Geochemical signatures of the early Campanian ...

Palaeogeography, Palaeoclimatology, Palaeoecology 433 (2015) 191–200

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Geochemical signatures of the early Campanian belemnite Belemnellocamax mammillatus from the Kristianstad Basin in Scania, Sweden Anne Mehlin Sørensen a,⁎, Clemens V. Ullmann a,b, Nicolas Thibault a, Christoph Korte a a b

Department of Geosciences and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, Dk-1350 Copenhagen K, Denmark University of Exeter, Camborne School of Mines and Environment and Sustainability Institute, Penryn Campus, Treliever Road, Penryn, Cornwall TR10 9FE, UK

a r t i c l e

i n f o

Article history: Received 8 September 2014 Received in revised form 12 May 2015 Accepted 26 May 2015 Available online 3 June 2015 Keywords: Belemnites Late Cretaceous Palaeoenvironment δ13C δ18O Element ratios

a b s t r a c t Macrofossil calcite plays a central role for palaeoenvironmental reconstructions because its preservation state and geochemical signatures can be checked using a variety of screening techniques. Reconstructions of the ecology and seawater composition from such data require knowledge about species-specific compositions of well-preserved calcitic fossils. Comprehensive datasets, recording averages, intraspecimen and intraspecific variability of geochemical proxies in fossil calcite, however, are still largely lacking. Here we report δ13C and δ18O values, as well as Mg/Ca, Sr/Ca and Mn/Ca ratios of 204 aliquots sampled along transects through three rostra of the late early Campanian belemnite species Belemnellocamax mammillatus. Geochemical signatures of partially altered samples are consistent to those reported for early marine calcite cementation and show trends towards moderately higher Mn/Ca, heavier δ13C and lower Sr/Ca. In contrast, Mg/Ca ratios and δ18O values remain similar to projected original values. Median δ18O values of −0.3 to 0.0‰ are comparable to the heaviest values of coeval oysters from the same localities, and internal variability of the well-preserved parts of the rostra is low. These findings suggest that B. mammillatus lived mostly in the deep part of the basin where seasonal changes were low and water temperatures cool. The δ13C is offset from oysters and most coeval belemnite species from the same region, and from bulk rock values of other European localities by several permil towards lighter values. This offset suggests that a strong vital effect impacted the carbon isotopes of the investigated species during calcification. Average Mg/Ca ratios of 11.5 mmol/mol and Sr/Ca ratios of 2.13 mmol/mol (n = 155) are found for the well-preserved parts of the belemnite rostra. The intraspecimen variability of Mg/Ca and Sr/Ca ratios between 7 and 20% (2 relative standard deviations) is similar amongst the specimens, but intraspecific variability of Mg/Ca is significantly higher than for Sr/Ca. Average Sr/Ca ratios of 2.1 ± 0.2 mmol/mol and Mg/Ca of 11 ± 3 mmol/mol are proposed for the late early Campanian B. mammillatus of the Kristianstad Basin. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The calcite rostra of belemnites sensu stricto (Fuchs, 2012) are common fossils in Jurassic and Cretaceous sedimentary strata and are globally found from the late Early Jurassic until the Late Cretaceous (Doyle and Bennett, 1995). Their presumably stream-lined morphology in combination with numerous arm hooks and sharp beaks suggests that they were fast swimmers and effective predators (e.g., Price and Sellwood, 1997; Klug et al., 2010; Mutterlose et al., 2010). Despite ongoing efforts in constraining belemnites habitats (e.g. Pirrie et al., 2004; Wierzbowski and Joachimski, 2007; Malkoc and Mutterlose, 2010; Ullmann et al., 2014) the ecology of many belemnite species remains ⁎ Corresponding author. Tel.: +45 35322448. E-mail addresses: [email protected] (A.M. Sørensen), [email protected] (C.V. Ullmann), [email protected] (N. Thibault), [email protected] (C. Korte).

http://dx.doi.org/10.1016/j.palaeo.2015.05.025 0031-0182/© 2015 Elsevier B.V. All rights reserved.

poorly constrained. It is still debated, whether belemnites were active swimmers occupying a wide range of water depths like modern nautiloids, or nektobenthic, similar to the modern Sepia (Rexfort and Mutterlose, 2009). Seasonal and daily migratory behaviour as well as preferred water depths are largely unknown and may have varied among different taxa. Seasonal migrations between different habitats and daily migrations between deeper and shallow waters are described from modern cephalopods such as squids and nautiloids and are thus common for this molluscan class (Carlson et al., 1984; Sauer et al., 1992; Akyol and Sen, 2004; Kiyofuji and Saitoh, 2004; Watanabe et al., 2004; Lindberg and Pyenson, 2007; Dunstan et al., 2011). The endoskeletal parts of the belemnites have been widely employed for geochemical investigations and were taken as the first isotopic reference material for carbon and oxygen isotope ratios in carbonates (Epstein et al., 1951). Their comparatively large size and their visible growth rings make them promising targets for studies of

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A.M. Sørensen et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 433 (2015) 191–200

palaeoseasonality and first attempts to resolve annual temperature variability date back to the 1950s (Urey et al., 1951). Because of their ill-constrained habitats, the palaeoenvironmental significance of geochemical fluctuations in the rostra has been debated (e.g. Stevens and Clayton, 1971; van de Schootbrugge et al., 2005; Zakharov et al., 2011). While many macrofossil datasets are available for the Jurassic (e.g., Dera et al., 2011; Korte and Hesselbo, 2011) and for the early Cretaceous (e.g., McArthur et al., 2004; Price et al., 2011), the Late Cretaceous is still poorly covered (but see Voigt and Wiese, 2000; Voigt et al., 2003; Vonhof et al., 2011). Only relatively few isotopic datasets on calcitic shells for the Campanian time interval exist (Ando et al., 2013) and large datasets combining oxygen and carbon isotope ratios with element concentrations of monospecific, coeval, belemnites, are entirely lacking. So far only one study provides isotopic data on Campanian belemnites (Niebuhr and Joachimski, 2002). Comprehensive multi-proxy datasets are necessary for the interpretation of ontogenic geochemical trends in belemnites and their palaeoenvironmental significance. The aim of the present study is to document high resolution oxygen and carbon isotopic signatures as well as Mg/Ca, Sr/Ca and Mn/Ca ratios from three specimens of the Campanian B. mammillatus from Scania, Southern Sweden. The geochemical data are used to assess the preservation state of the fossils. The data from well-preserved parts of the rostra are then taken to estimate the intraspecimen- and intraspecific variability of geochemical proxies in B. mammillatus and to investigate the ecology and life cycle of this species. Oxygen isotope ratios of B. mammillatus are compared to those of coeval oysters, belonging to the species Acutostrea incurvas, representing the local benthic signal.

A Greenland

Baltic Shield Norway Sweden B Kristianstad Basin

emergent land 500 km

B onshore Late Cretaceous of the Kristianstad Basin Ivö Klack

Åsen

Helsingborg Kristianstad Maltesholm Copenhagen

2. Geological setting

Malmö During Late Cretaceous times the Earth experienced greenhouse conditions and the global sea level was up to 100 m higher than today (Sahagian et al., 1996; Kominz et al., 2008). Large areas of southern Sweden were transgressed during a number of intercontinental transgressive events and the small Kristianstad Basin was developed along the northern margin of the extensive epeiric Chalk Sea which covered most of NW Europe during this time interval (Fig. 1). The exposed marine Cretaceous succession in the basin is less than 200 m thick and consists mainly of skeletal carbonate gravel, sand and silt of late early Campanian to early late Campanian age (Christensen, 1975; Surlyk and Sørensen, 2010). Southern Sweden has been tectonically quiescent since the Campanian without significant uplift or subsidence (Norling and Bergström, 1987). Palaeogeographic reconstructions show that the Kristianstad Basin was open towards the SE and probably without marine connection to the west (Surlyk and Sørensen, 2010). The palaeolatitude was about 50°N (Smith et al., 1994) and the climate was warm temperate to subtropical (Surlyk and Christensen, 1974; Surlyk and Sørensen, 2010). 2.1. Localities The studied belemnite rostra were collected at three localities in the Kristianstad Basin, representing three different palaeoenvironments. All three rostra stem from the species Belemnellocamax mammillatus, which in the Kristianstad Basin defines the latest early Campanian B. mammillatus biozone (~82 Myr) (Christensen, 1997). 2.1.1. Ivö Klack The locality is an abandoned kaolin and limestone quarry located in the northern part of the Island of Ivö (Fig. 1). The uppermost lower Campanian succession comprises 22–24 m of coarse-grained skeletal carbonates composed mainly of fragments of bivalves, bryozoans, brachiopods, echinoids, and dominated by oysters. The carbonates were deposited along a steep, irregular rocky shore with abundant

Trelleborg

Ystad

25 km

Fig. 1. A: Palaeogeographical map of northern Europe during the Late Cretaceous. The location of the Campanian Kristianstad Basin is indicated by an asterix, based on Ziegler (1990) and Surlyk and Sørensen (2010). B: Onshore extent of Late Cretaceous sediments from the Kristianstad Basin, based on Norling and Bergström (1987).

encrusted boulders and hummocks, and with oysters forming banks along the shoreline (Surlyk and Sørensen, 2010; Sørensen et al., 2012).

2.1.2. Maltesholm The locality is an abandoned limestone quarry adjacent to the northern margin of the Linderödsåsen horst (Fig. 1; Christensen, 1975; Sandström, 2001). The uppermost lower Campanian succession consists of skeletal sandstones with abundant fragments of rudists, calcareous algae and belemnites, and few other fossil remains. The locality represents the deeper, more open coastal waters of the overall shallow basin.

2.1.3. Åsen The locality is an abandoned clay pit, presently used as a refuse dump, situated at the northern margin of the Kristianstad Basin (Fig. 1). The uppermost lower Campanian marine succession consists of unconsolidated quartz-sand. The sediment was deposited in a sheltered coastal environment, presumably with murky waters (Sørensen et al., 2013). A fluvial system was probably located nearby indicated by the topography of the basement rock (i.e., the Höljeån valley) and the presence of early Campanian fluvial sediments underlying the marine sediments, (Friis and Skarby, 1981; Rees, 1999).


2.2. Geographical distribution of Belemnellocamax mammillatus B. mammillatus is abundant in the Kristianstad Basin, where it is used to biostratigraphically define the latest early Campanian, and covers about 90–95% of the belemnite fauna (Christensen, 1975, 1997). All growth stages occur, suggesting that the species lived and bred in the basin (Christensen, 1997). The species is very rare outside the Kristianstad Basin with a little more than 100 specimens recorded from the eastern part of the Russian Platform, about a dozen specimens recorded from northern Germany and Poland, two specimens from the northeastern part of the Ukrainian syneclise, and a single specimen recorded in a drill core near Poznan, Poland (Christensen, 1975, 1986, 1997; Olszewska, 1990). Christensen (1997) suggests that those specimens may be regarded as stray individuals outside their normal habitat, since all fossils found outside the basin were classified as adults. This interpretation indicates migrations of B. mammillatus to the south and east.

193

Element ratios were measured using an Optima 7000 DV ICP-OES (Perkin Elmer). Sample solutions remaining from stable isotope measurements were diluted to 25 μg/g Ca and measured against matrix matched, synthetic multi element calibration solutions, using a three point calibration. Reproducibility and accuracy of Mg/Ca, Sr/Ca and Mn/Ca ratios were checked with the reference materials JDo-1 and JLs-1. Measured average element ratios deviate less than 2% from ratios computed from Imai et al. (1996) apart from Mg/Ca in JLs-1 (8%). Reproducibility (2 rsd) is better than 3% for all ratios apart from Mn/Ca in JLs-1 (8% due to low Mn/Ca ratio of ~ 30 μmol/mol in this reference material). Common detection limits (3 sd of baseline) correspond to 0.1 mmol/mol for Mg/Ca; 4 μmol/mol for Sr/Ca and 2 μmol/mol for Mn/Ca. The shell ultrastructure was investigated using a FEI Quanta 250 Scanning electron microscope at the Geological Museum Copenhagen. 4. Results

3. Material and methods

4.1. Belemnites

Three belemnite rostra of the species B. mammillatus, AS-AA-3 from Åsen, AS-I-3 from Ivö Klack, and AS-M-2 from Maltesholm, were cut transversely about 1 cm away from the pseudoalveolus towards the posterior end (Fig. 2). This position was chose to access the most complete sequence of growth bands and to avoid potential growth raterelated effects on element incorporation (Ullmann et al., 2015). The cut surface was polished and continuous samples were taken using a hand-held drill from the outer surface to the center of the rostra (Fig. 2). The resulting spatial resolution is ~120 μm. To avoid sampling of diagenetically altered material as best as possible, profiles through the rostra were chosen in areas without cracks and least decolouration. Since the object of the study was the derivation of data for the complete life span of the belemnite, rim and apical line were not avoided for sampling despite their commonly less well-preserved geochemical signatures (Podlaha et al., 1998; McArthur et al., 2007; Ullmann et al., 2013a, 2015; Ullmann and Korte, 2015). Additionally, a total of 114 samples from 4 oyster specimens of the species Acurostrea incurva, two from Åsen and two from Ivö Klack, were measured to allow comparison with the local benthic signal to geochemical signatures of the belemnites. Samples were taken by scratching calcite off the shell using a stainless steel scalpel (Ullmann et al., 2010, 2013b). Each sample was analyzed for carbon and oxygen isotope ratios and element ratios (Mg/Ca, Sr/Ca and Mn/Ca) at the University of Copenhagen, using protocols of Ullmann et al. (2013a, b). Stable isotope ratios were determined on sample amounts of ~ 0.6 mg using an IsoPrime gas source IRMS, using the continuous flow technique described in Spötl and Vennemann (2003). Reproducibility of δ18O and δ13C values are 0.18‰ and 0.07‰, respectively (2 sd; n = 96).

Most of the calcite of the three investigated rostra is massive. A few millimeter wide zone around the apical line, however, is partly highly porous with a partially calcite-cemented series of rings connected by calcite laths (Fig. 3). This structural arrangement is similar to the wall and pillar arrangement of Sepia officinalis (Rexfort and Mutterlose, 2006). Geochemical signatures of all three specimens of B. mammillatus follow similar trends and show comparable ranges (Supplementary Table 1, Fig. 4). Carbon isotope ratios in the three specimens range from − 2.4 to + 1.7‰ and generally show the highest values close to the apical line and the rim, while the intermediate area is relatively depleted in 13C (Fig. 4). Compared to the carbon isotope profiles, the oxygen isotope ratios are much less variable, ranging from −0.8 to + 0.4‰. Significant drifts in oxygen isotope ratios throughout the profiles occur in all three specimens, but their magnitude is limited to less than 0.5‰ (Fig. 5). Geochemical proxies show dissimilar behaviour between samples with elevated Mn/Ca ratios and samples with low Mn/Ca ratios (Fig. 6). Changing patterns of correlations between Mn/Ca with other geochemical proxies around a limit of 0.02 mmol/mol for Mn/Ca are particularly clear for specimen AS-AA-3 (Fig. 6). Sr/Ca profiles of all three specimens show their minima at the apical line, corresponding to enrichments in manganese of up to 0.39 mmol/mol (Fig. 6). Similar depletions of Sr are observed at the rims of specimens AS-I-3 and ASM-2, where Mn/Ca ratios reach values comparable to those at the apical line. Sr/Ca ratios are higher and relatively stable in parts of the shell, where Mn/Ca ratios are lower than 0.02 mmol/mol. In the zone of low Mn/Ca ratios, relative variability of Sr/Ca ratios is b20% (2sd) in all

Fig. 2. Sections through rostra of Belemnellocamax mammillatus sampled for this study: A: Sample AS-AA-3 from Åsen. B: Sample AS-I-3 from Ivö Klack. C: Sample AS-M-2 from Maltesholm. Black lines indicate the traces of the sample transects. Samples are oriented with the ventral side to the right.

194


Fig. 3. SEM images of rostrum AS-AA-3 close to the apical line. Scale bars are 50 μm the section is perpendicular to the apical line. A: Incompletely mineralized growth bands connected by pillars of calcite. Hollow parts are partly cemented by inorganic calcite. B: close-up of A showing the architecture of the central part of the belemnite rostrum, resembling the wall and pillar structure observed in the cuttlefish Sepia officinalis.

specimens and median ratios are high with values of 2.07, 2.16 and 2.00 mmol/mol, respectively for specimens AS-AA-3, AS-I-3 and AS-M-2. Mg/Ca ratios show similar trends as Sr/Ca ratios, but do not respond uniformly to enrichments in manganese at the rims and apical line (Figs. 4, 6). The internal variability of Mg/Ca ratios in the zones with Mn/Ca ratios b0.02 mmol/mol is similar to that of the Sr/Ca ratios. Median Mg/Ca ratios of samples with Mn/Ca ratios b0.02 mmol/mol, however, vary more between the three specimens, with values of 10.9, 12.4 and 9.8 mmol/mol, respectively for AS-AA-3, AS-I-3 and AS-M-2 (Fig. 7). Systematic fluctuations of Mg/Ca ratios over length scales of 1–2 mm are observed for specimens AS-AA-3 and AS-I-3, but not for specimen AS-M-2, which shows the overall lowest Mg/Ca ratios (Fig. 4).

show comparable ranges of Sr/Ca (0.48 to 1.19 mmol/mol) and Mg/Ca (1.6 to 9.5 mmol/mol), but Mn/Ca ratios in Ivö Klack are distinctly lower (0.01 to 0.14 mmol/mol) than in Åsen (0.10 to 0.40 mmol/mol). δ13C values in oysters from Ivö Klack range from + 0.6 to + 2.9‰ and approximate + 1.5 to + 2.0‰, where Mn/Ca ratios are highest. δ13C values in oysters from Åsen show higher values of + 1.9 to + 3.7‰ with values of ~ +3‰, where Mn/Ca ratios are highest. In both localities, the lightest oxygen isotope values are associated with comparatively low Mn/Ca ratios and high Sr/Ca ratios, whereas samples with δ18O values N −1.0‰ preferentially show Mn enrichment and Sr depletion (Fig. 8).

4.2. Oyster data

5.1. Belemnite preservation

δ18O values in oysters from Ivö Klack and Åsen range from −2.2 to − 0.2‰ (Supplementary Table 1; Fig. 8). Samples from both localities

The Mn/Ca ratio is used to assess the preservation quality of fossil calcite, because primary manganese concentrations in biogenic calcite

5. Discussion

AS-AA-3

AS-I-3

AS-M-3

δ 13C, δ 18O ‰ PDB

2 1 0 -1 -2

Mn/CaSr/Ca Mg/Ca mmol/mol

-3

0.40 0.35

3.0 2.5

0.20 0.15

2.0 1.5 1.0

0.00

13 12 11 10

0.10 0.05

15 14

0.30 0.25

16

0.5 0.0

9 8

apical line

rim

apical line

rim

apical line

rim

Fig. 4. Geochemical results from profiles through rostra of Belemnellocamax mammillatus. High δ13C values are observed at the apical line and the rims, coinciding with low Sr/Ca ratios and high Mn/Ca ratios. δ18O values show little variation.


AS-AA-3

0.2

specimens at Mn/Ca ratios N0.02 mmol/mol, corresponding to the expected diagenetic depletion of Sr (Fig. 6). Because covariation of geochemical tracers with Mn/Ca ratios changes significantly above 0.02 mmol/mol we use only samples with Mn/Ca ratios below this limit for palaeoenvironmental interpretation. This Mn/Ca limit is considerably lower than values normally employed for identifying alteration (0.09 mmol/mol: Malkoc and Mutterlose, 2010 or 0.18 mmol/mol: Price and Rogov, 2009; Nunn and Price, 2010; Ullmann et al., 2013a). It corresponds to critical Sr/Mn ratios of alteration around 100 mol/mol, which is higher than the value of 50 mol/mol employed by Rosales et al. (2004). Overall very low Mn concentrations in diagenetic carbonates are indicative of calcite precipitation from oxic waters that show low Mn concentrations (Algeo and Maynard, 2004) and/or diagenetic fluids lacking an abundant manganese source. Samples showing the strongest Mn enrichments have Sr/Ca ratios ~ 0.6 mmol/mol (Figs. 4, 6). Such values are expected for inorganic precipitation from a liquid with a sea water-like Sr/Ca ratio of ~8.5 mmol/mol at a pH of 8.3 and precipitation rates below 10−7 mol/m2/s (Tang et al., 2008; DePaolo, 2011). The Sr/Ca ratio of ~ 8.5 mmol/mol for modern seawater (de Villiers, 1999) is likely comparable to seawater Sr/Ca during the Campanian (Lear et al., 2003), making a seawater-like liquid a potential source for the diagenetic carbonate. The δ13C values in the rostra rise to a maximum of +1.7‰ with increasing Mn/Ca, approaching values which are similar to latest early Campanian European chalk bulk carbonate values of ~ 2–2.5‰ (Jarvis et al., 2006; Voigt et al., 2010). No strong effect on δ18O values can be observed in the significantly altered material indicating that the diagenetic calcite was precipitated from water, whose δ18O value and temperature were similar to the sea water in which the belemnites lived (Figs. 4, 6). All geochemical trends therefore indicate early marine cementation as the process of post-depositional alteration. Recrystallization or further cementation of the porous apical zone of the specimens was prevented by tectonic quiescence and the minor influence of groundwater flow in the sedimentary strata.

0.0

5.2. Belemnite ecology

0.4

δ 18O ‰ PDB

0.2 0.0 -0.2 -0.4 -0.6 -0.8 0

10

20

30

40

50

60

70

80

rim

sample number

apical line

AS-I-3 0.4

δ 18O ‰ PDB

0.2 0.0 -0.2 -0.4 -0.6 -0.8 0

10

20

apical line

30

40

50

60

70

rim

sample number AS-M-2

0.4

δ 18O ‰ PDB

195

-0.2 -0.4 -0.6 -0.8 0

apical line

10

20

30

sample number

40

rim

Fig. 5. Oxygen isotope profiles for the three specimens. Blue band shows LOESS fit with 95% confidence interval; hatched area denotes zone with Mn/Ca ratios N20 μmol/mol. Significant trends in oxygen isotope ratios throughout the profiles are observed for all specimens.

are typically low and manganese is incorporated into calcite during post-depositional processes (e.g. Brand and Veizer, 1980; Ullmann and Korte, 2015). The spatial association of manganese enrichments in the rostra with porous, partially cemented calcite close to the apical line is indicative of a diagenetic origin (Fig. 6). Diagenetic enrichments of manganese are focused to the apical line and rim of the specimens, but in all three specimens minor increases of Mn/Ca in the intermediate part of the rostrum are observed (Fig. 4). Such diagenetic patterns are normal for belemnites and controlled by the permeability of the rostra (e.g. Podlaha et al., 1998; McArthur et al., 2007; Ullmann et al., 2013a, 2015). Another geochemical indication for diagenesis can be taken from Sr concentrations, which are reduced during alteration (e.g. Ullmann and Korte, 2015), leading some authors to use the Sr/Mn ratio as an indicator for the preservation state of the samples (e.g. Rosales et al., 2004). A negative correlation of Mn/Ca ratios with Sr/Ca ratios is observed in all

5.2.1. Oxygen isotopes The use of oxygen isotopes from skeletal remains as proxies for palaeo-temperatures is based on the assumption that the skeletal calcite is precipitated close to oxygen isotope equilibrium with the surrounding water. This has been asserted for many living cephalopods, bivalves and brachiopods (Taylor and Ward, 1983; Carpenter and Lohmann, 1995; Brand et al., 2003; Rexfort and Mutterlose, 2006; Lukeneder et al., 2008; Ullmann et al., 2010, 2013b) and most authors assume that this was also the case for belemnites (e.g., Tan et al., 1970; Price and Sellwood, 1997; Wierzbowski and Joachimski, 2007, 2009). The small range of δ18O values in all studied specimens suggests that ambient water temperatures remained stable within a few degrees indicating limited seasonality in their habitat in the Kristianstad Basin or migration to waters of similar temperature (Stevens et al., 2015). The average δ18O value of − 0.17‰ (n = 149) for well-preserved samples of the late early Campanian B. mammillatus is almost identical to that found for mid Campanian (78–75.5my) Belemnitella mucronata from northern Germany (−0.12‰, n = 54; Niebuhr and Joachimski, 2002). Niebuhr and Joachimski (2002), using the Anderson and Arthur (1983) calibration and an estimate of −1‰ SMOW for ambient water, estimate that their average δ18O corresponds to seawater temperatures of 12.5 °C. This temperature is cooler than what would be expected for near-shore environments of the Kristianstad Basin where the presence of hermatypic corals indicates warmer waters (Sørensen et al., 2011). A few species of hermatypic corals can live in temperatures as low as 15 °C, but most require warm water due to their symbiosis with algae (Wells and Hill, 1956; Kaiser et al., 2005). Today most hermatypic corals are found in waters with temperatures above 18 °C and with maximum development between 26 °C and 28 °C (Jokiel and Coles, 1977). Rudistid


AS-AA-3

AS-I-3

103

Mn/Ca (µmol/mol)

Mn/Ca (µmol/mol)

103

102

101

100

102

101

100 -3

-2

-1

0

1

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-2

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-3

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Mn/Ca (µmol/mol) 1.0

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Mg/Ca (mmol/mol)

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3

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-2

δ13C ‰ PDB

Mn/Ca (µmol/mol)

Mn/Ca (µmol/mol)

Mn/Ca (µmol/mol)

0

103

10

101

δ13C ‰ PDB

103

8

102

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δ13C ‰ PDB

Mn/Ca (µmol/mol)

AS-M-2 103

Mn/Ca (µmol/mol)

196

102

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100 0.0

0.5

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1.5

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Sr/Ca (mmol/mol)

Fig. 6. Cross plots of δ13C values, Mg/Ca and Sr/Ca ratios against Mn/Ca ratios for the three specimens. Significant changes of co-variation in Mn/Ca ratios with other element ratios and carbon isotopes are observed at 20 μmol/mol. These changes are especially clear for specimen AS-AA-3.

0. 5

0. 4

16 14

0. 0

0. 2

2. 5

-0 .2

-0 .4 -2 .0

-2 .5

-3 .0

Mg/Ca mmol/mol

-1 .5

δ 18O ‰ PDB

δ 13C ‰ PDB

0. 0

10 8 6

Sr/Ca mmol/mol

12

-0 .5

-1 .0

3. 0

2. 0

1. 5

1. 0

4 -0 .6

0. 5 2

-0 .8

0

0. 0

Fig. 7. Box plots of carbon and oxygen isotope ratios as well as Mg/Ca and Sr/Ca ratios from the profile parts showing good preservation (Mn/Ca b20 μmol/mol). Left: AS-AA-3; Middle: ASI-3, Right: AS-M-2. Boxes show the second and third quartile with the median value and whiskers denote the whole range of values measured. Values are generally similar, with much larger variability for carbon isotope ratios than for oxygen isotope ratios and better agreement of Sr/Ca ratios than Mg/Ca ratios.


Ivö

160 140

Mn/Ca µmol/mol

120 100

Sr/Ca mmol/mol 1.19

80 60

0.84 40

0.48

20 well-preserved belemnite calcite

0 -2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

18

O ‰ V-PDB

Åsen

400 350

Mn/Ca µmol/mol

300 250

Sr/Ca mmol/mol 1.19

200 150

0.84 100

0.48

50 well-preserved belemnite calcite

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from the higher average Mn/Ca ratios in oysters from Åsen as compared to those from Ivö Klack (Fig. 8), indicating that the Åsen specimens were situated closer to the palaeo-shore (Almeida et al., 1998; Ullmann et al., 2013b). Oyster samples from Åsen - yielding relatively low Mn/Ca ratios and high Sr/Ca ratios - also show lower average δ13C and δ18O than the Ivö Klack specimens (Fig. 8, Supplementary Table 1), compatible with a slight influence of 13C and 18O depleted riverine input. The B. mammillatus specimen from Åsen shows similar isotopic signals as the specimens from Ivö Klack and Maltesholm making it unlikely that the belemnite precipitated significant amounts of calcite at the Åsen locality. It is more probable that the precipitation occurred mainly further away from the shoreline and that the animal either migrated to Åsen at the end of its life span or was transported to Åsen as part of the taphonomic process. In order to explain the stable isotope signatures of belemnites, characteristic migratory behaviour of these animals has been inferred, where the early ontogenetic phases may reflect a shallow, warmer water habitat, and later stages the deeper, cooler water habitat (Mutterlose et al., 2010; Mettam et al., 2014). This would also explain their common concentrations on bedding planes, known as belemnite battlefields, as post-spawning mass mortality (Doyle and MacDonald, 1993). The high concentration of B. mammillatus at Åsen may indicate that this locality represents a belemnite battlefield. The geochemical signals of the calcitic rostra close to the apical line and the rim, representing the juvenile and the latest life stage of B. mammillatus, are affected by post-depositional alteration and do not represent a primary signal (Fig. 5). Calcite precipitated just after the belemnite hatched and just before it died after spawning at Åsen is thus not available for further discussion of hypothetical belemnite battlefields and post-spawning mortality. The occurrence of all growth stage of B. mammillatus at the investiagted localities, however, speaks against the interpretation as belemnite battlefields.

0 -2.5

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O ‰ V-PDB

Fig. 8. Cross plots of Mn/Ca ratios and δ18O values for fossil oysters from Ivö Klack and Åsen. Circles show specimen one and diamonds specimen two of each locality. The colouration of the symbols is according to the Sr/Ca ratio in the sample. Alteration of the oyster material is indicated by Mn enrichments and Sr depletion and seems to coincide with a trend toward δ18O values of ~−1.0 to −0.5‰. In both plots, the grey shaded area indicates the range of oxygen isotope ratios and Mn/Ca ratios observed in well-preserved parts of B. mammillatus from the Kristianstad Basin.

bivalves that have comparable needs for warm water environments are also common at Ivö Klack and Maltesholm and are the northernmost rudists known so far (Sørensen et al., 2012). It is thus expected that temperatures were at the lower limit of what hermatypic corals and rudists could tolerate. In studies where similar oxygen isotope ratios were found for belemnites and benthic organisms, a nektobenthic life style of the belemnites has been suggested (Anderson et al., 1994; Wierzbowski and Joachimski, 2007, 2009; Wierzbowski et al., 2009). In other studies belemnites indicate lower temperatures than expected and even lower than those of contemporary benthic brachiopods or bivalves (e.g. Voigt et al., 2003; Fürsich et al., 2005; Dera et al., 2011). In concert with these latter publications, the belemnite δ18O values of the present study are heavier and more stable compared to contemporary benthic oysters of the Kristianstad Basin (Fig. 8). This finding implies a restricted range of seasonality and cooler temperatures compatible with the notion that belemnites calcified predominantly in deeper parts of the basin. Another line of evidence comes from the specimen collected at Åsen. This locality was most likely situated close to a fluvial system and calcite formed there should most likely show different isotopic signals compared to calcite formed near Ivö Klack and Maltesholm due to the riverine imprint on the ambient water. This imprint can be deduced

5.2.2. Carbon isotopes The δ13C values in B. mammillatus from the Kristianstad Basin are comparatively light with median values of − 1.72 to − 0.88‰. There are several independent lines of evidence indicating that these negative carbon isotope values are generated by a strong vital effect of B. mammillatus: 1) δ13C values in B. mammillatus deviate by two to five permil from coeval bulk rock δ13C reported for the UK (Jarvis et al., 2006) and Germany (Voigt et al., 2010). 2) The compilation of carbon and oxygen isotope analyses on the latest early Campanian to earliest Maastrichtian belemnite assemblage of the Kristianstad basin from Ljungberg (2004) delineates two groups of species (Fig. 9). One group corresponds mainly to 13C-depleted values of B. mammillatus and Belemnellocamax balsvikensis specimens, whereas the other group presents a range of δ13C values which is comparable with coeval bulk rock δ13C values (Fig. 9). If these δ13C values represent the primary sea-water signal, then a simple environmental interpretation would suggest that B. mammillatus and B. balsvikensis would have mostly lived in deep waters with lighter δ13C signatures (c.f. Ullmann et al., 2014). However, B. mammillatus is rare outside the Kristianstad Basin and all growth stages of the species are represented in the studied area, indicating that specimens of this species lived and died in the overall shallow Kristianstad Basin. The co-occurrence of the different belemnite species in the Basin and the lack of a difference in their oxygen isotope composition rather indicate that all belemnite species of the Kristianstad area lived in similar habitats (Fig. 9). 3) δ13C values of B. mammillatus are significantly lighter than values observed in coeval oyster specimens from the Ivö Klack and Åsen localities. 4) Diagenetic alteration of the porous central part of the belemnites generates a positive correlation of Mn/Ca ratios with δ13C values, whereas δ18O values do not show clear diagenetic changes. These trends are compatible with early marine cementation with ambient water carrying a DIC signature more positive than the primary signature recorded in the belemnite rostra. The sum of these observations makes it likely that δ13C

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3 2

δ 13C ‰ PDB

1 G. quadrata scaniensis† B. alpha† B. grossouvrei† B. mucronata† B. lanceolata† B. balsvikensis† B. mammilatus† B. mammilatus this study

0 -1 -2

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Fig. 9. Cross plot of carbon isotope and oxygen isotope ratios of latest Early Campanian belemnites from the Kristianstad Basin. The different species show similar oxygen isotope ratios but two groups of belemnites can be distinguished according to their carbon isotope ratios. Belemnellocamax mammillatus and Belemnellocamax balsvikensis (blue shaded area) show a relative depletion of ~2.5‰ with respect to the other belemnite species (grey shaded area), indicating a strong vital effect for carbon isotope ratios of these species. †: Data from Ljungberg (2004). Full species names in sequence: Gonioteuthis quadrata scaniensis; Belemnitella alpha, Belemnellocamax grossouvrei, Belemnitella mucronata, Belemnella lanceolata, Belemnellocamax balsvikensis, Belemnellocamax mammillatus.

values of B. mammillatus (and B. balsvikensis) are influenced by a strong vital effect, which is not observed in other coeval species. The lack of a similar vital effect on oxygen isotope ratios in B. mammillatus points to metabolic controls on carbon isotope fractionation rather than kinetic effects during calcite formation (Auclair et al., 2004; McConnaughey and Gillikin, 2008). The observed differences in δ13C signatures of different coeval belemnite species highlight the importance of taxonomy of investigated specimens when using δ13C values of belemnite rostra for palaeoenvironmental interpretations.

5.2.3. Element ratios The composition of seawater has been interpreted to have changed considerably through time due to tectonic effects and biological activity with far-reaching effects on marine carbonate producing organisms (Stanley and Hardie, 1998; Steuber and Veizer, 2002; Lear et al., 2003; Ullmann et al., 2013c). Apart from theoretical computation (e.g. Stanley and Hardie, 1998) and few experimental data derived from calcium carbonate veins and evaporites (Coggon et al., 2010 and references therein; Rausch et al., 2013), reconstructions of past seawater composition rely on the interpretation of the chemical composition of fossil carbonates. Due to their abundance in the sedimentary record and biostratigraphical ties, belemnites can successfully be used as such a seawater proxy (Steuber and Veizer, 2002; Ullmann et al., 2013c). The reliability of the reconstruction, however, depends on the quality and understanding of the fossil data set at hand. The data for B. mammillatus presented here constitute the first baseline for a Late Cretaceous belemnite taxon. To function as an indicator for past seawater composition, it needs to be shown that the incorporation of trace elements into fossil calcite is without any other major contributing factor than seawater composition, is predictable, uniform, and can be calibrated by modern analogues. Mg/Ca and/or Sr/Ca ratios in belemnite rostra have been proposed as potentially controlled by calcification temperature (e.g. Rosales et al., 2004; McArthur et al., 2007; Li et al., 2012).This interpretation, however, is still debated and findings on single rostra argue against a temperature control linking δ18O with Mg/Ca or Sr/Ca ratios (Li et al., 2013; Ullmann et al., 2015). None of the specimens of B. mammillatus show strong correlations between δ18O values and Mg/Ca or Sr/Ca ratios (Fig. 4). The highest observed correlation coefficient is r2 = 0.34 for Mg/Ca and

δ18O in specimen AS-I-3. These generally weak correlations between δ18O and Mg/Ca ratios are compatible with the recent findings of Li et al. (2013) and Ullmann et al. (2015) who concluded that Mg/Ca ratios do not depend strongly on temperature in Jurassic and Early Cretaceous belemnites. Relatively low internal variabilities of Mg/Ca and Sr/Ca ratios in single specimens of B. mammillatus of ≤20% (2rsd) facilitate potential reconstructions of seawater composition from the chemical composition of their rostra. High-resolution Mg/Ca and Sr/Ca ratios can be measured from very small aliquots allowing the identification of the best preserved material. The Mg/Ca variability of median values of 24% (2rsd, n = 3) between the different B. mammillatus specimens is greater than the intra-specimen variability, suggesting that a comparatively large number of individuals needs to be analysed in order to generate reliable results. For Sr/Ca ratios a low intraspecific variability with a mean value of 2.1 ± 0.2 mmol/mol (2sd, n = 3) is recorded. Average Mg/Ca ratios of 11 ± 3 mmol/mol and Sr/Ca ratios of 2.1 ± 0.2 mmol/mol are proposed as an average for element/ratios in B. mammillatus. Relative changes in seawater composition can be traced by belemnite records without a modern tie point. Inter-specific differences in element incorporation, however (e.g. McArthur et al., 2007; Wierzbowski and Rogov, 2011; Li et al., 2013), may bias such records (but see Ullmann et al., 2013c). A reconstruction of the absolute chemical composition of past seawater from element ratios in belemnite rostra is more complicated because of the lack of a representative modern analogue with a calcite rostrum. Studies employing the calcite of rostra for seawater reconstructions have to rely on cross calibrations with a representative number of specimens of other taxa for which modern analogues exist (Steuber and Veizer, 2002; Ullmann et al., 2013c). Oyster data generated for the present study indicate that where Mn/Ca ratios are lowest, Sr/Ca ratios are comparable and Mg/Ca ratios lower than in modern oysters (Supplementary Table 1, Fig. 8; Ullmann et al., 2013b and references therein) which is expected from the inferred seawater composition of the Campanian (Stanley and Hardie, 1998; Lear et al., 2003; Rausch et al., 2013). Delineation of precise numbers for a calibration of the belemnite data, however, is hampered by partly uncertain preservation state of the oyster material and a relatively small sample set. Future studies of additional fossil taxa and groups, spanning a longer time interval may enable their use for a robust reconstruction of sea water chemistry during this time interval.

6. Conclusions We have presented here the first isotopic and element ratios from well-preserved late early Campanian belemnite rostra of B. mammillatus of the boreal Kristianstad Basin (Sweden, ~ 50°N palaeolatitude). Diagenetic effects mostly overprint the calcite near the apical line and rims of the rostra and are interpreted to be caused by early marine cementation. Diagenetic liquids had low Mn concentrations, so that altered materials show unusually low Mn/Ca ratios as low as 0.02 mmol/mol. The δ18O values of B. mammillatus suggest a nektobenthic life habit in a low seasonality area in the deep part of the Kristianstad Basin and that they spent only short time at shallower water or were reworked. The δ13C values of belemnites B. mammillatus and B. balsvikensis are strongly influenced by vital effects. This finding highlights the necessity of taxonomic determinations of belemnites when using carbon isotopes for reconstructions for palaeoenvironmental interpretations. Average Sr/Ca ratios of 2.1 ± 0.2 mmol/mol and Mg/Ca of 11 ± 3 mmol/mol are proposed for the late early Campanian B. mammillatus of the Kristianstad Basin. These values constitute a first baseline for the average elemental composition of early Campanian rostra and potential future seawater reconstructions using these element ratios. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.palaeo.2015.05.025.


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