Maurice Renard, Marc De Rafélis, Laurent Emmanuel, Catherine Beltran, Michel Moullade, Guy Tronchetti - PDF

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Fluctuations of sea-water chemistry during Gargasian (Middle Aptian) time. Data from trace-element content (Mg, Sr, Mn, Fe) in hemipelagic carbonates from La Marcouline Quarry (Cassis, SE France) Maurice

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Fluctuations of sea-water chemistry during Gargasian (Middle Aptian) time. Data from trace-element content (Mg, Sr, Mn, Fe) in hemipelagic carbonates from La Marcouline Quarry (Cassis, SE France) Maurice Renard, Marc De Rafélis, Laurent Emmanuel, Catherine Beltran, Michel Moullade, Guy Tronchetti To cite this version: Maurice Renard, Marc De Rafélis, Laurent Emmanuel, Catherine Beltran, Michel Moullade, et al.. Fluctuations of sea-water chemistry during Gargasian (Middle Aptian) time. Data from trace-element content (Mg, Sr, Mn, Fe) in hemipelagic carbonates from La Marcouline Quarry (Cassis, SE France). Carnets de Geologie, Carnets de Geologie, 2007, CG2007 (A03), pp hal HAL Id: hal https://hal.archives-ouvertes.fr/hal Submitted on 24 Jul 2007 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Fluctuations of sea-water chemistry during Gargasian (Middle Aptian) time. Data from trace-element content (Mg, Sr, Mn, Fe) in hemipelagic carbonates from La Marcouline Quarry (Cassis, SE France) Maurice RENARD 1,2 Marc de RAFÉLIS 1 Laurent EMMANUEL 1 Catherine BELTRAN 1 Michel MOULLADE 3,4 Guy TRONCHETTI 3 Abstract: In the Lower Aptian historical stratotype area (Cassis-La Bédoule, SE France), a geochemical study of the Gargasian (Middle Aptian) marl-limestone alternations of the La Marcouline quarry complements data already obtained from Bedoulian (Early Aptian) sediments there. Nannoconids are the main carbonate producers in both limestones and marls. Although diagenetic minerals, such as ankerite (2.5%) are present in small amounts, the trace-element content of bulk carbonate is very close to that of Nannoconus spp. so geochemical sequences can be defined. The long-term evolution of trace-element content was not affected by diagenetic processes, variations in carbonate mineralogy, or a change of carbonate producers. An increase of around 500 ppm in the strontium content of bulk carbonate occurs between the base of the Cabri zone (late Bedoulian) and the Algerianus zone (late Gargasian). This evolution is linked to fluctuations in seawater Sr/Ca ratios caused by variability in the influx of hydrothermal and river waters, by changes in the ratio of aragonite/calcite production and by shifts in sea level. The eustatic sequence Aptian 4, its parasequences and its key surfaces (sequence boundaries, maximum flooding surface) are clearly reflected in the evolution of the bulk-carbonate contents of manganese. Key Words: Aptian; Gargasian; pelagic carbonates; magnesium; strontium; manganese; iron; Nannoconus; seawater Sr/Ca; chemostratigraphy; sequence stratigraphy Citation : RENARD M., RAFÉLIS M. de, EMMANUEL L., BELTRAN C., MOULLADE M. & TRONCHETTI G. (2007).- Fluctuations of sea-water chemistry during Gargasian (Middle Aptian) time. Data from trace-element content (Mg, Sr, Mn, Fe) in hemipelagic carbonates from La Marcouline Quarry (Cassis, SE France).- Carnets de Géologie / Notebooks on Geology, Brest, Article 2007/03 (CG2007_A03) Résumé : Fluctuations de la chimie de l'eau de mer au cours du Gargasien (Aptien Moyen). Apports des teneurs en éléments traces (Mg, Sr, Mn et Fe) des carbonates hémipélagiques de la carrière de La Marcouline (Cassis, Sud-Est France).- Dans la région du stratotype historique de l'aptien inférieur (Cassis-La Bédoule, SE France), les sédiments alternants du Gargasien (Aptien supérieur) de la carrière de la Marcouline ont été étudiés du point de vue géochimique. Le dosage des éléments traces (Sr, Mg, Mn et Fe) du carbonate total a permis de compléter les données déjà obtenues sur le Bédoulien et de définir une zonation chimiostratigraphique du Gargasien. Les Nannoconus sont les producteurs carbonatés principaux aussi bien dans les bancs calcaires que dans les bancs marneux. Les différences géochimiques observées entre les marnes (enrichies en Sr et Mg) et les calcaires (enrichis en Mn et Fe) ne peuvent être réduites à une variation des producteurs ou à une 1 Université P. et M. Curie, JE 2477 Biominéralisations et Paléoenvironnements et CNRS FR32, Case 116, 4 place Jussieu, Paris Cédex 05 (France) 2 3 Université de Provence, Centre de Sédimentologie & Paléontologie, CNRS UMR 6019, Centre Saint-Charles, Marseille Cedex 3 (France) 4 Museum d'histoire naturelle de Nice, 60 bd Risso, Nice (France) Manuscript online since July 2, diagenèse différentielle. En dépit de la présence de traces d'ankérite d'origine diagénétique (2,5%), l'enregistrement des éléments-traces au sein du carbonate total reste très proche de celui des fractions pures en Nannoconus et permet de définir les séquences géochimiques. Dans la continuité du processus initié dans le Bédoulien supérieur (base de la zone à Cabri), les teneurs en strontium croissent régulièrement durant le Gargasien. Au total une augmentation de l'ordre de 500 ppm survient entre la base de la zone à Cabri (Bédoulien supérieur) et la zone à Algerianus (Gargasien supérieur). Elle traduit une variation du rapport Sr/Ca de l'eau de mer, à mettre en relation avec le bilan des apports hydrothermaux et fluviatiles, le rapport de la sédimentation aragonitique à la sédimentation calcitique et les variations du niveau marin. Les teneurs en manganèse ont permis de mettre en évidence la séquence eustatique Aptien 4 et les paraséquences qui la composent, ainsi que de localiser ses surfaces clefs (limites de séquence et surface d'inondation maximale). Mots-Clefs : Aptien ; Gargasien ; carbonates pélagiques ; magnesium ; strontium ; manganèse ; fer ; Nannoconus ; Sr/Ca de l'eau de mer ; chimiostratigraphie ; stratigraphie séquentielle Introduction MOULLADE & TRONCHETTI (2004) and MOULLADE et alii (2004) described the general setting of La Marcouline quarry, where the succession of Gargasian (Middle Aptian) beds is exposed continuously, and they specified its lithostratigraphic and biostratigraphic relationships to the Bedoulian (Lower Aptian) of the Cassis-La Bédoule area. Both the marls and the limestones of the Gargasian beds of La Marcouline quarry were sampled and analyzed for stable carbon and oxygen isotopes (see KUHNT & MOULLADE, 2007) and for the trace elements in bulk carbonate (see results and discussion below). Samples were washed in distilled water, crushed and then dissolved in acetic acid (1N). Trace elements were analyzed by atomic absorption (Hitachi Z8100 spectrometer) using the method described by RENARD & BLANC (1971; 1972) and RICHEBOIS (1990). Analytical accuracy is around 5%. All data are listed in Appendix 1. Gargasian sequences in La Marcouline section are defined geochemically and labeled in continuity with the Bedoulian ones as proposed in the same geographic area by RENARD & de RAFÉLIS (1998). Results 1- CaCO 3 contents and relationship with bedding and stratonomy In La Marcouline quarry, Gargasian sedimentation is of a hemipelagic type in which marl and marly limestone deposits alternate regularly. Contacts between the two lithologies are transitional. Overall, the thickness of limestone beds relative to marly ones decreases upward (Fig. 1). A parasequence pattern is superimposed on this general trend and there are nine parasequences of momentary thickening upward in limestone beds: Parasequence S1: Beds 1 to 6 (incomplete sequence), Parasequence S2: Beds 7 to 14 (the upper boundary of this sequence corresponds to the transition between the Cabri (Luterbacheri) and Ferreolensis zones (MOULLADE et alii, 2005), Parasequence S3: Beds 15 to 26 (this parasequence ends with a well-defined triplet of limestone: beds 22, 24, 26), Parasequence S4: Beds 27 to 31 Parasequence S5: Beds 32 to 38 (this sequence includes the Ferreolensis/Barri zone boundary), Parasequence S6: Beds 39 to 52, Parasequence S7: Beds 53 to 64, Parasequence S8: Beds 65 to 86 (this sequence includes the Barri/Algerianus zone boundary), Parasequence 9: Beds 87 to 90 (the first steps of the last visible parasequence). We note that the thickness of each parasequence increases upward throughout the observed Gargasian: Parasequence 2 occupies 2.5 m whereas parasequence 8 spans 11 m. Figure 1: Lithology, sequence pattern and CaCO 3 content in the Gargasian beds of La Marcouline quarry. Blue line and circles: CaCO 3 contents in the marls (soft beds) ; red line and circles: CaCO 3 contents in the limestones (hard beds). Biostratigraphic scales from (1) ROBAZYNSKI & CARON (1995) and MOULLADE et alii (2002) and (2) from MOULLADE et alii (2005). 2 3 The CaCO 3 content of each bed is in accord with the marl/limestone alternations as delineated in the outcrop scale (Fig. 1). The CaCO 3 content of the limestone beds ranges from 64.4% to 87.5% (mean 77.6%) and the CaCO 3 content of the marly beds from 53% to 77.4% (mean 62.8%). In both limestones and marls, the CaCO 3 content decreases upward in the section (more markedly in parasequences 6 through 9). A comparison with Bedoulian data (Fig. 2) from the same area (RENARD & de RAFÉLIS, 1998; RENARD et alii, 2005) shows that the CaCO 3 content of the Gargasian beds at La Marcouline is intermediate between that of the strata of early Bedoulian age (Kuznetsovae and Blowi foraminiferal zones) and that of the sequence of late Bedoulian age (Cabri zone). CaCO 3 content appear to be more variable during the Gargasian than during the Bedoulian. This pattern is caused by the fact that in the Bedoulian interval only limestones were sampled whereas in the Gargasian both limestones and marls were collected. Figure 2: Evolution in the CaCO 3 content of Bedoulian and Gargasian sediments from the Cassis area (La Bédoule, Les Tocchis and La Marcouline). 4 The difference in the amount of CaCO 3 in limestone and marl is not constant throughout the Gargasian succession (Fig. 1). The mean difference is about 15% but values range from 30.2% (between beds 25 and 26) to 0.4% (between beds 85 and 86). Sequences with a markedly greater amount of CaCO 3 in limestone over that in marl (beds 4-5, 23 to 26, 53-54, 78-79) alternate with sequences in which the difference is slight (beds 7-8, 27 to 33, 39-40, and 82 to 86). In these successions, the hard beds , called limestones in outcrop, may have a CaCO 3 content nearly the same as or even lower than the soft beds (i.e. marls ). These fluctuations define 5 sequences that start with small differences in the CaCO 3 content of the two rock types and are more or less related to the pattern of the stratonomic parasequences described above (Fig. 1): CaCO 3 sequence 1 encompasses stratonomic sequences 2 and 3, CaCO 3 sequence 2 subsumes stratonomic sequences 3 to 5, CaCO 3 sequence 3 includes stratonomic sequences 6 and 7, CaCO 3 sequence 4 takes in stratonomic sequence 8, and CaCO 3 sequence 5 corresponds with stratonomic sequence 9. BELTRAN (2006) and KUHNT & MOULLADE (2007) show that these fluctuations in CaCO 3 content have a MILANKOVITCH-like frequency. This finding led to their interpretation as reflections of oscillations in the orbit of the earth that affect climate and in turn the rate and characteristics of sedimentation. BELTRAN (2006) and BELTRAN et alii (2007) have carried out a detailed sedimentological study focused on the interval spanning beds 29 to 32. In this interval the compositions of marl and limestone beds are very similar in terms of carbonate particles and type of producers (Fig. 3). The carbonate fraction is composed of a heterogeneous assemblage of calcareous nannofossils made up of coccoliths (around 8%, mainly Watznauearia barnesae and few Rhagodiscus spp., Biscutum spp. and Zeugrhabdotus spp.), and nannoconids (around 20-25%), a small quantity of planktonic foraminiferal debris, structureless carbonate particles, i.e. carbonate macrocrystals (around 8%), and calcitic microcrystals (around 60%). Structureless carbonate particles of uncertain origin Calcareous Nannofossils Ankerite macro- Calcite macro- Micarbs particles particles Coccoliths Nannoconus sp. 8 Marls Watznaueria sp. Rhagodiscus sp. Biscutum sp. Others Fragments Limestones Watznaueria sp. Rhagodiscus sp. Biscutum sp. Others Fragments Figure 3: Mean composition of carbonate fraction in marls and limestones (volume %, from BELTRAN, 2006). Carbonate macrocrystals (8.5% of the carbonate fraction) range in size from 12 to 5 µm and their mineralogic composition includes calcite (6%) and ankerite (2.5%). MINOLETTI et alii (2005) described the same kind of macrocrystals in the Late Cretaceous pelagic sediments ( Marnes de Bidart ) under the Bay of Biscay (SW France). In both locations geochemical data suggest that these particles result from early diagenetic processes (see infra). If so, the effects of this diagenesis appear to be quite limited. Microcrystals are defined as calcitic particles (between 3 and 5 µm) with no biological shapes or microstructures detectable by optical and electronic microscopy. They correspond to the micarb of COOK & 5 EGBERT (1983), ERBA (1992), NOËL et alii (1994) and MATTIOLI & PITTET (2002). Oxygen and carbon isotope ratios and crystallographic data (DTA/TG) show that La Marcouline micarb are mainly minute fragments of Nannoconus spp. (BELTRAN, 2006). This shows that the carbonate fraction of La Marcouline sediments is dominanted by biogenic particles (around 90%) and that in both marls and limestones, Nannoconus spp. are the main carbonate producers (around 80% by volume). The non-carbonate portion of the samples is mainly clays and quartz. In both marls and limestones the clay fraction consists of illite (31%) and kaolinite (29%). The lithological contrasts between limestones and marls are caused by dilution cycles which reflect periodic changes in the supply of non-carbonate material during uniform carbonate duction (BELTRAN, 2006; BELTRAN et alii, 2007). Figure 4: Evolution of Mg content in bulk carbonate in Bedoulian to Gargasian sediments from the Cassis area (La Bédoule, Les Tocchis and La Marcouline). For outcrop locations see MOULLADE et alii (2004). 6 2- Magnesium 2.a- Long-term evolution and comparison with Bedoulian sediment contents At La Marcouline the magnesium content of the carbonates of Gargasian age is relatively higher ( ppm, mean: 4306 ppm) than that of Bedoulian strata in the same Cassis region (Fig. 4, RENARD & de RAFÉLIS, 1998). In these underlying beds the Mg contents ranges consistently from 2500 to 4500 ppm (mean = 3726 ppm), but in the sediments spanning the transition from the Deshayesi to the Furcata ammonite zones, a higher content (5000 to 6300 ppm) is related to the late Bedoulian anoxic event OAE1a. Compared to the geochemical data obtained for Bedoulian deposits (RENARD & de RAFÉLIS, 1998), the Gargasian samples show stronger fluctuations in both Mg and carbonate content. These differences may be caused by the different sampling procedures chosen. Indeed, the Bedoulian samples were taken from limestone beds only whereas the Gargasian ones were collected equally from both lithologies. Figure 5: Sr and Mg contents in the bulk carbonate of Gargasian beds in the La Marcouline quarry. Blue circles: Marls (soft beds); red circles: Limestones (hard beds). 7 2.b- Relationships between Mg content and lithology In a general way the evolution of the Mg content of the bulk carbonate is in phase with changes in lithology (Fig. 5): Marls have a higher contents (mean 4557 ppm, average ppm) than limestones (mean 4039 ppm, average ppm). Nevertheless, the long-term evolution of Mg content throughout the Gargasian succession may not be related to lithology alone. In the lower part of the sequences (beds 2 to 6, 22 to 26, 46 to 48, 57 to 60) many samples of both marl and limestone are low in magnesium while in the uppermost portion (from bed 64 to top) the content of magnesium in both marl and limestone increases in a similar manner. 2.c- Mg geochemical sequences in the La Marcouline succession. Throughout the Gargasian succession at La Marcouline there is an overall trend toward an increase in Mg contents from ppm at the base to ppm at the top (Fig. 5). The Mg content increases in five positive excursions, each ended by a negative shift. The lowest samples figured are the uppermost portion of the last geochemical sequence described by RENARD & de RAFÉLIS (1998) in Bedoulian outcrops. The first Gargasian Mg sequence runs from bed 3 (3313 ppm) to bed 22 (base of a well-characterized triplet in outcrop, 3113 ppm) and reaches its highest reading, 4818 ppm, in bed 17. This first sequence is attributed to the Cabri (Luterbacheri) and early Ferreolensis zones and is labelled Garg.Mg1, in accordance with the numbering system of RENARD & de RAFÉLIS (1998). In the second sequence, (Garg.Mg2), from bed 22 to 36 (3738 ppm), the Mg content reaches a maxinum of 4447 ppm in the upper part of bed 31. The third Mg sequence (Garg.Mg3) spans beds 36 to 46 (3098 ppm) with a maximum in bed 39 (5402 ppm). In the Garg.Mg4 sequence, Mg values rise to 5748 ppm in bed 46 and the upper boundary is again a negative shift, recorded in bed 58 (3206 ppm). The last well-exposed Mg sequence (Garg.Mg5) spans beds 58 to 86 (5041 ppm) with the highest Mg content (6046 ppm) in bed 82. At La Marcouline quarry, only the initial stages of the Garg.Mg6 sequence are accessible. 3- Strontium 3.a- Long-term evolution and comparison with Bedoulian sediment contents The increase in Sr in the sediments of the Bedoulian type locality (Fig. 6, RENARD & de RAFÉLIS, 1998) from the base of the Cabri Zone upward continues throughout the La Marcouline Gargasian series, rising from 626 ppm in bed 1 to 896 ppm in bed b- Relationship between Sr content and lithology The correlation of Sr content with variations in lithology is less obvious than it is for Mg (Fig. 5). Nevertheless, in marls the content is slightly higher (mean 711 ppm, average ppm) than in limestones (mean 699 ppm, average ppm). The effects of lithology are clear only at and near the base of the outcrop (base to bed 22) where the difference in the Sr content of marls and limestones is on the order of 40/50 ppm. In the higher sequences the long-term increase in Sr content screens difference caused by changes in lithology. 3.c- Sr geochemical sequences at La Marcouline (Fig. 5) As for Mg, the overall increase in Sr upward is supplemented by positive excursions marking the geochemical sequences. The first (Garg.Sr1) indicates an increase in Sr from 605 ppm (bed 3) to maxima of 732 ppm in beds 9 and 13 followed by a decrease to 588 ppm (bed 22). The negative shift in bed 22 (base of the outcrop triplet) seems to be a major geochemical break. The second positive excursion (Garg.Sr2) extends from bed 23 to bed 36 (646 ppm) with a maximum in bed 31 (759 ppm). The highest value of Garg.Sr3 is in bed 49 (758 ppm) and the sequence ends in bed 58 (681 ppm). We note that this sequence covers two magnesium sequences (Garg.Mg3 and 4). The upper boundary of sequence Garg.Sr4 is located at bed 86 (800 ppm). Its maximum content is recorded in bed 81 (896 ppm). 8 Figure 6: Evolution of Sr content in bulk carbonate in Bedoulian to Gargasian sediments from the Cassis area (La Bédoule, Les Tocchis and La Marcouline). 3.d- Correlation of Sr and Mg contents The development of the Sr and Mg profiles follow a similar course, so are easily comparable (Fig. 7); the long-term trends both show an upward rise but lithology had a greater effect on Mg than on Sr. Figure 4shows the correlation between these two trace elements.
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