Chironomid assemblages

Productive chironomid samples were obtained from Strata C and E (Figs. 1 and 2); results from those sufficiently abundant to justify further analysis are shown in Figure 3. Stratum E chironomid assemblages are relatively species poor, comprising taxa typical of warm, eutrophic waters with submerged aquatic vegetation; low oxygen conditions existed, either in a deep, thermally stratified lake or in a shallow lake containing large amounts of decaying vegetation. Stratum C chironomid assemblages are relatively species rich, indicative of relatively shallow, well-oxygenated waters with submerged aquatic vegetation, possibly influenced by stream inflow, and dominated by widespread, eurytopic taxa that can occur in both warm and temperate waters; they include cold stenothermic taxa, but the overall indications are of cool-temperate rather than cool climate. Further details are given in the Supplementary Material, Section 2.

Figure 3.

Summary of chironomid occurrences recorded from MIS 11 strata at Hoxne.

It is possible that thermophilic taxa present in Stratum C (“Arctic Bed”) were reworked from earlier sediments deposited during a warmer period (e.g., Stratum E). Indeed West (Reference West1956, p. 313) suggested this possibility when considering the vegetation record at Hoxne, identifying the pollen spectrum of a clay mud pebble from Stratum C as being from HoIId (uppermost Early Temperate, within Stratum E; redesignated HoIIc by Turner and West [Reference Turner and West1968]). However, the Stratum E chironomid assemblages, which represent a warm climate, are dominated by Chironomus plumosus-type, and yet this taxon is hardly present in any of the Stratum C samples, which would be expected if there had been reworking of sediments. Equally, the temperate taxa that are well represented in the Stratum C sediments, especially Paratanytarsus penicillatus-type and Microtendipes pedellus-type, are absent from the samples analysed from Stratum E, suggesting these taxa have not been reworked.

CTF method palaeotemperature reconstructions

All reconstructed ranges are mean monthly air palaeotemperatures; the CTF method was used to reconstruct only July temperatures. Results are detailed in Table 2 and illustrated in Figures 3 and 4. Stratum E results are +19.1°C ± 1.4 and +19.3°C ± 1.8; equivalent to a combined range (MOR) of +17.8°C to +20.5°C. Stratum C results range from +13.4°C ± 1.1 to +15.0°C ± 1.1, equivalent to a combined MOR of +13.9°C to +14.5°C.

Table 2.

Chironomid transfer function palaeotemperature reconstructions.^a

^a See Figs. 1 and 2 for locations and stratigraphic context of samples.

^b MOR, mutual overlap range.

Figure 4.

Multiproxy palaeotemperature reconstructions for MIS 11 at Hoxne.

Ostracod assemblages

Ostracod assemblages were recovered from Strata F, E, C, and B (Figs. 1 and 2); details are given in the Supplementary Material, Section 3. The assemblages are composed entirely of typically freshwater taxa (Table 3), although many of those can tolerate slightly brackish waters, and are consistent with lacustrine and (in the case of Stratum B) fluvial depositional environments (Supplementary Material, Section 3).

Table 3.

Summary of ostracod occurrences recorded from MIS 11 strata at Hoxne, with calibrations used in the mutual ostracod temperature range method where available.

^a An asterisk (*) denotes extinct species.

MOTR method palaeotemperature reconstructions

All reconstructed ranges are mean monthly air palaeotemperatures. The calibrations used (Table 3) are from Horne et al. (Reference Horne, Curry, Mesquita-Joanes, Horne, Holmes, Rodriguez-Lazaro and Viehberg2012a), with two exceptions. In the case of C. lacustris, an adjustment was made to take account of living records in Lake Biwa, Japan (Smith and Janz, Reference Smith and Janz2009), raising the maxima of its mean January and July ranges by 1°C and 3°C, respectively. In the case of Limnocythere suessenbornensis, found in Stratum F and considered to be extinct in Europe, an unpublished calibration of the living North American species Limnocythere friabilis was applied, based on recent work by DJH with Alison Smith (Kent State University, Ohio, USA) that suggests that the two species are synonymous. Extinct species (Cyclocypris obunca, Ilyocypris quinculminata, Leucocythere batesi, Limnocythere falcata, Scottia browniana) are not included in the analyses, because they cannot be calibrated. Two living species are not used due to taxonomic uncertainties; attempted calibrations of Ilyocypris gibba are considered unreliable, because it has frequently been confused with other congeneric species, and there are insufficient living records (only two) of Paralimnocythere compressa in NODE and OMEGA to permit its calibration. At least three other species are present in our Hoxne assemblages, lumped together as Ilyocypris spp. in Table 3; one of these may be Ilyocypris bradyi Sars, 1890, another Ilyocypris decipiens Masi, 1905, while the third may be a new species, and further study will be necessary before their taxonomy and occurrences are well enough understood to include them in our analyses. All the reconstructions produced 100% overlap of calibrated ranges (i.e., no outliers with nonmutual ranges). Results are illustrated in Figures 4 and 5 and detailed in Table 4.

Figure 5.

Stratum C, Hoxne: palaeotemperature reconstructions from mutual ostracod temperature range (MOTR), beetle mutual climatic range (BMCR), and chironomid transfer function (CTF) methods, with the addition of plant threshold temperatures and ostracod shell δ¹⁸O and δ¹³C measured in shells of Cytherissa and Ilyocypris. Note that the BMCR and CTF sample intervals were determined from a different numbering sequence matched to the equivalent ostracod sample numbers shown on the left of the figure (see Fig. 2 for details).

Table 4.

Mutual ostracod temperature range results from Hoxne.^a

^a See Figs. 1 and 2 for locations and stratigraphic context of samples.

Stratum F results for each of three samples are provided in Table 4; the MOTR ranges for the combined assemblage being January −8°C to −3°C and July +16°C to +23°C. Stratum E results from three separate sample assemblages are likewise given in Table 4, the combined MOTR reconstruction being January −8°C to +3°C and July +12°C to +21°C.

High-resolution sampling of Stratum C yielded ostracod assemblages from 36 consecutive samples, comprising a mixture of taxa commonly regarded as cold climate indicators (e.g., L. falcata, Limnocythere sanctipatricii, C. lacustris) and warm/temperate indicators (e.g., Metacypris cordata, Darwinula stevensoni). It cannot be ruled out that at least some of the warm-climate taxa were introduced into Stratum C by reworking from older (interglacial) lake sediments exposed around the margins during times of lowered lake level. However, because all the taxa included in the MOTR analysis do have overlapping, mutual ranges (i.e., they could have coexisted) and there are no reliable criteria (e.g., preservational differences) for distinguishing between in situ and reworked specimens, it had to be assumed that they are all in situ and thus valid for the MOTR reconstruction; potential problems of reworking are considered further in the “Discussion” section. Accordingly, MOTR reconstructions for Stratum C, based on 10 calibrated taxa, are presented in Figure 5 and Table 4. The sample-by-sample reconstructions appear to demonstrate considerable variation in climatic conditions through the sequence. Because the actual temperatures could be anywhere within the ranges reconstructed, it is possible that they could have stayed between narrower limits throughout the interval, with little or no variation, but comparison with the other proxy results suggests that this is unlikely.

The MOTR ranges for the combined Stratum C assemblage are January −7°C to +1°C and July +14°C to +19°C. The lower extreme of the July MOTR is 1° cooler than the lower extreme of Benardout's (Reference Benardout2015) July result for Stratum C (+15°C to +19°C), a discrepancy that can be explained by our decision not to include I. decipiens in our analyses because of taxonomic uncertainties within the genus Ilyocypris. Also worthy of note are the occurrences of three extinct taxa with palaeoclimatic significance: L. batesi, L. falcata, and I. quinculminata. The first two are considered to be cold-climate indicators by association with other taxa (Whittaker and Horne, Reference Whittaker, Horne, Whittaker and Hart2009). Ilyocypris quinculminata was considered to be a warm indicator by association with other taxa (Whittaker and Horne, Reference Whittaker, Horne, Whittaker and Hart2009), so its sporadic occurrence in Stratum C (and absence from the other strata) seemed anomalous. Because it is extinct, its temperature ranges cannot be calibrated, but two occurrences for which MOTR results have been published more recently offer an insight into its palaeotemperature tolerances. At Boxgrove (MIS 13), it was found in an assemblage that provided palaeotemperature reconstructions, based on a combination of MOTR and herpetile-mutual climatic range (MCR) methods, of −4°C to +4°C (January) and +15°C to +20°C (July), which in comparison with modern temperatures for that vicinity are consistent with similar summers but suggest colder winters (by at least 2°C) than today (Holmes et al., Reference Holmes, Atkinson, Darbyshire, Horne, Joordens, Roberts, Sinka and Whittaker2010). In MIS 11 at Ebbsfleet (Southfleet Road), it occurred in association with extant ostracod taxa that together yielded MOTR values of −4°C to −1°C (January) and +17°C to +21°C (July) (Whittaker et al., Reference Whittaker, Horne, Wenban-Smith and Wenban-Smith2013), which for that region suggests warmer summers but colder winters (by at least 4°C) than today (a more continental climate). We conclude, tentatively, that I. quinculminata may be regarded as an indicator of a temperate/cool continental climate similar to that of northern Germany and Poland today, with (in comparison with the maritime/oceanic climate of present-day England) warmer summers and colder winters.

Stratum B1 ostracods were sparse, yielding wide-ranging MOTR results from two samples (Table 4); the combined MOTR reconstructions are January −8°C to +13°C and July +10°C to +25°C.

Beetle assemblages

Productive beetle samples were obtained from Strata C, D, E, and F (Figs. 1 and 2). The beetle assemblage of Stratum F was obtained and described by Coope (Reference Coope, Singer, Gladfelter and Wymer1993) from a single sample of laminated drift mud collected at the base of the sequence in West's (Reference West1956) section 100 (Fig. 1). Stratum F yielded 82 taxa, of which 69 could be named to species, the assemblage being dominated by cold-adapted species (e.g., Pycnoglypta lurida, Holoboreaphilus nordenskioeldi, Tachinus caelatus, Simplocaria metallica). There were also more temperate, less cold-adapted species present, such as Bembidion guttula and Colymbetes fuscus, although the ranges of both of these extend north of the Arctic Circle in Fennoscandia at the present day.

Stratum E beetle samples from BH 02/1 (Figs. 1 and 2) yielded only sparse assemblages, likely due to small sample size. The Stratum D beetle assemblage, originally published by Coope (Reference Coope, Singer, Gladfelter and Wymer1993), is from a single block of woody peat taken from Cutting XXIII of the 1971–1974 excavations (Wymer, Reference Wymer, Singer, Gladfelter and Wymer1993; Fig. 1); it is very diverse and clearly associated with temperate conditions.

Stratum C beetle assemblages were obtained from Area VII samples (Figs. 1 and 2) and first published by Ashton et al. (Reference Ashton, Lewis, Parfitt, Penkman and Coope2008). Table 5 shows critical species from Stratum C that have significance because none of them lives in Britain today. Two climatically significant species in this assemblage are not represented in the BugsCEP database; Hippodamia arctica is a high boreal species living in Fennoscandia and northern Russia with an outpost in the Caucasus Mountains (Brundin, Reference Brundin1934), where it is found on Betula nana–rich Empetrum heath, while S. metallica is a boreo-alpine species, ranging from Greenland and Fennoscandia and the high mountains of central Europe (Horion, Reference Horion1953), that feeds exclusively on mosses.

Table 5.

Calibrated present-day mean monthly temperature ranges of selected beetle taxa from Stratum C, Hoxne.^a

^a Data based on BugsCEP envelopes with the exception of P. lurida and O. boreale, which are based on updated envelopes generated using GBIF, which demonstrate improved winter temperature envelope values (Milne, Reference Milne2016). Only a restricted number of revised envelopes are available via this source.

It is interesting to observe the presence in Stratum C of a few species that also occur in the underlying Stratum D, suggesting the possibility of reworking. It is noteworthy that the lowermost assemblages of Stratum C (374 and 373; Table 6) are cool temperate, indicating warmer summer conditions than the rest of the sequence, but still very cold winter conditions; there is a small assemblage of wood/tree-associated species (3.6% of the terrestrial fauna) as well as a number of dung beetles and species associated with open ground (4.5% of the terrestrial fauna). The abundance of individuals and diversity of taxa in these samples is more typical of cool-temperate conditions, including a range of ground beetles, hydrophilid and staphylinid beetles, leaf beetles, and weevils. A wide and diverse range of aquatic and semi-aquatic vegetation is indicated by the reed beetles, not seen again higher in the sequence. These include Donacia dentata, which is found on Sagittaria sagittifolia and occasionally Potamogeton spp. (Hyman, Reference Hyman1992); it is not an alpine taxon, and its distribution in Britain today is largely southern (Hyman, Reference Hyman1992; Koch, Reference Koch1992); similarly, Plateumaris affinis is not an alpine species and, along with Donacia thalassina, is found on club rushes and sedges (Carex spp., Scirpus spp.) (Hyman, Reference Hyman1992; Koch, Reference Koch1992). Donacia semicuprea is found on Glyceria spp. (Bullock, Reference Bullock1993). Donacia aquatica cocoons on Ranunculus lingua, Sparganium simplex, Glyceria spp., and Carex spp. (Stainforth, Reference Stainforth1944), similar habitat for Donacia cinerea. This rich waterside vegetation fauna is also found in Stratum D, although in a more abundant and diverse assemblage. There are also, however, several species that indicate very cold winter conditions, including Boreaphilus henningianus and H. nordenskioeldi.

Table 6.

Beetle mutual climate range results from Hoxne.^a

^a See Figs. 1 and 2 for locations and stratigraphic context of samples.

^b The % overlap indicates the proportion of calibrated species in the assemblage that have mutual (i.e., overlapping) temperature ranges.

The other species common to Strata C and D is the saproxylic Stenoscelis submuricatus, which appears in both lowermost samples from Stratum C. Today, it lives in the sapwood of Populus and Salix, but also on Alnus, Fraxinus, and Fagus (Koch, Reference Koch1992). It is clearly a temperate species that has been found in other Hoxnian sites (e.g., Nechells, Warwickshire) as well as MIS 7, 9, and 5e sites. It was very abundant at the top of Stratum D. Its presence in Stratum C may seem anomalous but is not necessarily inconsistent with the assemblage, as its ability to live on a variety of tree hosts (including Salix, notwithstanding that Salix polaris is a tundra shrub rather than a tree) may have allowed it to be more tolerant of cool conditions. The original view, based on research by Gaunt et al. (Reference Gaunt, Coope, Osborne and Franks1972) and Hoffmann (Reference Hoffmann1954), was that this is a Mediterranean species, but more recent records show that it can also be found in western and eastern Europe across to Switzerland, Austria (Folwaczny, Reference Folwaczny, Freude, Harde and Lohse1983), Hungary (Szénási, Reference Szénási2014), montane regions of France and the Pyrenees (Hoffmann, Reference Hoffmann1954), and northern coastal France (GBIF, 2018). A review by Whitehouse (Reference Whitehouse2006) of many non-British Holocene fossil cossinine weevils, a group to which S. submuricatus belongs (note that some authors prefer to assign the species to the genus Rhyncolus), suggests their modern distribution is more likely to be influenced by availability of quality habitat rather than temperature per se. Recent species distribution models of Rhyncolus and other saproxylic taxa (Whitehouse, N., Milne, G., Cameron, A., unpublished data) suggest that many appear responsive to continental conditions and are especially adversely impacted by wetter conditions rather than cold or warm temperatures per se (see further discussion on Rhyncolus, in the “BMCR Method Palaeotemperature Reconstructions” section). They are, however, always associated with tree habitats. Several other species attest to the presence of trees, including Agrilus sp., whose larvae develop under the bark or wood of deciduous trees, while Eledona agricola lives on various fungi growing on deciduous trees, principally Polyporus sulphuraeus on species of Salix. There are, therefore, grounds for reconsidering whether S. submuricatus signifies warm conditions; its biology and ecology could be more in keeping with the rest of the faunal assemblage, indicative of cooler summer temperate conditions and cold winter conditions. There are three species typical of northern boreo-montane conditions, including P. lurida, Helophorus obscurellus, and S. metallica (the first two not now living in the British Isles). Simplocaria metallica is found in montane and alpine areas of central Europe (Koch, Reference Koch1989); while P. lurida is found as far south as Denmark, Germany, and Poland (Koch, Reference Koch1989; Gusarov, Reference Gusarov1995); and H. obscurellus is an Asiatic species, found as far west as the Kanin Peninsula, Russia (Coope, Reference Coope1994). None is associated with high arctic conditions, and all have been found associated with late glacial deposits during cooler interstadial periods; P. lurida, for instance, can be found in summer warmth up to +24.5°C and winter temperatures as mild as −0.4°C (Table 5). They are not inconsistent with cooler temperate conditions indicated by the rest of the assemblage, suggesting continental conditions of cold winters but moderately warmer summers during the early stages of Stratum C, compared with later samples from this stratum.

In summary, despite some ecological similarities between Stratum C samples 374–373 and Stratum D, on balance, those from Stratum C appear to constitute a coherent in situ assemblage, rather than one contaminated with reworked material, indicating cold winter temperatures and cool summer temperatures of sufficient spring and summer warmth to allow some tree growth, perhaps in the form of Salix and other cool-tolerant species (e.g., Betula) in the basal portions.

Additional notes on selected climatically significant species from Strata F and C (none of which occurred in Strata E or D) are given in the Supplementary Material, Section 4.

BMCR method palaeotemperature reconstructions

The BMCR results are illustrated in Figures 4 and 5 and summarized in Table 6.

Previously published BMCR results for Stratum F, based on 24 species, were January −26°C to −9°C and July +10°C to +12°C (Coope, Reference Coope, Singer, Gladfelter and Wymer1993). To explain the mixture of taxa (warm and cold) from this interval, Coope (Reference Coope, Singer, Gladfelter and Wymer1993) discussed the possibility that some were reworked, but considered it likely that the mixture was due to fairly rapid climate change in the interval represented by Stratum F, with July temperatures initially about +10°C but quickly rising to +15°C. However, rerunning Coope's (Reference Coope, Singer, Gladfelter and Wymer1993) list in BugsCEP (36 species, 89% overlap, which means 11% had outlier palaeotemperature ranges that were not mutual with the 89%) produced revised reconstructions suggesting substantially warmer climate (January −11°C to −4°C, July +15 to +16°C). Failure to find 100% overlap likely supports the idea that this interval encompassed a period of rapid climate change, such that the sampled assemblage does not represent a single snapshot of stable climate.

Stratum E yielded only nine beetle species from four small samples of the lacustrine sediments, and because so few predatory or scavenging species were found, no MCR reconstruction could be made. However, they include several species, not in the MCR database, which indicate that the temperatures at the time were at least as warm and possibly warmer than those of the present day and likely to be reasonably continental. Notable amongst these is the non-British Rhyncolus elongatus, a saproxylic species associated with decaying, rotting pine (Koch, Reference Koch1992). Its distribution is today in central and eastern parts of northern and central Europe up to southern Fennoscandia (Whitehouse, Reference Whitehouse1997, Reference Whitehouse2000). Recent exploratory research on this species and other members of this genus by N. Whitehouse, G. Milne, and A. Cameron (unpublished data), using the Maxent software maximum-entropy approach for modeling species niches and distributions, with 18 bioclimatic variables, supports the idea that its predicted climatic niche based on modern distributions is largely in eastern Europe and southern Sweden. Temperature seasonality appears to be the strongest driver of its predicted climatic niche, followed by mean diurnal range and minimum temperatures of the coldest month. This suggests that while seasonality is an important determinant of the distribution of R. elongatus, it is also sensitive to daytime temperature extremes and winter cold. Another species in the assemblage supports temperate conditions: the deathwatch beetle Xestobium rufovillosum is not found in its natural environment, represented by present climate, in northern Britain (Alexander, Reference Alexander1994).

The originally published BMCR results for Stratum D (Coope, Reference Coope, Singer, Gladfelter and Wymer1993; Ashton et al., Reference Ashton, Lewis, Parfitt, Penkman and Coope2008) were January −10°C to +6°C and July +15°C to +19°C. The revised results, based on 25 species, are January −10°C to + 1°C and July +16°C to +19°C (96% overlap), showing a July lower limit that is 1°C higher and a January upper limit 5°C lower than the original ones. The suboptimum overlap is likely due to minor in-sequence climate change (although there is no obvious evidence for this in the species list) and/or because the full climate range of one or more species has not been fully captured (Milne, Reference Milne2016).

Stratum C reconstructions using BugsCEP are presented on a sample-by-sample basis (Fig. 5, Table 6), as well as overall (Fig. 4), using the combined assemblages to represent Stratum C. Using 23 species of predatory or general scavenging beetles, combined BMCR results are January −16°C to −2°C and July +12°C to +14°C (82.6% overlap), indicative of somewhat warmer (though still markedly colder than present-day) summer conditions than quoted by Ashton et al. (Reference Ashton, Lewis, Parfitt, Penkman and Coope2008) (July at or <10°C) but consistent with their winter reconstruction (January about −15°C). The fact that 100% overlap was not achieved suggests that climate variation occurred during the times represented by the relatively coarse sampling interval, and this is borne out by the BMCR sample-by-sample reconstructions (Fig. 5) and supported by evidence from the high-resolution ostracod results described earlier.

The sample-by-sample reconstruction for Stratum C (Fig. 5, Table 6) demonstrates clear differences in climatic conditions through the sequence. Working up from the base of the sequence, where summer temperatures were only slightly cooler (+13°C to +15°C in sample 374) than today, we see cooling to +9°C to +11°C (sample 369), a mean July temperature drop of at least 2°C alongside a slight amelioration of winter temperatures through the same interval, with the upper limit of the January range rising from −13°C (sample 374) to −9°C (sample 369). Also noteworthy is the step between the reconstructions for samples 373 and 372, which show no overlap of their reconstructed temperature ranges, indicating a significant and relatively rapid change involving summer cooling and winter warming and suggesting a transition from continental to oceanic climate. It must also be noted, however, that the size of the winter envelopes is large for these samples and extends to very low values. The coldest envelopes are particularly controlled by two species that can tolerate extremely cold winters, Boreaphilus henningianus and H. nordenskioeldi, both of which are recorded as being associated with winter temperatures as low as −34°C and −35°C, respectively (Table 5). It is impossible to determine where exactly within an envelope the true temperature was situated, but it seems likely, given these species also occur in adjacent assemblages that give slightly warmer reconstructed temperatures, that conditions were never as cold as the lowest limits that the reconstructions might imply. The upper samples provide reconstructions that suggest climatic warming, with ranges of January −16°C to 0°C and July +10°C to +14°C indicated by the topmost assemblage (sample 328). Note that combining the sample-by-sample results to obtain overall representative ranges for Stratum C produces MGRs (January −13°C to −12°C, July +11°C to +13°C) that differ from those obtained by applying the BMCR method to the combined assemblage from all of the Stratum C samples (January −16°C to −2°C, July +12°C to +14°C), which produced an overlap of only 82.6%; this demonstrates that low sampling resolution may have merged short-term climatic changes to which the beetle taxa responded, resulting in gaps between the calibrated ranges of some included taxa and thence a lower overlap percentage for the whole assemblage.

Importantly, this BMCR reconstruction does not use plant-associated species and is therefore independent of (as well as consistent with) the overall beetle assemblage qualitative evidence (discussed earlier). The diversity of wood-loving taxa evident in Stratum D is absent from the rest of Stratum C, and conditions are cooler than Stratum D, becoming increasingly cooler over time (although S. submuricatus reappears in the upper portions of the sequence); therefore, the Stratum C assemblages represent an increasingly cooling community that is largely distinct from the temperate community represented by Stratum D.

In summary, the sample-by-sample reconstruction suggests a previously unknown summer cooling event coincident with winter amelioration within Stratum C. However, its registration within winter temperatures is much less clear due to the much larger envelope sizes generated by the present BugsCEP MCR package. New envelopes that have recently been developed and tested independently using modern modeling techniques (Milne, Reference Milne2016) suggest that the current envelopes tend to overestimate winter cold and summer warmth when compared against 12 modern test sites and species lists. The new envelopes are not yet fully operational, so are not used here.

Finally, the types of species used for each sample BMCR suggest the reconstructions are robust. A common assumption of the BMCR methodology has been that increasing the number of taxa used in a reconstruction will give a more precise (i.e., narrower) temperature range (Atkinson, T., personal communication, 2013). Modern testing work has shown that there is no discernible relationship between the number of taxa used in the BMCR reconstruction and the obtained BMCR temperature ranges for summer or winter (Milne, Reference Milne2016), but that understanding species niche traits is crucial. As Russell Coope highlighted both vocally and in print, “species should not be treated merely as ciphers in some purely mathematical model” (Coope, Reference Coope2010, p. 1511); thus, simply including more beetles in a reconstruction will not necessarily produce a better result. What may be more pertinent is whether the species are eurytherms (climate generalists) or stenotherms (climate specialists): the type of species used in a reconstruction has huge importance for the output. It is thus vitally important that users of any reconstruction system understand the climatic and ecological conditions associated with the taxa used in order to understand the robustness of a reconstruction. In this reconstruction, a high number of stenotherms have been used, increasing the confidence levels of the reconstruction.

Palaeoclimatic implications of flora

Several plant indicator taxa recorded at Hoxne (West, Reference West1956) are indicative of threshold temperatures (Figs. 4 and 5). Of the pollen taxa, Hedera (assumed to be Hedera helix; West, Reference West1956) requires mean air temperatures of −1.5°C or higher in January and +13°C or higher in July (Iversen, Reference Iversen1944; West, Reference West1956). Ilex (assumed to be Ilex aquifolium; West, Reference West1956) requires a mean January temperature of at least −0.5°C or 0°C (the latter value is used herein) (Iversen, Reference Iversen1944; West, Reference West1956; Walther et al., Reference Walther, Berger and Sykes2005). Typha latifolia needs a mean July temperature of +13°C or above (Isarin and Bohncke, Reference Isarin and Bohncke1999). All three taxa occur in Strata C, D, and E at Hoxne, with the last mentioned being found additionally in Stratum F (West, Reference West1956). Minimum mean July air temperature requirements of four plant macrofossil species were obtained from Isarin and Bohncke (Reference Isarin and Bohncke1999): B. nana (+7°C), Ceratophyllum demersum (+13°C), Miriophyllum spicatum (+10°C), and Sanguisorba officinalis (+9°C). All four occur in Stratum C, and C. demersum also occurs in Strata D, E, and F (West, Reference West1956).

Another potentially significant plant taxon, the water fern Azolla filiculoides, was commented on by West (Reference West1956), who found it commonly in Stratum C (but suggested that it may have been reworked from older strata), abundantly in Stratum D, and occasionally in the middle and upper parts of Stratum E. He considered its palaeoclimatic value doubtful, as it is widely distributed in the Americas and has an “erratic” behaviour in western Europe, and he noted that it survives under ice but is killed by winter frosts. On the other hand Hall (appendix A in Coxon, Reference Coxon1985) noted the occurrence of Azolla megaspores at nearby Athelington as being of particular interest, as it is typical of Hoxnian zones II and III, and because he considered it indicative of somewhat higher summer temperatures than at the present day. Experimental evidence has shown that adult plants survived at −2°C but were killed at −4°C in the laboratory, but they can survive air temperatures (or being encased in ice) of −5°C or even lower in the wild, given the right microclimatic conditions (Janes, Reference Janes1998). It apparently grows best at +15°C to +20°C and high irradiance (Janes, Reference Janes1998), although its optimum temperature for nitrogen fixation and oxygen evolution is +25°C (Wong et al., Reference Wong Fong Sang, Van Vu, Kijne, Thanh Tam and Planque1987). Populations in Poland have been observed to survive frosts down to −22°C (Szczęśniak et al., Reference Szczęśniak, Błachuta, Krukowski and Picińska-Fałtynowicz2009). In the light of this evidence, its status as a thermophile in the context of the British Quaternary is questionable, to say the least; consequently, we have not included it in our analyses.

Molluscan assemblages

Nonmarine molluscs were obtained from the same samples processed for other proxies as part of the Ancient Human Occupation of Britain (AHOB) project excavations (Ashton et al., Reference Ashton, Lewis, Parfitt, Penkman and Coope2008), from Strata E, D, C, and B2 (Figs. 1 and 2); see the Supplementary Material, Section 1, for further details. The first analysis of nonmarine molluscan remains spanning the complete Hoxne sequence was undertaken by White (Reference White2012), and relevant data are published here for the first time (Table 7). Stratum F data from the quantified faunal list of Sparks (Reference Sparks1956) have been included to complete the Hoxne molluscan succession, but it should be noted that they came from a more marginal part of the lake than the other material.

Table 7a.

Freshwater Mollusca from Hoxne, Suffolk; relative abundance (%) values used to plot Fig. 6.^a Note that Table 7 is in four parts: 7a–d.

Table 7b.

Table 7c.

Table 7d.

^aData from Sparks (1956) and White (2012). See Figs. 1 and 2 for locations and stratigraphic context of samples.

The Stratum F freshwater assemblage is dominated by gastropods, namely Gyraulus laevis (35.8%) and Gyraulus crista (34.4%), with subordinate numbers of Valvata piscinalis (8.0%) and Radix balthica (7.8%). Hippeutis complanatus is also well represented (3.9%). Several other gastropod taxa were recorded at low frequencies. Bivalves, predominantly Pisidium nitidum, account for only 9.8% of the overall assemblage. The molluscan evidence from Stratum F is indicative of a relatively shallow lacustrine environment, rich in aquatic vegetation and inhabited by species tolerant of colder conditions (White, Reference White2012).

Assemblages from the overlying sediments of Stratum E are dominated by the freshwater gastropod V. piscinalis, which constitutes more than 90% of most samples and is indicative of a relatively deep-water habitat. Also present at low frequencies throughout Stratum E is the extinct bivalve Pisidium clessini, which occurs in several middle Pleistocene interglacial periods and is a member of the “Rhenish suite” of aquatic molluscs characteristic of Hoxnian pollen substage HoIIb (White et al., Reference White, Preece and Whittaker2013). None of the molluscan taxa present within Stratum E are obligate thermophiles, but all are compatible with the temperate climate indicated by the Hoxne pollen record (West, Reference West1956; Turner, Reference Turner1970).

The molluscan assemblage of Stratum D includes V. piscinalis, Bithynia tentaculata, Anisus leucostoma, and indeterminate species of Pisidium (Fig. 6). The shells from this part of the sequence were in relatively poor condition, with a distinctive dark stain indicative of the peaty sediments from which they were recovered. It is not clear whether this assemblage represents an in situ fauna or material derived from the underlying lacustrine deposits. Only the early part of the period represented by Stratum D, during which lake water levels were still reasonably high (cf. Coope, Reference Coope, Singer, Gladfelter and Wymer1993), would have provided a suitable habitat for V. piscinalis and B. tentaculata.

Figure 6.

Selected freshwater Mollusca from Hoxne Strata F to B.

Mollusc shells recovered from Stratum C in Area VII were relatively poorly preserved, and a large proportion could not be identified to species level, leading to underrepresentation of species. The assemblages from Stratum C contain no obligate cold-adapted species but can be easily distinguished from the underlying temperate faunas from the earlier parts of the Hoxne sequence by the dominance of various species of Pisidium and the reduced abundance of V. piscinalis (Fig. 6). Similar bivalve-dominated faunas have been reported from other British cold-stage localities, such as Beetley (Preece, in West, Reference West1991), and in modern situations such as Scandinavian lakes (Kuiper et al., Reference Kuiper, Økland, Knudsen, Koli, Proschwitz and Valovirta1989; Økland, Reference Økland1990). Shells representing species common in the underlying sediments, such as Valvata cristata and B. tentaculata, were probably reworked from older deposits, although alternative interpretations are possible (see “Discussion: Reworking in Stratum C Assemblages”). However, several bivalve species present in Stratum C, notably Pisidium amnicum and Pisidium henslowanum, can be regarded as part of an in situ fauna, because with the exception of single specimens of the latter recorded in Stratum F and the basal sample of Stratum E, they are absent from the earlier part of the Hoxne sequence (Fig. 6).

In the fluvial deposits of Stratum B2, the molluscan assemblages include a more diverse gastropod fauna dominated by V. piscinalis and B. tentaculata and are broadly similar to the temperate fauna encountered in Stratum E (Fig. 6).

Calculation of the ratio of Bithynia shells to opercula has proved to be a useful means of assessing the energy dynamics of depositional environments (e.g., potentially discriminating between lacustrine and fluvial conditions) and can contribute to assessment of reworking. At Hoxne, the ratio of Bithynia shells to opercula within Strata F and E is balanced at close to 1:1 in most samples, indicative of minimal postmortem sorting and suggestive of a low-energy depositional environment consistent with a lake. In contrast, ratios of 1:3 and 1:4 typify samples recovered from Stratum C and may be considered to support the idea that Bithynia is a reworked element in these assemblages. Stratum C Samples in which only opercula are represented might indicate some decalcification of the sediments. It is also possible, however, that the Bithynia in Stratum C are in situ but were subject to postmortem sorting in the lake environment. Alexandrowicz (Reference Alexandrowicz1999) described a process whereby shells of dead Bithynia float for a more prolonged period of time than the opercula, which detach more quickly and sink, while the shells are driven shoreward by wind and wave processes. In the fluvial Stratum B2, evidence for higher-energy conditions is provided by Bithynia shell:opercula ratios of 1:10 (White, Reference White2012). These interpretations should be treated cautiously, as the numbers on which the shell:opercula ratios are based are generally very low (20 specimens or fewer) in samples from Strata E, D, and C, although many of the assemblages from Stratum B2 are big enough (>50) for the ratios to be considered more reliable.

Climatic significance of the molluscan assemblages

The molluscan record from Hoxne does not provide strong evidence for significant climatic variation due to the relatively broad tolerances of most of the taxa represented. Moreover, as noted earlier, several features of the record provide critical evidence relating to the potential for reworking and the taphonomic integrity of assemblages from Stratum C.

The most climatically significant taxon present in Stratum F is the gastropod G. laevis, which is characteristic of late-glacial or early postglacial molluscan faunas. It is common in the “Arctic Fresh-water Bed” at Ostend, Norfolk (Parfitt et al., Reference Parfitt, Coope, Field, Peglar, Preece and Whittaker2010), and a fauna dominated by G. laevis was reported in association with an early postglacial pollen profile at Tottenhill (Ventris, Reference Ventris1996). Similar freshwater assemblages dominated by G. laevis and G. crista have been documented from both late Anglian/early Hoxnian and late Devensian/early Holocene sequences (Kerney, Reference Kerney and Shotton1977). Given that the molluscan evidence from Stratum F is derived from a single bulk sample, it is difficult to compare with the data provided by the beetle faunas from this part of the Hoxne sequence, but they are not incompatible.

In the overlying temperate lacustrine deposits of Stratum E there are no obligate cold- or warm-adapted species that would place significant constraints on palaeoclimatic reconstructions. One species that provides some evidence for prevailing temperatures is V. cristata, which is not known from any Pleistocene cold-stage deposits. However, it occurs only as a rare element of the molluscan assemblages from Stratum E due to the deep-water environment represented by this unit, which is suboptimal for V. cristata. The limited molluscan fauna preserved in the Stratum D sediments provides little in the way of climatic information.

The molluscan assemblages from Stratum C contain no obligate cold-adapted species but can easily be distinguished from the preceding parts of the Hoxne sequence by the dominance of various species of the bivalve genus Pisidium. Similar faunas have been reported from other cold-stage localities such as Beetley (Preece, in West, Reference West1991) and in modern habitats such as Scandinavian lakes (Kuiper et al., Reference Kuiper, Økland, Knudsen, Koli, Proschwitz and Valovirta1989; Økland, Reference Økland1990). The presence of V. cristata in Stratum C at higher frequencies than the underlying Stratum E deposits has been attributed to reworking (cf. White, Reference White2012), although the rarity of this species in the upper part of the Stratum E deposits suggests that these shells might have been derived from sediments now missing from the sequence (i.e., the hiatus between Strata D and C). Alternatively, if they are genuinely part of the Stratum C fauna, their occurrence together with cold-tolerant taxa requires explanation. Another gastropod species known only from interglacial and interstadial sequences in Britain is B. tentaculata; this taxon is primarily represented in Stratum C by opercula, suggestive of reworking, although as noted earlier, other explanations are possible.

The assemblages from Stratum B2 are very similar to those from Stratum E, indicating a return to temperate climatic conditions. The similarity of the assemblages in these units is somewhat surprising, given the marked sedimentologic differences indicative of lacustrine (Stratum E) and fluvial environments (Stratum B2). The absence of P. henslowanum from Stratum B is noteworthy, given than it is a common species within Stratum C and prefers moving-water environments.

Small vertebrate assemblages from Stratum C

Small vertebrates were obtained from successive samples through Stratum C (Figs. 1 and 2); additional specimens were extracted from larger subsamples collected for insect remains (see the Supplementary Material, Section 1, for further details). A summary of the small vertebrates and other fossils recovered from the sieved samples is given in Table 8 and Figure 7. Bone fragments are not distributed uniformly through the sequence, with concentrations of bones in some horizons separated by samples that are virtually devoid of vertebrate material. Fish remains are the most abundant component, with identified specimens that include scales of Perca fluviatilis (perch), a tooth of Esox lucius (pike), pharyngeal teeth of cyprinids (carp family), and the distinctive spines of Pungitius pungitius (nine-spined stickleback). The bulk samples processed for insects yielded tiny pharyngeal bones of Alburnus alburnus (bleak) and Phoxinus phoxinus (minnow). Taken together, the general composition, low diversity of identified species, and relatively small size of the fishes imply a small waterbody, such as a pool or backwater.

Figure 7.

Abundance of small vertebrate and other fossil remains in Stratum C (Area VII). Conspicuous organic seams correspond to peaks in the abundance of plant and other fossils remains in samples 314, 311, 308–307, and 304. Chalk stringers and lenses of chalky and gravelly sand occurring toward the top of the sequence (equivalent to samples 304–301) are associated with reworked Chalk Group fossils (corals, ostracods), pre-Quaternary fish remains, and marine molluscs. Occasional Mesozoic ostracods and foraminifera are also found in the lower organic-rich horizons.

Table 8.

Small vertebrate remains from Stratum C, Area VII sample column, Hoxne.^a

^a See Figs. 1 and 2 for locations and stratigraphic context of samples.

Only a few small mammal specimens were recovered from Stratum C. Among the more complete specimens, the presence of Dicrostonyx sp. (collared lemming) was determined from a molar fragment (sample 304) and a second upper molar (M²) fragment (sample 301). At least one other microtine rodent (vole) is represented by material too fragmentary to identify more precisely. For further details and discussion see the Supplementary Material, Section 5.

Taphonomy and environmental implications of the Stratum C vertebrates

Taking the assemblage as a whole, the vertebrate species recovered from Stratum C appear to be climatically coherent. The assemblage includes Dicrostonyx, a cold-indicating lemming of tundra and mammoth steppe (Guthrie, Reference Guthrie1968), together with fish species (P. fluviatilis, E. lucius, P. pungitius) that have geographic ranges that today extend above the Arctic Circle. The cyprinids could also belong to the same cold-stage assemblage. Although many cyprinid species are restricted in their ability to colonise the far north due to insufficiently high summer water temperatures in order to spawn successfully (Wheeler, Reference Wheeler1977), other cyprinids (including bleak, A. alburnus, and minnow, P. phoxinus, both found in Stratum C) can survive for long periods in ice-covered water and are found in rivers and lakes in the tundra, some as far north as the Arctic Ocean. The closest region to Hoxne where the geographic ranges of these taxa overlap today is located at the southern limit of the tundra south of the Kanin Peninsula. Today, this area is characterised by extremely cold winters (mean air temperatures of the coldest month between −14°C and −8°C) and summers with temperatures of the warmest month between +7°C and +14°C. Whether these estimates can be applied to Stratum C, however, depends on two critical assumptions: first, that the vertebrate assemblage represents animals that lived in the vicinity of the site during the period of accumulation of Stratum C, and second, that conditions did not change significantly during the deposition of this unit.

The first of these assumptions was investigated by examining the stratigraphic occurrence of different elements of the fossil assemblages recovered from the bulk samples (Fig. 7). Sedimentologically, Stratum C consists of silty muds interspersed with thin layers of organic detritus and, in its upper part, lenses of chalky gravel and sand. These coarser horizons represent episodic pulses of erosion and redeposition of older sediments exposed on the margins of the basin. These sediments contain a mix of fossil material, some of which (Mesozoic corals, ostracods and foraminifera, and Cenozoic marine molluscs) have undergone multiple cycles of transportation and redeposition, with initial erosion from the bedrock, transportation by glacial processes, and winnowing before burial at the depositional site. Plio-Pleistocene “crag” and glacial deposits (Lowestoft Till and “Corton beds”) at the edge of the Hoxne basin (Lord, Reference Lord and Dixon2012) provide a probable source for these reworked fossils. The horizons in Stratum C containing reworked pre-Quaternary fossils also correspond to peaks in the abundance of Quaternary fish, small mammals, and plant macrofossils, the last including well-preserved seeds and wood fragments up to 5 cm in diameter. This mix of debris appears to have been transported by pulses of higher-energy currents that washed the detritus into the basin. Marginal exposures of the Hoxnian lake muds provide a possible source for the Quaternary plant macrofossils (Stratum D) and fish remains (Stratum E). However, there is no obvious difference in the state of preservation (color differences, weathering, abrasion) of the vertebrate remains that identifies a reworked component in the Quaternary assemblage. With respect to taphonomy, while a few fish specimens in the Stratum C assemblage may be reworked from the interglacial deposits, the dominance of taxa (pike, perch, nine-spined stickleback) that are otherwise rare in the earlier lake muds suggests that the Stratum C accumulation reflects an essentially indigenous fauna. Similarly, the lemming teeth are unlikely to have been reworked from the interglacial sediments, and their presence in Stratum C plausibly indicates severe climatic conditions, at least during the deposition of the upper part of that unit.

Stable isotopes

Details of δ¹⁸O and δ¹³C measurements of ostracod (Ilyocypris and Cytherissa) shells from Stratum C are shown in Figure 5 and Table 9. The oxygen isotope composition of lacustrine carbonate is controlled by the temperature and isotopic composition of lake water. The isotopic composition of lake water, in turn, is determined by the composition of rainwater and inputs from groundwater or surface runoff, catchment effects (sensu Gat and Lister Reference Gat, Lister, Frengel, Stauffer and Weiss1995), and any evaporative enrichment. For ostracod shells, there is generally a positive offset from oxygen isotope equilibrium that is taxon specific (e.g., von Grafenstein et al., Reference von Grafenstein, Erlernkeuser and Trimborn1999; Decrouy, Reference Decrouy, Horne, Holmes, Rodriguez-Lazaro and Viehberg2012). For C. lacustris, the offset is well characterised as +1.2‰ ± 0.23 (von Grafenstein et al., Reference von Grafenstein, Erlernkeuser and Trimborn1999). For Ilyocypris, the other genus analysed here, there is no published offset. Moreover, we do not have sufficient numbers of paired analyses from Hoxne to be able to estimate the effect with confidence. However, values for paired analyses of Candona neglecta, which has a well-established vital offset, and I. bradyi in Belis and Aritztegui (Reference Belis and Ariztegui2004) have a mean difference of −1.78 ± 0.80 (n = 5), indicating that the vital offset for the latter is smaller than that for the former. We used a bootstrap technique (Efron and Tibsharini, Reference Efron and Tibsharini1993) to estimate the mean of the vital effect for Ilyocypris as +0.41‰ ± 0.31. The fact that the δ¹⁸O values for C. lacustris are more positive than those for Ilyocypris for the two levels at Hoxne for which we have values for both species is at least consistent with this result.

Table 9.

Carbon and oxygen stable isotope results (‰) from Stratum C ostracod shells, Hoxne.^a

^a See Figs. 1 and 2 for locations and stratigraphic context of samples.

^b Sample 327 is a test sample from the same level as 362.

The carbon isotope composition of lacustrine carbonates is strongly controlled by the δ¹³C of dissolved inorganic carbon (DIC): fractionation of carbonate isotopes between carbonate and DIC is minimally affected by temperature and, for ostracod shells, appears to be an equilibrium process (Keatings et al., Reference Keatings, Heaton and Holmes2002). The isotopic composition of DIC is, in turn, controlled by the δ¹³C of groundwater inputs, exchange with atmospheric CO₂, and processes occurring in the lake, especially the preferential uptake of ¹²C during aquatic photosynthesis and the return of ¹³C-depleted organic matter from plants as a result of decay (Talbot, Reference Talbot1990).

The δ¹⁸O values for Ilyocypris fall within a fairly narrow range of −2.47‰ to −1.08‰, which equates to −2.88‰ to −1.49‰ at oxygen isotope equilibrium. Owing to taxonomic uncertainties and limited information on life cycles, it is difficult to be precise about the calcification season of the Ilyocypris specimens that were analysed. However, given that reconstructed temperatures were lower than present day temperatures during the deposition of Stratum C, especially during winter, it seems reasonable to assume that the ostracods were more likely to have reached adulthood during the warm season: this is supported for Ilyocypris sp. by limited evidence from Lake Geneva (Decrouy, Reference Decrouy2009). If this is correct, the calculated δ¹⁸O for Hoxne lake water during Stratum C times (based on reconstructed July temperature of 14°C) would have been −2.3‰, substantially higher than precipitation. Present-day precipitation of eastern England has a δ¹⁸O value of around −7‰ (Darling et al., Reference Darling, Bath and Talbot2003), and values during the colder climate of Stratum C times may have been even lower. Higher reconstructed δ¹⁸O values for the Hoxne Lake are best explained by evaporative enrichment. The limited number of δ¹⁸O values for C. lacustris provides partial support for these conclusions (Fig. 5, Table 9). Two of the levels yield similar results to those derived from analyses of Ilyocypris, assuming warm-season calcification, whereas for the remaining two, values are much more negative. However, even for the most negative values, reconstructed δ¹⁸O values for lake water (around −5.5‰) are higher than those for precipitation. Whether these values reflect variations in the lake environment during the formation of Stratum C remains unclear. Our assumption that temperatures remained constant during this interval is also questionable in view of the MOTR evidence for variation. However, substantially lower temperatures would be required for the ostracods to have calcified in water with a δ¹⁸O value close to the likely values of contemporary rainfall. We therefore conclude that Hoxne lake water was subject to evaporative enrichment throughout much or all of the time represented by Stratum C.

The carbon isotope values are more difficult to interpret, as they are controlled by a number of variables, as discussed earlier, and we have limited constraint on any one of these. Our conclusions must therefore remain speculative. Carbon supply to DIC in the Hoxne lake could have come from chalk, soil CO₂ (ultimately derived from catchment vegetation), and aquatic vegetation, potentially modified by aquatic photosynthesis and equilibration with atmospheric CO₂. Carbon in groundwater would have δ¹³C value in the range −16‰ to −10.5‰, assuming a 50:50 mixture of carbon from chalky till (δ¹³C for Upper Chalk Formation = +2‰ to +3‰: Jarvis et al., Reference Jarvis, Gale, Jenkyns and Pearce2006) and soil organic matter (δ¹³C = −24‰ to −24‰ in areas of C3 vegetation: Smith and Epstein, Reference Smith and Epstein1971; Deines, Reference Deines, Fritz and Fontes1980). Ostracod shells that calcified in equilibrium with this DIC would have δ¹³C values between −15‰ and −9.5‰. However, ostracods from Hoxne Stratum C have δ¹³C values between −3.1‰ and −5.7‰, indicating that other sources of carbon and/or processes must have affected DIC composition. The addition of ¹³C-depleted carbon to the DIC pool from respiration or decay of aquatic plants would decrease the δ¹³C of DIC. However, the uptake of ¹²C by aquatic plants for photosynthesis and equilibration with atmospheric CO₂ both have the potential to increase δ¹³C values (e.g., Talbot, Reference Talbot1990). Without further evidence, it is difficult to be certain which was the dominant process, although partial equilibration with atmospheric CO₂, which is a slow process, is consistent with a relatively long residence time for the lake, a condition that would favour evaporative enrichment of water isotopes, which is consistent with the oxygen isotope values and some of the faunal assemblage data.

MPC palaeotemperature reconstructions

The acronyms used in this section are explained in the “Methods,” but a summary look-up table is provided for convenience (Table 10). The MPC approach produced MORs for stratum C (July) and Strata D, E, and F (all January and July), while Stratum C (January) required acceptance of the only MGR result. Only a single proxy method (MOTR) was available for Stratum B, and no proxies usable for quantitative palaeotemperature reconstruction were available from Strata A and G. These results are presented here in chronological order from bottom to top and illustrated in Figure 4. For comparison, present-day mean monthly air temperature values for the vicinity of Hoxne are +3.0°C for January and +16.5°C for July.

Table 10.

Look-up table of method acronyms used in the multiproxy consensus palaeotemperature reconstructions section of the text.

Stratum C

The January reconstruction uses a combined BMCR overlapping the MOTR to produce an MOR of −7°C to −2°C, indicating winters between 5°C and 10°C colder than today. However, accommodating the two floral indicator taxa, which have minimum thresholds higher than the upper limit of the MOR by 0.5°C (H. helix) and 2.0°C (I. aquifolium), results in an MGR of −2°C to 0°C, indicating winters between 3°C and 5°C colder than today. This gap is accommodated within the variability apparent in the high-resolution record from Stratum C (discussed below). Whether the MOR or the MGR is accepted, the indications are clear that Stratum C summers and winters were definitely colder than during the interglacial period. The upper limit of the combined BMCR for July coincides with the lower limit of the MOTR, producing an MOR of precisely +14°C, indicating summers 2.5°C cooler than today. All three CTF ranges and the floral indicator taxa for Stratum C are consistent with this result.

In the other sedimentary units, it has necessarily been assumed that the sampled interval is representative of the unit as a whole, but in the case of Stratum C, high-resolution sampling has produced a detailed ostracod record that reveals variability within the unit. Reconstructed July palaeotemperature ranges from beetles and chironomids (neither of which were sampled at the same high resolution as the ostracods) show some overlap and some gaps when mapped onto the detailed MOTR results (Fig. 5, Table 11). The gaps may be the result of a lack of exact equivalence (both geographic within the Hoxne site and stratigraphic within Stratum C) in sampling for the different proxies, in view of the possibility of climate variation within this interval, as shown by the high-resolution MOTR results. We cannot rule out the possibility that at least some of the variation in the MOTR results may be due to “noise” attributable to low numbers of taxa recovered from some samples (the maximum number of species used for the MOTR analysis in a single sample was nine, but in a few cases only one or two could be used and inevitably produced very wide ranges), nor can we completely rule out contamination of the assemblages with reworked specimens. The BMCR results, although based on samples mostly of coarser stratigraphic resolution than the MOTR data and covering only parts of the succession, similarly show variation within Stratum C. However, the July results from the BMCR and CTF methods show a general correspondence with the minimum curve of the July MOTR results; similarly the January BMCR results mostly fall around the minima of the January MOTR results, although there are gaps in the lowest samples, the BMCR method indicating colder temperatures than the MOTR method. This suggests that the January and July MOTR minima may offer the best representation of the actual palaeotemperatures. The occurrences of the extinct ostracod I. quinculminata, which we consider to be possibly indicative of continental climate with warmer summers and colder winters than today, correspond to some extent with some of the warmer July intervals with cooler January temperatures.

Table 11.

Stratum C individual proxy-based palaeotemperature reconstructions and multiproxy consensus (MOR in bold, MGR in italics) results.^a

^a For sample locations and equivalents, see Figs. 1 and 2; for acronyms, see Table 10.

The maxima of the January BMCR results generally fall substantially below the threshold temperature indicated by pollen taxa H. helix and I. aquifolium, but in one case (a beetle sample equivalent to ostracod samples 343–345), the BMCR maximum matches the pollen taxa thresholds. Unfortunately, we have no way of determining precisely from West's (Reference West1956) pollen data the equivalence of his Stratum C samples with ours; his pollen diagrams for Stratum C (West, Reference West1956, figs. 22–24) show sporadic rather than continuous occurrences of the two pollen taxa. Molluscs and vertebrates recovered from Stratum C are generally consistent with the quantitative reconstructions; Dicrostonyx is indicative of tundra and mammoth steppe, while the temperature ranges of the suggested modern analog region for the fish assemblage has a July maximum that matches our July MOR, and although its January maximum is 6°C colder than the low end of our January MGR, this is easily accommodated within the variability shown by the high-resolution MOTR and BMCR results.