Palaeogeography, Palaeoclimatology, Palaeoecology 365–366 (2012) 124–135

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Palaeogeography, Palaeoclimatology, Palaeoecology

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A high-resolution shallow marine record of the Toarcian (Early Jurassic) OceanicAnoxic Event from the East Midlands Shelf, UK

Bryony A. Caswell ⁎, Angela L. CoeDepartment of Environment, Earth and Ecosystems, Centre for Earth, Planetary, Space and Astronomical Research, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK

⁎ Corresponding author at: School of Environmental ScNicholson Building, Liverpool, L69 3GP, UK. Tel.: +44945196.

E-mail address: B.A.Caswell@liverpool.ac.uk (B.A. Ca

0031-0182/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.palaeo.2012.09.021

a b s t r a c t

a r t i c l e i n f o

Article history:Received 2 September 2011Received in revised form 20 August 2012Accepted 18 September 2012Available online 25 September 2012

Keywords:Toarcian (Early Jurassic)BivalvesOceanic anoxic eventEast Midlands ShelfCarbon isotopesBiostratigraphy

The global deposition of organic rich-mudrocks, a mass extinction, and marked geochemical changes in thesedimentary rocks and fossils deposited during the early Toarcian indicates a period of extreme environmentalchange and an oceanic anoxic event. This study investigates at a high-resolution the environmental and bioticchanges that occurred during the event in a shallow-marine area using rocks deposited on the East MidlandsShelf, UK. We present a new graphic log, geochemical data (δ13Corg, total organic carbon, CaCO3, total sulphur andtotal nitrogen), and benthic macroinvertebrate ranges from North Quarry, Holwell, Leicestershire, UK.Comparison of the ammonite ranges between the Cleveland Basin (Yorkshire) and EastMidland Shelf successions inthe UK shows that there is a hiatus on the shelf across the Dactylioceras semicelatum–Cleviceras exaratum Subzoneboundary. The new high-resolution geochemical data show that an ~−5‰ δ13Corg excursion occurs similar toother lower Toarcian sections in the world, δ13Corg shifts ‘A’ to ‘C’ are missing in the hiatus, but that δ13Corg shift‘D’ is present.Similar to the Cleveland Basin, the C. exaratum Subzone of the East Midlands Shelf succession is dominated bythree epifaunal bivalve species, but their ranges differ significantly between the successions. An increase inthe faunal diversity occurs within the upper C. exaratum Subzone at Holwell and other UK sections. The bioticdata indicate that conditions on the East Midlands Shelf were more hospitable than in the Cleveland Basin.The bivalve populations on the shelf may have provided a source of new recruits for the basins.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Geochemical and stratigraphical studies have shown that the earlyToarcian (Early Jurassic) was a period of considerable environmentalchange (e.g. Cohen et al., 2004; Cohen et al., 2007; Hesselbo et al.,2007) and that it coincided with the mass extinction of both marineand terrestrial biota (Benton, 1995; Little and Benton, 1995; Aberhanand Fürsich, 1996; Hori, 1997; Gahr, 2005; Bambach, 2006; Zakharovet al., 2006; Caswell et al., 2009). The event is associated with wide-spread burial of marine carbon (Jenkyns et al., 2002; Fig. 1A), and alarge negative carbon isotope excursion in marine organic matter(δ13Corg) and marine carbonates (δ13Ccarb) in successions depositedin the major oceans at that time (Fig. 1A; Boreal Ocean: Suan et al.,2011; Tethys Ocean: Hollander et al., 1991; Jiménez et al., 1996;Hesselbo et al., 2000; Jenkyns et al., 2001; Röhl et al., 2001; Kempet al., 2005; van Breugel et al., 2006; Cohen et al., 2007; Hesselboet al., 2007; Suan et al., 2008; Hermoso et al., 2009; Hesselboand Pienkowski, 2011; and Pacific Ocean: Al-Suwaidi et al., 2010;Caruthers et al., 2011; Gröcke et al., 2011). Additionally, a negative

iences, University of Liverpool,1517 954390; fax: +44 1517

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δ13C excursion has been observed within fossilised wood (Hesselboet al., 2000; McElwain et al., 2005; Hesselbo et al., 2007) andphytoclast separates (wood fragments and leaf cuticles; Hesselboand Pienkowski, 2011) indicating that the event also influenced theatmospheric system. The negative δ13C excursion is synchronouswith evidence for higher seawater surface temperatures (7 to 13 °C;McArthur et al., 2000; Bailey et al., 2003; Gomez et al., 2008; Suanet al., 2008), enhanced rates of global chemical weathering (Cohenet al., 2004), widespread marine anoxia (Pearce et al., 2008), andhigher relative sea-level (Hesselbo and Jenkyns, 1998).

The event was interpreted to represent an oceanic anoxic event(OAE) by Jenkyns (1988) based on the widespread deposition oforganic-rich lithologies (Fig. 1A–B). Geochemical proxy evidence hassince shown that the water column was euxinic and that the extentof euxinia fluctuated (e.g. degree of pyritisation (Raiswell et al.,1993), total organic carbon/total sulphur ratio (Raiswell et al., 1993;Kemp et al., 2011), Re/Mo, [Mo], and Mo-isotopes (Pearce et al.,2008)), and that the global areal extent of marine anoxia periodicallyincreased at this time (Pearce et al., 2008). The presence of biomarkersfrom phototrophic anoxygenic marine bacteria from several samplesdeposited during the carbon isotope excursion shows that marineeuxinia locally extended into the euphotic zone (Schouten et al.,2000; Pancost et al., 2002; Schwark and Frimmel, 2004; van Breugelet al., 2006).

A

localities with shallow-marine and terrestrial facies showing evidence of the early Toarcian eventlocalities with deep sea faciesa

localities with continental shelf organic-rich facies

C

B

Fig. 1. Palaeogeographical maps for the early Toarcian. (A) Global palaeogeographical map showing distribution of organic-rich facies modified from Cohen et al. (2007) and ref-erences therein. Box shows (B). (B) Toarcian palaeogeography of NW Europe showing the widespread distribution of Toarcian organic-rich facies and landmasses, and the presentday coastline, modified from Loh et al. (1986). PH = Pennine High; WH = Welsh High; CM = Cornubian Massif; EMS = East Midlands Shelf; and WB = Wessex Basin. (C) Map ofthe area around Holwell, Leicestershire showing the roads, Lower Jurassic outcrop and geological exposures discussed.

125B.A. Caswell, A.L. Coe / Palaeogeography, Palaeoclimatology, Palaeoecology 365–366 (2012) 124–135

A high-resolution study of the succession deposited in the Cleve-land Basin (Fig. 1B) and now exposed at Hawsker Bottoms and PortMulgrave [NZ 947079 and NZ 799178], near Whitby, Yorkshire, UKhas shown that the negative δ13Corg excursion occurs in four abruptshifts termed ‘A’ to ‘D’ (Kemp et al., 2005; Cohen et al., 2007). Thishigh-resolution record from the Yorkshire section has become an infor-mal reference section for the Toarcian OAE. The abrupt δ13Corg shiftshave since been recognized in the Paris Basin (Fig. 1B; Hermoso et al.,2009). Similar abrupt shifts in δ13Corg can be observed in the recordfrom the Lusitanian Basin, Portugal (Hesselbo et al., 2007), and in δ13Cof phytoclast separates (wood fragments and leaf cuticles that are ofterrestrial origin) from the Polish Basin representing the atmosphericsignal (Hesselbo and Pienkowski, 2011).

To date, shallow-marine sections of the Toarcian OAE have notbeen investigated at a high-resolution. However, these sections arekey to understanding how the event influenced the biota and thepalaeoenvironment. Here we present a new high-resolution graphiclog, total organic carbon (TOC), calcium carbonate (CaCO3), totalsulphur (TS) and total nitrogen (TN) abundances, together with car-bon isotope ratios, as well as palaeontological data for the ToarcianOAE from a well preserved section in Leicestershire, UK. These newgeochemical data have been used to interpret this shallow-marinesuccession and to correlate the section with the deeper marine refer-ence succession exposed at Port Mulgrave and Hawsker Bottoms,Yorkshire (Fig. 2) and other sections further afield.

1.1. Geological setting

During the Jurassic, the area that now forms Leicestershire, UKwas situated on the East Midlands Shelf (Fig. 1B), which covers~60 000 km2, and formed a relatively high area on the edge of theLondon-Brabant Massif (Fig. 1B). The Cleveland Basin was to thenorth of the shelf and the Wessex Basin to the south (Fig. 1B). TheToarcian succession on the East Midlands Shelf represents fairlyshallow-marine conditions ranging from just below storm wave-

base to shallow water depths which facilitated the formation of bothferruginous and carbonate-apatite ooids.

Lower Toarcian mudrocks are patchily exposed in a number ofplaces in Leicestershire, UK including: a disused railway cutting at Tilton[SK762055] and preserved quarry faces at Harston [SK843305], Brown'sHill Quarry [SK472234], Holwell and North Quarry [SK740236], Holwell(Figs. 1C and 2). The best preserved exposures are those near Holwell.North Quarry contains a more complete and less weathered successionof the Whitby Mudstone Formation than Brown's Hill Quarry and sowas chosen for this high-resolution study. The quarries at Holwell areboth Regionally Important Geological Sites and are part of a naturereserve managed by the Leicestershire and Rutland Wildlife Trust.

1.2. Lithostratigraphy and biostratigraphy

At the Holwell quarries the lowermost part of the Toarcian isrepresented by the Marlstone Rock Bed, which comprises ~1 m ofsandstone overlain by ~4.4 m of oolitic ironstone (Clements, 1989;Fig. 2). TheMarlstone Rock Bed is overlain by ~9 m of dark-greymud-stones which comprise the laterally extensive Whitby MudstoneFormation. The upper Toarcian (Hildoceras bifrons Zone and higher) isnot preserved, and these facies are thought to have been removed byerosion prior to deposition of the Middle Jurassic (Simms, 2004).

The ammonite biostratigraphy for the Toarcian is well established(Howarth, 1992 and references therein) and the stratigraphically com-plete succession in Yorkshire forms the basis for the lower Toarcian bio-stratigraphy in NW Europe (Howarth, 1992; Fig. 2A). The Clevicerasexaratum Subzone of the Subboreal Province, which is themain intervalof interest in the present study, contains a formally recognized succes-sion of ammonite species (Howarth, 1992) that were defined usingthe Yorkshire section (Fig. 2A). This succession of ammonites has beenused to divide the subzone into three informal divisions (Howarth,1992; Fig. 2A) characterised by the range of the index species. These di-visions have been designated formal biohorizons by Page (2003) thoughthese are not widely accepted. From the base of the subzone these am-monite divisions are: the Eleganticeras elegantulum, C. exaratum, and

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127B.A. Caswell, A.L. Coe / Palaeogeography, Palaeoclimatology, Palaeoecology 365–366 (2012) 124–135

Cleviceras elegans (it should be noted that the ammonite, C. exaratum,after which the subzone is named has a very limited range within thesubzone, e.g. Fig. 2A). The ranges of these ammonite index species havebeen used in the correlation presented in this paper (Fig. 2).

The ammonite biostratigraphy at the Holwell quarries has notbeen studied in detail, but some specimens have been collected byR. G. Clements (personal communication; Fig. 2D–E), and the posi-tions of these specimens were recorded to the nearest bed. For thisreason the ammonite biostratigraphy for the nearby (b11 km), andlithologically similar, exposures at Tilton railway cutting and HarstonQuarry described by Howarth (1980, 1992) (Fig. 2B–C) is used to pro-vide further information on the likely relative age of the beds atHolwell.There are some microfossil data from Tilton (Fig. 2C; Wilkinson, 2001)and Brown's Hill Quarry (Fig. 2D; Wilkinson, 2002) which have alsobeen used to interpret the relative age of the beds at Holwell.

Howarth (1980, 1992) showed that on the East Midlands Shelf, in-cluding at Tilton, the top 1–3 m of the Marlstone Rock Bed representsthe Dactylioceras tenuicostatum Zone, and the lower 3–6 m representsthe Pleuroceras spinatum Zone. Therefore, the Pliensbachian–Toarcianstage boundary occurs within theMarlstone Rock Bed rather thanwith-in the ‘transition bed’ (Fig. 2C) as previously postulated (Wilson andCrick, 1889; Hallam, 1955). This stage boundary position is supportedby microfossil data from Tilton (Fig. 2C; Wilkinson, 2001) and Brown'sHill Quarry, Holwell (Fig. 2D; Wilkinson, 2002).

At Tilton,within theD. tenuicostatum Zone, there is only evidence forthe Dactylioceras semicelatum Subzone (Fig. 2C). Although at HarstonQuarry (Fig. 1C) all but the lowermost subzone of the D. tenuicostatumZone is present (Howarth, 1980, 1992; Fig. 2B). Abundant Tiltonicerasantiquum occur in the top 0.20 m of the Marlstone Rock Bed at Tilton(Fig. 2C, Howarth, 1992) and in the top 0.08 m at Harston (Fig. 2B).This ammonite represents a useful marker species because its strati-graphic range (Howarth, 1992) is limited to the uppermost part of theD. semicelatum Subzone. In Yorkshire its first occurrence is at the baseof bed 32; from there up to 0.10 m above the base of bed 32 it occursabundantly forming shell plasters; and between 0.10 and 1.41 mabove the base of bed 32 it occurs only rarely. The top of the abundantoccurrence of T. antiquum is ~0.80 mbelow δ13Corg shift ‘A’ in Yorkshire.

At Tilton, Howarth (1980, 1992) (Fig. 2C) did not find any speci-mens of E. elegantulum or C. exaratum but the ammonite Harpocerasserpentinum which is found in both the C. exaratum and C. elegans di-visions of the C. exaratum Subzone, was found between 1.40 m and3.0 m above the base of the Whitby Mudstone Formation (Fig. 2C;Howarth, 1980, 1992). The upper part of the H. serpentinum occur-rence overlaps with C. elegans (1.85 m and 2.00 m above the base ofthe Whitby Mudstone Fm. (Fig. 2C) at Tilton). Overall, this suggeststhat the E. elegantulum division is missing, but that parts of theC. exaratum and C. elegans divisions are present. Although Howarth

Fig. 2. Stratigraphical correlation of the lower Toarcian exposures in Yorkshire (Cleveland BBottoms and Port Mulgrave, Yorkshire, (B) Harston Quarry, (C) Tilton railway cutting, (D) Band bed numbers for Yorkshire (Howarth, 1962, 1973); graphic log (Kemp et al., 2005; Kempfive sections (Howarth, 1992); filled circles indicate the middle of the beds where the amAmmonite biostratigraphy, lithology, and ammonite species occurrences for Harston and Ticommunication), and are also reproduced in Simms (2004) with the permission of R. G. Clemfrom the present study (see Fig. 3 for details); lithostratigraphy (Clements, 1989), but notethe Marlstone Rock Bed. Ammonite occurrences and bed numbers for Brown's Hill and Nor(prefix ‘JF’) for Tilton and Brown's Hill Quarry are based on the foraminifera data of WilkinsPl. spinatum Zone (Pliensbachian), JF12b = D. tenuicostatum Zone (Toarcian), JF13 = C. exaand 4.1. Position of the stage boundary and ammonite zonal boundaries at Holwell are uncbiostratigraphy at Tilton. Grey broken horizontal lines represent the correlation betweenlines are used; from the base these are: Pl. hawskerense–P. paltum Subzone boundary, abundextent of the range of H. falciferum (for the Holwell quarries the biostratigraphic correlationSection 4.1). Biostratigraphic correlations that are less certain than others are indicated byviations: Pl. spinatum = Pleuroceras spinatum (Brugière), Pl. apy. = Pleuroceras apyrenum (B(Orthodactylites) tenuicostatum (Young & Bird), Pr. paltum = Protogrammoceras paltum ((Moxon), D. clev. = D. (O.) clevelandicum Howarth, D. semi. = D. (O.) semicelatum (Sim(Young & Bird), T. antiquum = Tiltoniceras antiquum (Wright), E. elegantulum = ElegantC. elegans = Cleviceras elegans (J. Sowerby), and P. hetero. = Phylloceras heterophyllum (J.Note that the Yorkshire section (a) is at a different vertical scale to the other four sections.

(1980, 1992) did not find C. exaratum and E. elegantulum at Tilton itis possible that the lowermost 1.30 m of the Whitby Mudstone Fm.from which no ammonites have been recovered yet represents thelower and middle parts of the C. exaratum Subzone (Fig. 2C). AtHarston Quarry E. elegantulum has not been found, but fragments ofC. cf. exaratum occur in a bed 0.05 m thick at the base of the WhitbyMudstone Fm. (Howarth, 1980, 1992; Fig. 2B). Therefore, all of theE. elegantulum division and probably some of the C. exaratum divisionare also likely to be missing from the Harston section.

At the Holwell quarries, C. exaratum occurs in the lowermost 3 m(beds 10–14; Fig. 2D–E) of the Whitby Mudstone Fm., and Harpocerasfalciferum occurs from bed 18 upwards (Fig. 2D–E). To date,E. elegantulum, C. elegans, Hildaites murleyi, and H. serpentinum havenot been reported from either Holwell quarry (R. G. Clements, person-al communication).Microfossil data indicating foraminiferal zone JF13(Wilkinson, 2002; Fig. 2D) at Brown's Hill Quarry is consistent withthe presence of the C. exaratum division at Holwell.

Taking into account the ammonites recorded from the Holwell,Tilton and Harston sections, it is interpreted that, compared toYorkshire, the uppermost part of the D. semicelatum Subzone abovethe abundant occurrence of T. antiquum and at least the lowermostpart of the C. exaratum Subzone (E. elegantulum division) is absentthroughout Leicestershire. At Brown's Hill Quarry, Holwell a bed withabundant belemnites occurs at the top of the Marlstone Rock Bed(bed 7, Fig. 2D) at the level of this proposed hiatus; this probably repre-sents awinnowed deposit. Similar age strata are also reportedlymissingin Northamptonshire (Howarth, 1978).

The position of the C. exaratum–H. falciferum Subzone boundary atHolwell is uncertain. The presence of H. falciferum in bed 18 (Fig. 2D;R. G. Clements personal communication) indicates that it must lie be-tween the top of bed 14 (where the highest C. exaratum was found)and the base of bed 18. At Brown's Hill Quarry beds 12 to 19 (Fig. 2D)are devoid of ostracods and foraminifera and so cannot be used to inter-pret the microfossil biostratigraphy of the upper part of the successionat Holwell (Hylton, 2000; Wilkinson, 2002).

1.3. Palaeontology

At both Holwell quarries the lowest 1 m of the Whitby MudstoneFm. (beds 10–11; Fig. 3) contains bivalves, ammonites and echinoids(Clements, 1989); and concentrated in the basal few centimeters(bed 8; Fig. 3) are abundant fish remains and occasional insects(Clements, 1989). These immediately overlie the belemnite bed (bed7; Fig. 3) at the top of the Marlstone Rock Bed. The stratigraphicallyequivalent organic-rich mudstones at the base of the C. exaratumSubzone at Tilton also contain the remains of insects (R. G. Clements,personal communication). In Northamptonshire the Abnormal Fish

asin; A) and in the area around Leicestershire (East Midlands Shelf; B–E). (A) Hawskerrown's Hill Quarry, Holwell, and (E) North Quarry, Holwell. Ammonite biostratigraphy, 2006); and ammonite species ranges for those species which are used to correlate themonites have been recorded (but the true occurrences are shown for T. antiquum).

lton (Howarth, 1980, 1992). Bed numbers for Tilton are from R. G. Clements (personalents. Lithology for Brown's Hill Quarry (Clements, 1989). Lithology for North Quarry isthat Carney et al. (2004) assigned the sandstone to the Dyrham Formation rather thanth Quarry, Holwell (R. G. Clements, personal communication). The foraminiferal zoneson (2001, 2002; and correlate with the ammonite biostratigraphy as follows: JF12a =ratum Subzone, JF14 = H. falciferum–Harpoceras fibulatum Subzones), see Sections 1.2ertain and are based on the available biostratigraphy and the lithology and ammonitethe ammonite biostratigraphy for the five different successions; four different brokenant occurrence of T. antiquum, lowest extent of the range of C. exaratum, and the lowesttakes into account the carbon isotope correlation presented in Fig. 4 and discussed in

‘?’. The hiatus proposed in Sections 1.2 and 4.1 is indicated. Ammonite subzone abbre-uckman), Pl. hawsk. = Pleuroceras hawskerense (Young & Bird), D. ten. = DactyliocerasBuckman), D. crosbeyi = D. (O.) crosbeyi (Simpson), Hi. murleyi = Hildaites murleyipson), H. falcif. = Harpoceras falciferum (J. Sowerby), C. exar. = Cleviceras exaratumiceras elegantulum (Young & Bird), H. serp. = Harpoceras serpentinum (Schlotheim),Sowerby). Pliens. = Pliensbachian; TB = transition bed; MRB = Marlstone Rock Bed.

fmcsand

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Oolitic ironstone

Grey claystoneYellow claystone

Iron rich mudstoneMudstone

Highly fissileModerately fissilePoorly fissile

Limestone Nodules (var. lithologies)Silty mudstone

Brown marlstoneLithology:

Iron ooidsNodules with ooids

B. b

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B. r

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16

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-24-30-31 -25-29 -26-27-28

0 10 20 30 40 50 60 70

2 4 6 8 10 12

0 1 2 3 4 5 6

0 0.15 0.25 0.300.100.05 0.20

δ13Corg (‰ VPDB)

Fig. 3. Graphic log for the Whitby Mudstone Formation at North Quarry, Holwell, Leicestershire. Ammonite biostratigraphy and bed numbers are from R. G. Clements (personalcommunication). Uncertainty on the stratigraphic position of the C. exaratum–H. falciferum subzone boundary is represented by the shading in the subzone column. Speciesrange and abundance data are from the present study (diamonds joined by lines). Fossil abundance is a direct measure of the number of specimens found. δ13Corg, CaCO3, TOC,TS and TN are from the present study (raw data are provided in the supplementary information). Between 1.075 m and 1.30 m, δ13Corg is not plotted, the values ranged from−6 to −23.3‰ and are presented in Supplementary Fig. 2.

128 B.A. Caswell, A.L. Coe / Palaeogeography, Palaeoclimatology, Palaeoecology 365–366 (2012) 124–135

Bed, containing fish remains, immediately overlies theMarlstone RockBed and is interpreted to represent the middle to upper C. exaratumSubzone (Howarth, 1978, 1992). At the level interpreted as the baseof the C. exaratum Subzone in Harston and Denton Park quarries,Lincolnshire, shell beds occur that contain abundant broken bivalveshells, large numbers of belemnites rostra, and large Tiltonicerasantiquum specimens (Howarth, 1980; Fig. 2B). A fish bed (the SaurianFish Bed) of equivalent age exposed near Ilminster, Somerset containsabundant fish, insects (Odonata, Neuroptera, Orthoptera, Homoptera,Diptera, and Coeloptera), crustaceans, ammonites, belemnites, andteuthoids (Moore, 1867).

At the Holwell quarries the overlying ~1.5 m of fissile organic-richmudrocks (beds 10–14; Fig. 3) contain a low diversity fauna of flat-tened ammonites and bivalves (Clements, 1989). The bivalves includethe key species Bositra radiata (Goldfuss), Bositra buchi (Roemer), andPseudomytiloides dubius (Sowerby), which are found in high abun-dance in the Yorkshire exposures (Caswell et al., 2009). The upperpart of the North Quarry section (H. falciferum Subzone, beds 16–19;Fig. 3) contains a diverse fauna of micro- and macrofossils (thisstudy and R. G. Clements, personal communication).

Hylton (2000) studied the foraminifera from Tilton and found thatthe H. falciferum Subzone contained a low diversity fauna that was

dominated by a small opportunist Reinholdella? planiconvexa in bed 12and beds 14–16 (Fig. 2C). At Tilton and Holwell there is an interval inthe middle of the Whitby Mudstone Formation (beds 12–13 at Tilton,and beds 14–15 at Brown's Hill; Fig. 2C and D, respectively) withinwhich foraminifera are rare or absent (Hylton, 2000; Wilkinson, 2001,2002). The facies which represent the H. falciferum Zone at Tilton con-tain abundant ammonites, P. dubius, and the gastropod Coelodiscusminutus (Schubler) (Hallam, 1967). Additionally, fish scales, insectand teuthoid remains, small brachiopods, echinoid spines and sporadicoccurrences ofNucula sp.,Astarte sp.,Meleagrinella substriata (Münster),and B. radiata occur (Hallam, 1967).

A diverse and abundant fauna, comprising a mixture of adult formsof small species (i.e. micromorphs) and juveniles of larger species ofbrachiopods, bivalves, echinoids, crinoids, ophiuroids, holothurians,gastropods, and scaphopods occurs in beds 17 and 19 at North Quarry(R. G. Clements, personal communication). Faunas of micromorphicbrachiopod species of C. exaratum Subzone age have been reportedfrom other contemporaneous sections in the UK, France (Vörös,2002), and further afield (e.g. Morocco and Normandy (Vörös,2002)). For example, an abundant fauna of the small brachiopodsOrthotoma globulina (Davidson) andNannirhynchia pygmaea (Morris),and echinoid spines have been observed within the C. exaratum

129B.A. Caswell, A.L. Coe / Palaeogeography, Palaeoclimatology, Palaeoecology 365–366 (2012) 124–135

Subzone at Alderton Hill Quarry, Gloucestershire (Buckman, 1922;Simms, 2004). At Ilminster, Somerset, UK the ‘Leptaena Clay’, whichrepresents the base of the C. exaratum Subzone (Howarth, 1992),contains abundant minute (millimetre sized) brachiopods includ-ing Koninckella bouchardi (Davidson), Cadomella moorei (Davidson),N. pygmaea and O. globulina (Moore, 1867; Hallam, 1967). Based onthe shell morphology, and the fact that larger specimens of these bra-chiopod species have never been found elsewhere, Ager (1990) con-cluded that they were adult forms and so represent micromorphicspecies specifically suited to ‘living in sulphurous black muds’. Fur-thermore, the spines and plates of echinoids and a ‘mostly dwarfed’(i.e. stunted) fauna of one gastropod and 17 different bivalve speciesare reported from near Ilminster, including P. dubius and B. radiata,by Moore (1867), but the reason(s) they are stunted is unclear.

2. Materials and methods

All weatheredmaterial was removed from a vertical channel about1 mwide down the face of Holwell North Quarry in an area where thevegetation was not covering the face. The section was graphicallylogged using a ladder to reach the upper part of the face. The collectionof palaeontological data was limited within some beds due to thedifficulty of accessing large bedding plane surfaces (particularly inthe clay facies). Using a small pocket knife on the cleaned faces,fresh mudrock samples were collected every 1.25 cm for geochemicalanalyses. The samples were dried at 35 °C and crushed using an agatepestle and mortar. A total of 337 samples were analysed for totalorganic carbon (TOC), calcium carbonate (CaCO3), total sulphur (TS)and total nitrogen (TN) abundance using a CNS-2000 Leco ElementalAnalyser. Analytical precision was better than 0.02 wt.% for carbon,0.11 wt.% for sulphur and 0.02 wt.% for nitrogen based on inter-runanalyses of the in house standard. The samples analysedwere collectedevery 1.25 cmbetween−0.0625 mand3.075 m, every 2.5 cmbetween3.10 m and 4.63 m, and every 10 cm between 4.65 m and 6.75 m(Fig. 3).

One hundred and seventy three samples were analysed for carbonisotopes at a stratigraphic resolution of 2.5 cm between−0.06 m and2.70 m, a resolution of 5 cm between 2.75 m and 4.00 m, and at a res-olution of 10 cm between 4.00 m and 6.75 m (Figs. 3 and 4). Thesesamples were decarbonated using 1 M HCl, rinsed and dried at35 °C and then analysed using a Geo-20-20 mass spectrophotometerwith an ANCA elemental analyser preparation system (PDZ EuropaScientific). Carbon isotope values are reported relative to the ViennaPeedee belemnite standard (VPDB). Standard in house referenceswere calibrated against the international standard IAEAN3. Analyticalprecision for carbon isotope analyses was 0.08‰ based on inter-runanalyses of an in house standard.

3. Results

The succession at North Quarry, Holwell, Leicestershire comprises~9 m of sedimentary deposits, mainly mudrock; these form theWhit-by Mudstone Fm. and represent part of the H. falciferum Zone (Fig. 3).The mudrocks directly overlie the oolitic ironstone of the MarlstoneRock Bed which forms the quarry floor and represents part of theD. tenuicostatum Zone (Fig. 2).

The basal ~1.5 m of the Whitby Mudstone Fm. comprises yellowand grey mudstones, and this is overlain by ~3.0 m of dark-greymud-stones of varying fissility (Fig. 3). The upper ~4.0 m of the successionconsists of medium-greymudstones. Towards the top of the successionmarlstones occur which contain calcium carbonate nodules, carbonate-apatite ooids and a small amount of ferruginous ooids (Fig. 3). The suc-cession contains several iron-rich mudstone horizons (Fig. 3).

The North Quarry facies has relatively high TOC content of up~10 wt.% (Fig. 3), and the highest TOC (~6–10 wt.%; Fig. 3) valuesoccur within the highly fissile mudstones of the upper part of bed

14 and the lower part of bed 15. The fissile mudstone facies on eitherside (lower part of bed 14 and the upper part of bed 15, beds 16 and17) also have relatively high TOC with values between 4 and 8 wt.%.TS, TN, and CaCO3 abundance are also highest between beds 14 and17 (Fig. 3). All raw data are provided in Supplementary Table S1.

The North Quarry succession records a δ13Corg of ~−5‰within the C. exaratum Subzone. Carbon isotope ratios range from−24.85‰ to −29.98‰ through the section (Fig. 3), and are on aver-age ~1‰ higher than those from the succession in Yorkshire (Fig. 4).The negative carbon isotope excursion is overlain by an interval withfairly constant values of ~−26‰ and the upper part of the measuredsection averages −25.6‰ with slightly more variation (Fig. 3).Between 1.075 m and 1.30 m (Fig. 3) there is a sharp change to anom-alously high δ13Corg values for marine organic matter. These δ13Corgdata are also highly variable, ranging from −6‰ to −23‰ (Supple-mentary data Fig. 2). Repeat analyses were performed on samplescollected from contemporaneous facies that were separated laterallyby ~50 m; these had similar anomalous δ13Corg values (Supplementa-ry Table S1 and Fig. 2). Furthermore, TOC and CaCO3 within this inter-val are very low and show low amplitude variations (Fig. 3).

The succession at North Quarry is dominated by three epifaunal bi-valve species B. buchi, B. radiata, and P. dubius (Fig. 3). The infaunal sus-pension feeding bivalve Pleuromya sp., and the deposit feeding bivalvesPalaeonucula hammeri (Defrance) and, a single small (~2 mm) speci-men of, Dacryomya ovum (Sowerby) were also found in the upperpart of the succession (Fig. 3). The gastropods Levipleura blainvillei(Goldfuss) and Ptychomphalus expansus (Sowerby) also occur in lowabundance (Fig. 3). Species diversity and abundance are relatively lowduring the main negative carbon isotope excursion (Fig. 3).

4. Discussion

The early Toarcian palaeoenvironmental change is prominentlyrecorded at North Quarry, Holwell by changes in lithology, bulk rockgeochemistry, a −5‰ δ13Corg excursion, and changes in the biota. Theaverage TOC content within the North Quarry succession (~4 wt.%) isgenerally lower and the CaCO3 abundance higher (~18 wt.%) than thatof the Yorkshire succession (TOC~8 wt.%, CaCO3~5 wt.%, Kemp et al.,2011; Fig. 4). The TS abundance within the North Quarry deposits iscomparable to those of the Yorkshire section (Kemp et al., 2011). Theδ13Corg−5‰ excursion in North Quarry is of smaller overall magnitude(~−7‰, Fig. 4; Kemp et al., 2005) and varies in detail from that from theYorkshire succession. This is interpreted to be the result of twohiatuses;the first at the D. semicelatum–C. exaratum Subzone boundary and sec-ond in the upper part of the C. exaratum Subzone. The hiatuses arediscussed further in Section 4.1 and are shown in Fig. 4. The anomalous-ly high δ13Corg values and unusual CaCO3 and TOC values between1.075 m and 1.30 m (Fig. 3) at North Quarry are interpreted to be theresult of a diagenetic overprint and are not discussed further.

4.1. Correlation between Leicestershire and Yorkshire and other Toarciansections

The combined ammonite species occurrences from Tilton, Harston,and Holwell in Leicestershire indicate that there is a hiatus representingat least the top of the D. semicelatum and the base of the C. exaratumSubzones (Fig. 2). This is because, in Leicestershire, the highest stratawithin the D. semicelatum Subzone are at the T. antiquum level, and thelowest strata in the overlying C. exaratum Subzone contain C. exaratumfor which the lowest occurrence in the type section in Yorkshire is nearthe middle of the subzone (Fig. 2A). Thus the Eleganticeras elegantulumdivision is not represented by strata in Leicestershire. Foraminiferalindex species at the base of the Whitby Mudstone Fm. and the topof the Marlstone Rock Bed (Wilkinson, 2002; Fig. 2D) at Holwell areconsistent with this interpretation of the ammonite biostratigraphy(foraminiferal zones JF12b and 13, Wilkinson, 2002, Fig. 2D).

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Fig. 4. Correlation between the composite section fromHawsker Bottoms and Port Mulgrave, Yorkshire (left hand side) and Holwell Quarries, Leicestershire (right hand side). Graph-ic log, ammonite biostratigraphy, and bed numbers for North Quarry, Holwell are as for Fig. 3. The lowermost ~3.6 m is from Brown's Hill Quarry recordedwhen it was better exposed(Clements, 1989). Graphic log for the Yorkshire succession (Kemp et al., 2005; Kemp 2006); ammonite species ranges, ammonite biostratigraphy and bed numbers (Howarth, 1962,1973, 1992). Range data for Bositra buchi, Bositra radiata, and Pseudomytiloides dubius for Yorkshire and the oxygenated interval (Caswell et al., 2009). Bositra buchi, B. radiata, andP. dubius range data for Leicestershire are from this study; the interval containing diverse fauna is based on data from R. G. Clements (personal communication). CaCO3 (grey)and δ13Corg (black) data for Yorkshire (Kemp et al., 2005, 2011), and data for Leicestershire are from the present study; carbon isotope shifts ‘A’ to ‘D’ (Kemp et al., 2005; Cohenet al., 2007). Dashed horizontal grey lines show the correlation between Yorkshire and Leicestershire, and are based upon ammonite and foraminifera biostratigraphy, variationin δ13Corg and CaCO3 abundance. Diagonal lines indicate intervals where strata are interpreted to be missing (see also Supplementary Fig. 1). Dashed horizontal black line representsthe position of the C. exaratum–H. falciferum subzone boundary. Key to the lithology and species abundances for the Holwell North Quarry section are as for Fig. 3. Ammonite rangesfor Holwell (shown as broken bars) are based on the ammonite occurrences of R. G. Clements (personal communication). Ammonite abbreviations are as for Fig. 2.

130 B.A. Caswell, A.L. Coe / Palaeogeography, Palaeoclimatology, Palaeoecology 365–366 (2012) 124–135

Using the ammonite biostratigraphy and ranges discussed aboveas a framework for correlation of the δ13C record from Leicestershireand Yorkshire, the most plausible interpretation is that δ13C shifts ‘A’,‘B’ and ‘C’ recognized in Yorkshire are missing in the hiatus betweenthe D. semicelatum and C. exaratum Subzones in Leicestershire. Wetherefore interpret the large δ13Corg shift in the lowermost 0.2 m ofthe North Quarry section as δ13Corg shift ‘D’ of Cohen et al. (2007) inYorkshire (Fig. 4). This correlation is supported geochemically bythe preceding distinctive rapid increase in carbon isotope variation(Fig. 4). The identification of carbon isotope shift ‘D’ indicatesthat the hiatus in Leicestershire is represented by 4.6 m of strata inYorkshire (between −2.0 m and +2.6 m; Figs. 2A and 4). Carbonisotope shift ‘D’ appears to be 2‰ larger in North Quarry than inYorkshire, but this is likely to be because the sampling resolution of2.5 cm in Yorkshire has artificially aliased the signal.

The upper part of the C. exaratum Subzone in Leicestershire is difficultto correlate at high-resolution because of the lack of biostratigraphicallysignificant ammonites, microfossils and a distinctive carbon isotope sig-nature. The large 4‰ step towards higher δ13Corg values (from −29‰

to −25‰) between beds 11 and 13 (Fig. 4) at Holwell suggests thatbed 13 upwards is equivalent to the uppermost part of bed 38 or higherin Yorkshire (Fig. 4). This implies that there may be a second hiatus atHolwell equivalent to most of beds 36, 37 and 38 in Yorkshire (Fig. 4).The diagenetically altered level within beds 12 and 13 at Holwellcould potentially be related to this hiatus. The presence of the bio-stratigraphically significant ammonite species H. falciferum in bed 18at Holwell places an upper limit on the possible position of theC. exaratum–H. falciferum Subzoneboundary (Fig. 4). This subzonebound-ary therefore occurs between the top of bed 14 and the base of bed 18 atNorth Quarry (Fig. 4), but the precise position cannot be determined fromthe existing biostratigraphical data nor the new δ13Corg data presentedherein.

Correlation of the new high-resolution shallow water δ13C recordfrom Leicestershire presented in this study with δ13C records from anumber of other key early Toarcian sections from around the world(Fig. 5A–J) shows three features: (1) the magnitude of the negativeδ13Corg excursion in Holwell (East Midlands Shelf) is similar to thatfor the NW Germany and Yorkshire (NW German and Cleveland

131B.A. Caswell, A.L. Coe / Palaeogeography, Palaeoclimatology, Palaeoecology 365–366 (2012) 124–135

Basins, respectively) allowing for the resolution of the records and themissing strata at Holwell, but the absolute δ13Corg values are slightly off-set; (2) the δ13Corg excursion in the Mochras borehole (Cardigan BayBasin) is of similar magnitude and absolute δ13Corg values to Holwell(Fig. 5A and D) but both sections are offset by ~1‰ compared to York-shire; (3) on the East Midlands Shelf there is no clear positive δ13Corgexcursion near the C. exaratum–H. falciferum Subzone boundary com-pared to the δ13Ccarb and δ13Corg records from Mochras (Fig. 5C), theδ13Ccarb record from Peniche, Portugal (Lusitanian Basin; Fig. 5H), andδ13Corg record from Katsuyama, Japan (Fig. 5J; Gröcke et al., 2011). Sim-ilarly Yorkshire only shows a low amplitude positive excursion at thislevel (Fig. 5B) and Jenkyns et al. (2002) attributed this to condensationor incomplete strata within this section.

The difference in magnitude of the negative δ13Corg excursionbetween the East Midlands Shelf and the Cleveland and NW GermanBasins is most likely due to the absence of the upper D. semicelatum–

lower C. exaratum Subzone facies and hence the minimum δ13Corg

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-32 -30 -28 -2-34

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δ13Corg (‰ VPDB)

δ13Ccarb (‰ VPDB) δ13Cc

strata missing?

strata missing

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Fig. 5. Correlation of δ13C data between Leicestershire and nine other marine sections fromShelf (this study); (B) composite from Hawsker Bottoms and Port Mulgrave, Whitby, ClevelMochras borehole, Cardigan Bay Basin (Jenkyns and Clayton, 1997); (E) Dotternhausen, NWal., 2009); (G) Brody-Lubienia, Polish Basin (Hesselbo and Pienkowski, 2011); (H) Peniche,Basin, (Al-Suwaidi et al., 2010); and (J) Katsuyama, Japan (Gröcke et al., 2011). All sectionsusing short broken black horizontal lines to represent the biostratigraphic boundaries;Correlation between the carbon isotope data and the biostratigraphy are based upon those

values represented by shifts ‘A’ and ‘B’ (Figs. 4 and 5B) from theEast Midlands Shelf. The offset between the East Midlands Shelf andCardigan Bay Basin compared to the Cleveland Basin may either bedue to: the ‘seawater ageing effect’ whereby the seawater on top ofthe shelf had a more positive isotopic composition because water cir-culation was less restricted than within the basins; or may reflect agreater incorporation of terrestrial organic matter given that, terres-trial organic matter generally has higher average δ13C values, andthe sediments were deposited in a more proximal setting on theEast Midlands Shelf or close to the Welsh High (Fig. 1B). This secondhypothesis is supported by the hydrogen index data for the CardiganBay section (Baudin et al., 1989; Jenkyns and Clayton, 1997). Howev-er, variations in the proportions of terrestrial versus marine organicmatter alone are insufficient to explain the −5‰ change in δ13Corgin the Leicestershire section because the carbon isotope ratios offossilised wood are only offset by up to +2‰ from that of organicmatter from the same facies (Hesselbo et al., 2000).

VPDB)

-32

(G) Brody-Lubienia δ13 Cphytoclast (‰ VPDB)

-4

??? ?

??

(I) Arroyo Lapa δ13 Corg (‰ VPDB)

-30 -28 -26 -24 -22 -32 -30 -28 -26 -24 -22

-32-30-28-26-24-22

(J) Katsuyama(H) Penicheerre-Couy borehole

-34

6

-3 -2 -1 0 1 2 3 41 0 1 2 3 4 5arb (‰ VPDB) δ13Ccarb (‰ VPDB) δ13Ccarb (‰ VPDB)

around the world. Carbon isotope data are: (A) North Quarry, Holwell, East Midlandsand Basin (Cohen et al., 2004; Kemp et al., 2005; Littler et al., 2010); (C) and (D) fromGerman Basin (Röhl et al., 2001); (F) Sancerre-Couy borehole, Paris Basin (Hermoso etPortugal, Lusitanian Basin (Hesselbo et al., 2007); (I) Arroyo Lapa, Argentina, Neuquénare plotted against the ammonite biostratigraphy for Yorkshire (Howarth, 1962, 1973)ammonite abbreviations are as for Fig. 2; P–T = Pliensbachian–Toarcian boundary.presented by the authors cited above for each dataset.

132 B.A. Caswell, A.L. Coe / Palaeogeography, Palaeoclimatology, Palaeoecology 365–366 (2012) 124–135

4.2. Biotic and sea-level changes associated with the Toarcianpalaeoenvironmental change

The fauna of the abnormal and saurian fish beds (i.e. the abundantinsect remains, fish remains, belemnite rostra, concentrations of brokenshells, echinoids, teuthoids and crustaceans) at the D. tenuicostatum–

C. exaratum Subzone boundary in Lincolnshire, Northamptonshire,Leicestershire, and Ilminster are interpreted to indicate the large-scalemortality of pelagic fauna (discussed below) and/or possible furtherconcentration through reworking by increased current activity consis-tent with the onset of relative sea-level rise.

The high abundance of fossilised fish remains at the D. tenuicostatum–

C. exaratum Subzone boundary in the shallow marine sections is consis-tent with elevatedmortality and preservation duringwidespread euxiniathat developed over the time period represented by carbon isotope shifts‘A’, ‘B’ and ‘C’ in Yorkshire and is evidenced by changes in the Mo isotopecomposition (Pearce et al., 2008) and the presence of isorenieratane(Schouten et al., 2000; Pancost et al., 2002; Schwark and Frimmel, 2004;van Breugel et al., 2006). De-oxygenation in the modern seas and oceanshas a range of impacts on the marine biota including mass mortalities ofbenthic and pelagic organisms, decreased species diversity and increaseddominance by opportunistic organisms (e.g. Diaz, 2001).

The unusual occurrence of abundant insect remains at the base ofthe Whitby Mudstone Fm. in shallow marine sections across the UKmay simply be due to the suitability of the conditions of preservationand proximal nature of the deposits, though it is unclear why abun-dant insect remains are not commonly found in other Jurassicshallow-marine fine-grained deposits. It therefore seems more likelythat the contemporaneous rapid environmental changes that occurredduring the early Toarcian, such as temperature (e.g. Bailey et al., 2003)and pCO2 increases (e.g. Hesselbo et al., 2000; McElwain et al., 2005;Hesselbo et al., 2007; Hesselbo and Pienkowski, 2011), had significanteffects on the early Toarcian insect populations. Experimental andpresent day observational studies show that elevated levels of pCO2

reduce the nutritional quality (C:N) of plants which affect herbivorousinsects by increasing larval development times andmortality (Coviellaand Trumble, 1999 and references therein). Over the longer termthese changes may result in local extinctions, and alter species popu-lation structures and geographic ranges (Coviella and Trumble, 1999).Currano et al. (2008) showed that substantial changes in patterns ofinsect herbivory occurred during the Paleocene–Eocene Thermal Maxi-mum and these changes were linked with increasing mean global tem-peratures and pCO2.

There are four lines of evidence indicating relative sea-level rise inthe C. exaratum Subzone which could have been associated withreworking and concentration of the fossils at this level: (i) Mg/Ca ratiosand O-isotopes (Bailey et al., 2003) indicate global warming of 7–13 °Cat the start of the Toarcian event a consequence of which would havebeen the thermal expansion of seawater and/or the melting of montaneglaciers leading, in turn, to relative sea-level rise; (ii) sequence strati-graphical studies of the Lower Jurassic strata indicate relative sea-levelrise at this time (Hesselbo and Jenkyns, 1998); (iii) relative sea-levelrise in the C. exaratum Subzone across the EastMidlands Shelf is indicatedby the change in facies from the Marlstone Rock Bed with ferruginousand carbonate-apatite ooids, to the remanié bed (bed 7 at North Quar-ry) and clay facies of the Whitby Mudstone Formation (beds 8–13 atNorth Quarry); (iv) the higher TOC and CaCO3 abundance in Leicester-shire between beds 14 and 17 could indicate decreased dilution byclay minerals due to continued relative sea-level rise over the EastMidlands Shelf as global temperatures continued to rise. Subsequentrelative sea-level fall in the upper part of the H. falciferum Subzone(beds 20–21) at North Quarry, Holwell is indicated by the occurrenceof carbonate-apatite (Horton et al., 1980) and ferruginous (mostlychamosite and berthierine) ooids. These ferruginous ooids are wide-spread across the East Midlands Shelf (Tilton (Howarth, 1980), Harston(Horton et al., 1980), Northampton (Howarth, 1992), Nettleton,

Lincolnshire (Bradshaw and Penney, 1982) and twelve borehole sec-tions across the Midlands (Horton et al., 1980)).

Hylton (2000) showed a noteworthy absence of foraminiferawithinthe H. falciferum and the lower H. bifrons zones in Dotternhausen, SWGermany, the Truc de Balduc, S France, Yorkshire, and Tilton. Thiswidespread absence of foraminifera across the NW Europe is consistentwith the recorded ostracod and foraminifera extinctions at theD. semicelatum–H. falciferum zone boundary (e.g. fig. 4 of Caswell etal., 2009) across the Boreal and Tethyan realms, and further afield,and is probably a consequence of oceanic anoxia.

Several of the macroinvertebrate species found in the lowerToarcian of North Quarry, Holwell also occur in Yorkshire; theseinclude L. blainvillei, P. expansus, D. ovum, B. buchi, B. radiata andP. dubius (Tate and Blake, 1876; Little, 1996). Similar to the biota ofthe Yorkshire succession (Caswell et al., 2009; Fig. 4) the dominanceby the epifaunal bivalves B. buchi, B. radiata, and P. dubius in thelower Toarcian facies in North Quarry is likely to be due to thesespecies' adaptions to the low oxygen conditions. P. dubius is the onlybivalve found in high abundance during the extreme conditionsin Yorkshire (C. exaratum Subzone; as indicated by geochemicalproxies), and is only ever found in organic-rich facies during otherperiods of geologic time suggesting that it is specialised for de-oxygenated conditions (Caswell, 2009; Caswell et al., 2009). Basedon the occurrences and abundances of the three species in Yorkshire(Caswell et al., 2009) P. dubius appears to have the highest toleranceof de-oxygenation followed by B. radiata and B. buchi.

Pseudomytiloides dubius and B. radiata appear to have been ex-treme opportunists, with short life spans and fast generation times,well adapted to de-oxygenated conditions. Other adaptations mayhave included a long-lived planktotrophic larval stage and a facultativepseudoplanktonic life habit for P. dubius (Caswell, 2009; Caswell et al.,2009). The monospecific Bositra communities probably representmore hospitable, possibly dysoxic, conditions (Caswell et al., 2009).

In Holwell the benthos of the upper part of the C. exaratumSubzone and the lower part of the H. falciferum Subzone is more di-verse than in Yorkshire (Fig. 4), and the local species ranges of thedominant bivalves within the two successions differ considerably. InHolwell North Quarry P. dubius is common but is only abundant atthe boundary between beds 14 and 15 (Fig. 4). Whereas in YorkshireP. dubius occurs in high abundance and completely dominates thebenthos during the negative carbon isotope excursion (C. exaratumSubzone; Fig. 4), and the lower half of the H. falciferum Subzone(Caswell et al., 2009). However, at North Quarry B. buchi and B. radiatadominate during the middle to upper part of the δ13Corg excursion.

In Yorkshire the first occurrence of B. buchi is later (near theC. exaratum–H. falciferum Subzone boundary; Fig. 4; Caswell et al.,2009) than in Leicestershire (Fig. 4), and it occurs abundantly insome horizons within the H. falciferum Subzone in both sections. AtNorth Quarry B. radiata only occurs in abundance within the mostorganic-rich facies in the upper part of the C. exaratum Subzone(beds 14–15; Fig. 4). Based on the correlation presented herein(Fig. 4), this is coincident with the beginning of the relatively oxygen-ated interval in Yorkshire (Caswell et al., 2009; Fig. 4). In YorkshireB. radiata occurs in two discrete intervals at the beginning andend of the negative carbon isotope excursion (Caswell et al.,2009; Fig. 4). Bositra radiata becomes extinct at the C. exaratum–

H. falciferum Subzone boundary in Yorkshire (Caswell et al., 2009). Itis not clear what the upper limits of the range of B. radiata is at Holwellrelative to the biostratigraphy however it occurs up to the middle ofbed 17 (Fig. 4). This upper occurrence of B. radiata may representthe position of the C. exaratum–H. falciferum Subzone boundary inHolwell as it does in Yorkshire. Furthermore, this may represent thefinal occurrence of B. radiata in the UK.

The palaeontological data indicate that conditions in Holwell wereleast hospitable during the negative carbon isotope excursion (Fig. 4).However, the higher faunal diversity in Leicestershire and the

133B.A. Caswell, A.L. Coe / Palaeogeography, Palaeoclimatology, Palaeoecology 365–366 (2012) 124–135

differences in the stratigraphic ranges of B. buchi, B. radiata andP. dubius throughout the negative carbon isotope excursion suggestthat conditions were generally more hospitable than in Yorkshire.The lower TOC of the North Quarry deposits supports this interpreta-tion. The facies of the C. exaratum Subzone of the East Midlands Shelfindicate that they were deposited in shallower water relative tothose of the Cleveland Basin and so the water column is likely tohave undergone a greater degree of mixing leading to greater oxygena-tion and thus a relatively more diverse biota.

The return to organic-poor clay facies and the diverse and abundantfauna (including brachiopods, bivalves, echinoids, crinoids, ophiuroids,holothurians, gastropods, and scaphopods) of many small individualswithin beds 17 and 19 (Fig. 4) in North Quarry and further afield, indi-cate that conditions are likely to have become relatively more oxygen-ated at the time of deposition and a more diverse benthic fauna couldsettle and establish. The small size of the fauna is curious and couldrepresent some micromorphic species, adapted for these environmen-tal conditions, similar to the brachiopods of the Leptaena Clay fromIlminster, UK (Section 4.1), or juvenile assemblageswhichwere period-ically killed off before attaining maturity, or adult organisms withstunted growth due to the inhospitable environment. For example,Fürsich et al. (2001) found that the average size of lower Toarcian bi-valve species in east-central Spain were between 7.5% and 61% smallerthan their other Jurassic occurrences. The small size of these bivalvespecieswas interpreted as an instance of stunting attributed to an oligo-trophic environment (Fürsich et al., 2001). Further detailed collectingand study of the Toarcian East Midlands Shelf populations would beneeded to determine whether the small faunas are stunted adults.The brachiopod faunas, such as the micromorphic species, found atIlminster and on the EastMidlands Shelf represent the furthest northerlyextent of the family Koninckidae and have been taken to indicate anortherly invasion of a Tethyan fauna and the associated radiation ofthe family (Vörös, 2002). This invasion is probably linkedwith the trans-gression of warmer Tethyan waters.

Despite the geochemical similarities between the basin and theshelf, the macrofauna show that overall on the East Midlands Shelfconditions were more hospitable. Bivalve populations on the shelfcould have represented a source of new recruits for populations inthe basin once palaeoenvironmental conditions had improved in theaftermath of the intense anoxic or euxinic periods.

5. Conclusions

• Geochemical analyses of the lower Toarcian deposits from NorthQuarry, Holwell, Leicestershire show that the average TOC contentis ~4 wt.% and the average CaCO3 content is ~18 wt.%. The TOC is~4 wt.% lower and the CaCO3 is ~13 wt.% higher than the lowerToarcian reference section in Yorkshire. These data, and the faunalassemblage, are consistent with relatively shallower water deposi-tion. A negative δ13Corg excursion of ~−5‰ is evident from thelower Toarcian deposits at North Quarry, and this excursion is ofsimilar magnitude to the negative δ13Corg excursion within theYorkshire section.

• The ammonite ranges indicate that between Leicestershire andYorkshire there is a significant hiatus that spans from at least theupper part of the D. semicelatum to the lower part of the C. exaratum(E. elegantulum division) subzones, but that significant parts of themiddle to upper C. exaratum Subzone (C. exaratum and C. elegansdivisions) are present. The precise position of the C. exaratum–

H. falciferum Subzone boundary is uncertain but probably occursbetween beds 15 and 18 at North Quarry, Holwell.

• The δ13Corg record allows further refinement of the correlation be-tween Yorkshire and Leicestershire and indicates that the base ofbed 8 to the top of bed 11 in North Quarry, Holwell correspondsto the strata between 2.6 and 4.4 m above the base of bed 33 atPort Mulgrave and Hawsker Bottoms, Yorkshire.

• The negative δ13Corg shift ‘D’ identified from the Yorkshire succes-sion (Kemp et al., 2005; Cohen et al., 2007) also occurs within theNorth Quarry, Holwell succession, however it is ~2‰ greater inmagnitude at Holwell. At North Quarry δ13Corg is offset by ~+1‰from Yorkshire, but is similar to the Cardigan Basin section atMochras; this offset probably represents the seawater ageing effectand/or dilution by terrestrial organic matter.

• The lower C. exaratum Subzone facies deposited on the EastMidlands Shelf contain abundant well-preserved belemnites, fishremains, and insects which is consistent with mortality associatedwith the extreme environmental changes in the biosphere overthe time interval spanning δ13Corg shifts A to C.

• The benthic macro-invertebrate species composition of the Leices-tershire succession is very similar to the Yorkshire succession, andis dominated by the epifaunal opportunists P. dubius, B. radiata,and B. buchiwhich were well adapted to oxygen deficiency. Howev-er, overall, Leicestershire is more faunally diverse than Yorkshire.

• The local occurrences of P. dubius, B. buchi, and B. radiata are differ-ent within the Leicestershire and Yorkshire successions. The maindifferences were: P. dubius occurs in significantly lower abundanceand does not dominate during the main negative carbon isotope ex-cursion in Leicestershire; B. buchi and B. radiata occur in higherabundance and dominate during the negative carbon isotope excur-sion in Leicestershire; and B. buchi occurs earlier in Leicestershireand B. radiata occurs later. Palaeontological data from beds 16–17in North Quarry, Leicestershire and at contemporaneous intervalsin other UK sections show that a diverse fauna occurs indicatingthat conditions improved relatively rapidly after the end of the neg-ative carbon isotope excursion.

• Palaeontological and geochemical data indicate that conditions dur-ing the negative carbon isotope excursion in Leicestershire weremore hospitable than in Yorkshire, and that the benthic fauna recov-ered more quickly in Leicestershire. The East Midlands Shelf mayalso have provided a source of new recruits for some fauna.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.palaeo.2012.09.021.

Acknowledgements

This work was funded by the National Environment ResearchCouncil through a PhD studentship to Bryony A. Caswell. We wouldlike to thank the Leicestershire and Rutland Wildlife Trust (http://www.lrwt.org.uk/) for their permission to collect field samples fromNorth Quarry. We would like to thank Caroline Douglas, DavidWeatherby, Alison C. Hunt, and Patricia L. Clay for their help withfieldwork. We are grateful to Mabs Gilmour for her help with thecarbon isotope analyses, and Mark Evans at the Leicester Museum.Particular thanks to Roy Clements for sharing his knowledge of theHolwell section, and for his comments on an earlier version of thismanuscript. We would also like to thank Finn Surlyk and anonymousreviewers for their comments on earlier versions of this manuscript.

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