AMQ32(1) 5-13 Ragaini et al + ring.pub Available online http://amq.aiqua.it ISSN (print): 2279-7327, ISSN (online): 2279-7335 Alpine and Mediterranean Quaternary, 32 (1), 2019, 5 - 13 MINERALOGY AND OXYGEN ISOTOPE PROFILE OF PELECYORA GIGAS (VENERIDAE, BIVALVIA) FROM TUSCAN PLIOCENE Luca Ragaini 1, Federica Ficini 1, Giovanni Zanchetta 1, Eleonora Regattieri 1,2, Natale Perchiazzi 1, Luigi Dallai 2 1Dipartimento di Scienze della Terra, Università di Pisa, Pisa, Italy. 2Istituto di Geoscienze e Georisorse, IGG-CNR, Pisa, Italy. Corresponding author: L. Ragaini ABSTRACT: a specimen with joined valves of Pelecyora gigas, an extinct species, was collected in a sandy layer of Early Plioce- ne age in the northern part of the Siena Basin (Tuscany, Italy). XRD data demonstrated that the original mineralogical composition of the specimen was aragonitic and it maintained substantially the original structure and composition. Neglecting possible changes in sea salinity during different seasons, we can estimate, using oxygen isotope composition a maximum seasonal temperature differences of ca. 9 °C experienced during the life of the individual. An approximate estimation of past sea water composition al- lows to calculate an average temperature of 23.0±2.7 °C for the water where the shell lived, whereas calculated temperature ex- tremes are 18.5 °C for the colder season and 27.6 °C for the warmer. These data are in a good agreement with those proposed on the basis of the Pliocene Mediterranean taxa nowadays living along the western African shores. Keywords: Pelecyora gigas, shell mineralogy, isotopic composition, Pliocene temperatures, Tuscany. 1. INTRODUCTION Many factors may influence the biogeography of shallow marine benthic molluscs such as nutrients, pre- dation, competition, substrate, natural barrier, ocean circulation (Monegatti & Raffi, 2001; Silva et al., 2006). However, it is largely accepted that sea surface temper- ature (SST) plays the major role in controlling the latitu- dinal distribution of benthic organisms on the continental slope (Hall, 1964; Petuch, 2004). In this context the critical factor does not lie as much in the temperatures suitable for the survival of specimens but rather the duration of the time interval in which the sea water is at the temperature required for successful reproduction and early growth (Raffi, 1986; Raffi et al., 1985; 1989). Even though the Pliocene period appears to have been, on average, warmer than present (Jansen et al. 2007), it is characterized by climatic cooling phases recorded in the Northern Hemisphere by paleontological and isotopic data (Stanley, 1986; Stanley & Ruddiman, 1995; Mudelsee and Raymo, 2005; Haug et al., 2005; Lisiecki & Raymo, 2007; Lawrence et al., 2009). The two major cooling events occurred at about 3.1 Ma and 2.7 Ma (the latter just older than the recently revised Plio-Pleistocene boundary, Gibbard et al., 2009) and caused pulses of extinction and local disappearance among the Mediterranean mollusc fauna (Monegatti & Raffi, 2001; Monegatti et al. 2002; Monegatti & Raffi, 2010). This fact results in a Pliocene decrease of the mollusc diversity because faunal impoverishment is not balanced by recovery phases, probably owing to an upwelling phenomenon along the Northwest African coasts, which appears to have intensified since the Mid- dle Pliocene (Cita & Ryan, 1979; Sarnthein et al., 1992; Monegatti & Raffi, 2001). During the Early-early Middle Pliocene the Mediter- ranean Basin was populated by a thermophilic malaco- fauna thriving in a tropical sea with estimated mean maximum SSTs over 24–25 °C for at least five to six months every year (Monegatti & Raffi, 2001; Silva et al., 2010); the molluscan fauna is categorized as MPMU1 by Raffi & Monegatti (1993) and Monegatti & Raffi (2001). At least as far as molluscan fauna is concerned, this temperature regime remains substantially stable up to about 3.1 Ma, when the first cooling event, even though not so severe as that at 2.7 Ma, has dramatic results for the Early Pliocene thermophilic fauna. The disappear- ance of Strombus coronatus and the drastic decrease of the taxonomic diversity of Conidae and Terebridae are the most relevant events among gastropods whereas bivalves offer a more detailed and quantitative approach thanks to the hundreds of infra- and circalittoral species https://doi.org/10.26382/AMQ.2019.01 6 Ragaini L. et al. taken into account by Monegatti & Raffi (2001). In correspondence to, or just after, the 3.1 Ma event, a percentage of bivalves varying from 15% to 23% (more than 50 and no more than 80 species) disappears from the Mediter- ranean (Raffi & Monegatti, 1993), owing to both regional disappearances and true extinc- tions (sensu Raffi et al., 1985; 1989). Among 13 and 17 species typical of MPMU 1 are still living along the West African coasts south of latitude 20-22°N (i.e. the tropical Mauritanian- Senegalese province of Hall, 1964) but this stock appears to be negligible in comparison with that of the extinct taxa. Pelecyora gigas is a large, thick-shelled bivalve typical of the Early Pliocene tropical environments in the Mediterranean Basin, that becames extinct during, or just after, the 3 Ma cooling event. This species has been little studied and research results are restricted to the systematic perspective and are mainly concentrated in the 1960s and 1970s (Tavani & Tongiorgi, 1963; Palla, 1966; Malatesta, 1970, among others). The aim of the present study is: - to analyze the shell composition of Pelec- yora gigas to investigate its original mineralogy; - to reconstruct the shell growth pattern; - to measure the oxygen isotope composition of the shell and to interpret these data with regard to climate conditions of the Mediterranean during the Early Plio- cene. Concerning the last point, stable isotope analysis of the Plio-Pleistocene molluscs in Tuscany was pio- neered in the late 60s to 70s of the last century and then almost completely forgotten (Longinelli et al., 1961; Vergnaud Grazzini, 1968). To better understand and constrain future climate change in a high CO2 world, it is essential to study potential CO2 concentration ana- logues in Earth’s history and the climate at that time. The Pliocene epoch prior to the intensification of North- ern Hemisphere glaciation at about 2.75 Ma is a likely candidate for such a high CO2 ana- logue (e.g. Bartoli et al., 2011) and this should encourage to a reappraisal of the study of the Pliocene marine successions preserved in Tuscany. 2. GEOLOGICAL AND PALEONTOLOGICAL DATA The P. gigas specimen was gathered from a Pliocene sandy level outcropping near Colle d’Arbia (Monteaperti, Siena) in the north- ern part of the Siena Basin (Tuscany, Italy). This is part of a broader tectonic depression, NNW–SSE oriented, traditionally interpreted as a half-graben formed in the framework of late Miocene-Quaternary polyphase extension- al tectonics of Northern Apennines (Bossio et. al., 1993; Carmignani et al., 2001; Brogi, 2011) (Fig. 1). The Neogene sedimentary infill of the Siena Basin starts with a Miocene (late Messinian) suc- cession overlying the pre-Neogene bedrock composed of several metamorphic and non-metamorphic tectonic units (Brogi, 2011). Miocene sediments are in turn over- lain by Pliocene ones with an intervening angular uncon- formity (Costantini et al., 2009). Pliocene deposits of the northern part of this basin, mainly consisting of sands with gravel and mud intercalations, have been recently mapped as a succession of four depositional sequences (Martini & Aldinucci, 2017; Martini et al., 2011) ranging from early Zanclean to Piacenzian in age. In particular the sequence S3, the fossil bearing unit, ranges within the late Zanclean-early Piacenzian time span (Martini & Aldinucci, 2017). From the taphonomic point of view, the fossil speci- Fig. 1 - Location map of the fossiliferous outcrop (scale map 1:10.000). Fig. 2 - P. gigas just after the extraction. Note the original horizontal position. 7 Mineralogy and oxygen isotope of Pelecyora gigas men displays conjoined valves in a nearly horizontal position, which reflects a post mortem dislodging due to wave/current sweeping action (Fig. 2) However, the presence of an articulated individual with nearly complete valves displaying one of these with the ventral margin broken but nearly whole (i.e. each piece of the shell is juxtaposed with the surrounding ones) along with the general good state of shell preser- vation allow us to infer low energy conditions after the exhumation consistent with minimal post-mortem dis- turbances, very little reworking and negligible transpor- tation. Inconspicuous signs of bioerosion and encrusta- tion suggest a rapid burial, which also promotes the conservation with articulated valves. P. gigas is a large and thick-shelled venerid reach- ing the largest dimensions known within this genus, a characteristic which seems typical of species living in tropical environments (Marasti & Raffi, 1980). Regarding the paleoecological evidence, this species has been frequently recognized in inner subtidal sandy bottoms (Bernasconi & Robba, 1993; D’Alessandro et al., 2004; Ferrero et al., 2005), a substrate preference consistent with the grain size (sand 80.07%, silt 17.41%, clay 2.51%) of the siliciclastic sediment of the Colle d’Arbia outcrop. P. gigas is considered a suspension-feeder, which lived as a sluggish active shallow-burrower, a life habit suggested by the shell thickness, shape and size along with the pallial sinus form (Stanley, 1970). Howev- er, the relationship between the burrowing depth and the shell/pallial sinus morphology discussed in the pioneer- ing work of Stanley (1970) is not sufficient to be used as a clue to reconstruct the burial depth of extinct species. Taking into account both the horizontal position dis- played by the specimen and the average level of sedi- ment reworking in shallow water sandy bottoms (about 15-20 cm), it seems reasonable to consider the above measure as the burrowing depth attained by our individ- ual of P. gigas. A closer relationship between burrowing depth and pallial sinus length is suggested by Kondo (1987) on the basis of the ratio of pallial sinus to shell length (Pallial Sinus Index, PSI). This index is very use- ful in active burrowing, suspension-feeding siphonate bivalves and may be also useful for extinct species. The PSI calculated for our P. gigas specimen corresponds to a burrowing depth of 1-2 times the shell length, a result consistent with that previously inferred. In terms of the bionomic model of Peres & Picard (1964), this venerid is considered an infralittoral species referable to Fine Well Sorted Sand (SFBC) biocenosis (Bernasconi & Robba, 1993; D’Alessandro et al., 2004). P. gigas ranges from the Early Miocene to the Mid- dle Pliocene. During the Pliocene this venerid is wide- Fig. 3 - Closeup of the X-ray pattern region hosting major diffraction peaks for aragonite and calcite, showing that aragonite is the major phase in the analysed sample. spread over the whole Mediterranean Basin from the eastern (Turkey) to the western (Andalusia) part (Erunal -Erentoz, 1958; Vera-Pelaez et al., 1995) with a sole Atlantic finding along the Moroccan coast (Lecointre, 1952). 3. SHELL MINERALOGY Apart from the presence of trace components, the mineralogical composition of bivalve shells ranges from pure aragonite to pure calcite, the latter being a rather rare case. Usually calcite secreting bivalves are those with an epifaunal life habit whereas those with an infau- nal life habit form wholly aragonitic shells. In particular, calcitic continous layers have been recognized only in some taxa, such as Eupteriomorphs, Hippuritoida (a group of extinct rudists), some mytiloids and chamoids (Carter, 1980; Esteban-Delgado et al., 2008). Within Veneridae, the shell mineralogy is generally uniform being composed of minute aragonite crystals, with some exceptions being representatives of Saxidomus, Pro- tothaca and Irus where small calcitic structures, mainly described as “conellae”, are present in the outer part of the outer shell layer (Carter et al., 1998). Most of the studies concerning Veneridae shell mineralogy are focussed on the living taxa (Shimamoto, 1986; Glover & Taylor, 2010) and very few data are available on fossil species, such as P. gigas. In order to identify the mineral phases in the skeleton of this taxon, an X-ray powder diffraction analysis was carried out. The shell sample was crushed into small pieces and then ground in an agate mortar, with subsequent milling under acetone to produce a fine powder (grain size nearly 5-10 μm) which was loaded into a capillary 0.7 mm in diameter. X-ray diffraction data were obtained using a Brucker D8 Advance diffractometer, equipped with primary Ge (111) monochromator and with a Braun PSD linear detector. The running conditions were: 40 kV, 40 mA, 2θ range 21° to 70°, 2θ step 0.0157°, meas- urement speed 6 sec/step PSD measurement width 4°. Experimental data were examined through the EVA Bruker software: in Fig. 3 is shown the diffraction pattern after background subtraction, together with the PDF reference patterns for calcite (blue bars, PDF 83-1762) and aragonite (red bars, PDF 41-1475) One can notice that aragonite (red bars) is clearly the major phase, and that the presence of calcite, if any, is very minor. To precisely evaluate the presence of calcite, the experimental data were elaborated through a Rietveld refinement, performed with the Topas-Academic pro- gram (Coelho, 2018) for mineralogical quantitative anal- ysis for mixtures including both crystalline and amor- phous phases. In a Rietveld refinement, the difference between the observed and calculated intensities is mini- mized at every 2θ point of the observed powder diffrac- tion pattern, namely: The weight fractions of the phases in a mixture can be derived from the refined scale factors of each phase Sp, through the expression: where W is the relative weight fraction of phase p in a mixture of n phases, and S, Z, M, and V are, respective- ly, the Rietveld scale factor, the number of formula units per cell, the mass of the formula unit (in atomic mass units) and the unit cell volume (in Å3). Structural models for the Rietveld refinement were taken from literature (calcite: Effenberger et al., 1981; aragonite: Dal Negro & Ungaretti, 1971), and only arag- onite cell parameters were refined. During the refinement, a parameter taking into account asymmetry, determined from the refinement of SRM 675 NIST standard, was introduced and not re- fined. Background was modelled with a 12-terms Cheby- chev function, and a pseudo-Voigt function was used for peak shape. A common Lorentzian parameter for crys- tallite size was introduced and refined for aragonite and calcite to model peak width. Final agreement factors for the refinement (Fig. 4) were satisfactory, with the figures Rwp =2.817 %, Rp =2.167 %, Gof =1.087. In conclusion, shell mineralogical data based on x- ray diffraction suggest that our specimen of P. gigas was originally aragonitic and reasonably retained its pristine mineralogy; therefore, it seems very unlikely that diagen- esis has reset the shell isotopic composition (Dodd & Stanton, 1976). 4. OXYGEN ISOTOPE COMPOSITION The external shells were drilled in an ontogenetic sequence, from the umbo towards the ventral margin (e.g. Krantz et al., 1987) using a Dremel microdrill equipped with a 1 mm bit, with average distance be- tween the center of the holes of ca. 1.5 mm. 44 samples were obtained (Fig.5). Stable isotope analysis on the obtained powder was performed using a Gas Bench II (Thermo Scientific) coupled to a Delta XP IRMS (Finnigan) at the Institute of Geosciences and Earth Resources of the Italian Nation- al Research Council (IGG-CNR) in Pisa (Italy). Car- bonate samples of ca. 0.15 mg of CaCO3 were dissolved in H3PO4 (105%) for one hour at 70°C. Sample results were corrected using the International Standard NBS-18 and a set of 3 internal standards, previously calibrated using the international standards NBS-18 and NBS-19. Isotopic results are reported using the conventional δ‰ notation, with reference to the V- PDB standard; δ18O values of water are quoted with the reference to V- SMOW. Analytical uncertainty (±1σ) for δ18O was ±0.20‰. The equilibrium oxygen isotopic composition of marine biogenic carbonates primarily depends on the temperature of carbonate deposition and on the oxygen isotope composition of the surrounding sea-water (Epstein et al., 1953). The latter is globally influenced by continental ice volume and locally by salinity variations (i.e. mixing with freshwater and/or evaporation, e.g. 8 Ragaini L. et al. Shackleton, 1987). With only the δ18O of car- bonate, neither the δ18O of water nor pale- otemperature can be reliably determined with- out independent quantification of one or the other. Although complex in detail, oxygen isotope composition of samples collected along the shell growth have been demonstrat- ed to be related to changes of seasonal condi- tions like in change in temperature and salinity (e.g. Schöne et al., 2007; Leng et al., 1998; Collareta et al., 2018). Therefore, analysing isotopic composition along the shell growth, it is possible to calculate the seasonal changes in temperature, reasonably assuming constant isotopic composition of sea water during the year. Moreover, for most bivalves, precipita- tion close to equilibrium can be assumed (Wefer & Berger, 1991) even in absence of calibration of a single species. There is dis- cussion concerning the best equation for cal- culating paleotemperature (Patterson et al., 1993; Kim & O’Neil, 1997; White et al., 1999; Zanchetta et al., 2005) and the ideal would be to have specific equation for each species. However, different calibrations usually differ only slightly in the term of rate of change in dα/dt, where α is the fractionation factor be- tween water and calcite and t is the tempera- ture (White et al., 1999) but can differ for the absolute value of temperature determined. For the reconstruction of the paleotem- peratures (SST °C) from shell oxygen isotopes we used the equation for aragonite given by Grossman & Ku (1986), with a correction for the conversion of V-SMOW to V-PDB (Dettman et al.,1999). SST(ºC)=20.60-4.34[δ18Oshell-VPDB-(δ18Owater-VSMOW-0.27)] (1) This implies that for a constant isotopic composi- tion of seawater the Δδ18O/ΔT ~0.23‰/°C. The average δ18O values measure along the profile is 0.06 ±0.61 ‰ the maximum values and the minimum are 1.12‰ and -0.99‰ respectively. This implies a max- imum difference in temperature during the life of the mollusc of ca. 9.2°C. At least three clear seasonal oscil- lations are visible (Fig. 6) in the δ18O profile. At the edge of shells the ontogentic reduction of shells growth (e.g. Aguirre et al., 1998) complicate the identification of sea- sonal cycles at the resolution adopted. However, it seems reasonable to assume that the specimen repre- sents more than 2 yr of δ18O record. To calculate past “absolute temperature” is difficult because of the absence of accurate evaluation of past 9 Fig. 4 - Rietveld refinement plot, closeup of the 2θ range 25-50°; dark blue line represents the observed pattern, red line the calculated pattern. Their difference line is represented by the lower trace, with vertical marks showing the calculated positions of Bragg reflections for aragonite and calcite. Fig. 5 - The specimen of P.gigas showing the stable isotope sub-sampling tracks. Mineralogy and oxygen isotope of Pelecyora gigas sea water isotopic composition. Current δ18O values of western Mediterranean can range from ca. +1.3 to +1.4‰ (Pierre, 1999). Mediterranean is today consid- ered a concentration basin due to the deficit in the hy- drological budget and this has been probably true al- ready during the Pliocene (Bianchi et al., 2012). This implies that local isotopic composition is higher than the ocean average owing to evaporation. In addition, it must be considered that Pliocene sea level was higher than today associated to lower isotopic composition of sea- water for a lower mass of continental ice. Multiple lines of evidence suggest that the sea level during the middle Pliocene was >20 m above present sea level and a val- ue of +25 m is often adopted in numerical climate model simulations (Dwyer and Chandler, 2009; Miller et al., 2012; Rohling et al., 2014 and reference therein). In many estimate for the Quaternary Period, the relation of ca. 0.009 ‰/m of eustatic sea level variation has been found (Rohling et al., 2014). Assuming an eustatic component of +25 m, the isotopic water should be on average ca. -0.25 ‰ lower than today. More re- cently, estimates of isotopic composition of global ocean for the eustatic component suggest an average value ca. -0.5‰ less than today for the Pliocene at around ca. 3 Ma (Rohling et al., 2014). These differences in esti- mate may insert some incertitude in the final tempera- ture estimate of ca. 1°C or more, if the sea level was higher than assumed. The subdued variance of oxygen isotopic time series on benthic foraminifera during the interval of 3-5 Ma (Lisieki & Raymo, 2005, 2007) sug- gest a very minor “ice volume effect” (Shackleton, 1987); nevertheless this can introduce additional uncer- tainty to our estimation. Assuming a similar relation between salinity and oxygen isotope composition of sea water in western Mediterranean, a reasonable estimate using the dis- cussed figures would be ca. +0.9‰ for the oxygen iso- topic composition of western Mediterranean sea water. This does not account for local changes in salinity and in the following calculation, we neglect seasonal change in salinity. This can introduce further error in the pro- posed calculation. Therefore, the average temperature obtaining from these figures is 23.0±2.7°C, whereas extremes calculated temperature are 18.5°C and 27.6°C respectively. The average maximum temperature ob- tained for each cycle is 26.6±0.7°C, whereas the mini- mum is 19.3±0.7°C. P. gigas belongs to the MPMU1 mollusc group including a stock of thermophilic taxa that no longer exist in the Mediterranean waters. About fifteen species of this stock (named “ Mediterranean Pliocene Tropical Survivors”, MPTSs) are living along the coast of West Tropical Africa south of latitude N 21° (Cape Blanc) (Monegatti & Raffi, 2001), the northern limit of the North- ern Alternance Region of Le Loeuff & Von Cosel (1998). Taking into account that the areas currently populated by these taxa are characterized by SSTs over 24°C for at least five to six months per year and never cooler than 19°C (data from NOAA-CIRES Climate Diagnostic center) and that the MPTS and the most common tropi- cal extinct species were living everywhere in the Medi- terranean Basin before 3.1 MA, an analogous SST pat- tern has been suggested for MPMU1 time interval (Monegatti & Raffi, 2001, 2007; Silva et al., 2006, 2010). Our temperature estimations based on P. gigas isotope composition are in very good agreement with the aforementioned data and strongly support typical tropi- cal conditions in the Mediterranean during the Zanclean- early Piacenzian. This fact confirms the thermophilic character of P. gigas and that its extinction was caused by the first Pliocene climatic cooling at around 3.1 MA. 5. CONCLUSION In this paper we describe the mineralogy and the oxygen isotope composition of a fossil shell of Pelecyora gigas found in a Pliocene sandy layer in central Tuscany (central Italy). P. gigas is an extinct thermophilic species of the Lower Pliocene of the Mediterranean Basin, dis- appearing during the first cooling step at ca. 3 Ma (Monegatti & Raffi, 2001). New data on mineralogy indi- cate that the shell was aragonitic as are most bivalves. Petrography and XRD indicate that the selected speci- men was perfectly preserved and suitable for stable isotope analyses. Neglecting possible changes in sea salinity during different seasons, δ18O values allow cal- culating a maximum seasonal temperature differences during the life of the shell of ca. 9°C. A crude estimation of past sea water composition allows to calculate an average temperature of 23.0±2.7°C, whereas extremes of calculated temperature are 18.5°C for the colder sea- son and 27.6°C for the warmer. These preliminary data indicate that P. gigas would represent an interesting archive to reconstruct seasonal climatic variation during the warmer part of the Pliocene, a period which, to some extent, represents a potential analogues for future pro- jection owing to increasing of atmospheric CO2 concen- tration toward values typical of the Pliocene (Bartoli et al., 2011). ACKNOWLEDGEMENTS This paper is dedicated to the memory of Antonello Bonadonna, for his dedication to Quaternary geology and the paleontology, his important contributions to ge- ology and paleontology will always be remembered. GZ wants to dedicate this paper to his unforgettable master and his political passion. This project is partially funded by: Project PRA (2017/18) “Palaeoclimatic palaeoenvironmental evolu- tion of the Apuan area since the Last Glacial Maximum”. (leader C. Baroni), funded by Pisa University. REFERENCES Aguirre M.L., Leng M.J., Spiro B. (1998) - Variation in isotopic composition (C, O and Sr) of Holocene Mactra isabelleana (Bivalvia) from the coast of Buenos Aires Province, Argentina. The Holocene, 8, 613-621. Bartoli G., Hönisch B., Zeebe R.E. (2011) - Atmospheric CO2 decline during the Pliocene intensification of Northern Hemisphere glaciations. Paleoceanogra- phy, 26, PA4213. Bernasconi M., Robba E. (1993) - Molluscan paleoecol- ogy and sedimentological features: an integrated approach from the Miocene Meduna section, 10 Ragaini L. et al. valves (unionidae). Geochimica et Cosmochimica Acta, 63, 1049-1057. Dodd J.R., Stanton R.J. (1976) - Paleosalinities within a Pliocene bay, Kettlement, California: A study of the resolving power of isotopic and fauna techniques. Geological Society of American Bulletin, 87, 51-64. Dwyer G.S., Chandler M.A. (2009) - Mid-Pliocene sea level and continental ice volume based on coupled benthic Mg/Ca palaeotemperatures and oxygen isotopes. Philosophical Transactions Royal Society A, 367, 157-168. Effenberger H., Mereiter Κ., Zemann J. (1981) - Crystal structure refinements of magnesite, calcite, rhodo- chrosite, siderite, smithonite, and dolomite, with discussion of some aspects of the stereochemistry of calcite type carbonates. Zeitschrift für Kristallog- raphie . 156, 233-244. Epstein S., Buchsbaum R., Lowenstam H. A., Urey, H. C. (1953) - Revised carbonate-water isotopic tem- perature scale. Geological Society of America Bulletin, 64(11), 1315-1326. Erunal-Erentoz L. (1958) - Mollusques du Néogène des Bassins de Karaman, Adana et Hatay (Turquie). Publications de l’Institut d’Etudes et Recherches minières de Turquie, 4, 1-232. Esteban-Delgado F.J., Harper E.M., Checa A.G., Rodri- guez-Navarro A.B. (2008) - Origin and expansion of foliated microstructure in pteriomorph bivalves. Biological Bullettin, 214(2), 153-165. Ferrero E., Merlino B., Provera A., Martinetto E. (2005) - Associazione a molluschi marini e vegetali terrestri del Pliocene di Castellengo, Biella, Italia NW). Rendiconti Società Paleontologica Italiana, 2, 87- 106. Gibbard P.L., Head M.J., Walker M.J.C., and The Sub- commission on Quaternary Stratigraphy (2009) - Formal ratification of the Quaternary System/ Period and the Pleistocene Series/Epoch with a base at 2.58 Ma. Journal of Quaternary Sciences, 25(2), 96-102. Glover E.A., Taylor J.D. (2010) - Needles and pins: acic- ular crystalline periostracal calcification in venerid bivalves (Bivalvia: Veneridae). Journal of Mollus- can Studies, 76(2), 157-179. Grossman E.L., Ku T.L. (1986) - Oxygen and carbon isotopic fractionation in biogenic aragonite: tem- perature effects. Chemical Geology, 59, 59-74. Hall C. (1964) - Shallow-water marine climates and mol- luscan provinces. Ecology, 45, 226-234. Haug G.H., Ganopolski A., Sigman D.M., Rosell-Mele A., Swann G.E., Tiedemann R., Jaccard S.L., Bollmann J., Maslin M.A., Leng M.J., Eglinton G. (2005) - North Pacific seasonality and the glaci- ation of North America 2.7 million years ago. Na- ture, 433, 821-825. Kim S.-T., O’Neil J.R. (1997) - Equilibrium and nonequi- librium oxygen isotope effect in synthetic car- bonates. Geochimica and Cosmochimica. Acta, 61, 3461-3475. Kondo Y. (1987) - Burrowing depth of infaunal bivalves - observation of living species and its relation to shell morphology. Transactions and Proceedings of the Palaeontological Society of Japan, 148, 306- northern Italy. Palaeogeography, Palaeoclimatolo- gy, Palaeoecology, 100, 267-290. Bianchi C.N., Morri C., Chiantore M., Montefalcone M., Parravicini V., Rovere, A. (2012) - Mediterranean Sea biodiversity between the legacy from the past and a future of change. In: Life in the Mediterrane- an Sea: a look at habitat changes. Nova Publisher, 1-55. Bossio A., Costantini A., Lazzarotto A., Liotta D., Mazzanti R., Mazzei R., Salvatorini G., Sandrelli F. (1993) - Rassegna delle conoscenze sulla strati- grafia nel Neoautoctono toscano. Memorie della Società Geologica Italiana, 49, 17 98. Brogi A. (2011) - Bowl-shaped basin related to low-angle detachment during continental extension: The case of the controversial Neogene Siena Basin (central Italy, Northern Apennines). Tectonophys- ics, 459, 54-76. Carmignani L., Decandia F.A., Disperati L., Fantozzi P.L., Kligfield R., Lazzarotto A., Liotta, D. Meccheri M. (2001) - Inner Northern Apennines. In: Gian Battista, Vai, Peter Martini, I. (eds.), Anatomy of an Orogen: The Apennines and Adjacent Mediterra- nean Basins,. Kluwer Academic publications, 197- 214. Carter J.G. (1980) - Guide to bivalve shell microstruc- tures. In: Rhoads, D. C. and Lutz, R. A. (eds.), Skeletal growth of aquatic organism. Plenum Press, New York, 645-670. Carter J.G., Barrera E., Tevesz M.J.S. (1998) - Thermal potentiation and mineralogical evolution in the Bivalvia (Mollusca). Journal of Paleontolology, 72, 991-1010. Cita M.B., Ryan W.B.F. (1979) - Late Neogene environ- mental evolution. Proceedings ODP Initial Reports, 47, 447-460. Coelho A.A. (2018) - TOPAS and TOPAS Academic: an optimization program integrating computer algebra and crystallographic objects written in C++. Jour- nal of Applied Crystallography, 51(1), 210-218. Collareta A., Regattieri E., Zanchetta G., Lambert O., Catanzariti R., Bosselaers M., Covello P., Varola A., Bianucci, G. (2018) - New insights on ancient cetacean movement patterns from oxygen isotope analyses of a Mediterranean Pleistocene whale barnacle. Neues Jahrbuch für Geologie und Paläontologie-Abhandlungen. 288(2), 143-159. Costantini A., Decandia F. A., Lazzarotto A., Liotta D., Mazzei R., Pascucci V., Salvatorini G., Sandrelli F. (2009) - Carta Geologica d’Italia alla Scala 1:50.000, Foglio 296-Siena, Tipografia A.T.I., APAT-Roma, pp. 129. D’Alessandro A., Massari F., Davaud E., Ghibaudo G. (2004) - Pliocene-Pleistocene sequences bounded by subaerial unconformities within foramol ramp calcarenites and mixed deposits (Salento, SE Ita- ly). Sedimentary Geology, 166, 89-144. Dal Negro A., Ungaretti L. (1971) - Refinement of the crystal structure of aragonite. American Mineralo- gist, 56, 768-772. Dettman D.L., Reische A.K., Lohmann K.C. (1999) - Controls on the stable isotope composition of sea- sonal growth bands in aragonitic fresh-water bi- 11 Mineralogy and oxygen isotope of Pelecyora gigas 323. Krantz D.E., Williams D.F., Jones D.S. (1987) - Ecologi- cal and paleoenvironmental information using stable isotope profiles from living and fossil mol- luscs. Palaeogeography, Palaeoclimatology, Pal- aeoecology, 58, 249-266. Lambeck K., Rouby H., Purcell A., Sun Y., Sambridge M. (2014) - Sea level and global ice volumes from the last glacial maximum to the Holocene. Pro- ceedings of the National Academy of Science, 111, 15296-15303. Lawrence K.T., Herbert T.D., Brown C.M., Raymo M.E., Haywood A. M. (2009) - High amplitude variations in North Atlantic sea surface temperature during the early Pliocene warm period, Paleoceanogra- phy, 24, PA2218. Lecointre G. (1952) - Recherches sur le Néogène et le Quaternaire marins de la côte atlantique du Ma- roc. Service Geologique, Notes et Memoires. 99, 1 -198. Le Loeuff P., von Cosel R. (1998) - Biodiversity patterns ofn the marine benthic fauna of the Atlantic coast of tropical Africa in relation to hydroclimatic condi- tions and paleogeographic events. Acta Oecologi- ca, 19(3), 309-321. Leng M. J., Heaton T. H., Lamb H. F., Naggs F. (1998) - Carbon and oxygen isotope variations within the shell of an African land snail (Limicolaria kambeul chudeaui Germain): a high-resolution record of climate seasonality?. The Holocene, 8(4), 407- 412. Lisiecki, L.E., Raymo M.E. (2005) - A Pliocene- Pleistocene stack of 57 globally distributed benthic δ18O records. Paleoceanography, 20, PA1003. Lisiecki, L.E., Raymo M.E. (2007) - Plio-Pleistocene climate evolution: Trends and transitions in glacial cycle dynamics, Quaternary Science Reviews, 26, 56-69. Longinelli A., Tongiorgi E., Trevisan L. (1961) - Oxygen isotopic composition of some Pelecypoda from two Calabrian faunas. Bollettino della Società Paleon- tologica Italiana, 1, 1-6. Malatesta A. (1970) - Malacofauna pliocenica umbra. Memorie per servire alla descrizione della carta Geologica d’Italia, 13, pp. 498. Marasti I., Raffi S. (1980) - Extinction of polysyringian bivalves in the Mediterranean Pliocene. In: AAVV, Volume dedicato a Sergio Venzo, Grafiche STEP Parma, 107-115. Martini I., Aldinucci M. (2017) - Sedimentation and basin -fill history of the Pliocene succession exposed in the Northern Siena-Radicofani Basin (Tuscany, Italy): a sequence-stratigraphic approach. Rivista Italiana di Paleontologia e Stratigrafia, 123(3), 407 -432. Martini I., Aldinucci M., Foresi L.M., Mazzei R., Sandrelli F. (2011) - Geological map of the Pliocene suc- cession of the Northern Siena Basin (Tuscany, Italy). Journal of Maps, 2011, 193-204. Miller K.G., Wright J.D., Browning J.V., Kulpecz A., Kominz M., Naish T.R., Cramer B.S., Rosenthal Y., Peltier R., Sosdian S. (2012) - High tide of the warm Pliocene: Implications of global sea level for Antarctic deglaciation. Geology, 40(5), 407-410. Monegatti P., Raffi S. (2001) - Taxonomic diversity and stratigraphic distribution of Mediterranean Pliocene bivalves. Palaeogeography, Palaeoclimatology, Palaeoecology, 165(3-4), 171-193. Monegatti P., Raffi S. (2007) - Mediterranean-Middle Eastern Atlantic Façade: molluscan biogeography and ecobiostratigraphy throughout the Late Neo- gene. In: Avila S.P., de Frias Martins AM. (eds) Proceedings of the first Atlantic Islands Neogene, Acoreana Supplements, 5, 132-154. Monegatti P., Raffi S. (2010) - The Messinian marine molluscs record and the dawn of the eastern Atlan- tic biogeography. Palaeogeography, Palaeoclima- tology, Palaeoecology, 297, 1-11. Monegatti P., Canali G., Bertoldi R., Albianelli A. (2002) - The classical Late Piacenzian Monte Falcone-Rio Crevalese section (Northern Italy): palynological evidence and biomagnetostratigraphic constraints for climate cyclicity and local moolusc extinctions. Geobios, M.S. 24, 219-227. Mudelsee M., Raymo M.E. (2005) - Slow dynamics of the Northern Hemisphere glaciation, Paleoceanog- raphy, 20, PA4022. Negri A., Amorosi A., Antonioli F., Bertini A., Florindo F., Lurcock P. C., Marabini S., Mastronuzzi G., Regattieri E., Rossi V., Scarponi S., Taviani M., Zanchetta G., Vai G.B. (2015) - A potential Global Stratotype Section and Point (GSSP) for the Ta- rentian Stage, Upper Pleistocene, from the Taran- to area (Italy): Results and future perspectives. Quaternary International, 383, 145-157. Palla P. (1966) - Lamellibranchi pliocenici della Bassa Val d’Elsa. Rivista Italiana di Stratigrafia e Paleon- tologia, 72(2), 397-458. Patterson W.P., Smith G.R., Lohmann K.C. (1993) - Continental paleothermometry and seasonality using the isotopic composition of aragonite otoliths of freshwater fishes. In: Swart, P.K., et al., (eds.), Climatic Change in Continental Isotopic Records, Geophysical Monograph Series., 78, 191-202. Peres J.M., Picard J. (1964) - Nouveau manuel de bio- nomie bentique de la Méditerranée. Recent Tra- vaux Station Marine d’Endoume, 31, 1-137. Petuch E.J. (2004) - Cenozoic seas: the view from East- ern North America. CRC Press, Boca Raton, USA, pp. 328. Pierre C. (1999) - The oxygen and carbon isotope distri- bution in the Mediterranean water masses. Marine Geology, 153, 41-55. Raffi S. (1986) - The significance of marine boreal mol- luscs in the Early Pleistocene faunas of the Medi- terranean area. Palaeogeography, Palaeoclimatol- ogy, Palaeoecology, 52, 267-289. Raffi S., Monegatti P. (1993) - Bivalve taxonomic diversi- ty throughout the Italian Pliocene as a tool for cli- matic oceanographic and stratigraphic inferences. Ciencias da Terra (UNL) 12, 45-50. Raffi S., Stanley S.M., Marasti R. (1985) - Biogeographic patterns and Plio-Pleistocene extinction of Bivalvia in the Mediterranean and Southern North Sea. 12 Ragaini L. et al. Paleobiology, 11, 368-388. Raffi S., Monegatti P., Marasti R. (1989) - East Atlantic molluscan province boundaries and Mediterranean Neogene extinctions. In: Di Geronimo, I.S. (ed.), Atti 3° simposio di Ecologia e Paleoecologia delle Comunita` bentoniche, Catania, 321-332. Rohling E.J., Foster G.L., Grant K.M., Marino G., Roberts A.P., Tamisiea M.E., Williams F. (2014) - Sea-level and deep-sea-temperature variability over the past 5.3 million years. Nature, 508, 477- 482. Sarnthein M., Pflaumann U., Ross R., Tiedemann R., Winn K. (1992) - Transfer functions to reconstruct ocean palaeoproductivity: a comparison. In: Sum- merhayes, C.P., Prell, W.L., Emeis, K.C. (eds.), Upwelling Systems: Evolution since the EarlyMi- ocene, Geologica Society Special Publications, 63, 463-497. Schöne B., Rodland D.,Wehrmann A., Heidel B., Oschmann W., Zhang Z., Fiebig J., Beck L.A. (2007) - Combined sclerochronologic and oxygen isotope analysis of gastropod shells (Gibbula cine- raria, North Sea): life-history traits and utility as a high-resolution environmental archive for kelp forests. Marine Biology, 150, 1237-1252. Shackleton N.J. (1987) - Oxygen isotopes, ice volume and sea level. Quaternary Science Reviews 6, 183 -190. Shakun J.D., Lea D.W., Lisiecki L.E., Raymo M.E. (2015) - An 800-kyr record of global surface ocean δ18Oand implications for ice volume-temperature coupling. Earth and Planetary Science Letters, 426, 58-68. Shimamoto M. (1986) - Shell microstructure of the Ven- eridae (Bivalvia) and its Phylogenetic Implications. Tohoku University Science Reports, 56(1), 1-39. Silva C.M. Da, Landau B.M., Domènech R., Martinell J. (2006) - Pliocene Atlanto-Mediterranean biogeog- raphy of Patella pellucida (Gastropoda, Patelli- dae): Palaeoceanographic implications. Palaeoge- ography, Palaeoclimatology, Palaeoecology, 233 (3), 225-234. Silva C.M. Da, Landau B.M., Domènech R., Martinell J. (2010) - Pliocene Atlantic molluscan assemblages from the Mondego Basin (Portugal): Age and pal- aeoceanographic implications. Palaeogeography, Palaeoclimatology, Palaeoecology, 285(3-4), 248- 254. Stanley S.M. (1970) - Relation to shell form to life habit of Bivalvia. Geological Society American Memoirs, 125, pp. 296. Stanley S.M. (1986) - Anatomy of a regional mass ex- tinction: Plio-Pleistocene decimation of the West- ern Atlantic bivalve fauna. Palaios, 1, 17-36. Stanley S.M., Ruddiman W.F. (1995) - Neogene Ice Age in the North Atlantic Region: Climatic Changes, Biotic Effects, and Forcing factors. In: AAVV, Ef- fects of Past Global Change on Life, National Academy Press, 118-133. Tavani G., Tongiorgi M. (1963) - La fauna miocenica delle Arenarie di Ponsano. Palaeontographia Itali- ca, 58, 1-41. Vergnaud Grazzini C. (1968) - Analyses isotopiques de faunes malacologiques plio-pléistocenes des envi- rons de Pise. Vie et Milieu, Serie B, Océanogra- phie 19(2B), 233-272. Vera-Pelaez J.L., Lozano-Francisco M.C., Muniz-Solis R., Gili C., Martinell J., Domènech R., Palmqvist P., Guerra-Merchan A. (1995) - Estudio preliminar de la malacofauna del Plioceno de Estepona (Málaga, España). Iberus,13, 93-117. Wefer G., Berger W.H. (1991) - Isotope paleontology: growth and composition of extant calcareous spe- cies. Marine Geology, 100, 207-248. White R.M.P., Dennis P.F., Atkison T.C. (1999) - Ex- perimetnal calibration and field investigation of the oxygen isotopic fractionation between biogenic aragonite and water. Rapid Communication in Mass Spectrometry, 13, 1242-1247. Zanchetta G., Leone G., Fallick A.E., Bonadonna F.P. (2005) - Oxygen isotope composition of living land snail shells: data from Italy. Palaeogeography, Palaeoclimatology, Palaeoecology, 223, 20-33. 13 Ms. received: July 20, 2018 Final text received: January 8, 2019 Mineralogy and oxygen isotope of Pelecyora gigas 14