conuloidea

G. opercularis

Summer

aff. aff.

A. A.

R. R.

M.

multistriata

P.

.

R. R.

A.

P

T. T.

squamulosa ampla squamulosa

C.

W C

Winter

Modernseasonal range

MOLLUSCA: BIVALVIA MOLL: GASTROPODA BRACHIOPODA

[Fig. 14 – for 2-column reproduction; colour required] Supplementary Material A Click here to download Background dataset for online publication only: Supplementary Material A - Vignols et al..xlsx

Supplementary Material B Click here to download Background dataset for online publication only: Supplementary Material B - Vignols et al..docx