Stable isotope and trace element geochemistry of Upper Cretaceouscarbonates and belemnite rostra (Middle Campanian, north Germany)

La géochimie des isotopes stables et des éléments traces des carbonateset des rostres de bélemnite d’âge Crétacé supérieur (Campanien moyen,

Allemagne du nord)

Stabile Isotopen- und Spurenelement-Geochemie oberkretazischerKarbonate und Belemniten-Rostren (Mittel-Campan, Nord-Deutschland)

Birgit Niebuhra,*, Michael M. Joachimskib

aJulius-Maximilians-Universität, Institut für Paläontologie, Pleicherwall 1, 97070 Würzburg, GermanybUniversität Erlangen-Nürnberg, Institut für Geologie und Mineralogie, Schlossgarten 5, 91054 Erlangen, Germany

Received 19 December 2000; accepted 18 June 2001

Abstract

Trace element contents and stable isotopic composition of Middle Campanian marl-limestone rhythmites and belemnite rostra ofBelemnitella mucronata were investigated. High strontium and low iron as well as manganese and magnesium contents of belemnite calciteand bulk rock samples suggest no diagenetic overprint. However, the orange-coloured cathodoluminescence of coccolith-rich sedimentsindicates diagenetic cementation and/or recrystallization. The non-luminescent belemnite rostra reveal an extraordinary preservation of themicrostructures that is interpreted to have been favoured by a silification of the outer rim of the belemnite rostra. Carbon isotope ratios ofthe coccolith limestones and belemnite rostra are comparable, with higherδ13C variations observed for belemnite calcite. The 1.5–2‰depletion inδ18O of the marl-limestone rhythmites relative to belemnite calcite is explained by diagenetic alteration of the sediments.Palaeotemperatures, calculated from theδ18O values of the well-preserved belemnite rostra, are around 12.5± 2 °C and suggest rather lowsea-surface temperatures for the Middle Campanian epicontinental sea of north Germany assuming a water depth of less than 100 m.© 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.

Résumé

Le contenu des éléments en trace et la composition des isotopes stable ont été étudiés sur les rythmites marnes-calcaires et les rostres debélemnite (Belemnitella mucronata) d’âge Campanien Moyen. Les teneurs élevées en strontium ainsi que les faibles proportions du fer,magnésium et manganèse contenus dans la calcite des bélemnites et la roche totale indiquent l’absence d’une signature diagénétique. Cepen-dant les observations en cathodoluminescence des sédiments riches en coccolithes montrent l’existence d’une cimentation diagénétiqueet/ou recristallisation. La non luminescence des bélemnites révèle un excellent état de préservation des microstructures interprétée comme lerésultat de la silicification de la paroi externe des rostres. Les proportions des isotopes de carbone des calcaires à coccolithes et des rostres debélemnite sont comparables, montrant une variation élevée duδ13C observées au niveau de la calcite des bélemnites. La réduction de1.5–2‰ enδ18O des rythmites marnes-calcaires à coccolithes par rapport à la calcite des bélemnites s’explique par une altérationdiagénétique des sédiments. Les paléotempératures calculées à partir des valeurs enδ18O des rostres donne des valeurs de 12.5± 2 °C etsuggèrent des températures relativement basses à la surface de la mer au Campanien Moyen épicontinentale de l’Allemagne du nord, sug-gérant une profondeur d’eau n’excédant pas les 100 m. © 2002 E´ditions scientifiques et médicales Elsevier SAS. Tous droits réservés.

* Corresponding author.E-mail address: niebuhr@mail.uni-wuerzburg.de (B. Niebuhr).

Geobios 35 (2002) 51–64

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Zusammenfassung

Kalk/Mergel-Rhythmite und Belemniten-Rostren von Belemnitella mucronata aus dem norddeutschen Mittel-Campan wurden isotopen-und elementgeochemisch untersucht. Die hohen Strontium- und niedrigen Eisen-, Mangan- und Magnesium-Gehalte der Belemniten undSedimente lassen keine diagenetische Überprägung erkennen. Allerdings weist die orangefarbene Kathodenlumineszenz auf einediagenetische Rekristallisation und/oder Zementation der Coccolithen-reichen Karbonate hin. Die nicht-lumineszenten Belemniten-Rostrenzeigen eine außergewöhnlich gute mikrostrukturelle Erhaltung, welche durch eine randliche Silifizierung der Rostren begünstigt wurde. DieSchwankungsbreite in den δ13C-Werten ist in den Rostren wesentlich höher als im Sediment, was auf eine biologische Kohlenstoff-Fraktionierung der Belemniten hinweisen könnte. Die im Vergleich zu den Belemniten-Rostren um 1.5–2‰ geringeren δ18O-Werte derKalk/Mergel-Rhythmite werden als diagenetische Signale gewertet. Die anhand der δ18O-Werte der Belemniten ermittelten Paläotempera-turen liegen bei 12.5 ± 2 °C und implizieren recht kühle Oberflächenwasser-Temperaturen für das mittelcampane Epikontinental-MeerNord-Deutschlands, dessen Wassertiefe 100 m nicht überstieg. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.

Keywords: Upper Cretaceous; North Germany; Carbon and oxygen isotopes; Belemnites; Palaeotemperatures; Carbonate diagenesis

Mots clés: Crétacé supérieur; Allemagne du nord; Isotopes stable de carbone et d’oxygéne; Bélemnites; Paléotempératures; Diagénése des carbonates

Schlüsselworte: Oberkreide; Nord-Deutschland; Kohlenstoff- und sauerstoff-isotopie; Belemniten; Paläotemperaturen; Karbonat-diagenese

1. Introduction

Secular variations in the carbon isotopic composition ofUpper Cretaceous carbonates proved to be a powerful toolfor the correlation of time-equivalent sequences (e.g.Schlager et al., 1987; Voigt and Hilbrecht, 1997) and mayhelp to decipher changes in palaeoceanographic conditions(e.g. Marshall, 1992; Mitchell et al., 1997). Isotope studieson Cenomanian carbonate successions demonstrated thatindividual sections can be correlated by means of theirδ13Ccarb records (Gale et al., 1993). However, many studiesconcentrated on the Cenomanian to Turonian time period(e.g. Schlager et al., 1987; Gale et al., 1993; Mitchell et al.,1997; Voigt and Hilbrecht, 1997) and only few data areavailable for the Santonian to Maastrichtian interval ofnorth and central Europe (e.g. Schönfeld et al., 1990).

The oxygen isotopic composition of Mesozoic and espe-cially Palaeozoic seawater is one of the most controversiallydebated subjects in stable isotope geochemistry (Land,1995; Veizer, 1995; Veizer et al., 1999). Low-magnesiumcalcitic brachiopod and belemnite shells are preferentiallyused to reconstruct the oxygen isotopic composition of pastoceans. However, beginning with the early isotopic study ofJurassic and Early Cretaceous belemnites by Bowen (1966),it has become evident that the isotopic ratios measured onbelemnite calcite for a given time interval vary for severalpermil (Podlaha et al., 1998). It is unclear whether thisvariation reflects primary differences in the palaeoenviron-ment (e.g. Lowenstam, 1961) or is due to vital fractionationeffects (e.g. Sælen and Karstang, 1989).

This contribution presents a stable isotope and traceelement study of a Middle Campanian coccolith limestone/marl sequence and coeval belemnite rostra. Transmitted-lightand cathodoluminescence microscopy in combination withtrace element analysis was used to unravel a potentialdiagenetic alteration of the primary isotopic signals.

2. Geological setting

The Upper Cretaceous of Lower Saxony (north Ger-many) is exposed in several secondary marginal salt domedepressions and in the intervening broad synclines. Thethickest and most complete sequence is developed in theLehrte West Syncline east of Hannover (Fig. 1) where amore than 500 m thick succession of marl-limestone rhyth-mites is exposed in several large active cement quarries. Tothe north of the Lehrte area, the marl-limestone rhythmitesare interfingering with white chalks, whereas to the east

Fig. 1. Schematic geological map of the Hannover area (north Germany)showing the distribution of Upper Cretaceous sediments and location of theinvestigated sections in the Lehrte West Syncline.Carte géologique du secteur de Hannover (Allemagne du nord) montrant ladistribution des sédiments Crétacés supérieur et localisation géographiquedes coupes étudiées dans le la partie ouest du synclinal de Lehrte.

52 B. Niebuhr, M.M. Joachimski / Geobios 35 (2002) 51–64

marls, greensands, and conglomerates are developed (Nie-buhr, 1995).

A more than 125 m thick Middle Campanian successionwas studied in the Lehrte area which comprises a timeinterval of approximately 2.7 my. During Campanian timesthe Lehrte area was situated at palaeolatitudes of approxi-mately 45°N (Voigt, 1996). The palaeogeographic setting,microfacies of the marl-limestone rhythmites, foraminiferalplankton/benthos ratio as well as a minor abundance ofkeeled planktonic foraminifers (< 5%) suggest that waterdepths did not exceed 100 m.

3. Belemnite palaeoecology

All investigated belemnite rostra belong to Belemnitellamucronata mucronata (SCHLOTHEIM, 1813), the classicindex fossil of the Boreal Middle Campanian (Christensen,1996). In north Germany, B. mucronata occurs indepen-dently of marine facies in chalks, limestones, marls, green-sands, and conglomerates (Niebuhr, 1995). B. mucronatacan be found in high abundances in the studied marl-limestone rhythmites of the Lehrte West Syncline whichrepresent the stratum typicum for B. mucronata mucronata(see Christensen et al., 1975). The length of individualbelemnite rostra varies from 3 to more than 10 cm.

The belemnite rostra consist of the rostrum cavum, wherethe alveolus, a conical cavity at the anterior part, is situated,and the rostrum solidum (Fig. 2). The majority of fossilizedrostra are originally impervious (e.g. Bandel et al., 1984)consisting of low-magnesium calcite, though some arereported to have been originally partly aragonitic andporous (Spaeth, 1971, 1975; Spaeth et al., 1971). Importantshell morphologic characteristics of the rostrum solidum arethe following.

• The apical line, which is the axis of the rostrum andshows the trajectory of the apex during successivegrowth stages. In B. mucronata, it exactly marks thecentre of the rostrum and is not ventrally displaced.Some authors consider the apical line as an originallyorganic feature, formed by radial growth of calciteprisms which where connected to organic fibres (Ban-del et al., 1984). Due to the possibility of diageneticalteration, this structure was not sampled in the studiedspecimens.

• The rings, which have traditionally been interpreted asconcentric alternations of organic (laminae obscurae)and inorganic layers (laminae pellucidae) (Müller-Stoll,1936), whereby the laminae obscurae are considered tohave been modified by diagenesis in most cases(Müller-Stoll, 1936; Spaeth, 1971).

• The radial structures, which have been formed bycalcite crystals traversing from the apical line up to theouter border of the rostrum and cross the rings more orless perpendicularly. Due to their coarse crystalline

nature, they were often attributed to recrystallization(Spaeth, 1971).

Belemnites are interpreted to have had a nectonic way oflife (Spaeth, 1975). The delicate structure of the siphuncleapparatus limited the habitat depth range of belemnitesbecause it cannot withstand high water pressures (Bandel etal., 1984). Furthermore, the presence of well-developedeyes and an ink sack makes only sense for animals inhab-iting the upper light-saturated part of the water column.Therefore, palaeotemperatures calculated from δ18O data ofbelemnite calcite are generally interpreted as sea-surfacetemperatures (e.g. Podlaha et al., 1998). A comparison ofδ18O values of belemnite rostra and benthic organisms suchas articulate brachiopod shells could help to answer thequestion whether belemnites were thriving in surface wa-ters. However, brachiopods occur only rarely in the inves-tigated sediments, are thin-shelled, and were thus notavailable for stable isotope analysis.

4. Material and methods

4.1. Sediment

Samples for trace and main elements as well as stableisotope analysis were taken at 0.5 and 1 m intervals,respectively. Polished thin-sections were used for microfa

Fig. 2. Schematic sections through the rostrum of Belemnitella mucronata(modified after Sælen, 1989).Coupe schématique à travers le rostre de Belemnitella mucronata (modifiéd’après Sælen, 1989).

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cies and cathodoluminescence (CL) investigations. Nanno-facies and diagenetic features were investigated on freshfractures using a Carl Zeiss DSM 962 scanning electronmicroscope (SEM) at the Institut für Biologie, University ofWürzburg. After powdered samples were processed to melttablets, main and trace element contents of the marl-limestone rhythmites were determined by X-ray fluores-cence analysis (XFA) using a Philips PW 2400 spectrometerat the Institut für Chemie und Biologie des Meeres, Univer-sity of Oldenburg.

4.2. Belemnites

Nearly 80 belemnite rostra were collected from the samehorizons as the bulk sediment samples. The rostra were cutperpendicular to their length axis. In order to study theinternal texture and state of preservation, polished thin-sections were investigated by transmitted-light (TL) and CLmicroscopy. The non-luminescent parts of 36 individualbelemnite rostra were sampled with a 0.4 mm drill bit for atotal of 54 stable isotope analyses.

4.3. Both sediment and belemnites

Trace element contents (Mn, Fe, Sr, Mg) of belemniterostra and bulk sediment were determined by ICP-AESanalysis at the Institut für Geologie und Mineralogie,University of Erlangen-Nürnberg. Stable isotope ratios ofpowdered belemnite calcite and bulk sediment were mea-sured with a carbonate preparation line (Carbo-Kiel I)connected on-line to a Finnigan Mat 252 mass-spectrometerat the Institut für Geologie und Mineralogie, University ofErlangen-Nürnberg. All isotopic values are reported in thestandard δ-notation relative to V-PDB. Calcite palaeotem-perature values were calculated using the equation ofAnderson and Arthur (1983): t (°C) = 16 – 4.14(δc – δw) + 0.13(δc – δw)2, assuming a δ18O value of –1‰for non-glacial Upper Cretaceous seawater (e.g. Savin,1977). External precision was checked by multiple analysisof an internal laboratory standard and is better than 0.05(± 1 σ) for δ18O and δ13C.

5. Results

5.1. Facies, petrography and geochemistry of marland limestone beds

Siliceous sponges (Lithistida and Hexactinellida) as wellas belemnites and irregular echinoids are the most commonmacrofossils. The carbonates are calcisphere packstones,calcisphere–foraminifera wackestones, and mudstones withforaminifera and ostracodes (Fig. 3(2)). Low-magnesiumcalcitic coccoliths represent the main carbonate component,SEM investigations reveal that individual coccoliths areembedded in a micritic matrix (Fig. 3(3, 4)). In general, the

marly interbeds show a high primary porosity. Diageneticintergranular microspar occurs, as a subordinate constituent,only in a few limestone beds.

The carbonate content of the marl and limestone bedsvaries from 72 to 90% with rhythmic variations observed ona 0.5–1.5 m scale (Fig. 3(1, 6)). Discrete bedding planes arenot developed. Terrigenous input was constantly low andvariations in the carbonate content of the marl-limestonerhythmites are interpreted to have been initiated by climati-cally induced variations in calcareous plankton productivityof coccolithophorids and calcispheres (Niebuhr, in prep.),which secrete low-magnesium calcite. The carbonates con-tain less than 0.5% Mg (Table 1) and, therefore, lie farbelow the limit of less than 5% magnesium carbonate(Brand and Veizer, 1980). Manganese and iron contentsrange from 120 to 230 ppm and 350 to 1380 ppm, respec-tively. The strontium content varies from 850 to 1900 ppm(Table 1; Fig. 6) and is much higher than expected forlow-magnesium calcite (see Morse and MacKenzie, 1990),arguing against a prominent diagenetic loss of Sr. However,the bulk sediment reveals a brownish- (dull) to orange-coloured cathodoluminescence (Fig. 5(3, 4, 7, 8)) thatclearly points to diagenetic recrystallization and/or cemen-tation of the coccolith muds.

5.2. Microstructure and trace element contentof belemnite rostra

Transmitted-light microscopy reveals that the outermost(and at the rostrum cavum also central) rim of all investi-gated rostra is silicified (Figs. 4(1, 2, 5, 6, 8) and 5(3, 4, 7,8)). The inner belemnite calcite was protected from latercarbonate diagenesis by a surficial early diagenetic silifica-tion appearing as non-luminescent quartzine-lutecite(length-slow chalcedony) spherules (Fig. 4(2)). It is as-sumed that the silica was derived from abundant siliceoussponge spicules. The radial orientation of the relativelycoarse calcitic crystals, typical of belemnite rostra, is clearlyvisible (Fig. 4(8)). The calcite crystals are only looselyconnected and, most probably during sample preparation,occasionally broke off at the crystal faces (Fig. 4(3, 4)). Anoriginal porosity of belemnite rostra is as yet unknown.

CL investigations show that, on average, 95% of indi-vidual belemnite rostra are non-luminescent (Fig. 5(1, 6)).Even the calcite crystal faces are not overgrown by lumi-nescent diagenetic cements. The apical line generally re-flects an orange- to yellowish-coloured luminescence, sur-rounded by one to two weak luminescent concentric rings(Figs. 4(3, 4) and 5(1, 2)). In addition, only few belemniterostra show non-luminescent discontinuous rings in themiddle to outer areas of the rostrum covering generally lessthan half of the girth of the rostrum (Figs. 4(4, 7) and 5 (3,4)). Microscope investigations show the spherulitic natureof the radially oriented coarse calcite crystals which cutthrough the rings more or less perpendicularly (Fig. 4(4, 7)).

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Rings in belemnite rostra are generally considered asprimary growth stages (e.g. Spaeth et al., 1971). Theyellow- to orange-coloured luminescence of the (inner)growth rings is explained by remineralization of intercrys-talline organic matter (laminae obscurae), increase in per-meability, and precipitation of brightly luminescent diage-netic calcite under reducing conditions (Sælen, 1989).

However, and in contrast to other published studies (Sælen,1989; Elorza et al., 1997; Podlaha et al., 1998), most partsof the investigated rostra are not characterized by concentricgrowth rings. This observation suggests that primaryinter- or intracrystalline organic carbon content may havebeen relatively low and homogeneous throughout therostra. The investigated Middle Campanian belemnites are

Fig. 3. Middle Campanian marl-limestone rhythmites of the Lehrte West Syncline east of Hannover. (1) TEUTONIA I quarry showing a 30 m thicksuccession. (2) Characteristic microfacies: calcisphere wackestone with foraminifers and ostracodes (width of photo = 2 mm). (3) SEM photomicrograph ofcalcareous marl: note high porosity and mosaic of coccolith debris and carbonate mud (scale = 5 µm). (4) SEM photomicrograph of coccolith limestone: notehigher abundance and size of diagenetic microspar crystals in comparison to (3) (scale = 5 µm).Les rythmites marnes-calcaires de l’âge Campanien moyen de la partie ouest du synclinal de Lehrte, est de Hannover. (1) La carrière de TEUTONIA I montre30 m d’épaisseur. (2) Les caractéristiques du microfaciès: wackestone à calcisphère avec des foraminifères et des ostracodes (largeur de la photo = 2 mm).(3) Photo au MEB des marnes calcaires: il faut noter la porosité élevée et la mosaïque des débris de coccolithes et les calcaires fins (échelle = 5 µm). (4)Photo au MEB des calcaires à coccolithes: noter l’abondance et la taille des cristaux de microspars diagénétiques en comparaison avec la partie 3(échelle = 5 µm).

Table 1Calcium, magnesium, strontium, manganese, and iron content of time-equivalent bulk sediment and belemnite rostra of the same horizons, / = samples notanalysed.Contenu en calcium, magnésium, strontium, manganèse et fer d’un temps équivalent du sédiment total et des rostres de bélemnite du mêmehorizon, / = échantillons non analysés

bulk sediment belemnite rostraCa Mg Sr Mn Fe Fe Mn Sr Mg Ca(%) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (%)

E30 (sed.) 31.79 4100 967 146 1063 21 <4 1318 2046 37.48 (bel.) E30E32 (sed.) 32.20 3919 932 149 1086 160 <4 1218 / / (bel.) E32E39 (sed.) 32.50 3920 1030 158 1172 400 <4 1206 2062 37.55 (bel.) E39C53 (sed.) 30.45 4342 953 133 1141 91 <4 1423 2289 37.02 (bel.) C53F44 (sed.) 35.15 3317 1226 147 1221 135 <4 1287 2099 38.85 (bel.) F44F31 (sed.) 35.36 3437 856 141 1026 130 <4 1342 1928 36.38 (bel.) F31F20 (sed.) 33.29 3920 1157 124 940 52 <4 1287 2131 37.93 (bel.) F20F12 (sed.) 34.23 3800 1163 131 1147 29 <4 1295 2113 36.63 (bel.) F12F10 (sed.) 34.91 3597 1197 127 1096 68 <4 1195 / / (bel.) F10

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Fig. 4. Cross-sections of rostra of Belemnitella mucronata under transmitted light (TL) and crossed nicols (CN). (1–8) Thin-sections. (1, 2) Bed C-1, ×40(1-TN, 2-CN). Alveolar section of rostrum cavum (right) with internal sediment (left). Note silicified zone (Si) of quartzine-lutecite spherules. (3) BedF10, ×40, TL. Apical line and continuous rings in the central area of the rostrum solidum. Note radial orientation of loosely connected calcite crystals. (4)Bed F50, ×40, TL. Apical section (left) and two discontinuous rings (arrowed) of the early-to-middle growth stage of the rostrum solidum. (5, 6) Bed F94, ×10(5-TL, 6-CN). Two discontinuous rings (arrowed) of middle-to-late growth stages of the rostrum solidum, covering less than half of the girth. Boring (B)occurred prior to silification. (7) Bed F94, close-up of (5) and (6), ×100, CN. The coarse calcite crystals are visible as different shades of grey and pass overthe discontinuous rings without break. (8) Bed C-3, ×10, CN. Most common preservation of the radial region of the rostrum solidum: very small apical line(left), no rings, radial orientation of narrow calcite crystals, which cover nearly the whole radius up to the outermost silification front (right).Coupe transversale des rostres de Belemnitella mucronata sous la lumière transmise (LT) et nicols croisés (NC). (1–8) Lames minces. (1, 2) Banc C-1, ×40(1-LT, 2-NC). Coupe alvéolaire du rostrum cavum (droite) avec du sédiment interne (gauche). Noter la zone silicifiée de quartzine-lutecite sphérules (Si).(3) Banc F10, ×40, LT. Ligne apicale et bordures continues dans la zone centrale de rostrum solidum. Noter l’orientation radiale des cristaux de calcite enconnexion. (4) Banc F50, ×40, LT. Coupe apicale (gauche) et deux bordures discontinues (fléchée) du stade primaire à moyen de croissance de rostrumsolidum. (5, 6) Banc F94, ×10 (5-LT, 6-NC). Deux bordures discontinues (fléchée) du stade moyen à ultérieur de croissance de rostrum solidum, couvrantmoins de la moitié de la circonférence de la cavité mise en place avant la silicification. (7) Banc F94, proche de (5) et (6), ×100. Les gros cristaux de calcitesont visibles sous NC avec une différence de gris et passe à travers les bordures discontinues sans cassure. (8) Banc C-3, ×10, NC. Préservation de la zoneradiale du rostrum solidum avec une petite ligne apical (gauche) pas de bordures, orientation radiale des cristaux de calcite qui couvrent le canal radialjusqu’au front de silicification (droite).

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Fig. 5. Cross-sections of rostra of Belemnitella mucronata and bulk sediment with varying cathodoluminescent characteristics (CL). (1–8) Thin-sections. (1,2) Bed F50, close-up of Fig. 4(4), ×40 (1-TL, 2-CL). Apical section of rostrum solidum with bright yellow-coloured luminescence (y) of diagenetic cementsin the apical line area and in a single weak ring of the early growth stage. Note that fracture (F) within the belemnite calcite is non-luminescent and thereforenot replaced by diagenetic cements. (3, 4) Bed C-1, close-up of Fig. 4(3, 4), ×40 (3-TL, 4-CL). Alveolar section of rostrum cavum with brownish toorange-coloured luminescence (b) of internal sediment and innermost belemnite calcite up to the protecting silicified zone (Si). Most of the belemnite calcite(below) does not show any luminescence; neither the calcite crystals, nor the grain boundaries, nor the discontinuous rings (arrowed). (5, 6) Bed C-3, close-upof Fig. 4(8), ×40 (5-TL, 6-CL). Outermost portion of the apical region of the rostrum solidum shows dull orange CL with bright orange rims (o) up to thesilicified zone (Si). Protected belemnite calcite (left) is non-luminescent. (7, 8) Bed F110, ×100 (7-TL, 8-CL). Alveolar section of rostrum cavum with centralsilicified zone (Si) and internal sediment, partly with brownish to orange-coloured luminescence (b). Note that fracture (F) within the belemnite calcite isnon-luminescent.Coupe transversale des rostres de Belemnitella mucronata et du sédiment total avec des variations des caractéristiques de la cathodoluminescences (CL):(1–8) Lames minces. (1, 2) Banc F50, proche de Fig. 4(4), ×40 (1-LT, 2-CL). Coupe apical du rostrum solidum avec une luminescence claire à jaune (y) duciment diagénétique au niveau du domaine de la ligne apicale et dans le premier cercle du stade primaire de croissance. Noter la fracture (F) à l’ intérieurde la calcite non luminescente et, non remplacée par le ciment diagénétique. (3, 4) Banc C-1, proche de Fig. 4(3, 4), ×40 (3-LT, 4-CL). Coupe alvéolaire durostrum cavum avec une couleur de luminescence brune à orange (b) du sédiment interne et la partie calcitique interne de la bélemnite jusqu’au la zonesilicifiée protégée (Si). La plupart de la calcite de bélemnite ne montre pas de luminescence ainsi que pour les cristaux de calcite, les limites de grains etles bourdures discontinues (fléchée). (5, 6) Banc C-3, proche de Fig. 4(8), ×40 (5-LT, 6-CL). La partie externe de la zone apicale du rostum solidum montreune couleur gris orange CL avec des limites oranges claires (o) jusqu’à la zone silicifiée (Si). La calcite de bélemnite est protégée (gauche) et nonluminescente. (7, 8) Banc F110, ×100, (7-LT, 8-CL). Section alvéolaire du rostum cavum avec la zone centrale silicifiée (Si) et le sédiment interne montrantune luminescence en partie brunâtre à orange (b). Noter la fracture (F) à l’ intérieur de la bélemnite, la calcite est non luminescente.

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Fig. 6. Carbonate content of bulk sediment and strontium content, carbon and oxygen isotopic composition of Middle Campanian bulk sediment andbelemnite rostra. At left scale, every 10th sample (= 5 m) is numbered. δ18O palaeotemperatures calculated after Anderson and Arthur (1983).Le contenu en carbonate du sédiment total et les teneurs en strontium, composition isotopique du carbone et de l’oxygène du sédiment total et des rostresde bélemnite d’âge Campanien moyen. L’échelle de gauche représente les échantillons numérotés tous les 5 m. δ18O paléotempératures calculée d’aprèsAnderson et Arthur (1983).

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considered to be extraordinarily well preserved, which mayin part be explained by the fact that silification shelteredbelemnite calcite from carbonate diagenetic alteration.

The belemnite calcite has concentrations of 0.2% Mg(Table 1) and, therefore, the rostra consist of low-magnesium calcite. In contrast to metastable aragonite andhigh-magnesium calcite, low-magnesium calcite is a stablecalcite modification and has a high potential to preserve itsprimary geochemical signals. The strontium content ofbelemnite calcite ranges from 1160 to 1420 ppm and isalmost equivalent to that measured for the bulk sediment(Table 1; Fig. 6). Iron content is between 20 and 400 ppm,and manganese content of the belemnite rostra is below thelevel of detection (< 4 ppm Mn).

5.3. Carbon and oxygen isotopic composition of belemniterostra and bulk sediment

The isotopic composition of belemnite calcite and bulksediment ranges from –0.6 to +0.4‰ δ18Obel. and –2.6to –1.6‰ δ18Osed. as well as +1.4 to +3.6‰ δ13Cbel.

and +1.9 to +2.5‰ δ13Csed., and shows no prominent varia-tions across the investigated time span of approximately2.7 my (Fig. 6). δ13C and δ18O values of belemnite rostraand bulk sediment do not show any correlation (Fig. 7(1)).Most of the values obtained from belemnite δ18O lie withinthe higher portion of the low-magnesium calcite (LMC)area, those from bulk sediment δ18O within the lowerportion for calcium carbonate precipitated in isotopic equi-librium with ambient seawater (0 to –2‰ δ18O and 0to +4‰ δ13C; see Morrison and Brand, 1986). Furthermore,we observe no correlation between carbonate content of the

sediment and δ13C and δ18O values of either bulk sedimentor belemnite rostra (Fig. 7(2, 3)).

The detailed investigation of an individual belemniterostrum (eight measurements along a transect from the outerrim to the apical line) reveals values ranging from +1.4to +2.8‰ δ13C and –0.6 to 0 ‰ δ18O (Fig. 8). Belemniterostra sampled from individual horizons show approxi-mately the same variation in δ13C and δ18O (Fig. 9).

6. Discussion

Petrographic studies suggest that low-magnesium calciticcoccoliths and calcispheres represent the main carbonatecomponent of the investigated Middle Campanian marl-limestone rhythmites. The sediments are interpreted to havebeen deposited under open marine conditions below stormwave base. SEM investigations reveal that the marl andlimestone beds are only weakly cemented. The subordinatecontribution of diagenetic microspar, high primary porosity,and high strontium content seems to suggest only minordiagenetic alteration of the coccolith muds. Accordingly, theδ18O values might be used to calculate sea-surface tempera-tures for Middle Campanian ocean water. However, thebrownish- to orange-coloured cathodoluminescence pointsto recrystallization and/or cementation of the carbonates.Mn2+ is considered as the major activator of orange-coloured luminescence (Machel et al., 1991), whereasincrease in the Fe2+ content of diagenetic calcite will resultin a dark red- to brownish-coloured luminescence (dull).Manganese content of the luminescent marl-limestonerhythmites is significantly higher than manganese content of

Fig. 7. Comparison of stable isotopic composition. (1) δ13C, δ18O of bulk sediment and belemnite rostra. The LMC (low-magnesium calcite) area definesthe limit for calcium carbonate precipitated in isotopic equilibrium with ambient seawater under most near-surface conditions (after Morisson and Brand,1986). (2) Bulk sediment and belemnite rostra δ13C in comparision to carbonate content of the bulk sediment. (3) Bulk sediment and belemnite rostra δ18Oin comparision to carbonate content of the bulk sediment.Comparaison entre la composition des isotopes stables. (1) δ13C, δ18O du sédiment total et des rostres de bélemnite. La zone ombrée LMC (calcite faiblementmagnésienne) définie le limite des carbonates de calcium précipités dans en équilibre isotopique avec une eau de mer ambiante sous des conditions de surface(après Morisson et Brand, 1986). (2) δ13C du sédiment total et des rostres de bélemnite par rapport au contenu en carbonate du sédiment total. (3) δ18O dusédiment total et des rostres de bélemnite par rapport au contenu en carbonate du sédiment total.

B. Niebuhr, M.M. Joachimski / Geobios 35 (2002) 51–64 59

Fig. 8. Variation in δ13C and δ18O along a transect within a single belemnite rostrum from bed C60.Variation des valeurs de δ13C et δ18O le long d’un transect à l’ intérieur du rostre de bélemnite du banc C60.

60 B. Niebuhr, M.M. Joachimski / Geobios 35 (2002) 51–64

Fig. 9. Inter-specimen variation in δ13C and δ18O of belemnite rostra from the same horizons (a, bed F20; b, bed F94; c, bed C-3; d, bed C-1).Inter-spécimen variation des valeurs de δ13C et δ18O des rostres de bélemnite à partir des même horizons (a, banc F20 ; b, banc F94 ; c, banc C-3 ; d, bancC-1).

B. Niebuhr, M.M. Joachimski / Geobios 35 (2002) 51–64 61

the non-luminescent belemnite rostra and may explain theorange-coloured luminescence of the carbonates. SinceMn2+ is only available for substituting Ca2+ in the calcitelattice under reducing conditions, a diagenetic recrystalliza-tion and/or cementation of the carbonates has to be as-sumed. This assumption is supported by the fact that δ18Ovalues of non-luminescent belemnite rostra are enriched in18O by 1.5 to 2‰ in comparison to δ18O values of thecarbonates. Precipitation of diagenetic calcite or microsparduring early burial is expected to result in a decrease inδ18O as a consequence of increasing burial temperatures.Therefore, the interpretation of these data might result inerroneous palaeotemperatures.

The δ13C signals of the marl-limestone rhythmites (+1.9to +2.5‰) compare well with the δ13C data of the belemnites rostra (+1.4 to +3.6‰). This observation is explainedby the relatively high preservation potential of the carbonisotopic composition of carbonates in case the diageneticstabilisation or cementation proceeds in a diageneticallyclosed system. However, δ13C values of belemnite calciteshow a larger spread in comparison to the δ13C values ofbulk sediment. This larger variation in δ13C could beattributed to a vital fractionation effect of the living belem-nites. Wefer (1985) reported non-equilibrium fractionationsfor modern cephalopods Nautilus and Sepia with the shellcalcite being depleted in 13C in comparison to expectedequilibrium values. In contrast, Morrison and Brand (1986)reported that modern cephalopods do not exhibit a vitalfractionation effect. Studies on modern organisms with vitalfractionation effects frequently reveal a positive correlationof δ13C and δ18O (Wefer, 1985; Carpenter and Lohmann,1995) which is not observed in the studied Middle Campa-nian belemnites. In conclusion, since belemnites have nomodern representatives, it is difficult to decide whether theobserved variation in δ13C is the consequence of a non-equilibrium fractionation effect or not.

In contrast to carbon isotopes, the δ18O values of thebelemnite rostra reflect minor variability and are enriched in18O in comparison to the δ18O values of the carbonates. Inaddition, the detailed investigation of a single belemniterostrum reveals that the intra-specimen variation in δ18O(–0.6 to 0‰) is comparable to the variation in δ18O of theentire investigated belemnite population (–0.6 to +0.4‰).These observations support the conclusions that the δ18Osignals of the carbonates have been diagenetically over-printed and that the belemnite rostra probably preservedtheir primary δ18O signals.

In the Middle Campanian time interval (78–75.5 my;duration see Ehrmann, 1986), palaeotemperatures calcu-lated from the belemnite δ18O values reflect variationswithin a 12.5 ± 2 °C range (Figs 6 and 7(1)) and are surpris-ingly low taking into account that water depths of the UpperCretaceous shelf sea did probably not surpass 100 m.Oxygen isotopic data of Inoceramus prisms from the UpperCampanian chalk of Kronsmoor located 150 km north of the

Lehrte West Syncline (see Fig. 1) were reported by Schön-feld et al. (1990) and Schönfeld and Burnett (1991).Calculated average Inoceramus δ18O palaeotemperaturesfor the Upper Campanian (75–72 my) increase from 14 to17 °C. In contrast, belemnite surface water palaeotempera-tures from the uppermost Campanian of Kronsmoor indicatea mean value of 12.3 °C (Niebuhr, in prep.). The fact thatindividual Inoceramus prisms show δ18O values rangingfrom –0.2 to –1.6‰ and that the prisms are overgrown bysmall microspar crystals (Schönfeld et al., 1990) may pointto a diagenetic overprint of the Inoceramus δ18O signals.Interestingly, Elorza et al. (1997) reported a comparablepattern for Middle Campanian to Early Maastrichtian ino-ceramid shells and belemnite rostra from NE Belgium (seeFig. 1) with lower δ18O values measured for benthic ino-ceramids (–0.5 to –3.8‰) and higher δ18O values for nekticbelemnites (–0.9 to +0.4‰). The lower δ18O values wereexplained by an increasing diagenetic overprint of theluminescent inoceramid shells and prisms. Similar to thisstudy, belemnite palaeotemperatures from the Middle toUpper Campanian of NE Belgium indicate a mean value of12.5 °C (Elorza et al., 1997). δ18O data measured onthe < 63 µm fraction of Late Campanian coccolith mudsfrom a latitude of 45 to 47°S translate into sea-surfacetemperatures of 12.5 °C (Clarke and Jenkyns, 1999). Al-though these data have to be evaluated carefully due to apotential diagenetic overprint, their reported palaeotempera-tures compare well to temperatures reported in this studyand may support the conclusion that sea-surface waters inmid-latitudes during Campanian times were relatively cool.

The fact that neither δ18O nor δ13C values of thebelemnite rostra show a strong correlation with the carbon-ate content of the bulk sediment indicates that the inferredvariations in nannoplankton productivity were probablyneither induced by changes of sea-surface water tempera-ture nor by variations in nutrient availability.

7. Conclusions

Although measured strontium content and nannofaciesanalysis suggest that the carbonates may have retained theirprimary geochemical signatures, the orange-colouredcathodoluminescence indicates diagenetic alteration. Incontrast, the non-luminescent belemnite rostra preservedtheir primary microstructures and geochemical signaturesand can be used for reconstruction of δ18O palaeotempera-tures of Middle Campanian sea-surface water. In northGermany and NE Belgium (Elorza et al., 1997), situated atmid-latitudes and representing a water depth < 100 m, be-lemnite δ18O values indicate relatively cool sea-surfacepalaeotemperatures of approximately 12.5 °C in the Middleand Upper Campanian (78–72 my). In mid-latitudes ofthe southern hemisphere (43–47°S), palaeotemperaturesgenerally warmed into the earliest Campanian, before cool-ing for several degrees into the late Campanian

62 B. Niebuhr, M.M. Joachimski / Geobios 35 (2002) 51–64

(76–71.5 my), where a temperature of 12.5 °C is character-istic (Clarke and Jenkyns, 1999). Interestingly, the ‘UpperCretaceous cooling trend’ seems to have started earlier thanpreviously assumed.

Acknowledgements

We thank Markus Wilmsen, Würzburg for many fruitfuldiscussions. Max Bülow, Cairon, and Abderrazak El Albani,Poitiers are thanked for the translation of the Frenchabstract. August Ilg, Franz Fürsich, and Wolfgang Os-chmann are acknowledged for their critical reviews andadvice. The study of B.N. was financially supported by agrant of the Deutsche Forschungsgemeinschaft (Ni 546/1).

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