First record of the Valanginian positive carbon isotope anomaly in the Mendoza shelf , Neuquén Basin , Argentina : palaeoclimatic implications *

The Tithonian-Valanginian time interval in the Mendoza Shelf (Neuquén Basin, Argentina) is well exposed in the Río Salado, Puesto Loncoche and Cuesta del Chihuido sedimentary sections. From those localities, more than fifty preserved oyster shells of the genus Aetostreon sp. were selected and sampled in order to perform the first δ13C curves for this particular time interval. Mineralogical and cathodoluminiscence properties, inner micromorphology of the valves, added to major and trace element geochemistry were analyzed in order to highlight the best C-O isotopic preservation. The δ13C isotope curves show values varying between 0 and -3‰ VPDB for the Tithonian-Berriasian basal section, and a positive excursion of ~2.4-2.7‰ VPDB in the Valanginian upper section. This δ13C up section trend is here considered in order to reveal eminent correlations with other sections from the Neuquén Basin, as well as the Weissert Event from the Tethys area, also on the basis of their ammonite faunal zones. The palaeotemperatures obtained from δ18O preserved values, added to a detailed sedimentological study suggest that observed δ13C anomaly may responds to a global climatic change from warm and dry to warm and humid conditions.

The origin and features of this anomaly has been the subject of controversy.Lini et al. (1992) and Erba et al. (2004) link it to an increase in atmospheric CO 2 as a result of volcanism (especially the basalts of the Paraná-Etendeka emissions), which would have led to global warming.This hypothesis is supported by the mineralogical studies of Duchamp-Alphonse et al. (2011), who analyzed an increased kaolinite content related to a warm, humid climate in sedimentary deposits.On the other hand, Van de Schootbrugge et al. (2000), Pucéat et al. (2003), and Price and Mutterlose (2004) propose a cold climate for the anomaly, based on palaeotemperatures derived from δ 18 O data in belemnites and fish tooth enamels.Such hypothesis is supported by glendonites (Kemper, 1987;Tarduno et al., 2002), dropstones, as well as by the nature of calcareous nannofossils (Melinte and Mutterlose, 2001).Erba et al. (2004) link the positive δ 13 C excursion with the presence of black shales rich in organic carbon in the Southern Alps and in the Pacific Ocean, reinterpreting the Valanginan anomaly as an ocean anoxic event called the Weissert Event.Westermann et al. (2010) propose that the anoxic conditions would be circumscribed only to a few regions.Recently, Föllmi (2012)  change in climate conditions from arid to humid in a warm context, classifying the Weissert Event as an episode of environmental change.
The combination of a decrease in shallow-marine carbonate production coupled with the enhanced burial of organic matter on the continent has been suggested to explain the global positive carbon excursion (Westermann et al., 2010).Enhanced detrital and dissolved continental fluxes to the ocean probably boosted marine primary productivity in marginal marine and epicontinental settings (Föllmi, 1995;Duchamp-Alphonse et al., 2007).
The main objective of this contribution is to provide the first record of δ 13 C data from the Tithonian-Valanginian interval of the Mendoza Shelf (Neuquén Basin), with special emphasis on the positive excursion of the Valanginian.The palaeogeographic position of the section is of particular interest, since it was deposited in a near-shore setting.Palaeotemperature data obtained from δ 18 O in calcareous fossils is also shown.Moreover, the integration of stable isotope data and the analyzed sedimentological aspects of the outcrops studied can contribute to the understanding of the climatic context of the Valanginian carbon excursion registered globally.

Neuquén Basin
The Neuquén Basin is located in west-central Argentina, and it constitutes the main oil basin in the country (Fig. 1).Two main depositional areas can be recognized in the Neuquén Basin: the Neuquén Embayment to the south, and the narrow Mendoza Shelf to the north (Doyle et al., 2005).The development of the Mendoza Shelf is favoured by the obliquity between the Andean arc and the axis of the basin, which is oriented NW-SE, and it is characterized by a significant reduction in the width of the basin (Spalletti et al., 2000;Doyle et al., 2005).In this area, an important sedimentary succession (4,000 m) was deposited during the Upper Jurassic-Lower Cretaceous interval, initially called "Mendociano" by Groeber (1947) and currently known as "Mendoza Group".It comprises the Vaca Muerta, Chachao and Agrio formations of the Mendoza Shelf (Mombrú et al., 1976) (Fig. 2).
The Vaca Muerta Formation was originally defined by Weaver (1931) to designate a set of Tithonian layers consisting of dark grey calcareous shales (Spalletti et al., 2015).This unit represents the stratigraphic succession of the Mendoza Group with the largest extension, the highest degree of lithological uniformity and continuity and of huge regional economic importance due to its potential oil and phosphate content (Leanza et al., 1977).The Vaca Muerta Formation is underlain by the continental deposits of the Tordillo Formation, and the contact between both formations is marked by the Tithonian transgression (Leanza, 1981).In the Mendoza Shelf, overlying the Vaca Muerta Formation, marine carbonates and shales of the Chachao Formation are recorded.The Vaca Muerta FIG. 2. Chronostratigraphic scheme of the Neuquén Basin (Upper Jurassic-Lower Crateceous).Based on Groeber (1946) and Gulisano and Gutiérrez Pleimling (1994).
First record of the Valanginian positive carbon isotope anomaly in the Mendoza shelf, Neuquén...

Geological setting of the studied dataset
Fieldwork was carried out in three localities of the Mendoza Shelf, near the city of Malargüe (Fig. 1): Río Salado, Puesto Loncoche and Cuesta del Chihuido.In these localities, the sedimentary succession studied is composed of mixed ramp deposits (Mitchum and Uliana, 1986;Legarreta and Uliana, 1991;Spalletti et al., 2000;Kietzmann et al., 2008; among others) of the Vaca Muerta and Chachao formations (Tithonian-Valanginian).

Río Salado area
In the Río Salado area (35°11' S and 69°46' W), a mixed succession of sedimentary rocks with a vertical coarsening-upward trend has been described (Doyle et al., 2005).This unit consists of a 176-m-thick succession composed of an alternation of black shales and carbonates.Carbonates are arranged in 10-70-cm-thick layers composed of bioclastic wackestones, packstones, floatstones and rudstones (Fig. 3).The formational boundaries and environmental characterization of these deposits were proposed by Doyle et al. (2005).
The Vaca Muerta section at Río Salado area is characterized by the presence of black shales interbedded with marls and scarce wackestones and packstones near of the base.Based on these lithological attributes and the recognized trace fossil associations, Doyle et al. (2005) interpreted this section as the result of sedimentation under an anoxic to suboxic seafloor in a basinal environment.This siliciclastic and carbonate particles may have been deposited from suspension under an anoxic to suboxic seafloor, with evidences of storm reworking at the top.The middle interval consists of black shales, marls, heterolithic beds and packstones, with primary sedimentary structures (such as lamination and hummocky cross-stratification), and deformational structures (water escape features, ball-and-pillow), all this features strongly suggest deposits of a distal outer ramp system.The upper part of Vaca Muerta Formation is characterized by black shales and marls, marking a restoration of the basinal conditions.Meanwhile, the Chachao Formation consists of floatstones, rudstones, shales and marls.The skeletal carbonates are interpreted as oysters shell beds that developed in a proximal outer ramp (Doyle et al., 2005).

Puesto Loncoche area
In the Puesto Loncoche area (35°36' S and 69°37' W), the studied deposits were previously described in detail by Kietzman et al. (2008).The same vertical coarsening-upward trend observed in the Río Salado section is recognized here, accompanied by a thickening-upward arrangement that culminates in a 25-m-thick oyster bank, corresponding to Chachao Formation (Fig. 3).
A detailed palaeoenvironmental analysis of the Vaca Muerta Formation in the Puesto Loncoche area proposed by Kietzmann et al. (2008) divides the unit, from base to top, into four sub-environments: basin, distal outer ramp, proximal outer ramp and distal middle ramp.The basin sub-environment is composed of black shales, massive marls and siltstones, together with laminated mudstones and wackestones deposited by decantation in anoxicsuboxic environments.The distal outer ramp consists of black shales, marls, nodular mudstones and bioclastic wackestones (massive and laminated).The fine-grained siliciclastic and mixed siliciclastic carbonate deposits were essentially accumulated from suspension fall out.The outer ramp bioclastic wackestones represent distal storm-induced layers.In the proximal outer ramp, in addition to the association described previously, the above mentioned authors identified floatstones and packstones interpreted as proximal tempestites.Finally, the distal middle ramp is composed by an association of suspension fall out deposits (shales, marls, nodular mudstones) and storm induced deposits (f loatstones, massive bioclastic packstones and wackestones).Subsequently, the palaeoenvironmental interpretations were later reinforced with taphonomic, biofacies and sequence stratigraphic studies (Kietzmann and Palma, 2009;Kietzmann et al., 2014;Kietzmann et al., 2015).The Chachao Formation is represented, in this area (Fig. 3), by an inner ramp environment, constituted by a coarse oyster bioclastic buildup of rudstones and floatstones, which shows vertical accumulation and a lateral stacking pattern after described by Palma and Lanés (2001).Within these bodies, slight signs of reworking were recognized from the original oyster banks, and can be characterized as an autoparabiostrome according to Kershaw (1994).Palma and Lanés (2001) performed a taphofacies analysis of carbonate bodies of the Chachao Formation, describing patterns of accumulation and dividing this unit into two sections: the lower section, with a low degree of reworking of the oyster beds, and the upper section, with a progressive increase in the intensity of disarticulation, bioerosion and fragmentation of the remains, accompanied by an increase in the matrix content.

Cuesta del Chihuido area
In the 63-m-thick Cuesta del Chihuido study section (35°44' S and 69°34' W), only the top of the Vaca Muerta Formation and the entire Chachao Formation crop out (Kietzmann et al., 2015).This section shows vertical arrangement, similar to the one described in Puesto Loncoche (Fig. 3).The Vaca Muerta Formation was studied in this region by Kietzmann et al. (2015), the cusp deposits were described as an oyster intercalation forming autoparabiostromes composed of bioclastic floatstones and rudstones, bioturbated wackestones, packstones, laminated wackestones and marls.This section is interpreted as a bioclastic middle ramp to proximal outer ramp, and an oyster biostrome dominated middle ramp.

Sampling
Three sedimentary sections from three selected areas were studied in detail in order to recognize the sedimentary facies, fossil content and their palaeoenvironmental features.Fifty-seven (57) samples of fossil oysters (11 from Río Salado, 29 from Puesto Loncoche and 17 from Cuesta del Chihuido) were selected from sedimentary logs, taking as the main criteria the sampling equidistance (subject to fossil presence) and the degree of fossil preservation.Samples of Aetostreon sp. were selected to obtain accurate C and O isotope data and also to avoid the variations due to the "vital effect" (Fig. 4).Moreover in order to constrain these data with previous results obtained from samples of the same genus in other localities of the Neuquén Basin (Aguirre- Urreta et al., 2008).
Samples were carefully selected based on their mesoscopic features.The best criteria for choosing the fossil material were the taphonomic characteristics (avoiding material which shows a high degree of fragmentation and corrasion), and the size (microscopic observations showed that the larger individuals had better preservation of the internal layers); therefore, specimens larger than 5 cm were selected.

Petrography
Chips from the innermost oyster shell were covered with Au and analysed by SEM-EDS with a FEI Quanta 200 SEM (Laboratorio de Investigaciones de Metalurgia Física, Universidad Nacional de La Plata (UNLP), Argentina) with the objective of recognizing the original microtextures.In addition, sixteen thin sections in dorsoventral position were analysed under cathodoluminescence CITL Technosyn MKIII microscope of the Centro de Investigaciones Geológicas (CONICET-UNLP).Manganese emits a characteristic dull to bright luminescence during cathodic excitation of the calcitic shell layers and induces a typical orange colour, indicative of diagenetic alteration, while primary calcite sectors appear neither luminescent nor blue (Ullmann and Korte, 2015;Marshall, 1992).

Geochemistry
Twenty-eight elemental analyses were performed in ~50 mg of powdered oyster samples (12 from Puesto Loncoche, 6 from Río Salado and 10 from Cuesta del Chihuido).The dissolution of the carbonate phase (5% HNO 3 solution for 2-3 h) was analyzed by ICP-MS, using a Perkin-Elmer ICP-MS fitted with a Meinhardt concentric nebulizer of the Centro de Investigaciones Geológicas geochemistry laboratories (CONICET-Universidad Nacional de La Plata) to determine Mn and Sr concentrations in ppm.Precision and reproducibility for all elements analyzed are better than 10%, based on replicate measurements of laboratory calcite and dolomite standards.
Fifty-seven microsamples of 0.1 gr of oysters (one sample per oyster) were taken with a microdrill.The microsamples were analyzed for C and O isotopes (29 from Puesto Loncoche, 11 from Río Salado and 17 from Cuesta del Chihuido).These analyses were performed at the Núcleo de Estudos Geoquímicos-Laboratório de Isótopos Estáveis (NEG-LABISE) of the Departamento de Geología, Universidade Federal de Pernambuco, Brazil.Extraction of CO 2 gas from selected unaltered samples was performed in a high-vacuum line after reaction with 100% orthophosphoric acid at 25 °C for one day.Released CO 2 was analyzed after cryogenic cleaning in double inlet, triple-collector SIRA II or Delta V Advantage mass spectrometers and results are reported in δ notation in per mil (‰) relative to the VPDB standard.The uncertainties of the isotope measurements were better than 0.1‰ for carbon and 0.2‰ for oxygen, based on multiple analyses of an internal laboratory standard (BSC).The δ 13 C and δ 18 O values obtained were calibrated against the internationally accepted International Atomic Energy Association carbonate standard NBS-19.

Palaeotemperatures
Palaeotemperature values obtained from the calcite samples were estimated using the modified version of the Epstein palaeotemperature equation (Epstein et al., 1953), given by Anderson and Arthur (1983), and then shown by Pirrie et al. (2004) as: T(°C)=16.0-4.14(δc-δw)+0.13(δc-δw) 2 In this equation, δc=δ 18 O (VPDB) of the analyzed carbonate at 25 °C, and δw=δ 18 O (SMOW) of the water in which the carbonate precipitated, relative to the Standard Mean Ocean Water international standard.While δc is measured, δw has to be estimated as -1.2‰, the global average for periods of limited or no glaciation (Shackleton and Kennett, 1975).The δ 18 O values higher than -5‰ VPDB were considered appropriate to estimate palaeotemperatures as seen in similar works (e.g., Zakharov et al., 2011).Those higher values indicate a low degree of diagenetic alteration (Brand and Veizer, 1981) and are therefore more accurate to show original features.First record of the Valanginian positive carbon isotope anomaly in the Mendoza shelf, Neuquén...

Results
From the photomicrographs obtained by SEM, oysters show foliated growth in layers with smoothtextured surfaces (Fig. 5).Cathodoluminescence photomicrographs make it possible to distinguished luminescent and non-luminescent zones (Fig. 6).Non-luminescent areas coincide with the growth of a foliated type of oysters, and were particularly selected for isotope microsampling.
Manganese, Sr, δ 13 C, and δ 18 O values are shown in Table 1.The Mn/Sr ratio is below 1.5 (Table 1), except for three samples (PL88, CDC35 and CDC38, excluded from further interpretation), which are as high as Mn/Sr=6 and are considered as probably being diagenetically altered.Strontium concentrations vary between 300 and 700 ppm, very close to the data recorded by Korte et al. (2009) and Korte and Hesselbo (2011) for Jurassic oysters, with anomalously low values of up to 135 ppm.
Manganese concentrations reach 100 ppm on average, with some values of up to 282 ppm, which are also similar to those obtained by Korte et al. (2009) and Korte and Hesselbo (2011) for Jurassic oysters.
with those of foliated growth, indicate a better degree of preservation, as mentioned above.
The data obtained from Mn and Sr showed there was no relative Sr impoverishment or Mn enrichment produced by diagenetic alterations in most of the samples, with three exceptions (samples PL 88, CDC 35 and CDC 38, Table 1) which show higher values of Mn, and depleted Sr, which can be associated with probable diagenetic alteration.Furthermore, the degree of correlation between the different variables was calculated considering the values of stable isotopes and Mn and Sr concentrations (Fig. 7 and Table 2).
The lack of covariation between δ 13 C and Mn/Sr (Fig. 7a and Table 2) and δ 13 C versus Sr (Fig.   First record of the Valanginian positive carbon isotope anomaly in the Mendoza shelf, Neuquén... and Table 2) for the three localities studied suggest a high degree of isotopic preservation for the stable carbon isotopes (Marshall, 1992).Furthermore, the diagram of δ 18 O versus Mn shows no correlation in the Puesto Loncoche and Cuesta del Chihuido localities, while in the Río Salado area it shows a slight positive correlation (Fig. 7c and Table 2).

Chemostratigraphy
The δ 13 C values were represented in curves parallel to the sedimentary logs in order to compare the segments of the same age from the three localities under study (Fig. 8).The δ 13 C curve for the Río Salado sector shows values obtained from the uppermost strata of the Chachao Formation, taking into consideration the presence and preservation of the fossil samples.In Puesto Loncoche, the δ 13 C curve represents the complete section; however, data from the basal section of the Vaca Muerta Formation is not represented due to the absence of fossils of the genus Aetostreon sp.In Cuesta del Chihuido, the δ 13 C curve represents the complete section of the Chachao Formation and the upper part of the Vaca Muerta Formation.The δ 13 C values from the early Tithonian (V.mendozanus Ammonite Zone) to the early Valanginian (N.wichmanni Ammonite Zone) range from slightly negative to ∼0‰, with a single value that reaches 1‰.In the early Valanginian-early late Valanginian succession (O.atherstoni Ammonite Zone), a marked positive excursion in δ 13 C values is recognized throughout the three studied sections, ranging between 0.94 and 2.53‰ (Río Salado), from 1.16 to 2.66‰ (Puesto Loncoche) and from 0.91 to 2.13‰ VPDB (Cuesta del Chihuido), with a decrease towards the upper sedimentary section (top of the Chachao Formation).
Estimated seawater palaeotemperatures, obtained from the δ 18 O values (Table 1, Fig. 9), are on average 25 °C (26 °C in Río Salado, 25 °C in Puesto Loncoche and 25 °C in Cuesta del Chihuido); this value coincides with those specified for the Neuquén Basin by Lazo et al. (2008), based on isotopic studies in fossil oysters of the same genus, and Lazo et al. (2005), on the basis of fossiliferous associations of the Pilmatué Member of the Agrio Formation located to the south of the basin, in Province of Neuquén.

Discussion
The C and O-isotope chemostratigraphic curves compare variations from three different sections of the Chachao Formation (Fig. 7), Mendoza shelf area, which are well correlated with previous data presented by Aguirre-Urreta et al. (2008) andGómez Peral et al., (2012) et al. (2012) obtained a δ 13 C curve with values that on average are of +1.6‰ with a positive excursion of +2.9‰.This analysis of oysters and micrite corresponds to deposits from Buta Ranquil (37º06'00''-69º49'00'') characterized by the subzone of K. attenuatum (Schwarz et al., 2011).Therefore, the positive carbon anomaly recorded in O. arthestoni Zone deposits is clearly established in different areas of the Neuquén Basin, from the Mendoza Platform (this study) to the central area of the basin (Aguirre- Urreta et al., 2008;Gómez Peral et al., 2012).
The isotopic results indicate an average seawater palaeotemperature of 25º C, indicating a warm context for the Tithonian-Valanginian interval in the Neuquén Basin, which is consistent with data provided by Lazo et al. (2005Lazo et al. ( , 2008) ) for other locality in the same basin.Duchamp-Alphonse et al. (2011) also proposed warm temperatures based in studies of kaolinite content of Valanginian sequences.Lini et al. (1992) and Erba et al. (2004) arrived at the same conclusion but linked to increase in pCO 2 from volcanism, especially the Paraná-Ethendeka continental flood-basalts.In the analyzed sections there is no evidence of vertical changes in temperature, so the enrichment in the δ 13 C ratio to the top of the succession cannot be related to sudden changes in temperature.
Furthermore, black shales rich in organic matter or other evidence of anaerobic conditions are absent in the studied sections, which make to avoid a connection of the δ 13 C anomaly with an ocean anoxic event (OAE).
Föllmi (2012) postulated that the δ 13 C anomaly was in response to a change in the Cretaceous climate from warm and dry to warm and wet, which had occurred in the mid-Valanginian (Busnardoites campylotoxus Zone-Saynoceras verrucosum Zone, in the Tethys area).Among other factors, this variation in humidity regime would have produced a significant increase in the detrital input from the continent to the sea, with the consequent drowning of the carbonate platform (Westermann et al., 2010).The fact that the δ 13 C anomaly is located just on the top of the buildup of Chachao Formation, reinforce the approach that this ultimate demise was in response to the climatic change suggested by Föllmi (2012).In this sense, Gómez Peral et al. (2012) showed a C isotope curve for Mulichinco Formation, located in a deeper portion of the basin (north of Neuquén).They place the Valanginian positive δ 13 C anomaly in the top of the unit, which coincides with the basal siliciclastic parasequence immediately above the carbonate bodies that characterize the middle section of Mulichinco Formation.Furthermore, it is noticeable that the positive δ 13 C excursion in the Neuquén Basin occurs as a regional event, which can be correlated with an increase in the detrital supply to the marine system, and as consequence this cause the cease in the development of the carbonate deposits.Therefore, it should not be ruled out, this event reflects a change towards higher humidity conditions as suggested by Föllmi (2012).
On the other hand, in the studied sections of the basin represented in the Mendoza Shelf area, the δ 13 C anomaly shows no correlation with a facies change.In this case, the anomaly is recorded at the top of the biogenic carbonate bodies of the Chachao Formation and it is considered as signal of its imminent collapse.

Conclusions
Based on the foregoing, it can be concluded that: The samples of Aetostreon sp.selected for C-O isotope analysis show a high degree of preservation validated by geochemical, cathodoluminescence and scanning electron microscopy studies.
The water palaeotemperature estimations for the Mendoza Shelf in the Neuquén Basin area during the Tithonian-Valanginian indicate a warm period, with seawater temperatures around ∼25 °C on average, and sudden temperature changes are not recorded.
The occurrence of the well-established mid-Valanginian positive δ 13 C excursion is documented, and it is recognized in the upper section of the Chachao Formation and characterized by the O. atherstoni Ammonite Zone in the Río Salado, Puesto Loncoche and Cuesta del Chihuido areas.
The positive δ 13 C anomaly can be temporally correlated with the previously defined chemostratigraphic studies in other localities of the Neuquén Basin and can be also related to the event defined for the Tethys area.
The absence of black shale deposits or other indications of anaerobic conditions in Chachao Formation make it impossible to link the anomaly with an ocean anoxic event in the Mendoza Shelf.First record of the Valanginian positive carbon isotope anomaly in the Mendoza shelf, Neuquén...
The Chachao Formation, especially the upper section constituted by thick carbonates with a wide faunal diversity, can be correlated with the contemporary deposits of Buta Ranquil, where a marked decrease in carbonate content with the consequent drowning of the carbonate platform can be observed.
Finally, it can be concluded that the Upper Chachao Formation may constitute a scenario linked to an episode of environmental change, in which the positive δ 13 C excursion must be related to a change in a warm context from arid to humid conditions.
FIG. 1.To the left, the Neuquén Basin, located in west-central Argentina.Record of the Weissert Event in the Neuquen Basin: Cerro La Parva area (Aguirre Urreta et al., 2008), Buta Ranquil area (Gómez Peral et al., 2012 ).To the right, the study area, with the location of the studied sections: Río Salado, Puesto Loncoche and Cuesta del Chihuido.

FIG. 4 .
FIG. 4. Selected samples of Aetostreon sp. with a high degree of preservation.Notice the low grade of corrasion on external walls, this taphonomic feature suggest a better conservation of the inner layers.

FIG. 5 .
FIG. 5. Scanning electron microscopy images of oysters from the Puesto Loncoche sedimentary succession.It is possible to observe the "smooth textured" surfaces.

FIG. 6 .
FIG. 6. Cathodoluminescence microscope images of oyster samples.Luminescent (right) and non-luminescent (left) areas are recognized;the non-luminescent areas match the foliated growth of oysters. 7b

FIG. 8 .
FIG. 8. Sedimentary sections with the δ 13 C curves of the three areas under study (Río Salado in green, Puesto Loncoche in red and Cuesta del Chihuido in blue).Towards the top of the sequence, a carbon positive excursion can be recognized.