Middle Miocene seasonal temperature changes in theStyrian basin, Austria, as recorded by the isotopiccomposition of pectinid and brachiopod shells

Ana-Voica Bojar a;�, Hartmut Hiden a, Alois Fenninger a, Franz Neubauer b

a Institute of Geology and Paleontology, Karl-Franzens University, Heinrichstrasse 26, A-8010 Graz, Austriab Institute of Geology and Paleontology, Paris-Lodron University, Hellbrunnerstrasse 32, A-5020 Salzburg, Austria

Received 28 August 2002; received in revised form 21 August 2003; accepted 22 September 2003

Abstract

Using the isotope record on pectinid and brachiopod shells, we reconstructed Middle Miocene palaeotempera-tures for a shelf environment in the Styrian basin, Austria. Bivalve shells ofMacrochlamys sp., collected from differenthorizons of the Lower Badenian Leitha limestone, show N

18O values in the range of 33 to 0x (PeeDee Belemnite).The large intrashell variability of 3x indicates significant seawater temperature fluctuation. The results suggest thatdespite the high geographical latitude (ca. 40‡N), the climate in southern Austria was warm (subtropical) with apronounced seasonality at that time. In contrast, the N18O isotopic composition of a pectinid (Flabellipecten sp.) andan indeterminate terebratulid brachiopod, from the siliciclastic deposits overlying the Leitha limestone, rangesbetween 0 and 31x. This indicates cooler mean annual temperatures and reduced seasonal variations. A 39Ar/40Arage on fresh volcanic biotites shows a value of 14.2 7 0.1 Ma. The tuffs containing the biotites are located below thelayer containing the terebratulid and the Flabellipecten sp. The stable isotope and radiogenic data suggest that thedrop in temperature is in direct relationship with the East Antarctic Ice Sheet expansion, which started at ca. 14 Ma.Due to the variation in temperature, the subtropical fauna in the Retznei quarry disappeared during the EarlyMiocene and the biogenic limestones were replaced by clastic sediments. The N

18O isotopic profiles from threepectinids collected from the Leitha limestone indicate that these grew within ca. 1.5 years. Growth interruptionsoccurred during the warm season because during this period the pectinids very likely used their energy for the growthand maturation of gonads. Carbon isotopic compositions vary between 0 and 2x. Statistical tests show that forsome of the analyzed shells, the cyclicity of the N

13C profiles may be explained by the temperature-dependentfractionation between air CO2 and dissolved bicarbonate.? 2003 Elsevier B.V. All rights reserved.

Keywords: stable isotopes; Styrian basin; Middle Miocene; climate change

1. Introduction

An important interval in the global climatic andcryospheric development of the Cenozoic was theEarly to Middle Miocene, from 17 to 12 Ma. The

0031-0182 / 03 / $ ^ see front matter ? 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0031-0182(03)00662-X

* Corresponding author. Fax: +43-(316)-380-9871.E-mail addresses: ana-voica.bojar@unigraz.at

(A.-V. Bojar), alois.fenninger@unigraz.at (A. Fenninger),franz.neubauer@sbg.ac.at (F. Neubauer).

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Palaeogeography, Palaeoclimatology, Palaeoecology 203 (2004) 95^105

www.elsevier.com/locate/palaeo

climatic optimum near the Early/Middle Miocenewas followed by global cooling at around 14 Ma,the latter being concomitant with the expansionof the East Antarctica Ice Sheet (Shackleton andKennett, 1975; Savin et al., 1985; Miller andKatz, 1987; Miller et al., 1991; Woodru¡ andSavin, 1989, 1991; Wright et al., 1992; Flowerand Kennett, 1994; Pagani et al., 2000; Billupsand Schrag, 2002). Thus, the Middle Miocene ischaracterized by climatic changes which resultedin a rapid shift from relative high-latitude warmthto high-latitude refrigeration.In this study mollusc and brachiopod shells

have been used to evaluate palaeoclimatic param-eters for a shelf environment during the MiddleMiocene. The studied outcrop, which is strati-graphically well documented, belongs to the Styr-ian basin. This basin was part of the Paratethysrealm, a land-locked remnant sea which formedsubsequent to the collision of Europe and Africa-derived microplates.When molluscs grow, their shells become bio-

geochemical recorders of climatic and environ-mental conditions during their lifetimes. Previousstudies have shown that the calcitic shell of pecti-nids (Arthur et al., 1983; Krantz et al., 1984,1987; Krantz, 1990; Barrera et al., 1990; Weferand Berger, 1991; Schein et al., 1991; Jones andAllmon, 1995; Hickson et al., 1999; Hickson etal., 2000) and brachiopods (Lowenstam, 1961;Lepzelter et al., 1983; Mii and Grossman, 1994;Carpenter and Lohmann, 1995; Buening et al.,1998; Picard et al., 1998) are suitable to palae-oclimatic reconstruction, as they secret theirskeleton in oxygen isotopic equilibrium with sea-water. Because pectinids support little salinity var-iation (Amler et al., 2000), they are particularlysuitable for reconstructing water palaeotempera-tures.A radiogenic age of a tu¡ intercalation from

the studied outcrop made possible to correlatethe evaluated Miocene temperatures and seasonalvariations with the interpreted oceanographicchanges occurring worldwide at that time. More-over, the N18O pro¢les measured on molluscs havebeen used to evaluate growth rates and to deter-mine the relationship between growth interruptionand seasonal variation. The processes that may

have produced the variations in the N13C pro¢les

were also discussed.In this study we have investigated the potential

of the oxygen isotope pro¢les along pectinid andbrachiopod shells in order to: (1) reconstruct theMiddle Miocene climatic changes for a shelf en-vironment situated within the Paratethys realm,and (2) study the link between seasonal temper-ature variation and life history.

2. Geological situation

The Styrian basin, formed during the eastwardtectonic extrusion and orogen-parallel extension,is related to the last collisional phase between theAdriatic and the European continental plates(Neubauer and Genser, 1990). There are severalpublished studies concerning the stratigraphy, li-thology and palaeogeography of the basin depos-its (Ebner and Sachsenhofer, 1991; Ro«gl, 1998;Gross, 2000; Ro«gl, 2001). The sedimentationstarted in Ottnangian times (Early Miocene); thepreserved thickness of the deposits reaches ca.4 km.The ‘Retznei Zementwerk’ quarry is situated

not far away from the border to Slovenia (Fig.1). The lithology and faunal assemblage havebeen described by Friebe (1990, 1991) and Fritzand Hiden (2001) and are of Badenian age (forthe Paratethys timescale, see Ro«gl, 1996). Frombottom to top, the following lithofacies werefound (Fig. 2):(1) sands and silts with marl boulders of Kar-

pathian and/or Badenian age with Gastrochaenaand Lithophaga. For this facies Friebe (1991) as-sumed deposition in a sub- to intertidal environ-ment.(2,3) coral carpets (with Porites, Tarbellastraea,

Acanthastraea, Montastraea, Sparidae, Balistidaeand shark teeth) with intercalations of sand withcrustaceans.(4) algal debris. According to Friebe (1991),

lithofacies (2^4) were deposited at a depth abovethe fair weather-wave base.(5) bioclastic rhodolith limestones with various

fossils, e.g. pectinids, oysters, red algae.(6) marly limestone.

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(7) sands and silts with molluscs and crabs.At the base of lithofacies (7), a discontinuous

tu¡ intercalation was described (Hauser, 1951).The tu¡ contains unaltered phenocrysts of feld-spars and fresh biotites, and was deposited duringan eruption of one of the volcanic centers situatedfarther to the east (e.g. Ebner and Sachsenhofer,1995). The lithofacies levels (5^7) were deposited

at depths less than the storm waves base. Forlithofacies (7), an ichnofauna (Ophiomorpha, Sco-lithos and Planolites) was described by Hiden(1995). This fauna is characteristic of the transi-tion from nearshore to shoreface, and indicatesdepths around 10 m (Dodd and Stanton, 1990).Facies (1^6) are characterized by the occurrenceof various coral fauna. In contrast, this coral fau-

Fig. 1. Geological map of the Styrian basin (after Flu«gel and Neubauer, 1984).

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na is not found in facies (7), but echinoids(Schizaster), crustaceans (Calianassa, Portunus)were found. This change is interpreted as indicat-ing a transition from a subtropical to a temperatefaunal assemblage.

3. Materials and methods

In this study we analyzed three large calciticshells of Macrochlamys sp., one shell of Flabelli-pecten sp. and one of an indeterminate terebratul-id collected from di¡erent horizons of the outcrop(Fig. 2). Macrochlamys, Flabellipecten and the ter-ebratulid were benthic organisms. The pectinidshells consist of an external foliated layer, an in-

Fig. 2. Simpli¢ed lithostratigraphic section of the Retznei quarry (after Friebe, 1990). M1, M2, M3, F1 and B1 indicate the loca-tions of the analyzed shells.

Fig. 3. Internal structure of the Macrochlamys shell. All thesamples were taken from the outer foliated layer, along thedorso-ventral axis.

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termediate ¢ne crystalline layer and an internalfoliated layer (Watabe, 1988). Study of thin sec-tions under polarized light shows preservation ofthe internal structure of the pectinid shells (Fig.3). Microbeam analyses indicate that all measuredshells consist entirely of low-magnesium calcite,implying that the primary mineralogical composi-tion has been preserved. Cathodoluminescencemay also serve as an indicator of eventual alter-ation because the manganese content, the com-mon activator of luminescence in calcite, increasesas skeletal carbonates are diagenetically altered(Brand and Veizer, 1980). Thin sections of thestudied shells show no luminescence. For thesereasons, we consider that the e¡ect of diagenesiswas minimal and that the data can be used forpalaeoclimatic reconstruction.Samples of powdered shell of Macrochlamys sp.

(M1, M2, M3), Flabellipecten sp. (F1) and theterebratulid (B1) were analyzed for N

18O and

N13C. The shells were sequentially sampled alongthe dorso-ventral axis (average sample separation1.5^2 mm) using a 0.5-mm drill bit. Isotopic anal-yses were performed using an automatic Kiel IIpreparation line and a Finnigan MAT Delta PlusMass Spectrometer. NBS-19 and an internal lab-oratory standard were analyzed continuously foraccuracy control. Standard deviation was 0.1xfor both N

18O and N13C. All isotopic results are

reported in per mil, relative to the PeeDee Belem-nite (PDB) standard. The 40Ar/39Ar laser-ablationtechnique was applied to several fresh grains ofbiotite phenocrysts from the tu¡ level (sampleAVB-2). The technique is described in detail byLiu et al. (2001). The biotite concentrate of sam-ple AVB-2 records a plateau age of 14.2 7 0.1 Ma.Neither extraneous argon nor disturbances of theage pattern were detected. The age is thereforeconsidered to be geologically signi¢cant, record-ing the age of the volcanic eruption.

Fig. 4. Oxygen and carbon isotopic pro¢les from the Macrochlamys (M1, M2 and M3) external shell surface. The calculated tem-peratures are displayed for each measured pro¢le. For M1, two parallel pro¢les were measured.

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4. Results

The resulting N18O and N

13C values are plottedon the y-axis against shell length from the originof growth (Figs. 4 and 5). The N18O values of M1,M2 and M3 show cyclical patterns ranging be-tween approximately 33.5 to 0x. The N13C val-ues in the shells range between 0 to 2.2x. To testthe reliability of the data, two di¡erent pro¢les forM1 were measured along the dorso-ventral axis.Except for the umbo region, the results for thetwo pro¢les are very similar. For F1 and B1,the N

18O values range from 31 to 0%, and theN13C values from 0.5 to 2.2x.

5. Discussion and conclusions

5.1. Oxygen isotope pro¢les

Previous studies have shown that pectinidsgrown in various climatic settings, such as tropicaland subtropical (Jones and Allmon, 1995; Hick-son et al., 2000), temperate (Krantz et al., 1987;Hickson et al., 1999) and Antarctic (Barrera et al.,1994) climates, precipitating their shells in equilib-rium with the seawater. Moreover, pectinids aswell as brachiopods populate only environmentswith little salinity variation. Consequently, whentheir shells are not diagenetically altered, the N18O

Fig. 5. Oxygen and carbon isotopic pro¢les from the indeterminate terebratulid (B1) and Flabellipecten (F1) external shell surface.The calculated temperatures are displayed for the measured pro¢le.

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values of calcite can be interpreted in terms ofseasonal changes in water temperatures, with low-er values of calcite representing summer and high-er values winter conditions.For M1, M2 and M3 abrupt variations of N18O

values can be observed in the umbo region. Thesemay be related to vital e¡ects and disequilibriumduring the early, fast growth period of the shells(Wefer and Berger, 1991). Palaeotemperatureswere calculated using the equation of O’Neil etal. (1969) for the water^calcite system. Takinginto account that at that time Paratethys was re-lated to the oceanic domain, and that within theStyrian realm there was no massive freshwaterinput, we assume an interglacial value of30.7x for the N

18O value of water (Lear et al.,2000). Excluding the umbo region, the calculatedtemperatures range between approximately 13‡and 26‡C and indicate a subtropical environmentand strong seasonal variations. Previous studieshave shown that for temperate regions, growthcessations, marked by prominent rings on theshell surface, occur during the cold season (Tanet al., 1988; Dare and Deith, 1991; Hickson et al.,2000). The studied Macrochlamys shells show thatgrowth began during the warm season, and thatgrowth interruptions on the external surface wereassociated with the next warm season (Fig. 4).The growth interruptions are likely related tothe warm season because during this period thepectinids used their energy for the growth andmaturation of gonads (Amler et al., 2000; Man-dic, pers. commun.). Ansell (1968) has shown thattemperature is the major factor controlling metab-olism and hence growth rate of pectinids. If thecycle in oxygen isotopes re£ects growth rate, thenthe pectinid shells analyzed in this study reachedtheir preserved length within approximately 1^2years. This is faster than the growth rate deter-mined for modern sea pectinids from temperateregions (Krantz et al., 1984; Schein et al., 1991),but ¢ts with growth rates determined by Jonesand Allmon (1995) for a ca. 3-Ma-old Carolina-pecten arboreus from a subtropical environment.At about 14 Ma, a signi¢cant change in the

N18O value of the ocean occurred due to a majorwidespread of the Antarctic Ice Sheet. FollowingLear et al., 2000, for calculating the temperature

distribution for F1 and B1, we assumed a 30.1xvalue of water. In this case, the temperatures cal-culated from the N

18O isotopic pro¢les acrossboth F1 and B1 show lower values, between 15and 19‡C and reduced seasonal temperature var-iation (Fig. 5). The data indicate change in theannual temperature distributions across, belowand above the tu¡ level.Radiogenic dating of the tu¡ layer situated in

the uppermost part of the marly limestone layermakes it possible to correlate this climatic shiftwith the worldwide palaeoceanographic events ofthat time. 39Ar/40Ar dating of the fresh biotitesgives a plateau age of 14.2 7 0.1 Ma. The positionof this sample is within the clastic deposits, justbelow the sand layers where the Flabellipecten sp.and the terebratulid come from. The radiogenicdata ¢t well with the beginning of the spread ofthe Antarctic Ice Sheet, for which an age of ca. 14Ma has been assumed (Shackleton and Kennett,1975; Savin et al., 1985; Billups and Schrag,2002). Thus, we consider that the shift from asubtropical climate to much cooler annual tem-peratures, as well as the shift from biogenic car-bonates to the dominantly clastic deposits, arerelated to the widespread of the East AntarcticaIce Sheet. This event constituted one of the majorpalaeoceanographic events of Miocene times.

5.2. Carbon isotope pro¢les

The N13C values of brachiopods and molluscs

are usually similar to those of ambient dissolvedinorganic carbon (DIC), but cases of N13C deple-tion also occur (Epstein et al., 1952; Craig, 1953;Mook, 1971). These depletions, due to skeletalincorporation of respired CO2 into the shell skel-eton, are related to metabolic activities (Aharon,1991; Wefer and Berger, 1991; Tanaka et al.,1986; Krantz et al., 1987). More recent studies(McConnaughey et al., 1997; Hickson et al.,1999) have shown that aquatic invertebrates usingrespiration incorporate little respired CO2 intotheir shells. However, this issue is still under dis-cussion (Owen et al., 2002). Thus, for this groupof organisms, the N

13C values of the shells areprimarily controlled by the N13C of DIC, and sec-ondary by the metabolic rates.

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For marine waters, the main inorganic dis-solved carbon species is HCO3

3 , the occurrenceof dissolved CO2 being subordinate. Thus, theN13C values of shells will re£ect the N

13C compo-sition of dissolved HCO3

3 . The calcite^bicarbon-ate fractionation is not temperature-dependent for

carbon (Romanek et al., 1992), but the isotopefractionation between dissolved HCO3

3 and airCO2 is temperature-dependent (Mook, 1974).For the analyzed shells, the amplitude of temper-ature-dependent vHCO3

3 -CO2 values ¢t the am-plitude of N13C values measured in calcite (Fig. 6).

Fig. 6. Calculated vHCO33 ^CO2 values and the N

13C values measured along shells.

Table 1Statistical tests of the correlation between the vHCO3

3 ^CO2 variation and the N13C values measured along shells

Sample Number of measurements Pearson correlation Spearman correlation

M1 52 P6 0.001 P6 0.001Cor= 0.81 Cor= 0.82

M2 70 P=0.06 P=0.83Cor= 0.22 Cor= 0.02

M3 55 P=0.01 P=0.06Cor= 0.37 Cor= 0.25

B1 15 P=0.05 P=0.06Cor= 0.52 Cor= 0.5

F1 16 P=0.33 P=0.34Cor= 0.27 Cor= 0.26

P=P-value; Cor= correlation coe⁄cient

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Moreover, some of the N13C pro¢les show cyclic-

ity, in phase with the N18O values. Therefore, for

temperatures calculated using the calcite ther-mometer, we have statistically tested the presenceof the correlation between the calculatedvHCO3

3 ^CO2 and the measured N13C values

along the shells, using both the Pearson andSpearman tests (Table 1). The data indicate thatfor M1, M3 and B1 there is a correlation betweenthe two data sets. In this case the cyclical £uctua-tion of the N13C values of calcite may be explainedconsidering the temperature-dependent fractiona-tion between dissolved HCO3

3 and air CO2. Rel-atively low water depths and a favorable mixingof waters may have favored the seasonal equili-bration between CO2 and HCO3

3 as well. For M2and F1 the correlation between the two data setsis poor, most probably due to local, seasonalevents which can be poorly documented.

Acknowledgements

Financial support for this study was providedby FWF Project 13029-Geo. V. Atudorei, Z.D.Sharp (Albuquerque) and F. Vaida (Harvard)are thanked for interesting discussions and com-ments. The authors wish to acknowledge O. Man-dic (Wien) and W. Piller (Graz) for useful adviceinto the biology of molluscs. We thank also J.Hickson and A. Longinelli for constructive com-ments that greatly improved the manuscript. An-gela Poly is thanked for general text revisions.

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