Dela Pierre, F.

Natalicchio, M.

English

Institutional Research Information System University of Turin

GEOGACETA, 70, 2021Copyright© 2021 Sociedad Geológica de España / www.geogaceta.com 19Sulfate isotope composition of Messinian evaporites in the Piedmont basin (Italy)Composición isotópica del sulfato de las evaporitas Messinienses de la cuenca del Piamonte (Italia)Javier García-Veigas1, Luis Gibert2, Dioni I. Cendón3, Francesco Dela Pierre4, Marcello Natalicchio4 y David Artiaga11 CCiTUB, Scientific and Technological Centers, Universitat de Barcelona, 08028 Barcelona, Spain; garcia_veigas@ub.edu; dartiaga@ub.edu2 Departament de Mineralogia, Petrologia i Geologia Aplicada, Universitat de Barcelona, 08028 Barcelona, Spain; lgibert@ub.edu3 ANSTO, Australian Nuclear Science and Technology Organisation, Kirrawee DC, NSW 2232, Australia; dce@ansto.gov.au4 Dipartimento di Scienze della Terra, Università di Torino, 10125 Torino, Italy; francesco.delapierre@unito.it; marcello.natalicchio@unito.it ABSTRACT The Piedmont basin (NW Italy) records a Messinian Salinity Cri-sis (MSC) succession including a selenite gypsum deposit assigned to the Primary Lower Gypsum (PLG, MSC stage 1). Strontium isotope ratios are in the range of the PLG deposits of the Mediterranean area. Sulfate isotope compositions of vertically oriented selenite gypsum beds, in the lower part of the succession, are similar to those reported in other PLG deposits. However, flattened branching selenite cones in the upper part show higher isotope compositions, mainly in δ34S values, suggesting intense BSR conditions, stronger than reported in other PLG deposits. We interpret this chemical shift during deposition of the upper part of the PLG as the result of increased marine restric-tion assisted by the marginal position of this basin in the Adriatic Gulf during the Apennine and Alpine uplifts. RESUMEN La cuenca del Piamonte (NW Italia) contiene una serie Messi-niense que incluye una unidad de yeso selenítico atribuida al PLG (MSC estadio 1). La isotopía del estroncio confirma esta asignación. La isotopía del sulfato de los yesos seleníticos de desarrollo vertical de la parte inferior de la serie es comparable a la de otras series PLG del Mediterráneo. Sin embargo, los conos de desarrollo horizontal de la parte superior de la serie muestran composiciones isotópicas mayores, especialmente en δ34S, sugiriendo una intensa actividad bacteriana (BSR) no observada en otros depósitos PLG. Interpre-tamos esta diferencia como resultado de mayores condiciones de restricción marina de la cuenca del Piamonte debidas a la posición marginal de esta cuenca en el Golfo Adriático durante el levanta-miento de los Apeninos y los Alpes.IntroductionDuring the Messinian Salinity Crisis (MSC), the Mediterranean recorded the deposition of a large volume of evapori-tes (Fig. 1A). Based on seismic reflection profiles, more than 1 million of km3 of salts have been estimated below the cu-rrent Mediterranean (Ryan, 1973). MSC evaporites consist mainly of gypsum in circum-Mediterranean basins and in-clude chlorides in the deepest seafloor. The enticing idea of a desiccated Medi-terranean transformed in a giant salina suggested by Hsü et al. (1973) is today, fifty years after its proposal, under dis-cussion. Pending on the drilling of the deep Mediterranean, direct observations and studies of the MSC evaporites are limi-ted to gypsum deposits outcropping in western (Spain), central (Italy, Sicily), and eastern (Greece, Cyprus, Turkey) margins of the current Mediterranean. The chro-no-stratigraphic framework of the MSC (CIESM, 2008), mainly based on Sicily and extrapolated to the deep Mediterranean areas, propose three evolutionary sta-ges. During MSC stage 1 (5.97-5.60 Ma) selenite gypsum deposits of the Primary Lower Gypsum (PLG) formed in shallow peripheral basins while organic-rich sha-les and carbonate-rich beds accumulated in deep basinal areas. The acme of the MSC took place in MSC stage 2 (5.60-5.55 Ma), and is characterized by subae-rial exposure and erosion of the periphe-ral basins with the development of the Messinian Erosional Surface (MES). This surface becomes a correlative conformity in the deep basins located at the base of the Resedimented Lower Gypsum (RLG). Halite deposits with K-Mg salts preci-pitated in the intermediate deep Calta-nissetta basin (Sicily) at this time. Upper Gypsum and Lago Mare deposits formed during the MSC stage 3 (5.55-5.33 Ma). The Upper Gypsum, consisting of selenite gypsum beds intercalated within marls, was deposited in Sicily and in the eastern Mediterranean (Cyprus), while evapo-rite-free clastic sediments deposited in the western basins (Spain). The reestabli-shment of fully marine conditions at the base of the Zanclean (5.33 Ma) marks the end of the MSC.The MSC succession of the Piedmont basin was one of the reference sections used for the theory of a catastrophic desiccated Mediterranean (Sturani, 1976). The recent revision of the paleontological data in this basin (Carnevale et al., 2019) points to the persistence of marine ste-nohaline organisms throughout the MSC supporting the ‘deep-water deep-basin’ model for the Mediterranean.The isotope compositions of the dis-solved sulfate in seawater (δ34SSW and δ18OSW) and of the related marine preci-pitated gypsum (δ34Sgyp and δ18Ogyp) have changed through geological times. Sul-fate isotope compositions of δ34S ~ 22‰ and δ18O ~ 12‰ should be expected for late Miocene marine evaporites. Howe-ver, different but homogeneous sulfate isotope compositions have been repor-ted for MSC evaporites in different wes-tern Mediterranean basins (García-Vei-gas et al., 2018). PLG deposits (MSC stage 1) in Betic basins (Spain), and RLG depo-sits (MSC stage 2) from one section in Si-cily provide narrow isotopic ranges (δ34S: 22 - 24‰ and δ18O: 12 - 15‰). The isoto-pe composition of UG evaporites (MSC Geogaceta, 70 (2021), 19-22 Fecha de recepción: 20/10/2020ISSN (versión impresa): 0213-683X      Fecha de revisión: 23/04/2021ISSN (Internet): 2173-6545    Fecha de aceptación: 28/05/2021GEOGACETA, 70, 2021 J. García-Veigas, L. Gibert, D.I. Cendón, F. Dela Pierre, M. Natalicchio y D. Artiaga 20 Petrología y Geoquímica / Petrology and Geochemistrysive beds of vertically oriented selenite crystals, up to 10m thick, in the lower part of the succession (SW sector, Arnulfi sec-tion), to discrete beds, up to 2m thick, of flattened conical structures of branching selenites (Lugli et al., 2010) surrounded by a terrigenous matrix in the upper part (NE sector, Pollenzo section). A singular marker bed (SKB, Sturani key-bed) occurs between the lower and upper parts allowing regional correla-tion (Fig. 2). The SKB consists of flattened conical gypsum structures of branching selenite crystal aggregates growing in a fine-grained laminated gypsum.Samples for isotope analyses were recovered from the Arnulfi and Pollenzo sections (Fig.2).Strontium isotope ratiosSeven 87Sr/86Sr determinations in gypsum beds (Fig. 2) are close to 0.7089. Such narrow range of values is typical of - Sant’Agata Fossili Marls. Outer shelf to slope shale and marl couplets formed under progressively more restricted con-ditions.- PLG deposits (MSC stage 1). Seleni-te gypsum beds intercalated with shales. Towards the basin depocenter, gypsum beds disappear passing into carbona-te-rich layers and finally into organic-rich shales and marls.- Valle Versa Chaotic Complex (MSC stage 2). Chaotic gypsum and carbonate blocks interpreted as RLG deposits.- Cassano Spinola Conglomerates (MSC stage 3). Fluvio-deltaic and lacus-trine conglomerates.Alba Messinian section PLG gypsum beds outcrop in the sou-thwestern most part of the basin, close to the town of Alba (Fig. 1B). Gypsum li-thofacies change laterally and vertically (Figs. 1C and 2), from bedded and mas-stage 3) in Sicily and Cyprus provide si-milar isotope sulfur values (δ34S ~ 23‰) but significant higher values for oxygen isotope compositions (δ18O: 17 – 19‰). A stratified deep-water deep-basin model for the MSC Mediterranean is pro-posed in García-Veigas et al. (2018) based on the homogenous sulfate isotope com-positions. The aim of this work is to obtain accurate isotopic profiles of the norther-nmost MSC succession, in the Piedmont basin, and compare them with those re-ported for other MSC successions.The MSC succession in the Piedmont basin.The Piedmont basin (NW Italy, Fig. 1) is a wedge-top basin located in the in-ner side of the SW Alpine arc. The basin is filled with upper Eocene – late Mioce-ne sediments. The Messinian succession (Dela Pierre et al., 2011; Natalicchio et al., 2013) consists, in ascending order, of: Figure 1. A: Distribution of Messinian evaporites in the Mediterranean. B: Location of studied sections in the Piedmont Basin. C: Schematic cross section of the Messinian deposits in the Piedmont basin. Not to scale. PLG: Primary Lower Gypsum. (After Dela Pierre et al. 2011).Figura 1. A: Situación de las evaporitas del Messiniense en el Mediterráneo. B: Situación de las secciones estudiadas en la cuenca del Piamonte. C: Diagrama idealizado, sin escala, de los depósitos Messinienses en la cuenca del Piamonte. PLG: Yesos Inferiores Primarios. (Modificado de Dela Pierre et al., 2011).GEOGACETA, 70, 2021Isotopy of MSC evaporites in the Piedmont basin (Italy)21Petrología y Geoquímica / Petrology and Geochemistrypronounced in the conical structures of the upper part, only exposed in Pollen-zo, reaching values up to 27‰ for δ34S and up to 16‰ for δ18O. Such maximum values are higher than those reported in other PLG/RLG deposits suggesting a significant increase of the BSR rate in the Piedmont basin relative to other con-temporaneous Mediterranean gypsum deposits.The MSC Piedmont basin: a very restricted evaporite basin Unlike homogeneous isotope signa-tures in other MSC deposits, the PLG in the Piedmont is characterized by: (1) si-milar isotopic enrichments in the beds composed of vertically oriented selenite crystals of the lower part of the succes-sion; (2) late Miocene marine values in the SKB in the margins of the basin shif-ting to higher values towards the depo-the PLG deposits in the Mediterranean area confirming that the studied succes-sion corresponds to the MSC stage 1.Sulfate isotope compositionsSulfate isotope composition (Fig. 2) of the beds consisting of vertically orient-ed selenite crystals in the lower part (4 beds in Arnulfi, 2 beds in Pollenzo) shows a narrow range, with similar average δ34S (22.8‰ in Arnulfi, 23.4‰ in Pollenzo) and δ18O (12.4‰ in Arnulfi, 12.7‰ in Pol-lenzo) values.Important differences exist in the av-erage values obtained for the SKB bed in Arnulfi (δ34S: 22.4‰; δ18O: 12.5‰) with respect to the same bed in Pollenzo (δ34S: 25.1‰; δ18O: 14.5‰).Conical structures of non-vertically oriented selenites of the upper part, only present in Pollenzo, show higher aver-age isotopic values (δ34S: 26.0‰; δ18O: 15.1‰).δ34S values in the lower part of the Piedmont evaporites (~ 23‰) are sim-ilar to those found in other MSC PLG/RLG deposits. However, δ18O values (~ 12‰) overlap with those expected for late Miocene marine evaporites, being in the lower range reported for other PLG/RLG deposits (12 - 15‰). These data sug-gest that, vertically oriented selenites of the lower part formed from the ‘gypsum saturated layer’ affected by bacterial sul-fate reduction (BSR) as proposed in other MSC Mediterranean sections (García-Vei-gas et al., 2018).Sulfate isotopes of the SKB bed in the more marginal Arnulfi section match with ‘normal’ marine values (δ34S ~22‰; δ18O ~12‰). However, moving towards the depocentre, in Pollenzo, the same bed shows different values (δ34S ~25‰; δ18O ~14‰), with similar δ18O, but with higher δ34S values than those reported for other PLG/RLG deposits (δ34S ~23‰). Iso-topic enrichments, mainly in 34S, are more Figure 2. Sulfate isotope profiles (δ34S and δ18O) and 87Sr/86Sr ratios in Messinian gypsum beds of the Arnulfi and Pollenzo sections (Pied-mont basin). Data from the Ag1 basal gypsum bed, in the Arnulfi section, correspond to a fallen block. The yellow box indicates the ranges reported for other MSC sections in Western Mediterranean (García-Veigas et al., 2018).Figura 2. Perfiles isotópicos de sulfato (δ34S and δ18O) y valores de 87Sr/86Sr en los yesos Messinienses de las secciones de Arnulfi y Pollenzo (cuenca del Piamonte). Datos de la capa de yeso inferior en la sección de Arnulfi (Ag1) corresponden a un bloque caído. En amarillo: rangos registrados en otras cuencas messinienses del Mediterráneo Occidental (García-Veigas et al., 2018).GEOGACETA, 70, 2021 J. García-Veigas, L. Gibert, D.I. Cendón, F. Dela Pierre, M. Natalicchio y D. Artiaga 22 Petrología y Geoquímica / Petrology and GeochemistryAcknowledgementsResearch funded by the projects CGL-2013-42689 and CGL-2016-79458 of the Spanish Government. The authors are indebted to Francesca Lozar for the field trip, sampling and discussions, and to Es-ther Sanz Montero and Jesús Soria Min-gorance for their constructive reviews. ReferencesCarnevale, G., Gennari, R., Lozar, F., Nata-licchio, M., Pellegrino, L and Dela Pierre (2019). Bolletino della Società Paleonto-logica Italiana 58, 109-140.CIESM (2008). CIESM Whorkshop Mono-graphs 33, 168pDela Pierre, F., Bernardi, E., Cavagna, S., Clari, P., Gennari, R., Irace, A., Lozar, F., Lugli, S., Manzi, V., Natalicchio, M., Ro-veri, M. and Violanti, D. (2011). Palaeo-geography, Paleoclimatology, Palaeoe-cology 310, 238-255.Dela Pierre, F., Clari, P., Natalicchio, M., Ferrando, S., Giustetto, R., Lozar, F., Lu-gli, S., Manzi, V., Roveri, M and Violanti, D. (2014) Marine Geology 355, 71-87.García-Veigas, J., Cendón, D.I., Gibert, L., Lowenstein, T.K. and Artiaga, D. (2018). Marine Geology 403, 197-214.Hsü, K.J., Ryan, W.B.F. and Cita, M.B. (1973). Nature 242, 240-244.Sturani, C. (1976). Memorie della Società Geologica Italiana 16, 11-25.Lugli, S., Manzi, V., Roveri, M. and Sch-reiber B.Ch. (2010). Palaeogeography, Paleoclimatology, Palaeoecology 297, 83-99.Ryan, 1973. In: Drooger, C.W. (Ed.) Mes-sinian events in the Mediterranean, 26-38.Natalicchio, M., Dela Pierre, F., Clari, P., Birgel, D., Cavagna, S., Martire, L. and Peckman, J. (2013). Palaeogeography, Paleoclimatology, Palaeoecology 390, 68-80.from anoxic brines particularly deve-loped in the Piedmont basin. Further inwards the Piedmont basin, gypsum disappears passing to marls and carbo-nates with filamentous fossils formed in anoxic to euxinic conditions (Dela Pierre et al., 2014). Higher terrigenous inputs in the upper cycles point to major hydrologic changes, more humid climate conditions, and a general shallowing trend in the NW termination of the Adriatic Gulf (Dela Pierre et al., 2011).A marked brine stratification occurred in the Piedmont basin during develop-ment of the branching selenite cones of the upper cycles. These beds can be interpre-ted as growing in a little thick gypsum-sa-turated layer where vertically oriented se-lenite crystals cannot grow. This particular gypsum-saturated layer was developed between an upper well-oxygenated mari-ne water mass with increasing continental inputs, and a lower anoxic water mass in which strong reducing conditions avoided gypsum precipitation.The Piedmont basin, during the MSC, is interpreted as having formed in a strongly restricted and stratified basin, located within the northward end of a much larger evaporite basin, restricted and brine-stratified as well: the MSC deep-water deep-basin Mediterranean. ConclusionsThe lower PLG cycles in the Piedmont basin, characterized by beds of vertically oriented selenite crystals, were formed from the same ‘intermediate gypsum-sa-turated layer’ developed in the MSC stra-tified Mediterranean (García-Veigas et al. 2018) and can be compared with other PLG sections in the Mediterranean. Although marine inputs extend through the complete evaporitic succes-sion, the upper PLG beds in the Piedmont basin were formed in strongly reducing conditions developed in this more res-tricted basin.center; and (3) enrichments, up to 5‰ in δ34S and 4‰ in δ18O, relative to expected for marine evaporites, in the upper cycles only developed towards the depocenter of the basin.  The different isotope values of the upper gypsum beds in the Piedmont sections point to specific restriction con-ditions in the Piedmont basin compared to other contemporaneous MSC basins. Following the ‘deep-water stratified model’ proposed in García-Veigas et al. (2018), the lower gypsum cycles of verti-cally oriented selenites could have been formed in the intermediate gypsum-sa-turated layer of the stratified Medite-rranean water mass as is interpreted for other PLG deposits. Most likely, an important hydrolo-gical change occurred in the Piedmont basin since deposition of the SKB gyp-sum bed. The sharp change observed in the gypsum lithofacies of this bed is accompanied by ‘normal’ oceanic sul-fate isotopic values towards the basin margins, whereas strong isotopic enrich-ments, mainly in 34S, occur towards the depocenter. This change suggests local hydrochemical conditions in the Pied-mont basin compared to other contem-poraneous Mediterranean basins. Proba-bly, the intermediate gypsum-saturated layer where vertically oriented selenites formed, withdrew towards deeper Medi-terranean areas. However, marine inputs of the Mediterranean well-mixed upper layer (García-Veigas et al., 2018) must re-mained feeding the Piedmont basin as indicated by 87Sr/86Sr ratios.Gypsum lithofacies change in the upper beds to small isolated cones of non-vertical oriented branching seleni-tes. Strontium isotopes in these upper beds point to marine connections. However, higher sulfate isotope compo-sitions suggest strong 34S enrichments by BSR processes, much higher than those reported in other PLG/RLG deposits. We interpret the upper gypsum beds as formed under reducing conditions 

Sulfate isotope composition of Messinian evaporites in the Piedmont basin (Italy).

https://iris.unito.it/bitstream/2318/1865149/1/Garcia%20Veigas%20et%20al.%202021.pdf

Sulfate isotope composition of Messinian evaporites in the Piedmont basin (Italy).

Abstract

Similar works

Full text

Available Versions

Institutional Research Information System University of Turin