Proceedings of the Geologists’ Association 127 (2016) 699–711

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Isotopic seawater temperatures in the Albian Gault Clay of the

Boulonnais (Paris Basin): palaeoenvironmental implications

Brahimsamba Bomou a,b,c,*, [1_TD$DIFF]Jean-Francois Deconinck c, Emmanuelle Puceat c,Francis Amedro d,c, Michael M. Joachimski e, Frederic Quillevere f

a Universite de Corse Pascal Paoli, Faculte des Sciences et Techniques, Campus Grimaldi, BP 52, 20250 Corte, Franceb CNRS, UMR 6134, SPE, 20250 Corte, Francec Universite de Bourgogne, UMR 6282 CNRS Biogeosciences, 6 Bd Gabriel, 21000 Dijon, Franced 26 rue de Nottingham, 62100 Calais, Francee GeoZentrum Nordbayern, Friedrich-Alexander Universitat Erlangen-Nurnberg, Schlossgarten 5, 91054 Erlangen, Germanyf Universite Claude Bernard Lyon 1, UMR 5276 CNRS Laboratoire de Geologie de Lyon: Terre, Planetes, Environnement, 2 rue Raphael Dubois,

69622 Villeurbanne Cedex, France

A R T I C L E I N F O

Article history:

Received 25 March 2015

Received in revised form 12 August 2015

Accepted 13 August 2015

Available online 4 September 2015

Keywords:

Albian

Gault Clay Formation

Oxygen isotopes

Palaeotemperature

Foraminifera

Selachian teeth

Fish teeth

Belemnites guards

Paris Basin

A B S T R A C T

Oxygen isotopes were measured on several types of fossil hardparts from the Gault Clay Formation

including benthic and planktonic foraminifera, belemnite guards, and fish small-teeth. Belemnites d18O

values indicate low temperatures (13.5–19.3 8C) with an increase from the Middle to Late Albian.

Foraminifera provide variable d18O values, some too low to be relevant in terms of temperature (until

42 8C). These low values probably result from a diagenetic alteration of the foraminiferal tests even

though SEM observations revealed well-preserved microstructures. However, higher foraminiferal d18O

values recorded in some levels indicate temperatures in the range of previously published estimates for

the Albian at comparable palaeolatitudes. In these levels, temperatures inferred from benthic and

planktonic foraminiferal d18O range between 15–17 8C and 27–30 8C respectively, during the Middle–

Late Albian interval. This slight increase in temperature is coherent with the long-term warming that

characterises the Aptian–Cenomanian interval. The temperature difference between sea-surface and

bottom waters fits well with a deposition at a palaeodepth of about 180 m in lower offshore

environments, assuming a temperature gradient with depth comparable to the modern one in similar

epicontinental tropical environments. Fish small-teeth indicate a temperature range from 22 to 28 8Cconsistent with previously published data from planktonic foraminifera, with a greater variability

recorded during the late than during middle Albian. This correspondence suggests that small-teeth

assemblages may be dominated by pelagic fishes, thus recording upper ocean temperatures. Finally, the

markedly lower temperatures recorded by the belemnite guards compared to other analysed materials

suggest a necto-benthic mode of life of belemnites.

� 2015 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved.

1. Introduction

Seawater temperature of Mesozoic oceans and epicontinentalseas are frequently estimated using oxygen isotopes of biogeniccalcite produced by various groups of marine organisms. Theseinclude bivalves (oysters, e.g. Brigaud et al., 2008 or rudists,Steuber et al., 2005), foraminifera (Huber et al., 1999; Wilson andNorris, 2001; Bornemann et al., 2008; Erbacher et al., 2011),

* Corresponding author at: Universite de Corse Pascal Paoli, Faculte des Sciences

et Techniques, CNRS, UMR 6134, SPE, Campus Grimaldi, BP 52, 20250 Corte, France.

Tel.: +33 04 20 20 21 96.

E-mail address: brahimsamba.bomou@gmail.com (B. Bomou).

http://dx.doi.org/10.1016/j.pgeola.2015.08.005

0016-7878/� 2015 The Geologists’ Association. Published by Elsevier Ltd. All rights re

brachiopods and belemnites guards (e.g. Van de Schootbruggeet al., 2000; Rosales et al., 2004; McArthur et al., 2007; Dera et al.,2011; Price et al., 2013; Stevens et al., 2014). Although foraminif-era, brachiopods and belemnites are composed of stable low Mgcalcite (LMC), some works have shown that primary calcite mayencounter recrystallisation during burial diagenesis (e.g. Pearsonet al., 2001). Consequently, prior to isotopic analyses, carefulexamination of the preservation state of the biogenic hardparts isrequired (e.g. Niebuhr and Joachimski, 2002). Typically, molluscsand brachiopods are studied under cathodoluminescence and/oranalysed for their trace element concentrations (Mn, Fe, Sr: e.g.

Mutterlose et al., 2012), while the preservation state of foraminif-era is checked using optical microscope (Wilson et al., 2002;Moriya et al., 2007) and scanning electron microscopy (SEM;

served.

[(Fig._1)TD$FIG]

Fig. 1. Geological map of the studied area (after Chantraine et al., 2003) and location of the Wissant section.

B. Bomou et al. / Proceedings of the Geologists’ Association 127 (2016) 699–711700

Barrera et al., 1987; Pearson et al., 2001). Each type of organismcarries specific information on temperature and environmentalconditions. Oysters and rudists living in shallow coastal environ-ments record sea-surface temperatures (SST) or upper-oceantemperatures (typically 0–100 m), depending on the depth of thedepositional environment (Surge et al., 2001; Steuber et al., 2005;Brigaud et al., 2008). Seasonal variations of temperature andsalinity can be inferred through microanalyses of bivalve shells (e.g.

Steuber et al., 2005). Foraminifera record temperatures at variousdepths of the water column, from the upper surface to the deepocean, depending on the depth habitat of the planktonic andbenthic taxa (Barrera and Savin, 1999; Spero et al., 2003; Friedrichet al., 2012; Birch et al., 2013). Belemnites belong to an extinctgroup and although several studies have recently suggested thatthey may have recorded intermediate to deeper water tempera-tures (Dutton et al., 2007; Wierzbowski and Joachimski, 2007; Deraet al., 2009), their depth-related ecology is still debated (e.g. Priceand Page, 2008; Rexfort and Mutterlose, 2009; Price et al., 2009).

Oxygen isotope ratios of biogenic apatite constitute anotherproxy that can be used to reconstruct temperatures of past oceans.Selachian fish tooth enamel is more resistant to a secondarydiagenetic alteration than biogenic carbonates (Lecuyer et al.,2003; Puceat et al., 2003). Selachian teeth d18O has been thereforeused to determine Mesozoic upper surface temperatures in theTethyan realm. Yet, such records are often produced at lowtemporal resolution because large selachian teeth are not commonin the sedimentary record. In addition, selachian fish teeth aremineralised in less than a season, which generates variability ind18O of several teeth from a specific horizon (Lecuyer et al., 2003;Puceat et al., 2003). The resolution of these records may beimproved by analysing selachian or teleostean small teeth whichappear far more common in sediments (Dera et al., 2009).

Sedimentary sections yielding well-preserved benthic andplanktonic foraminifera, together with belemnite guards and fishteeth, are still very scarce. As a consequence, in most studies, the

determination of past oceans seawater temperatures is usuallybased on oxygen isotope data originating from only one or twofossil groups. Some recent studies have focused on the comparisonbetween belemnite and foraminiferal d18O data but generally donot include bivalves of fish tooth d18O data (e.g. Dutton et al., 2007).In the present paper, we compare stable isotope data frombelemnites, fish teeth and benthic and planktonic foraminifera,which remarkably co-occur throughout the Gault Clay Formation(Middle and Upper Albian) that outcrops near the city of Wissant inthe northern France (Fig. 1). The studied section was chosen mainlybecause it contains a large diversity of a priori very well-preservedshells (Knight, 1997). Our main objective is to compare and discussMiddle and Late Albian seawater temperatures deduced from co-occurring biogenic calcite and apatite from various fossils groups.In addition, the comparison of belemnite d18O data with thosefrom benthic and planktonic foraminifera may further ourknowledge of the ecology and depth habitat of belemnites.

2. Geological setting

The Wissant section is exposed along coastal cliffs in theBoulonnais area (Northern Paris Basin, Fig. 1). From the Jurassic–Cretaceous transition (Purbeckian facies) to the Aptian, the Anglo-Paris Basin was characterised by a trend to continental facies(Purbeck-Wealden facies; Allen, 1998). Transgressive UpperAptian/Lower Albian glauconitic sands (greensands) were depos-ited in epicontinental marine environments (shore-face). In theBoulonnais, glauconitic dark-green sandstones, which depositedbetween the late Aptian and the early Albian, are occasionallyexposed on the beach at low spring tides while the overlyingargillaceous ‘‘Gault Clay Formation’’ of middle and late Albian inage (Owen, 1975; Gale et al., 1996; Hart, 2000) is exposed at thebase of the coastal cliffs. These ones are formed mainly by early andmid-Cenomanian chalks. The Wissant section (Fig. 2) was firstdescribed in details by Destombes and Destombes (1938), Amedro

[(Fig._2)TD$FIG]

Fig. 2. Litho- and biostratigraphy of the Wissant section (P1 to P6 = phosphatic nodule beds; W1 to W14 = studied samples).

Modified from Robaszynski and Amedro (1993).

B. Bomou et al. / Proceedings of the Geologists’ Association 127 (2016) 699–711 701

B. Bomou et al. / Proceedings of the Geologists’ Association 127 (2016) 699–711702

and Destombes (1978) and Robaszynski et al. (1980) and theninterpreted in a sequence stratigraphic framework (Amedro et al.,1981; Amedro, 1992; Robaszynski and Amedro, 1993; Amedro andRobaszynski, 1997; Amedro, 2009a,b). The section shows dark-grey homogeneous clays dominantly composed of illite, smectitesand kaolinite (Holtzapffel, 1984). The absence of sandy facies withsedimentary structures indicating hydrodynamic features sug-gests a deposition below the storm wave base, in a quiet, loweroffshore environment (Robaszynski et al., 1980; Knight, 1997). Theabundance of benthic fauna (bivalves, gastropods, echinoids,foraminifera, ostracods) and bioturbation indicate oxic conditionsat the sediment/seawater interface (Knight, 1997). However, thecommon occurrence of pyrite suggests that bacterial sulphatereduction was active early after deposition (Gale et al., 1996). TheGault Clay Formation is overlain by the lowermost CenomanianGlauconitic Marl (locally called ‘‘Tourtia’’) and by the lowerCenomanian marly chalk becoming less and less argillaceous up-section. The succession records an overall increasing water-depthfrom the Aptian (10–50 m) to the middle Cenomanian (200–300 m). According to the litho- and biofacies, the Gault ClayFormation was deposited in open-marine environments during amajor transgressive trend (Amedro and Robaszynski, 1997). Thetwelve metres-thick section of Albian clays is condensed, and thusexceptionally fossiliferous. Sediments contain abundant bivalves,gasteropods, ammonites, scaphopods, echinoderms and bothbenthic and planktonic foraminifera, these later being dominatedby Globigerinelloides, Hedbergella, Guembelitria, and Heterohelix. Thehigh abundance of planktonic foraminifera and the large diversityof the invertebrate fauna point to an open marine environment. Inthe following, we therefore assume a normal salinity of seawater,although we acknowledge that limited seasonal fluctuations linkedto variations in precipitation and evaporation remain likely. Thearea was located in a large seaway between the Brabant-Rhenish-Bohemian massif and the Armorican massif (Ziegler, 1990) at anestimated palaeolatitude of 358–408 N (Dercourt et al., 1993;Fenner, 2001; Amedro et al., 2014, Fig. 3).

[(Fig._3)TD$FIG]

Fig. 3. Palaeogeographic map of the Anglo-Paris basin d

Ammonite biostratigraphy is well established, and althoughhiatuses and condensed intervals highlighted by several horizonsof phosphate nodules commonly occur (Gale et al., 1996; Knight,1999), all ammonite zones from the Hoplites dentatus toMortoniceras inflatum Zones have been identified (Fig. 2).

The very fine-grained lithology limits fluid circulation duringdiagenesis and thus favours preservation of the fossil hardparts,which appears exceptional at a first view (ammonites arecommonly found with their nacre layer). The total burial depthof the Gault Clay Formation of the Boulonnais can be estimated tohave been less than 500 m including the overlying chalk(Cenomanian to Santonian and probably Campanian and Maas-trichtian chalks that have been eroded) and thin tertiary deposits.This is consistent with the occurrence of abundant smectites,which indicates a negligible influence of thermal diagenesis(Holtzapffel, 1984).

3. Material and methods

Seventeen samples of clays have been collected throughout thesection. After washing and sieving, individual benthic, planktonicforaminifera and fish small teeth (millimetre-sized teeth) werepicked from the >125 mm size fraction under a binocular lens.Hedbergella spp. (planktonic foraminifera) and Gavelinella spp.(benthic foraminifera) were selected for isotope analyses becausethey were frequent in all samples and because these taxa havebeen commonly used for isotope analyses, allowing furthercomparisons (Norris and Wilson, 1998; Fassell and Bralower,1999). In order to select glassy specimens, benthic and planktonicforaminifera were first observed using optical microscope (Fig. 4).They were secondly SEM-analysed to further control theirpreservation state (Plate 1). In each sample, for stable isotopeanalyses, �40 and 10 specimens of planktonic and benthicforaminifera respectively, were picked from the 125–250 mmand >250 mm size fractions. All these picked specimens weredevoid of sediment infilling and micro-recrystallisation (see Plate

uring the Middle Albian (after Amedro et al., 2014).

[(Plate_1)TD$FIG]

Plate 1. SEM photomicrographs of benthic and planktonic foraminiferal tests. Benthic foraminifera Gavelinella sp.: (1): W3 (�5500); (3): W5 (�270); (4): W5 (�8000); (5):

W7 (�230); (6): W7 (�5000); (9): P6 (�30); (10): P6 (�4500). Planktonic foraminifera Hedbergella sp.: (2): W3 (�3500); (7): W8 (�3500); (8): W8 (�15,000); (11): W11bis

(�500); (12): W11bis (�4500). Photomicrograph 2–6 and 9–12 showing well preserved outer surface, unlike the view 7 and 8 which exhibit secondary calcite or

recrystallisation. The cross section view through test chamber (photomicrograph 1) showing no evidence of recrystallisation.

[(Fig._4)TD$FIG]

Fig. 4. Planktonic and benthic glassy foraminifera photographed with an optical microscope: (1) Hedbergella sp. from level W7; (2) Gavelinella sp. from level W7.

B. Bomou et al. / Proceedings of the Geologists’ Association 127 (2016) 699–711 703

Table 1Isotopic data (oxygen) from belemnite guards (Neohibolithes minimus) from the

Gault Clay of the Wissant section (Boulonnais, Northern France).

Sample (m) Ammonite zone Belemnite guards

d18O (% V-PDB) Temperature (8C)

(Anderson and

Arthur, 1983)

12.2 M. inflatum �1.03 16.1

12.1 M. inflatum �1.13 16.5

12.1 M. inflatum �1.38 17.6

12 M. inflatum �1.51 18.2

9.75 M. inflatum �1.26 17.1

9.75 M. inflatum �1.28 17.2

8 M. pricei �1.39 17.6

8 M. pricei �1.32 17.3

6.75 M. pricei �1.4 17.7

5.5 D. cristatum/M. pricei �0.99 15.9

5.5 D. cristatum/M. pricei �0.62 14.5

1.5 H. dentatus �0.93 15.7

1.5 H. dentatus �0.38 13.5

Sample (m) Ammonite zone Variability in Belemnite guard

d18O (% V-PDB)

W10bis (6.75 m) M. pricei �1.82

�1.74

�1.75

�1.68

�1.33

�1.14

W13 (9.75 m) M. inflatum �1.13

�1.45

�1.19

�1.16

�1.24

�1.26

12.1 m M. inflatum �1.35

�1.57

�1.23

�1.20

�1.81

�1.46

�1.36

�1.31

B. Bomou et al. / Proceedings of the Geologists’ Association 127 (2016) 699–711704

1). Typically, 10–30 small teeth pooled from each sample level(mass ranging between 1 and 1.5 mg) were analysed. The recoveredteeth were too small to separate enamel from dentine and they werethus analysed as a whole. Large shark teeth are rare, since in thirtyyears only three of them were collected by one of us (FA). Enamelfrom these three large teeth was isolated and it was analysed.

Table 2Isotopic data (oxygen) from benthic and planktonic foraminifera from the Gault Clay o

[30_TD$DIFF]Depth

(m)

Sample Ammonite

[10_TD$DIFF]zone

Benthic foraminifera Planktonic forami

[26_TD$DIFF]d18O

(% V-PDB)

Temperature [12_TD$DIFF](8C)

(Barras et al.,

2010)

d18O

(% V-PDB)

Temp

(Erez

Luz,

11.9 W14 M. inflatum �1.47 17.2 �4.09 30.0

9.75 W13 M. inflatum �4.76 32.3 �6.07 39.3

8.25 W12 M. pricei �5.03 33.6 �6.3 40.4

8.15 W11bis M. pricei �4.68 32.0 �6.29 40.4

8 P6 M. pricei �5.19 34.3 �6.52 41.5

7.75 W11 M. pricei �3.44 26.2 �4.52 32.0

6.75 W10bis M. pricei �5.27 34.7 �6.59 41.8

5.37 W9 D. cristatum �1.18 15.8 �3.63 27.8

4.12 W7 D. biplicatus �1.06 15.3 �3.61 27.7

3.81 W5 D. niobe �3.08 24.6 �4.28 30.9

3.44 W4 D. niobe �2.65 22.6 �4.64 32.5

3.12 W3 A. intermedius �2.08 20.0 �4.25 30.7

The shaded data correspond to samples unaffected by diagenesis. DT = difference of tem

Common belemnites guards (Neohibolithes minimus) have beensampled in the field as often as possible. Prior to stable isotopeanalyses, polished sections of belemnite guards were examined fortheir preservation state using cathodoluminescence microscopy(8200MKII Technosyn cathodoluminescence coupled to an Olym-pus microscope). After careful mapping of luminescent and non-luminescent areas of each rostrum, only non-luminescent partscorresponding to pristine calcite were microsampled using adental-drill (0.05 and 0.1 mg).

Oxygen isotope analyses for both calcite (foraminifers, belem-nites) and apatite (selachian and fish small teeth) were performedat the GeoZentrum Nordbayern (University of Erlangen-Nurem-berg/Germany). Carbonate powders were reacted with 100%phosphoric acid at 75 8C using a Kiel III online carbonatepreparation line connected to a ThermoFinnigan 252 mass-spectrometer. All values are reported in per mil relative to V-PDB by assigning a d13C value of +1.95% and a d18O value of�2.20% to NBS19. Reproducibility was checked by replicateanalysis of laboratory standards and was �0.05% (1s) for bothd18O and d13C. Fish tooth apatite was dissolved in nitric acid andchemically converted into Ag3PO4 using a modified version of themethod described by O’Neil et al. (1994). Oxygen isotope ratios weremeasured on CO using a High Temperature Conversion ElementalAnalyzer (TC-EA) connected online to a ThermoFinnigan Delta plusmass spectrometer. Oxygen isotope compositions of fish tooth apatiteare reported in the delta notation relative to V-SMOW (ViennaStandard Mean Ocean Water). All phosphate d18O data have beennormalised using a value of NBS120c of 21.7% (Halas et al., 2011).Reproducibility of analyses (1s) was better than �0.2% (1s).

4. Results

Belemnite (N. minimus) d18O values range between �1.5% and�0.4% V-PDB (Table 1). Internal variability within a rostrum hasbeen examined on three different rostra using multiple analyses ondifferent growth areas (Fig. 5). In agreement with Rosales et al.(2004), only limited d18O variations are recorded within the rostra(<0.7%; Fig. 5). In four levels, 2 rostra for each ones, have beenanalysed, and yield similar values (difference of 0–0.5% betweenrostra; Fig. 6). Belemnite d18O values appear slightly higher in themiddle Albian (�0.4% to �1%) than in the late Albian (�1.1% to�1.5%, Fig. 6).

Benthic foraminiferal d18O values range between �1.1% and�5.3% (Table 2). Values comprised between �2% and �3% arerecorded at the base of the section in the Anahoplites intermedius

and Dimorphoplites niobe ammonite zones (middle Albian; Amedro

f the Wissant section (Boulonnais, North of France).

nifera D [31_TD$DIFF]T8 (T8C Planktonic

� T8C Benthic)

(Erez and Luz, 1983;

Barras et al., 2010)

erature [12_TD$DIFF](8C)

and

1983)

Temperature [12_TD$DIFF](8C)

(Bemis et al., 1998)

[32_TD$DIFF]Eq. (1)

(low-illumination)

Temperature [12_TD$DIFF](8C)

(Bemis et al., 1998)

[33_TD$DIFF]Eq. (2)

(high-illumination)

30.0 26.3 12.8

39.5 35.8 7.0

40.6 36.9 6.9

40.6 36.8 8.4

41.7 37.9 7.2

32.1 28.3 5.7

42.0 38.3 7.2

27.8 24.1 12.0

27.7 24.0 12.5

30.9 27.2 6.3

32.7 28.9 10.0

30.8 27.0 10.8

perature between deep and surface waters.

[(Fig._5)TD$FIG]

Fig. 5. Variability in d18O values (% V-PDB) of three belemnite rostra (Neohibolithes minimus) from levels W10bis, W13 and 12.1 m.

B. Bomou et al. / Proceedings of the Geologists’ Association 127 (2016) 699–711 705

et al., 1981), followed by higher average d18O values of �1.2%during the upper part of the middle Albian (Dimorphoplites

biplicatus and Dipoloceras cristatum Zones; Fig. 6). Benthicforaminifera yield markedly lower d18O values of about �5% inthe early Late Albian (Mortoniceras pricei and part of the M. inflatum

Zones), although one sample shows a higher value of �3.4%(Fig. 6). One benthic foraminiferal sample collected higher in the M.

inflatum Zone exhibits a higher d18O value of�1.5% comparable tovalues recorded in the late Middle Albian (Fig. 6). Planktonicforaminiferal d18O values record a very similar evolution althoughthey are markedly offset by about �1.5%, with d18O rangingbetween �3.6% and �6.6% (Table 2).

The three analysed large selachian teeth display an overallincreasing trend of d18O values (Fig. 6). At the base of the EarlyAlbian (P1), the first selachian tooth shows a d18O value of 19.7%.The second, from the Middle Albian (D. niobe Zone), yields a d18Ovalue of 20.7%, and the third in the Upper Albian (M. inflatum

Zone = C. auritus subzone) shows a d18O value of 21.8% (Table 3).Enough small teeth for isotope analyses have been collected fromten samples. In comparison to the large teeth, the d18O values ofthe small teeth are quite constant, mostly in the 20.1–20.9% rangewith the exception of two samples located close to the M. pricei andM. inflatum Zones boundary, which yield values around 19.6%(Fig. 6).

[(Fig._6)TD$FIG]

Fig. 6. Evolution of d18O of belemnites, foraminifera (benthic and planktonic) and fish small teeth from the Gault Clay in the Wissant section. Low d18O values of foraminifera

result in unrealistically high temperatures and are probably due to diagenetic alteration (see text).

Table 3Isotopic data (oxygen) from selachians and fish small teeth from the Gault Clay of the Wissant section (Boulonnais, Northern France).

Depth (m) Sample Ammonite zone d18OPO4(% SMOW) Temperature (8C) (Puceat et al., 2010)

Fish microteeth 9.75 W13 M. inflatum 20.5 24

8.25 W12 M. pricei 19.7 28

8.15 W11bis M. pricei 19.6 28

8 P6 M. pricei 20.9 23

7.75 W11 M. pricei 20.9 23

6.75 W10bis M. pricei 21.0 22

5.37 W9 D. cristatum 20.2 25

3.44 W4 D. niobe 20.3 25

3.12 W3 A. intermedius 20.1 26

1.62 W1 H. dentatus 20.5 24

Selachian teeth 9.75 W13 M. inflatum 21.8 19

4 P4 D. niobe/D. biplicatus 20.7 23

0.47 P1 C. floridum 19.7 28

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5. Discussion

5.1. Reconstruction of seawater temperatures at Wissant

Several fractionation equations have been published to recon-struct seawater temperatures from planktonic foraminiferal d18O,as offsets have been shown to occur between different species ofplanktonic foraminifera (e.g. Spero et al., 2003; Pearson, 2012;Birch et al., 2013). As different species of Hedbergellids have beenmixed in this study for the analyses, the general equation of Erezand Luz (1983) has been applied to estimate seawater tempera-ture. For comparison, temperatures calculated with the equations

of Bemis et al. (1998) for both low-illumination conditions (Eq. (1))and for high-illumination (Eq. (2)) are presented (Table 2). Forbenthic foraminifera, the equation of Barras et al. (2010) is used forthe 200–250 mm fraction. The equation of Anderson and Arthur(1983) has been applied for belemnites assuming that theseorganisms, as modern cephalopods, precipitated calcite inequilibrium with seawater (Auclair et al., 2004; Rexfort andMutterlose, 2009), and Puceat et al. (2010) for fish teeth apatite. Anoxygen isotope composition of seawater (dw) of �1% V-SMOWwas assumed considering the Albian as an ice-free period(Shackleton and Kennett, 1975), converted to �1.27% PDB whenrequired in the equations (Hut, 1987; Barras et al., 2010).

B. Bomou et al. / Proceedings of the Geologists’ Association 127 (2016) 699–711 707

Temperatures calculated using belemnite d18O values arebetween 13.5 and 18.2 8C (Table 1 and Fig. 7). From the base tothe top of the Gault Clay Formation, estimated temperaturesgenerally increase notably at the base of the M. pricei Zone. A[(Fig._7)TD$FIG]

Fig. 7. Comparison of palaeotemperatures reconstructed from belemnite,

similar trend was previously reported from the Gault ClayFormation of South England by Gale and Owen (2010). A reversetrend is however observed from the selachian teeth, which suggesta cooling trend from ca. 28 8C to ca. 19 8C (Table 3 and Fig. 7). This

benthic and planktonic foraminifera, selachian and small teeth d18O.

B. Bomou et al. / Proceedings of the Geologists’ Association 127 (2016) 699–711708

trend, based only on three analyses and apparently in conflict withour belemnite data, may be explained by the fact that shark teethgrowth occur during one season and that reconstructed tempera-tures do not necessarily reflect annual average temperature. Thedata from the Wissant section thus indicate that a single selachiantooth per level cannot be used to approach the evolution ofpalaeotemperatures.

Fish small teeth indicate temperatures ranging from ca. 22 to ca.

28 8C without any clear trend from base to top of the section(Fig. 7). The data may suggest a cooling trend during the early LateAlbian, while belemnites record a warming. This apparentcontradiction may be related to the facts that (1) the small teethbelonged to different species of fish living at different water-depths (2) our analyses of small teeth included both enamel anddentine, with the latter component being less resistant todiagenesis. Despite these difficulties, assuming that temperaturesinferred from fish small teeth d18O reflect overall sea surfacetemperatures, our results seem to be consistent with the generallyaccepted Albian sea-surface temperature at low to mid-latitudes(Norris and Wilson, 1998; Norris et al., 2002).

Most foraminifera analysed from Wissant yield low d18O values,resulting in high, unrealistic temperatures, reaching more than35 8C for benthic foraminifera and more than 41 8C for planktonicforaminifera while Cretaceous SST probably never exceeded 33–34 8C (Norris et al., 2002). Temperatures calculated using theequation for high-illumination of Bemis et al. (1998) are lower butstill reach about 38 8C. Some temperatures reconstructed fromboth planktonic and benthic foraminiferal d18O are within thetemperature range calculated from fish small teeth and belemniteguards, respectively (Fig. 7). In the upper part of the Middle Albian,at least two samples of benthic and planktonic foraminifera fromthe D. biplicatus and D. cristatum Zones yield temperaturesconsistent with those inferred from belemnites and fish smallteeth. Temperatures deduced from foraminiferal d18O measure-ments within these zones (around 15–16 8C from benthicforaminifera, and 28 8C from planktonic foraminifera) are closeto those previously published from Albian sediments deposited inthe North Atlantic Ocean at comparable palaeolatitudes (Huberet al., 2002; Petrizzo et al., 2008). In the topmost part of the section(M. inflatum Zone), an additional sample yields reasonabletemperatures (17 8C from benthic foraminifera, and around 308from planktonic foraminifera).

5.2. Significance of low foraminiferal d18O values

Most of the planktonic and benthic foraminiferal d18O valuesanalysed in this work give anomalously high palaeotemperatures,despite their apparent good preservation state under SEM. Lowd18O values can be explained either by a recrystallisation of theforaminiferal tests or by lower d18O values of sea water due to thesupply of freshwater during heavy rainfall as it is often the case intropical regions (e.g. Gagan et al., 1994).

While there is no faunal and ecological evidence for freshwaterinput in the depositional environment of the Gault Clay (Gale andOwen, 2010), the middle–late Albian transition is characterised byhigher proportions of detrital clay minerals (chlorite, illite andkaolinite) and a relative depletion of smectites (Holtzapffel, 1984).A coeval decrease of glauconite grains and phosphate noduleshorizons suggests higher sedimentation rates during the lateAlbian than during the middle Albian. Detrital supply may resultfrom enhanced continental runoff suggesting more humid climateconditions at a regional scale. For example, a coeval enhanceddetrital input was also noticed in the Gault Clay of the Isle of Wight(Gale et al., 1996) and in the Lower Saxony basin in Germany (Kuhnet al., 2001). Yet, if salinity changes occurred during the studiedtime interval, they are unlikely to explain the anomalously low

d18O values of part of the foraminiferal samples. If increased runoffhad an impact on d18O of local seawater, lower seawater d18Ovalues should not only be mirrored in the tests of benthic andplanktonic foraminifera, but also in the belemnite guards and thefish teeth recovered from the same levels. As a result, we argue thatthe low d18O values of the benthic and planktonic foraminiferacannot be explained by increased freshwater inputs.

Although SEM observations of foraminifera did not reveal anyobvious diagenetic recrystallisation of the calcite (Plate 1), the testswere also studied under cathodoluminescence to acquire addi-tional information on their preservation state (Plate 2). All testsexhibit an orange luminescence, usually a criterion to identifysecondary alteration as calcite precipitated under oxidisingconditions has low Mn2+ concentrations, Mn2+ being the mainactivator of an orange coloured luminescence. However, it shouldbe noted that Mn2+ incorporation in the tests of recent benthicforaminifera results in an orange luminescence (Barbin et al., 1991;Barbin, 2013). Consequently, the luminescence of the benthicforaminifera of the Gault Clay cannot be taken as an argument for adiagenetic overprint, even if it cannot be ruled out. By contrast,recent planktonic foraminifera generally show no luminescence(Barbin, 2000). As a consequence, a diagenetic alteration at least ofthe planktonic foraminifera from the Gault Clay is likely. As apronounced positive correlation between benthic and planktonicforaminiferal d18O is observed throughout the section, the effectsof a diagenetic influence on the benthic foraminifera are likely aswell.

One feature that remains unclear concerns the existence oflevels (W7, W9 and W14; Fig. 7) in which planktonic and benthicforaminifera yield temperatures in agreement with those inferredfrom fish small teeth and belemnite guards, respectively, that tendto suggest that the foraminiferal tests in these levels are well-preserved. Such a difference in the preservation of the geochemicalsignal of foraminifera is surprising given that all the sampled levelsare rich in clay that are impermeable and thus supposed to limitpercolation of diagenetic solutions, and that no major change in thenature of sediments can be observed between the level that yieldanomalously low d18O values of foraminifera and the others.

5.3. Insights on belemnites habitat

Except for one temperature derived from a large shark toothfrom the M. inflatum Zone, temperatures calculated from belem-nite d18O are lower than all palaeotemperatures reconstructedfrom the different hardparts (Fig. 7). The low temperatures are alsocomparable to temperatures measured on three benthic forami-niferal samples collected from levels that may not have beenimpacted by diagenesis as suggested by the d18O values ofplanktonic foraminifera, which show calculated SSTs in the rangeof previously published values for this period of time (Norris andWilson, 1998; Norris et al., 2002). These features are in agreementwith a necto-benthic mode of life for belemnites as it has beensuggested in several recent publications (Dutton et al., 2007;Wierzbowski and Joachimski, 2007; Dera et al., 2009; Alberti et al.,2012; Price et al., 2012). The temperatures inferred from thebelemnite guards tend to be slightly lower than those derived fromthe supposedly preserved benthic foraminifera, although the dataset remains limited. If the temperatures deduced from benthicforaminifera in these three levels are reliable, the only way toexplain this would be to consider that at some point of their life,belemnites lived at a greater depth or at higher palaeolatitudes.

5.4. Climate change at the Middle–Late Albian transition

Detailed studies of oxygen isotopes throughout the Albian stageshow a progressive warming of both deep and surface waters

[(Plate_2)TD$FIG]

Plate 2. Cathodoluminescence photographs of benthic and planktonic foraminifera. Note that orange luminescence occurs both in benthic and planktonic samples from levels

with low oxygen (W7 level; +4 m) and high oxygen (W11 level; +6 m) isotopic signatures. (For interpretation of the references to colour in this plate legend, the reader is

referred to the web version of this article.)

B. Bomou et al. / Proceedings of the Geologists’ Association 127 (2016) 699–711 709

(Huber et al., 1995, Norris and Wilson, 1998; Fassell and Bralower,1999; Fenner, 2001; Price and Hart, 2002, Steuber et al., 2005; Priceet al., 2012). Lower Albian deposits show evidence of a relativelycool climate (e.g. Pirrie et al., 2004) that represents the end of therelatively cool interval which characterises the Aptian (Price,1999). At Wissant, this trend appears to be recorded also in bottomwaters by belemnites that should live at a relatively constant waterdepth.

The parallelism between planktonic and benthic foraminiferaisotopic curves is an interesting feature (Fig. 7). Surprisingly,fluctuations in d18O of benthic foraminifera show greateramplitudes than the d18O values of planktonic foraminifera.Indirectly, as a reverse relationship was expected, this featureconfirms the important impact of diagenesis on the isotopic signal.Interestingly, where a diagenetic influence is indicated byminimum d18O values, the difference between planktonic andbenthic foraminifera d18O is reduced. In the 3 levels presentingplanktonic foraminifera that yielded d18O values in the range ofpreviously published values, the difference between sea surfaceand deeper water temperatures calculated from planktonic andbenthic foraminifera, respectively (12–13 8C) is only slightlyhigher than the temperature difference given by belemnites andfish small teeth (6–11 8C). Such a difference between sea surfaceand bottom temperatures would be consistent with a deposition ata palaeodepth in the order of 170–180 m assuming a modernthermal gradient of about �1 8C/14 m (Picard et al., 1998). Thisestimate is consistent with the offshore facies (deposition belowstorm wave base) of the Gault Clay, but significantly deeper thanthe palaeodepth (40–60 m) proposed by Knight (1997) and Galeand Owen (2010).

d18O values of belemnites and fish small teeth show moderatefluctuations in the middle Albian, while larger fluctuations areobserved during the late Albian. These characteristics may resultfrom relatively stable climate conditions occurring during themiddle Albian, which contrast with a more variable climateassociated with carbon cycle perturbations and probably climatechanges in the Milankovitch frequency band (Erba et al., 1992;Erbacher et al., 2011) occurring during the late Albian.

6. Conclusions

The oxygen isotope data presented in this paper are difficult tointerpret, as far as foraminifera carry a palaeoclimatic signaloccasionally overprinted by secondary diagenetic alteration whoseorigin remains to be established. However, d18O of belemnites, fishsmall teeth and of some of the foraminiferal samples show thatmiddle and late Albian deep water and sea-surface temperaturescan be estimated at around 13–18 8C, and 22–30 8C respectively.Fluctuating SSTs are consistent with previous temperatureestimates for the Albian at comparable palaeolatitudes.

In the case of the Gault Clay of Wissant, belemnite guards andfish small teeth are the most reliable hardparts to reconstructdeep-water and sea-surface temperatures. Indeed, SEM examina-tion of foraminiferal tests does not seem sufficient to rule out anydiagenetic recrystallisation. But cathodoluminescence does, atleast for the benthics.

Finally, from the middle to the late Albian, the minor increase intemperature and the increasing difference between deep andsurface waters temperatures corresponds well with the progres-

B. Bomou et al. / Proceedings of the Geologists’ Association 127 (2016) 699–711710

sive global warming and gradual sea level rise that characterisedthe Aptian to Cenomanian time slice [3_TD$DIFF].

Acknowledgements

We warmly thank Francoise Magniez-Jannin (Universite deBourgogne) for the training and the determination of planktonicand benthic foraminifera, Claudie Josse (Universite de Bourgogne)for SEM analyses, Henri Cappetta (Universite de Montpellier) forthe fish teeth determination[5_TD$DIFF], Claudia Baumgartner (Universite deLausanne) for help with the cathodoluminescence and AndreVillard (Universite de Neuchatel) for foraminifers thin sections.The manuscript greatly benefited from suggestions from HubertWierzbowski and an anonymous reviewer who are warmlyacknowledged.

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